geochemical constraints on the formation of late cenozoic...
TRANSCRIPT
ELSEVIER Marine Geology 138 (1997) 149-169
Geochemical constraints on the formation of Late Cenozoic ferromanganese micronodules from the central Arctic Ocean
Bryce L. Winter *, Clark M. Johnson, David L. Clark
Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USA
Received 15 April 1996; accepted 10 February 1997
Abstract
In order to determine geochemical compositions of Late Cenozoic Arctic seawater, oxide fractions were chemically separated from 15 samples of hand-picked ferromanganese micronodules (50-300 pm). The success of the chemical separation is indicated by the fact that ~97% of the Sr in the oxide fraction is seawater-derived. Rare-earth element (REE) abundances of the Arctic micronodule oxide fractions are much lower than those of bulk Fe-Mn nodules from other ocean basins of the world (e.g., 33 vs. 145 ppm Nd), but the Arctic oxides are enriched in Ce relative to Nd (Ce,/Nd, = 2.2 f 0.5) and have convex-upward, shale-normalized REE patterns ( NdN/GdN =0.61 f 0.06, GdN/YbN = 1.5 + 0.2, NdN/Yb, = 0.9 f 0.2), typical of other hydrogenous and diagenetic marine Fe-Mn-oxides. Bulk sediment samples from the central Arctic Ocean have REE abundances and patterns that are characteristic of those of post-Archean shale. Non-detrital fractions (calcite +oxide coatings) of Recent Arctic foraminifera have REE abundances and patterns similar to those of Recent foraminifera from the Atlantic Ocean.
Electron microprobe analyses (n = 178) of transition elements in 29 Arctic Fe-Mn micronodules from five different stratigraphic intervals of Late Cenozoic sediment indicate that oxide accretion occurred as a result of hydrogenetic and diagenetic processes close to the sediment-seawater interface. Transition element ratios suggest that no oxide accretion occurred during transitions from oxic to suboxic diagenetic conditions. Only K is correlated with Si and Al, and ratios of these elements suggest that they are associated with illite or phillipsite. Ca and Mg are correlated with Mn, which indicates variable substitution of these elements from seawater into the manganate phase. The geochemical characteristics of Arctic Fe-Mn micronodules indicate that the REEs of the oxide fractions were ultimately derived from seawater. However, because of minute contributions of Sr from siliciclastic detritus during diagenesis or during the chemical leaching procedure, Sr isotope compositions of the oxide fractions cannot be used to trace temporal changes in the *‘Sr/“Sr ratio of Arctic seawater or to improve the chronostratigraphy. 0 1997 Elsevier Science B.V.
Kevwords. 87Sr/86Sr; Arctic Ocean; Ferromanganese nodules; Marine sediments; Rare-earth elements
1. Introduction
Fe-Mn-oxides are abundant and widespread marine precipitates that are important for under-
* Corresponding author.
standing marine geochemical cycles, submarine hydrothermal processes, and paleoceanography, including ocean circulation patterns, bioproductiv- ity, and the extent and location of oxygen mini- mum zones (e.g., Piepgras et al., 1979; Dymond et al., 1984; Halbach, 1986; Olivarez and Owen,
0025-3227/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZ SOO25-3227(97)00013-3
150 B.L. Winter et al. / Marine Geology 138 (1997) 149-169
1989; German et al., 1990; Hein et al., 1992a,b; Peucker-Ehrenbrink et al., 1994; Abouchami and Goldstein, 1995). Because of their scientific and potentially economic importance, Fe-Mn-oxides from ocean basins other than the Arctic have been extensively investigated. Minor amounts of Fe-Mn-oxide in the form of micronodules occur throughout much of the Late Cenozoic sediment in the central Arctic Ocean (Clark et al., 1980). Because polar ice plays a principal role in global oceanic and atmospheric circulation patterns, the Arctic Ocean has a strong influence on global climate evolution (cf. Clark, 1990; Aagaard and Carmack, 1994), and therefore, it is important to understand the sedimentologic details of the Arctic Ocean. In this investigation we place constraints on the origin of Fe-Mn micronodules (FMMNs) from the central Arctic Ocean (Fig. 1) by deter- mining the transition, minor, and rare-earth ele- ment (REE), and Sr isotope variations of the oxide and silicate fractions. Furthermore, REE abundances of Arctic foraminifera and bulk silicate sediment are measured and compared with both the micronodule data and published data for sedi- ment from other ocean basins. The data are used to evaluate the source of REEs and the potential of FMMNs to accurately record variations in the isotope composition of Arctic seawater throughout the Late Cenozoic.
Fig. 1. Location of Fletcher (FL in Table 1) and CESAR 11
cores in the central Arctic Ocean.
2. Origin of marine Fe-Mn-oxides
Seafloor Fe-Mn-oxides chiefly occur in the form of nodules and micronodules, crusts that are deposited on rock substrates, and coatings on sediment particles. Marine Fe-Mn-oxides are formed by precipitation from hydrothermal solu- tions, precipitation from ambient seawater (hydrogenous), and diagenetic processes (cf. Hein et al., 1992a). Each formation process may reflect a characteristic tectonic setting, and be distin- guished by mineralogy, growth rate, and chemical composition (cf. Halbach, 1986; Hein et al., 1992b).
2.1. Hydrogenous Fe-Mn-oxides
Hydrogenous Fe-Mn precipitates usually form distal to active spreading centers in regions that have low bulk sedimentation rates (< 10 mm kyr -‘) and moderate to low biologic productivity (<50-100 g C m-’ yr-i; cf. Muller et al., 1988; Cronan et al., 1991). Hydrogenous deposits accu- mulate very slowly ( l-l 5 mm Myr - ‘; Dymond et al., 1984; Mangini, 1988) by direct precipitation of colloidal hydrous metal oxide particles from near-bottom seawater in the form of crusts, nod- ules, and micronodules (cf. Halbach, 1986). Hydrogenous Fe-Mn precipitates are primarily composed of &MnO,, which is intergrown with FeOOH . nH,O, and in nodules is commonly intergrown with authigenic aluminosilicates (Burns and Burns, 1977; Hein et al., 1987). Hydrogenous accretion results in Fe-Mn-oxides that are highly enriched in REEs and transition metals (Mn/Fe < 2, Mn/Ni = 30-50; e.g., Calvert and Price, 1977; Elderfield et al., 1981; Clauer et al., 1984; Dymond et al., 1984).
2.2. Diagenetic Fe-Mn-oxides
Marine FeeMn macronodules (0.08- 15 cm) and micronodules (40-1000 urn) are commonly geo- chemically and mineralogically similar ( Addy, 1978, 1979; Staffers et al., 1981; Kunzendorf et al., 1989; Kunzendorf et al., 1993). Most Fe-Mn nodules and micronodules contain metals that are derived from diagenetic reactions near the sedi-
B.L. Winter et al. 1 Marine Geology 138 (1997) 149-169 151
ment-water interface. Reactions involving the degradation of organic matter are the most impor- tant (Dymond et al., 1984; Lyle et al., 1984; Halbach, 1986), as they enrich Fe-Mn-oxides in transition elements (particularly Mn, Ni, Cu, and Zn). Such reactions are unlikely to contribute REEs, which are principally derived from seawater (see below). The main oxide phase produced during diagenetic accretion is a 10-A manganate (i.e., todorokite; Halbach, 1986).
2.2.1. Oxic diagenesis Fe-Mn accretion in the surficial oxic zone of
marine sediments involves three possible reactions (cf. Dymond et al., 1984): ( 1) aerobic oxidation of organic matter and dissolution of biogenic components; (2) reaction between amorphous Fe-Mn-hydroxides and biogenic silica, which will fractionate Fe and Mn as Fe combines with silica to form clay minerals and Mn is released for incorporation into nodules; and (3) equilibrium adsorption of metals on nodule surfaces from other sedimentary phases. Oxic diagenetic accre- tion typically produces Fe-Mn-oxide that has Mn/Fe = 5-10 and Mn/Ni = 15-20 at the rate of lo-50 mm Myr-’ (cf. Dymond et al., 1984). Enrichment of Cu, Ni, and Zn in Fe-Mn-oxides occurs primarily during oxic rather than suboxic diagenesis, because Cu, Ni, and Zn are slightly more soluble (i.e., greater redox potential) than Mn.
2.2.2. Suboxic diagenesis Suboxic diagenetic reactions occur during the
transition from O,-dominated respiration to condi- tions where compounds such as NO,- and MnO, are the terminal electron acceptors during the decomposition of organic matter (Froelich et al., 1979). Fe--Mn-oxide, chiefly composed of todoro- kite, accretes at the rate of 100-200 mm Myr-’ during suboxic diagenesis and has Mn/Fe=20-70 and Mn/Ni=60-200 (Dymond et al., 1984). Under suboxic conditions, unstable Mn-oxide dis- solves (i.e., is reduced), and then subsequently this Mn is incorporated into growing nodules under more oxic conditions. Because Ni and Cu appear to stabilize the todorokite crystal structure (Bischoff et al., 1981; Usui et al., 1989), the
unstable Mn-oxide that is reduced during suboxic diagenesis will tend to be impoverished in Ni and Cu, which will further increase the Mn/( Ni + Cu + Zn) ratio of growing suboxic nod- ules (Bischoff et al., 1981; Dymond et al., 1984). The diffusion rate of Mn from the Mn reduction zone (N 10-l 5 cm below the sediment surface, Klinkhammer, 1980) through the overlying oxic sediment layer to nodules growing at the sediment surface is at least an order of magnitude less than Mn-Fe-oxide accumulation rates (Callender and Bowser, 1980). Consequently, steady-state upward diffusion of metals from the Mn reduction zone cannot explain the extreme Mn enrichments in nodules (cf. Dymond et al., 1984; Lyle et al., 1984). It has been suggested that episodic, high- productivity events may produce transient manga- nese reduction in the upper few centimeters of the sediment column, and that this explains Mn enrich- ment in nodules (Dymond et al., 1984).
3. Arctic sedimentology and stratigraphy
Late Cenozoic (Late Miocene through Holocene) sediment from the central Arctic Ocean (Fig. 1) is divided into 16 lithostratigraphic units (A3 to M; Fig. 2) that are correlated over several hundred thousand square kilometers in several hundred cores (Clark et al., 1980; Minicucci and Clark, 1983; Mudie and Blasco, 1985; Morris et al., 1985). Late Cenozoic Arctic Ocean sediment on the ridges and rises (i.e., <2500-m water depth; e.g., Alpha-Mendeleyev Ridge and Chukchi Rise) is composed of - 80% silty mud and w 20% sandy mud. The silty mud contains an average of -5% coarse (> 63 urn) sediment (Clark et al., 1980) and primarily consists of sediment rafted by sea ice (i.e., -2-m-thick ice that forms on the continental shelves and transports primarily silt and clay; Reimnitz et al., 1992), but also pelagic and authi- genie components (e.g., Fe-Mn micronodules). Sandy mud layers are distinctive markers that contain a higher percentage (g-25%) of dropstones (0.25-3 mm), and are interpreted to be sediment rafted by glacial ice (i.e., ice bergs that calve off of continental glaciers and deposit sediment of all size ranges) (Clark et al., 1980; Darby et al., 1989).
152 B.L. Winter et al. /Marine Geology 138 (1997) 149-169
I A2a I
Fig. 2. Late Cenozoic lithostratigraphy and chronostratigraphy of the central Arctic Ocean. Ages of the stratigraphic units are based on extrapolation from magnetic reversals using a sedi- mentation rate of 1 mm kyr I.
Late Cenozoic Arctic Ocean sediment younger than -2.4 Ma (units A to M; Fig. 2) is cyclic, composed of alternating sandy and silty mud. These alternating coarse and fine units are interpre- ted to be directly linked to glacial and interglacial periods, respectively (Clark et al., 1980; Boyd et al., 1984; Darby et al., 1989). Central Arctic Ocean sediment older than -2.4 Ma (units Al, A2, A3) is composed of silty mud that has virtually no dropstones, and is interpreted to be pre-glacial sediment that was probably deposited by sea ice (Clark, 1996).
Fe-Mn micronodules (50-300 urn in diameter) are a common component of the sand-sized
(> 63 urn) fraction of the silty mud, and are partic- ularly abundant in units A3 to A, parts of I, and all of unit IS (Fig. 2). Although Fe-Mn micronod- ules (FMMNs) can comprise up to 15-25 wt% of the sand-sized fraction, they comprise ~2 wt% of the total sediment. The FMMNs are friable aggre- gates of Fe-Mn-oxides and other sedimentary components, including quartz, illite, feldspar, chlo- rite, and foraminifera as identified microscopically, by X-ray diffraction, and transmission electron microscopy. Most Arctic micronodules do not display the microbanding in back-scatter electron imaging that is characteristic of Fe-Mn nodules from other ocean basins (e.g., Sugisaki et al., 1987). However, some Arctic FMMNs have a rim relatively enriched in Fe and Mn that is -25% of the radial thickness of the micronodule. X-ray diffraction analysis of FMMNs from three different stratigraphic intervals in the Arctic Ocean yielded strong peaks at 2.4 and 1.4 A and no reflections between 9.5 and 10 A, indicating that &MnO, is the predominant oxide phase (Burns and Burns, 1977), and consequently most of the Fe-Mn-oxide probably has a hydrogenous origin.
4. Analytical procedures
In order to determine the environment of Fe-Mn accretion, which is important for inferring the geochemistry of Arctic seawater, it is necessary to chemically isolate the oxide fraction of the FMMNs, because detrital material comprises 28-83 wt% of the micronodules (i.e., after hand- picking; Table 1). Experiments conducted in order to determine an optimum leaching procedure to separate the oxide and silicate fractions are described in Appendix A.
4.1. Analysis of Fe-&In micronodule samples
Hand-picked micronodules from 15 strati- graphic intervals were weighed (averaging 4-8 mg, Table 1 ), finely crushed, and then sequentially leached with 1 M ammonium acetate (NH,OAc) and 1% acetic acid (HOAc) before dissolving the oxide fractions with a mixture of 0.2 M ammonium oxalate and 0.2 M oxalic acid (AO-OA; see
Tab
le
1 L
ocat
ion,
st
ratig
raph
ic
posi
tion,
an
d sa
mpl
e m
ass
(in
mg)
da
ta
for
fora
min
ifer
a,
Fe-M
n m
icro
nodu
les
and
bulk
se
dim
ents
fr
om
the
Alp
ha
Rid
ge,
Cen
tral
A
rctic
O
cean
Sam
ple
No.
St
ratig
raph
ic
unit
Cor
e (d
epth
) L
atitu
de
Lon
gitu
de
Wat
er
dept
h T
otal
sa
mpl
e So
lubl
e”
Silic
ate
Perc
ent
Perc
ent
(.S
) (“
W)
(m)
mas
s (m
g)
mas
s m
ass
solu
ble
silic
ate
1 -pl
ank
uppe
rmos
t M
FL
-200
(O
&lc
m)
80
10.5
5 17
2 19
.63
3048
27
.58
22.9
4 4.
64
83.2
16
.8
2-pl
ank
uppe
rmos
t M
FL
-474
(O
-l
cm)
85 2
0.88
11
0 00
.23
1647
28
.30
23.9
6 4.
34
84.7
15
.3
2-be
nthi
c up
perm
ost
M
FL-4
74
(O-l
cm
) 85
20
.88
110
00.2
3 16
47
12.1
1 9.
04
3.07
74
.6
25.4
Fe-M
n tn
icro
nodu
lr5
3 m
iddl
e K
FL
-286
(1
06-1
09
cm)
84 0
0.84
14
4 02
.17
2316
6.
10
4.01
4 lo
wer
K
FL
-286
(1
12-1
15cm
) 84
00.
84
144
02.1
7 23
16
3.18
2.
54
5 m
iddl
e I
FL-2
86
(202
-205
cm
) 84
00.
84
144
02.1
7 23
16
6.62
1.
47
6 m
iddl
e G
FL
-443
(2
20-2
21
cm)
85
57.9
6 12
1 07
.71
2436
5.
09
1.23
1 E
/F
boun
dary
C
ESA
R
11 (
207-
209
cm)
85
50.9
0 10
8 21
.20
1380
6.
67
2.65
8 m
iddl
e D
FL
-443
(2
89-2
90
cm)
85
57.9
6 12
1 07
.71
2436
6.
63
2.46
9 lo
wer
D
FL
-275
(2
06-2
07
cm)
83
30.2
3 14
9 58
.64
2884
7.
04
2.42
10
C/D
bo
unda
ry
CE
SAR
11
(21
0-21
1 cm
) 85
50
.90
108
21.2
0 13
80
3.57
2.
51
11
uppe
rmos
t A
C
ESA
R
11 (
2252
28
cm)
85
50.9
0 10
8 21
.20
1380
1.
75
_
12
uppe
r A
C
ESA
R
11 (
2277
228
cm)
85
50.9
0 10
8 21
.20
1380
4.
11
2.76
13
A2-
A3
FL-3
80
(285
-286
cm
) 84
37
.54
128
27.8
9 24
01
15.6
4 4.
26
14
mid
dle
Al
CE
SAR
11
(276
-277
cm
) 85
50
.90
108
21.2
0 13
80
3.97
1.
97
15
uppe
r A
2 C
ESA
R
11 (
3333
334
cm)
85
50.9
0 10
8 21
.20
1380
8.
78
3.34
16
mid
dle
A2
CE
SAR
11
(39
5396
cm
) 85
50
.90
108
21.2
0 13
80
17.1
8 5.
65
17
low
er
A2
CE
SAR
11
(44
6-44
7 cm
) 85
50
.90
108
21.2
0 13
80
54.0
0 9.
06
18
uppe
r A
3 C
ESA
R
11 (
447-
448
cm)
85
50.9
0 10
8 21
.20
1380
4.
73
1.74
1.75
65
.7
0.64
79
.9
5.15
22
.2
3.86
24
.2
4.02
39
.7
4.17
37
.1
4.62
34
.4
1.06
70
.3
1.35
67
.2
11.3
8 27
.2
2.00
49
.6
5.44
38
.0
11.5
3 32
.9
44.9
4 16
.8
2.99
36
.8
Bul
k se
ditn
nr t:
19
uppe
rmos
t M
FL
-300
(0
-l
cm)
85
18.5
3 14
4 02
.85
2082
20
uppe
rmos
t M
FL
-508
(2
-3
cm)
84 0
7.50
11
2 13
.00
1866
21
uppe
rmos
t M
FL
-530
(O
-l
cm)
84 4
9.70
09
9 30
.60
1985
22
mid
dle
K
FL-4
28
(81-
82
cm)
86 0
3.18
13
4 35
.28
2271
23
mid
dle
K
FL-5
08
(107
-108
cm
) 84
07.
50
112
13.0
0 18
66
24
mid
dle
G
FL-4
28
(205
-206
cm
) 86
03.
18
134
35.2
8 22
71
25
low
er
F FL
-443
(2
68-2
69
cm)
85
57.9
6 12
1 07
.71
2436
26
low
er
D
FL-2
75
(206
6207
cm
) 83
30
.23
149
58.6
4 28
84
27
uppe
r A
l FL
-284
(2
54-2
55
cm)
83 4
7.34
14
5 50
.92
2681
28
uppe
r A
2 FL
-420
(4
17-4
18
cm)
84 4
6.59
12
2 55
.14
2248
29
mid
dle
A2
CE
SAR
11
(38
1-38
2 cm
) 85
50
.90
108
21.2
0 13
80
30
mid
dle
A3
CE
SAR
11
(49
7749
8 cm
) 85
50
.90
108
21.2
0 13
80
28.7
B
h
20.1
77.8
2
75.8
7
60.3
9
62.9
R
65.6
29
.7
5 _,
_ ,3
32.8
9
72.8
g
50.4
$
62.0
z
67.1
:
83.2
:
63.2
2 2 P 2 Y
)
Solu
ble
repr
esen
ts
the
calc
ite
frac
tion
and
oxid
e fr
actio
n fo
r fo
ram
inif
era
and
mic
rono
dule
sa
mpl
es,
resp
ectiv
ely.
154 B.L. Winter et al. / Marine Geology 138 (1997) 149-169
Appendix A). The silicate residues were then rinsed with water, dried, weighed, and spiked with mixed 87Rb-84Sr and REE tracers before dissolving in a mixture of concentrated HF and HNO, (see Appendix A). The mass of the oxide fractions is closely approximated by the difference between the total mass of the micronodules and the mass of the silicate residue. The solutions of the oxide fractions were split into two approximately equal aliquots; one was spiked with mixed 87Rb-84Sr and REE tracers, whereas the second aliquot was used for measuring high-precision Sr isotope com- positions without spike being present. Fe and Mn were collected during the Sr elution chemistry of the unspiked aliquots for concentration determin- ations by ICP-AES; this procedure results in mini- mum concentrations, as some Fe and Mn may be lost during column chemistry. We estimate the analytical error of the Fe and Mn contents deter- mined in this manner to be < lo-15%. Pb and Nd isotope compositions, and REE concentrations were determined on the same spiked solutions by isotope dilution.
4.2. Analysis offoramimfera and bulk sediment samples
Foraminifera, which were separated from the bulk sediment by hand-picking (Table 1 ), were cleaned with 1 M NH,OAc in an ultrasonic bath for several hours to remove easily exchangeable cations. Foraminifera samples were then repeatedly rinsed with water in an ultrasonic bath to remove any adhered sediment and as much of the sediment trapped inside the foraminifera chambers as pos- sible. Cleaned foraminifera were leached with 10% HOAc for N 1 h, which dissolved the calcite and presumably much of the oxide coatings (i.e., the non-detrital fraction), and then analyzed. The remaining silicate residue was dried, weighed, spiked, and then dissolved in a mixture of concen- trated HF and HNO, for 2 days in closed Teflon vials on a hot plate.
Unleached bulk sediment samples (“total” in Table 2) were rinsed repeatedly with water before spiking and dissolving with concentrated HF and HNO, in sealed Teflon vials on a hot plate for 3 days. The silicate fraction of bulk sediment samples
(“silicate” in Table 2) was isolated by first leaching with 10% HOAc for 4 h, and then leaching two times with 0.2 M AO-OA for 2 h in darkness while in an ultrasonic bath. The remaining silicate fraction was rinsed three times with water in an ultrasonic bath before drying, weighing, spiking, and finally dissolving with concentrated HF and HN03 in metal-jacketed Teflon bombs for 2 days at 200°C.
4.3. Instrumentalprocedures
4.3.1. Mass spectrometry Sr isotope ratios (n = 120) were measured on a
VG Instruments Sector 54 mass spectrometer using dynamic, multi-collector analysis in the University of Wisconsin Radiogenic Isotope Laboratory. Ratios were normalized to 86Sr/88Sr =0.1194, and Rb interference was continuously monitored at mass 85 and was always negligible. Twenty analy- ses of NBS-987 run during this time period yielded *‘Sr/*‘Sr = 0.710232 ( 2osE = 0.000015). REE con- centrations were determined by isotope dilution mass spectrometry using static multi-collector analysis. Based on analyses of rock standards (BCR-l), analytical error is estimated to be < +2%, except Nd which is < 20.5%. Total procedural blanks (Rb < 35 pg, Sr < 85 pg, Nd < 60 pg) have no influence on the results.
4.3.2. Electron microprobe Micronodules from five stratigraphic intervals
were analyzed as grain mounts with a Cameca SX50 electron microprobe using a 3-l.un beam, 15-kV accelerating voltage, 0.02~uA beam current, and integrating for 20 s (Table 3). As is commonly the case with electron microprobe analysis of Fe-Mn nodules, the element totals were less than 100% (e.g., Sugisaki et al., 1987); this probably is the result of incomplete elemental analysis (e.g., H,O, CO,), difficulty in obtaining smooth, pol- ished surfaces, and because the oxide matrix is porous at the microscopic level. Because of these problems, we use elemental ratios of the micro- probe data in our interpretations. ICP-AES analy- ses and averages of the microprobe analyses for Fe and Mn contents for samples from the same stratigraphic interval are in general agreement.
Table 2
B.L. Winter et al. / Marine Geology I38 (1997) 149-169 155
Rare-earth element, Fe, Mn, Sr, and Rb elemental data, and Rb-Sr isotope data for foraminifera, Fe-Mn micronodules and bulk
sediments from the Alpha Ridge, Central Arctic Ocean
Samplea Ce Nd Sm Eu Gd Dy Er Yb % Fe % Mn Sr Rb 87Sr/86Sr s7Rb/a6Sr
Foraminifera:
1 -plank. 10.0 4.65 1.05 0.240 0.969 0.809 0.396 0.296 -
1 -residue 48.9 19.1 2.99 0.608 2.15 2.22 1.65 1.87 -
2-plank. 4.95 5.72 1.33 0.311 1.37 1.11 0.551 0.404 -
2-residue 83.9 16.5 3.06 0.687 2.58 2.51 1.49 1.55 ~
2-benthic 3.08 2.24 0.51 0.131 0.509 0.440 0.240 0.189 -
FeeMn mi ‘cronodules:
3-oxide
3-residue
4-oxide
5-oxide
5-residue
6-oxide
6-residue
duplicate 7-oxide
7-residue
duplicate I-oxide
duplicate R-residue
Y-oxide
9-residue
1 O-oxide
lo-residue
ll-oxfres
12-oxide
duplicate 12-residue
13-oxide
duplicate 13-residue
1Coxide
duplicate 14-residue
15-oxide
15-residue
16-oxide
duplicate 16-residue
1 ‘J-oxide
17-residue
1 S-oxide
1 g-residue
95.8 21.1 4.52 1.18 4.58 4.41 2.46 2.17 3.58
97.5 38.2 6.12 1.30 4.26 3.75 2.60 2.85 -
101 15.6 3.45 1.56 3.76 3.77 2.03 1.80 4.19
277 77.4 16.3 3.92 16.2 14.6 7.26 6.19 15.0
47.3 19.3 3.28 0.650 2.32 1.92 1.15 1.24 ~
151 43.5 9.00 2.22 9.32 8.43 4.24 3.18 6.35
55.4 22.6 3.71 0.859 2.67 2.50 1.61 1.74 - _ _ _ _ _ _
86.1 13.6 2.98 0.706 3.074 3.06 2.11 1.56 5.58
48.0 19.3 3.06 0.862 2.146 2.03 1.41 1.55 - _ _ _ _ _ _ _ _
146 33.0 7.02 1.65 6.76 11.4 3.89 2.37 5.05 _ _ _ _ _ _ _ _
57.09 23.1 3.80 0.842 2.78 2.64 1.81 1.90 ~
133 37.5 8.62 2.00 8.63 7.58 4.03 3.18 5.30
62.1 25.3 4.12 0.790 2.84 2.48 1.71 1.86 ~
74.8 15.5 3.45 0.846 3.60 3.29 1.71 1.47 2.82
81.4 33.5 5.53 1.25 3.80 3.39 2.32 2.42
122 45.2 9.07 3.30 9.31 - - 4.14 ~
79.2 25.4 5.50 1.34 5.57 5.31 2.88 2.49 3.86 _ _ _ _ _
134 59.3 ~ ~ ~ 6.04 3.92 4.12 -
1564 37.2 8.81 2.19 - 10.7 5.74 4.81 12.8 _ _ _ _ _ _
224 32.0 5.64 1.23 4.07 2.98 1.66 1.63 -
84.0 25.5 5.58 1.41 5.80 5.35 3.03 2.89 4.37 _ _ _ _ _ _
54.5 22.9 3.84 0.920 3.35 2.56 1.73 1.79 -
126 27.3 6.01 1.46 6.43 5.92 3.27 3.05 5.19
50.5 21.1 3.43 0.853 2.39 2.15 1.38 1.46 -
126 23.1 5.56 1.77 7.29 7.93 4.48 3.95 7.46 _ _ _ _ _ _ _ _
58.9 28.2 4.94 1.33 3.88 2.93 1.73 1.74
268 51.5 13.1 3.41 15.7 16.2 8.62 7.25 11.5
51.2 21.5 3.78 0.808 2.94 2.37 1.43 1.40 ~
166 40.4 8.74 2.07 9.45 8.77 4.87 4.61 7.80
51.9 20.6 3.29 0.733 2.28 1.83 1.13 1.18 -
2.23
4.80
11.4
7.63 _
_
10.12 _
_
6.50 _
9.44
5.60
4.38
_ 23.7
_ 4.91
_ 7.02 _
11.4 _
34.7 _
13.2
_ 138 _
_ 218
26.0
121
71.8
202
_ 143
143
55.9
212
164
267
265
31.8
31.9
298
218 _ _ 248 110
_ _ 232 138
51.7
216
77.4 _ 211
233
189
54.0
117 _
5.52
195
3.20
27
92
7.10
110 _
10.9
135
135
12.9
138
10.5
98.6
200
127
12.7
12.7
260
8.15
4.86
109
5.26 _
108
20.6
78.8 _
103
0.709208 rt 08 _ 0.718887i 10 2.45
0.709204 + 08 _
0.709201 kO8
0.711012+18
0.724573 & 10
0.709941+23
0.710350* 14
0.719395* 10
0.709716+_ 19
0.715656k 10
0.715658 k 10
0.710936+25
0.714104+11
0.714100 *09
0.710326+22
0.710313+21
0.715835&09
0.710179+21
0.715947* 10
0.709563 +_21
0.715069_fO9
0.712640* 11
0.710568 + 10
0.710611+ 10
0.715389F 11
0.709529 + 09
0.709534*09
0.713077* 10
0.709461_+21
0.709458 k21
0.714730+09
0.709219+ 10
0.715080+08
0.709009~ 10
0.708996 + 11
0.714837+09
0.709021_+07
0.713154*34
0.709154+10
0.716713+ 10
_ 2.59
0.36
2.21
0.286
1.58
2.72
2.73
0.668 _
1.89
1.75
2.17
1.39
1.16
1.15
2.53
0.108
1.29
._ 1.71
0.272
1.46 0.20 _.
1.48 0.256
1.21 _. 5.52
156 B.L. Winter et al. /Marine Geology 138 (1997) 149-169
Table 2 (continued)
Bulk sediment:
19-total 80.2 29.9 5.68 1.21 4.64 4.05 2.25 2.13 - - - _ _
20-total 98.0 35.2 6.82 1.41 4.49 5.21 2.92 2.73 - 111 133 0.725062 07 + 3.45 20-silicate 101 35.1 6.16 1.24 4.93 4.49 2.68 2.68 132 129 0.725009 kO8 2.83 21-total 105 35.0 6.54 1.48 5.58 4.69 2.58 2.40 - ~ - _ _
22-total 108 37.4 7.33 1.55 5.98 5.72 3.25 3.09 - 127 131 0.720089 09 _+ 3.00 22-silicate 93.2 31.2 5.39 1.10 4.26 4.01 2.50 2.55 158 135 0.720398 +08 2.46 23-total 76.3 31.8 6.13 1.34 5.21 5.07 3.04 2.86 ~ - 106 120 0.723861+09 3.29 duplicate _ _ ~ 106 0.723849 If: 10 3.29 24-total 78.3 31.8 6.18 1.28 3.66 4.78 2.75 2.71 ~ - 123 122 0.717787*08 2.86 duplicate - _ _ _ ~ 123 - 0.717778 kO8 - 24-silicate 81.9 32.0 5.66 1.14 4.51 4.20 2.59 2.64 - - 145 124 0.717279*08 2.47 25-silicate 66.1 28.0 4.82 0.966 3.77 3.53 2.24 2.31 - 136 112 0.717408_+09 2.37 duplicate ~ _ _ _ _ - 140 - 0.717385+09 - 26-silicate 82.0 34.8 6.03 1.21 4.64 4.37 2.80 2.92 - ~ - 142 0.717057+ 10 - 27-total 78.4 34.2 6.72 1.42 4.77 5.14 3.00 2.93 - - 137 123 0.714057 +09 2.59 27-silicate 70.7 30.2 5.28 1.07 3.97 3.51 2.27 2.42 ~ - 157 116 0.713772*07 2.14 duplicate - _ _ _ _ ~ 153 0.713784+08 - 28-silicate 91.1 36.0 6.15 1.22 4.67 4.06 2.51 2.58 ~ ~ 129 138 0.722878 + 09 3.10 29-silicate 62.3 26.5 4.48 0.928 3.24 3.03 1.92 2.03 189 108 0.715659+09 1.65 30-silicate 62.9 27.4 4.65 0.944 3.44 3.10 2.03 2.14 - ~ 153 118 0.716293-&08 2.24
“w = non-detrital fraction of planktonic foraminifera separates; benthic = non-detrital fraction of benthic foraminifera separates; & = oxide fraction of Fe-Mn micronodule separates; residue = silicate residue fraction of foraminifera and micronodule separates; 11 -ox + res is the total dissolution (oxide + silicate fractions) of a micronodule separate; total = bulk, non-leached sediment; silicate = silicate fraction of bulk sediment samples. See Table 1 for location and stratigraphic information for each sample. REE, Rb, and Sr concentrations (in ppm) were determined by isotope dilution; Fe and Mn concentrations (weight W) were determined by ICP-AES on the same solutions. In-run precisions given for Sr isotope analyses are 2-sigma errors. -=not analyzed.
5. Processes of Fe-Mn-oxide precipitation
Distinctions between hydrogenous, oxic diage- netic, and suboxic diagenetic processes of Fe-Mn- oxide precipitation have been proposed on the basis of the relative abundances of Mn, Fe, and (Ni + Cu + Zn) on ternary diagrams (Bonatti et al., 1972; Dymond et al., 1984). Fe-Mn micronodules were analyzed from five different stratigraphic intervals in Late Cenozoic sediment from the central Arctic Ocean (Table 3). Five micronodules (21 analyses) from interval 1 (Table 3) all plot very close to the hydrogenous end-member composition (Fig. 3A). Eight micronodules from interval 2 (Table 3; 57 analyses) and two micronodules from interval 3 (Table 3; 10 analyses) all plot between the hydrogenous and oxic diagenetic end-member compositions (Fig. 3B and C), indicating a mix- ture of transition metals derived by these two processes. Analyses of the rim and core are distin- guished in Fig. 3C for those micronodules from
intervals 2 and 3 that have a rim in back-scatter electron (BSE) imaging. These data (Fig. 3C) indi- cate that the chemically zoned FMMNs initially grew by hydrogenous processes, followed by accre- tion of the rims during oxic diagenesis.
Most (68%) of the 37 analyses of seven FMMNs from interval 4 (Table 3) plot near the Mn apex (Fig. 3D), indicating that accretion chiefly occurred during suboxic diagenesis. The remaining analyses (32%) from interval 4 plot near the hydrogenous end-member composition. Given the scatter between hydrogenous and suboxic end- member compositions (Fig. 3E), six FMMNs (43 analyses) from interva2 5 (Table 3) probably formed by sub-oxic and hydrogenous processes. However, one micronodule (#7, 9 analyses) from interval 5 has a chemical composition that ranges between the hydrogenous and oxic diagenetic end- members (Fig. 3E).
The transition element relations illustrated in Fig. 3 are well-defined for FMMNs from a particu-
B.L. Winter et al. /Marine Geology 138 (Z997) 1495169 157
Fe Mn
Intervals 2 &
Mn
Fe Mn Fe Mn
Fig. 3. Fe/Mn/(Cu+Ni +Zn) x 10 ternary diagrams illustrating the mode of Fe-Mn accretion (Bonatti et al., 1972; Dymond et al.,
1984) for Late Cenozoic FMMNs from the Arctic Ocean.
A. Microprobe analyses (n=21) of five micronodules from Interval I (FL-286, 231-234 cm, upper unit K).
B. Microprobe analyses (n=27) of five micronodules that are homogeneous in back-scatter electron imaging (BSE) from Interval
2 (CESAR 11, 210-211 cm, boundary of units C and D) and Interval 3 (CESAR 11, 395-396 cm, lower unit A2).
C. Analyses (n=40) of five FMMNs from Intervals 2 and 3 that are zoned in BSE imaging.
D. Analyses (n = 37) of seven micronodules from Interval 4 (CESAR 11, 4433444 cm, lower unit A2). E. Microprobe analyses (n =42) of 8 FMMNs from Interval 5 (CESAR 11, 443-444 cm, lower unit A2). Analyses of micronodule
Tab
le
3 E
lect
ron
mic
ropr
obe
data
fo
r th
e ox
ide
mat
rix
of
Fe-M
n m
icro
nodu
les
from
th
e A
lpha
R
idge
, C
entr
al
Arc
tic
Oce
an
Sam
ple”
M
gO
A12
03
SiO
z K
zO
CaO
co
M
n Fe
N
i cu
Z
n M
n/Fe
Inte
rval
1
(FL
-286
, 23
1-23
4 cm
; up
per
K):
#1 (
5)
1.44
kO.2
8 8.
47 +
0.95
22
.8 f
15
.2
#2 (
3)
1.41
kO
.3
35.0
2_+1
.4
622.
2k8.
7 #3
(1)
1.
51
10.3
48
.4
#4 (
6)
1.46
+_0.
31
8.75
k3.8
5 19
.4&
12
.4
#5 (
6)
2.19
kO.2
4 8.
80+1
.14
17.9
k4.6
Inte
rval
2
(CE
SAR
11
, 21
0-21
1 cm
; C
/D
boun
dary
):
#l-c
ore
(3)
1.67
kl.1
1 7.
7lkO
.94
23.8
k8.4
#l-r
im
(8)
2.58
kO.6
3 8.
5912
.68
35.4
+ 17
.5
#2 (
3)
2.23
+ 1
.65
10.4
k2.5
34
.5+
11.1
#3-c
ore
(3)
0.98
kO.1
5 5.
19kO
.6
812.
5kO
.6
#3-r
im
(3)
4.48
kO
.44
10.5
t1.7
17
.8 k
O.8
#4 (
6)
4.23
kO.5
1 8.
36kO
.72
12.9
kl.O
#5 (
6)
2.67
+ 1.
54
11.3
k5.4
30
.7 k
18
.4
#6-c
ore
(4)
1.77
+ 1.
01
5.93
kO.9
6 12
.Okl
.8
#6-r
im
(3)
2.54
k1.1
3 12
.9k2
.7
22.4
k3.6
#7-c
ore
(4)
1.11
+0.3
3 7.
8lk2
.79
27.8
* 18
.8
#7-r
im
( 1)
1.
43
12.7
6 28
.28
#‘l-
rim
(5
) 4.
78kO
.58
9.62
+2.2
1 15
.3k3
.9
#8 (
8)
2.83
kO
.25
8.9O
k4.0
6 19
.4+9
.8
Inte
rval
3
(CE
SAR
11
, 39
5-39
6 cm
; lo
wer
A
2):
#1 (
4)
1.64
kO.2
41
3.6k
3.3
25.1
k3.
1
#2-c
ore
(3)
1.16
+0.5
41
4.4k
5.7
29.5
k6.8
#2
-rim
(3
) 2.
61 k
O.8
4 7.
121-
4.3
319.
6+
16.0
1.45
kO.3
3
1.09
kO.4
9
1.80
1.34
kO.6
1.31
kO
.23
1.11
+0.3
5
1.65
kO
.45
3.35
k 2
.82
0.98
kO.1
9
1.43
* 0
.54
1.07
+0.
30
1.44
kl.4
8
0.91
kO
.15
2.22
+ 0
.79
1.33
kO
.38
2.59
1.32
kO.5
6
1.48
+ 0
.98
2.09
_+0.
87
1.21
kO
.72
1.05
+ 0
.72
0.88
+ 0
.33
0.67
+0.
08
0.78
31.1
0+0.
41
0.91
io
.22
0.26
+0
.09
0.43
kO.1
5
0.37
*0.1
4
0.29
kO.0
8
0.63
+0.1
1
0.58
k 0
.05
0.36
+0
.26
0.42
kO.1
4
0.34
+0.0
6
0.67
k
0.36
0.34
1.03
*0.1
5
0.43
* 0
.07
0.85
+0.1
7
0.45
+_0.
17
1.42
f0.6
9
<O.l
<O.l
<O.l
0.16
,O.ll
to.1
<O.l
<O.l
to.1
10.1
0.16
,0.0
5
0.19
+0.0
3
0.28
+0.1
5
to.1
<O.l
<O.l
<O.l
<O.l
co.1
<O.l
<O.l
<O.l
6.18
*1.6
0 5.
1lkl
.75
10.0
9 <O
.l <O
.l 1.
3kO
.3
6.37
kO.9
3 2.
54kO
.36
0.12
_+0.
06
to.1
to
.1
2.5
+0.4
3.69
4.
21
< 0.
09
10.1
<O
.l 0.
9 6.
59k2
.68
4.67
kl.6
7 0.
14&
0.09
<O
.l to
.1
1.4k
O.5
7.
56+
1.06
4.
27kO
.49
0.24
+0.0
7 <O
.l <O
.l 1.
8kO
.3
11.2
k7.3
13.9
k4.1
11.2
25.3
7.87
kO
.97
2l.O
kO.8
21.6
kO.9
11.6
k4.9
10.3
+ 3
.0
9.39
f
1.28
7.16
k2.3
4
3.31
22.9
k3.3
16.1
k2.
6
4.13
k2.0
7
5.88
+ 3
.29
4.97
k1.8
1
4.72
+
1.02
4.53
+0.7
9
5.85
* 1.
54
5.35
k2.1
0
4.69
+0.
56
5.81
&
1.88
4.99
*
1.07
6.02
4.38
+0
.49
4.75
kl.1
8
0.75
+0
.78
1.15
+0.4
1
1.00
+0.7
2
0.28
+0.2
1
2.11
+0.0
4
1.95
*0.2
0
0.72
k 0
.37
0.62
+0.5
5
0.57
+0.
32
0.24
kO.1
6
<0.0
9
2.29
kO.5
4
1.22
kO.2
3
<O.l
0.15
+0.0
5
10.1
to.1
0.43
kO.1
9
0.58
kO.1
3
0.19
+0.0
6
0.25
+0.1
6
0.48
+0
.09
<O.l
<O.l
0.22
+0.1
0
<O.l
<O.l
0.26
kO.1
0
0.17
*0.0
5
<O.l
0.41
kO
.13
0.48
iO.0
9
0.12
&0.
06
0.20
+0.1
7
0.19
*0.0
9
<O.l
<O.l
0.20
*0.0
2
0.14
*0.0
3
2.5k
O.4
2.8k
l.l
2.5_
+ 1.
2
1.8k
O.5
4.7f
0.6
3.9+
0.8
2.5k
l.4
2.3
+0.9
1.9+
0.8
1.4k
O.3
0.55
5.3+
1.1
3.6k
O.9
8.08
k3.0
2 8.
07kl
.24
0.13
+0.1
6 10
.1
<O.l
l.OkO
.3
4.55
+1.0
0 5.
3Okl
.72
c 0.
09
<O.l
<O.l
0.9k
O.2
19
.9kl
1.7
5.96
kO.7
6 l.O
S+O
.66
<O.l
10.1
3.
6k2.
4
B. L. Winter et al. / Marine Geology I38 (1997) 149-169 159
3’--3c+-+-H3c
d d 6 d d d d d d d d 6 d vvvvvvvvvvvvv
‘34-+---4-e3
dddddddddddd vvvvvvvvvvvv
160 B.L. Winier et al. / Marine Geology 138 (1997) 149-169
lar stratigraphic interval and suggest that accretion resulted from hydrogenesis, or a combination of hydrogenous and diagenetic processes. The sequence of oxide accretion processes for Arctic FMMNs that have distinct zones in BSE imaging (Fig. 3C) is in agreement with that inferred for partially buried nodules from other ocean basins (e.g., Dymond et al., 1984; Calvert and Piper, 1984). Oxide accretion commonly begins with hydrogenous precipitation at the sediment-water interface, whereas diagenetic reactions become a more important source of transition metals just below the sediment surface (Halbach, 1986). In the Arctic Ocean, both oxic diagenetic and suboxic diagenetic reactions are inferred from the trans- ition-element data, but there is no evidence for Fe-Mn accretion occurring during a change from oxic to suboxic conditions (Fig. 3). The data are consistent with previous studies that suggest that diagenetic Fe-Mn accretion occurs close to the sediment-water interface (i.e., within a depth of several centimeters) under either oxic or suboxic bottom water conditions, and that the level of bottom water oxygenation is primarily dependent on shallow-water biologic productivity (Dymond et al., 1984; Lyle et al., 1984; Halbach, 1986).
Mn and Ni in Arctic FFMNs that contain a mixture of hydrogenous and oxic diagenetic com- ponents (intervals 2 and 3) are directly correlated (R2=0.89, n =68; Fig. 4). Nodules from the east- ern equatorial Pacific that primarily accreted during suboxic diagenesis have inversely correlated Mn and Ni (Dymond et al., 1984), whereas FMMNs from the Arctic Ocean that have a strong suboxic component (intervals 4 and 5) are compar- atively enriched in Mn, but very impoverished in Ni (Fig. 4). Arctic FMMNs that primarily accreted during suboxic diagenetic conditions con- tain a hydrogenous component, whereas the Pacific suboxic nodules (Dymond et al., 1984) initially accreted during oxic diagenesis, which is the princi- pal process that concentrates Ni. Arctic hydroge- nous FMMNs (interval 1) have relatively low transition-metal contents, as is typical for this formation process (Dymond et al., 1984). Mn-Zn and Mn-Cu relations are similar to those of Mn-Ni (Fig. 4) for the respective accretion pro- cesses, except that the absolute enrichment of Ni
1
0 0.2 0.4 0.6 0.8
NVAl
Fig. 4. Ni/Al vs. Mn/Al diagram for 29 Fe-Mn micronodules (164 analyses) from 5 different stratigraphic intervals in the central Arctic Ocean.
in the hydrogenous-oxic diagenetic FMMNs is much greater than that of Cu and Zn.
Arctic FMMNs have Mn/Fe ratios for the different oxide accretion processes that are consis- tent with ranges reported for nodules from other ocean basins (cf. Dymond et al., 1984; Halbach, 1986). The average Mn/Fe ratio for the hydroge- nous Arctic FMMNs (interval 1, Table 3) is 1.6 f. 0.6, whereas FMMNs that are composed of a mixture of hydrogenous and oxic diagenetic components (intervals 2 and 3, Table 3) have an overlapping, but wider range (Mn/Fe = 2.8 + 1.6). FMMNs that grew primarily during suboxic dia- genesis (intervals 4 and 5, Table 3) have the highest Mn/Fe ratios ( 12.1+ 10.9), and because many of these micronodules have a hydrogenous compo- nent, Mn/Fe ratios have considerable variability.
6. Light element associations
Si and K are both directly correlated with Al (Table 3; Si-Al: r2 =0.71 when normalized to Mn, n = 177; K-Al: r2 = 0.73 when normalized to Mn, II= 177). The mean Si/Al (1.52.2), K/Si (O.ll-0.23), and K/Al (0.20-0.29) weight ratios for Arctic FMMNs from the five sampled intervals
B.L. Winter et al. I Marine Geology 138 (1997) 149-169 161
are most consistent with those of illite (Si/Al = 1.2-1.4, K/Si=0.2-0.35, K/Al=O.3-0.45) and phillipsite (Si/Al= 1 J-2.7, K/Si =0.3-0.6, K/Al = 0.5-0.9). We interpret these relations to indicate that most Si, Al, and K in the oxide fractions of the Arctic FMMNs are associated with sub-um- sized aluminosilicate material (authigenic or detri- tal ) that is intergrown with Fe-Mn-oxides. Dymond et al. ( 1984) noted strong correlations between Al (and Si) and Cu, the later of which is principally enriched during oxic diagenetic accre- tion; they interpreted these correlations to indicate that authigenic aluminosilicate precipitation is cou- pled with oxic diagenetic Fe-Mn accretion. Other than Fe, which is weakly correlated with Al (r2= 0.59 when normalized to Mn, II = 177), no other analyzed elements show a significant correlation with Al or Si in the Arctic FMMNs; this may suggest that some Fe is associated with aluminosili- cate material in the oxide fractions.
Magnesium contents in the Arctic FMMNs cor- relate with Mn (?=0.81 when normalized with Al, n = 177), and Mg does not correlate with Fe, which are similar to the relations noted for nodules from other ocean basins. These relationships are best explained by variable substitution of Mg, probably derived from seawater, into the todorokite structure (Burns and Bums, 1977; Bischoff et al., 1981; Turner and Buseck, 198 1). Calcium in the Arctic FMMNs is better correlated with Mn (Fig. 5) than it is with Fe, which may suggest that Ca is present in the todorokite structure. End-member hydroge- nous micronodules (interval 1) define a Ca-Mn correlation trend (r2 = 0.77 when normalized to Al, y1=21) that has a lower slope (i.e., lower Mn/Ca ratio) than the trend (r2 = 0.77, 12 = 147) defined by the hydrogenous-suboxic and hydrogenous-oxic micronodules (intervals 2-5; Fig. 5); this may in part be explained by the relatively low Mn contents of hydrogenous Fe-Mn precipitates. There is no clear relationship between Ca and Fe, and without P data it is difficult to determine if phosphate is the primary carrier phase of Ca.
7. Seawater origin of leached fraction
Sr isotope data for Arctic FMMNs demonstrate that our oxide leach fractions reflect >97% sea-
15
$ 10
0
0 0.2 0.4 0.6 0.8
CdAl
Fig. 5. Ca/Al vs. Mn/AI diagram for Fe-Mn micronodules from 5 different stratigraphic intervals in the central Arctic Ocean. The 6 analyses that have Mn/Al>20 (see Fig. 4) fall on the hydrogenoussoxic and hydrogenous-suboxic correlation (n = 147) shown here and are used to calculate the correlation statis- tics, but they were not included in this diagram in order to illustrate the different slope of the correlation defined by the hydrogenous micronodules at low Mn/AI ratios.
water Sr. Bulk Arctic sediment and the silicate fractions of FMMNs have similar average Sr contents ( Srbulk = 137 f 2 1 ppm, Srresidue = 198 + 60 ppm; Table 2) and ranges of *‘Sr/*?$r ratios (0.7131-0.7251). Oxide fractions of the FMMNs have markedly less radiogenic and far less variable *7Sr/s6Sr ratios (0.7090-0.7110, mean = 0.70986) relative to the silicate fractions. In addition, the oxide fractions have much lower 87Rb/86Sr ratios (0.1-0.4) than the FMMN silicate fractions and bulk sediment samples (87Rb/*6Sr = 1.2-3.4). The lowest 87Sr/86Sr ratios of the oxide fractions are similar to the Sr isotope range inferred for seawater during the Late Cenozoic (0.70900-0.70925; cf. Farrell et al., 1995). However, because of low Sr contents (Sr = 26-233 ppm) in the oxide fractions compared to most other seawater precipitates (Sr=500 to > 1000 ppm), even minor contribu- tions (2-3%) of very radiogenic Sr from the detrital silicate fractions, during diagenesis or laboratory leaching, will result in a substantial increase (~0.0002) in the 87Sr/86Sr ratio of the FMMN oxide fractions. An increase of 0.0002 in the
162 B.L. Winter et al. / Marine Geology 138 (1997) 149-169
La Ce Nd SmEuGd Dy Er Yh Lu
h4icronodule . Silicate Fraction
h.
60.2’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ La Ce Nd SmEuGd Dy Er YhLu
Silicate Fraction
11 1 I I I I I1 I I I I I
La Ce Nd SmEuGd Dy Er Yb Lu
Average Bulk Sediment D.
Tj:,iy=g2g~ 1 R La Ce Nd SmEuGd Dy Er nJ Lu
Fig. 6. A. Average and range (shaded area) of shale-normalized REE patterns for the oxide fractions of Fe-Mn micronodules (n = 15) from Late Cenozoic sediments of the central Arctic Ocean. Pertinent average REE ratios (+ 1~) are: Ceu/Nd,= 2.3 kO.5, Ndu/Gd,=0.61 kO.06, Eu,/Eu,* = 1.02 kO.06, GdN/YbN= 1.5 +0.2, Ndu/Ybu =0.9 +0.2 (N denotes shale-nor- malization); these ratios are very similar to those of Fe-Mn macronodules (Piper, 1974; Elderfield et al., 1981; Glasby et al., 1987; Nath et al., 1992; Pattan and Banakar, 1993), micronod- ules (Addy, 1979; Kunzendorf et al., 1989), and crusts (Aplin, 1984; De Carlo and McMurtry, 1992) from other ocean basins.
“Sr/*%r ratio is -20 times the precision that is required (-JO.OOOOl) for applying Sr isotopes to Late Cenozoic seawater studies. We cannot, there- fore, use Sr isotope compositions of FMMNs to refine Arctic chronostratigraphy. In contrast, Sr isotope compositions of foraminifera from the Arctic Ocean are consistent with the Sr isotope variations in the lower-latitude oceans (Winter et al., 1996a).
8. Rare-earth element variations
8.1. Bulk sediments
All bulk sediment samples from the central Arctic Ocean have similar REE contents (Nd = 33 f 2 ppm; C3 + REE = 57 + 4 ppm; Table 2), and they have REE patterns that are typical of those for post-Archean shale (Taylor and McLennan, 1985). Chondrite-normalized bulk sediment samples are LREE enriched (NdcN/YbcN = 4.0-5.2; CN denotes chondrite normalization), have relatively flat HREE patterns (Gd,,/Yb,, = l&1.9), and have negative Eu anomalies [ Eu/Eu* = Et&( SmCN*Gd,,)‘.’ = 0.70-0.751. In order to recognize and illustrate subtle varia- tions, REE contents of the different components will hereafter be discussed in terms of normaliza-
The REE patterns of Arctic micronodule oxide fractions are distinctly different, however, from those of hydrothermal depos- its from other ocean basins (Ce,/Nd,=0.3 kO.2, NdN/YbN= 0.7kO.2, EuN/EuN*= - , 1 2 5 2. Clauer et al., 1984; Ruhlin and Owen, 1986; Barrett and Jarvis, 1988; German et al., 1993; Mills and Elderheld, 1993).
B. Average and range (shaded area) of shale-normalized REE patterns for the silicate fraction of Arctic Fe-Mn micronodules (n= 14; Nd,/Gdu= 1.3kO.l; Dy,/Yb,=0.8fO.l).
C. Shale-normalized REE patterns for the silicate and non- detrital fraction (i.e., Fe-Mn coatings plus calcite lattice) of foraminifera samples from the Arctic Ocean. Pertinent average REE ratios for the non-detrital fractions of Arctic foraminifera arc: Ndu/Yb,= 1.3, Gd,/Yb,= 1.86, Ce,/Ndu=0.66, which are very similar to those of Atlantic foraminifera (NdN/YbN = 0.9, Gd,/YbN = 1.39, Ce,/Nd,=0.57; Palmer, 1985).
D. Average REE patterns for bulk (i.e., non-leached) sedi-
ment (n=7; NdJYb,= 1.2kO.l; GdN/YbN= 1.2kO.l) and the silicate fraction of bulk sediment (n=9) from the central
Arctic Ocean.
B. L. Winter et al. / Marine Geology 138 (1997) 149-169 153
tion to shale (North American Shale Composite, NASC; Haskin et al., 1968; Gromet et al., 1984). All bulk Arctic sediments have relatively flat shale- normalized REE patterns (Fig. 6).
8.2. FMMN oxide fractions
REE contents of the oxide fractions (Table 2) of Arctic FMMNs are more variable (Nd = 33 f 16 ppm, X3 + REE = 63 + 32 ppm) than the REE contents of the bulk sediments, although the average REE contents are similar. REE contents of the oxide fractions of Arctic FMMNs are considerably lower than those of most Fe-Mn nodules (Nd = 145 + 45 ppm, X3 + REE = 290 ) 75 ppm) and hydrogenous crusts (Nd = 240 f 80 ppm, X3 + REE =455 * 140 ppm) from other ocean basins (Fig. 6). REE abundances of Arctic FMMNs are most similar to those of nodules from the southeast equatorial Pacific (Bauer and Peru Basins), which have some of the lowest REE contents reported in the literature for Fe-Mn nodules (Nd = 55 IfI 20 ppm; Elder-field and Greaves, 1981; Glasby et al., 1987). REE patterns for all Arctic FMMN oxide fractions are similar, but are distinctly different from the relatively flat patterns of the bulk sediments (Fig. 6). The oxide fractions have convex-upward patterns centered on Gd, positive Ce anomalies (Ce,/NdN > 1 ), and NdN/YbN = l.OkO.3, which are typical of pre- viously analyzed Fe-Mn nodules, micronodules (hydrogenous and diagenetic), and hydrogenous crusts from other ocean basins (Fig. 6).
The low REE contents of Arctic FMMNs are probably explained, at least in part, by Fe contents that are at the low end of the range measured for nodules from other ocean basins (Table 2; Fig. 7). REE contents are directly correlated with Fe contents in marine nodules (Fig. 7), which indi- cates that REEs are primarily hosted by a hydroge- nous Fe-oxyhydroxide phase. The low Fe contents may be the result of: (1) a decreased introduction of Fe (i.e., from fluvial, aeolian, or hydrothermal processes) to the perennially ice-covered Arctic Ocean, (2) ocean circulation patterns that inhibit delivery of colloidal material to bottom sediments on Arctic ridges and rises (e.g., Alpha-Mendeleyev Ridge), or (3) the vertical transport process of
25 1
s0&c8ntl8/ P8Cmc 0 I I
0 100 200 300 400
pm Nd
Fig. 7. Weight percent Fe vs. Nd content (ppm) for marine Fe-Mn-oxides. Oxide fractions of Arctic micronodules are denoted by the so/id dots. Data for macronodules from other oceans are from Addy (1979), Elderfield and Greaves (198 1). Elderfield et al. (1981) Glasby et al. (1987) and Nath et al. (1992). Central Pacific crust data are from Aplin (1984).
metals by organically-coated particles through the highly stratified Arctic Ocean. Alternatively, the low abundance of hydrogenous metals in the Arctic FMMNs may be related to growth rates.
8.3. Silicate fractions of bulk sediment, FMMN, andforaminifera samples
Silicate fractions of bulk sediment (Nd = 31+3 ppm, X3+REE=51&6ppm) and FMMNs (Nd=28+10ppm, C3+REE=3919ppm) have REE abundances that are similar to the non- leached bulk sediment samples, but silicate frac- tions of foraminifera samples have distinctly lower REE contents (Nd = 18 ppm, I;3 + REE = 29 ppm; Table 2). The later probably reflects a greater proportion of silt-sized quartz, which is not easily removed during cleaning of the foraminifera. All leached silicate fractions have similar REE patterns (i.e., concave-upward centered on Dy; Fig. 6), which is approximately the mirror image of the FMMN oxide fractions. Mass-balance calculations indicate that bulk Arctic FMMNs (oxide + silicate fractions) have REE contents (Nd = 26 + 4 ppm, C3 + REE = 46 + 7 ppm) and patterns that are vir- tually identical to the bulk sediments, except for positive Ce anomalies (Ce,/NdN = 1.8 fO.4). The
164 B.L. Winter et al. J Marine Geology 138 (1997) 149-169
MREE depletion of the silicate fractions of Arctic sediment components (Fig. 6) is a common pattern of silicate sediment that has been leached by weak acids (Ohr et al., 1991, 1994; Awwiller, 1994; Weber et al., 1995), and is the result of partial dissolution of REE-rich oxide and/or phosphatic phases during chemical leaching.
8.4. Non-detritalfraction offoraminifera
The non-detrital fractions (calcite + oxide coat- ings) of Arctic foraminifera samples (Table 2) that dissolved in 10% HOAc have REE concentrations (Nd=2.2-5.7 ppm, C3 +REE=4.3-10.8 ppm) and patterns (Fig. 6) that are similar to 15 fora- minifera samples from the Atlantic Ocean (Nd = 1.1-4.7 ppm, X.3 + REE = 2.0-9.3 ppm) analyzed by Palmer (1985). Palmer (1985) demonstrated that most (- 90%) REEs in the non-detrital frac- tion of Atlantic foraminifera are present in Fe-Mn- oxides that coat the foraminifera, and only a minor proportion occurs in the calcite lattice; our data are consistent with this interpretation (Fig. 6).
9. Origin of REEs in arctic Fe-Mn-oxide fractions
REE contents of bulk nodules from lower-lati- tude oceans are commonly correlated with Fe, Ca, and P contents (Calvert and Price, 1977; Elderfield et al., 1981; Glasby et al., 1987; Nath et al., 1992; this work - Fig. 7), which has led previous workers to propose that REEs are hosted by either a single ferriphosphate phase, or two phases con- sisting of phosphate (diagenetic and/or bone phos- phate) and Fe-oxyhydroxide. REE patterns do not yield insight into the composition of the host phase, because concentrates of phosphate and Fe-Mn-oxide commonly have similar convex- upward (i.e., MREE enriched) REE patterns. Regardless of the specific REE carrier phase, there is strong evidence that REEs in Fe-Mn nodules in general, and the Arctic Ocean in particular, are ultimately derived from seawater, even for nodules that have a strong diagenetic component:
(1) REE patterns for most Fe-Mn-oxide nod- ules are very similar to those of hydrogenous precipitates that encrust rock substrates (i.e.,
convex upward with CeN/NdN > 1 and NdN/Yb, = l.Of0.3; Fig. 6).
(2) REEs are inversely correlated or have no correlation with K, Al, or Si (Calvert and Price, 1977; Glasby et al., 1987; Nath et al., 1992), indicating that silicate material (detrital and authi- genie) does not contribute significant amounts of REEs to marine Fe-Mn precipitates as a result of inclusion during growth or transfer during diagenesis.
(3) Diagenetic Fe-Mn nodules have lower REE contents than nodules that are chiefly hydrogenous (Nath et al., 1992). Furthermore, there is no correlation between REE contents and burial depth for the Arctic FMMN oxide fractions, sug- gesting that REEs are not progressively added to nodules during burial diagenesis.
(4) REE contents are commonly directly corre- lated with Fe contents (Fig. 7), abundance of 6- MnO,, and Ce enrichment, but inversely correlated with Mn/Fe ratios and abundance of todorokite. These relationships indicate that REEs are concen- trated in the hydrogenous, Fe-oxyhydroxide phase of nodules (Elderfleld et al., 1981; Glasby et al., 1987; Nath et al., 1992).
(5) Nd isotope compositions of Fe-Mn nodules and foraminifera coatings from the lower-latitude oceans are nearly identical to those of the overlying seawater (Goldstein and O’Nions, 1981; Palmer and Elderfield, 1985; Ben Othman et al., 1989) and in the central Pacific are dramatically different than the detrital silicate fraction of the encom- passing sediment (cf. Jones et al., 1994).
(6) Sr isotope compositions of the oxide fraction of nodules are very close to that of contemporane- ous seawater from which the oxide precipitated (Goldstein and O’Nions, 1981; Clauer et al., 1984; Futa et al., 1988; Ingram et al., 1990; Amakawa et al., 199 1; this study) and are commonly much less radiogenic than the encompassing silicate sediment.
Seawater derivation of REEs in the Arctic oxide fractions implies that Nd isotope compositions and probably Pb isotope compositions of the micronodule oxide fractions and non-detrital frac- tions of foraminifera samples represent those of Arctic seawater from which the Fe-Mn-oxides precipitated (Winter et al., 1995, 1996b).
B.L. Winter et al. /Marine Geology 138 (1997) 149-169 165
10. Conclusions
Fe-Mn-oxide micronodules are a common, but minor component of Late Cenozoic sediment from the central Arctic Ocean. Fe-Mn accretion in each stratigraphic interval occurred initially by hydroge- nous processes, and then usually continued during oxic or suboxic diagenesis close to the sedi- ment-water interface. The transition element data do not support continued oxide growth during deeper burial. Because the mode of Fe-Mn-oxide accretion is primarily controlled by the flux of biogenic components, further detailed analysis of FMMNs ( Fe-Mn micronodules) may provide general information concerning temporal varia- tions in shallow-water biologic productivity in the Arctic Ocean, the latter of which is virtually unknown in Arctic sediments older than -2.4 Ma (cf. Clark, 1996).
Sr isotope compositions of the FMMN oxide fractions are close to those of Late Cenozoic seawater (i.e., >97% seawater Sr) and are much less radiogenic than FMMN silicate fractions and bulk sediments. These relations indicate that chem- ical leaching, utilizing ammonium oxalate and oxalic acid, successfully concentrates the hydroge- nous portion of Fe-Mn nodules. However, minor contributions of Sr from detrital components result in an increase in the *‘Sr,YZr ratios of the oxide fractions and prevents their use for chronostrati- graphic purposes. Silicate fractions and bulk sedi- ments have very high “Sr/?Sr ratios, which suggests derivation from ancient continental crust.
Bulk sediments from the central Arctic Ocean have REE contents and patterns that are typical of post-Archean shale. Silicate fractions of bulk sediment, FMMNs, and foraminifera samples have concave-upward shale-normalized REE patterns, which are typical of leached silicate sediment and suggest partial dissolution of phosphatic and/or oxide phases during chemical separation in the laboratory. Oxide fractions of Arctic FMMNs and the non-detrital fractions of Arctic foraminifera have convex-upward REE patterns that are charac- teristic of the REE patterns of Fe-Mn nodules, micronodules, hydrogenous crusts, and foramini- fera coatings from other ocean basins. This distinc- tive REE pattern, the low 87Sr/86Sr ratios, and the
fact that REE contents do not increase with burial depth, indicate that the REEs of the Arctic micro- nodule oxide fractions were ultimately derived from seawater.
Acknowledgements
We thank Jan Boyer for conducting the chemical leaching experiments. We thank John Foumel for assistance with electron microprobe analyses. We thank Ed Sholkovitz and an anonymous reviewer for their comments. This work was supported by NSF grants OPP-9122741 (D.L.C. and C.M.J.), OPP-9400254 (D.L.C. and B.L.W.), EAR-9406684 (C.M.J.), EAR-9 105966 (C.M.J.), and EAR-9304455 (C.M.J.).
Appendix A
In order to determine the optimum procedure to chemically separate the oxide and silicate frac- tions of Arctic Fe-Mn micronodules (FMMNs), several leaching experiments were conducted using Sr isotopes because of the very large difference between the Sr isotope composition of seawater (87Sr/86Sr ~0.709) and detrital silicate sediment (87Sr/86Sr ~0.720). Consequently, Sr isotope com- positions will be exceptionally sensitive to any detrital contamination of the hydrogenous fraction.
Procedures Approximately 350 mg of FMMNs, which were hand-picked
from core CESAR 11 (416-420 cm depth; Fig. l), were finely powdered and split into four fractions for one bulk analysis and three sequential leaching experiments (#l, #2, and #3 in Table 4) using different strengths of acetic acid (l%, lo%, and 20% HOAc, Table4). All reagents that were utilized had low blank levels, and did not influence the results. The bulk micronodule aliquot, which was not leached, was weighed and then dissolved with concentrated HF and HNO, in a closed Teflon vial on a hot plate for 2 days. The remaining three micronodule aliquots were first leached with 1 M ammonium acetate (NH,OAc) for 24 h in an ultrasonic bath at room temperature, which removes easily exchangeable cations. After centrifuging, the NH,OAc leachate and one water rinse were pipetted off and combined for analysis. Each of the residues were then leached with a different strength of HOAc (Table 4)
166
Table 4
B. L. Winter et al. / Marine Geology I38 (1997) 149-169
Sr isotope data for sequential leaching of Fe-Mn micronodules from the Central Arctic Ocean (416420-cm depth in the CESAR 11 core)
Exp. # 1 -*l% HOAc EXP. # 2 ~ *lo% HOAc
*‘Sr/s6Sr % total Sr
NH,OAc 0.709175 20.51 HOAc* 0.709219 5.34 AO-OA 0.708808 18.82 1 MHCl 0.710390 1.97 Residue (HF ) 0.716175 53.37 Total dissolution 0.713533 142 ppm Sr
87Sr/*6Sr total Sr
0.709255 0.70 0.709534 0.64 0.708919 2.22 0.710478 0.18 0.714974 96.25
EXP. # 3 - *20% HOAc
*‘Sr/a6Sr % total Sr
0.709285 0.80 0.709529 0.64 0.708975 0.21 0.710460 0.32 0.714419 98.04
Oxide fraction noted in bold.
for 4 h in an ultrasonic bath at room temperature. HOAc dissolves calcite and leaches labile Sr. After centrifuging, the HOAc leachate and three water rinses were pipetted off and combined. The remaining residue for each of the three experi- ments was leached for 2 h in darkness in an ultrasonic bath at room temperature with a mixture of 0.2 M ammonium oxalate and 0.2 M oxalic acid (AO-OA), which dissolves Mn-oxides and amorphous Fe-oxide, but does not attack crystalline goe- thite or Fe-rich smectite (Landa and Gast, 1973; Heath and Dymond, 1977). After centrifuging, rinsing, and decanting the AO-OA leachates, the residues were leached with 1 M HCl for 4 h at room temperature in an ultrasonic bath in order to evaluate the effect of a stronger acid. Finally, after centrifuging and pipetting off the HCl leachates and one water rinse, the remaining residues (primarily silicates) were dried, weighed, and dissolved with concentrated HF and HNO, in closed Teflon vials on a hot plate for 2 days. Before converting to a chloride form for column chemistry, the leachates at each stage were passed through au acid-cleaned l-urn filter to ensure removal of any suspended residue material.
Results and interpretations The 87Sr/86Sr ratios of the 1 A4 NH,OAc leachates and the
1% HOAc leachate range from 0.70918 to 0.70929 (Table 4), which is very similar to that of modem seawater (0.70918) and is interpreted to reflect removal of surface-adsorbed Sr derived from modem pore fluids. The 10% and 20% HOAc leachates have 87Sr/86Sr ratios (0.70953) that are higher than that of modem seawater (Table4), suggesting that stronger HOAc leaches some radiogenic Sr from detrital grains, in addition to adsorbed modem seawater Sr. The Fe-Mn-oxide fractions dissolved by AO-OA (after removal of adsorbed seawater Sr) have the lowest s’Sr/86Sr ratios (0.70881-0.70898). There is a strong correlation between the Sr isotope composition, percent total Sr of the oxide fraction, and the strength of the preceding HOAc leach. The ?jr/s6Sr ratio of the AO-OA leachate that was preceded by the weakest HOAc leach (1%) is the least radiogenic (0.70881), and therefore, this leachate most closely approximates the Sr isotope composition of the seawater from which the oxide precipitated. AO-OA leachates of oxide frac-
tions that were previously leached in 10% and 20% HOAc have lower proportions of Sr, and are therefore more susceptible to slight contamination by radiogenic Sr from silicates. These data indicate that some radiogenic Sr from silicates is always leached by the 0.2 M AO-OA mixture, and consequently, measured *‘Sr/s’Sr ratios of the oxide fractions must be consid- ered maximum 87Sr/86Sr ratios of the parent seawater. 1 M HCl leachates of the residue after dissolution of the oxides have markedly higher Sr isotope ratios (0.71039-0.71048, Table4), indicating that this aggressive leaching results in significant contributions of Sr from detrital silicates. Complete dissolution of the final silicate residue in HF and HNO, yields lower *‘Sr,@Sr ratios for the two aliquots previously leached in stronger HOAc, in agreement with 10% and 20% HOAc having leached significant amounts of radiogenic Sr from the silicate fraction.
References
Aagaard, K., Carmack, E.C., 1994. The Arctic Ocean and cli- mate: a perspective. In: Johannessen, O.M., Muench, R.D., Overland, J.E. (Eds.), The Polar Oceans and Their Role in Shaping the Global Environment. Am. Geophys. Union, Geophys. Monogr. 85, 5-20.
Abouchami, W., Goldstein, S.L., 1995. A lead isotopic study of Circum-Antarctic manganese nodules. Geochim. Cos- mochim. Acta 59, 180991820.
Addy, S.K., 1978. Distribution of Fe, Mn, Cu, Ni and Co in coexisting manganese nodules and micronodules. Mar. Geol. 28, M9-M19.
Addy, S.K., 1979. Rare earth element patterns in manganese nodules and micronodules from northwest Atlantic. Geochim. Cosmochii. Acta 43, 1105-l 115.
Amakawa, H., Ingri, J., Masuda, A., Shimizu, H., 1991. Iso- topic compositions of Ce, Nd, and Sr in ferromanganese nodules from the Pacific and Atlantic Oceans, the Baltic and Barents Seas, and the Gulf of Bothnia. Earth Planet. Sci. Lett. 105, 554-565.
B.L. Winter et al. /Marine Geology 138 (1997) 149-169 167
Aplin, A.C., 1984. Rare earth element geochemistry of Central ment composition and sedimentary processes in the Arctic Pacific ferromanganese encrustations. Earth Planet. Sci. Lett. Ocean. In: Hearman, Y. (Editor), The Arctic Seas. van 71, 13-22. Nostrand Reinhold, New York, NY, pp. 657-720.
Awwiller, D.N., 1994. Geochronology and mass transfer in Gulf Coast mudrocks (south-central Texas, U.S.A.): Rb-Sr, Sm-Nd and REE systematics. Chem. Geol. 116, 61-94.
Barrett, T.J., Jarvis, I., 1988. Rare-earth element geochemistry of metalliferous sediments from DSDP Leg 92: The East Pacific Rise transect. Geochim. Cosmochim. Acta 67, 243-259.
Ben Othman, D., White, W.M., Patchett, J., 1989. The geo- chemistry of marine sediments, island arc magma genesis, and crust-mantle recycling. Earth Planet. Sci. Lett. 94, 1-21.
Bischoff, J.L., Piper, D.Z., Leong, K., 1981. The aluminosilicate fraction of North Pacific manganese nodules. Geochim. Cos- mochim. Acta 45,2047-2063.
Bonatti, E., Kraemer, T., Rydell, H., 1972. Classification and genesis of submarine iron-manganese deposits. In: Horn, D.R. (Ed.), Papers on a Conference About Ferromanganese Deposits on the Ocean Floor. Natl. Sci. Found., Washington, DC, pp. 149-166.
De Carlo, E.H., McMurtry, G.M., 1992. Rare-earth element geochemistry of ferromanganese crusts from the Hawaiian Archipelago, central Pacific. Chem. Geol. 95, 235-250.
Dymond, J., Lyle, M., Finney, B., Piper, D.Z., Murphy, K., Conard, R., Pisias, N., 1984. Ferromanganese nodules from MANOP sites H, S, and R - Control of mineralogical and chemical composition by multiple accretionary processes. Geochim. Cosmochim. Acta 48,931-949.
Elderfield, H., Greaves, M.J., 1981. Negative cerium anomalies in the rare element patterns of oceanic ferromanganese nod- ules. Earth Planet. Sci. Lett. 55, 163-170.
Elderfield, H., Hawkesworth, C.J., Greaves, M.J., Calvert, S.E., 198 1. Rare earth element geochemistry of oceanic ferroman- ganese nodules and associated sediments. Geochim. Cos- mochim. Acta 45, 513-528.
Boyd, R.F., Clark, D.L., Jones, G., Ruddiman, W.F., McIn- tyre, A.. Pisias, N.G., 1984. Central Arctic Ocean response to Pleistocene earth-orbital variations. Quat. Res. 22, 121-128.
Bums, R.G., Bums, V.M., 1977. Mineralogy. In: Glasby, G.P. (Ed.), Marine Manganese Deposits. Elsevier, Amsterdam, pp. 185-245.
Callender, E., Bowser, C.J., 1980. Manganese and copper geo- chemistry of interstitial fluids from manganese nodule-rich pelagic sediments of the northeastern equatorial Pacific Ocean. Am. J. Sci. 280, 1063-1096.
Calvert, S.E., Piper, D.Z., 1984. Geochemistry of ferromanga- nese nodules from DOMES Site A, northern equatorial Pacific: Multiple diagenetic metal sources in the deep sea. Geochim. Cosmochim. Acta 48, 1913-1928.
Calvert, S.E., Price, N.B., 1977. Geochemical variation in ferro- manganese nodules and associated sediments from the Pacific Ocean. Mar. Chem. 5, 43-74.
Clark, D.L., 1990. Arctic Ocean ice cover; geologic history and climatic significance. In: Grantz, A., Johnson, L., Sweeney, J.F. (Ed%), The Geology of North America. Geol. Sot. Am. L, pp. 53-62.
Farrell, J.W., Clemens, SC., Gromet, L.P., 1995. Improved chronostratigraphic reference curve of late Neogene seawater %/*‘%r. Geology 23, 403-406.Froelich, P.N., Klinkham- mer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B., May- nard, V., 1979. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagen- esis. Geochim. Cosmochim. Acta 43, 1075-1090.
Futa, K., Peterman, Z.E., Hein, J.R., 1988. Sr and Nd isotopic variations in ferromanganese crusts from the Central Pacific: Implications for age and source provenance Geochim. Cos- mochim. Acta 52,2229-2233.
German, C.R., Klinkhammer, G.P., Edmond, J.M., Mitra, A.. Elderfield, H., 1990. Hydrothermal scavenging of rare-earth elements in the ocean. Nature (London) 345, 516-518.
German, CR., Higgs, N.C., Thomson, J., Mills, R., Elderfield, H., Blusztajn, J., Fleer, A.P., Bacon, M.P., 1993. A geochem- ical study of metalliferous sediment from the TAG hydro- thermal mound, 26”08’N, Mid Atlantic Ridge. J. Geophys. Res. 98, 9683-9692.
Clark, D.L., 1996. The Pliocene record in the central Arctic Ocean. Mar. Micropaleontol. 27, 157-164.
Clark, D.L., Whitman, R.R., Morgan, K.A., Mackey, S., 1980. Stratigraphy and glacial-marine sediment of the Amerasian Basin, Central Arctic Ocean. Geol. Sot. Am., Spec. Pap. 181, 57 pp.
Glasby, G.P., Gwozdz, R., Kunzendorf, H., Friedrich, G., Thijssen, T., 1987. The distribution of rare earth and minor elements in manganese nodules and sediments from the equa- torial and S.W. Pacific. Lithos 20, 97-113.
Goldstein, S.L., O’Nions, R.K., 1981. Nd and Sr isotope rela- tionships in pelagic clays and ferromanganese deposits. Nature (London) 292, 324-327.
Gromet, L.P., Dymek, R.F., Haskin, L.A., Korotev, R.L., 1984. The “North American shale composite”: Its compilation, major and trace element characteristics. Geochim. Cos- mochim. Acta 48, 2469-2482.
Clauer, N., Stille, P., Bonnot-Courtois, C., Moore, W.S., 1984. Nd-Sr isotopic and REE constraints on the genesis of hydro- thermal manganese crusts in the Galapagos. Nature (London) 3 11, 743-745.
Halbach, P., 1986. Processes controlling the heavy metal distri- bution in Pacific ferromanganese nodules and crusts. Geol. Rundsch. 75 (l), 235-247.
Cronan, D.S., Hodkinson, R.A., Miller, S., 1991. Manganese nodules in the EEZ’s of island countries in the southwestern equatorial Pacific. Mar. Geol. 98, 425-435.
Darby, D.A., Naidu, A.S., Mowatt, T.C., Jones, G., 1989. Sedi-
Haskin, L.A., Haskin, M.A., Frey, F.A., Wildman, T.R., 1968. Relative and absolute terrestrial abundances of the rare earths. In: Ahrens, L.H. (Ed.), Origin and Distribution of the Elements. Pergamon, New York, NY, pp. 889-912.
Heath, G.R., Dymond, J., 1977. Genesis and transformation of
168 B. L. Winter et al. I Marine Geology 138 (1997) 149-169
metalliferous sediments from the East Pacific Rise, Bauer Deep, and Central Basin, northwest Nazca plate. Geol. Sot. Am. Bull. 88, 723-733.
Hein, J.R., Morgenson, L.A., Clague, D.A., Koski, R.A., 1987. Cobalt-rich ferromanganese crusts from the Exclusive Eco- nomic Zone of the United States and nodules from the Oce- anic Pacific. In: Scholl, D.W., Grantz, A., Vedder, J.G. (Eds.), Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins - Beaufort Sea to Baja California. Circ.-Pac. Count. Energy Miner. Resour., Earth Sci. Ser. 6, 261-283.
Hein, J.R., Bohrson, W.A., Schulz, M.S., Noble, M., Clague, D.A., 1992a. Variations in the fine-scale composition of a central Pacific ferromanganese crust: Paleoceanographic implications. Paleoceanography 7, 63-77.
Hein, J.R., Schulz, M.S., Gein, L.M., 1992b. Central Pacific cobalt-rich ferromanganese crusts: Historical perspective and regional variability. In: Keating, B.H., Bolton, B.R. (Eds.), Geology and Offshore Mineral Resources of the Central Pacific Basin. Circ.-Pac. Count. Energy Miner. Resour., Earth Sci. Ser. 14, 261-283.
Ingram, B.L., Hein, J.R., Farmer, G.L., 1990. Age determin- ations and growth rates of Pacific ferromanganese deposits using strontium isotopes. Geochim. Cosmochim. Acta 54, 1709-1721.
Jones, C.E., Halliday, A.N., Rea, D.K., Owen, R.M., 1994. Neodymium isotopic variations in North Pacific modem sili- cate sediment and the insignificance of detrital REE contri- butions to seawater. Earth Planet. Sci. Lett. 127, 55-66.
Klinkhammer, G.P., 1980. Early diagenesis in sediments from the eastern equatorial Pacific, II. Pore water metal results. Earth Planet. Sci. Lett. 49, 81-101.
Kunzendorf, H., Gwozdz, R., Glasby, G.P., Stoffers, P., Renner, R.M., 1989. The distribution of rare earth elements in manganese micronodules and sediments from the equato- rial and southwest Pacific. Appl. Geochem. 4, 183-193.
Kunzendorf, H., Glasby, G.P., Stoffers, P., Pltiger, W.L., 1993. The distribution of rare earth elements and minor elements in manganese nodules, micronodules and sediments along an east-west transect in the southern Pacific. Lithos 30,45556.
Landa, E.R., Gast, R.G., 1973. Evaluation of crystallinity in hydrated ferric oxides. Clays Clay Miner. 21, 121-130.
Lyle, M., Heath, G.R., Robbins, J.M., 1984. Transport and release of transition elements during early diagenesis: Sequential leaching of sediments from MANOP Sites M and H, Part I. pH 5 acetic acid leach. Geochim. Cosmochim. Acta 48, 1705-1715.
Mangini, A., 1988. Growth rates of manganese nodules and crusts. In: Halbach, P., Friedrich, G., von Stackelberg, U. (Eds.), The Manganese Nodule Belt of the Pacific Ocean. Ferdinand Enke, Stuttgart, pp. 142-151.
Mills, R., Eldertield, H., 1993. A dual origin for the hydrother- mal component in a metalliferous sediment core from the Mid-Atlantic Ridge. J. Geophys. Res. 98, 9671-9681.
Minicucci, D.A., Clark, D.L., 1983. A late Cenozoic stratigra- phy for glacial-marine sediments of the eastern Alpha Cordil- lera, central Arctic Ocean. In: Molnia, B.F. (Ed.), Glacial-
Marine Sedimentation. Plenum, New York, NY, pp. 331-365.
Morris, T.H., Clark, D.L., Blasco, SM., 1985. Sediments of the Lomonosov Ridge and Makarov Basin: A Pleistocene stratigraphy for the North Pole. Geol. Sot. Am. Bull. 96, 901-910.
Mudie, P.J., Blasco, S.M., 1985. Lithostratigraphy of the CESAR cores. In: Jackson, H.R., Mudie, P.J., Blasco, S.M. (Eds.), Initial Geologic Report on CESAR - The Canadian Expedition to Study the Alpha Ridge, Arctic Ocean. Geol. Surv. Can. Pap. 84-22, 59-100.
Muller, P.J., Hartmann, M., Suess, E., 1988. The chemical envi- ronment of pelagic sediment. In: Halbach, P., Friedrich, G., von Stackelberg, U. (Eds.), The Manganese Nodule Belt of the Pacific Ocean. Ferdinand Enke, Stuttgart, pp. 70-90.
Nath, B.N., Balaram, V., Sudhakar, M., Phiger, W.L., 1992. Rare earth element geochemistry of ferromanganese deposits from the Indian Ocean. Mar. Chem. 38, 185-208.
Ohr, M., Halliday, A.N., Peacor, D.R., 1991. Sr and Nd iso- topic evidence for punctuated clay diagenesis, Texas Gulf Coast. Earth Planet. Sci. Lett. 105, 110-126.
Ohr, M., Halliday, A.N., Peacor, D.R., 1994. Mobility and fractionation of rare earth elements in argillaceous sedi- ments: Implications for dating diagenesis and low-grade metamorphism. Geochim. Cosmochim. Acta 58,289-312.
Olivarez, A.M., Owen, R.M., 1989. REE/Fe variations in hydrothermal sediments: Implications for the REE content of seawater. Geochim. Cosmochim. Acta 53, 757-762.
Palmer, M.R., 1985. Rare earth elements in foraminifera tests. Earth Planet. Sci. Lett. 73, 285-298.
Palmer, M.R., Elderlield, H., 1985. Variations in the Nd iso- topic composition of foraminifera from Atlantic Ocean sedi- ments. Earth Planet. Sci. Lett. 73, 299-305.
Pattan, J.N., Banakar, V.K., 1993. Rare earth element distribu- tion and behavior in buried manganese nodules from the Central Indian Basin. Mar. Geol. 112, 303-312.
Peucker-Ehrenbrink, B., Hofmann, A.W., Hart, S.R., 1994. Hydrothermal lead transfer from mantle to continental crust: the role of metalliferous sediments. Earth Planet. Sci. Lett. 125, 129-142.
Piepgras, D.J., Wasserburg, G.J., Dasch, E.J., 1979. The iso- topic composition of Nd in different ocean masses. Earth Planet. Sci. Lett. 45, 223-236.
Piper, D.Z., 1974. Rare earth elements in ferromanganese nod- ules and other marine phases. Geochim. Cosmochim. Acta 38, 1007-1022.
Reimnitz, E., Marincovich, L., Jr., McCormick, M., Briggs, W.M., 1992. Suspension freezing of bottom sediment and biota in the Northwest Passage and implications for Arctic Ocean sedimentation. Can. J. Sci. 29, 693-703.
Ruhlin, D.E., Owen, R.M., 1986. The rare earth element geo- chemistry of hydrothermal sediments from the East Pacific Rise: Examination of a seawater scavenging mechanism. Geochim. Cosmochim. Acta 50, 393-400.
Staffers, P., Glasby, G.P., Thijssen, T., Shrivastava, P.C., Mel- guen, M., 1981. The geochemistry of coexisting manganese nodules, micronodules, sediments and pore waters from five
B.L. Winter et al. / Marine Geology I38 (1997) 149-169 169
areas in the equatorial and SW. Pacific. Chem. Erde 40, 213-297.
Sugisaki, R., Ohashi, M., Sugitani, K., Suzuki, K., 1987. Com- positional variations in manganese micronodules: a possible indicator of sedimentary environments. J. Geol. 95,433454.
Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp.
Turner, S., Buseck, P., 198 1. Todorokites: a new family of natu- rally occurring manganese oxides. Science 212, 10241027.
Usui, A., Mellin, T.A., Nohara, M., Yuasa, M., 1989. Structural stability of marine 10 A manganates from the Ogasawara (Bonin) Arc: Implication for low-temperature hydrothermal activity. Mar. Geol. 86, 41-56.
Weber, E.T., Owen, R.M., Dickens, G.R., Halliday, A.N., Jones, C.E., Rea, D.K., 1995. Significance of middle REE
depletion in the eolian component of North Pacific sedi- ments. Eos (Trans. Am. Geophys. Union) 76, S172.
Winter, B.L., Clark, D.L., Johnson, CL., 1995. REE, Nd, Sr, and Pb isotope geochemistry of Late Pleistocene ferromanga- nese micronodules and sediment from the central Arctic Ocean. Eos (Trans. Am. Geophys. Union) 76, F319.
Winter, B.L., Clark, D.L., Johnson, C.L., 1996a. Late Cenozoic Sr isotope evolution of the Arctic Ocean: constraints on water mass circulation with the lower latitude oceans. Deep- Sea Res. (submitted ).
Winter, B.L., Johnson, C.L., Clark, D.L., 1996b. Sr, Nd, and Pb isotope variations of seawater and silicate sediment throughout the Late Cenozoic in the central Arctic Ocean: Implications for sediment provenance and the source of trace metals in seawater. Geochim. Cosmochim. Acta (submitted).