calcareous nannofossil bioevents and … in the mediterranean sea (castradori, 1993; sprovieri et...

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Marine Micropaleontology

Calcareous nannofossil bioevents and environmental control on

temporal and spatial patterns at the early–middle Pleistocene

Patrizia Maiorano*, Maria Marino

Dipartimento di Geologia e Geofisica-Universita di Bari, Via E. Orabona, 4-70125 Bari, Italy

Received 13 April 2004; received in revised form 9 August 2004; accepted 10 August 2004

Abstract

Quantitative distributions of calcareous nannofossils are analysed in the early–middle Pleistocene at the small Gephyrocapsa

and Pseudoemiliania lacunosa zone transition in deep-sea cores from the Mediterranean Sea and North Atlantic Ocean (Ocean

Drilling Program [ODP] Sites 977, 964 and 967, Deep Sea Drilling Project [DSDP] Site 607). The temporal and spatial mode of

occurrence of medium-sized gephyrocapsids and reticulofenestrids has been examined to refine biostratigraphic constraints and

evaluate possible relationships of stratigraphic patterns to environmental changes during a period of global climatic

deterioration. The timing of bioevents has been calibrated using high-resolution sampling and correlation to the y18O record in

chronologically well-constrained sections. Newly identified events and ecostratigraphical signals enhance the stratigraphic

resolution at the early–middle Pleistocene. The first occurrence (FO) of intermediate morphotypes between Pseudoemiliania

and Reticulofenestra (Reticulofenestra sp.) is proposed as a reliable event within marine isotope stage (MIS) 35 or at the MIS

35/34 transition. The distribution of Reticulofenestra asanoi is characterized by rare and scattered occurrences in its lowest

range, but the first common occurrence (FCO) is consistently identified at MIS 32 or 32/31; the last common occurrence (LCO)

of the species is a distinctive event at MIS 23. In the studied interval, Gephyrocapsa omega dominates among medium-sized

Gephyrocapsa. The FO of G. omega and contemporaneous re-entry of medium-sized gephyrocapsids at the lower–middle

Pleistocene transition are diachronous between the Atlantic Ocean and Mediterranean Sea and from the western to eastern

Mediterranean. In the Mediterranean, the LO of G. omega falls at MIS 15, insolation cycle 54 and is isochronous among the

sites. Abundance fluctuations of G. omega show notable relations to early–middle Pleistocene climate changes; they

considerably increase in abundance at the interglacial stages, suggesting warm water preferences. Gephyrocapsa omega

temporarily disappears during the glacial MIS 22 and MIS 20. Above MIS 20, an impoverishment in G. omega and in the total

abundance of medium-sized gephyrocapsids occurs. A decrease in abundance of G. omega is observed between the western Site

977 and the easternmost Site 967 in the Mediterranean Sea, as a possible response to high salinity and/or low nutrient content.

Possible environmental influences on the distribution of R. asanoi and of Reticulofenestra sp. are discussed.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Pleistocene; calcareous nannofossils; biochronology; temporal distribution; Mediterranean Sea; Atlantic Ocean

0377-8398/$ - s

doi:10.1016/j.m

* Correspon

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53 (2004) 405–422

ee front matter D 2004 Elsevier B.V. All rights reserved.

armicro.2004.08.003

ding author. Tel.: +39 80 544 3455; fax: +39 80 544 2625.

ess: [email protected] (P. Maiorano).

P. Maiorano, M. Marino / Marine Micropaleontology 53 (2004) 405–422406

1. Introduction

The Pleistocene was characterized by significant

orbitally forced climate fluctuations, which are

well recorded in the sedimentary record through

changes in sediment properties, fossil commun-

ities, chemical and isotopic characteristics. In

more detail, during the early–middle Pleistocene

transition, the Earth climatic system experienced a

major change related to intensification of the

Northern Hemisphere glaciation, with a significant

shift toward more intense glacial conditions,

occurred between 0.9 and 0.6 Ma, together with

the development of 100 ky glacial–interglacial

oscillations (Ruddiman et al., 1989). Quantitative

biostratigraphic techniques, high-resolution sam-

pling and calibration of bioevents through the

astronomical tuning of the cyclic patterns in the

sedimentary record, have led to a high-quality

biochronological framework of the Pleistocene

record (Shackleton et al., 1990; Hilgen, 1991;

Hilgen et al., 1993; Lourens et al., 1996).

However, the early–middle Pleistocene transition

falls within the Pseudoemiliania lacunosa zone,

which has the lowest biostratigraphic resolution of

the entire Pleistocene. Previous studies suggested

that calcareous nannofossil biostratigraphy could

be improved by collecting further data on poorly

known taxa and on ecostratigraphical signals. The

Fig. 1. Location map of

availability of precise Pleistocene chronology

provides a powerful reference frame for this.

Moreover, the increasing documentation of the

ecological preferences of modern coccolithophorids

can be a key tool for interpreting distinctive

distribution patterns of calcareous nannofossils,

during the past, in terms of response to palae-

oenvironmental changes.

The aim of this study is to understand charac-

ters in the stratigraphic and spatial patterns of

selected calcareous nannofossils through the small

Gephyrocapsa and Pseudoemiliania lacunosa zones

in high-quality Mediterranean and extra-Mediterra-

nean deep-sea cores, within a well-constrained

temporal framework. In particular, the study focu-

ses on the distribution of medium-sized gephy-

rocapsids and selected reticulofenestrids, on their

correlation to oxygen isotopes fluctuations and on

geographical differences in the abundance of taxa,

in order to discuss the potential stratigraphical

value of new or contradictory bioevents, their

patterns of diachrony and their possible relation

to environmental parameters.

2. Material and methods

Three Mediterranean deep-sea sections located

along an east–west transect (ODP Sites 977, 964,

the studied sites.

Table 1

Summary of the sites investigated in this study and related references

Studied sites Reference

Leg/Site Location, latitude,

longitude

Previous calcareous

nannofossil

studies

Oxygen isotope

stratigraphy

Sapropel stratigraphy,

insolation cycle

Age model

DSDP 94/

Site 607

Eastern Atlantic,

418 N, 338 WTakayama and Sato (1987),

Raffi et al. (1993),

Wei (1993), Raffi (2002)

Ruddiman et al.

(1986)

/ Mix et al.

(1995)

ODP 161/

Site 977

Western Mediterranean,

368 N, 18 Wde Kaenel et al. (1999) von Grafenstein

et al. (1999)

Murat (1999), de

Kaenel et al. (1999)

von Grafenstein

et al. (1999)

ODP 160/

Site 964

Eastern Mediterranean,

368 N, 17 E

Sprovieri et al. (1998),

Maiorano et al. (2004)

Howell et al.

(1998),

Sprovieri et al.

(1998)

Sakamoto et al.

(1998), Emeis

et al. (2000)

Howell et al.

(1998)

ODP 160/

Site 967

Eastern Mediterranean,

348 N, 328 ERaffi (2002) Kroon et al.

(1998)

Kroon et al. (1998) Kroon et al.

(1998)

P. Maiorano, M. Marino / Marine Micropaleontology 53 (2004) 405–422 407

967) and a reference mid-latitude extra-Mediterranean

section (DSDP Site 607) located in the North Atlantic

Ocean (Fig. 1) were selected for this study. Age

control, oxygen isotope stratigraphy and sapropel

stratigraphy when possible, are available in the

sections (Table 1). A total of 664 samples were

analysed in the interval across marine oxygen isotope

stages 35 to 15 in all the sections. Sample spacing

varied from mean values of 10 and 20 cm at Sites 607,

964, 967 to 40 cm at Site 977 to obtain approximately

one sample per 3–5 ky at all sites, according to the

different sedimentation rates, ranging from mean

values of about 150 m/Ma at Site 977 to 25–40 m/

Ma at the other sites. Frequently, samples used for

oxygen isotopes have been investigated. Simple smear

slides were prepared from unprocessed samples

according to standard technique (Bown and Young,

1998) and analyzed under a polarized light micro-

scope at 1000� magnification. Quantitative data were

collected by counting the number of specimens per

unit area (Backman and Shackleton, 1983). About 150

fields of view having approximately the same density

of nannofossils (nearly 50–60 specimens N4 Am) were

investigated, and abundances were plotted as number

of specimens/mm2.

3. Taxonomic remarks

The taxonomic criteria for unambiguous recog-

nition of Reticulofenestra asanoi and intermediate

morphotypes between Reticulofenestra and Pseudoe-

miliania are discussed in recent papers (Marino,

1996; Maiorano et al., 2004). Circular to subcircular

reticulofenestrids larger than 6 Am are referred to R.

asanoi (Plate 1, Figs. 1–4); the number of slits (up to

four) on the distal shield as taxonomic definition of

the species (de Kaenel et al., 1999) is not followed

because we found that it cannot be consistently

applied in sediments with varying preservation state.

Intermediate morphotypes between Reticulofenestra

and Pseudoemiliania are distinguished from R.

asanoi and are indicated here as Reticulofenestra

sp. (Plate 1, Figs. 5–8); they are larger than 5 Am,

exhibit a subcircular outline and a prominent collar.

The size of the central opening is wider than in R.

asanoi and smaller than in Pseudoemiliania, repre-

senting about 30% of the total length of the

reticulofenestrid; only a few slits are visible on the

distal shield. The absence of slits in R. asanoi is an

important taxonomic criterion if compared to Retic-

ulofenestra sp.

As far as Gephyrocapsa is concerned, it is

worthwhile mentioning that taxonomic concepts vary

between different authors. Syntheses on the equiv-

alence among gephyrocapsid taxonomic units are

given by Flores et al. (1999) and Raffi (2002). In

particular, gephyrocapsids are generally subdivided

by using biometric criteria, mainly among biostratig-

raphers and/or grouped in morphological associations

(probable genotypes) having different environmental

preferences (Bollmann, 1997), as frequently adopted

Plate 1. Light microscope photographs of selected calcareous nannofossil species. XP—crossed polarized light, PL—parallel light. Scale bar

represents 5 Am.

1–2. Reticulofenestra asanoi Sato and Takayama, 1 XP, 2 PL, ODP Hole 967A, 4–4, 90 cm.

3–4. Reticulofenestra asanoi Sato and Takayama, 3XP, 4 PL, DSDP 607, 5–6, 88 cm.

5–6. Reticulofenestra sp., 5 XP, 6 PL, ODP Hole 967A, 4–3, 110 cm.

6–7. Reticulofenestra sp., 6 XP, 7 PL, ODP Hole 967A, 4–4, 90 cm.

8–9. Pseudoemiliania lacunosa (Kamptner) Gartner, 8 XP, 9 PL, ODP Hole 967A, 4–2, 90 cm.

10. Gephyrocapsa omega Bukry, XP, ODP Hole 967A, 3–3, 50 cm.

11. Gephyrocapsa oceanica s.l. sensu Rio, 1982, XP, ODP Hole 967A, 4–2, 30 cm.


in late Pleistocene to recent. In this study, the

classification of gephyrocapsids follows the morpho-

metric criteria of Raffi et al. (1993), which is a

powerful subdivision for stratigraphic purposes;

medium-sized Gephyrocapsa includes all early and

middle Pleistocene gephyrocapsids 4–5.5 Am in size.

However, Gephyrocapsa omega (gephyrocapsids N4

Am in size with a high angle bridge) is also

distinguished within medium-sized Gephyrocapsa to

enhance the data set on stratigraphic meaning of

Pleistocene temporary disappearances of the species

(Marino et al., 2003; Maiorano et al., 2004) and of its

Fig. 2. Quantitative patterns of selected calcareous nannofossils and oxygen isotope stratigraphy at Site 607. Position of re-entrance of medium-

sized Gephyrocapsa (reemG) is indicated according to Raffi (2002). Oxygen isotopes and interglacial stage numbers are from Ruddiman et al.

(1989). FO—first occurrence; FCO—first common occurrence; LO—last occurrence; LCO—last common occurrence; td—temporary

disappearance.


LO in the Mediterranean Sea (Castradori, 1993;

Sprovieri et al., 1998).

4. Results

The present results mainly focus on quantitative

distributions of Reticulofenestra sp., Reticulofenestra

asanoi , total medium-sized Gephyrocapsa and

Gephyrocapsa omega, which are major components

of the investigated Pleistocene calcareous nannofossil

assemblage. The studied interval corresponds to the

transitional record through the small Gephyrocapsa

and Pseudoemiliania lacunosa zones. The recognized

calcareous nannofossil events, their correlation with

oxygen isotope stages and estimated biochronology are

listed in Table 2. The discussion that follows outlines

the mode of occurrence of the recognized bioevents,

from older to younger, between the mid-latitude

Atlantic site and the Mediterranean sections (Figs. 2–

5), as well as from western to eastern Mediterranean,

and evaluates correlation of bioevents among different

areas (Fig. 6). Comparison of abundance patterns to

oxygen isotope stratigraphy is examined (Figs. 7 and 8)

in an attempt to understand possible relations of the

evolutionary and/or ecostratigraphical events to envi-

ronmental parameters.

4.1. Stratigraphical patterns

4.1.1. FO of Reticulofenestra sp.

Reticulofenestra sp. is very rare in its lowermost

distribution, and this is a consistent pattern among the

Mediterranean sites (Figs. 3–5). The first occurrence

(FO) of the species can be identified and correlated

among the sections (Fig. 6); it always predates the

lowest occurrences of Reticulofenestra asanoi and

falls at marine isotope stage (MIS) 35 at Sites 607,

977, 967 or at MIS 35/34 at Site 964. At Sites 977 and

967, where sapropel stratigraphy is available and

Fig. 3. Quantitative patterns of selected calcareous nannofossils and oxygen isotope stratigraphy at Site 977. Sapropel layers are according to

Murat (1999) and the corresponding insolation cycles are from de Kaenel et al. (1999). Oxygen isotope stratigraphy and stage assignments are

from von Grafenstein et al. (1999). For abbreviations, refer to Fig. 2.


correlated to the insolation cycles (i-cycle), the event

is located between i-cycles 110 and 112. The

calibrations of the event are well comparable among

the Sites 607, 977 and 967 (Table 2); at Site 964, the

age of the FO of Reticulofenestra sp. is about 29 ky

younger than at the other sites. This difference is

unlikely related to the slightly higher error in the age

estimate at Site 964, which is on the order of 6 ky

(Table 2), and can be the result of comparing different

age models. The uppermost distribution of Reticulo-

fenestra sp. is variable between the studied sections

(Figs. 2–5), and therefore, neither the last common

occurrence (LCO) nor the LO can be identified with

certainty.

Close morphological affinities between Reticulofe-

nestra sp. and Reticulofenestra asanoi as well as

remarkable similarity in abundance patterns of the

taxa suggest that they may be phylogenetically

relationed.

4.1.2. FCO of Reticulofenestra asanoi

The distribution of Reticulofenestra asanoi is

restricted to the lower–middle Pleistocene transition,

as documented by several authors (Matsuoka and

Okada, 1989; Sato et al., 1991; Wei, 1993; Marino,

1996; de Kaenel et al., 1999; Fornaciari, 2000; Flores

et al., 2000; Raffi, 2002; Maiorano et al., 2004).

However, wide disagreement exists in previous

correlations of FO and LO of the species to oxygen

isotope data. In our opinion, these discrepancies can

be related to indistinct concepts between R. asanoi

and Reticulofenestra sp. In all the sections, R. asanoi

is rare and discontinuous at the beginning of the

distribution (Figs. 2–5); the first common occurrence

(FCO) of the species is more distinctive than the

absolute FO, and it is recorded at MIS 31/32 at Sites

607, 977 and 964 (Figs. 2–4) or at MIS 32 at Site 967.

The event falls slightly above i-cycle 102 at Sites 964

and 967 (Fig. 6). It is noteworthy that a younger

Fig. 4. Quantitative patterns of selected calcareous nannofossils at Site 964. Position of sapropel layers is from Sakamoto et al. (1998); i-cycles

are from Emeis et al. (2000). Oxygen isotope data are from Howell et al. (1998); interglacial stage numbers are indicated according to Sprovieri

et al. (1998) with slight revision after Maiorano et al. (2004). For abbreviations refer to Fig. 2.


correlation of FCO of R. asanoi at Site 964 to MIS 30

(Maiorano et al., 2004) is revised here as a conse-

quence of the higher sampling resolution adopted in

the present study, which has considerably improved

the recognition of the event. The age of FCO of R.

asanoi is significantly comparable among the sections

(Table 2). In our opinion, the previous older calibra-

tions of the FO of R. asanoi at MIS 33/34 at Site 977

(mbsf 183.53–182.62, de Kaenel et al., 1999) and at

MIS 35 at Site 607 (mcd 47.85–48.66, Wei, 1993) are

probably caused by different taxonomic criteria

adopted by different authors.

4.1.3. LCO of Reticulofenestra asanoi

The significant decrease in abundance (LCO) of

Reticulofenestra asanoi is recorded in all the inves-

tigated sites and appears as a more reliable event with

respect to the absolute LO; above the LCO, the

distribution of R. asanoi is characterized by discon-

tinuous occurrences of the species (Figs. 2–5), and the

abundances are generally lower than 2 specimens/

mm2. The event is recorded at MIS 23 at all the

investigated sites (Fig. 6), in agreement with Maior-

ano et al. (2004), and slightly predates the severe MIS

22 glaciation. Biochronologic data on the LCO of R.

asanoi are comparable among the sections (Table 2)

inasmuch as differences in the age assignments range

between 1 and 19 ky. The age of the LCO of R. asanoi

is correlatable to data on the LO of the species at MIS

23 or 22/23 from Raffi (2002). On the other hand, the

event is over 30 ky older if compared with the age of

the LO (not LCO), from Wei (1993) at Site 607, where

the event has been correlated to MIS 22. Previous

correlations of the LO of R. asanoi to MIS 19 (de

Kaenel et al., 1999) are most probably caused by

different taxonomic concepts.

4.1.4. Re-entry of medium-sized Gephyrocapsa and

FO of Gephyrocapsa omega

The re-entry of medium-sized Gephyrocapsa and

the contemporaneous FO of Gephyrocapsa omega are

well recognizable events in the studied sites (Figs.

Fig. 5. Quantitative patterns of selected calcareous nannofossils and oxygen isotope stratigraphy at Site 967. Sapropel layers and corresponding

insolation cycles, oxygen isotope stratigraphy and glacial stage numbers are from Kroon et al. (1998). For abbreviations refer to Fig. 2.


2–5) and define the small Gephyrocapsa and Pseu-

doemiliania lacunosa zonal boundary (Rio et al.,

1990). The abundance patterns of total medium

Gephyrocapsa essentially match that of G. omega in

all the sections, confirming that the reappearance of the

medium-sized gephyrocapsids is largely composed of

G. omega and subordinately of Gephyrocapsa oce-

anica s.l. (sensu Rio, 1982); Gephyrocapsa carib-

beanica represents a minor component in the studied

sections. With regard to the available biochronolog-

ical data set, the middle Pleistocene reappearance of

medium-sized Gephyrocapsa is known to be consis-

tently diachronous at different latitudes, between MIS

29 and 25, and is considered as a possible migratory

event from low to mid-high latitudes (Wei, 1993;

Raffi et al., 1993; Flores et al., 1999; Raffi, 2002). In

the Mediterranean, the event is substantially recorded

in the interval between MIS 26 and MIS 25

(Castradori, 1993; de Kaenel et al., 1999; Raffi,

2002; Maiorano et al., 2004). At Site 607, the event

falls at MIS 27 (Raffi, 2002). In the Mediterranean,

our results indicate that at the westernmost Site 977,

the re-entrance of medium-sized Gephyrocapsa and

the FO of G. omega are characterized by very rare

occurrences correlated to MIS 26/27 transition below

i-cycles 92 (Fig. 3), before the abrupt increase

recorded at MIS 25 within i-cycle 88. At the eastern

sites, the FO of G. omega is sudden and occurs at

MIS 25 between i-cycles 90–92 at Site 964 and

within sapropel layer correlated to i-cycle 90 at Site

967 (Fig. 6). Correlation of bioevents to oxygen

isotopes (Fig. 6) and biochronological data (Table 2)

suggests a diachrony from Atlantic to Mediterranean,

as well as from western to eastern Mediterranean, on

the order of 30–60 ky between Atlantic and Medi-

terranean and of about 30 ky within the Mediterra-

nean. It is worth noting that various sampling

resolution in the sections may affect evaluation of

slight diachrony; in this interval, temporal resolution

in the Mediterranean sites is slightly lower at Site 964

Fig. 6. Position of bioevents relative to age models adopted at the investigated sites. Rsp—Reticulofenestra sp.; Ra—Reticulofenestra asanoi; reemG—re-entry of medium

Gephyrocapsa; Gom—Gephyrocapsa omega; td—temporary disappearance. Dashed lines indicate selected i-cycles. Ages of sapropel layers are relative to their midpoints.

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(4.5 ky) but is comparable between Sites 977 and 967

(about 3 ky), supporting the value of the estimated

diachrony of the event (Table 2). However, it cannot

be excluded that uncertainty in the evaluation of small

age constraints in diachrony may be also affected by

comparing different age models. The abrupt increase

(FCO) of G. omega at MIS 25 appears to be a more

distinctive pattern rather than the FO in the Medi-

terranean; however, it is variously correlated between

the sites with respect to sapropel stratigraphy, result-

ing younger in age in the western Mediterranean than

in the eastern basin, and therefore, the FCO is not

suggested in replacement of the standard and widely

adopted FO.

4.1.5. Temporary disappearance of Gephyrocapsa

omega

An interval of temporary disappearance of

medium-sized Gephyrocapsa is well known in the

small Gephyrocapsa zone (Gartner, 1977) in the

lower Pleistocene. More recently, Marino (1996),

Marino et al. (2003), Maiorano et al. (2004) discussed

the potential biostratigraphic value of an additional

interval of temporary disappearance of Gephyrocapsa

omega in the Pseudoemiliania lacunosa zone.

Although intervals of temporary disappearance can

be considered as unconventional and unreliable

biostratigraphic markers, inasmuch as they may be

strongly environmentally controlled, they are often

well documented in the calcareous nannofossil assem-

blage in different time intervals. They are widely used

as biostratigraphic proxies although their causes are

not always explained.

Our results allow the data set on the occurrence of

temporary disappearances of Gephyrocapsa omega in

different geographic areas to be extended. The taxon

clearly shows a consistent pattern among the inves-

tigated sites; two intervals of remarkable rarity or

absence of the species can be observed independently

among sections indicative of diverse water masses

(Figs. 2–5), suggesting that they can be considered as

a primary variation in the nannofossil assemblage. In

fact, both the older interval of temporary disappear-

ance (td 1) and the younger one (td 2) appear as

essentially correlated to MIS 22 and 20, respectively

(Fig. 6), that represent two severe oxygen isotope

glacial signals of the middle Pleistocene. In more

details, td 1 is a short-term ecostratigraphic signal

(about 15 ky) and occurs from MIS 23/22 or MIS 22

to MIS 22/21 transition or MIS 21 in the Mediterra-

nean sites, with the ages of bottom and top of

temporary disappearance in good agreement among

all the investigated sections (Table 2) and with an

error in the age estimates not exceeding 2 ky. The

beginning of td 2 is well comparable among the sites

(Fig. 6, Table 2) occurring at MIS 21/20 or MIS 20 at

Site 964. This result is consistent with data from a

southern Apennine foredeep section (Maiorano et al.,

2004). The end of td 2 has a slightly variable

extension in the sites (Fig. 6, Table 2), ranging from

MIS 19/20 to MIS 19/18; however, difference in the

age assignments is not higher than 29 ky and is

unlikely related to sampling resolution inasmuch as

the estimated error is not higher than 7 ky (Table 2).

4.1.6. LO of Gephyrocapsa omega

Gephyrocapsa omega is still present in the open

oceans today; the LO of the species is known only in

the Mediterranean Sea, where it was first identified by

Castradori (1993) at 0.58 Ma and correlated to MIS 15

at Site 964 (Sprovieri et al., 1998). In the present

study, the LO of G. omega is also recognized at Sites

977 and 967 for the first time and correlated to MIS

15 in both sections, in agreement with previous

results. Biochronological data obtained in this study

indicate a high synchroneity of the event with an

estimated age ranging from 570 to 577 ky (Table 2)

and correlated to i-cycle 54 at Sites 977 and 964 (Fig.

6). The slight differences with previous age assign-

ments can be ascribed to different references used for

age evaluation; however, the best biochronologic

datum with the lower age uncertainty (F2 ky) among

the investigated records is that obtained at Site 977.

4.2. Environmental control on stratigraphic and

geographic patterns

High-resolution sampling, correlation to oxygen

isotope stratigraphy and comparison of the quantita-

tive distribution of gephyrocapsids and reticulofenes-

trids among different sites suggest a possible

environmental control on abundance patterns.

As shown in Fig. 7, Gephyrocapsa omega, which

represents almost the total medium-sized Gephyro-

capsa, shows significant variations in abundance in

the open ocean Atlantic site and in the Mediterranean

Table 2

List of calcareous nannofossil events: depth, correlation to marine oxygen isotope (MIS) and age evaluation

Calcareous

nannofossil event

Site 607 Site 977 Site 964 Site 967

Depth

(mcd)

MIS Age (Ma) Depth

(mbsf)

MIS Age (Ma) Depth

(rmcd)

MIS Age

(Ma)

Depth

(rmcd)

MIS Age (Ma)

LO G. omega 90.1 15 0.577F0.002 24.07 15 0.575F0.008 22.26 15 0.57F0.004

End td 2 31.64 19 0.778F0.004 127.77 20/19 0.79F0.001 29.88 19 0.786F0.0002 27.46 19/18 0.761F0.007

Beginning td 2 33.31 21/20 0.816F0.002 131.37 21/20 0.809F0.001 30.63 20 0.799F0.001 29.04 21/20 0.817F0.0004

End td 1 34.51 22/21 0.862F0.001 140.9 21 0.86F0.001 31.99 22/21 0.863F0.002 30.24 21 0.864F0.0003

Beginning td 1 35.09 23/22 0.88F0.002 142.6 22 0.874F0.002 32.36 22 0.88F0.001 31.2 22 0.887F0.001

LCO R. asanoi 36.23 23 0.912F0.003 147.65 23 0.913F0.002 32.93 23 0.906F0.004 31.43 23 0.894F0.004

FO G. omega 41.32* 27* 1.005F0.005* 160.67 27/26 0.985F0.001 34.31 25 0.965F0.004 33.19 25 0.956F0.002

FCO R. asanoi 44.78 32/31 1.087F0.003 178.3 32/31 1.079F0.002 37.36 32/31 1.089F0.0003 36.39 32 1.089F0.002

FO Reticulofenestra sp. 47.41 35 1.17F0.003 192.05 35 1.178F0.003 38.56 35/34 1.148F0.006 38.75 35 1.168F0.003

mcd =meters composite depth; mbsf =meters below sea floor; rmcd = revised meters composite depth.

* Data from Raffi (2002) and refer to re-entry of medium Gephyrocapsa.

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Fig. 7. Comparison of abundance patterns of medium-sized gephyrocapsids and y18O stratigraphy among the investigated sites. Shaded areas indicate intervals of temporary

disappearance of medium-sized Gephyrocapsa.

P.Maiorano,M.Marin

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eMicro

paleo

ntology53(2004)405–422

416

Fig. 8. Comparison of abundance patterns of Reticulofenestra sp., Reticulofenestra asanoi and y18O stratigraphy among the investigated sites.

P.Maiorano,M.Marin

o/Marin

eMicro

paleo

ntology53(2004)405–422

417


sections. In particular, a positive correlation is noted

between increase in abundance of gephyrocapsids and

the interglacial stages (Fig. 7), suggesting warm water

preferences of the taxon. In this context, the two

intervals of temporary disappearance of G. omega at

MIS 22 and MIS 20 appear to be a response to glacial

conditions (Fig. 7). In fact, the glacial MIS 22 and 20

represent intense cold stages during the middle

Pleistocene climate deterioration, when the perennial

ice condition replaced the seasonal ice condition in the

Arctic Ocean (Herman and Hopkins, 1980; Worsley

and Herman, 1980) from 0.9 to 0.7 Ma (Pisias and

Moore, 1981; Prell, 1982; Ruddiman et al., 1986). It is

worth noting that gephyrocapsids are considered

dissolution-resistant coccoliths (Roth and Berger,

1975; Roth and Coulbourn, 1982; Roth, 1994;

Bollmann et al., 1998) so it is unlikely that the

temporary disappearances of G. omega are related to

dissolution. A similar argument was made by McIn-

tyre and McIntyre (1971) and Gartner (1988) for the

small Gephyrocapsa zone. An environmental influ-

ence on the distribution of gephyrocapsids is also

observed in changes in the abundances of gephyr-

ocapsids within the Mediterranean; an impoverish-

ment of G. omega and of all medium-sized

Gephyrocapsa occurs above MIS 20 (Fig. 7). Mean

abundances of G. omega change from about 500

specimens/mm2 (Sites 977 and 964) or 120 (Site 967)

during interglacial stages 25–21 to about 70 speci-

mens/mm2 (Sites 977 and 964) or 8 (Site 967) during

interglacial 19–15, suggesting that its distribution was

affected by the climate cooling of the glacial

Pleistocene. Further considerations arise from com-

parative analysis of the abundance of G. omega

between the cores. A noticeable decrease in abun-

dance is observed between the western and the eastern

Mediterranean (Fig. 7); mean values in the total

abundance of G. omega range from about 280

specimens/mm2 at Site 977 and Site 964 to about 65

at Site 967 during interglacial stages (MIS 25–15).

This pattern is probably a response to increase in

salinity and/or decrease in nutrient content in the

eastern basin (Rohling et al., 2000; Bethoux et al.,

1999).

Our results suggest that during the early–middle

Pleistocene, Gephyrocapsa omega was a warm water

taxon having higher abundances in low salinity and

high nutrient waters. The temperature dependence of

G. omega is not known in the fossil record; however,

our data seem to be in agreement with studies from the

recent, suggesting warm water preferences for gephyr-

ocapsids having high angle bridge (McIntyre et al.,

1970; Bollmann, 1997; Takahashi and Okada, 2000).

Moreover, comparable environmental preferences are

widely known for Gephyrocapsa oceanica Kamptner

(N3 Am). In fact, in many studies, G. oceanica is

proved to be a warm water taxon both in the

Pleistocene record (Thierstein et al., 1977; Gartner,

1988; Flores et al., 1999; Sprovieri et al., 2003) and in

the recent (Okada and Honjo, 1973; Honjo and Okada,

1974; Geitzenauer et al., 1977; Okada and McIntyre,

1979; Kleijne et al., 1989; Giraudeau, 1992; Jordan

and Chamberlain, 1997; Jordan and Winter, 2000;

Hagino et al., 2000; Findlay and Giraudeau, 2000) and

to prefer high nutrient content (Winter, 1982; Mitchell-

Innes and Winter, 1987; Gartner, 1988; Houghton and

Guptha, 1991; Kinkel et al., 2000; Andruleit and

Rogalla, 2002; Cortes et al., 2001) and low salinity

(Kleijne, 1993; Jordan and Winter, 2000). In the

Mediterranean, it is absent in the column water and

surface sediments from the eastern basin and is

considered a tracer for Atlantic surface waters (Knap-

pertsbusch, 1993).

Unlike gephyrocapsids, abundance of reticulofe-

nestrids, although highly variable through the sec-

tions, is not clearly related to oxygen isotope

fluctuations and shows increase both in glacial and

interglacial stages (Fig. 8). The abundances of

Reticulofenestra asanoi and of Reticulofenestra sp.

are higher in the Atlantic than in the Mediterranean

and decrease eastward within the Mediterranean

basin. Although no ecological preferences are known

for these taxa, this trend may be a response to increase

in salinity, suggesting a preference for normal salinity

waters. However, further data need to be collected on

this topic.

5. Conclusion

The quantitative data collected from the Mediter-

ranean and North Atlantic deep-sea sections allowed

to outline characters in the distributions of selected

calcareous nannofossils at the early–middle Pleisto-

cene transition. Correlation between abundance pat-

terns of calcareous nannofossils, oxygen isotopes and


sapropel stratigraphy enabled testing of the strati-

graphical value of bioevents to provide biochrono-

logic data and to evaluate patterns of diachrony.

Possible relations of evolutionary and/or ecostrati-

graphical events to environmental parameters have

also been inferred.

The results on the distribution of Reticulofenestra

sp. and Reticulofenestra asanoi allow few consid-

erations on the previous inconsistent calibrations of

both the lowest and highest occurrences of R.

asanoi to marine isotope stages. Unambiguous

taxonomy is necessary to differentiate Reticulofe-

nestra sp. from R. asanoi; the two taxa have

comparable abundance patterns and similar mor-

phology, and their phylogenetic relationship cannot

be excluded. However, the occurrences of specimens

of Reticulofenestra sp. slightly below the lowest

occurrence of R. asanoi may be responsible of the

controversial results in previous findings. Clear

taxonomic criteria allow us to propose the FO of

Reticulofenestra sp. as reliable event occurring at

MIS 35 or MIS 35/34 transition. The distribution of

R. asanoi is characterized by very rare and scattered

occurrences in the lower part of the range and

suggests a low reproducibility of the FO of the

species; on the other hand, the increase in abun-

dance of the species (FCO) can be identified at MIS

32 or 31/32. The uppermost distribution of R.

asanoi indicates a high reliability of the LCO of

the species at MIS 23, well comparable to previous

data.

In the investigated interval, the medium-sized

Gephyrocapsa are almost exclusively represented by

Gephyrocapsa omega, and their distribution appears

significantly environmentally controlled. The reap-

pearance of medium-sized gephyrocapsids (contem-

poraneous to the FO of G. omega) in the middle

Pleistocene, which is known to follow a latitudinal

trend, is probably also affected by a migratory trend

from the Atlantic to the Mediterranean Sea and by a

regional environmental gradient (salinity and/or

nutrient) within the Mediterranean. This is sug-

gested by a pattern in diachrony of the event

between Atlantic and western Mediterranean, as

well as between western and eastern Mediterranean,

on the order of 30–60 ky between Atlantic and

Mediterranean and of about 30 ky within the

Mediterranean.

The two intervals of temporary disappearance of

Gephyrocapsa omega correlated to the glacial MIS

22 and MIS 20 remarkably improve the stratigraphic

constraints at the lower–middle Pleistocene transition

if high resolution data (3–5 ky) are available and are

interpreted as an ecological response to significant

glacial conditions. The positive correlation between

increases in the abundance of G. omega and

interglacial fluctuations suggests that the species

may provide valuable signals of paleoclimatic

amelioration in the middle Pleistocene record

through MIS 25–15. In this framework, the impov-

erishment of G. omega and of all medium-sized

Gephyrocapsa from MIS 22/20 transition upwards

can be interpreted as a response to the global

climatic deterioration. Moreover, the eastward

decrease of G. omega within the Mediterranean

seems to be related to increased salinity and

decreased nutrients.

Acknowledgements

We wish to thank the Ocean Drilling Program for

providing samples of all the investigated sites. We are

deeply indebted to J. Henderiks and J. Young for the

careful review of the manuscript and their valuable

suggestions, which greatly improved the early ver-

sion of the paper. This research was financially

supported by MIUR Grant 60% (2002) to A.

D’Alessandro.

Appendix A. Taxonomic list

All taxa cited in the paper are listed below

Gephyrocapsa Kamptner, 1943

G. oceanica Kamptner, 1943

G. oceanica s.l >4 Am sensu Rio, 1982

G. caribbeanica Boudreaux and Hay, 1967

G. omega Bukry, 1973

Pseudoemiliania, Gartner, 1969

P. lacunosa (Kamptner, 1963) Gartner, 1969

Reticulofenestra Hay et al., 1966

R. asanoi Sato and Takayama, 1992


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