james et al 2005 arag.pdf

JOURNAL OF SEDIMENTARY RESEARCH, VOL. 75, NO. 3, MAY, 2005, P. 454–463Copyright q 2005, SEPM (Society for Sedimentary Geology) 1527-1404/05/075-454/$03.00 DOI 10.2110/jsr.2005.035

WHERE HAS ALL THE ARAGONITE GONE? MINERALOGY OF HOLOCENE NERITIC COOL-WATERCARBONATES, SOUTHERN AUSTRALIA

NOEL P. JAMES,1 YVONNE BONE,2 AND T. KURTIS KYSER1

1 Queen’s University, Kingston, Ontario K7L 3N6, Canada2 Adelaide University, Adelaide, South Australia 5005, Australia

ABSTRACT: Surficial carbonate sediments on the southern continentalshelf of Australia are cool-water in aspect and composed of biogenicparticles produced largely during the late Quaternary. Current under-standing is that such sediments are calcite-dominated, as were theirolder Cenozoic counterparts. The Holocene fraction of these sedimentsin modern open-shelf, neritic environments between 30 and 350 meterswater depth is, however, 50% to 80% aragonite. Scant evidence ofsignificant former aragonite in many cool-water carbonate sedimentaryrocks implies that most aragonite is lost before such sediments exit themarine diagenetic environment. Although marine dissolution must betaking place in such settings, the conundrum is exacerbated becauseseawater over the shelf in southern Australia is saturated with respectto aragonite. It is proposed that the aragonite, from skeletons of gas-tropods, infaunal bivalves, and certain bryozoans, is dissolved in theshallow subsurface, probably as the byproduct of bacterial degradationof sedimentary organic matter. As a consequence, the geological andpaleontological record of many cool-water carbonates is strongly bi-ased, and the inferred original calcitic composition of such sedimentsis the product of early diagenetic taphonomic loss, not selective bio-genic productivity. The net result is not only dissolution of aragonitebut also neomorphism of Mg-calcite to calcite with a marine geochem-ical signature. Synsedimentary aragonite loss, by removing CaCO3 thatis usually available for calcite cementation during meteoric diagenesis,leads to retarded lithification of these cool-water carbonates until deep-ly buried. Such removal of a significant carbonate fraction during de-position likely contributes to the low rates of cool-water sediment ac-cumulation.

INTRODUCTION

Cool-water carbonates, those neritic sediments that form in marine wa-ters whose bottom temperature is generally ,208C, are an important partof the depositional spectrum in modern oceans (Lees and Buller 1972;Nelson 1988; James 1997) and were so throughout the Phanerozoic (Jamesand Clarke 1997). Such sediments have a distinctive suite of characteristicsthat include biotic composition, sedimentary structures, paleontology, andmineralogy. It is almost axiomatic that these sediments are formed by asuite of invertebrate organisms and lesser algae whose skeletons are dom-inantly calcite with varying amounts of magnesium as represented mostlyby molluscs, benthic foraminifers, coralline algae, bryozoans, and echino-derms, with increasingly numerous planktic foraminifers in deep water.Aragonite, when present, is minor and mostly localized to the inner shelf.In this way, they are distinct from warm-water, photozoan, carbonate sed-iments that are everywhere rich in aragonite (Milliman 1974; Bathurst1975; Tucker and Wright 1990). These conclusions are reinforced by stud-ies of relatively recent Cenozoic cool-water carbonates (Nelson 1978; Bet-zler et al. 1997; James 1997), which show that the sediments were domi-nated by calcite biofragments and larger fossils. Such sediments are gen-erally poorly cemented with few aragonitic molds, even after prolongedresidence in meteoric diagenetic environments, indicating that aragonitewas not abundant when they entered the meteoric diagenetic realm.

To quantify the character of these Holocene sediments, samples fromtwo large sectors of the cool-water depositional realm on the continentalshelf off southern Australia, the Great Australian Bight (GAB) and the

Lacepede Shelf (LS) (Fig. 1), were analyzed for mineralogy. The resultsare totally unexpected. The Holocene sediments contain significantly morearagonite than anticipated. The purpose of this paper is to report theseresults, interpret their meaning, and discuss implications for better under-standing the ancient rock record. The reason why aragonite is lost in cool-water neritic environments where seawater is saturated with respect to thiscarbonate phase is not clear. Recent studies, however, suggest that aragonitemay be dissolved as a byproduct of bacterial degradation of organic matter.If such marine dissolution is widespread in the cool-water realm, it impliesthat many calcite-dominated, cool-water sedimentary rocks may have con-tained much more aragonite originally than their preserved record indicates.

SETTING

The southern continental margin of Australia is largely veneered by car-bonate sediment. There are few sources of terrigenous clastic material ex-cept from the River Murray, which empties onto the Lacepede Shelf (Fig.1). Sediment on the shelf proper is generally thin, less than 10 m thick(Boreen and James 1993; James et al. 1994), but it thickens dramaticallyin slope environments (Passlow 1997; Hine et al. 1999; James et al. 2004).The latitude-parallel shelf is vast, ;2000 km in length along the southernmargin. Sediments investigated are from ;30 to ;350 meters water depth(mwd). The relatively narrow shoreface shallower than 30 mwd was notstudied. Details of the individual shelf sectors can be found in James et al.(1992), James et al. (1994), James et al. (1997), and James et al. (2001).

Oceanography

The region is a cool-water, high-energy, storm-dominated setting inwhich grains are typically moved continuously to water depths of ;70 mand disturbed by storms to depths of ;200 m. Shelf water masses, whichvary in character throughout the year, are mixtures of cold Southern OceanWater, warm Indian Ocean Water, saline waters produced within adjacentgulfs, and waters formed in the Great Australian Bight, all of which flowto the southeast. Southern Ocean Water is generated around Antarctica andflows eastward as the West Wind Drift, and intrudes onto the shelf duringsummer months.

Sea water temperatures on that part of the shelf in the Southern Ocean,200 mwd are generally ,208C, except for the GAB, where innermost-shelf bottom-water temperatures over the shallow Roe Terrace (Fig. 1) riseto just over 238C during summer. A strong thermocline develops duringsummer months but is destroyed by winter storms. Waters on the slopebelow 200 m are generally ,158C. These trends are perturbed by seasonalupwelling (Fig. 1), particularly along the SE part of the Lacepede Shelf(Bonney Shelf) and eastern and western GAB, where cold, nutrient-richSouthern Ocean waters flood onto the shelf during summer, lowering in-shore water temperatures to 15–178C. Otherwise downwelling predomi-nates across the entire region.

The GAB is also profoundly affected by the Leeuwin Current (Fig. 1) astream of warm, relatively low-salinity, tropical, surface water that flowssouthward along the west coast of the continent, around the southwestcorner, and eastward into the Great Australian Bight (Cresswell 1991). Itextends as far east as about the middle of the GAB. It is a winter current,first entering Southern Ocean waters in May and gradually disappearing inSeptember–October. When flowing, the Leeuwin Current impedes any po-

455ARAGONITE DISSOLUTION IN COOL-WATER CARBONATES, SOUTHERN AUSTRALIA

FIG. 1.—A) Map of Australia with location of B) Lacepede Shelf and C) Great Australian Bight Shelf. B) Bathymetric chart of Lacepede Shelf with major currentpatterns outlined. C) Bathymetric of Great Australian Bight with major current patterns outlined.

tential upwelling. The Great Australian Bight Current (informal name)(Fig. 1) comes from a warm (17–198C in winter; 19–228C in summer),saline (35.9–36.4‰) watermass in the central and western Great AustralianBight (Rochford 1986). This water drifts southeastward and occupies muchof the shelf and slope break east of the southern end of Eyre Peninsulayear-round.

The environment of the Lacepede Shelf is overall colder, and althoughwaters from Gulf St. Vincent (19–208C) spill into the northern part of theshelf, summer surface waters are generally ,178C to 20 mwd with thethermocline base at ;80 mwd, below which waters are ,128C. Winterwaters are mixed, with surface ocean temperatures of 178C and ,148C at200 mwd.

The generally southeast flow of all these surface waters is matched bythe Flinders Current (Fig. 1), a westward, geostrophic underflow of coolerintermediate depth waters (Bye 1972). This style of circulation exchangessaline shelf and gulf waters with fresher, cooler open ocean waters.

Surface Sediment

The sediments, although predominantly carbonate, are palimpsest, a mix-ture of relict, stranded, and Holocene grains (James et al. 1997) (Fig. 2).Relict, brown- to orange-stained, typically abraded carbonate particles (Fig.2C) are compositionally out of equilibrium with the modern oceanographicclimate and are thought to have formed during intermediate sea-level standsduring Marine Isotope Stages 3 and 4 (James et al. 1997). Relict particlesare generally intraclasts, in that biofragments are filled with fine-grained

carbonate (bioclasts) or are several particles cemented together (lithoclasts).Stranded carbonate grains (Fig. 2B), lightly stained, buff-colored particlesthat are variably abraded are interpreted to have formed mostly during thelate stages of Marine Isotope Stage 2 transgression (;19,000–1,000 yearsB.P.). This conclusion is supported by the abundance of coralline algaegrains, far below the photic zone and 14C dates (n 5 6) ranging from 8,960to 16,350 B.P. Holocene fragments and fossils (Fig. 2A), distinguished bytheir white, or brightly colored character, lack of stain, and typically un-abraded surfaces (even delicate zooecia are commonly preserved), formedin the last 8,000 years or less, during the current sea-level highstand.

Sediment ranges from coarse sand and gravel on the shelf to sand andfine muddy sand on the upper slope. The main Holocene components arebryozoans, molluscs, and benthic foraminifers, with coralline algae andserpulids common in shallow water, together with ubiquitous but not abun-dant brachiopods and echinoderms. Molluscs are typically more numerousthan bryozoans on the inner and middle shelf, (,100 mwd) whereas theopposite is true on the outer shelf and upper slope, with some upper-slopedeposits being almost all bryozoans. Planktic foraminifers become volu-metrically more important as water depth increases off the shelf. Other lessimportant components are, serpulid worm tubes, brachiopods, corals, bar-nacles, ostracodes, and sponge spicules. Muds are typically silt-rich andgenerally rich in macerated fragments of coarser biofragments together withcoccoliths and ascidian spicules.

Sediments in the Great Australian Bight, from 328S to 358S (James etal. 1994; James et al. 1997; James et al 2001) are generally warm-temperate

456 N.P. JAMES ET AL.

FIG. 2.—Photographs of the different particle types. A) Holocene grains, generallyunabraded and biofragmented (GAB location 338 11.69 S, 1248 55.29 E; depth 54m). B) Stranded grains, moderately abraded and slightly Fe-stained (GAB location348 34.819 S, 1238 37.749 E, depth 98 m). C) Relict grains, highly abraded and Fe-stained bioclasts and lithoclasts (GAB 348 07.99 S, 1348 30.19 E, depth 80 m).

(sensu Betzler et al. 1997). This is supported by the water temperatures todepths of 300 m, which vary between 158C and 198C, and by the benthicforaminifer biota, which contains some large taxa in the west includingHeterstigina, and an Amphisorus–Marginopora association in the east (Liet al. 1996b; Li et al. 1998; Li et al. 1999). This biota further confirms thatthe environment overall is somewhat oligotrophic. There are, however, nophotosynthetic calcareous organisms such as calcareous green algae orzooxanthellate corals.

The Lacepede Shelf and contiguous Bonney Self to the southeast (Fig.1) are farther south (368S to 398S) than the Great Australian Bight. Thesediments (James et al. 1992) although similar to the GAB, contain abun-dant quartzose sand on the inner shelf, reflecting deposition from the RiverMurray during the last glacial maximum lowstand. There are, however, nowarm-temperate indicators such as large benthic foraminifers in the sedi-ment (Li et al. 1996a; Li et al. 1996b). The environment overall is cool-temperate and more mesotrophic (Li et al. 1996a).

METHODS

Sample Acquisition

This study is based on information obtained during the following cruisesof CSIRO R.V. Franklin: Lacepede Shelf, FR03/89 in March 1989 andFR02/91 in January 1991; Lincoln Shelf FR06/94 in June and July 1994;Great Australian Bight, FR07/95 July 1995 in the western GAB and FR03/98 March/April 1998 in the eastern GAB. A total of 181 sediment sampleswere utilized. Most were obtained with a simple bucket (Bleys Dredge)with a volume of ;20 liters, or from a large epibenthic sled with a volumeof ;220 liters. Samples are a mixture of all surface and subsurface materialto a depth of ;10 cm. Navigation was by Global Positioning System(GPS), transit satellite, radar, and dead reckoning. The average accuracyof sample coordinates varies from meters to several tens of meters.

Laboratory Analysis

General attributes of the coarse silt-, sand-, and gravel-size sedimentfractions were logged on board with samples reexamined in the laboratoryunder binocular microscope, where, using ;1.0 g of material, Holoceneparticles were separated from stained relict and stranded grains. Other silt-and clay-size particles were not analyzed because they generally form asmall proportion of the sediment on this high-energy shelf and because thedifferent particle types could not be differentiated in such fine material.The relative percentage of different biofragments was estimated using vol-umetric comparison charts (Flugel 1982; Tucker 1988). The percentage ofdifferent bryozoan growth forms (Bone and James 1993) was also tabulatedduring binocular analysis.

X-ray diffraction analyses were carried out using standard techniques(Tucker 1988). The carbonate minerals present were aragonite, low-mag-nesium calcite (LMC 5 ,4 mole% MgCO3), and high-magnesium calcite(HMC 5 .4 mole% MgCO3). Most of the HMC contained between 4and 12 mole% MgCO3, and so is often referred to as intermediate-mag-nesium calcite (IMC) (cf. James 1997). Each sample was ground in ethanolto guard against phase change for 7 to 10 minutes. The dried powder wasthen put into a back-packed mount and analyzed under CoKa radiationbetween 258 and 408 2u at a rate of 0.28/minute for 10 seconds per incre-ment. The relative proportion of different carbonate minerals was deter-mined by analyzing peak areas following the techniques of Milliman (1974)and Tucker (1988). Peak areas were calculated using the program X-pertHighscore (version 1.0a; Philips Analytical B.V.). Standards were from twosources: (1) in-house biogenic skeletons analyzed by XRD; aragonite fromcoral or gastropod; LMC from brachiopod or bivalve; HMC from corallinealgae; and (2) a set of biogenic materials utilized by Swart et al. (2002) intheir calibration. The percentage of carbonate minerals from sediment sam-ples was compared to results from these standards. Samples were run in


FIG. 3.—Chart of Great Australian Bight Shelf with sample sites and percent aragonite in Holocene sediments. Inset triangle Holocene sediment mineralogy as determinedby XRD (99 samples; 30–300 meters water depth).

batches of 14 with a standard always included. When no aragonite waspresent, the sample was rerun with pure quartz to confirm peak position.Accuracy and precision is estimated at 65% (n 5 5).

As a check, samples that contained .70% aragonite as determined byXRD (n 5 60) were resampled and thin sections prepared from the Ho-locene grains. These sections were stained with Clayton Yellow using thetechnique of Choquette and Trusell (1978) to separate aragonite from cal-cite and enable the visual, qualitative differentiation between LMC andHMC. Results indicate that XRD analyses consistently show aragonite val-ues greater by 10% compared to those estimated from thin section analysis.Thus, on balance, values of aragonite should be viewed as 610%.

RESULTS

General Mineralogy

Holocene sediments on both shelves are mixtures of aragonite, HMC,and LMC (Appendices 1 and 2; see Acknowledgments). They are, however,overwhelmingly dominated by aragonite (Figs. 3, 4). The bulk of the sed-iment samples analyzed contain .50% aragonite (GAB 5 75%; LS 582%). In both areas there is rarely .50% HMC or .30% LMC.

Compositionally on the GAB, the aragonite components (as measured in18 thin sections) range from 100% molluscs and 0% bryozoans to 15%molluscs and 85% bryozoans, averaging 63% molluscs, 35% bryozoans,and 2% other grains. On the LS (as measured in 21 thin sections) the rangesare similar, from 100% molluscs and no bryozoans to 14% molluscs and86% bryozoans, averaging 58% molluscs and 39% bryozoans with 3%other particles.

Spatially on the GAB most of the shelf environment ,200 mwd has.50% Holocene aragonite particles (Fig. 3), except for inshore facies westof the Roe Plain, which are dominated by coralline algal rhodolites (HMC).

Much of the central shelf ,100 mwd and parts of the inner shelf ,50mwd contain .70% aragonite, with large areas having .80%. Sedimentcontaining the highest amount of aragonite is on the eastern and westernparts, whereas the central region has between 60 and 70% aragonite. Dis-crete areas of the outer shelf and upper slope (directly off Eucla and offthe Head of the Bight; Fig. 3) are low (,50%) in aragonite.

A similar pattern is evident on the LS, where areas with .70% aragoniteextend across the mid to outer shelf and comprise the whole shelf off Robe(Fig. 4). Again, like the GAB, there are areas of the outer shelf that areconspicuously low in aragonite directly adjacent to regions of high ara-gonite content.

Aragonite Components

The aragonite components in these carbonates are limited to certain mol-luscs, some bryozoans, serpulid worm tubes, and ascidian (tunicate) spic-ules. All gastropods and most infaunal bivalves are aragonite.

Molluscs.—The coarse fraction (.2 mm) of sediment samples at allstations was examined to assess the general nature of the mollusc assem-blage. Mollusc fragments in the sand-size fraction are all pieces and couldnot be ascribed to any particular genus. Whereas it is not certain that mol-luscs in the coarse fraction uniformly supply particles to the sand-size frac-tion via biofragmentation and physical breakage, it is a broad measure ofthe character of the biota. Genera on the GAB, the number of stations atwhich they were found, and their mineralogy is presented in Table 1.

Gastropods are aragonite and the biota is fairly diverse containing 24genera. The most recurring elements are columellas of a variety of forms.The most common gastropods are the suspension and detritus feeders Ca-lyptraea (which in life is mostly attached to dead bivalves), the periwinkleClanculus, various turitellids, olives, mitres, sponge-dwelling siliquarids


FIG. 4.—Chart of Lacepede Shelf with sample sites and percent aragonite in Holocene sediments. Inset triangle Holocene sediment mineralogy as determined by XRD(82 samples; 30–300 meters water depth).

(Tenagodus), and cowries (Cyprea). Predatory forms include moon shells(Polinices), pheasant shells (Phasianella), and rock shells (Murex).

Bivalves are also diverse, with 37 genera identified. The only bivalvewhose shell is mostly calcite is the scallop Chlamys, and it is widespreadand numerous. Those with a mixed calcite–aragonite shell are file shells(Lima, Limaria, Limatula), mussels (Modiolus, Mytilus, Brachiodontes),and pen shells (Pinna and Myadora). Bivalves with aragonite shells arewholly infaunal and overall more diverse than epifaunal forms. The mostubiquitous and abundant are the cockles Katalysia and Glycymeris. Addi-tional cockles such as Fulvia, Mactra, Callucina, and Spisula are also foundat numerous localities. Other widespread forms are the tellin Tellina, thetrigonid Neotrigonia, the nut shells Callista, Notocallista, and Nuculana,the chionids Circumphalus and Placamen, the basket shell Corbula, andthe lantern shell Myodora.

Taken together aragonitic gastropods and bivalves dominate the Holo-cene macromollusc assemblage. Aragonite is also contributed to the sedi-ment by those epifaunal forms with a mixed mineralogy.

Bryozoans.—Such an analysis is not practical for bryozoans because asignificant proportion of the biota disintegrates into sand-size particles upondeath, and so analysis of the coarse fraction is genera-specific. The problemis simplified, however, because there are relatively few aragonitic bryo-zoans; most are calcitic. Using the morphological terminology of Bone andJames (1993), aragonitic bryozoans are found in the foliose, encrusting,erect rigid flat robust branching, fenestrate, and vagrant groups (Bone andJames 1993, 1997). Examples of totally aragonitic bryozoans are Parmu-laria sp. (foliose), Caleschara sp. (encrusting), Adeona sp. (fenestrate), andSelenaria sp. (vagrant). Bimineralic bryozoans include Schizoporella sp.(encrusting) and Adeonellopsis sp. (erect rigid, flat robust branching). Somespecies of Adeonellopsis have a central core of HMC along the axis of thebranch, comprising ;10% of the skeletal volume.

On the LS, living bryozoans are strongly depth partitioned; ,130 mwdthey are a mixture of aragonitic and IMC forms; .130 mwd almost all ofthe bryozoans are either IMC or LMC with few if any aragonitic types(Bone and James 1993). This is also reflected somewhat by the general


TABLE 1.—GAB Molluscs.

Bivalves–AragoniteBivalves–Aragonite

and Calcite Gastropods—Aragonite

28 Katelysia25 Glycymeris11 Tellina09 Fulvia07 Neotrigonia

11 Lima07 Modiolus04 Limaria03 Brachidontes02 Pinna

15 Calyptraea07 Clanculus06 Turitellids06 Columella04 Oliva

07 Mactra06 Notocallista05 Placamen04 Circomphalus04 Callista

02 Myadora02 Malleus02 Limatula01 Spondylus01 Mytilus

04 Mitra03 Tenagodus03 Conus03 Polinices02 Phasianella

04 Nuculana04 Corbula02 Spisula

01 Anomia 02 Murex02 Cypraea01 Vermicularia

02 Myadora02 Callucina01 Venerupis01 Sunetta01 Solen

Bivalves—Calcite

34 Chlamys

01 Triton01 Serpulorbis01 Penion01 Niotha01 Latirus

01 Paphia01 Donax01 Cardita01 Arca

01 Gazameda01 Eubittium01 Diala01 Bulla

01 Antigona01 Amesodesma01 Acrosterigma

01 Bedeva01 Batillaria

01 5 number of stations in which this genera occurs; Acrosterigma 5 species

overall mineralogy of Holocene particles when plotted against depth (Fig.5A, B). These diffuse trends of increasing aragonite content also reflect anincreasingly higher proportion of molluscs in the sediment (Fig. 5C, D).

Although many aragonitic components are fresh, a large proportion ofthe skeletal particles, regardless of taxa, are conspicuously more intensivelybored than their HMC and LMC counterparts. Such apparent mineralogicalspecificity parallels that from many other neritic environments, both warm-water and cool-water (Bathurst 1975; Smith and Nelson 2003). Some ara-gonitic skeletons also illustrate minor chalkification (Bone and James 1993)indicating incipient dissolution.

Areal Distribution of Aragonite in Southern Australia

Holocene aragonite is generally highest (.70%) in areas where Holo-cene material comprises ,50% of the surficial sediment; i.e., most of thesediment is relict intraclasts. The relative proportion of molluscs is alsogreatest in this area, with the mollusc/(mollusc 1 bryozoan) ratio being.50%. James et al. (2001) suggest that in the GAB these are areas ofsomewhat reduced carbonate production, largely because of a warm, nu-trient-depleted water mass that slowly moves across the area during latewinter and spring. Otherwise there is no clear relationship between ocean-ography as currently understood and the proportion of aragonite in thesediments. Areas of Holocene aragonite on the outer GAB and LS, forexample, vary dramatically along strike but are not directly related to areasof upwelling or downwelling. This may, in part, be due to the relativeproductivity of organisms with differing mineralogies, i.e., more gastropods(aragonite) than coralline algae (Mg-calcite) or visa versa.

ODP drilling on the central GAB slope indicates extensive off-shelf sed-iment transport throughout the Plio-Pleistocene (James et al. 2001; Fearyet al. 2004). These fine carbonate sands and muds contain 20–30% ara-gonite to depths of ;200 meters below the sea floor (Swart et al. 2002).Persistence of aragonite here is interpreted to be due to the highly salineporewaters. The aragonite decreases downhole and finally disappears atbetween 250 m and 400 m subsurface, depending upon the site. This de-crease down-core is interpreted to reflect dissolution and reprecipitation asLMC cement from modified marine porewaters. Nevertheless, even thoughanalyses are of bulk sediment and so include relict grains, these finer slopesediments contain significant aragonite. Furthermore, this situation has per-sisted over at least the last 300,000 years.

INTERPRETATION AND DISCUSSION

It is now almost axiomatic that a majority of Cenozoic, cool-water, het-erozoan carbonate sediments, the bridge into the older rock record, weredominated by skeletal calcite mineralogies with little aragonite originally(Nelson 1988; James 1997). This is because those cool-water carbonatesthat have only seen marine and meteroic fluids, are mostly unlithified andcomposed of large well-preserved skeletons. In contrast, aragonite-rich,warm-water, photozoan neritic carbonates, which have been sequestered insimilar meteroic environments and altered to LMC, have variably preservedskeletons, numerous molds, and abundant LMC cement in molds and in-terparticle spaces, generally resulting in good lithification (Bathurst 1975;James and Choquette 1990). Yet the results from this study highlight thefact that Holocene sediments on at least two extensive cool-water carbonateplatforms contain high proportions of aragonite. What is the reason for thisdisconnect? If these aragonitic particles had been abundant in Cenozoicsediment when it was altered to LMC in the meteoric environment, thenthere would be many molds and much cement. The implication is thataragonite disappeared while the sediment was still soft and before it wassubject to percolating meteoric waters. Aragonite must have dissolved ei-ther: (1) on the seafloor or in the shallow subsurface because overlyingseawater was undersaturated with respect to aragonite, or (2) in the shallowsubsurface because marine porewater was modified such that it was un-dersaturated with respect to aragonite.

Synsedimentary diagenesis of neritic carbonates is generally thought ofas the combined effect of chemical and biological processes that lead tomicrite envelopes, total micritization, and facies-specific cementation inwarm-water environments (Bathurst 1971; Tucker and Wright 1990; Jamesand Choquette 1990), and maceration or seafloor dissolution in cool-watersettings (Alexandersson 1978, 1979; Nelson et al. 1988; Freiwald 1995;James 1997). The principal control on carbonate dissolution in seawater ismineralogy (Bathurst 1975; Morse and Arvidson 2002). Secondary controlrevolves around multiple intrinsic variables (Martin 1999; Smith and Nel-son 2003) such as skeletal robustness (Davies et al. 1988; Smith et al.1992), grain size (Walter and Burton 1990), organic coatings, intraskelatalpore space, and amount of intercrystalline organic material (Freiwald1995), as well as size and shape of crystallites (Henrich and Wefer 1986).

The saturation state of Southern Ocean seawater with respect to aragoniteand calcite in the region of the Great Australian Bight (GAB) and LacepedeShelf (LS) was calculated using the CSIRO water analysis database andSEACARB package (Gattuso and Proye 2004). Values for alkalinity wereestimated from temperature and salinity following Millero et al. (1998).The dissolved inorganic carbon (DIC) values come from a regression ofobserved Southern Ocean DIC values with t, s, nitrate, phosphate, silicate,and oxygen. Saturation state was calculated as V, which is ion activityproduct/Ksp. Given errors in the predictive equation for DIC and alkalinity,errors in the V values are ;60.15. V values were calculated for 100,200, 300, and 1000 m water depth.

Results indicate that seawater in this region is oversaturated with respectto both aragonite and calcite to depths of 1000 m. Varagonite values at 100mwd are 2.8 to 3.2, being highest in the central GAB and lowest on theouter GAB and LS. Values decrease with depth to ;2.80 at 200 mwd,;2.65 at 300 mwd, and 1.35 to 1.45 at 1000 mwd. Vcalcite values over thesame depth range vary from ;4.6 at 100 mwd to ;2.20 at 1000 mwd.Thus, from the perspective of solution chemistry, seawater overlying mod-ern neritic sediments in this region is more than 23 oversaturated withrespect to aragonite, confirming that aragonite is not dissolving because thewatermass is undersaturated.

Much recent work has underscored the role of early carbonate dissolu-tion, irrespective of the saturation state of surrounding seawater. Such pro-cesses are particularly apparent in the water column (Milliman et al. 1999)and in the first few meters of sediment below the seafloor (Sanders 2003),part of the concept of taphonomic loss (Kidwell and Bosence 1991; Martin


FIG. 5.—Percentage aragonite in Holocene sediment samples versus percentage of molluscs on A) the Great Australian Bight shelf and B) Lacepede Shelf. Percentageof aragonite in Holocene sediments collected at varying water depths on C) the Great Australian Bight shelf and D) Lacepede Shelf.

1999). At the same time it has been further realized that dissolution, ce-mentation, and neomorphism can occur in the deeper marine subsurfacerealm, to depths of several hundred meters (Brachert and Dullo 2000; Ma-lone et al. 2001; Melim et al. 2002). Such changes leading to under satu-ration are brought about by the oxic to anoxic microbial degradation ofsedimentary organic matter (Froelich et al. 1981) via several pathways,including: (1) aerobic oxidation of organic matter, (2) oxidation of reducedby-products and H2S, (3) sulfate reduction, and (4) anaerobic methane ox-idation. Such processes are decoupled from overlying bottom waters, andhave little relationship to the saturation state of the ambient seawater. Wal-ter and Burton (1990), Walter et al. (1993), Patterson and Walter (1994),and Ku et al. (1999) demonstrated that in photozoan sediments from Florida

and the Bahamas, where seawater is oversaturated with respect to aragoniteyet pore waters are saturated or undersaturated, approximately 50% of thesediment has undergone dissolution.

The precise importance of such effects on the modern southern Austra-lian shelf cannot be assessed because of the overall thinness of the sectionon the shelf proper (Boreen and James 1993; James et al. 1994), and thepaucity of thick vibracore sections. Nonetheless, in the presence of satu-rated seawater, the conclusion that much aragonite has been removed inthe shallow marine subsurface by dissolution related to microbial respira-tion is compelling.

While the foregoing appears to confirm the loss of aragonite from thedepositional system, what is the fate of Mg-calcite components? Most such


FIG. 6.—Sketch illustrating the mineralogicalchanges envisaged for shallow marine carbonatesediments in a cool-water neritic environment asthey are buried in the shallow subsurface. Suchprocess may be applicable to other cool-watercarbonate settings today and in the past.

particles are intermediate-Mg-calcite, with between 4 and 12 mole%MgCO3, and, other attributes being equal, their relative solubility is lessthan that of aragonite (Burton and Walter 1987).

When surveying information from the modern New Zealand shelf, Nel-son et al. (1988) noted that there appeared to be a ‘‘slow-leakage’’ ofmagnesium from calcite skeletons through time, implying transformationfrom higher-Mg to lower Mg-calcite. Data from their Table 4 suggests anoverall neomorphism to LMC. The most telling observations, however,come from the Cenozoic of southern Australia, where Kyser et al. (1998)conclude that, on the basis of stable and radiogenic isotope analyses, theoriginal HMC (mostly IMC) skeletons neomorphosed to LMC in seawater.This marine diagenesis was facilitated by slow accumulation rates, rela-tively fine grain size, and high original porosities. In short, the sedimentshad altered to LMC long before they entered the meteoric realm wherethey now reside.

When integrated with the information on aragonite dissolution it wouldseem that the process overall is not just aragonite loss, but also HMCneomorphism to LMC (Fig. 6). The resultant sediment has no aragonite(or record of it) and is all LMC, with a marine isotope signature.

An important caveat: these findings do not imply that all cool-watercarbonates lack any indication of original aragonite. Evidence of formeraragonite, usually as skeletal molds, is present in Cenozoic nearshore facies(James and Bone 1989; Lukasik et al. 2000) and in deposits with relativelyhigh accumulation rates (Nelson et al. 2003), and it is even preserved asaragonite proper in muddy Cenozoic sediments with low porosity and per-meability (Reeckmann 1981; Lukasik et al. 2000). Such examples notwith-standing, most cool-water carbonates show scant evidence of any originalaragonite.

IMPLICATIONS FOR THE OLDER ROCK RECORD

In cool-water carbonates, it has long been suspected that the aragonitefraction was lost from the sediment relatively early (Beu et al. 1971). Spe-cifically, Nelson (1978) noted that dissolution must have occurred beforedeep burial because no skeletal molds were preserved in Cenozoic NewZealand cool-water limestones, yet faunal assemblages should have includ-ed aragonite-secreting organisms. This concept has been reemphasized byBetzler et al. (1997), who stress that the absence of micrite envelopes (cf.Bathurst 1966) in cool-water carbonate sediments results in lack of evi-dence of any dissolved aragonitic skeletons in the rock record. Brachertand Dullo (2000), in their study of early marine diagenesis of periplatformcarbonates off the Queensland Plateau, further concluded that original cool-

water carbonates may have contained much more aragonite than is com-monly assumed. Such a concept has further support in the older rock record,where Cherns and Wright (2000), Wright et al. (2003), and Wright andCherns (2004) have shown convincingly that all the original aragoniticskeletal elements of some Silurian and Jurassic neritic carbonates wereremoved while the sediment was still soft.

If the conclusion that aragonite is lost early in sedimentary history iscorrect, then there are major implications for the older rock record. First,it implies that for sediments such as those on the Southern Australian shelf,40–90% of the carbonate that is produced never gets into the geologicalrecord. Thus, not only is the record biased and lacking in any vestige ofaragonitic components, there is also no aragonite for shallow meteoric ce-mentation (cf. James and Bone 1989). Lithification is delayed until sub-stantial burial and chemical compaction. Second, part of the low sedimen-tation rates calculated for neritic cool-water carbonates probably reflectboth diagenetic loss as well as benthic production rate. As stressed bySanders (2003) it would further appear that areas of high sedimentation(accumulation) rates would favor preservation of aragonite into the sub-surface whereas the opposite would be true for regions of low sedimenta-tion rates (cf. Nelson et al. 2003). Likewise, aragonite molluscs and bryo-zoans should be preferentially preserved at hardgrounds where they are‘‘frozen’’ into the sediment before any potential ‘‘soft-sediment’’ disso-lution. This is especially evident where erect rigid, flat robust branchingbimineralic bryozoans such as Adeonellopsis are now only a thin core ofLMC (HMC originally) and the rest of the void is filled with carbonatemud, or gastropod molds are filled with mud or marine cement (Nelsonand James 2000). Otherwise, the sediments outside the hardgrounds containno evidence whatsoever of aragonite components.

SUMMARY AND CONCLUSIONS

1. Holocene surficial, neritic carbonate sediments between water depths of30 to 350 m on two large sectors of the southern margin of Australia,the Great Australian Bight, and the Lacepede Shelf, are composed pre-dominantly of aragonite, with lesser amounts of low-magnesium andhigh-magnesium calcite. Such aragonitic biogenic particles are princi-pally from the skeletons of gastropods, infaunal bivalves, and a restrict-ed number of bryozoan genera.

2. Lack of such aragonite or aragonitic diagenetic products (molds, LMCcement) in the rock record of Cenozoic neritic cool-water carbonatepaleoenvironments implies that these particles are lost early in diage-netic history, before the sediments enter the burial or meteoric diagenetic


environments. Because seawater in the region is oversaturated with re-spect to aragonite, the process that results in such loss is interpreted tobe dissolution tied to the microbial degradation of sedimentary organicmatter.

3. The calcite-dominated mineralogy of many Cenozoic and older cool-water carbonates is likely a function of early preferential dissolution asopposed to original composition.

4. The paleontological record of cool-water carbonates is likely biased, andsediments originally contained much more aragonite than would be de-duced from examination of the rock record.

ACKNOWLEDGMENTS

This research is supported by the Natural Sciences and Engineering ResearchCouncil of Canada (NPJ, TKK), and the Australian Research Council (YB). Wethank the CSIRO Division of Oceanography and captains and crew of the R.V.Franklin for their help and support during research cruises. R. Matear kindly cal-culated saturation values for aragonite and calcite in Southern Ocean waters. Lab-oratory analyses were conducted by M. Coyne and C. Koebernick. The manuscriptwas improved through reading and comment by P. Pufahl.

The data described in this paper have been archived, and are available in digitalform at the SEPM data archive, URL: http://www.sepm.org/archive/[email protected]; email: [email protected].

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Received 18 June 2004; accepted 11 October 2004.

APPENDIX 1.—Results—Great Australian Bight (GAB)

Sample No.Latitude

(S)Longitude

(E)Water Depth

(m)Aragonite

(%)HMC(%)

LMC(%)

ACM051ACM052ACM053ACM054ACM055

33819.9833812.8834827.3434824.9934823.82

134815.04134816.46132848.54132851.25132851.31

71.565

500234176

7579545677

1611423719

910

474


34831.5533844.0932853.3934800.0533859.94

132859.01133815.97133848.25132825.32132826.78

1219462

288174

6970876264

2420

53331

710

855


33847.0733824.0933806.98

na33830.76

132835.12132851.11133805.82

na131843.33

1239482na

195

6680814439

2817165141

6335

20ACM075ACM076ACM077ACM078ACM079

33821.3432855.3132836.6332812.2432800.11

131848.92132802.71132813.81132827.70132834.90

1209174.56345

5882745874

3613214018

65528


32822.0532852.0932816.0131838.9431857.89

131831.00131815.68130851.92130827.98130827.98

7089685056

6880796672

1416

91621

184

1218

7

APPENDIX 1.—Continued

Sample No.Latitude

(S)Longitude

(E)Water Depth

(m)Aragonite

(%)HMC(%)

LMC(%)


32830.0332856.1433815.9833822.3533826.18

130828.24130828.23130828.22130828.06130828.12

66.594

150260

na

9271343758

627554723

22

111619

ACM099ACM102ACM104ACM105

33838.7833821.4732855.9832838.06

130828.37129830.33129829.96129829.99

1007200.5

8065

33678881

827

216

596

103

GAB002GAB003GAB005GAB006GAB011

32842.1232819.1032806.1632804.4032825.01

127830.79127830.00128829.09128829.12129859.77

49.530404060

8774797482

713152015

613

663

GAB021GAB022GAB023GAB024

33821.5933819.9933819.7933818.31

128828.29128828.07128827.88128829.08

492.5347343305

37423836

40364744

23221520


33819.1733819.283382033819.7933816.04

128829.13128829.26128829128829.06128831.74

295309358205148

3437434137

4442464411

2221111552


33813.0833802.5832840.0532859.65

na

128828.67128828.71128828.90128800.39

na

16790.554.587na

3642656872

4743292724

1715

654


33816.4033810.1632859.0332840.4832828.02

127829.77127829.69127811.19126838.92126822.79

153.5100

76.55142.5

6577836871

3120122820

43549


32824.0033824.8433824.5933853.6833853.57

126813.98125858.04125858.00125821.88125821.81

41143.5136.5305182

2685774953

7415204144

003

103


33852.5533852.473383633826.3033811.59

125821.79125820.92125811125804.98124855.20

156118.5

71.56254

5352656468

4745333531

03211


33816.7633808.2232846.9732857.5533812.92

125818.16125857.73125825.23124846.61124822.94

61.5na504246

5973444463

3323565625

8400

12GAB068GAB070GAB086GAB087GAB088

33837.9033854.8734826.1934832.5434834.81

124822.99124823.07123848.54123840.17123837.74

5973.5909098

6469716561

2326223513

13570

26

PL94-01PL94-02PL94-03PL94-04PL94-05

33843.2733856.1934807.9334820.8334827.48

134854.98134841.15134830.08134817.32134810.14

5172779594

6576908495

2116

712

1

148344

PL94-06PL94-07PL94-08/09PL94-10PL94-12

34840.3434846.9634854.9034855.5734844.92

133856.53133849.60133843.00133840.98135819.42

111115205450

44

7865494176

18283924

8

47

123516

PL94-13PL94-14PL94-15PL94-16PL94-17

34848.3234856.9335807.0735814.2435816.54

135816.32135810.73135803.15134857.85134854.80

2599

113120124

7467776055

2311203337

322

378

PL94-18PL94-19PL94-20PL94-21PL94-22

35822.1535823.9235825.7135826.4235827.26

134851.40134849.94134849.04134848.49134848.41

138148165310445

7258394762

2234384328

68

231010

na 5 not available.

2

APPENDIX 2.—Results—Lacepede Shelf (LS)

Sample No.Latitude

(S)Longitude

(E)Water Depth

(m)Aragonite

(%)HMC(%)

LMC(%)


38826.1038840.7538811.2238822.7838821.83

141826.01141840.75141808.05140843.39140843.39

65207

28273176.5

5948605060

240

253723

1752151317


38816.1438807.9338813.1337846.4737851.25

140844.61140845.74140824.13140812.92139859.80

13130

19135

130

6049756048

252612.54040

152512.5

012


37852.4937853.1837826.8137830.6037827.51

139856.06139854.06139849.18139824.94139827.03

179278

38240130

8364628373

92631

824

810

793

ACM026ACM029

35855.0636804.00

137854.07137803.14

4857

7062

3032

06

VH89-01VH89-02VH89-03VH89-04VH89-05

36856.5736855.5936855.4636845.7536837.26

137829.16137839.47137838.84137847.55137857.50

171127123

6768

5165546764

4227332628

78

1378

VH89-06VH89-07VH89-08VH89-09VH89-10

36828.2536819.5036810.5836801.8835853

138807.20138817.50138827.18138837.13138847

6562575247

6490499148

99

358

36

271

161

16VH89-11VH89-23VH89-25VH89-34VH89-40

3584436807.6136822.4736841.7436809.94

138857.30138800.21137846.03138822.45139825.86

4258746141

9467827971

52813

927

155

122

VH89-41VH89-42VH89-43VH89-44VH89-45

36818.5936827.3236835.9136846.3236850.19

139817.66139807.47138856.68138846.57138842.57

5053555257

7583965577

1512

14322

105321

VH89-46VH89-47VH89-50VH89-53VH89-54VH89-56

36854.5036858.4637804.1236842.1836834.3537807.92

138836.94138832.58138856.31139820.67139830.21139822.37

677791473962

726681606557

282514183032

095

225

11VH89-59VH89-60VH89-61VH89-62VH89-63

37833.1037823.2937821.8837820.3937817.40

139825.39139830.81139836.00139837.42139841.07

16782516240

8669918771

1031

71012

4023

17VH89-64VH89-68VH89-85

37831.2836843.5536806.80

139825.84137818.29138828.60

249113

56

867186

92412

552

VH91-105VH91-107VH91-108VH91-109VH91-111

36800.8336857.1936856.3536853.4236853.02

137832.04137835.63137836.06137839.47137839.95

1064425281106105

6038455363

2538343327

1524211410

VH91-112VH91-116VH91-117VH91-118VH91-119

36853.0837809.9037807.7437807.1137807.18

137839.61138834.43138834.71138834.74138834.77

97434305200243

5135403546

3432363338

1533243216

VH91-121VH91-122VH91-130VH91-134VH91-136

37804.9037804.1537803.7137803.0437853.53

138833.33138833.56138805.26138814.16137857.84

148136113113

77

5834767269

2930162126

1336

875

VH91-137VH91-138VH91-139VH91-140VH91-141

36857.6337859.7237802.3037803.2937804.40

137857137856.62137856.54137855.79137855.52

8697

141170335

8279767257

1317242843

54000

VH91-147VH91-148VH91-149VH91-150VH91-156

36829.536833.8836836.3736837.6137836.21

136856.10136854.38136854.08136854.36140802.61

108123129137

29

6383577751

3714332342

03

1007

VH91-157VH91-158VH91-159VH91-160VH91-165VH91-166

37838.6637843.2337847.1337847.79

nana

139858.60139853.75139847.67139846.72

nana

6083

240285

3070

595160466470

654936451622

6049

208

na 5 not available

james et al 2005 arag.pdf

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