Michel, J. et al., (2011). Modern heterozoan carbonates from a eutrophic tropical shelf (Mauritania).pdf

Journal of Sedimentary Research, 2011, v. 81, 641655

Research Article

DOI: 10.2110/jsr.2011.53

MODERN HETEROZOAN CARBONATES FROM A EUTROPHIC TROPICAL SHELF (MAURITANIA)

JULIEN MICHEL,* GUILLEM MATEU VICENS,{ AND HILDEGARD WESTPHAL{

MARUM and Department of Geosciences, Universitat Bremen, Leobener Strae, 28359 Bremen, Germany

e-mail: [email protected]

ABSTRACT: Heterozoan or foramol production is typical in extratropical carbonate sedimentary systems. However, undermesotrophic to eutrophic conditions, heterozoan carbonates also form in tropical settings, but such heterozoan tropicalsedimentary systems are poorly understood. Nevertheless, distinction between tropical and extratropical heterozoan carbonatesin ancient successions is crucial for accurate paleoenvironmental and paleoclimate reconstructions. Here, surficial Holoceneand Pleistocene sediments of the northern Mauritanian shelf are studied as an example of a tropical eutrophic carbonatedepositional system (11 mg?L21 Chl-a [chlorophyll-a]). Upwelling nutrient-rich waters push onto the wide Mauritanian shelf,where they can warm up to in excess of 25uC. This condition favors production of heterozoan carbonates dominated by bivalvesand foraminifers, even in this tropical setting. In addition, sediments are provided by eolian input from the desertic hinterland.The resulting sediments are carbonate and mixed carbonatesiliciclastic facies, in which the carbonates are characterized by amixture of tropical and cosmopolitan taxa. Benthic photosynthetic biota are absent while suspension-feeding organisms aredominant. This foramol grain association on a shelf scale is reminiscent of cool-water carbonates, therefore recognition ofwarm-water heterozoan carbonates relies on key taxa related to tropical waters within the biota assemblages associated with ahighly productive environment.

INTRODUCTION

Formation of carbonate sediment is related to biological activity withinthe ecosystem of a given depositional environment. Beside temperature,multiple factors such as salinity, type and availability of substrate,nutrient concentration, water depth, light penetration, water energy,seawater chemistry, and siliciclastic supply control the rates and style ofbiogenic carbonate production (Hallock and Schlager 1986; Carannanteet al. 1988; Pomar 2001; Mutti and Hallock 2003; Pomar et al. 2004;Wright and Burgess 2005; Pomar and Hallock 2008; Westphal et al.2010). In consequence, carbonate sediments are multiparameter archivesof environmental conditions that can be useful for reconstructingpaleoecology and paleoclimate.

In the modern, most tropical settings are characterized by oligotrophicwarm waters. In such settings, carbonates are produced predominantly byautotrophic biota such as calcareous green algae and mixotrophic biota(sensu Hallock 1981), such as zooxanthellate corals. Such associationshave been termed chlorozoan by Lees and Buller (1972) and photozoanby James (1997). In contrast, carbonate sediments from temperate topolar regions are dominated by heterotrophic biota, corresponding to theforamol association of Lees and Buller (1972) and the heterozoanassociation of James (1997) (e.g., Nelson and Bornhold 1983; Freiwald

1993; Freiwald and Henrich 1994; Henrich et al. 1995; Rao 1996).However, foramol or heterozoan associations not only form in cool tocold-water settings but occur in all climate belts from the poles to thetropics (Lees and Buller 1972; Lees 1975; Mutti and Hallock 2003; Wilsonand Vecsei 2005). As a consequence, interpreting heterozoan versusphotozoan or foramol versus chlorozoan occurrences as indicative ofcold or temperate versus tropical conditions can result in misleadingpaleoclimatic and paleoenvironmental interpretations (cf. Edinger et al.2002; Pomar et al. 2004).

An increasing number of ancient examples of tropical heterozoancarbonates have recently been described (Neogene: Brandano and Corda2002; Pomar et al. 2004; Triassic: Hornung et al. 2007; Pennsylvanian:Samankassou 2002). Recognition and interpretation of such occurrencesrequires detailed facies description, including taxonomic determination ofthe skeletal components. In addition to detailed study of the rock record,modern analog studies provide a means for better understanding of high-nutrient tropical settings.

Modern heterozoan carbonate depositional systems in tropical tosubtropical latitudes in most cases are related to oceanographic upwellingcausing elevated nutrient levels (Hallock and Schlager 1986; James 1997;cf. Westphal et al. 2010). At the same time, the cool upwelling waterslower the water temperature in tropical seas so that the effect of highnutrient concentrations is overshadowed by the effect of coldertemperatures and cannot be studied independently (cf. Halfar et al.2004). In contrast to this general pattern, on the wide shelf off northernMauritania cool, upwelling nutrient-rich waters warm to tropicaltemperatures, creating a warm-water eutrophic ecosystem. Whereas inthe southern part of this area (the wide Golfe dArguin), carbonatesediment is diluted by large amounts (up to 63%) of eolian silt, in thenorthern part carbonate content reaches up to 93% (Michel et al. 2009).

* Present Address: Universite de Provence, Laboratoire de Geologie des

Syste`mes et des Reservoirs Carbonates, case 67, 3 place Victor Hugo, 13331

Marseille Cedex 3, France

{ Present Address: Dipartimento di Scienze della Terra, Universita` di RomaLa Sapienza, Ple Aldo 7 Moro, 5. I-00185 Roma, Italy

{ Present Address: Leibniz-Center for Tropical Marine Ecology, Fahrenheit-strasse 6, 28359 Bremen, Germany

Copyright E 2011, SEPM (Society for Sedimentary Geology) 1527-1404/11/081-641/$03.00

In this paper, the facies of this eutrophic tropical sedimentary system arestudied with focus on ecological data to gain a better understanding ofthis type of carbonate despositional environment and to improve theinterpretation of ancient counterparts.

STUDY AREA

The narrow (, 65 km) continental shelf of northwest Africa broadensoffshore northern Mauritania to the Golfe dArguin, with a width ofsome 150 km (Fig. 1). This extensive gulf hosts the shallow BancdArguin, with water depths of less than 10 m and in many areas less than5 m (Piessens 1979; Sevrin-Reyssac 1993; Wolff et al. 1993). We here referto the inner shelf as the area from the coastline to the abrupt 20 m breakin slope, which borders the shallow Banc dArguin. The shelf below thisbathymetric break down to 50 mwd (meters water depth) we refer to asmid shelf. The mid shelf is about 50 km wide in the northern part of thestudy area, whereas it narrows to a few kilometers in width in the south,where it is cut by submarine canyons. The outer shelf we define here asthe area below 50 mwd down to the shelf break at 100150 mwd.

The waters off northern Mauritania are among the most productivemarine areas in the world (11 mg?L21 Chl-a [chlorophyll-a]; Quack et al.2007) and are important fishing grounds (Binet et al. 1998). The origin ofthe high productivity lies in the elevated nutrient levels. Oceanic upwellingcauses elevated concentrations of phosphate and nitrate on the shelf (upto 1.3 and 18 mmol.L21 in the surface waters, respectively; Quack et al.2007), whereas influx of eolian dust is thought to result in elevated ironconcentrations (e.g., Ohde and Siegel 2010).

Oceanic upwelling along the northwest African coastline stretches from12u N to 33u N (Mittelstaedt 1991; Van Camp et al. 1991), and is reflectedin the presence of cool-water carbonates in most parts of this region(Summerhayes et al. 1976). Between 20u N and 25u N, oceanic upwellingoccurs year round. Seasonality in the Golfe dArguin, however, inducesan increasing influence of the Guinea Current during summer, related tolatitudinal movements of trade winds that causes sporadic interruptionsof upwelling (Mittelstaedt 1983). Where the upwelling waters enter theBanc dArguin, minimum temperatures are 16uC. In the shallow GolfedArguin, however, the upwelling waters warm to subtropicaltropicaltemperatures; on the bank, water temperature exceeds 25uC in summerand does not drop below 18uC in winter (Sevrin-Reyssac 1993; Quack etal. 2007). At the same time, the trophic level remains high, resulting inmesotrophic to eutrophic warm-water conditions.

The Golfe dArguin is characterized by high water energy caused byswell from the northwest, waves, tides, and wind-driven and deepcurrents. Whilst trade winds drive surface currents southwards (speed ofup to 50 cm?s21), undercurrent flows northward along the slope and shelfbreak up to 20 cm?s21 (Shaffer 1974; Mittelstaedt 1991). On thenorthwest African shelf, tidal currents are usually weak; they are strongonly inshore, as on top of the Banc dArguin (up to 150 cm?s21 in narrowchannels; Piessens 1979; Mittelstaedt 1991). Wind-generated waves arethought to influence the seafloor on the shallow bank (Piessens 1979).Swell from north or northwest displays wave lengths of 250 m (measuredon aerial photos) and is thought to play a major role in sedimentremobilization off northwest Africa (cf. Summerhayes et al. 1976;Piessens 1979).

FIG. 1.Location and bathymetry of theGolfe dArguin study area between Cap Blancand Cap Timiris. Note the large expanse ofshallower water (, 10 mwd) in the center of thegulf, known as the Banc dArguin. This areaincludes water depths of less than 5 m. Large-scale surface currents (gray arrows) and domi-nant wind-induced surface current direction(large gray arrow) are shown (modified fromDomain 1985; Hanebuth and Lantzsch 2008;Michel et al. 2009).

642 J. MICHEL ET AL. J S R

The sedimentary system of the Golfe dArguin is characterized bymobile sand waves in the northern part, including the Baie du Levrier.This coarse-grained sediment is largely biodetrital, and carbonatecontents reach up to 93% (Michel et al. 2009). On the Banc dArguin,most material consists of skeletal sand and in coastal parts locally ofcarbonate-poor sediments (Piessens and Chabot 1977; Piessens 1979). Inthe Baie de Saint-Jean, where hypersaline conditions occur, and in rarecases in coastal regions of the Baie du Levrier, ooids are present(Koopmann et al. 1979; Stein 1980). Along the pronounced bathymetricbreak of the shallow Banc dArguin, to the outer, deeper part of the GolfedArguin, bioclastic sandy material accumulates. Towards the south,finer-grained sediment dominates that is largely composed of eolian dust,and carbonate contents are below 50% (Michel et al. 2009). In thesouthernmost part of the Golfe dArguin, the shelf is incised by a series ofsmall canyons, which on the slope merge to the Timiris canyon (Shaffer1974; Krastel et al. 2004).

MATERIAL AND METHODS

The samples studied here were collected during two cruises of R/VPoseidon (Westphal et al. 2007; Zonneveld et al. 2010) Sampling of theshallow Banc dArguin was not feasible during these cruises because ofthe shallow water depth (, 10 m) and migrating bedforms thatprohibited the research vessel from entering this area. Sampling thereforetook place on the off-bank shelf, based on the consideration that in a soft-bottom bioclastic sedimentary system information on the updip

carbonate production is recorded in the downdip sediment as a resultof downslope transport.

A total of 74 surface samples were taken with a van Veen grab and abox corer (Fig. 2). Van Veen grab recovers materials up to 20 cm belowthe sedimentwater interface. Study of the box cores was restricted to thesurface sediment layers. The sediment is strongly bioturbated and thusconsidered homogeneous, and no attempt was made to quantify live anddead shells apart from a visual estimate; the whole resulting assemblagesare studied qualitatively for environmental interpretation. Carbonatecontent as a percentage of the bulk sediment is determined using theCarbometer method, i.e., by measuring the increase in gas pressure afterreacting the ground sample with HCl (Muller and Gastner 1971).Multiple measurements lead to an internal error of less than 1%. Thecarbonate mineralogical composition of 74 samples was determined usinga Bragg-Brentano X-ray diffractometer (XRD). The percentage of thecarbonate minerals (aragonite, high-Mg calcite 5 HMC, minimum of4 mol. % MgCO3, and low-Mg calcite 5 LMC, maximum of 4 mol. %MgCO3; see Flugel 2004) relative to total carbonate content wascalculated from peak area ratios using calibration curves. Noncarbonatecomponents were not further investigated.

For grain-size analyses, bulk samples (n 5 61) were wet sieved at63 mm. The coarse-grained fraction was split into different sub-fractionswith a sonic sifter (63125 mm, 125250 mm, 250500 mm, 5001,000 mm,and . 1,000 mm, corresponding to very fine, fine, medium, coarse, andvery coarse-grained sand and gravel, respectively). Further dry sievingwas performed on the bulk material of selected samples to determine the

FIG. 2.Distribution of carbonate contentsof bulk sediment surface samples in theGolfe dArguin.

TROPICAL HETEROZOAN CARBONATE 643J S R

respective amounts of the very coarse-grained sand (1,0002,000 mm) andgravel (. 2,000 mm) fractions.

The sedimentary components of 39 samples were analyzed quantita-tively to determine composition. Sorting was determined on thin sectionsof bulk sediments. These samples are from the shallow subtidal zone (1050 mwd) along the Banc dArguin (n 5 34), the top of the bank (n 5 1),and from deeper water settings (70155 mwd, n 5 4). Samples from thedeeper shelf (. 155 mwd) and the Arguin and Timiris mud wedges werenot analyzed for composition. A minimum of 300 components per grain-size fraction . 125 mm was determined, and included tallies of: red algae,planktonic foraminifers, benthic foraminifers, bivalves, gastropods,pteropods, scaphopods, bryozoans, decapods, barnacles, ostracods,solitary corals, echinoderms, alcyonarian spicules, sponge spicules,serpulids, aggregated worm tubes, fish remains, fecal pellets, carbonateclasts (aggregate grains including organically aggregated grains andintraclasts), unidentified bioclasts (biogenic grains of non-determinableorigin), and siliciclastic grains (mostly quartz). Mollusks and foraminiferswere classified to species level where possible for obtaining a reliableenvironmental interpretation of the sedimentary facies. For the determi-nation of the mollusks, the systematics of Nikle`s (1950), Gofas et al.(1985), Cosel (1995, unpublished data), and Ardovini and Cossignani(2004) was used; for foraminifers, the systematics of Colom (1950, 1974),Loeblich and Tappan (1987), Colom and Mateu (2000), and Ellis andMessina (1940) was employed.

Sedimentary facies were determined on the basis of grain size andcomposition of the sediment. Hierarchical cluster analysis (dendrogramusing Wards method and the Euclidean distance measure based onpercentages of grain-size fractions, carbonate content, and composition)was performed to statistically distinguish groups of samples using PASTversion 2.01 for Windows (Hammer et al. 2001). Non-determinable grainswere not included in the statistical analysis. These unidentified grains,however, are considered in carbonate content and grain-size data; theyare further taken into account for sedimentary interpretations (i.e.,hydrodynamics and time averaging).

Radiocarbon ages were determined for mollusk shells that werestrongly bored, physically eroded, and stained, on the basis of theassumption that they have been reworked and may represent maximumages of the material in the surface sediment. The measurements wereundertaken by AMS 14C at the University of Poznan, Poland. The raw14C dates were calibrated using CALIB version 5.0.1 (Stuiver and Reimer1993) and the calibration curve Marine04 incorporating a reservoir-agecorrection of 400 years (Hughen et al. 2004).

RESULTS

Carbonate Content and Grain Size

The carbonate content in the sediments of the Golfe dArguin rangesfrom 35 to 93% (n 5 74; 53 samples . 50%; Fig. 2, Tab 1). Thesedimentary system thus is a mixed carbonatesiliciclastic system; itincludes a carbonate-dominated area in the northern study area, wherecarbonate content reaches values up to 7093%. In the vicinity of CapBlanc, carbonate content locally is lower (4462%) as a result of quartzsand dunes migrating into the sea. With increasing water depth,carbonate contents decrease from . 70% to , 50% before increasingagain between 100 and 200 mwd to . 80%. The central study area ischaracterized by intermediate values between 4570%, whereas in thesouthern part values of 3550% are typical.

Grain-size distribution shows a pattern similar to that of the carbonatecontent (Fig. 3, Tab 1). In the north, the most abundant grain sizes aremedium- to coarse-grained sand and even coarse-grained sand to gravelsouth of Cap Blanc. In the southern part of the study area, mud and veryfine- to fine-grained sand dominate. The coarse-grained fraction iscomposed of biogenic carbonate components whereas the silt fraction is

siliciclastic-rich (for the grain-size distribution in the silt fraction, seeMichel et al. 2009).

Component Analysis of the Loose Sediment

The clear dominance of mollusks and foraminifers in most samples(Fig. 3) places the sediment of the Golfe dArguin in the foramol grainassociation of Lees and Buller (1972) and the heterozoan association ofJames (1997). Bivalves, small benthic and planktonic foraminifers,barnacles, and echinoderms are the most common bioclasts in thesediments (Fig. 3). Bivalve shells are the most abundant carbonate grainsin 33 out of 39 samples and in none of the samples constitute less than19% of the identified bioclastic grains (Tab 1; in the following,abundances of identified bioclasts are given as relative percentages ofbioclastic grains excluding unidentified bioclasts and siliciclastic grains).Of the six samples with less than 30% bivalves, one from the Baie duLevrier is dominated by barnacle fragments and five from the southernpart of the Golfe dArguin are dominated by benthic foraminifer tests(Fig. 3). Small benthic foraminifers are present in most parts of the gulfand are most abundant in the southern part of the study area. Planktonicforaminifers constitute . 5% of the identified bioclasts in nine samples,four of which are from the outer shelf and five are from the mid shelf.Barnacle fragments are present predominantly in the shallowest samplesin the northern part of the Golfe dArguin and on the Banc dArguin.Echinoderm fragments are common, but abundance . 5% is restricted tothe Baie du Levrier and the mid shelf.

Less common bioclasts include gastropods, which are ubiquitous butexceed 5% of the identified grains in only six samples, four from thesouthernmost part of the area. Scaphopods are rare. Aggregated worm-tube fragments show abundances . 5% in the Baie du Levrier and on thesouthern mid shelf. Crustaceans other than barnacles are rare and includedecapods and ostracods. Bryozoan fragments occur in all samples but donot exceed 5% of the identified bioclasts. Fish remains, fecal pellets,organically aggregated grains that might represent fragments ofaggregated worm tubes, and sponge spicules are rare, while alcyonarianspicules, serpulids, pteropods, and ahermatypic coral fragments are veryrare. Fragments of red algae and intraclasts (counted as aggregates inTable 2) were observed exclusively in the sample from the shallow BancdArguin.

Each mollusk and foraminifer assemblage is composed of a mixture ofcosmopolitan and tropical species. Most bioclasts consist of fragmentedshells. Unidentified bioclasts range between 1 and 42% of the sedimentgrains . 125 mm (Tab 1). The abundance of siliciclastic grains is highlyvariable and ranges from 0 to 70% of the bulk of the identified grains.Most siliciclastic grains are composed of quartz.

Mineralogical Analysis of the Carbonate Fraction

Aragonite constitutes 2181% of the bulk sediment (mean of 42%) and4594% of the total carbonate portion (mean of 68%) and thus is thedominant carbonate mineral (Fig. 4). Only four samples contain less than50% aragonite in the total carbonate portion. The highest aragonitecontent is found (1) in shallow water depths along the Banc dArguin and(2) at greater water depths around the shelf break. The lowest aragonitecontents in the carbonate fraction are found in the vicinity of Cap Blancbetween 15 and 40 mwd, in an area located between 35 and 65 mwd offthe northern Banc dArguin, and in four isolated samples farther south(Fig. 4).

LMC averages at 13% (335%) of the bulk sediment and 23% (551%)of the total carbonate. Only three samples located directly south of CapBlanc contain more LMC than aragonite. HMC constitutes an average of5% (021%) of the bulk sediment and 8% (041%) of the total carbonate.The highest HMC content is present in a sample from the central openshelf.


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s;ALC,alcyonarian

spicules;SPO,sponge

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0.0

0.0

0.4

934.

195

.70.

311

535

0.0

32.0

12.1

45.1

1.7

0.2

0.0

0.7

0.2

0.0

1.3

0.0

2.2

0.0

0.3

0.0

0.6

0.1

0.8

0.2

2.7

690.

287

.212

.711

540

0.0

29.8

35.0

19.2

2.2

0.0

0.0

0.0

0.0

0.0

1.0

0.0

0.0

0.0

0.0

1.0

0.1

8.8

0.6

0.5

1.5

530.

278

.521

.311

547

0.0

12.5

14.1

64.1

0.8

1.0

0.0

1.5

0.0

0.0

0.0

0.0

3.3

0.0

0.3

0.0

0.1

1.1

0.0

0.1

1.1

910.

498

.01.

611

549

0.0

2.0

25.6

67.6

1.7

0.0

0.1

0.0

0.0

0.0

0.0

0.0

1.2

0.0

0.0

0.0

0.3

0.1

0.0

0.7

0.7

810.

798

.50.

811

591

6.9

0.0

1.1

34.4

3.3

0.0

0.0

0.5

0.0

14.1

0.0

0.0

1.0

0.0

0.0

0.4

0.0

0.0

0.0

8.7

29.8

505.

994

.10.

011

593

0.0

2.1

37.5

19.5

6.2

0.0

0.0

0.1

0.1

1.1

3.6

0.0

9.2

0.0

2.9

0.0

5.2

0.2

0.2

0.0

12.1

390.

130

.969

.011

594

0.0

0.8

28.4

19.2

5.5

0.0

0.0

2.9

0.1

4.1

2.5

0.0

18.4

0.0

0.7

0.0

5.7

0.1

0.0

0.0

11.5

390.

128

.871

.111

595

0.0

21.7

33.9

19.1

9.9

0.6

0.4

3.6

0.0

0.1

0.3

0.9

2.9

0.0

0.0

0.0

2.8

0.4

3.1

0.4

0.0

505.

169

.425

.611

597

0.0

0.0

3.8

12.7

1.7

0.0

0.0

0.1

0.0

10.5

0.0

0.0

1.0

0.0

0.0

0.0

0.1

0.0

0.0

0.0

70.1

550.

998

.90.

111

601

0.0

0.0

0.0

83.2

1.4

0.0

0.0

0.3

0.0

11.0

0.0

0.0

0.9

0.0

0.0

0.4

0.0

0.0

0.0

0.1

2.6

7513

.486

.20.

411

602

0.0

0.0

0.2

78.2

1.8

0.0

0.2

0.5

0.4

10.9

0.0

0.0

0.9

0.0

0.0

0.0

0.1

0.0

0.0

0.1

6.7

8521

.378

.50.

211

603

0.0

0.3

6.7

22.3

1.7

0.0

0.1

0.5

0.0

12.4

0.1

0.0

1.6

0.0

0.0

0.0

1.2

0.0

0.0

0.3

52.9

623.

296

.00.

711

604

0.0

0.0

2.0

24.6

1.4

0.0

0.0

0.3

0.1

27.5

0.0

0.0

5.7

0.0

0.0

0.0

4.7

0.0

0.0

0.3

33.2

451.

055

.643

.311

606

0.0

0.0

10.3

11.5

1.2

0.0

0.0

1.4

0.0

8.1

0.0

0.0

5.7

0.0

0.0

0.0

0.1

0.0

0.0

0.3

61.5

510.

399

.30.

511

607

0.0

0.0

14.1

47.7

0.3

0.3

0.0

0.9

0.0

10.7

1.5

0.0

2.2

0.0

0.0

1.8

0.0

0.0

0.0

0.9

19.4

511.

988

.29.

911

613

0.0

20.9

12.2

50.4

2.3

0.4

0.0

3.4

0.0

0.1

0.4

0.0

2.8

0.0

0.0

0.0

0.1

3.7

0.0

0.1

3.1

850.

698

.01.

411

614

0.0

9.9

20.2

45.5

3.2

0.6

0.0

2.7

0.4

0.7

1.9

0.0

1.5

0.6

0.0

0.6

1.8

0.7

0.0

2.1

7.7

62.5

2.3

88.3

9.3


14C Ages

The mollusk shells considered to be reworked relict material showincreasing ages with increasing water depth (Tab 2). In less than 40 mwd,ages reach a maximum of 300 cal yr BP, whereas in 50100 mwd agesbetween 9,500 and 15,500 cal yr BP occur.

SEDIMENTARY FACIES

Observation of the seafloor sediments reveals five sedimentary facies(Tab 3). This facies definition is defined by statistical analysis based ongrain size, carbonate content, and grain association (Fig. 5). Whereas thedistribution of two facies (F1 and F3) is restricted to shallow water

FIG. 3.Distribution of sediment grain sizes(bulk) and components (grains . 125 mm) ofsurface samples in the Golfe dArguin. BIV,bivalves; RAL, red algae; PFOR, planktonicforaminifers; BFOR, benthic foraminifers; BAR,barnacles; ECH, echinoderms; Other, otherbioclasts; QUA, quartz grains.

TABLE 2.Radiocarbon measurements and age calibration using Marine04 curve and a 400-years reservoir age of highly abraded mollusk shells fromsurface samples (mwd 5 meters water depth; Hughen et al. 2004).

Lab No. Sample Sample depth (mwd) Material 14C ages [14C yr BP]1s calibrated

[cal yr BP] Intercept [cal yr BP]

Poz-31069 GeoB11501 16.5 Donax burnupi valve Modern Modern ModernPoz-31070 GeoB11501 16.5 Bivalve piece 525 6 30 118238 180 6 60Poz-31073 GeoB11513 35.5 Gastropod piece 600 6 30 149290 220 6 70Poz-26879 GeoB11613 103.3 Bivalve piece 13350 6 70 1512715426 15275 6 150Poz-26880 GeoB11614 72.3 Bivalve piece 8870 6 50 94669565 9515 6 50Poz-26881 GeoB13018 37.0 Bivalve piece 640 6 30 255310 285 6 30Poz-26882 GeoB13019 53.0 Bivalve piece 10250 6 50 1117911262 11220 6 40


FIG. 4.Distribution of aragonite contents inthe Golfe dArguin as percentages of totalcarbonate. Aragonite, LMC (, 4 mol.%MgCO3), and HMC (. 4 mol.% MgCO3)percentages of the 74 samples are displayed inthe ternary plot showing the overwhelmingdominance of aragonite.

TABLE 3.Sedimentary facies (F) of the Golfe dArguin (Mauritania) based on a cluster analysis including grain size (dry wt%), carbonate content(CaCO3; dry wt%), and sediment composition of the samples.

FACIES SAMPLETEXTURE (%) AND

SORTING SEDIMENT COMPOSITIONCaCO3

(%)

F1Donax burnupi

GeoB11515, GeoB11516, GeoB11528,GeoB11529, GeoB11532, GeoB11534,GeoB11601, GeoB11602

gravel: 423; sand: 7696;mud: 01

moderately to well sorted

Quartz and barnacle-rich, bivalve-dominatedVariably fragmented and abraded shellsClean carbonate sand and gravel

7193

F2Bivalve

GeoB11513, GeoB11522, GeoB11524,GeoB11525, GeoB11530, GeoB11531,GeoB11533, GeoB11547, GeoB11549,GeoB11613


poorly to moderately sorted

Foraminifer-rich, bivalve-dominated, with variableamount of echinoderms

Highly fragmented, abraded, and bioeroded shellsClean carbonate sand

7191

F3Dune-influenced

GeoB11514, GeoB11526, GeoB11527,GeoB11591, GeoB11597, GeoB11603,GeoB11606


moderately to well sorted

Barnacle-rich, quartz- and bivalve-dominated, withvariable amount of benthic foraminifers andechinoderms

Variably fragmented and abraded shellsClean mixed carbonate-siliciclastic sand

4462

F4Dust-influenced

GeoB11511B, GeoB11511C, GeoB11511F,GeoB11511G, GeoB11535, GeoB11607,GeoB11614


very poorly to well sorted

Quartz-rich, benthic foraminifer- and bivalve-dominated,with variable amount of planktonic foraminifers

Mixed abraded-fresh shellsMixed carbonate-siliciclastic muddy sand to sand

3769

F5Mud-rich

GeoB11511D, GeoB11511E, GeoB11540,GeoB11593, GeoB11594, GeoB11595,GeoB11604


poorly to moderately sorted

Bivalve- and foraminifer-dominated, with variableamount of echinoderms and quartz

Mixed abraded-fresh shellsMixed carbonate-siliciclastic sandy mud to muddy sand

3956


depths, three facies (F2, F4, and F5) include deposits from a range ofwater depths (i.e., mid and outer shelf; Fig. 6). The facies distributionthus is not predominantly bathymetrycally defined but rather represents afacies mosaic (i.e., patches on the shelf). An environmental interpretationof the facies is based on the detailed taxonomical and ecological analysisof mollusks and foraminifers.

Coarse-Grained Donax burnupi-Dominated Sand Facies (F1; Fig. 7A)

Carbonate content of these sediments ranges from 71 to 93% (Fig. 6,Tab 3). Bivalves dominate (especially Donax burnupi), along withbarnacle fragments. These constituents are reflected in the coarse grainsizes, reaching from medium-grained sand to gravel; sorting is moderateto good. The carbonate grain association corresponds to a bimolskeletal assemblage sensu Hayton et al. (1995). Quartz grains are variablyabundant (019% of the grains . 125 mm). This facies is present insediment in the shallow northern part of the Golfe dArguin (1723 mwd)and in two samples located farther south at around 35 mwd (Fig. 6).

Interpretation and Environmental Conditions.The environment of thisDonax burnupi-dominated facies closely follows the ecological require-ments of the genus Donax, which prospers in high-energy, subtidalenvironments (cf. Ansell 1983). In the Golfe dArguin, these conditionsare associated with the swell from the northwest. The high trophicresources (i.e., phytoplankton) related to upwelling, and tropical tosubtropical climate, further provide preferable conditions for this bivalve(cf. Ansell 1983). In addition to the dominant D. burnupi shells, themollusk assemblage includes the bivalve species Crassatina marchadi and

Gari jousseaumeana, both of which have a clear tropical affinity (cf. Cosel1995). The low number of living organisms is interpreted to indicate thatcarbonate production occurs updip of the sampling locations (i.e., on theouter part of the Banc dArguin), from which the shells are shed andreworked. Quartz grains are supplied by migrating sand dunes of onshoreCap Blanc.

Bivalve Fragment-Dominated Sand Facies (F2; Fig. 7B, C)

The sediments consist of poorly to moderately sorted carbonatemedium and coarse-grained sand (7191% carbonate content) (Fig. 6,Tab 3). The grains of this facies are highly fragmented, abraded, andbioeroded (Fig. 7B, C). Besides bivalve fragments, benthic foraminifersare common. Planktonic foraminifers are common in three samples(Tab 1), two on the outer shelf. Echinoderm fragments range from 1 to17% of the identified bioclasts. This facies is a bimol skeletalassemblage sensu Hayton et al. (1995). It is present on the mid shelfand on the outer shelf in the northern part of the Golfe dArguin (Fig. 6).

Interpretation and Environmental Conditions.This bivalve fragment-dominated sand facies is present in two different settings, on the mid shelfand on the outer shelf. The main difference between the mid-shelf(Fig. 7B) and outer-shelf (Fig. 7C) deposits relate to the grade ofabrasion of the grains, which is more intense on the outer shelf, and to thetaxa present. In the mid-shelf deposits, Donax burnupi fragmentsdominate. In the outer-shelf sediments, shallow-water-related Erviliacastanea fragments (cf. Morton 1990) dominate, whereas the better-preserved mollusk shells are of deep-water origin (e.g., Anodontia

FIG. 5.Dendrogram of hierarchical clusteranalysis based on carbonate content, grain size(mud, sand, and gravel; dry wt%), and compo-sition (excluding unidentified bioclasts) from theloose-sediment analysis that statistically groupssimilar samples. The determination of thethreshold, which defines five groups of samplesand thus the five facies (F15), is based onsubjective observation of bulk sediment.


senegalensis, Mesalia flammifera). The 14C date of highly abraded bivalvematerial at 103 mwd indicates an age of 15.5 cal kyr BP (Tab 2). Thus,these outer-shelf deposits are interpreted as palimpsest sediments, whererelict grains have remained exposed on the seafloor for thousands ofyears.

The mollusk assemblage of this bivalve-fragment-dominated sandfacies is a mixture of cosmopolitan and northwest African taxa. Theendemic malacofauna includes both bivalves and gastropods (e.g., thenorthwest African species Mesalia mesal and Turritella bicingulata; cf.

Ardovini and Cossignani 2004); some clearly point to a tropicalenvironment (e.g., Modiolus nicklesi, Venus erronea, Persicula cingulata,and Persicula cornea; cf. Nickle`s 1950).

The foraminiferal assemblage is composed of a mixture of smallbenthic and planktonic taxa. The benthic association is dominated bysediment dwellers such as Pararotalia sp., non-keeled Elphidium spp.,miliolids, and textulariids. In addition, species such as Cibicides refulgens,Elphidium macellum, Lobatula lobatula, and Miniacina miniacea areabundant, generally associated with phytal substrates (cf. Langer 1993).

21N

20N

1915

1745 161517W

100m

50m

20m

1000

m

CapTimiris

CapBlanc

F1 F3 F4 F5Coarse

sand

Quartzbivalvesand

Siliciclasticbiv-foram

sand

Siliciclasticbiv-forammuddy

Seagrass

Ooids

)

Components

70-95 45-60 35-70 40-55

>1000500-1000

250-500125-250

63-125

Rare taxa (i.e., Cassidulina laevigata, bolivinids, buliminids, anduvigerinids) are related to low-oxygen conditions (cf. Bernhard and SenGupta 1999). Most of the benthic taxa present are ubiquitous and thusare not indicative of particular environmental conditions; however, thegenus Pararotalia indicates warm climatic conditions (cf. Murray 2006).

The planktonic taxa are mostly deep-water species (sensu Be 1977) thatlive at depths greater than 100 m (Globorotalia inflata and Neogloboqua-drina pachyderma). However, the epipelagic (, 50 mwd, sensu Be 1977)Globigerinoides trilobus is also abundant. Regarding their climaticsignificance, the planktonic foraminiferal association consists of anassemblage of cold (e.g., Globigerina bulloides and N. pachyderma), butmostly transitional (e.g., G. inflata) and subtropical to tropical (i.e.,Globigerina calida, Globorotalia crassula, Globorotalia menardii, andPulleniatina obliquiloculata) taxa. This group is interpreted to reflect thecomplex oceanographic situation of the region and is in agreement withprevious observations of the living planktonic foraminiferal assemblage(Miro 1973; Thiede 1975a, 1975b; Mateu 1979).

Quartz-Rich Bivalve Sand Facies (F3; Fig. 7D, E)

Sediments are moderately to well sorted and are dominated by fine- tocoarse-grained sand-size grains. Quartz is abundant and dilutes thecarbonate content, which ranges from 44 to 62% (Fig. 6, Tab 3). Themost common bioclasts are bivalves, along with barnacles. Some wholebarnacle shells are still attached to their substrate, such as bivalve shells.Small benthic foraminifers are more abundant in the samples dominatedby fine- to medium-grained sand from the close vicinity of Cap Blanc(Fig. 3). Echinoderm fragments range from 0 to 22% of the identifiedbioclasts. This mixed carbonateterrigenous material is variably frag-mented and abraded and is dominated by Donax burnupi shells close toCap Blanc (Fig. 4B) and by several other bivalve species on top of theBanc dArguin (e.g., Veneridae; Fig. 4C). Red algae fragments andintraclasts are present on top of the Banc dArguin.

Interpretation and Environmental Conditions.The environment of thismixed bivalve-quartz facies is similar to that of the Donax burnupi facies(i.e., shallow subtidal and swell-exposed), but this facies is additionallyinfluenced by quartz sediment from Cap Blanc. Also, this quartz-richfacies is dominated by finer grain sizes (i.e., fine- to medium-grainedsand). The accumulation of quartz probably corresponds to a submergedextension of Cap Blanc, as evident from the seafloor morphology (Fig. 1).On top of the Banc dArguin in the southern part of the Golfe dArguin,quartz grains are also abundant but coarser (i.e., medium- to very coarse-grained sand). The very shallow water of the Banc dArguin (4 mwd) isthe only location in the Golfe dArguin where photic-related benthiccarbonate grains (i.e., red algae fragments) were found (see also Piessens1979).

Suspension-feeding organisms (e.g., barnacles and the bivalve speciesD. burnupi) overwhelmingly dominate the bioclast composition. Warm-water-related taxa include the tropical bivalve Pitar belcheri and thetropicalsubtropical gastropods Marginella senegalensis and Persiculablanda in the vicinity of Cap Blanc, and the bivalves Carditamera contiguaand Diplodonta dautzenbergii on the Banc dArguin. The shallow

subtidal-related bivalves Dosinia exoleta and Carditamera contigua arepresent. Benthic foraminifers include sediment dwellers (e.g., non-keeledElphidium spp. and miliolids) and species associated with phytalsubstrates (sensu Langer 1993) such as Cibicides refulgens and Elphidiumcrispum.

Fine-Grained SiliciclasticBivalveForaminifer Sand Facies (F4;Fig. 7F, G)

The dominant grain size of this facies is very fine- and fine-grained sandwith minor amounts of mud and extremely rare gravel (Fig. 6, Tab 3).Sediments are poorly to well sorted; the sample from the Baie du Levrieris poorly sorted. Carbonate content ranges from 37% to 69%. Thecarbonate grain association corresponds to foramol sensu stricto:bioclasts consist mostly of mollusks (bivalves) and foraminifers. Thecontent of quartz grains ranges from 2 to 14% of the fraction . 125 mm.This facies is found on the mid shelf in the southern part of the GolfedArguin, in the Baie du Levrier, and on the northern outer shelf (Fig. 6).

Interpretation and Environmental Conditions.As for the bivalve-fragment-dominated sand facies, the fine-grained siliciclasticbivalveforaminifer sand facies is found in various settings of the shelf. Thismosaic-type distribution of facies is related to two parameters. In situcarbonate production provides variable grain associations and grain-sizespectra in the Golfe dArguin. Reworking of these clastic carbonates andof silicilcastics leads to resorting of the sediment according to hydraulicproperties of the grains. For example, fine-grained sediment is depositednot only below 50 mwd, where wave action is reduced (Fig. 7G) but alsoin protected settings of the Baie du Levrier (Koopmann et al. 1979).

The mollusk assemblage consists of a mixture of endemic northwestAfrican taxa and cosmopolitan species. Some bivalve species (e.g.,Cardiocardita ajar and Tellina densestriata) and gastropod species (e.g.,Periscula cingulata and Prunum annulatum) show a clear tropical affinity(cf. Nickle`s 1950; Cosel 1995). Other taxa, especially bivalves such asCuna gambiensis and Donax burnupi are well adapted to high-nutrientenvironments such as upwelling areas (cf. LeLoeuff and Cosel 1998;Branch et al. 2002).

The foraminiferal assemblage consists mostly of benthic species thatcorrespond to sediment dwellers (e.g., Cancris auriculus, non-keeledElphidium spp., miliolids, and textulariids) and species associated withphytal substrates (e.g., Cibicides refulgens, Lobatula lobatula, Miniacinaminiacea, Planorbulina mediterranensis, and Rosalina macropora; cf.Langer 1993; Murray 2006). Some other benthic taxa are related tolow-oxygen conditions (e.g., Bolivina sp. and Textularia spp.; cf.Bernhard and Sen Gupta 1999) or deep-water environments (i.e.,Planulina ornata; cf. Colom 1974).

In contrast, planktonic species are much scarcer and are dominated byepipelagic and mesopelagic taxa (e.g., Globigerina spp. and Globiger-inoides spp.). As for the other facies, cold-water species (e.g., Globigerinabulloides) coexist with transitional (e.g., Globorotalia inflata) and tropicalto subtropical (i.e., Globorotalia crassula and Globorotalia menardii) taxa.This situation is interpreted to reflect upwelling of deep, cool watermasses, mixing at the surface with warm water masses.

r

FIG. 7.Representative bulk sediments of the study area: A) F1, coarse-grained Donax burnupi-dominated sand facies, carbonate sand and gravel, GeoB11515,21 mwd; B) F2, bivalve fragment-dominated sand facies, carbonate sand, GeoB11547, 30 mwd; C) F2, bivalve fragment-dominated sand facies, relict carbonate sand,GeoB11613, 103 mwd; D) F3, quartz-rich bivalve sand facies, mixed carbonatesiliciclastic sand, GeoB11603, 25 mwd; E) F3, quartz-rich bivalve sand facies, mixedcarbonatesiliciclastic coarse sand, GeoB11591, 4 mwd; F) F4, fine-grained siliciclasticbivalveforaminifer sand facies, mixed carbonatesiliciclastic muddy sand,GeoB11511C, 31 mwd; G) F4, fine-grained siliciclasticbivalveforaminifer sand facies, mixed relict carbonatesiliciclastic muddy sand, GeoB11614, 72 mwd; H) F5,siliciclasticbivalveforaminifer muddy facies, mixed carbonatesiliciclastic sandy mud, GeoB11595, 41 mwd. B, barnacle; Bi, bivalve; Br, bryozoan; D, Donax burnupi; E,echinoderm; bF, benthic foraminifer; pF, planktonic foraminifer; G, gastropod; Q, quartz; W, aggregated worm tube. Scale is 5 mm for each picture.


SiliciclasticBivalveForaminifer Muddy Facies (F5; Fig. 7H)

Carbonate content of this facies is lowest and ranges from 39 to 56%(Fig. 6, Tab 3). This facies is characterized by high mud content, verypoor to moderate sorting, and a bivalve- and foraminifer-dominatedbiota with the exception of one sample from the Baie du Levrier that isdominated by barnacle fragments. Subordinate components of the. 125 mm fraction include echinoderms, aggregated worm tubes, andquartz. The mud-rich facies is present in the southernmost part of thestudy area, on the mid to outer shelf, and in the Baie du Levrier (Fig. 6).

Interpretation and Environmental Conditions.The mollusk assemblageconsists of a mixture of cosmopolitan, endemic northwest African, andtropical taxa. Bivalves Cuna gambiensis and Tellina densestriata andgastropods of the family Marginellidae such as Marginella glabella reflecttropical water temperatures (cf. Nikle`s 1950; Cosel 1995). Mollusksreflect the muddy substrate (e.g., Nuculana bicuspidata and Tellinacompressa). The occurrence of the bivalves Anodontia sp., Myrteaspinifera, and Thyasira flexuosa is interpreted to be related to low-oxygenconditions, which suggest high organic-matter concentrations in the fine-grained sediments.

The foraminiferal assemblage includes small benthic and planktonictaxa. Among the small benthic species, several low-oxygen-tolerant taxaare present, many of them associated with muddy bottoms (e.g.,Cassidulina laevigata, bolivinids, buliminids, and uvigerinids). Othersoft-bottom dwellers found in this facies are miliolids, textulariids, non-keeled elphidiids, and nonionids. The presence of Cibicididae (e.g.,Cibicides refulgens and Lobatula lobatula) and encrusting forms (i.e.,Miniacina miniacea and Planorbulina mediterranensis) often reported asepiphytic forms (cf. Langer 1993) may be related to the occurrence ofphytal substrates (i.e., seagrass) that have been described from inner partsof the Banc dArguin (Wolff and Smit 1990; Hemminga and Nieuwen-huize 1991).

The planktonic foraminifers are dominated by meso-epipelagic species.As in other facies, the planktonic foraminiferal assemblage consists of amixture reflecting different climatic requirements. Thus, subarctic totransitional species (e.g., Globigerina bulloides, Globorotalia inflata, andNeogloboquadrina pachyderma) co-occur with species adapted to sub-tropical to tropical conditions (e.g., Globigerina calida, Globorotaliacrassula, Globorotalia menardii, Globorotalia tumida, and Pulleniatinaobliquiloculata).

MINERALOGY OF HETEROZOAN CARBONATES

The anticipation of calcite being the dominant calcium carbonatemineral in heterozoan carbonates (whereas aragonite dominates photo-zoan associations, cf. Nelson 1988; James 1997; Flugel 2004) is notconfirmed for the infaunal bivalve-dominated grain association studiedhere, which is clearly aragonite-dominated (Fig. 4). Other aragonite-richheterozoan associations are known from modern cool-water depositsfrom New Zealand (Nelson et al. 1982; Gillespie and Nelson 1997) andthe South Australian Shelf (James et al. 2005). The dominance ofaragonite, together with the high organic content of the sediment, reducesthe preservation potential of the biotic association studied hereconsiderably (cf. Smith and Nelson 2003; Wright and Cherns 2008).

FACIES DISTRIBUTION AND DEPOSITIONAL SYSTEM

The sedimentary facies in the Golfe dArguin (Fig. 6) form a faciesmosaic rather than bathymetrically defined depositional belts. Eoliansediment input from the Sahara and Sahel spreads uniformly over theentire Golfe dArguin (cf. Stuut et al. 2005). The orientation of the BancdArguin relative to the swell from the northwest, coupled to theprevailing winds, leads to a northsouth-directed hydraulic regime

(Fig. 1) that results in winnowing, net sediment transport to the south,and deposition of the eolian silt in the southern part of the GolfedArguin. This sedimentation pattern is reflected by the overall northsouth trend from coarse-grained, carbonate-dominated sediments in thenorth to fine-grained, siliciclastic-dominated deposits in the south (Michelet al. 2009).

The carbonate deposits largely lack a photic-related zonation, becausemost of the benthic carbonate-secreting organisms are aphotic. Thebioclastic composition of the five facies therefore is similar throughout,dominated by shells and fragments of bivalves and foraminiferal tests. Asfor purely siliciclastic sediments and overshadowing the influence ofbiogenic production on grain sizes, the hydraulic regime largely controlsthe grain-size spectra and sorting of the sediment. Thus, the faciesarrangement reflects the interaction between: (1) carbonate production,controlled by the biology and ecology of the carbonate-secreting biota(e.g., the large portion of centimeter-size Donax burnupi shells present inthe northern shallow part of the Golfe dArguin; F1, F2, and F3), and (2)hydrodynamics and topography, which influence dispersion and distri-bution of the carbonate and noncarbonate sediments (e.g., greatercontents of mud and fine-grained-sand in the southern part of the GolfedArguin; F4 and F5; Fig. 6).

Reworking of sediment not only influences facies distribution, theresulting abrasion and fragmentation partly explains the large proportionof undeterminable bioclasts (up to 40%). On the outer shelf, most skeletalcoarse grains are relict material that may represent shallow-water(, 50 mwd), bivalve-rich deposits formed during the last transgression.The typically low sedimentation rate of open-shelf heterozoan carbonates(cf. Nelson 1988; James 1997) and the strong current regime (cf. Bein andFutterer 1977) might have been responsible for the exposure of thesesediments on the seafloor for thousands of years.

The depositional system of the Golfe dArguin became established onlyin the Holocene after submergence of the Banc dArguin around 7 kyr BP(Hanebuth and Lantzsch 2008). The sediment of this depositional systemtherefore composes a thin veneer. Seismic data from the shelf off the BancdArguin confirm that the Holocene sediment forms a layer of up to some10 m overlying an erosional surface at the top of the Pleistocenesuccession. The abraded relict grains that are dated between 15.5 cal kyrBP at 103 mwd to modern at 16.5 mwd (Tab 2) reflect the succeedingpostglacial sea-level rise.

IMPLICATIONS: HETEROZOAN WARM-WATER CARBONATES

The facies in the study area are dominated by bivalve fragments andforaminifers and thus are heterozoan carbonates sensu James (1997) andforamol carbonates sensu Lees and Buller (1972). Whereas this carbonatedepositional system displays facies also typical of nontropical systems(Tab 4), the oceanographic situation is clearly a warm-water eutrophicsetting. Seawater temperatures on the Banc dArguin (1829uC) aretropical, and those of the mid and outer shelf (1625uC) correspond to theboundary of tropical and warm-temperate conditions (cf. Flugel 2004;Tab 4). The carbonate mineralogy is aragonite-dominated, and ooids arefound in isolated coastal environments (e.g., Baie de Saint-Jean; Stein1980). The low-latitude conditions on the open shelf of the GolfedArguin are indicated throughout the entire shelf by tropical-related taxa(mollusks and foraminifers) that allow the biota to be distinguished fromtemperate counterparts.

Thus, the attributes of the Mauritanian shelf sedimentation do notsimply fit into common carbonate classifications, and demonstrate themultidimensional ecological control of carbonate sedimentation. TheGolfe dArguin thus might serve as modern analog for ancient carbonatesedimentary systems such as Paleozoic upwelling ramps (James 1997;Martindale and Boreen 1997), Cretaceous warm, high-nutrient mollusk-dominated systems (e.g., Carannante et al. 1995; Allmon 2007) or


Neogene carbonate ramps formed in nutrient-rich tropical waters (e.g.,Pomar et al. 2004; Brandano and Corda 2002). The depositionalenvironment of the Golfe dArguin is a tropical broad shelf influencedby upwelling. The nutrient-rich waters lead to increased planktonproductivity and subsequently to high turbidity, which is furtherincreased by dust input from the Sahara (Fig. 8). In addition, theseasonal variability following the southern boundary movements of thetrade winds (i.e., summer Guinea Current influence versus winter CanaryCurrent influence; Fig. 1) associated with the year-round upwellingoccurrence results in strong oceanographic instability and in annualvariations in sea-surface temperature of about 10uC in the GolfedArguin.

Variable and eutrophic conditions lead to a rather simple food web(opportunistic species and generalist predators) based on phytoplankton

as opposed to an oligotrophic food web based on benthic algae, detritus,and symbioses (cf. Wood 1993; Taylor 1997; Mutti and Hallock 2003;Pomar and Hallock 2008). Eutrophic conditions thus suppress photo-trophic and mixotrophic (phototrophic or heterotrophic) benthic organ-isms that are typical warm-water-related carbonate producers (i.e.,hermatypic corals, calcareous green algae, large foraminifers). Inconsequence, an interpretation of carbonate associations as tropical versusextratropical environments needs to include consideration of the trophicconditions. At a shelf scale, a heterozoan carbonate association over-whelmingly dominated by suspension and deposit feeders (e.g., infaunalbivalves, but also sponges) indicates an ecosystem of highly productiveenvironment. In this eutrophic, aphotic setting, latitude and/or climaticpaleointerpretation is based upon the recognition of taxa (e.g., tropical-related mollusks) with well-constrained paleogeographic distribution.

TABLE 4.Comparison of environmental and facies attributes characterizing tropical, subtropical, temperate-polar, and eutrophic tropicalsubtropical(Mauritanian-type) carbonates based on Nelson (1988), James (1997), and results of the present study.

Environmental and faciesparameters Tropical carbonates Subtropical carbonates Temperate-polar carbonates

Eutrophic tropicalsubtropicalcarbonates

Carbonate association photozoan photozoan/ heterozoan heterozoan heterozoanLatitude between 30 u N and 30 u S between 30 u N and 30 u S beyond 30 u N and 30 u S between 30 u N and 30 u SDepositional setting rimmed shelves or platforms open shelves or ramps open shelves or ramps open shelves or rampsTerrigenous supply low low low to high low to highMean water temperature . 22 uC 1822 uC , 18 uC 1822 uC (1629 uC)Trophic conditions oligotrophic oligotrophic to mesotrophic mesotrophic to eutrophic eutrophicPhotic conditions (shallow

water)euphotic oligophotic to euphotic oligophotic to aphotic aphotic to oligophotic

Reefs abundant rare rare or absent absentCarbonate content very high (. 90%) very high (. 90%) moderate to very high

(50100%)moderate to very high (50100%)

Ooids common rare or absent absent rareMajor skeletal carbonate

componentshermatypic corals, benthic

foraminifers, mollusks,calcareous green and redalgae

calcareous red algae, largebenthic foraminifers,bryozoans; few hermatypiccorals

bryozoans, bivalves, smallbenthic and planktonicforaminifers, barnacles,echinoderms, serpulids,brachiopods, sponges,calcareous red algae

bivalves, small benthic andplanktonic foraminifers,barnacles, echinoderms,serpulids; few calcareous redalgae

Terrigenous material rare rare rare to abundant rare to abundantShell preservation generally good poor to good poor to good poor to goodCarbonate mineralogy aragonite dominant low- and/or high-Mg calcite

dominantlow- and/or high-Mg calcite

dominantaragonite dominant

Diagenetic regime constructive (grainpreservation and chemicalprecipitation)

destructive (grain dissolutionand maceration,biodegradation)

destructive (grain dissolutionand maceration,biodegradation)

destructive (grain dissolution andmaceration, biodegradation)

FIG. 8.Distribution of the main skeletalgrains on the shelf of the Golfe dArguin andfactors controlling the neriticcarbonate production.


CONCLUSIONS

The modern sedimentary system presented here is unique in the modernworld, in that it is a eutrophic tropical carbonate depositional system.Nutrient concentration was previously shown to be an importantparameter of carbonate production (e.g., Hallock and Schlager 1986;Halfar et al. 2004); however, so far no tropical carbonate association hasbeen presented that is overwhelmingly dominated by bivalves. This casestudy presents a faunal assemblage characteristic of a eutrophicenvironment, namely suspension-feeder-dominated carbonate grainassociations. This demonstrates that the perception of temperature-related carbonate sedimentation models (tropical vs cool-water carbon-ates) is not sufficient. This interpretation has strong implications forpaleolatitude and paleoclimatic studies.

ACKNOWLEDGMENTS

Thanks are due to the crews and scientists of the R/V Poseidon-346 and R/V Poseidon-366 cruises and to the authorities of Mauritania for permission towork in their exclusive economic zone. Karl Gurs, Serge Gofas, and Rudovon Cosel are thanked for sharing their expertise on mollusk taxonomy. KarlGurs did not live to see this study completed. He is sorely missed. We aregrateful to Nereo Preto, John Reijmer, and Ralph Batzel for their support.This paper greatly benefited from the reviews by Werner Piller, Cam Nelson,an anonymous reviewer, AE Elias Samankassou, JSR editor Gene Rankey,and CE John E. Southard. The MARUM (DFG-Research Center/ExcellenceCluster The Ocean in the Earth System) is acknowledged for providinginfrastructure and support for this research. This study was supported by theGerman Science Foundation (We-2492/5). Two appendices are available fromJSR Data Archive, URL: http://sepm.org/pages.aspx?pageid5229.

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Received 16 April 2010; accepted 6 April 2011.


Michel, J. et al., (2011). Modern heterozoan carbonates from a eutrophic tropical shelf (Mauritania).pdf

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water carbonates

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tropical waters

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