Benthic foraminifera as indicators of hydrologic and environmental conditions in the Ross Sea (Antarctica)

E. Bertoni 1,2*, L. Bertello 1,3, L. Capotondi 1, C. Bergami 1, F. Giglio 1, M. Ravaioli 1, C. Rossi 1,3, and A. Ferretti 2

1Institute of Marine Science (ISMAR), National Research Council, Bologna, Italy2 Università di Modena & Reggio Emilia, Dipartimento Scienze della Terra, Modena, Italy

3 Università di Bologna, Dipartimento Scienze dellaTerra & Geologico Ambientali, Bologna, Italy*eri.bertoni@gmail.com

IntroductionIn general, the benthic foraminiferal distribution is related to numerous physical, chemical, and biological factors such as sediment grain-size, water temperature and salinity, oxygen and organic matter contents.Particularly, Antarctic waters are characterized by the unusual features of a low annual primary productivity in spite of a large population of consumers, and high accumulation rates of silica and organic carbon (Dunbar & Leventer, 1989; Nelson, 1992). Moreover, the preservation of calcareous foraminiferal tests in surface marine sediments is directly dependent from the CaCO3 concentration in bottom waters. The Ross Sea is characterized by a shallow calcium carbonate solution boundary at a depth of 500-550 m (Kennett, 1966; 1968) mostly due to low temperatures (0° to -2°C) and high salinities (34.75 to 35.00 ‰) of the bottom waters.Knowledge of the distribution of modern population in these areas is an useful tool for interpreting fossil assemblages, thus for reconstructing paleoceanographic history of Antarctica (Violanti, 1996).This study, present data on benthic foraminiferal assemblages from four box cores collected in different areas of the Ross Sea (Fig. 1) during the 2005 oceanographic cruise in the framework of the Italian Antarctic Research National Programme (PNRA).

Box core

Latitude Longitude Water depth

(m)

Core lenght(cm)

BC01 73°58.95'S 175°02.59'E 578 40

BC22 76°41.59'S 169°04.68'E 790 37

BC33 75°20.44'S 165°16.52'E 687 43

BC38 75°27.45'S 179°52.52'E 559 36

Materials and MethodsSediments samples were collected using a box corer, and then sub. All the cores were scanned for magnetic susceptibility and radiographed, then opened and decribed for visual sedimentological characteristics.One half was sampled every 0.5 cm. Biogenica silica contents were determined by the progressive dissolution methods (DeMaster, 1981). Quantitative analyses on benthic foraminifera were performed on samples washed through a 63 µm sieve. The sieved samples were subsequently divided by a microsplitter and for each sample micropalaeontological analyses have been performed on the benthic foraminifera assemblage (>125µm). At least 300 specimens of benthic foraminifera were counted and identified.

Adercotrima glomeratum

Hormosinella ovicula

Labrospira wiesnerii

Lagenammina difflugiformis

Miliammina arenacea

Nodulina subdentaliformis

Paratrochammina pseudotricamerata

Paratrochammina sp

Paratrochammina lepida

Portatrochammina wiesnerii

Pseudobolivina antarctica

Pseudotrochammina bullata

Recurvoides contortus

Rhabdammina

Reophax Group

Specie minori

Trochammina multiloculata

Trochammina group

Results and discussionSediment lithologies range between diamicton to surficial diatomaceous mud, the intermediate levels being glacial-marine sediment. The sedimentary sections include diatomaceous glacial-marine deposit over transitional (proximal grounding zone) glacial-marine sediment (Fig.2).The study revealed that the Ross Sea contains typical Antarctic foraminifera fauna with the dominance of agglutinated taxa (Fig. 3). Relatively elevated abundances, richness and diversity were common in the northernmost site, where the water column was characterized by relatively warmer intermediate waters and by the presence of the colder High Salinity Shelf Water (HSSW) occupying the deepest part of the basin. Here, the assemblage was dominated by Miliammina arenacea and the more abundant species were Trochammina quadricamerata and Lagenammina difflugiformis. In the southernmost site, richness and diversity were low and the most significant species were Trochammina sp., and Reophax sp. Moreover, elevated abundances, richness and diversity were common in the upper portion of the cores which represents the youngest climatic phase characterized by the presence of some calcareous specimens too. This may indicate a deeper Carbonate Compensation Depth, probably due to relatively stable and warmer environmental conditions.M. arenacea was ubiquitous in all the samples and sites, confirming its tolerance to cold corrosive bottom waters and salinity fluctuations as well as its uniquely high preservation potential..Results document that diversity of benthic foraminifera, number of specimens and variations in test morphology are related to regional differences in water properties (temperature, salinity, carbonate chemistry).Changes in foraminiferals distributions and in biogenic silica content (Fig. 4), had lead to the identification of three different intervals, that documents the transition from Ice-shelf to open marine conditions, and that may be corrrelated with the facies succession proposed by Domack (Fig. 3, 5). Radiocarbon dating are in progress, however a cronological frame, based on Domack et. al. allowed to set the transition from Ice-shelf to open marine conditions (intervals III/II in this study) in the Joides basin, at approximately 11 kyr BP. (Fig. 6).

Modified by Domack et. al., 1999

Modified by Domack et. al., 1999

M. arenaceaH. oviculaL. difflugiformis R. scorpiurus R. spiculifer P. antarctica A. glomeratum T. quadricamerataR. contortus

ReferencesDeMaster D.J. (1981): “ The supply and accumulation of silica in marine sediments”. Geochimica Cosmochimica Acta , 45, pp. 1715-1732.; Domack E. W., Jacobson E.A., Shipp S., Anderson J.B., (1999): “Late Pleistocene-Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 2˗ Sedimentologic and stratigraphic signature”. GSA Bulletin; October 1999; v. 111; no. 10; pp. 1517-1536.; Dunbar R.B. & Leventer A.R. (1989): “Biogenic sedimentation in McMurdo Sound, Antarctica”. Mar. Geol., 85 (2/4), 155-179, 8 figs., Amsterdam.; Kennet J. P. (1966) : “Foraminiferal evidence of a shallow calcium carbonate solution boundary, Ross sea, Antarctica”. Science, 153 (3732), 191-193, 2 figs., Washington; Kennet J. P. (1968): “The fauna of the Ross Sea. Pt. 6. Ecology and distribution of Foraminifera”. New Zeland Department of Science and Industrial Research Bullettin, 186, pp. 1-4.; Nelson D. (1992): “Biogeochemical cycles of organic and siliceus matter in the Southern Ocean”. In: Gallardo V.A., Ferretti O. & Moyano H.I. (Eds.) – Oceanografia in Antartide, 44-57, 4 figs., Conception.; Violanti D., (1996): “Taxonomy and distribution of recent benthic foraminifers from Terra Nova Bay (Ross Sea, Antarctica), Oceanographic Campaign 1987/1988”. Paleontographia Italica 1996; v. 83, PP. 25-71, tavv. 1-10.

Victoria Land

Victoria Land

Victoria Land

III II I

Victoria Land

Ross Sea

Fig.1

0.7 0.9 0.3 0.9 0.3 0.8 0.6 0.9

BC01 BC22 BC33 BC38

Porosity (%) Fig. 2

Fig.3

Fig.4 Fig.5 Fig.6

S.E.M images of benthic foraminiferal fauna, from the four cores .