Introduction

1. 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, Italy 2 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 Introduction In general, the benthicforaminiferaldistributionisrelatedtonumerousphysical, chemical, and biologicalfactorssuchassedimentgrain-size, water temperature and salinity, oxygen and organicmattercontents. Particularly, Antarcticwaters are characterizedby the unusualfeaturesof a low annualprimaryproductivity in spiteof a largepopulationofconsumers, and high accumulationratesofsilica and organiccarbon (Dunbar & Leventer, 1989; Nelson, 1992). Moreover, the preservationofcalcareousforaminiferaltests in surface marine sedimentsisdirectlydependentfrom the CaCO3concentration in bottomwaters. 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). Victoria Land Victoria Land Victoria Land Victoria Land Ross Sea III II I Fig.3 Results and discussion Sedimentlithologiesrangebetweendiamictonto 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 Miliamminaarenaceaandthe more abundant species were Trochamminaquadricamerata and Lagenamminadifflugiformis. 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). 0.3 0.7 0.6 0.3 0.9 0.8 0.9 0.9 Biogenicsilica % I Fig.1 Materials and Methods Sedimentssampleswerecollectedusing a box corer, and then sub. All the coreswerescannedformagneticsusceptibility and radiographed, thenopened and decribedforvisualsedimentologicalcharacteristics. Onehalfwassampledevery 0.5 cm. Biogenicasilicacontentsweredeterminedby the progressive dissolutionmethods (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. BC01 BC22 BC33 BC38 II Depth in core (cm) III BC01 BC22 BC33 BC38 Modified by Domack et. al., 1999 Modified by Domack et. al., 1999 Fig. 2 Porosity (%) References DeMaster D.J. (1981): “ The supply and accumulation of silica in marine sediments”. GeochimicaCosmochimicaActa , 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.; DunbarR.B. & Leventer A.R. (1989): “Biogenicsedimentation 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): “Biogeochemicalcyclesoforganic and siliceusmatter in the SouthernOcean”. In: GallardoV.A., Ferretti O. & MoyanoH.I. (Eds.) – Oceanografia in Antartide, 44-57, 4 figs., Conception.; Violanti D., (1996): “Taxonomy and distributionofrecentbenthicforaminifersfrom Terra Nova Bay (Ross Sea, Antarctica), OceanographicCampaign 1987/1988”. Paleontographia Italica 1996; v. 83, PP. 25-71, tavv. 1-10. Fig.4 Fig.5 Fig.6 T. quadricamerata L. difflugiformis H. ovicula M. arenacea R. scorpiurus R. spiculifer P. antarctica A. glomeratum R. contortus S.E.M images of benthic foraminiferal fauna, from the four cores .