Shelf, coastal and subglacial polar carbonates, East Antarctica

Modern and Pleistocene polar carbonates occur in East Antarctica in shelf, coast, lakes and marginal to underneath glaciers, associated mainly with glacigene muds, boulder tills and diamictites. Shelf carbonates (in Prydz Bay) are calcitic and unlithified, and consist mainly of sponges, bryozoans, echinoderms, bivalves and diatoms. Coastal carbonates (in the Vestfold Hills) are calcitic and contain faunal assemblages similar to those on the shelf, with calcareous algae, microbial mats, minor peloids and cements. Lake carbonates are aragonitic micrites and peloids. Carbonates close to glaciers (the Loken Moraines) are aragonitic and contain abundant ooids with intragranular fibrous cements. Subglacial carbonates are aragonitic micrites and peloids. Carbonate mineralogy changes from mainly low-Mg calcite in marine shelf to aragonite in brackish to freshwater dominated inland regions. Antarctic carbonate δ¹⁸O values (4.5 to -47‰ PDB) vary markedly due to frigid temperatures (0 to -2°C) and salinity (0 to 35‰) changes, as a result of meltwater dilution from adjacent glaciers. Their δ¹³C values (-9 to 8‰ PDB) also vary markedly due to exposure to atmospheric CO₂, the circulation of water masses and reaction of carbonate with CO₂ trapped in glacial ice. The regional distribution of carbonate sediments and their sedimentology, mineralogy, and δ¹⁸O and δ¹³C compositions indicate three types of glacial environments of formation. The first corresponds to a glacial stage and the formation of subglacial and bank carbonates, when the Antarctic ice sheet expanded onto the inner shelves. The second corresponds to interglacial stages and the formation of ice-marginal carbonates, during the retreat of the ice sheet from the inner shelf grounding line and accompanying the discharge of appreciable meltwater. The third corresponds to an interglacial oasis and the formation of coastal carbonates, proximal to distal lacustrine carbonates, and distal subglacial carbonates.