CARBONATE DEPOSITION ON THE ROSS SEA SHELF, ANTARCTICA

Quaternary carbonates on polar shelves provide analogs that will improve our ability to recognize and interpret ancient examples in the rock record, yet they remain understudied. This investigation integrates petrography, geochemistry, and radiocarbon dating to evaluate the origin, characteristics, and distribution of carbonate-dominated sediments recovered in 18 piston cores from the Ross Sea Shelf, Antarctica. Carbonate-bearing sediments include bryozoan, stylasterine, and barnacle floatstones and rudstones, which occur along the Mawson, Pennell and Ross Banks. Petrographic analysis of the <250 µm size fraction indicates that major components in sample matrices are diatoms, foraminifera, sponge spicules, and siliciclastic debris. Diatoms are mainly fragmented Miocene open-water and sea-ice forms. Examination of the >250 µm and >500 µm size fractions shows that the carbonate benthos is dominated by either stylasterine hydrocorals, barnacles, or bryozoans. Molluscs and echinoids are present but not abundant. Most skeletal grains are extensively bored and fragmented, indicating slow accumulation rates and long residence times on the sea floor. Geochemical analyses of the fine fraction revealed very low concentrations of marine organic matter. Carbonate concentrations (wt% CaCO₃) ranged from 0.5 to 53.4%, with the higher values occurring on the western, downcurrent bank margins. Radiocarbon analyses of stylasterine and bivalve shell fragments from 0-138 cm depth in 8 cores are clustered into three groups: >22 ka (n=9), at 2.6 ka (n=1), and <0.5 ka (n=2). These results highlight an age disparity between the carbonate grains and noncalcareous matrix components. Results indicate that carbonate deposition in the Ross Sea is highly localized in both space and time. Deposition was focused near the shelf break and along the western margins of banks on the NW Ross Sea Shelf. The main phase of carbonate deposition occurred during ice advance, at the transition from MIS 3 to MIS 2. These relationships indicate that carbonate communities were active at times and in places where input from other sediment sources was low. Results provide insights that will improve understanding controls on carbonate deposition in polar settings and will aid in interpretation of cold-water limestones found in the rock record.