TESTING THE MAGNESIUM/CALCIUM PALEOTEMPERATURE PROXY IN LATE QUATERNARY BRYOZOANS, ROSS SEA SHELF, ANTARCTICA

Calcareous fossils comprise a reservoir of information about the conditions that existed at the time of precipitation, but the study of biogenic carbonate at high latitudes, especially in polar latitudes, is incomplete. To evaluate the reliability of bryozoan calcite as a paleotemperature recorder, we analyzed well-preserved specimens from 15 piston cores collected on the outermost shelf of the Ross Sea, Antarctica. Elemental concentrations were determined using electron dispersive x ray analysis (EDX). This study determines paleotemperatures from the Mg/Ca in Late Quaternary bryozoans from the Antarctic applying the Cambridge Instruments CL5 EDX. We then compare δ¹⁸O paleotemperatures to the Mg/Ca paleotemperatures to evaluate the bryozoan paletemperature proxy. These sediments, which contain a low-diversity fossil assemblage dominated by bryozoans, stylasterine corals, and barnacles, formed episodically over the last 40 k.y. In this setting, carbonate production is most prolific during periods of ice advance, when siliciclastic sedimentation is arrested. When carbonate factories are inactive, fossil debris is subjected to infestation by bioeroders, dissolution, fragmentation, and physical reworking. The bryozoans examined here have 2.0 to 5.1 Mol% MgCO. Results indicate that the bryozoan Mg/Ca derived temperatures of -1 to -5°C have a smaller range than previous estimates of 1 to -8°C derived from δ¹⁸O values. Also, the range of temperature estimates from multiple specimens collected from the same sediment fraction exhibit temperature ranges of up to 4°C. This variance, still under investigation, may reflect either aspects of the paleotemperature proxies or time averaging of fossil debris within single samples. Results have major implications for the interpretation of Mg/Ca and oxygen isotope temperature estimates in polar settings like the Ross Sea Shelf.