PP41D-1431:
Effects of Seafloor Diagenesis on Planktic Foraminiferal Radiocarbon Ages

Thursday, 18 December 2014
Jody Wycech and Daniel Clay Kelly, University of Wisconsin Madison, Madison, WI, United States
Abstract:
Radiocarbon (14C) analysis of planktic foraminiferal calcite is widely used to study ocean-climate change over the past ~40 ka of Earth history. However, it is well known that planktic shell calcite typically yields 14C ages ~400 years older than those of bulk carbonate from the same sample. Such age discrepancies are problematic, and have been attributed to size-selective sediment mixing and/or differential dissolution of planktic shells within the sedimentary bioturbated zone. Another likely cause of such temporal offsets is the addition of secondary calcite to planktic shells via post-depositional diagenesis, but quantifying the deleterious effects of diagenesis on foraminiferal 14C ages has proven difficult owing to a paucity of suitable study materials. We address this problem by comparing 14C ages and δ13C values of planktic shells exhibiting a state of preservation (frosty) traditionally deemed acceptable for paleoceanographic studies to those of extremely well preserved (glassy) shells. Aliquots of frosty and glassy shells (>150 mm) of mixed-layer species (Globigerinoides ruber, Gs. sacculifer, Orbulina universa) were picked separately from a stratigraphic series of clay-rich samples recovered in a piston core taken atop Blake Ridge (northwestern Atlantic Ocean). Sample selection was guided by a foraminiferal δ18O record, which constrained the Last Glacial Maximum to ~100 cm core depth. Results support a diagenetic mechanism as glassy shells yield 14C ages that average ~2,000 ± 100 years younger than frosty shells from the same samples. Further, average δ13C of glassy shells is 0.6 ± 0.1‰ lower than that of frosty shells. Our findings indicate that 14C ages are artificially elevated by the dissolution of previously deposited (“old”) carbonate and its subsequent reprecipitation as secondary carbonate on younger foraminiferal shells at, and beneath, the seafloor – a phenomenon that has not been quantified prior to this study.