Effects of Seafloor Diagenesis on Planktic Foraminiferal Radiocarbon Ages

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.