Glassy vs. frosty forams and the low deglacial 14C/C anomaly

Abstract:

Measurements of foraminifera radiocarbon (14C/C) in the eastern tropical North Pacific (ETNP) suggest a dramatic lowering of seawater 14C/C during the last deglaciation (e.g., Marchitto et al. 2007; Lindsay et al. 2015; Rafter et al. 2018; 2019). One potential explanation for this anomaly is that the primary calcite of the foraminifera microfossils in the studied sediment cores is overlain by a radiocarbon-depleted authigenic calcite that applies a "frosty" appearance to the foraminifera microfossil. Prior work suggests that frosty foram 14C/C is lower than (presumably unaltered) "glassy" foram 14C/C (Wycech et al. 2016). Here we examine glassy and frosty foraminifera 14C/C from an ETNP core with previously established records of foraminifera abundance (to identify the influence of bioturbation) and anomalously low foraminifera 14C/C (measured in samples where glassy and frosty forams were not separated) (Rafter et al. 2018; 2019). Similar to the prior results (Wycech et al. 2018) we find that—on average—frosty foram 14C/C is lower (14C age is older) than glassy forams, but on the scale of hundreds of years—significantly lower than >8000 year offsets measured in the earlier work. Why the results from these two studies differ by an order of magnitude is unclear, but we will explore several potential explanations in this talk. Overall, our results suggest that the radiocarbon anomaly in deglacial ETNP foraminifera is not an artifact, but in fact documents a real change in seawater 14C/C.