Post-depositional changes in planktic foraminifera bulk density reflect deep equatorial Pacific carbonate ion concentrations since the Last Glacial Maximum

Abstract:

Reconstructing deep water carbonate ion concentration, [CO₃^2-], is critical to investigate carbon reorganization in the climate system on glacial-interglacial timescales, yet [CO₃^2-] changes over this timescale are still not well understood. Attempts to reconstruct deep ocean [CO₃^2-] largely focus on calcite dissolution in sediments and may be complicated by surface ocean conditions (e.g. surface ocean [CO₃^2-], particle size) and the fact that dissolution on the seafloor is not straightforward. For example, foraminifera shells dissolve from the inside-out and corrosive conditions in deep water may reprecipitate/alter calcite, which affects the interpretation of dissolution proxies. Evaluations of the interior structure and bulk density of single foraminifera shells using micro X-ray computed tomography (MicroCT) have recently been investigated as methods of quantifying Δ[CO₃^2-]. These methods are perhaps superior to other proxies because they do not rely on metrics set in the surface ocean (e.g. shell size/thickness) and allow for a carefully assessment of shell structure.

Here, we use bulk density (obtained from MicroCT scans) of three species of foraminifera, Trilobatus sacculifer, Pulleniatina obliquiloculata, and Neogloboquadrina dutertrei, to establish a modern core-top calibration for Δ[CO₃^2-] from equatorial Pacific cores that span a range of Δ[CO₃^2-]. We also explore changes in preservation since the Last Glacial Maximum using individual foraminifera obtained from a central pacific core. We find that the inner calcite of all three species is better preserved during the deglacial period and less well preserved in the recent Holocene. In Holocene specimens, fragile inner calcite is thin or absent, and blocky exterior calcite crystals are precipitated that are structurally distinct from primary calcite. We use laser ablation techniques to examine the trace element composition of blocky calcite precipitates and find they have distinct geochemistry from well preserved shells, suggesting diagenetic alterations affected by changing deep water carbonate conditions since the LGM.