GEOCHEMISTRY OF CULTURED AND PLANKTON TOW CAPTURED PLANKTIC FORAMINIFERA SUGGEST NON-SPINOSE SPECIES LIVE IN A PARTICULATE MICROHABITAT

Exploiting the trace element (TE) composition of non-spinose planktic foraminifera for paleoceanographic reconstructions requires an understanding of mechanisms responsible for the incorporation of TEs into the shell calcite. Culture experiments and plankton tow samples demonstrate that Ba/Ca in the deeper dwelling foraminifera Neogloboquadrina dutertrei reflect seawater Ba, but that ocean grown N. dutertrei Ba/Ca is higher than can be explained by seawater Ba alone (Fehrenbacher et al., 2018). Here, we present P. obliquiloculata culture data that suggests this species also incorporates Ba as a function of seawater Ba, yet Ba/Ca in plankton tow specimens are also too high to be explained by seawater Ba. Intrashell TE data (including Ba, Mn, and Zn) from other non-spinose species (G. menardii, G. glutinata, N. pachyderma, N. incompta, G. scitula) are also elevated and variable. We hypothesis the non-spinose species dwell in a microhabitat, such as particulate organic matter (POM), in the natural setting and this is responsible for the elevated and variable TE composition of these species.

We conducted culture experiments to test if non-spinose species (N. dutertrei, N. pachyderma, and G. glutinata) readily attach to and envelope their shell with POM. Specimens were collected from plankton nets and recovered for 48 hours in Falcon^© flasks. On day 3, a small particle of POM collected from a plankton net was pipetted onto the rhizopodia of 8 specimens. We used time-lapse photography and videos to record their behavior. In brief, the foraminifers rapidly (<10 mins) cover their shell with the POM or form a ‘ball’ of POM near the aperture. The following day, the specimens were observed floating. We rarely observe non-spinose species floating in culture and hypothesize the particulate helps them maintain buoyancy. The specimens had remnant particulate material either surrounding the shell in a thin layer or in a small ‘bulge’ near the aperture akin to a feeding cyst.

We hypothesize this behavior, attaching to particulates, is a strategy non-spinose foraminifers employ in the open ocean during early ontogeny. An organic aggregate microhabitat can explain the high specimen-to-specimen trace element variability. Elevated and variable intrashell Ba, Mn, and Zn in non-spinose species likely reflects the unique and variable chemistry of the particulate microhabitat (e.g. pH, oxygen, and interstitial seawater composition).