"Net result": using individual foraminifera from plankton tows to investigate the environmental variability recorded by different geochemical proxies

Individual foraminiferal analysis (IFA) has recently gained popularity in paleo-reconstructions as a tool to characterize environmental variability in the paleo-record. This technique targets select "time-slices" and leverages the range of geochemical proxies recorded by a suite of individual shells, which include reconstructions of paleo-monsoon intensity and the El Niño Southern Oscillation. Numerous studies have used live-culture, plankton tow, and sediment trap material to calibrate the relationship between oceanographic variables and pre-depositional shell chemistry. However, few studies have yet to explore the variability between individual foraminifera in a single contemporaneous population. In this study, we present paired geochemical measurements on individual Trilobatus sacculifer shells collected in depth-stratified plankton tows near Green Island, Taiwan (22.667ºN, 121.483ºE). We measure shell trace element ratios using LA-ICPMS depth profiling, which yields a Mg/Ca ratio for each shell, and then measure stable isotope ratios on the same individual shells. Using Mg/Ca ratios, we apply published proxy relationships to reconstruct calcification temperatures for individual T. sacculifer specimens, and use concurrent CTD data to estimate apparent calcification depth. We use measured environmental variables and water sample geochemistry to predict the range of values for stable isotope and trace element geochemistry, and we evaluate the variability in measured geochemistry among individual foraminifera. This study demonstrates the range of geochemical values that can be captured by a suite of living foraminifera shells during a single month, as well as the effect of oceanographic setting on variability within a geochemical proxy, providing a baseline for IFA applications. This study also highlights the need for proxy calibrations specific to single-shell geochemical analyses in select regional oceanographic settings.