The influence of seasonal calcification depth change on the planktonic foraminiferal stable oxygen isotope signal

Seasonal changes in temperature and habitat migration both leave large imprints on the stable oxygen isotope composition (818O) of planktonic foraminiferal tests, but little is known about how much the depth habitat change affects the calcite 818O on seasonal timescales, potentially introducing numerous errors to the study of seasonality applied individual foraminifera. In this study, two time series sediment trap moorings deployed in the northern South China Sea (SCS) from July 2012 to April 2013 and southern SCS (upwelling area) from April to October 2014 were used to infer seasonal calcification depth variations for four commonly used planktonic foraminiferal species, Globigerinoides ruber, Trilobatus sacculifer, without a sac-like final chamber, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei by analyzing their test 818O values. The findings demonstrate that planktonic foraminifera deposit their tests shallower in the winter and during cold water upwelling activity, and deeper in the summer, with a changing seasonal depth habitat for all species that tends to follow the isothermals. More importantly, we devised a new approach for quantifying the effect of seasonal calcification depth changes on the 818O signature of planktonic foraminiferal tests. After applying this method to our trap site and nine other published global sediment trap sites, the results show that foraminiferal seasonal 818O signal is strongly distorted (amplified or damped) by seasonal variations in their habitat depth, particularly for the species living in low latitude, attributed to the strong vertical temperature gradient relative to the seasonal temperature change at a given water depth. Surface water species that live in the homogeneous mixed layer may be able to provide the most comprehensive seasonality information. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the seasonal calcification depth variations of four common species of planktonic foraminifera by analyzing their stable oxygen isotope composition. The findings show that the depth habitat change affects the calcite 818O on seasonal timescales, and the seasonal depth habitat follows the isothermals. The research also proposes a new approach for quantifying the effect of seasonal calcification depth changes on the 818O signature of planktonic foraminiferal tests.