Decoupling temperature signal from biological noise in Mg/Ca variability of G. sacculifer as a first step towards developing a proxy for ocean seasonality

Amplitude of seasonal and inter-annual changes in seawater temperature and salinity is one of the major characteristics of the coupled ocean-atmosphere system. Reconstructing these crucial oceanographic parameters is a high priority but remains mostly unachieved. The primary obstruction in reconstructing seasonality is the presence of large and mostly unquantified biological effect on proxies for seawater temperature and salinity, such as Mg/Ca and δ18O compositions of foraminiferal shells. In this work we employed Secondary Iron Microprobe spectrometry (SIMs) to characterise variability in Mg/Ca compositions of planktonic foraminifera G. sacculifer from samples representing modern (sediment trap) and fossil (box-core) species assemblages. The sediment trap samples were collected offshore Somalia at bi-weekly intervals for one year, covering the whole seasonal cycle. To decouple seawater temperature signal from vital effect, two sediment trap samples, corresponding to the minimal and maximal sea water temperatures at the collection site, were analysed with SIMs. We also analysed G. sacculifer from core-top sample collected directly below the sediment trap site to compare between Mg/Ca variability in modern and fossil G. sacculifer populations. This information is used to investigate how accurate Mg/Ca variability is preserved in the sedimentary record and the processes that can potentially bias the signal. Based on the results of this work, we will discuss the potential for developing a quantitative proxy for past ocean seasonality as well as the methodology for generating paleoseasonality record.