[B-CG05] Frontier in diversity and ecology of protists and microfossils

[BCG05-03] Mixed layer thickness drives Mg/Ca-inferred temperature variability of planktic foraminifer Trilobatus sacculifer in the northern South China Sea

*Pei-Ting Lee¹, Yi-Chun Kuo¹, Yu-Heng Tseng¹, Chuan-Chou Shen², Sze Ling Ho¹ (1.Institute of Oceanography, National Taiwan University, Taipei, Taiwan, 2.Department of Geosciences, National Taiwan University, Taipei, Taiwan)

Keywords:Sediment trap, Individual foraminifera analysis, Mg/Ca, Mixed layer thickness

Mg/Ca ratio of foraminiferal tests varies as a function of ambient seawater temperature. Foraminiferal Mg/Ca records are commonly utilized in paleoclimate reconstruction, e.g., to infer upper water column stratification using paired Mg/Ca records based on foraminifera dwelling in the mixed layer and thermocline assuming a fixed habitat depth through time. This assumption, however, contradicts modern observations that indicate habitat depth shifts in response to seasonal hydrographic changes. To improve our understanding of the temporal variability and depth origin of foraminiferal Mg/Ca signal, we analyzed the Mg/Ca ratio of individual tests (IFA-Mg/Ca) of the mixed layer-dwelling species Trilobatus sacculifer from sediment trap samples deployed at two water depths (2000 m and 3500 m) in the northern South China Sea (nSCS) during the period 2017–2019. In the absence of a Mg/Ca-temperature calibration based on single-specimen measurements, our single-specimen Mg/Ca ratios were converted to temperatures using a calibration developed for multi-specimen measurements. This choice was justified as our newly developed cleaning protocol yields comparable Mg/Ca data for both single-specimen and the conventional multi-specimen measurements. Our Mg/Ca-inferred temperature time series exhibit a good agreement with the satellite sea surface temperature (SST) during cold seasons but indicate temperatures approximately 3 ºC lower during warm seasons, resulting in dampened seasonal cycles. Winter (Dec-Feb) temperatures derived from IFA-Mg/Ca data also show interannual differences due to ENSO variability, with temperatures similar to satellite SST recorded during the La Niña phase when the mixed layer was deeper. Notably, the distribution ofT. sacculifer IFA-Mg/Ca temperatures is generally unimodal in winter but bimodal in summer, indicating that T. sacculifer alters its habitat depth range seasonally. The bimodal distribution in summer months is probably a result of mixed layer-dwelling T. sacculifer thriving also at the deep chlorophyll maximum, which tends to form when the mixed layer is shallower in summer. Consequently, the T. sacculifer IFA-Mg/Ca temperature variability (spread in sample distribution) demonstrates a strong negative correlation with the mixed layer thickness. However, some IFA-Mg/Ca temperature data fall outside of the aforementioned relationship, potentially due to lateral transport. Our findings demonstrate that the temperature variability recorded by individual T. sacculifer test is indicative of variations in its habitat depth in response to hydrographic changes induced by seasonal monsoons and ENSO events in the nSCS. Therefore, IFA-Mg/Ca temperature variability is a promising indicator for the reconstruction of past changes in the upper water column thermal structure.