[B-PT02] Biomineralization and Geochemistry of Proxies

[BPT02-07] Nonthermal controls and seasonal biases in paleo-temperature proxies

*Yoshimi Kubota¹, Jun'ichi Okuno², Keisuke Yano³, Tsubasa Kohyama⁴, Sze Ling Ho⁵ (1.National Museum of Nature and Science, 2.National Institute of Polar Research, 3.The Institute of Statistical Mathematics, 4.Department of Information Sciences, Ochanomizu University, 5.Institute of Oceanography, National Taiwan University)

Keywords:Paleo-temperature proxy, Alkenone, Mg/Ca

The magnesium/calcium ratio (Mg/Ca) of planktonic foraminiferal shells and the unsaturation index of alkenone, an organic compound produced by haptophyte algae, have been widely used as seawater paleo-temperature indicators in paleoceanographic research. As various planktonic foraminifera species inhabit a wide range of water columns in the ocean, it is possible to reconstruct paleo-temperature at a greater variety of water depths and seasons/months if the characteristics of each species can be effectively utilized. Although these possibilities[SLH1] are recognized among paleoceanographic researchers, progress has been slow in exploring and realizing these possibilities. A previous study pointed out that the use of the Mg/Ca proxy has declined sharply, with ~50% fewer publications using planktonic foraminiferal Mg/Ca since 2011, caused by reliability issues of the Mg/Ca palaeothermometry. Several studies demonstrated that Mg/Ca depends on salinity and pH apart from water temperature. However, the reason why the use of Mg/Ca decreased is not only due to reliability issues but also because researchers cannot obtain realistic paleo-temperatures as expected. Mg/Ca is often published with the oxygen isotope ratio (δ¹⁸O). Still, there are many cases where data from the two proxies are inconsistent, especially for subsurface to middle-depth dwelling species.
Another potential bias of paleo-temperature proxies is the seasonal production of proxy carriers, an issue typically known as seasonality. However, the paleo-temperature proxy's seasonality has not yet been fully understood. Therefore, in this study, we compare the published paleo-temperature data for the late Holocene and instrumental data and discuss the seasonality of Mg/Ca and alkenone-based paleo-temperatures at selected 48 sites between 56ºS and 57ºN. Furthermore, we will examine the influence of seasonality on paleowater temperature reconstruction on the glacial-interglacial scale. In this presentation, we focus more on Mg/Ca but try to gain comprehensive insight into potential biases in the paleo-temperature proxies.