[M-IS14] 古気候・古海洋変動

[MIS14-24] Regional variability in the depth of the TEX₈₆ signal: Insights from Plio-Pleistocene records in the subtropical Pacific

★Invited Papers

*Syed Azharuddin¹、Sze Ling Ho¹、Jens Hefter²、Erin McClymont³、Jeroen Groenveld¹、Kenji Matsuzaki⁴ (1.Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan. 、2.Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany.、3.Department of Geography, Durham University, South Road, Durham DH1 3LE, UK.、4.Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.)

キーワード：Biomarkers, Plio-Pleiostocene, Sea Surface Temperature, Pacific Ocean, TEX86, UK'37

The Plio-Pleistocene Transition marks a critical shift from the warm Pliocene to the cooler Pleistocene, offering key insights into Earth's climate response to global ice volume and atmospheric CO₂ changes. However, discrepancies between widely used sea surface temperature (SST) proxies, such as U^K'₃₇ and Mg/Ca, highlight the need for alternative approaches. TEX₈₆, a proxy based on Thaumarchaeotal glycerol dialkyl glycerol tetraether (GDGT) lipids, offers a valuable alternative for warm climate reconstructions due to its broad temperature sensitivity and extensive spatial coverage. However, it remains uncertain whether TEX₈₆ reflects temperatures at the surface or in the sub-surface ocean due to limited data. To address this gap, we present TEX₈₆-derived temperature records spanning 3.4-2.4 Ma from three subtropical Pacific sites: ODP 1012 (Northeastern Pacific), ODP 1208 (Central North Pacific), and DSDP 593 (Southwestern Pacific), each characterized by distinct hydrographic settings. ODP 1012 is influenced by strong upwelling from the cold, nutrient-rich California Current, while DSDP 593 and ODP 1208 are affected by the warm East Australian Current and Kuroshio Extension, respectively, with ODP 1208 experiencing larger seasonal SST variability and stratification. We compare TEX₈₆ results with U^K'₃₇-derived SSTs, a well-established proxy at these sites. Additionally, we assume that both U^K'₃₇ and TEX₈₆ represent mean annual temperatures in these regions, as modern studies indicate no seasonal bias. All three sites exhibit predominantly warmer Pliocene temperatures compared to modern annual averages, followed by a cooling trend towards the Pleistocene. However, the relationship between U^K'₃₇ and TEX₈₆ varies regionally. At ODP 1012, U^K'₃₇ suggests a pronounced Pliocene-Pleistocene cooling, whereas TEX₈₆ show similar glacial-interglacial variations during both intervals. The U^K'₃₇-TEX₈₆ offset (ΔT) decreases significantly during the Pleistocene, exhibiting a distinct glacial-interglacial pattern. We observed larger ΔT during interglacial periods, while little to no offset during glacial periods. At DSDP 593, TEX₈₆ temperatures exceed the U^K'₃₇ values, with differing glacial-interglacial variations, suggesting both are influenced by different factors. In contrast, at ODP 1208, the proxies align closely within calibration uncertainty. GDGT [2/3] values (7-12 at ODP 1012; 6-10 at DSDP 593 and 5-6.5 at ODP 1208) indicate that TEX₈₆ reflects a mix of surface and subsurface temperatures at ODP 1012 and DSDP 593, whereas, at ODP 1208, it predominantly records surface temperatures. These findings suggest that at ODP 1012, TEX₈₆ reflects colder subsurface temperatures (0-200 m), while changes in ΔT indicate a well-mixed water column during the Pleistocene, supported by increased alkenone concentrations indicative of upwelling-driven productivity. At DSDP 593, U^K'₃₇ likely captures colder surface waters influenced by westerly winds, whereas TEX₈₆ reflects warmer subsurface conditions. At ODP 1208, TEX₈₆ effectively records SST with minimal subsurface bias. Overall, our results reveal regionally distinct upper-ocean temperature trends during the Plio-Pleistocene, shaped by changes in upwelling and ocean circulation. TEX₈₆ proves to be a reliable SST proxy in regions with stable ocean stratification, making it particularly valuable for reconstructing warm climates such as the Pliocene, where U^K'₃₇ and Mg/Ca proxies face limitations. This study underscores the importance of a multiproxy approach in paleotemperature reconstructions and refines the interpretation of TEX₈₆ across different oceanographic settings.