[M-IS14] 古気候・古海洋変動

[MIS14-23] Evidence of significant destratification of the Subantarctic Pacific during the past 3.3-2.4 million years

Li-Peng Wu¹、*Li Lo¹、Pei-Tzu Wu²、Chuan-Chou Shen¹、Horng-Sheng Mii³、Ludvig Löwemark¹ (1.Department of Geosciences, National Taiwan University、2.Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan、3.Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan)

キーワード：Southern Ocean, mid-Piacenzian Warm Period, Intensification of Northern Hemisphere Glaciation, Planktonic foraminifera, Mg/Ca ratio

The Earth's climate transitioned from the mid-Piacenzian Warm Period (mPWP, 3.3-3.0 Ma) to the intensified Northern Hemisphere Glaciation (iNHG, 3.0-2.7 Ma), with the Southern Ocean playing a key role in CO₂ regulation through ocean ventilation. While most studies focus on the Northern Hemisphere, this research examines planktonic foraminiferal assemblages, stable isotopes, trace metals, and sedimentary records from the Subantarctic Pacific to assess the Southern Hemisphere's contribution. Three intervals are identified: mPWP (3.3-3.0 Ma), iNHG (3.0-2.7 Ma), and the Subantarctic-dominant interval (<2.7 Ma). Key planktonic foraminiferal groups include Neogloboquadrina pachyderma (cold-water indicator), Globoconella spp. (thermocline indicators), and Globigerina bulloides (nutrient-enrichment indicator). During the mPWP, thermocline species dominated (~90%) but declined by 40% in the iNHG, while N. pachyderma increased. By the Subantarctic-dominant interval, N. pachyderma (~90%) reflected expanded cold-water conditions. G. bulloides increased by 30% at the mPWP's end and fluctuated with glacial-interglacial (G/IG) cycles, peaking during interglacials (~25% higher). These faunal shifts suggest ocean destratification at the end of the mPWP and increased surface productivity during the Subantarctic-dominant interval, supported by rising foraminiferal accumulation rates. Sedimentary records show a 90% decrease in CaCO₃ content and a 1% increase in total organic carbon (TOC) over time. Ice-rafted debris (IRD) rose significantly, reaching up to 200 pieces/cm²-kyr in the Subantarctic-dominant interval, aligning with the increased abundance of N. pachyderma, suggesting sea ice and ice sheet expansion. Stable isotope records indicate long-term environmental changes. Carbon isootpe values declined during the mPWP, stabilized in the iNHG, and became G/IG-dominant in the Subantarctic-dominant interval, with lower values in glacials linked to increased Circumpolar Deep Water (CDW) input. Oxygen isotope values show a cooling trend (>1 permil), with G/IG variability in N. pachyderma and Globoconella spp. from the Subantarctic-dominant interval. Seawater oxygen isotope increased from -0.5 to 2 permil, with glacials exhibiting ~1 permil higher values than interglacials. Mg/Ca-derived temperatures reveal complex patterns. N. pachyderma Mg/Ca ratios show strong G/IG fluctuations (~6^oC) in the Subantarctic-dominant interval, while G. puncticulata exhibits a long-term cooling (~6^oC) since the iNHG. G. inflata follows a similar trend, and G. bulloides shows a 2^oC temperature decline. These results highlight N. pachyderma's sensitivity to sea-ice expansion and G. bulloides' preference for stable thermal conditions. Overall, stable ocean stratification and minimal sea ice characterized the mPWP, underpinned by a well-defined thermocline. Since the iNHG, frontal shifts, destratification, and increased CDW upwelling have enhanced nutrient availability, boosting phytoplankton productivity and CO₂ sequestration. By 2.55 Ma, the Subantarctic Pacific emerged as a critical CO₂ sink, significantly contributing to global carbon storage during Pleistocene G/IG cycles.