Evidence of significant destratification of the Subantarctic Pacific during the past 3.3-2.4 million years

The Earth's climate transitioned significantly from the mid-Piacenzian Warm Period (mPWP, 3.3–3.0 Ma) to the intensified Northern Hemisphere Glaciation (iNHG, 3.0–2.7 Ma). The Southern Ocean, a major CO₂ sink, played a pivotal role in this shift by regulating CO₂ through ocean ventilation. While most research focuses on the Northern Hemisphere, this study investigates planktonic foraminiferal assemblages, stable isotopes, trace metals, and sedimentary records from the Subantarctic Pacific to explore 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, G. puncticulata and G. inflata), and Globigerina bulloides (nutrient-enrichment indicator). During the mPWP, thermocline species dominated (90%). The faunal assemblage underwent a transition during the iNHG, with a 40% decline in Globoconella spp. and accompanied growth of N. pachyderma. By the Subantarctic-dominant interval, N. pachyderma increased (~90%), reflecting expanded cold-water conditions. G. bulloides rose by 30% at the end of the mPWP and fluctuated with glacial-interglacial (G/IG) cycles, peaking during interglacials (~25% higher). These shifts suggest destratification at the mPWP’s end and higher surface productivity during the Subantarctic-dominant interval, supported by increased planktonic foraminiferal accumulation rates.

Sedimentary analysis reveals a long-term decrease in CaCO₃ content (90% reduction) and a slight increase in total organic carbon (TOC) content, showing a 1% growth throughout the research interval. Additionally, ice-rafted debris (IRD) production exhibits a pronounced increase, reaching a maximum of 200 pieces/cm²·kyr during the Subantarctic-dominant interval, and demonstrates a long-term upward trend throughout the research interval. This rise in the IRD aligns with the increased abundance of the cold-water species N. pachyderma, suggesting an expansion of sea ice and ice sheets.

Stable isotope records reveal long-term environmental changes. δ¹³C values decreased during the mPWP and stabilized during the iNHG but became G/IG-dominant in the Subantarctic-dominant interval, with lower values in glacial periods likely due to increased Circumpolar Deep Water (CDW) input. δ¹⁸O records suggest a cooling trend (over 1‰) throughout the interval, showing G/IG variability in N. pachyderma and Globoconella spp. during the Subantarctic-dominant period. The δ¹⁸O_seawater derived from planktonic foraminifera generally exhibits a long-term increasing trend from -0.5‰ to 2‰, with glacial periods showing approximately 1‰ higher δ¹⁸O_seawater compared to interglacial periods.

Mg/Ca-derived temperatures show complex patterns. While N. pachyderma Mg/Ca ratios reveal strong G/IG fluctuations (~6°C) during the Subantarctic-dominant interval, G. puncticulata exhibits a long-term cooling (~6°C) since the iNHG. G. inflata mirrors these trends, and G. bulloides shows a 2°C decline. These data highlight N. pachyderma’s sensitivity to sea-ice expansion and the thermal stability preferences of G. bulloides.

Overall, stable ocean stratification and minimal sea ice characterized the mPWP, underpinned by a well-stratified thermocline. Since the iNHG, frontal system shifts, destratification, and increased CDW upwelling have enhanced nutrient availability and phytoplankton photosynthesis, boosting CO₂ sequestration. By 2.55 Ma, the Subantarctic Pacific emerged as a critical CO₂ sink, driven by Pleistocene G/IG cycles and contributing significantly to global CO₂ storage.