Research reveals the role of biological and physical dynamics in CO2 release from the polar Southern Ocean during the last deglaciation.
In groundbreaking findings, scientists have reconstructed pivotal carbon dynamics in the polar Southern Ocean, highlighting its critical role in atmospheric CO2 fluctuations during the last deglaciation—from 18,000 to 11,700 years ago. The research, which utilized sediment core MD97-2106 from the Southwest Pacific, sheds light on how both biological and physical processes impacted air-sea CO2 exchanges across key climatic transitions.
The Southern Ocean is a vital region for regulating global carbon cycles, and its influence on historical atmospheric CO2 levels cannot be overstated. Notably, previous studies suggested that during the Late Pleistocene, the ocean contributed significantly to rising atmospheric CO2 levels as it released carbon long-sequestered in its depths through both biological respiration and physical exchanges.
Utilizing paired geochemical reconstructions, the study led by researchers Diana Dai and Jun Yu successfully bridged gaps in proxy data traditionally limiting investigations of the Southern Ocean's role. They focused on paired proxies such as phosphate and carbonate concentrations derived from benthic foraminifera, demonstrating that the Southern Ocean's biological carbon utilization was significantly reduced during the deglacial period.
One of the critical findings of the study revealed a decline in deep-water phosphate concentration ([PO43-]) of approximately 0.10 micromoles per kilogram and an increase in oxygen levels by about 75 micromoles per kilogram from the Last Glacial Maximum (LGM; 18–22 ka) to the late Holocene (LH; 4.2–0 ka). Consequently, the calculated PO4, alongside a substantial 16 micromoles per kilogram rise in deep-water carbonate concentration ([CO32-]), indicates that carbon sequestration dynamics evolved drastically within this period.
Moreover, the research found an increase in deep-water carbonate ion concentration ([CO32-]as) of approximately 39 micromoles per kilogram, suggesting that physiological and physical processes worked together in complex ways to influence atmospheric CO2 levels. The increase in phosphate and carbonate levels aligns with prior biological activity, indicating enhanced biological carbon uptake during the LGM.
Notably, during Heinrich Stadial 1 (HS1, 18.0–14.6 ka), researchers identified a rapid increase of about 0.4 micromoles per kilogram of PO4 towards the end of the period, alongside a significant 19 micromoles per kilogram rise in [CO32-]as. The findings suggest that nutrient dynamics during HS1 were influenced heavily by biological factors, providing extra context to understand the balance of carbon sequestration versus outgassing during key transitions.
Conversely, shifts were observed during the Antarctic Cold Reversal (ACR; 14.6–12.8 ka) when both PO4 and carbonate dynamics declined, indicating more efficient nutrient utilization within the Southern Ocean that suppressed atmospheric CO2 release. This contrasts with the Younger Dryas (YD; 12.8–11.7 ka) when heightened deep-water concentrations of PO4 and [CO32-]as pointed to increased returning nutrient supplies from deeper waters, which facilitated outgassing.
Throughout the study, the authors highlighted the need for improved understanding of how biogeophysical interactions in the Southern Ocean contribute to carbon cycling. They concluded, "Our reconstructions show that the partitioning of carbon between the deep Southern Ocean and the atmosphere via air-sea CO2 exchanges in the PAZ were mediated by biological and physical processes. This highlights the role of both these dynamics in influencing past climate patterns and current CO2 levels."
Hugely impactful, this study provides valuable insights into past ocean-atmosphere carbon interactions, shedding light on the various roles of biological and physical dynamics in controlling atmospheric CO2 and the implications of these interactions for future climate predictions.
