Paleoceanographic Evolution of the Antarctic Southern Ocean since the Mid-Pleistocene Transition

• Main Author: Hasenfratz, Adam P.
• Other Authors: Haug, Gerald, Jaccard, Samuel L., Martínez-García, Alfredo, Hodell, David A., Charles, Christopher D.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: ETH Zurich 2017
• Subjects: PALEOCEANOGRAPHY; BIOGEOCHEMISTRY; info:eu-repo/classification/ddc/550; Earth sciences; Antarctic; Southern Ocean; The Antarctic; Antarc*; Sea ice
• Online Access: https://hdl.handle.net/20.500.11850/181636; https://doi.org/10.3929/ethz-b-000181636

The Pleistocene epoch was characterized by orbitally-forced climate oscillations between warm stages and ice ages. The concentration of atmospheric CO2 (pCO2) has varied in step with these so-called glacial-interglacial cycles over at least the last 800 thousand years (kyr), with consistently 80–100 parts per million per volume (ppmv) lower pCO2 during ice ages. The Southern Ocean, a large water body that entirely encircles the Antarctic continent, exerts a dominant control on the partitioning of CO2 between the ocean interior and the atmosphere through its leverage on the efficiency of the biological pump. In the modern Southern Ocean, nutrient- and CO2-rich deep waters ascend to the surface ocean where iron limitation restricts the fixation of the major nutrients by phytoplankton, allowing for the evasion of deeply sequestered carbon to the atmosphere. In the Antarctic Zone of the Southern Ocean, south of the Antarctic Polar Front, the evasion of CO2 was reduced during ice ages by increased sea- ice cover and/or by a cooling-induced increase in stratification. In the northward Subantarctic Zone, a glacial increase in dust-derived iron was suggested to have stimulated marine export production, thereby contributing to enhanced deep ocean sequestration of carbon. While these two regions of the Southern Ocean provide a coherent two-part mechanism to explain the bulk of the glacial-interglacial pCO2 variations, the specific combination of processes modulating atmospheric pCO2 on longer time scales are not fully understood. The main focus of this thesis is on the mid-Pleistocene transition (MPT; ~1.2 to 0.7 million years ago (Ma)), when the climate cycles shifted from 41- to ~100-kyr periodicities in the absence of any substantial changes in the orbital parameters that control the amount of incoming solar radiation. Many of the proposed hypotheses are related to internal feedbacks within the climate system, and involve global cooling and an associated decline in glacial atmospheric CO2. Whereas evidence suggests ...