Thermohaline Circulation and Deep Ocean Carbonate Chemistry across the Mid-Pleistocene Transition

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The "Mid-Pleistocene Transition" (MPT) was the biggest reorganization of the global climate system in the recent geological past. It refers to the interval, occurring roughly a million years ago, when the lengths of ice age cycles shifted from ~40,000 to 100,000 years and ice ages became more extreme. During the MPT, atmospheric carbon dioxide (CO2) shifted significantly towards lower values, which would enhance climate cooling. Since the missing CO2 would have been stored in the ocean, it has been inferred that the decrease is a response to changes in the deep-ocean's circulation (also known as "thermohaline circulation"). This shift in the mode of climate variability occurred without significant change in the system's energy inputs (i.e., astronomical forcing), thus implying that non-linear responses of the climate system and/or internal changes within the ocean-atmosphere-cryosphere were ultimately responsible. This research will help quantify the interaction between deep-ocean circulation, the carbon cycle, and global climate change during this critical climatic period. Because the pre-MPT reflects a climate state characterized by elevated glacial atmospheric CO2 and smaller ice sheets, improving understanding of this transition will help scientists and policy makers to better predict and prepare for predicted high atmospheric CO2 levels in the future.Specifically, this research explores geological archives of past deep-ocean circulation and carbonate chemistry to help elucidate the history of ocean circulation and CO2 over the MPT. The work applies novel geochemical proxies (Nd isotopes, B isotopes and B/Ca ratios) to (1) reconstruct and quantify thermohaline circulation and CO2 changes in the deep-ocean and (2) apply these indicators to establish the nature and timing of this major climatic transition. Using sediment cores from different water depths in the northern, tropical, and south Atlantic will allow the research team to track lateral and vertical changes in the thermohaline circulation before, during and after the MPT. The Nd isotope data will complement the published benthic carbon stable isotope data, which also reflect thermohaline circulation but are also affected by biology (productivity), air-sea equilibration and changes in the global carbon budget. The project will also investigate deep-ocean CO2 storage by estimating seawater-pH and carbonate saturation from boron isotopes and B/Ca ratios in benthic foraminifers. Paired analysis of these proxies provides the opportunity to address the hypothesis that deep-ocean circulation facilitated the coeval drawdown of atmospheric pCO2 during glacial periods, which in turn enhanced high-latitude ice sheet growth, and established the boundary conditions for the development of 100-kyr cycles. Funding supports graduate and undergraduate research, and public outreach via the Lamont-Doherty Earth Observatory Annual Open House.