Deep‐sea oxygen depletion and ocean carbon sequestration during the last ice age

Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO2 concentrations by 80 to 100 ppm relative to interglacial levels. Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean's biological pump, in which case carbon storage in the deep sea would have been accompanied by a corresponding reduction in dissolved oxygen. We exploit the sensitivity of organic matter preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep central equatorial Pacific Ocean during the last glacial period (18,000-28,000 years BP) to have been within the range of 20-50 mu mol/kg, much less than the modern value of similar to 168 mu mol/kg. We further demonstrate that reduced oxygen levels characterized the water column below a depth of similar to 1,000 m. Converting the ice age oxygen level to an equivalent concentration of respiratory CO2, and extrapolating globally, we estimate that deep-sea CO2 storage during the last ice age exceeded modern values by as much as 850 Pg C, sufficient to balance the loss of carbon from the atmosphere (similar to 200 Pg C) and from the terrestrial biosphere (similar to 300-600 Pg C). In addition, recognizing the enhanced preservation of organic matter in ice age sediments of the deep Pacific Ocean helps reconcile previously unexplained inconsistencies among different geochemical and micropaleontological proxy records used to assess past changes in biological productivity of the ocean. Plain Language Summary Carbon dioxide (CO2) in Earth's atmosphere was lower during cold glacial periods of the last 800,000 years than during warm interglacial periods, by an amount equivalent to about one third of the preindustrial CO2 content of the atmosphere. It is thought that the ocean absorbed the CO2 missing from the atmosphere, but determining where and how the CO2 was stored in the ocean has remained a challenge. Photosynthesis in the surface ocean converts CO2 to organic matter, a portion of which sinks into the deep sea, where it is subsequently converted back to CO2 by respiration of the organisms that consume the organic matter. Stimulation of the biological uptake of CO2 in surface water, inhibition of the physical processes that raise deep waters back to the surface where they exchange gases with the atmosphere, or any combination of the two would enhance the storage of CO2 in the deep ocean while also reducing the concentration there of dissolved oxygen. Here we show that the oxygen concentration in deep waters of the Pacific Ocean was much lower during the last glacial period than today. The difference is sufficient to accommodate the CO2 removed from the atmosphere during the ice ages.