Abstract
Abstract The ∼100 k.y. cyclicity of the late Pleistocene ice ages started during the mid-Pleistocene transition (MPT), as ice sheets became larger and persisted for longer. The climate system feedbacks responsible for introducing this nonlinear ice sheet response to orbital variations in insolation remain uncertain. Here we present benthic foraminiferal stable isotope (δ18O, δ13C) and trace metal records (Cd/Ca, B/Ca, U/Ca) from Deep Sea Drilling Project Site 607 in the North Atlantic. During the onset of the MPT, glacial-interglacial changes in δ13C values are associated with changes in nutrient content and carbonate saturation state, consistent with a change in water mass at our site from a nutrient-poor northern source during interglacial intervals to a nutrient-rich, corrosive southern source during glacial intervals. The respired carbon content of glacial Atlantic deep water increased across the MPT. Increased dominance of corrosive bottom waters during glacial intervals would have raised mean ocean alkalinity and lowered atmospheric pCO2. The amplitude of glacial-interglacial changes in δ13C increased across the MPT, but this was not mirrored by changes in nutrient content. We interpret this in terms of air-sea CO2 exchange effects, which changed the δ13C signature of dissolved inorganic carbon in the deep water mass source regions. Increased sea ice cover or ocean stratification during glacial times may have reduced CO2 outgassing in the Southern Ocean, providing an additional mechanism for reducing glacial atmospheric pCO2. Conversely, following the establishment of the ∼100 k.y. glacial cycles, δ13C of interglacial northern-sourced waters increased, perhaps reflecting reduced invasion of CO2 into the North Atlantic following the MPT.
Highlights
Prior to the mid-Pleistocene transition (MPT), the growth and decay of Northern Hemisphere ice sheets responded to orbitally induced changes in insolation primarily on obliquity (~41 k.y.) time scales
After the MPT, the waxing and waning of these ice sheets was characterized by a generally slow buildup of continental ice followed by rapid glacial termination, defining a quasi-periodicity of ~100 k.y. (Hays et al, 1976; Maslin and Ridgwell, 2005)
Benthic foraminiferal carbon isotopes reflect a combination of biochemical and physical processes, including air-sea gas exchange in source water regions (Raymo et al, 1997; Lynch-Stieglitz et al, 1995), and eNd only acts as a proxy for source-water region and water mass mixing (e.g., Thomas and Via, 2007)
Summary
Prior to the mid-Pleistocene transition (MPT), the growth and decay of Northern Hemisphere ice sheets responded to orbitally induced changes in insolation primarily on obliquity (~41 k.y.) time scales. B/Ca to be used to reconstruct changing nutrient content and carbonate saturation state of ancient water masses, with a particular focus on ocean carbon dynamics of the Last Glacial Maximum and late Pleistocene (e.g., Boyle, 1992; Rickaby et al, 2010; Yu and Elderfield, 2007; Allen et al, 2015). We apply these same principles to our MPT records (see the Data Repository for methods and calibration details). We argue that the glacial-interglacial variations in B/Ca reflect changes in the saturation state of the bottom water mass, with minimal overprinting by local changes in productivity, and we discuss the significance of these below
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