Abstract
Abstract. Glacial–interglacial changes in bottom water oxygen concentrations [O2] in the deep northeast Atlantic have been linked to decreased ventilation relating to changes in ocean circulation and the biological pump (Hoogakker et al., 2015). In this paper we discuss seawater [O2] changes in relation to millennial climate oscillations in the North Atlantic over the last glacial cycle, using bottom water [O2] reconstructions from 2 cores: (1) MD95-2042 from the deep northeast Atlantic (Hoogakker et al., 2015) and (2) ODP (Ocean Drilling Program) Site 1055 from the intermediate northwest Atlantic. The deep northeast Atlantic core MD95-2042 shows decreased bottom water [O2] during millennial-scale cool events, with lowest bottom water [O2] of 170, 144, and 166 ± 17 µmol kg−1 during Heinrich ice rafting events H6, H4, and H1. Importantly, at intermediate depth core ODP Site 1055, bottom water [O2] was lower during parts of Marine Isotope Stage 4 and millennial cool events, with the lowest values of 179 and 194 µmol kg−1 recorded during millennial cool event C21 and a cool event following Dansgaard–Oeschger event 19. Our reconstructions agree with previous model simulations suggesting that glacial cold events may be associated with lower seawater [O2] across the North Atlantic below ∼ 1 km (Schmittner et al., 2007), although in our reconstructions the changes are less dramatic. The decreases in bottom water [O2] during North Atlantic Heinrich events and earlier cold events at the two sites can be linked to water mass changes in relation to ocean circulation changes and possibly productivity changes. At the intermediate depth site a possible strong North Atlantic Intermediate Water cell would preclude water mass changes as a cause for decreased bottom water [O2]. Instead, we propose that the lower bottom [O2] there can be linked to productivity changes through increased export of organic material from the surface ocean and its subsequent remineralization in the water column and the sediment.
Highlights
Oxygen is vital to all aerobic life
Hoogakker et al (2015) calculate that the total error associated with bottom water [O2] reconstructions using this method at mid to low latitudes is 17 μmol kg−1
Whilst export of diatoms to the deep ocean is not that efficient, accumulation of diatom deposits in sediments during Heinrich events (Lippold et al, 2009; Griffiths et al, 2013) could provide evidence that more organic-rich material was exported to greater water depths during these episodes. Based on this evidence we propose that the lower bottom water [O2] values at intermediate Ocean Drilling Program (ODP) Site 1055 during extreme millennial-scale cool events were driven by increased export production
Summary
Oxygen is vital to all aerobic life. Oxygen solubility in seawater is highly temperature dependent, with salinity playing a secondary role. The [O2] of a (deep or intermediate) water mass at a particular location is determined by its initial concentration at the region of sinking, the amount of respiration it has undergone, and mixing with other water masses. Both oxygen supply and consumption are driven by ocean circulation and biology (Schmittner et al, 2007). Climate models predict that oxygen concentrations in the ocean will decrease substantially in response to anthropogenic climate change.
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