Oxygen depletion recorded in upper waters of the glacial Southern Ocean

Abstract

Oxygen depletion in the upper ocean is commonly associated with poor ventilation and storage of respired carbon, potentially linked to atmospheric CO2 levels. Iodine to calcium ratios (I/Ca) in recent planktonic foraminifera suggest that values less than ∼2.5 μmol mol−1 indicate the presence of O2-depleted water. Here we apply this proxy to estimate past dissolved oxygen concentrations in the near surface waters of the currently well-oxygenated Southern Ocean, which played a critical role in carbon sequestration during glacial times. A down-core planktonic I/Ca record from south of the Antarctic Polar Front (APF) suggests that minimum O2 concentrations in the upper ocean fell below 70 μmol kg−1 during the last two glacial periods, indicating persistent glacial O2 depletion at the heart of the carbon engine of the Earth's climate system. These new estimates of past ocean oxygenation variability may assist in resolving mechanisms responsible for the much-debated ice-age atmospheric CO2 decline.

The devil's in the disequilibrium: multi-component analysis of dissolved carbon and oxygen changes under a broad range of forcings in a general circulation model

“…We suggest that this reflects a high sensitivity of O 2 to sea ice when sea ice coverage reaches very high fractions. This generally unrecognized potential for sea ice coverage to cause large oxygen undersaturation may have contributed to very low O 2 in the Southern Ocean during glacial periods, as suggested by foraminiferal I/Ca measurements (Lu et al, 2015).…”

“…We do not place a large degree of confidence in these values, given the likely sensitivity to poorly resolved details of sea ice dynamics (e.g., ridging, leads) and dense water formation. Nonetheless, the potential for very large disequilibrium oxygen under cold states prompts the hypothesis that very extensive sea ice cover over most of the exposure pathway in the Southern Ocean might have made a significant contribution to the low O 2 concentrations reconstructed for the glacial (Jaccard et al, 2016;Lu et al, 2015).…”

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

“…In contrast, nitrogen isotopic evidence has been interpreted as showing that the LGM Southern Ocean surface was almost completely devoid of NO 3 in summer (François et al 1997;Martínez-García et al 2014;Wang et al 2017), and the LGM deep ocean appears to have had significantly lower O 2 (Bradtmiller et al 2010;Galbraith and Jaccard 2015;Hoogakker et al 2015;Korff et al 2016) as expected given greater storage of respired carbon by the biological soft tissue pump. Recent evidence 1 3 even suggests that O 2 was quite low at 3 km depth in the LGM Southern Ocean (Jaccard et al 2016), and that this depletion may have extended to relatively shallow depths (Lu et al 2016). It seems possible that the Southern Ocean nitrogen isotopic records could have been misinterpreted if, for example, there had been isotopic enrichment due to denitrification in the O 2 -depleted glacial deep ocean.…”

Expanded oxygen minimum zones during the late Paleocene‐early Eocene: Hints from multiproxy comparison and ocean modeling

“…This is probably due to the low I/C org in plankton [ Elderfield and Truesdale , ], and contrasts to the patterns of major nutrients in organic particles, such as nitrate and phosphate, and δ 13 C in dissolved inorganic carbon (DIC). If the same mechanisms controlled iodine chemistry in the Paleogene, similar epifaunal benthic I/Ca values should be expected for coeval sites without a local OMZ, assuming that foraminiferal I/Ca values largely reflect seawater iodate levels [ Glock et al , ; Zhou et al , ; Lu et al , ].…”