Indian Ocean glacial deoxygenation and respired carbon accumulation during mid-late Quaternary ice ages.

Reconstructions of ocean oxygenation are critical for understanding the role of respired carbon storage in regulating atmospheric CO2. Independent sediment redox proxies are essential to assess such reconstructions. Here, we present a long magnetofossil record from the eastern Indian Ocean in which we observe coeval magnetic hardening and enrichment of larger, more elongated, and less oxidized magnetofossils during glacials compared to interglacials over the last ~900 ka. Our multi-proxy records of redox-sensitive magnetofossils, trace element concentrations, and benthic foraminiferal Δδ13C consistently suggest a recurrence of lower O2 in the glacial Indian Ocean over the last 21 marine isotope stages, as has been reported for the Atlantic and Pacific across the last glaciation. Consistent multi-proxy documentation of this repeated oxygen decline strongly supports the hypothesis that increased Indian Ocean glacial carbon storage played a significant role in atmospheric CO2 cycling and climate change over recent glacial/interglacial timescales.

Planktonic foraminifera organic carbon isotopes as archives of upper ocean carbon cycling.

The carbon cycle is a key regulator of Earth's climate. On geological time-scales, our understanding of particulate organic matter (POM), an important upper ocean carbon pool that fuels ecosystems and an integrated part of the carbon cycle, is limited. Here we investigate the relationship of planktonic foraminifera-bound organic carbon isotopes (δ13Corg-pforam) with δ13Corg of POM (δ13Corg-POM). We compare δ13Corg-pforam of several planktonic foraminifera species from plankton nets and recent sediment cores with δ13Corg-POM on a N-S Atlantic Ocean transect. Our results indicate that δ13Corg-pforam of planktonic foraminifera are remarkably similar to δ13Corg-POM. Application of our method on a glacial sample furthermore provided a δ13Corg-pforam value similar to glacial δ13Corg-POM predictions. We thus show that δ13Corg-pforam is a promising proxy to reconstruct environmental conditions in the upper ocean, providing a route to isolate past variations in δ13Corg-POM and better understanding of the evolution of the carbon cycle over geological time-scales.

Global sea-surface iodide observations, 1967-2018.

The marine iodine cycle has significant impacts on air quality and atmospheric chemistry. Specifically, the reaction of iodide with ozone in the top few micrometres of the surface ocean is an important sink for tropospheric ozone (a pollutant gas) and the dominant source of reactive iodine to the atmosphere. Sea surface iodide parameterisations are now being implemented in air quality models, but these are currently a major source of uncertainty. Relatively little observational data is available to estimate the global surface iodide concentrations, and this data has not hitherto been openly available in a collated, digital form. Here we present all available sea surface (<20 m depth) iodide observations. The dataset includes values digitised from published manuscripts, published and unpublished data supplied directly by the originators, and data obtained from repositories. It contains 1342 data points, and spans latitudes from 70°S to 68°N, representing all major basins. The data may be used to model sea surface iodide concentrations or as a reference for future observations.

Publisher Correction: Glacial expansion of oxygen-depleted seawater in the eastern tropical Pacific.

In this Letter, 'δ18C' should have been 'δ13C' in Fig. 3b, and the x axis should extend to 50 kyr rather than 40 kyr. This figure has been corrected online.

Glacial expansion of oxygen-depleted seawater in the eastern tropical Pacific.

Increased storage of carbon in the oceans has been proposed as a mechanism to explain lower concentrations of atmospheric carbon dioxide during ice ages; however, unequivocal signatures of this storage have not been found1. In seawater, the dissolved gases oxygen and carbon dioxide are linked via the production and decay of organic material, with reconstructions of low oxygen concentrations in the past indicating an increase in biologically mediated carbon storage. Marine sediment proxy records have suggested that oxygen concentrations in the deep ocean were indeed lower during the last ice age, but that near-surface and intermediate waters of the Pacific Ocean-a large fraction of which are poorly oxygenated at present-were generally better oxygenated during the glacial1-3. This vertical opposition could suggest a minimal net basin-integrated change in carbon storage. Here we apply a dual-proxy approach, incorporating qualitative upper-water-column and quantitative bottom-water oxygen reconstructions4,5, to constrain changes in the vertical extent of low-oxygen waters in the eastern tropical Pacific since the last ice age. Our tandem proxy reconstructions provide evidence of a downward expansion of oxygen depletion in the eastern Pacific during the last glacial, with no indication of greater oxygenation in the upper reaches of the water column. We extrapolate our quantitative deep-water oxygen reconstructions to show that the respired carbon reservoir of the glacial Pacific was substantially increased, establishing it as an important component of the coupled mechanism that led to low levels of atmospheric carbon dioxide during the glacial.

Oxygen depletion recorded in upper waters of the glacial Southern Ocean.

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.