North Atlantic Deep Water Production during the Last Glacial Maximum

Howe, Jacob N W; Piotrowski, Alexander M; Noble, Taryn L; Mulitza, Stefan; Chiessi, Cristiano Mazur; Bayon, Germain

doi:10.1038/ncomms11765

Cited by 134 publications

(181 citation statements)

References 60 publications

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“…This remains true in the glacial state for the Nordic and Labrador seas, but further south the contribution of NADW dwindles rapidly below 2 km depth. This simulated contraction of NADW co-occurs with the volumetric expansion of underlying Antarctic waters, and is consistent with the many paleoceanographic observations for shoaled NADW in the glacial North Atlantic (Curry and Oppo 2005;Adkins 2013;Lippold et al 2016), although it does not preclude the possibility that part of the apparent shoaling reflects a change in the remineralized carbon content of AABW (Gebbie 2014;Howe et al 2016). We also note the interesting possibility that, under glacial conditions, the Labrador Sea produced dense bottom waters that trickled downslope analogous to the modern Antarctic (Keigwin and Swift 2017), and which the model is incapable of simulating.…”

Section: Glacial Water Mass Characteristicssupporting

confidence: 86%

“…An Antarctic origin of these salty bottom waters would be consistent with reconstructions of the chemical composition of deep waters, primarily reflected by isotopic ratios in the shells of fossil benthic foraminifera. Both the stable carbon isotope ratio, δ 13 C (Boyle and Keigwin 1982;Duplessy et al 1984;Sarnthein et al 1994;Curry and Oppo 2005) and neodymium isotope ratio, εNd (Piotrowski et al 2005;Howe et al 2016) indicate that dramatic changes occurred in the Atlantic basin, with a volumetric expansion of the Antarctic chemical fingerprint under glacial conditions. Although the δ 13 C evidence is not unambiguous (Gebbie 2014), when viewed together with the spatial distribution of εNd changes, there is strong indication that the penetration depth of North Atlantic Deep Water (NADW) was reduced during the LGM (Lippold et al 2016).…”

mentioning

confidence: 99%

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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

Galbraith

Lavergne

2018

Clim Dyn

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Over the past few million years, the Earth descended from the relatively warm and stable climate of the Pliocene into the increasingly dramatic ice age cycles of the Pleistocene. The influences of orbital forcing and atmospheric CO 2 on landbased ice sheets have long been considered as the key drivers of the ice ages, but less attention has been paid to their direct influences on the circulation of the deep ocean. Here we provide a broad view on the influences of CO 2 , orbital forcing and ice sheet size according to a comprehensive Earth system model, by integrating the model to equilibrium under 40 different combinations of the three external forcings. We find that the volume contribution of Antarctic (AABW) vs. North Atlantic (NADW) waters to the deep ocean varies widely among the simulations, and can be predicted from the difference between the surface densities at AABW and NADW deep water formation sites. Minima of both the AABW-NADW density difference and the AABW volume occur near interglacial CO 2 (270-400 ppm). At low CO 2 , abundant formation and northward export of sea ice in the Southern Ocean contributes to very salty and dense Antarctic waters that dominate the global deep ocean. Furthermore, when the Earth is cold, low obliquity (i.e. a reduced tilt of Earth's rotational axis) enhances the Antarctic water volume by expanding sea ice further. At high CO 2 , AABW dominance is favoured due to relatively warm subpolar North Atlantic waters, with more dependence on precession. Meanwhile, a large Laurentide ice sheet steers atmospheric circulation as to strengthen the Atlantic Meridional Overturning Circulation, but cools the Southern Ocean remotely, enhancing Antarctic sea ice export and leading to very salty and expanded AABW. Together, these results suggest that a 'sweet spot' of low CO 2 , low obliquity and relatively small ice sheets would have poised the AMOC for interruption, promoting Dansgaard-Oeschger-type abrupt change. The deep ocean temperature and salinity simulated under the most representative 'glacial' state agree very well with reconstructions from the Last Glacial Maximum (LGM), which lends confidence in the ability of the model to estimate large-scale changes in water-mass geometry. The model also simulates a circulation-driven increase of preformed radiocarbon reservoir age, which could explain most of the reconstructed LGM-preindustrial ocean radiocarbon change. However, the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the LGM, and the model does not reproduce reconstructed LGM deep ocean oxygen depletions. These ventilation-related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes, but also raise the possibility that the LGM ocean circulation was not in equilibrium. Finally, the simulations display an increased sensitivity of both surface air temperature and AABW volume to orbital forcing under low CO 2 . We suggest that this enhanced orbital sensitivity contributed to the developm...

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Section: Glacial Water Mass Characteristicssupporting

confidence: 86%

mentioning

confidence: 99%

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

Galbraith

Lavergne

2018

Clim Dyn

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“…The new C isotope results support the same conclusion based on Nd isotopes (20). If this new source of bottom water resulted from brine rejection and not heat loss to the atmosphere, there may not have been the same climate impact as modern NADW production.…”

supporting

confidence: 76%

“…For example, Pa/Th results (2) and Nd results (17) from BR consistently show that the glacial mixture of bottom water must have contained a northern component. This finding is at variance with the prevailing view that there was little or no northern deep water present in the glacial North Atlantic (18,19), but a recent synthesis of Nd isotopes in the western Atlantic has also been interpreted as evidence for a glacial source of NADW (20). Furthermore, Böhm et al (21) found that some interstadials and the last interglacial on BR had lower eNd than in the modern ocean.…”

mentioning

confidence: 77%

Carbon isotope evidence for a northern source of deep water in the glacial western North Atlantic

Keigwin

Swift

2017

Proc. Natl. Acad. Sci. U.S.A.

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The prevailing view of western Atlantic hydrography during the Last Glacial Maximum (LGM) calls for transport and intermixing of deep southern and intermediate northern end members. However, δ13C and Δ14C results on foraminifera from a sediment core at 5.0 km in the northern subtropics show that there may have also been a northern source of relatively young, very dense, nutrient-depleted water during the LGM (18 ky to 21 ky ago). These results, when integrated with data from other western North Atlantic locations, indicate that the ocean was poorly ventilated at 4.2 km, with better ventilation above and below that depth. If this is a signal of water mass source and not nutrient storage, it would indicate that a previously unrecognized deep water end member originated along the western margin of the Labrador Sea, analogous to dense water formation today around Antarctica and in the Okhotsk Sea.

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“…Variations in the isotopic signature of Neodynium, for instance, indicate that AABW was more dominant in the deep ocean during the LGM and that its mixing with a glacial form of North Atlantic Deep Water (NADW) was more intense (Howe et al, 2016). These and other authors (Burckel et al, 2016) find further support for the presence of a shallower AMOC above 2500 m. Moreover, simulated distributions of carbon isotopes across a range of idealised circulations have shown that a shallower AMOC is necessary to optimise model-proxy agreement at the LGM (Menviel et al, 2016).…”

Section: Meridional Overturning Circulationmentioning

confidence: 99%

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle

et al. 2016

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Abstract. The ocean's ability to store large quantities of carbon, combined with the millennial longevity over which this reservoir is overturned, has implicated the ocean as a key driver of glacial-interglacial climates. However, the combination of processes that cause an accumulation of carbon within the ocean during glacial periods is still under debate. Here we present simulations of the Last Glacial Maximum (LGM) using the CSIRO Mk3L-COAL (Carbon-OceanAtmosphere-Land) earth system model to test the contribution of physical and biogeochemical processes to ocean carbon storage. For the LGM simulation, we find a significant global cooling of the surface ocean (3.2 • C) and the expansion of both minimum and maximum sea ice cover broadly consistent with proxy reconstructions. The glacial ocean stores an additional 267 Pg C in the deep ocean relative to the pre-industrial (PI) simulation due to stronger Antarctic Bottom Water formation. However, 889 Pg C is lost from the upper ocean via equilibration with a lower atmospheric CO 2 concentration and a global decrease in export production, causing a net loss of carbon relative to the PI ocean. The LGM deep ocean also experiences an oxygenation (> 100 mmol O 2 m −3 ) and deepening of the calcite saturation horizon (exceeds the ocean bottom) at odds with proxy reconstructions. With modifications to key biogeochemical processes, which include an increased export of organic matter due to a simulated release from iron limitation, a deepening of remineralisation and decreased inorganic carbon export driven by cooler temperatures, we find that the carbon content of the glacial ocean can be sufficiently increased (317 Pg C) to explain the reduction in atmospheric and terrestrial carbon at the LGM (194 ± 2 and 330 ± 400 Pg C, respectively). Assuming an LGM-PI difference of 95 ppm pCO 2 , we find that 55 ppm can be attributed to the biological pump, 28 ppm to circulation changes and the remaining 12 ppm to solubility. The biogeochemical modifications also improve model-proxy agreement in export production, carbonate chemistry and dissolved oxygen fields. Thus, we find strong evidence that variations in the oceanic biological pump exert a primary control on the climate.

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North Atlantic Deep Water Production during the Last Glacial Maximum

Cited by 134 publications

References 60 publications

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

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

Carbon isotope evidence for a northern source of deep water in the glacial western North Atlantic

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle

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