Deep water provenance and dynamics of the (de)glacial Atlantic meridional overturning circulation

Lippold, Jörg; Gutjahr, Marcus; Blaser, Patrick; Christner, Emanuel; Ferreira, Maria Luiza de Carvalho; Mulitza, Stefan; Christl, Marcus; Wombacher, Frank; Böhm, Evelyn; Antz, Benny; Cartapanis, Olivier; Vogel, Hendrik; Jaccard, Samuel L.

doi:10.1016/j.epsl.2016.04.013

Cited by 95 publications

(111 citation statements)

References 60 publications

(69 reference statements)

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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: 69%

“…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). This coherent dual-tracer signal of AABW expansion and contraction over glacial cycles extends back to the middle Pleistocene, and appears to have developed alongside the intensification of ice ages about 1 million years ago (Pena and Goldstein 2014).…”

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: 69%

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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“…Furthermore, biogenic opal has been shown to act as a strong scavenger of Pa [ Chase et al ., ] potentially influencing 231 Pa/ 230 Th on local scales. However, given opal contents below 4% in core GeoB16202‐2 (Figure c), a dominant effect of opal production on 231 Pa/ 230 Th seems unlikely, consistent with basin‐wide compilations [ Bradtmiller et al ., ; Lippold et al ., ; Lippold et al ., ].…”

Section: Resultsmentioning

confidence: 99%

Synchronous and proportional deglacial changes in Atlantic meridional overturning and northeast Brazilian precipitation

et al. 2017

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Changes in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high‐resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near‐linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.

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“…Given AAIW's role as a supplier of NADW that is exported at depth (Rintoul, ; Schmitz & McCartney, ), the finding of weaker AAIW influence in the northern tropics appears consistent with an overall weaker AMOC (Gu et al, ), regardless of whether (e.g., Gherardi et al, ; Lippold et al, ; McManus et al, ) or not (Lynch‐Stieglitz et al, , ) shallow overturning was more vigorous than today's. There are, however, other possibilities.…”

Section: Implications For the Amoc And Atmospheric Co2mentioning

confidence: 94%

Data Constraints on Glacial Atlantic Water Mass Geometry and Properties

Oppo

Gebbie

Huang

et al. 2018

Paleoceanog and Paleoclimatol

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The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.

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Deep water provenance and dynamics of the (de)glacial Atlantic meridional overturning circulation

Abstract: International audienc

Cited by 95 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

Synchronous and proportional deglacial changes in Atlantic meridional overturning and northeast Brazilian precipitation

Data Constraints on Glacial Atlantic Water Mass Geometry and Properties

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