Evolution of South Atlantic density and chemical stratification across the last deglaciation

Roberts, Jenny; Gottschalk, Julia; Skinner, Luke C; Peck, Victoria L; Kender, Sev; Elderfield, Henry; Waelbroeck, C.; Riveiros, Natalia Vázquez; Hodell, David A

doi:10.1073/pnas.1511252113

Cited by 68 publications

(106 citation statements)

References 58 publications

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“…The simulated response of the deep ocean circulation and stratification to atmospheric temperature change is consistent with differences between the present and LGM inferred from paleoproxy observations (3,6,12,13). A series of sensitivity experiments suggests that the dominant control over circulation and stratification changes is exerted by the atmospheric temperature around Antarctica.…”

Section: Discussionsupporting

confidence: 75%

“…Specifically, we recently showed that enhanced buoyancy loss around Antarctica is expected to lead to an increase in the abyssal stratification, an upward shift of NADW, and a clearer separation between NADW and southward-flowing AABW (11)-all in agreement with inferences made for differences in circulation and stratification between the present and LGM (3,6,12,13). This gives rise to the hypothesis that strong cooling around Antarctica led to an increased net freezing rate.…”

supporting

confidence: 83%

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Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Jansen

2016

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

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Earth's climate has undergone dramatic shifts between glacial and interglacial time periods, with high-latitude temperature changes on the order of 5-10 • C. These climatic shifts have been associated with major rearrangements in the deep ocean circulation and stratification, which have likely played an important role in the observed atmospheric carbon dioxide swings by affecting the partitioning of carbon between the atmosphere and the ocean. The mechanisms by which the deep ocean circulation changed, however, are still unclear and represent a major challenge to our understanding of glacial climates. This study shows that various inferred changes in the deep ocean circulation and stratification between glacial and interglacial climates can be interpreted as a direct consequence of atmospheric temperature differences. Colder atmospheric temperatures lead to increased sea ice cover and formation rate around Antarctica. The associated enhanced brine rejection leads to a strongly increased deep ocean stratification, consistent with high abyssal salinities inferred for the last glacial maximum. The increased stratification goes together with a weakening and shoaling of the interhemispheric overturning circulation, again consistent with proxy evidence for the last glacial. The shallower interhemispheric overturning circulation makes room for slowly moving water of Antarctic origin, which explains the observed middepth radiocarbon age maximum and may play an important role in ocean carbon storage.LGM | AMOC | stratification | cooling | sea-ice T he deep ocean today is ventilated mainly by two water masses. North Atlantic deep water (NADW) is formed in the North Atlantic before flowing southward at a depth of about 2-3 km and eventually rising back up to the surface in the Southern Ocean. Antarctic bottom water (AABW) is formed around Antarctica and spreads northward into the abyssal basins. The AABW that makes it into the Atlantic then slowly upwells into the lower NADW before returning southward and resurfacing again around Antarctica (1, 2). The glacial equivalent of NADW was likely confined to shallower depths, leaving more of the deep Atlantic filled with water masses originating primarily from around Antarctica (3-5). Moreover, the two water masses appear more distinct, with less mixing between them (6).Multiple studies have pointed toward the potential importance of sea ice and surface buoyancy fluxes around Antarctica in controlling changes in the deep ocean stratification and circulation between the present and Last Glacial Maximum (LGM) (7-11). Specifically, we recently showed that enhanced buoyancy loss around Antarctica is expected to lead to an increase in the abyssal stratification, an upward shift of NADW, and a clearer separation between NADW and southward-flowing AABW (11)-all in agreement with inferences made for differences in circulation and stratification between the present and LGM (3,6,12,13). This gives rise to the hypothesis that strong cooling around Antarctica led to an increased net freezing...

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Section: Discussionsupporting

confidence: 75%

supporting

confidence: 83%

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Jansen

2016

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

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“…Reconstructions of bottom water temperatures through oxygen isotope pore water analysis revealed a temperature decrease of around 2 • C at the Carnegie Ridge (Pacific) and the Ceara Rise (Atlantic) (Cutler et al, 2003) and, close to deep-water production sites, cooling of deep waters in the North Atlantic, South Pacific, and Southern Ocean by about 4-5, 2.5, and 1.5 • C (Adkins et al, 2002). Consistent with this, bottom water interglacial-glacial temperature changes have been inferred from Mg / Ca palaeo-thermometry (Dwyer et al, 1995, Skinner et al, 2003, Roberts et al, 2016. The modelled sea water temperatures may thus be somewhat higher than the observed ones, especially for the Southern Ocean.…”

Section: Resultssupporting

confidence: 64%

Ocean carbon cycling during the past 130 000 years – a pilot study on inverse palaeoclimate record modelling

2016

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Abstract. What role did changes in marine carbon cycle processes and calcareous organisms play in glacial-interglacial variation in atmospheric pCO 2 ? In order to answer this question, we explore results from an ocean biogeochemical general circulation model. We attempt to systematically reconcile model results with time-dependent sediment core data from the observations. For this purpose, we fit simulated sensitivities of oceanic tracer concentrations to changes in governing carbon cycle parameters to measured sediment core data. We assume that the time variation in the governing carbon cycle parameters follows the general pattern of the glacial-interglacial deuterium anomaly. Our analysis provides an independent estimate of a maximum mean sea surface temperature drawdown of about 5 • C and a maximum outgassing of the land biosphere by about 430 Pg C at the Last Glacial Maximum as compared to pre-industrial times. The overall fit of modelled palaeoclimate tracers to observations, however, remains quite weak, indicating the potential of more detailed modelling studies to fully exploit the information stored in the palaeoclimatic archive. This study confirms the hypothesis that a decline in ocean temperature and a more efficient biological carbon pump in combination with changes in ocean circulation are the key factors for explaining the glacial CO 2 drawdown. The analysis suggests that potential changes in the export rain ratio POC : CaCO 3 may not have a substantial imprint on the palaeoclimatic archive. The use of the last glacial as an inverted analogue to potential ocean acidification impacts thus may be quite limited.A strong decrease in CaCO 3 export production could potentially contribute to the glacial CO 2 decline in the atmosphere, but this remains hypothetical.

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“…Instead of abrupt and non-monotonic changes in the brine efficiency prescribed in Mariotti et al (2016), in the ONE_BRINE_130K experiment we assume that this efficiency is 0.75 at the LGM, 0 at present, and in between it follows the global ice volume. We do not claim that this scenario is more realistic, but at least it is more consistent with the findings of Roberts et al (2016). Figure 11 shows that the model with the brine rejection parameterisation and stratification-dependent vertical diffusivity simulates atmospheric 14 C in better agreement with empirical data then the standard version.…”

Section: Brine Rejection Mechanisms and Radiocarbon In The Ocean And mentioning

confidence: 52%

“…However, both sea level and the size of the Antarctic ice sheets remained essentially constant during this period and therefore there is no obvious reason for such large variations in the brine rejection efficiency. According to the interpretation of Roberts et al (2016), brine rejection remained efficient during most of the glacial termination and ceased only after 11 ka, when most of the glacial-interglacial CO 2 rise had already been accomplished. In the view of these uncertainties, we decided not to include parameterisations of the brine rejection mechanism in simulations of glacial cycles.…”

Section: Brine Rejection Mechanismmentioning

confidence: 99%

Simulation of climate, ice sheets and CO<sub>2</sub> evolution during the last four glacial cycles with an Earth system model of intermediate complexity

2017

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Abstract. In spite of significant progress in paleoclimate reconstructions and modelling of different aspects of the past glacial cycles, the mechanisms which transform regional and seasonal variations in solar insolation into long-term and global-scale glacial-interglacial cycles are still not fully understood -in particular, in relation to CO 2 variability. Here using the Earth system model of intermediate complexity CLIMBER-2 we performed simulations of the coevolution of climate, ice sheets, and carbon cycle over the last 400 000 years using the orbital forcing as the only external forcing. The model simulates temporal dynamics of CO 2 , global ice volume, and other climate system characteristics in good agreement with paleoclimate reconstructions. These results provide strong support for the idea that long and strongly asymmetric glacial cycles of the late Quaternary represent a direct but strongly nonlinear response of the Northern Hemisphere ice sheets to orbital forcing. This response is strongly amplified and globalised by the carbon cycle feedbacks. Using simulations performed with the model in different configurations, we also analyse the role of individual processes and sensitivity to the choice of model parameters. While many features of simulated glacial cycles are rather robust, some details of CO 2 evolution, especially during glacial terminations, are sensitive to the choice of model parameters. Specifically, we found two major regimes of CO 2 changes during terminations: in the first one, when the recovery of the Atlantic meridional overturning circulation (AMOC) occurs only at the end of the termination, a pronounced overshoot in CO 2 concentration occurs at the beginning of the interglacial and CO 2 remains almost constant during the interglacial or even declines towards the end, resembling Eemian CO 2 dynamics. However, if the recovery of the AMOC occurs in the middle of the glacial termination, CO 2 concentration continues to rise during the interglacial, similar to the Holocene. We also discuss the potential contribution of the brine rejection mechanism for the CO 2 and carbon isotopes in the atmosphere and the ocean during the past glacial termination.

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Evolution of South Atlantic density and chemical stratification across the last deglaciation

Abstract: International audienc

Cited by 68 publications

References 58 publications

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Ocean carbon cycling during the past 130 000 years – a pilot study on inverse palaeoclimate record modelling

Simulation of climate, ice sheets and CO<sub>2</sub> evolution during the last four glacial cycles with an Earth system model of intermediate complexity

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Evolution of South Atlantic density and chemical stratification across the last deglaciation

Abstract: International audienc

Cited by 68 publications

References 58 publications

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Ocean carbon cycling during the past 130 000 years – a pilot study on inverse palaeoclimate record modelling

Simulation of climate, ice sheets and CO&lt;sub&gt;2&lt;/sub&gt; evolution during the last four glacial cycles with an Earth system model of intermediate complexity

Contact Info

Product

Resources

About

Ocean carbon cycling during the past 130 000 years – a pilot study on inverse palaeoclimate record modelling

Simulation of climate, ice sheets and CO<sub>2</sub> evolution during the last four glacial cycles with an Earth system model of intermediate complexity