Similar rates of modern and last-glacial ocean thermohaline circulation inferred from radiochemical data

Yu, Ein-Fen; François, Roger; Bacon, Michael P.

doi:10.1038/379689a0

Cited by 369 publications

(355 citation statements)

References 42 publications

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“…The strength of the glacial overturning that has been implied from sedimentary records varies a great deal. Export rates of North Atlantic water into the Southern Ocean have been implied by sedimentary records to be similar or slightly higher to PD [Yu et al, 1996], while McManus et al [2004] infer a reduction of the Atlantic overturning by 30-40% (although probably less).…”

Section: Climate Modelmentioning

confidence: 99%

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Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere‐iceberg‐ocean model

2008

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[1] We introduce explicit icebergs from a dynamic and thermodynamic iceberg model into an intermediate complexity climate model, which includes the coupled atmosphere-ocean system. This modeling approach allows iceberg meltwater to be injected into the ocean on the basis of thermodynamical considerations along the iceberg trajectories. Icebergs are seeded from known ice sheets in both hemispheres. Adding icebergs to the present-day climate model has a minimal impact, but during the Last Glacial Maximum (LGM), Atlantic overturning strength is reduced by a third, while producing a model state that is consistent with a steady state climate. We test the sensitivity of the model at the LGM to additional Heinrich event-scale fluxes of icebergs from three possible sources: Hudson Strait, the Gulf of Saint Lawrence, and the Norwegian Channel Ice Stream (NCIS). The sensitivity of the ocean is similar for all locations, with differences dominated by the length of the iceberg meltwater pathways to the main ocean convection region. The NCIS events result in more variability and a distinctly different, more northerly, salinity anomaly. We compare these results to a more typical modeling approach, whereby meltwater is injected directly into the ocean at the iceberg source locations, and find that these floods overestimate the oceanic response compared to the iceberg events. Our results suggest that 0.3-0.4 Sv of additional freshwater flux, either as icebergs or freshwater, is required to shut down the North Atlantic meridional overturning, a larger freshwater flux than sometimes suggested because of the localized nature of the release of the freshwater.Citation: Levine, R. C., and G. R. Bigg (2008), Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere-iceberg-ocean model, Paleoceanography, 23, PA4213,

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Section: Climate Modelmentioning

confidence: 99%

“…Three conceptual models that have been put forward have been summarized by Boyle [1995] and Yu et al [1996]. The scenarios differ in the formation rate of North Atlantic Deep Water, and the resulting ocean states have also been discussed and modeled by Bigg et al [1998Bigg et al [ , 2000.…”

Section: Climate Modelmentioning

confidence: 99%

Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere‐iceberg‐ocean model

2008

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“…[36] The short residence time of deep waters in the Atlantic Ocean [Broecker et al, 1988] may make 231 Pa/ 230 Th ratios there less susceptible to the effects of variable productivity than in comparable areas of the Pacific or Indian Oceans [Yu et al, 1996;Marchal et al, 2000;Siddall et al, 2006]. Since 231 Pa has a longer residence time than 230 Th, 231 Pa remains available for export from an area after all 230 Th has been scavenged by particles.…”

Section: Comparison With Previous Paleoproductivity Studiesmentioning

confidence: 99%

“…Since 231 Pa has a longer residence time than 230 Th, 231 Pa remains available for export from an area after all 230 Th has been scavenged by particles. Therefore the southward advection of dissolved 231 Pa in newly formed North Atlantic Deep Water (NADW) competes with the removal of 231 Pa from the water column to the sediments in high-productivity areas [Yu et al, 1996]. This is consistent with our data, in that we do not see a strong correlation between high-productivity areas and high 231 Pa/ 230 Th ratios, which implies that advection may be at least part of the reason for spatially decoupled 231 Pa/ 230 Th ratios and opal fluxes.…”

Section: Comparison With Previous Paleoproductivity Studiesmentioning

confidence: 99%

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

et al. 2007

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The Silicic Acid Leakage Hypothesis (SALH) suggests that, during glacial periods, excess silicic acid was transported from the Southern Ocean to lower latitudes, which favored diatom production over coccolithophorid production and caused a drawdown of atmospheric CO2. Downcore records of 230Th‐normalized opal fluxes and 231Pa/230Th ratios from seven equatorial Atlantic cores were used to reconstruct diatom productivity over the past 30 ka (where a is years) and to test the SALH. Downcore records of 231Pa/230Th ratios and opal fluxes are highly correlated, suggesting that they constitute a production‐based record of opal flux. Opal flux records support the SALH in that glacial opal burial exceeded Holocene burial by 1.8 Gt opal/ka in the area 0°–40°W and 5°N–5°S. Earlier results from the eastern equatorial Pacific Ocean showed the opposite trend, with greater Holocene than glacial opal burial, but approximately the same magnitude of difference between glacial and interglacial opal burial. We suggest four (nonexclusive) scenarios to explain the data from both basins: (1) Increased upwelling in the equatorial Atlantic and El Niño–like conditions suppressing upwelling in the eastern equatorial Pacific altered the overall nutrient supply to both basins; (2) Si leaked from the Southern Ocean because of Fe fertilization, but was prevented from upwelling in the equatorial Pacific because of El Niño–like conditions during the LGM; (3) Si leaked from the Southern Ocean because diatom productivity was limited by increased sea ice extent and was again prevented from upwelling in the equatorial Pacific; or (4) changes in ocean circulation related to the decreased input of Agulhas water to the glacial South Atlantic provided excess dissolved Si to the equatorial Atlantic Ocean. A deglacial opal pulse is seen in both the Atlantic and Pacific Oceans. All four scenarios and the presence of the common deglacial opal pulse suggest a common driver in the Southern Ocean.

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“…Taken at face value, consistency of d 13 C amplitudes along the E. exigua isotope record with mean ocean d 13 C change would suggest that during glacial periods (MIS 2 and MIS 6) there was a persistent contribution of a well-ventilated water mass at intermediate depths at the Agulhas Plateau with a d 13 C signature similar to present-day NADW. A possible continued northern component water influence is in line with inferences from 213 Pa/ 230 Th [Yu et al, 1996], paired benthic-planktonic d 18 O [Matsumoto and Lynch-Stieglitz, 1999;Matsumoto et al, 2001] and detrital Nd isotopic records [Bayon et al, 2003] that all suggest only minor changes in water mass structure and circulation in the area on glacialinterglacial timescales. As well as the suggestion of only little change in the nutrient inventory for deep waters in the Southern Ocean derived from nearly constant glacial and interglacial foraminiferal Cd/Ca values in the region [Boyle, 1992;Oppo and Rosenthal, 1994;Boyle and Rosenthal, 1996].…”

Section: Stable Carbon Isotopesmentioning

confidence: 55%

Paleoceanography of the southern Agulhas Plateau during the last 150 ka: Inferences from benthic foraminiferal assemblages and multispecies epifaunal carbon isotopes

et al. 2007

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[1] Benthic foraminiferal assemblages and the carbon isotope composition of the epifaunal benthic foraminifera Epistominella exigua and Fontbotia wuellerstorfi have been investigated along core MD02-2589 located at the southern Agulhas Plateau (41°26.03 0 S, 25°15.30 0 E, 2660 m water depth). This study aims to evaluate changes in the benthic paleoenvironment and its influence on benthic d 13 C with a notable focus on E. exigua, a species associated with phytodetritus deposits and poorly studied in isotope paleoceanographic reconstructions. The benthic foraminiferal assemblages (>63 mm) show large fluctuations in species composition suggesting significant changes in the pattern of ocean surface productivity conceivably related to migrations of the Subtropical Convergence (STC) and Subantarctic Front (SAF). Low to moderate seasonality and relatively higher food supply to the seafloor are indicated during glacial marine isotope stages (MIS) 6, 4, and 2 and during MIS 3, probably associated with the northward migration of the SAF and confluence with the more stationary STC above the southern flank of the Agulhas Plateau. The lowest organic carbon supply to the seafloor is indicated from late MIS 5b to MIS 4 as a consequence of increased influence of the Agulhas Front (AF) and/or weakening of the influence of the STC over the region. Episodic delivery of fresh organic matter, similar to modern conditions at the core location, is indicated during MIS 5c-MIS 5e and at Termination I. Comparison of this paleoenvironmental information with the paired d 13 C records of E. exigua and F. wuellerstorfi suggests that organic carbon offsets d 13 C of E. exigua from ambient bottom water d 13 C DIC , while its d 13 C amplitude, on glacial-interglacial timescales, does not seem affected by changes of organic carbon supply to the seafloor. This suggests that this species calcifies preferentially during the short time span of the year when productivity peaks and phytodetritus is delivered to the seafloor. Therefore E. exigua, while offset from d 13 C DIC , potentially more faithfully records the amplitude of ambient bottom water d 13 C DIC changes than F. wuellerstorfi, notably in settings such as the Southern Ocean that experienced substantial changes through time in the organic carbon supply to the seafloor.Citation: Diz, P., I. R. Hall, R. Zahn, and E. G. Molyneux (2007), Paleoceanography of the southern Agulhas Plateau during the last 150 ka: Inferences from benthic foraminiferal assemblages and multispecies epifaunal carbon isotopes, Paleoceanography, 22, PA4218,

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Similar rates of modern and last-glacial ocean thermohaline circulation inferred from radiochemical data

Cited by 369 publications

References 42 publications

Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere‐iceberg‐ocean model

Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere‐iceberg‐ocean model

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

Paleoceanography of the southern Agulhas Plateau during the last 150 ka: Inferences from benthic foraminiferal assemblages and multispecies epifaunal carbon isotopes

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