Reduced deep ocean ventilation in the Southern Pacific Ocean during the last glaciation persisted into the deglaciation (original) (raw)
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Nature Communications, 2015
Consistent evidence for a poorly ventilated deep Pacific Ocean that could have released its radiocarbon-depleted carbon stock to the atmosphere during the last deglaciation has long been sought. Such evidence remains lacking, in part due to a paucity of surface reservoir age reconstructions required for accurate deep-ocean ventilation age estimates. Here we combine new radiocarbon data from the Eastern Equatorial Pacific (EEP) with chronostratigraphic calendar age constraints to estimate shallow sub-surface reservoir age variability, and thus provide estimates of deep-ocean ventilation ages. Both shallow- and deep-water ventilation ages drop across the last deglaciation, consistent with similar reconstructions from the South Pacific and Southern Ocean. The observed regional fingerprint linking the Southern Ocean and the EEP is consistent with a dominant southern source for EEP thermocline waters and suggests relatively invariant ocean interior transport pathways but significantly re...
Paleoceanography and Paleoclimatology, 2019
Coeval changes in atmospheric CO 2 and 14 C contents during the last deglaciation are often attributed to ocean circulation changes that released carbon stored in the deep ocean during the Last Glacial Maximum (LGM). Work is being done to generate records that allow for the identification of the exact mechanisms leading to the accumulation and release of carbon from the oceanic reservoir, but these mechanisms are still the subject of debate. Here we present foraminifera 14 C data from five cores in a transect across the Chilean continental margin between~540 and~3,100 m depth spanning the last 20,000 years. Our data reveal that during the LGM, waters at~2,000 m were 50% to 80% more depleted in Δ 14 C than waters at~1,500 m when compared to modern values, consistent with the hypothesis of a glacial deep ocean carbon reservoir that was isolated from the atmosphere. During the deglaciation, our intermediate water records reveal homogenization in the Δ 14 C values between~800 and~1,500 m from 16.5-14.5 ka cal BP to~14-12 ka cal BP, which we interpret as deeper penetration of Antarctic Intermediate Water. While many questions still remain, this process could aid the ventilation of the deep ocean at the beginning of the deglaciation, contributing to the observed~40 ppm rise in atmospheric pCO 2 .
Old radiocarbon ages in the southwest Pacific Ocean during the last glacial period and deglaciation
Nature, 2000
Marine radiocarbon (14C) dates are widely used for dating oceanic events and as tracers of ocean circulation, essential components for understanding ocean-climate interactions. Past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep-water and surface-water reservoirs, but the apparent age of surface waters (currently approximately 400 years in the tropics and approximately 1,200 years in Antarctic waters) might not be constant through time, as has been assumed in radiocarbon chronologies and palaeoclimate studies. Here we present independent estimates of surface-water and deep-water reservoir ages in the New Zealand region since the last glacial period, using volcanic ejecta (tephras) deposited in both marine and terrestrial sediments as stratigraphic markers. Compared to present-day values, surface-reservoir ages from 11,900 14C years ago were twice as large (800 years) and during glacial times were five times as large (2,000 years), contradicting the assumption of constant surface age. Furthermore, the ages of glacial deep-water reservoirs were much older (3,000-5,000 years). The increase in surface-to-deep water age differences in the glacial Southern Ocean suggests that there was decreased ocean ventilation during this period.
Paleoceanography and Paleoclimatology, 2019
Most conceptual models of ocean circulation during past glacial periods invoke a shallowed North Atlantic-sourced water mass overlying an expanded, poorly ventilated Southern Ocean (SO)-sourced deep water mass (Southern Component Water or SCW), rich in remineralized carbon, within the Atlantic basin. However, the ventilation state, carbon inventory, and circulation pathway of SCW sourced in the Pacific sector of the SO (Pacific SO) during glacial periods are less well understood. Here we present multiproxy data-including δ 18 O and δ 13 C measured on the benthic and planktic foraminifera Cibicidoides wuellerstorfi, and Neogloboquadrina pachyderma, and productivity proxies including percent CaCO 3 , total organic carbon, and Ba/Ti-from a sediment core located in the high-latitude (71°S) Pacific SO spanning the last 800 kyr. Typical glacial δ 13 C values of SCW at this core site are~0‰. We find no evidence for SCW with extremely low δ 13 C values during glacials in the high-latitude Pacific SO. This leads to a spatial gradient in the stable carbon isotope composition of SCW from the high-latitude SO, suggesting that there are different processes of deep-and bottom-water formation around Antarctica. A reduced imprint of air-sea gas exchange is evident in the SCW formed in the Atlantic SO compared with the Pacific SO. A spatial δ 13 C gradient in SCW is apparent throughout much of the last 800,000 years, including interglacials. A SO-wide depletion in benthic δ 13 C is observed in early MIS 16, coinciding with the lowest atmospheric pCO 2 recorded in Antarctic ice cores.
Paleoceanography, 1994
A reconstruction of late Pleistocene surface water carbon isotopic (•513C) variability is presented from Ocean Drilling Program (ODP) site 769 in the Sulu Sea in the western tropical Pacific. The Sulu Sea is a shallowly silled back arc basin with a maximum sill depth of 420 m. Site 769 was drilled on a bathymetric high in 3643 m of water and has average late Pleistocene sedimentation rates of 8.5 cm/kyr. The oxygen isotope record (•5180) of Globigerinoides ruber at site 769 shows a strong correlation with the SPECMAP stacked fi180 record, attesting to the continuity of sediment archive at the site. Surface fi13C displays consistent glacial-interglacial variability which averages-0.9%0 and has varied from 0.75 to 1.1%o over the last 800 kyr. Comparison to surface water fi13C records in the South China Sea and western tropical Pacific suggests that the glacial-interglacial surface fi13C variability is regional in scale. Planktonic fi13C data from ODP site 677 in the eastern Pacific is also coherent with the site 769. Additionally, we have found that the site 769 surface fi13C record is coherent at periods of 100 and 41 kyr with deepwater fi13C records from the Pacific. The highest correlation occurs with the deep eastern Pacific, where benthic fi13C data from cores RC13-110 and ODP site 677 closely match the Sulu Sea surface water record. We evaluate several possible controls of surface water •13C in the Sulu Sea that may explain the coherent timing with Pacific deepwater •513C records. These include variations in terrestrial organic matter flux to the basin, the upwelling of subsurface water and productivity changes, and the influx of western Pacific intermediate water to the Sulu Sea. Our preferred explanation involves a region of upper intermediate water upwelling in the far western Pacific which has been shown to outgas CO2 from subsurface waters into surface waters. Upwelling also occurs in the area of Panama Basin site 677. These equatorial upwelling zones could potentially provide a route by which Pacific intermediate water can modulate the •513C composition of certain Pacific surface water locations. Future reconstructions of late Pleistocene surface water •5•3C variability in the western Pacific and Indonesian seas will be required to further evaluate the source of the glacial-interglacial surface water •513C change. Introduction Late Pleistocene variations in the carbon isotopic composition of both Atlantic Ocean and Pacific Ocean deep waters have recently been shown to be coherent and inphase at periods of 100 and 41 kyr [Oppo et al., 1990; Rayrno et al., 1990; Mix et al., 1991]. Consistently higher õ13C values in the North Atlantic compared to the South Atlantic and in the tropical deep Pacific throughout the late Pleistocene indicate that the North Atlantic was always a source of nutrientdepleted North Atlantic Deep Water [Rayrno et al., 1990]. Deepwater fi13C changes in the North Atlantic are also inphase and coherent at 100-and 41-kyr periods (95% level) with the global õ•80 ice volume signal. This implies that late Pleistocene glacial-interglacial variations in oceanic carbon Copyright 1994 by the American Geophyscial Union. Paper number 93PA03216. 0883-8305/94/93PA-03216510.00 reservoirs and deepwater circulation can largely be related to ice volume changes. Changes in the surface water fi13C inferred from planktonic foraminifera from different sites have not been as interpretable or coherent as deepwater õ13C. Several independent factors have been shown to influence planktonic foraminiferal fi13C composition, including local and regional hydrography [Broecker and Peng, 1982], varying depth preferences [Fairbanks et al., 1982; Curry et al., 1983], and size dependent fractionation [Berger et al., 1978; Curry and Matthews, 1981; Oppo and Fairbanks, 1989], which can complicate interpretation of the õ13C data. Despite these potential complicating factors, several studies have demonstrated that in some tropical settings, planktonic fi13C data can be correlated on a regional scale. Oppo and Fairbanks [1989] report a consistent deglacial õ13C minimum seen in several planktonic records of tropical surface water which span the last deglaciation. They also found this planktonic õ 13 C minimum to be synchronous with intermediate water fi13C changes, suggesting a transfer of the 317 318 LINSLEY AND DUNBAR: PLANKTONIC •j13C HISTORY IN THE SULU SEA •il3C signal from intermediate water depths to the surface. Curry and Crowley [1987] correlated planktonic •il3C records from shallow-dwelling foraminifera in nutrient-depleted areas of the equatorial Atlantic and created a composite stacked record that depicts surface water carbon isotopic variability in the equatorial Atlantic. The difference between the tropical planktonic composite •i•3C stack and benthic •i•3C (A/5•3C) in general parallels the A/5•3C record generated by Shackleton and Pisias [1985] for eastern Pacific core V19-30. Shackleton et al. [1992] compiled three late Pleistocene planktonic •il3C records from the tropical western Pacific in order to reconstruct atmospheric pCO2 and refine the V19-30 record. This composite record resembles the V19-30 record, as well as the tropical Atlantic, and confirms that variations in atmospheric CO2 inferred from A/5•3C changes lead ice volume. The amplitude of the glacial-interglacial changes in this reconstruction are significantly smaller than the amplitude of CO2 observed in the ice core reconstructions [Barnola et al., 1987]. Shackleton et al. [1992] postulate that their reconstructed amplitude may be more representative of the "biological pump effect" or may be a result of the bioturbation effect on these western Pacific cores. To constrain further western tropical Pacific surface water •i•3C variability in the late Pleistocene, we have reconstructed the late Pleistocene history of surface water carbon and oxygen isotopic variability at Ocean Drilling Program (ODP) site 769 located in the Sulu Sea (Figures l a and b). Relatively high sediment accumulation rates have resulted in an expanded upper Pleistocene record at site 769. The Sulu Sea is a deep, back arc, silled basin located between Borneo and the '"
Paleoceanography, 1991
Stable isotopes in benthic foraminifera from Pacific sediments are used to assess hypotheses of systematic shifts in the depth distribution of oceanic nutrients and carbon during the ice ages. The carbon isotope differences between -1400 and -3200 m depth in the eastern Pacific are consistently greater in glacial than interglacial maxima over the last-370 kyr. This phenomenon of "bottom heavy" glacial nutrient distributions, which Boyle proposed as a cause of Pleistocene CO2 change, occurs primarily in the 1/100 and 1/41 kyr-1 "Milankovitch" orbital frequency bands but appears to lack a coherent 1/23 kyr-1 band related to orbital precession. Averaged over oxygen-isotope stages, glacial gradients from-1400 to -3200 m depth are 0.1%o greater than interglacial gradients. The range of extreme shifts is somewhat larger, 0.2 to 0.5%0. In both cases, these changes in Pacific •5•3C distributions are much smaller than observed in shorter records from the North Atlantic. This may be too small to be a dominant cause of atmospheric pCO2 change, rodess current models underestimate the sensitivity of pCO2 to nutrient redistributions. This dampening of Pacific relative to Atlantic õ13C 1Now at GEOMAR depth gradient favors a North Atlantic origin of the phenomenon, although local variations of Pacific intermediate water masses can not be excluded at present. Indian Ocean, Nature, 333, 651-655, 1988. Keir, R.S., On the late Pleistocene ocean geochemistry and circulation, Paleoceanography, 3, 413-446, 1988. Knox, F., and M. McElroy, Changes in atmospheric CO2: Influence of the marine biota at high latitude, J, Geophys. Res., 89. 4629-4637, 1985. Kroopnick, P., The dissolved O2-CO2-13C system in the eastern equatorial Pacific, Deep Sea Res., 21, 211-227, 1974. Kroopnick, P., The distribution of •3C of 5•CO2 in the world oceans, Deet• Sea Res., 32. 57-84, 1985. Labeyrie L.D., and J.-C. Duplessy, Changes in the oceanic C13/C 12 ratio during the last 140,000 years: High latitude surface water records, Palaeoeeoer. Palaeoclimatol. Palaeoecol., 50. 217-240, 1985.
Paleoceanography and Paleoclimatology, 2018
Published estimates of the radiocarbon content of middepth waters suggest a decrease in ventilation in multiple locations during the last glacial maximum (LGM; 24.0-18.1 ka). Reduced glacial ventilation would have allowed respired carbon to accumulate in those waters. A subsequent deglacial release of this respired carbon reservoir to the atmosphere could then account for the observed increases in atmospheric CO 2 and decline in atmospheric radiocarbon content. However, age model error and a release of 14 C-depleted mantle carbon have also been cited as possible explanations for the observed middepth radiocarbon depletions, calling into question the deep ocean's role in storing respired carbon during the LGM. Joint measurements of benthic foraminiferal carbon isotope values (δ 13 C) and cadmium/calcium (Cd/Ca) ratios provide a method for isolating the air-sea component of a water mass from changes in remineralization. Here we use benthic foraminiferal δ 13 C and Cd/Ca records from the eastern equatorial Pacific to constrain changes in remineralization and water-mass mixing over the last glacial-interglacial transition. These records are complemented with elemental measurements of the authigenic coatings of foraminifera to monitor postdepositional changes in bottom water properties. Our results suggest an increase of deep waters at midwater depths consistent with a shoaling of the boundary between the upper and lower branches of Southern Ocean overturning circulation. Additionally, our records demonstrate increased organic matter remineralization in middepth waters during the LGM, suggesting that respired carbon did accumulate in middepth waters under periods of reduced ventilation. Plain Language Summary Records of gas bubbles trapped in ice suggest that the concentration of CO2 in the atmosphere was lower during cold, glacial periods when ice covered more of Earth. These records show that both atmospheric CO2 and temperature increased as Earth's climate transitioned from last glacial period, about 20,000 years ago, to the warmer climate Earth has experienced over the last 10,000 years. Data suggest that the oceans are a significant source of this CO2. During ocean circulation, a water mass sinks from the surface of the ocean where it is no longer exchanging CO2 with the atmosphere, allowing the CO2 to accumulate in those waters. To investigate whether the oceans stored CO2 during the last glacial period, we developed records of past ocean chemistry using the shells of small marine organisms that record the chemistry of the water they are living in as they build their shells. Our results from the eastern equatorial Pacific indicate that CO2 accumulated in these waters during the last glacial, likely as a result of changes in ocean circulation. By understanding how the oceans and atmosphere have exchanged CO2 in the past, we better understand what role the oceans may play under modern CO2 increases. 14 C-depleted reservoir.