Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period (original) (raw)
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The southern ocean at the Last Glacial Maximum: A strong sink for atmospheric carbon dioxide
Global Biogeochemical Cycles, 2000
Analysis of satellite ocean color, sea surface temperature, and sea ice cover data reveals consistent patterns between biological production, iron availability, and physical forcings in the Southern Ocean. The consistency of these patterns, in conjunction with information on physical conditions during the last glacial maximum (LGM), enables estimates of export production at the LGM. The LGM Southern Ocean experienced increased wind speeds, colder sea surface and atmospheric temperatures, increased deposition of atmospheric dust, and a greatly expanded winter sea ice cover. These variations had strong effects on Southern Ocean ecology and on air-sea fluxes of CO2. The seasonal ice zone (SIZ) was much larger at the LGM (30 million km 2) than at present (19 million km2). The Antarctic Polar Front (PF) likely marked the northern boundary of this expanded SIZ throughout the Southern Ocean, as it does today in the Drake Passage region. A large northward shift in the position of the PF during glacial times is unlikely due to topographic constraints. North of the PF, the increased flux of aeolian dust during glacial times altered phytoplankton species composition and increased export production, and as a result this region was a stronger sink for atmospheric CO2 than in the modem ocean. South of the PF, interactions between the biota and sea ice strongly influence air-sea gas exchange over seasonal timescales. The combined influence of melting sea ice and increased aeolian dust flux (with its associated iron) increased both primary and export production by phytoplankton over daily-monthly timescales during austral spring/summer, resulting in a strong flux of CO2 into the ocean. Heavy ice cover would have minimized air-sea gas exchange over much of the rest of the year. Thus, an increased net flux of CO2 into the ocean is likely during glacial times, even in areas where annual primary production declined. We estimate that export production in the Southern Ocean as a whole was increased by 2.9-3.6 Gt C yr-• at the LGM, relative to the modem era. Altered seasonal sea ice dynamics would further increase the net flux of CO2 into the ocean. Thus the Southern Ocean was a strong sink for atmospheric CO2 and contributed substantially to the lowering of atmospheric CO2 levels during the last ice age. 1Now at National Center for Atmospheric Research, Boulder, Colorado.
Paleoceanography, 2015
Changes in ocean circulation structure, together with biological cycling, have been proposed for trapping carbon in the deep ocean during glacial periods of the Late Pleistocene, but uncertainty remains in the nature and timing of deep ocean circulation changes through glacial cycles. In this study, we use neodymium (Nd) and carbon isotopes from a deep Indian Ocean sediment core to reconstruct water mass mixing and carbon cycling in Circumpolar Deep Water over the past 250 thousand years, a period encompassing two full glacial cycles and including a range of orbital forcing. Building on recent studies, we use reductive sediment leaching supported by measurements on isolated phases (foraminifera and fish teeth) in order to obtain a robust seawater Nd isotope reconstruction. Neodymium isotopes record a changing North Atlantic Deep Water (NADW) component in the deep Indian Ocean that bears a striking resemblance to Northern Hemisphere climate records. In particular, we identify both an approximately in-phase link to Northern Hemisphere summer insolation in the precession band and a longer-term reduction of NADW contributions over the course of glacial cycles. The orbital timescale changes may record the influence of insolation forcing, for example via NADW temperature and/or Antarctic sea ice extent, on deep stratification and mixing in the Southern Ocean, leading to isolation of the global deep oceans from an NADW source during times of low Northern Hemisphere summer insolation. That evidence could support an active role for changing deep ocean circulation in carbon storage during glacial inceptions. However, mid-depth water mass mixing and deep ocean carbon storage were largely decoupled within glacial periods, and a return to an interglacial-like circulation state during marine isotope stage (MIS) 6.5 was accompanied by only minor changes in atmospheric CO 2. Although a gradual reduction of NADW export through glacial periods may have produced slow climate feedbacks linked to the growth of Northern Hemisphere ice sheets, carbon cycling in the glacial ocean was instead more strongly linked to Southern Ocean processes. Evidence on past Atlantic Ocean circulation derived from carbon isotope reconstructions has been used to suggest that ocean circulation is primarily responding to, rather than driving, Pleistocene climate change on orbital timescales. For example, Imbrie et al. [1992] placed deep Atlantic ventilation within a "late response" group of variables, with increased ventilation occurring~8 kyr behind precessional maxima in Northern Hemisphere insolation. More recently, Lisiecki et al. [2008] similarly proposed a lag of 6-11 kyr WILSON ET AL.
Paleoceanography, 1992
We present a new approach for paleoceanographic reconstruction of surface nutrient utilization in the southern ocean. It has been observed in the contemporary ocean that, due to preferential uptake of 14NO3-during photosynthesis, the 515N of planktonic organic matter increases with increasing nitrate depletion in surface waters. Our results demonstrate that the 515N signal produced in surface waters is reflected in the underlying surface sediments; core top 515N is inversely correlated with surface nitrate concentration along a transect across the Subtropical Convergence and the Polar Front in the southeast Indian Ocean. These results are consistent with a four-box model showing that the nitrogen isotopic composition of sinking organic matter depends on percent nitrate utilization in the euphotic zone. By comparing the 515N of surface sediments with that measured in the glacial sections of several cores, we infer changes in the intensity and latitudinal distribution of nitrate uptake in this region during the last glacial maxilnum. These preliminary results argue against increased biological uptake of nutrients in southern polar waters as a major mechanism for glacial lowering of atmospheric CO2. They also suggest that Subantarctic waters in Copyright 1992 by the American Geophysical Union. Paper number 92PA01573. 0883-8305 / 92 / 92PA-01573510.00 the SE Indian sector became more nutrient depleted as they migrated northward. Increased nitrate depletion might have also occurred slightly south of the glacial Polar Front. We use a six-box model to explore the possible impact of this observation on atmospheric CO2. Francois et al.: Southern Ocean Sediment Such findings prompted an extensive search in the sedimentary record of the southern ocean for evidence of the biogeochemical changes predicted by these models. Changes in surface nutrient concentrations were traced by 5•3C [Labeyrie and Duplessy, 1985; Charles and Fairbanks, 1990; Keigwin and Boyle, 1989] and Cd/Ca [Boyle, 1988; Keigwin and Boyle, 1989] in planktonic foraminifera and by Ge/Si in biogenic silica [Froelich et al., 1989; Mortlock et al., 1991], while estimates of past changes in primary production were obtained from accumulation rates of opal in sediments [Mortlock et al., 1991; Charles et al., 1991]. As a whole, however, results from these studies remain inconclusive. Calcite 5•3C in planktonic foraminifera was found to decrease during the last glacial maximum [Labeyrie and Duplessy, 1985; Charles and Fairbanks, 1990], suggesting higher nutrient levels in southern ocean surface waters during that period. On the other hand, Cd/Ca in the same foraminifera species stay relatively constant with time [Boyle, 1988; Keigwin and Boyle, 1989], suggesting no significant changes in surface nutrient concentration during deglaciation. Ge/Si ratios in biogenic opal suggest higher glacial silicate concentrations in Antarctic surface waters [Froelich et al., 1989; Mortlock et al., 1991], in apparent
Quantifying the ocean's role in glacial CO2 reductions
Climate of the Past, 2012
A series of Last Glacial Maximum (LGM) marine carbon cycle sensitivity experiments is conducted to test the effect of different physical processes, as simulated by two atmosphere-ocean general circulation model (AOGCM) experiments, on atmospheric pCO 2 . One AOGCM solution exhibits an increase in North Atlantic Deep Water (NADW) formation under glacial conditions, whereas the other mimics an increase in Antarctic Bottom Water (AABW) associated with a weaker NADW. None of these sensitivity experiments reproduces the observed magnitude of glacial/interglacial pCO 2 changes. However, to explain the reconstructed vertical gradient of dissolved inorganic carbon (DIC) of 40 mmol m −3 a marked enhancement in AABW formation is required. Furthermore, for the enhanced AABW sensitivity experiment the simulated stable carbon isotope ratio (δ 13 C) decreases by 0.4 ‰ at intermediate depths in the South Atlantic in accordance with sedimentary evidence. The shift of deep and bottom water formation sites from the North Atlantic to the Southern Ocean increases the total preformed nutrient inventory, so that the lowered efficiency of Southern Ocean nutrient utilization in turn increases atmospheric pCO 2 . This change eventually offsets the effect of an increased abyssal carbon pool due to stronger AABW formation. The effects of interhemispheric glacial sea-ice changes on atmospheric pCO 2 oppose each other. Whereas, extended sea-ice coverage in the Southern Hemisphere reduces the airsea gas exchange of CO 2 in agreement with previous theoretical considerations, glacial advances of sea-ice in the Northern Hemisphere lead to a weakening of the oceanic carbon uptake through the physical pump. Due to enhanced gas solubility associated with lower sea surface temperature, both glacial experiments generate a reduction of atmospheric pCO 2 by about 20-23 ppmv. The sensitivity experiments presented here demonstrate the presence of compensating effects of different physical processes in the ocean on glacial CO 2 and the difficulty of finding a simple explanation of the glacial CO 2 problem by invoking ocean dynamical changes. Published by Copernicus Publications on behalf of the European Geosciences Union. Clim. Past, 8, 545-563, 2012 www.clim-past.net/8/545/2012/ www.clim-past.net/8/545/2012/ Clim. Past, 8, 545-563, 2012 www.clim-past.net/8/545/2012/ Clim. Past, 8, 545-563, 2012
Glacial marine nutrient and carbon redistribution: Evidence from the tropical ocean
Geochemistry, Geophysics, Geosystems, 2011
The nature and timing of marine nutrient and carbon redistribution through a glacial cycle remains unclear. Understanding transfers to and from the surface and deep ocean reservoirs is important to explaining Pleistocene variation in atmospheric CO 2 content. Observations in the modern ocean show that the nutrient supply to the tropical upwelling regions depends on content of deep reservoirs and vertical mixing in the Southern and subantarctic oceans. Previous work in the Pacific demonstrated that nutrient supply to the tropics was reduced during the Glacial, consonant with reduced vertical mixing in the Southern Ocean. We examine the glacial record of the tropical Atlantic with the same methods used in the Pacific (N. dutertrei carbon isotope data combined with export production estimates to evaluate changes in thermocline nutrient content). In contrast to the Pacific, we find evidence for an increase in tropical Atlantic nutrient supply under glacial conditions. The source of nutrients can be traced to subantarctic surface waters and ultimately to an enriched abyssal reservoir. Bathymetrically forced vertical mixing could account for the transfer of nutrients from this reservoir in the S. Atlantic. The enriched reservoir developed in early MIS 4 (75 ka) and persisted until about 14.5 ka (Bolling/Allerod). This timing corresponds to shifts in atmospheric CO 2 content from intermediate to minimum (full glacial) levels at 75 ka and from intermediate to Holocene concentrations near 14.5 ka.
2019
We conduct a model-data analysis of the ocean, atmosphere and terrestrial carbon system to understand their effects on atmospheric CO 2 during the last glacial cycle. We use a carbon cycle box model "SCP-M", combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and biological productivity across marine isotope stages spanning 130 thousand years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea ice cover and shallow water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimised in each marine isotope stage, against proxy data for atmospheric CO 2 , δ 13 C and ∆ 14 C and deep ocean δ 13 C, ∆ 14 C and carbonate ion. Our model-data results suggest that global overturning circulation weakened at marine isotope stage 5d, coincident with a ∼25 ppm fall in atmospheric CO 2 from the penultimate interglacial level. This change was followed by a further slowdown in Atlantic meridional overturning circulation and enhanced Southern Ocean biological export productivity at marine isotope stage 4 (∼-30 ppm). There was also a transient slowdown in Atlantic meridional overturning circulation at MIS 5b. In this model, the last glacial maximum was characterised by relatively weak global ocean and Atlantic meridional overturning circulation, and increased Southern Ocean biological export productivity (∼-20 ppm during MIS 2-4). Ocean circulation and Southern Ocean biology rebounded to modern values by the Holocene period. The terrestrial biosphere decreased by ∼500 Pg C in the lead up to the last glacial maximum, followed by a period of intense regrowth during the Holocene (∼750 Pg C). Slowing ocean circulation, a cooler ocean and, to a lesser extent, shallow carbonate dissolution, contributed ∼-75 ppm to atmospheric CO 2 in the ∼100 thousand-year lead-up to the last glacial maximum, with a further ∼-10 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological productivity was one of the ingredients required to achieve the last glacial maximum atmospheric CO 2 level. The incorporation of longer-timescale data into quantitative ocean transport models, provides useful insights into the timing of changes in ocean processes, enhancing our understanding of the last glacial maximum and Holocene carbon cycle transition.
Quantifying the ocean's role in glacial CO2 reductions
A series of Last Glacial Maximum (LGM) marine carbon cycle sensitivity experiments is conducted to test the effect of different physical processes, as simulated by two atmosphere-ocean general circulation model (AOGCM) experiments , on atmospheric pCO 2. One AOGCM solution exhibits an increase in North Atlantic Deep Water (NADW) formation under glacial conditions, whereas the other mimics an increase in Antarctic Bottom Water (AABW) associated with a weaker NADW. None of these sensitivity experiments reproduces the observed magnitude of glacial/interglacial pCO 2 changes. However, to explain the reconstructed vertical gradient of dissolved inorganic carbon (DIC) of 40 mmol m −3 a marked enhancement in AABW formation is required. Furthermore, for the enhanced AABW sensitivity experiment the simulated stable carbon isotope ratio (δ 13 C) decreases by 0.4 ‰ at intermediate depths in the South Atlantic in accordance with sedimentary evidence. The shift of deep and bottom water formation sites from the North Atlantic to the Southern Ocean increases the total preformed nutrient inventory, so that the lowered efficiency of Southern Ocean nutrient utilization in turn increases atmospheric pCO 2. This change eventually offsets the effect of an increased abyssal carbon pool due to stronger AABW formation. The effects of interhemispheric glacial sea-ice changes on atmospheric pCO 2 oppose each other. Whereas, extended sea-ice coverage in the Southern Hemisphere reduces the air-sea gas exchange of CO 2 in agreement with previous theoretical considerations, glacial advances of sea-ice in the Northern Hemisphere lead to a weakening of the oceanic carbon uptake through the physical pump. Due to enhanced gas solubility associated with lower sea surface temperature, both glacial experiments generate a reduction of atmospheric pCO 2 by about 20–23 ppmv. The sensitivity experiments presented here demonstrate the presence of compensating effects of different physical processes in the ocean on glacial CO 2 and the difficulty of finding a simple explanation of the glacial CO 2 problem by invoking ocean dynamical changes.
The role of Southern Ocean processes in orbital and millennial CO2 variations – A synthesis
Quaternary Science Reviews, 2010
Recent progress in the reconstruction of atmospheric CO 2 records from Antarctic ice cores has allowed for the documentation of natural CO 2 variations on orbital time scales over the last up to 800,000 years and for the resolution of millennial CO 2 variations during the last glacial cycle in unprecedented detail. This has shown that atmospheric CO 2 varied within natural bounds of approximately 170-300 ppmv but never reached recent CO 2 concentrations caused by anthropogenic CO 2 emissions. In addition, the natural atmospheric CO 2 concentrations show an extraordinary correlation with Southern Ocean climate changes, pointing to a significant (direct or indirect) influence of climatic and environmental changes in the Southern Ocean region on atmospheric CO 2 concentrations.