Southern Ocean contribution to both steps in deglacial atmospheric CO2 rise

AbstractThe transfer of vast amounts of carbon from a deep oceanic reservoir to the atmosphere is considered to be a dominant driver of the deglacial rise in atmospheric CO2. Paleoceanographic reconstructions reveal evidence for the existence of CO2-rich waters in the mid to deep Southern Ocean. These water masses ventilate to the atmosphere south of the Polar Front, releasing CO2 prior to the formation and subduction of intermediate-waters. Changes in the amount of CO2 in the sea water directly affect the oceanic carbon chemistry system. Here we present B/Ca ratios, a proxy for delta carbonate ion concentrations Δ[CO32−], and stable isotopes (δ13C) from benthic foraminifera from a sediment core bathed in Antarctic Intermediate Water (AAIW), offshore New Zealand in the Southwest Pacific. We find two transient intervals of rising [CO32−] and δ13C that that are consistent with the release of CO2 via the Southern Ocean. These intervals coincide with the two pulses in rising atmospheric CO2 at ~ 17.5–14.3 ka and 12.9–11.1 ka. Our results lend support for the release of sequestered CO2 from the deep ocean to surface and atmospheric reservoirs during the last deglaciation, although further work is required to pin down the detailed carbon transfer pathways.

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Rapid shifts in circulation and biogeochemistry of the Southern Ocean during deglacial carbon cycle events

Science Advances ◽

10.1126/sciadv.abb3807 ◽

2020 ◽

Vol 6 (42) ◽

pp. eabb3807

Author(s):

Tao Li ◽

Laura F. Robinson ◽

Tianyu Chen ◽

Xingchen T. Wang ◽

Andrea Burke ◽

...

Keyword(s):

Southern Ocean ◽

Ocean Circulation ◽

Deep Sea ◽

Deep Ocean ◽

Biological Pump ◽

Last Deglaciation ◽

Proxy Data ◽

Ocean Ventilation ◽

Carbon Release ◽

The Last Deglaciation

The Southern Ocean plays a crucial role in regulating atmospheric CO2 on centennial to millennial time scales. However, observations of sufficient resolution to explore this have been lacking. Here, we report high-resolution, multiproxy records based on precisely dated deep-sea corals from the Southern Ocean. Paired deep (∆14C and δ11B) and surface (δ15N) proxy data point to enhanced upwelling coupled with reduced efficiency of the biological pump at 14.6 and 11.7 thousand years (ka) ago, which would have facilitated rapid carbon release to the atmosphere. Transient periods of unusually well-ventilated waters in the deep Southern Ocean occurred at 16.3 and 12.8 ka ago. Contemporaneous atmospheric carbon records indicate that these Southern Ocean ventilation events are also important in releasing respired carbon from the deep ocean to the atmosphere. Our results thus highlight two distinct modes of Southern Ocean circulation and biogeochemistry associated with centennial-scale atmospheric CO2 jumps during the last deglaciation.

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On new developments in accelerator mass spectrometry and how they promote our understanding of global carbon cycle dynamics during the last deglaciation

10.5194/egusphere-egu2020-12768 ◽

2020 ◽

Author(s):

Julia Gottschalk ◽

Robert F. Anderson ◽

David A. Hodell ◽

Alfredo Martinez-Garcia ◽

Alain Mazaud ◽

...

Keyword(s):

High Resolution ◽

Southern Ocean ◽

Bottom Water ◽

Deep Ocean ◽

Global Ocean ◽

Last Deglaciation ◽

Surface Ocean ◽

Ocean Atmosphere ◽

The Last Deglaciation ◽

Reservoir Age

Ocean-atmosphere 14C disequilibria of the surface and deep ocean reflect past changes in the efficiency of ocean-atmosphere CO2 exchange and ocean mixing, while it may also be related to variations in global-ocean respired carbon content. A full assessment of the oceanic mechanisms controlling deglacial changes in atmospheric CO2 is complicated by a lack of high-resolution 14C ventilation age estimates from the Southern Ocean and other key regions due to low foraminiferal abundances in marine sediments in those areas. Here we present high-resolution deglacial 14C ventilation age records from key sites in the Atlantic and Indian Sector of the Southern Ocean obtained by radiocarbon analyses of small benthic and planktic foraminiferal samples (<1 mg CaCO3) with the UniBe Mini-Carbon Dating System (MICADAS). Our analyses specifically circumvent foraminiferal sample size requirements related to &#8220;conventional&#8221; accelerator mass spectrometer analyses involving sample graphitization (>1 mg CaCO3 in most laboratories). Complementing multi-proxy analyses of sea surface temperature (SST) changes at these sites allow the construction of a radiocarbon-independent age model through a stratigraphic alignment of SST changes to Antarctic (ice core) temperature variations. We demonstrate the value of refining the age models of our study cores on the basis of high-resolution sedimentary U- and Th flux estimates, which allows an improved quantification of surface ocean reservoir age variations in the past. The resulting deep-ocean ventilation age changes are compared against qualitative and quantitative indicators of bottom water [O2] variations, in order to assess the role of Southern Ocean overturning dynamics in respired carbon changes at our study sites. We discuss the implications of our new radiocarbon- and bottom water [O2] data for the ocean&#8217;s role in atmospheric CO2 changes throughout the last deglaciation, and evaluate down-stream effects of southern high-latitude surface ocean reservoir age anomalies.

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Release of old carbon from the deep South Pacific during the last deglaciation

10.5194/egusphere-egu2020-4230 ◽

2020 ◽

Author(s):

Yuhao Dai ◽

Jimin Yu ◽

Patrick Rafter

Keyword(s):

Southern Ocean ◽

Deep Ocean ◽

South Pacific ◽

Deep South ◽

Last Deglaciation ◽

Age Model ◽

The Last Deglaciation ◽

The Antarctic ◽

First Time ◽

Underlying Processes

The release of old carbon via the Southern Ocean has been thought to contribute to the last deglacial atmospheric CO2 rise, but underlying processes are not fully understood, in part, due to insufficient high-fidelity radiocarbon (&#916;14C) reconstructions minimally complicated by age models and release of &#8220;dead carbon&#8221;. Here, we present a new deep-water &#916;14C record for a core located at 3.3 km water depth from the Southwest Pacific, based on a robust age model using planktonic Mg/Ca along with co-existing benthic 14C measurements. Our results confirm previous records that suggest enhanced ventilation in the Southern Ocean during Heinrich Stadial 1 and the Younger Dryas. For the first time, our data show a large &#916;14C decline during the Antarctic Cold Reversal, indicating strengthened stratification in the deep South Pacific. Our results strongly support that the deep ocean supplied old carbon to the atmosphere during the last deglaciation.

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

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1511252113 ◽

2016 ◽

Vol 113 (3) ◽

pp. 514-519 ◽

Cited By ~ 34

Author(s):

Jenny Roberts ◽

Julia Gottschalk ◽

Luke C. Skinner ◽

Victoria L. Peck ◽

Sev Kender ◽

...

Keyword(s):

Sediment Cores ◽

Seawater Temperature ◽

Deep Ocean ◽

South Atlantic ◽

Density Stratification ◽

Intermediate Water ◽

Benthic Foraminifer ◽

Last Deglaciation ◽

Physical Density ◽

The Last Deglaciation

Explanations of the glacial–interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a “chemical divide” between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ18O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22–2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ13C and foraminifer/coral 14C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.

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Hydrographic shifts south of Australia over the last deglaciation and possible interhemispheric linkages

Quaternary Research ◽

10.1017/qua.2021.12 ◽

2021 ◽

pp. 1-12

Author(s):

Matthias Moros ◽

Patrick De Deckker ◽

Kerstin Perner ◽

Ulysses S. Ninnemann ◽

Lukas Wacker ◽

...

Keyword(s):

Southern Ocean ◽

Southern Hemisphere ◽

Critical Role ◽

Temperature Response ◽

South Australia ◽

Last Deglaciation ◽

Tight Coupling ◽

Oceanic Fronts ◽

The Last Deglaciation ◽

Circumpolar Current

Abstract Northern and southern hemispheric influences—particularly changes in Southern Hemisphere westerly winds (SSW) and Southern Ocean ventilation—triggered the stepwise atmospheric CO2 increase that accompanied the last deglaciation. One approach for gaining potential insights into past changes in SWW/CO2 upwelling is to reconstruct the positions of the northern oceanic fronts associated with the Antarctic Circumpolar Current. Using two deep-sea cores located ~600 km apart off the southern coast of Australia, we detail oceanic changes from ~23 to 6 ka using foraminifer faunal and biomarker alkenone records. Our results indicate a tight coupling between hydrographic and related frontal displacements offshore South Australia (and by analogy, possibly the entire Southern Ocean) and Northern Hemisphere (NH) climate that may help confirm previous hypotheses that the westerlies play a critical role in modulating CO2 uptake and release from the Southern Ocean on millennial and potentially even centennial timescales. The intensity and extent of the northward displacements of the Subtropical Front following well-known NH cold events seem to decrease with progressing NH ice sheet deglaciation and parallel a weakening NH temperature response and amplitude of Intertropical Convergence Zone shifts. In addition, an exceptional poleward shift of Southern Hemisphere fronts occurs during the NH Heinrich Stadial 1. This event was likely facilitated by the NH ice maximum and acted as a coup-de-grâce for glacial ocean stratification and its high CO2 capacitance. Thus, through its influence on the global atmosphere and on ocean mixing, “excessive” NH glaciation could have triggered its own demise by facilitating the destratification of the glacial ocean CO2 state.

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The Driving Mechanisms on Southern Ocean Upwelling Change during the Last Deglaciation

Geosciences ◽

10.3390/geosciences11070266 ◽

2021 ◽

Vol 11 (7) ◽

pp. 266

Author(s):

Wei Liu ◽

Zhengyu Liu ◽

Shouwei Li

Keyword(s):

Southern Ocean ◽

Climate Model ◽

Model Simulation ◽

Early Stage ◽

Surface Wind ◽

Ekman Pumping ◽

Last Deglaciation ◽

Buoyancy Forcing ◽

Local Topography ◽

The Last Deglaciation

We explore the change in Southern Ocean upwelling during the last deglaciation, based on proxy records and a transient climate model simulation. Our analyses suggest that, beyond a conventional mechanism of the Southern Hemisphere westerlies shift, Southern Ocean upwelling is strongly influenced by surface buoyancy forcing and the local topography. Over the Antarctic Circumpolar Current region, the zonal mean and local upwelled flows exhibited distinct evolution patterns during the last deglaciation, since they are driven by different mechanisms. The zonal mean upwelling is primarily driven by surface wind stress via zonal mean Ekman pumping, whereas local upwelling is driven by both wind and buoyancy forcing, and is tightly coupled to local topography. During the early stage of the last deglaciation, the vertical extension of the upwelled flows increased downstream of submarine ridges but decreased upstream, which led to enhanced and diminished local upwelling, downstream and upstream of the submarine ridges, respectively.

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Different ocean states and transient characteristics in Last Glacial Maximum simulations and implications for deglaciation

Climate of the Past ◽

10.5194/cp-9-2319-2013 ◽

2013 ◽

Vol 9 (5) ◽

pp. 2319-2333 ◽

Cited By ~ 67

Author(s):

X. Zhang ◽

G. Lohmann ◽

G. Knorr ◽

X. Xu

Keyword(s):

Last Glacial Maximum ◽

Model Comparison ◽

Climate Model ◽

Deep Ocean ◽

Glacial Maximum ◽

Meridional Overturning Circulation ◽

Last Deglaciation ◽

New Paradigm ◽

Last Glacial ◽

The Last Deglaciation

Abstract. The last deglaciation is one of the best constrained global-scale climate changes documented by climate archives. Nevertheless, understanding of the underlying dynamics is still limited, especially with respect to abrupt climate shifts and associated changes in the Atlantic meridional overturning circulation (AMOC) during glacial and deglacial periods. A fundamental issue is how to obtain an appropriate climate state at the Last Glacial Maximum (LGM, 21 000 yr before present, 21 ka BP) that can be used as an initial condition for deglaciation. With the aid of a comprehensive climate model, we found that initial ocean states play an important role on the equilibrium timescale of the simulated glacial ocean. Independent of the initialization, the climatological surface characteristics are similar and quasi-stationary, even when trends in the deep ocean are still significant, which provides an explanation for the large spread of simulated LGM ocean states among the Paleoclimate Modeling Intercomparison Project phase 2 (PMIP2) models. Accordingly, we emphasize that caution must be taken when alleged quasi-stationary states, inferred on the basis of surface properties, are used as a reference for both model inter-comparison and data model comparison. The simulated ocean state with the most realistic AMOC is characterized by a pronounced vertical stratification, in line with reconstructions. Hosing experiments further suggest that the response of the glacial ocean is dependent on the ocean background state, i.e. only the state with robust stratification shows an overshoot behavior in the North Atlantic. We propose that the salinity stratification represents a key control on the AMOC pattern and its transient response to perturbations. Furthermore, additional experiments suggest that the stratified deep ocean formed prior to the LGM during a time of minimum obliquity (~ 27 ka BP). This indicates that changes in the glacial deep ocean already occur before the last deglaciation. In combination, these findings represent a new paradigm for the LGM and the last deglaciation, which challenges the conventional evaluation of glacial and deglacial AMOC changes based on an ocean state derived from 21 ka BP boundary conditions.

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Climate evolution at the last deglaciation: the role of the Southern Ocean

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2004.10.003 ◽

2004 ◽

Vol 228 (3-4) ◽

pp. 407-424 ◽

Cited By ~ 76

Author(s):

Cristina Bianchi ◽

Rainer Gersonde

Keyword(s):

Southern Ocean ◽

Last Deglaciation ◽

Climate Evolution ◽

The Last Deglaciation

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Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes

Ocean Science ◽

10.5194/os-7-533-2011 ◽

2011 ◽

Vol 7 (5) ◽

pp. 533-547 ◽

Cited By ~ 47

Author(s):

F. Fripiat ◽

A.-J. Cavagna ◽

F. Dehairs ◽

S. Speich ◽

L. André ◽

...

Keyword(s):

Southern Ocean ◽

Silicic Acid ◽

Mixed Layer ◽

Deep Water ◽

Biogenic Silica ◽

Polar Front ◽

Intermediate Water ◽

Subtropical Zone ◽

Isotopic Signatures ◽

The Antarctic

Abstract. Silicon isotopic signatures (δ30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products and with the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration northward, across the different interfaces, without significantly changing the AASW δ30Si composition. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, the δ30Si composition of the silicic acid pools is getting slightly, but significantly lighter from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This isotopic trend is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Through the use of δ30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is estimated to be 5.2 &pm; 1.3 and 1.1 &pm; 0.3 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively. This is in good agreement with previous estimations. Furthermore, summertime Si-supply into the mixed layer of both zones, via vertical mixing, is estimated to be 1.6 &pm; 0.4 and 0.1 &pm; 0.5 mol Si m−2, respectively.

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