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

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.

scholarly journals Deep ocean ventilation, carbon isotopes, marine sedimentation and the deglacial CO<sub>2</sub> rise

2011 ◽  
Vol 7 (3) ◽  
pp. 771-800 ◽  
Author(s):  
T. Tschumi ◽  
F. Joos ◽  
M. Gehlen ◽  
C. Heinze
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Isotope Composition ◽  
Deep Ocean ◽  
Atmospheric Co2 ◽  
Carbon Cycle Model ◽  
Ocean Ventilation ◽  
Marine Sedimentation ◽  
Sediment Formation ◽  
Ocean Carbon

Abstract. The link between the atmospheric CO2 level and the ventilation state of the deep ocean is an important building block of the key hypotheses put forth to explain glacial-interglacial CO2 fluctuations. In this study, we systematically examine the sensitivity of atmospheric CO2 and its carbon isotope composition to changes in deep ocean ventilation, the ocean carbon pumps, and sediment formation in a global 3-D ocean-sediment carbon cycle model. Our results provide support for the hypothesis that a break up of Southern Ocean stratification and invigorated deep ocean ventilation were the dominant drivers for the early deglacial CO2 rise of ~35 ppm between the Last Glacial Maximum and 14.6 ka BP. Another rise of 10 ppm until the end of the Holocene is attributed to carbonate compensation responding to the early deglacial change in ocean circulation. Our reasoning is based on a multi-proxy analysis which indicates that an acceleration of deep ocean ventilation during early deglaciation is not only consistent with recorded atmospheric CO2 but also with the reconstructed opal sedimentation peak in the Southern Ocean at around 16 ka BP, the record of atmospheric δ13CCO2, and the reconstructed changes in the Pacific CaCO3 saturation horizon.

scholarly journals Glacial CO<sub>2</sub> decrease and deep-water deoxygenation by iron fertilization from glaciogenic dust

2019 ◽  
Author(s):  
Akitomo Yamamoto ◽  
Ayako Abe-Ouchi ◽  
Rumi Ohgaito ◽  
Akinori Ito ◽  
Akira Oka
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Deep Water ◽  
Large Scale ◽  
Deep Ocean ◽  
Oxygen Solubility ◽  
Biological Pump ◽  
Surface Cooling ◽  
Iron Fertilization ◽  
Water Deoxygenation

Abstract. Increased accumulation of respired carbon in the deep ocean associated with enhanced efficiency of the biological carbon pump is thought to be a key mechanism of glacial CO2 drawdown. Despite greater oxygen solubility due to sea surface cooling, recent quantitative and qualitative proxy data show glacial deep-water deoxygenation, reflecting increased accumulation of respired carbon. However, the mechanisms of deep-water deoxygenation and contribution from the biological pump to glacial CO2 drawdown have remained unclear. In this study, we report the significance of iron fertilization from glaciogenic dust for glacial CO2 decrease and deep-water deoxygenation using our numerical simulation, which successfully reproduces the magnitude and large-scale pattern of the observed oxygen changes from the present to Last Glacial Maximum. Sensitivity experiments reveal that physical changes (e.g., more sluggish ocean circulation) contribute to only half of all glacial deep deoxygenation, whereas the other half is driven by enhanced efficiency of the biological pump. We found that iron input from the glaciogenic dust with higher iron solubility is the most significant factor for enhancement of the biological pump and deep-water deoxygenation. Glacial deep-water deoxygenation expands the hypoxic waters in the deep Pacific and Indian Ocean. The simulated global volume of hypoxic waters is nearly double the present value, which suggest that the glacial deep-water is sever environment for the benthic animals. Our model underestimated the deoxygenation in the deep Southern Ocean due to enhanced ventilation. The model-proxy comparison of oxygen change suggest that the stratified Southern Ocean is required for reproducing oxygen decline in the deep Southern Ocean. Enhanced efficiency of biological pump contributes to decrease of glacial CO2 by more than 30 ppm, which is supported by the model-proxy agreement of oxygen change. Our findings confirm the significance of the biological pump in glacial CO2 drawdown and deoxygenation.

scholarly journals Deep ocean ventilation, carbon isotopes, marine sedimentation and the deglacial CO<sub>2</sub> rise

2010 ◽  
Vol 6 (5) ◽  
pp. 1895-1958 ◽  
Author(s):  
T. Tschumi ◽  
F. Joos ◽  
M. Gehlen ◽  
C. Heinze
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Isotope Composition ◽  
Three Dimensional ◽  
Deep Ocean ◽  
Atmospheric Co2 ◽  
Carbon Cycle Model ◽  
Ocean Ventilation ◽  
Sediment Formation ◽  
Ocean Carbon

Abstract. The link between the atmospheric CO2 level and the ventilation state of the deep ocean is an important building block of the key hypotheses put forth to explain glacial-interglacial CO2 fluctuations. In this study, we systematically examine the sensitivity of atmospheric CO2 and its carbon isotope composition to changes in deep ocean ventilation, the ocean carbon pumps, and sediment formation in a global three-dimensional ocean-sediment carbon cycle model. Our results provide support for the hypothesis that a break up of Southern Ocean stratification and invigorated deep ocean ventilation were the dominant drivers for the early deglacial CO2 rise of ~35 ppm between the Last Glacial Maximum and 14.6 ka BP. Another rise of 10 ppm until the end of the Holocene is attributed to carbonate compensation responding to the early deglacial change in ocean circulation. Our reasoning is based on a multi-proxy analysis which indicates that an acceleration of deep ocean ventilation during the early deglaciation is not only consistent with recorded atmospheric CO2 but also with the reconstructed opal sedimentation peak in the Southern Ocean at around 16 ka BP, the record of atmospheric δ13CCO2, and the reconstructed changes in the Pacific CaCO3 saturation horizon.

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

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

&lt;p&gt;Ocean-atmosphere &lt;sup&gt;14&lt;/sup&gt;C disequilibria of the surface and deep ocean reflect past changes in the efficiency of ocean-atmosphere CO&lt;sub&gt;2&lt;/sub&gt; 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 CO&lt;sub&gt;2&lt;/sub&gt; is complicated by a lack of high-resolution &lt;sup&gt;14&lt;/sup&gt;C 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 &lt;sup&gt;14&lt;/sup&gt;C 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 (&lt;1 mg CaCO&lt;sub&gt;3&lt;/sub&gt;) with the UniBe Mini-Carbon Dating System (MICADAS). Our analyses specifically circumvent foraminiferal sample size requirements related to &amp;#8220;conventional&amp;#8221; accelerator mass spectrometer analyses involving sample graphitization (&gt;1 mg CaCO&lt;sub&gt;3&lt;/sub&gt; 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 [O&lt;sub&gt;2&lt;/sub&gt;] 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 [O&lt;sub&gt;2&lt;/sub&gt;] data for the ocean&amp;#8217;s role in atmospheric CO&lt;sub&gt;2&lt;/sub&gt; changes throughout the last deglaciation, and evaluate down-stream effects of southern high-latitude surface ocean reservoir age anomalies.&lt;/p&gt;

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

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

&lt;p&gt;The release of old carbon via the Southern Ocean has been thought to contribute to the last deglacial atmospheric CO&lt;sub&gt;2&lt;/sub&gt; rise, but underlying processes are not fully understood, in part, due to insufficient high-fidelity radiocarbon (&amp;#916;&lt;sup&gt;14&lt;/sup&gt;C) reconstructions minimally complicated by age models and release of &amp;#8220;dead carbon&amp;#8221;. Here, we present a new deep-water &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C 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 &lt;sup&gt;14&lt;/sup&gt;C 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 &amp;#916;&lt;sup&gt;14&lt;/sup&gt;C 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.&lt;/p&gt;

scholarly journals Poleward shift in the Southern Hemisphere westerly winds synchronous with the deglacial rise in CO2

2021 ◽  
Author(s):  
William Gray ◽  
Casimir de Lavergne ◽  
Robert Jnglin Wills ◽  
Laurie Menviel ◽  
Paul Spence ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Climate Models ◽  
Deep Ocean ◽  
Last Deglaciation ◽  
Westerly Winds ◽  
Poleward Shift ◽  
Deep Ocean Circulation ◽  
Southern Hemisphere Westerly Winds

Abstract The Southern Hemisphere westerly winds strongly influence deep ocean circulation and carbon storage1. While the westerlies are hypothesised to play a key role in regulating atmospheric CO2 over glacial-interglacial cycles2–4, past changes in their position and strength remain poorly constrained5–7. Here, we use a compilation of planktic foraminiferal δ18O from across the Southern Ocean and constraints from an ensemble of climate models to reconstruct changes in the westerlies over the last deglaciation. We find a 4.7° (2.9-6.9°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (about 20,000 years ago) relative to the mid-Holocene (about 6,000 years ago). Our reconstruction shows that the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO2. Experiments with a 0.25° resolution ocean-sea-ice-carbon model demonstrate that shifting the westerlies equatorward substantially reduces the overturning rate of the abyssal ocean, leading to a suppression of CO2 outgassing from the Southern Ocean. Our results establish a central role for the westerly winds in driving the deglacial CO2 rise, and suggest natural CO2 outgassing from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming8–10.

scholarly journals Southern Ocean contribution to both steps in deglacial atmospheric CO2 rise

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Thomas A. Ronge ◽  
Matthias Frische ◽  
Jan Fietzke ◽  
Alyssa L. Stephens ◽  
Helen Bostock ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Water ◽  
Deep Ocean ◽  
Polar Front ◽  
Intermediate Water ◽  
Last Deglaciation ◽  
Carbonate Ion ◽  
Carbon Transfer ◽  
The Last Deglaciation ◽  
Oceanic Carbon

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.

scholarly journals Asynchronous warming and δ18O evolution of deep Atlantic water masses during the last deglaciation

2017 ◽  
Vol 114 (42) ◽  
pp. 11075-11080 ◽  
Author(s):  
Jiaxu Zhang ◽  
Zhengyu Liu ◽  
Esther C. Brady ◽  
Delia W. Oppo ◽  
Peter U. Clark ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
General Circulation ◽  
Large Scale ◽  
Water Mass ◽  
Critical Role ◽  
Circulation Model ◽  
Deep Ocean ◽  
Last Deglaciation ◽  
The Last Deglaciation

The large-scale reorganization of deep ocean circulation in the Atlantic involving changes in North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) played a critical role in regulating hemispheric and global climate during the last deglaciation. However, changes in the relative contributions of NADW and AABW and their properties are poorly constrained by marine records, including δ18O of benthic foraminiferal calcite (δ18Oc). Here, we use an isotope-enabled ocean general circulation model with realistic geometry and forcing conditions to simulate the deglacial water mass and δ18O evolution. Model results suggest that, in response to North Atlantic freshwater forcing during the early phase of the last deglaciation, NADW nearly collapses, while AABW mildly weakens. Rather than reflecting changes in NADW or AABW properties caused by freshwater input as suggested previously, the observed phasing difference of deep δ18Oc likely reflects early warming of the deep northern North Atlantic by ∼1.4 °C, while deep Southern Ocean temperature remains largely unchanged. We propose a thermodynamic mechanism to explain the early warming in the North Atlantic, featuring a strong middepth warming and enhanced downward heat flux via vertical mixing. Our results emphasize that the way that ocean circulation affects heat, a dynamic tracer, is considerably different from how it affects passive tracers, like δ18O, and call for caution when inferring water mass changes from δ18Oc records while assuming uniform changes in deep temperatures.

On the impact of sea ice in a global ocean circulation model

1997 ◽  
Vol 25 ◽  
pp. 111-115 ◽  
Author(s):  
Achim Stössel
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Convective Activity ◽  
The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

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