Meridional Sea Surface Temperature Gradients in the Southern Indian Ocean over the Last Glacial Cycle
Lena M. Thöle 1,2 , E. Michel 3 , A. Auderset 4 , S. Moretti 4 , A. S. Studer 5 , A. Mazaud 3 , A. Martínez-García 4 , S. L. Jaccard 1
1 Institute of Geological Sciences and Oeschger Centre for Climate Change Research, University of Bern, Switzerland.
2 Marine Palynology and Paleoceanography, Department of Earth Sciences, Utrecht University, Netherlands.
3 LSCE, Gif-sur-Yvette, France. 4 Max Planck Institute for Chemistry, Climate Geochemistry Department, Mainz, Germany.
5 Department of Environmental Sciences, University of Basel, Switzerland.
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Take Home Message
Geographical Setting
We analyzed two new sediment cores close to the Kerguelen Plateau in the Southern Indian Ocean forming a latitudinal transect across the ACC.
SAZ: MD11-3357 (44°S, 80°E, 3349 m) AZ: MD11-3353 (50°S, 68°E, 1568 m)
Methods
What did we do?
SSTs were reconstructed based on (GDGT- based) TEX 86 L -index and calibrated for low temperatures after Kim et al. (2012) 7 .
Figure 2. Austral summer sea surface temperatures, oceanic fronts, and core locations.
Contact me: l.m.thole@uu.nl
@lena_thole
References
[1] Sarmiento et al. (2004) Nature 427, 56-59; [3] Orsi et al. (1995) Deep-Sea Res. I, 42, 641-673; [4] McCave et al. (2014) Nat. Geosci. 7, 113-116; [5] Mazaud et al. (2007) Geochem. Geophys. Geosyst., 8; [6] Kim et al. (2012) Environmental Microbiology 14(6), 1528-1543; [7] Jouzel et al. (2007) Science 317 (5839), 793-796; [8] Bereiter et al. (2015) Geophys. Res. Lett. 42, 542-549. [9] Saunders et al. (2018) Nature Geosciences 11, 650-655; [10] Studer et al. (2018) Nature Geosciences 11, 756-760.
Research Questions
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Background
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
200 300 400 500 600
2 2.5 3 3.5 4 4.5
4 4.5 5 5.5 6 6.5 7 7.5
250 260 270 280 290 300
0 2 4 6 8 10
0 2 4 6 8 10
Age (ka)
Age (ka)
SAZ-AZ SST gradient (°C)
∂
15
N
DB
(‰ air) CO
2
(ppm) D-I conductivity (µS cm
-1
) T i (%TSN) a)
c) b)
d)
wind intensity wind intensity
surface nutrient supply ACC flow strength
ACC shifts vs changes in the ACC strength The Antarctic Circumpolar Current (ACC) and upwelling intensity
Westerlies
SAMW AAIW LCDW
0
1000
2000
3000
Depth (m) 4000
5000
6000 80°S 70°S 60°S 50°S 40°S
NADW
Ekman
transport
UCDW sea ice
SB PF SAF STF
ACC
Antarctica Antarctic ice sheet
AABW
SST reconstructions
80˚S
60˚S
40˚S
20˚S
90˚W
0˚
90˚E
180˚E Ocean Data View / DIVA
0
5
10
15
20
25
30
80˚S
60˚S
40˚S
20˚S
90˚W
0˚
90˚E
180˚E
Sea surface temperature (°C)
MD11-3357 = SAZ
MD11-3353 = AZ
PF SAF
STF
0 2 4 6 8 10 12 14 16 18
0 20 40 60 80 100 120 140
-450 -440 -430 -420 -410 -400 -390 -380 -370 -360 -350
δ D ( ‰ vs SMOW)
Age (ka)
a)
CO
2
(ppm)
160 180 200 220 240 260 280 300 320
0 20 40 60 80 100 120 140
SST (°C)
Age (ka)
b)
c)
SAZ
AZ
Frontal shifts over glacial–interglacials Strengthening of the ACC during the Holocene
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! Glacial-interglacial SST amplitudes are about 8°C.
Lower SSTs during glacials may have strongly reduced the air-sea buoyancy flux, impeding CO 2 outgassing.
Reduced SST gradients during glacials may indicate a northward shift of the ACC. This would have reduced upwelling intensities and CO 2 outgassing.
During the Holocene, an increasing SST gradient may suggest a strengthening in the ACC flow speed, enhancing upwelling intensities and CO 2 outgassing.
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Figure 1. The ACC and upwelling dynamics in the Southern Ocean.
Both SST records show well defined glacial-interglacial temperature oscillations, allowing for robust intercomparison.
Core top SST values are ±2°C lower than modern annual mean SST (dots on y-axis).
This argues for a bias in our reconstruction towards summer SST, when productivity is highest.
SST reconstruction and atmospheric CO 2 concentration 8 show a high correlation.
Again, this, suggests a strong influence of Southern Ocean SST on CO 2 outgassing due to changes in the air-sea buoyancy flux.
Without paleosalinity reconstructions, we cannot make any assumptions about flow speed changes during cold periods.
SST gradient changes over glacial-interglacials are interpreted as a signal of shifting fronts.
Obliquity may have had a strong impact in modulating the SST gradient.
Reduced SST gradients during cold periods may suggest a northward shift of the ACC, reducing upwelling intensities.
During the MIS 5e, the smallest SST gradient together with high SSTs argues for a drastic southward shift of the ACC, possibly enhancing upwelling.
These ACC dynamics may have an important influence on CO 2 outgassing.
During the Holocene, density is mainly driven by temperature. SST gradients are interpreted as ACC flow speed changes.
Wind strength reconstruction from the Southern Ocean indicate intensifying westerlies over the Holocene 9 .
Nitrogen isotope reconstructions at our AZ core location suggest enhanced nutrient supply over the Holocene 10 .
Together with an increasing SST gradient over the Holocene, this argues for a stronger ACC flow speed, leading to more upwelling.
This argues for a crucial role of ACC strength on CO 2 release to the atmosphere.
Meridional SST gradients were determined by resampling the downcore records on a 2.5 ka resolution.
Age models were determined by aligning the newly generated SST records to the deuterium ice core data from Antarctica 8 .
Glacial-interglacial ACC changes have been hypothesized, but reconstructions remain ambiguous 4, 5 .
The ACC flow causes a net equatorward Ekman transport of surface water, which is replaced by upwelled deep water masses.
Sea surface temperatures (SST) and strength and position influence upwelling intensities and vertical mixing.
To better constrain glacial-interglacial dynamics of the ACC and their impact on upwelling intensities and, ultimately, CO 2 sequestration, we aim at addressing the following research questions:
How much did SSTs vary over the last glacial cycle?
Did the position and/or strength of the ACC vary?
How may these changes have
influenced atmospheric CO 2 ?
