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Paleoceanography

Rapid Reductions in North Atlantic Deep Water during the Peak of the Last Interglacial Period

Article: Eirik Vinje Galaasen et al. Science 343, 1129 (2014); DOI: 10.1126/science.1248667

Introduction

Fig 1. Ocean Circulation

Fig 1. Ocean Circulation

North Atlantic deep water forms primarily in more extreme northern latitudes due to the colder, saltier water with a higher density. When this flow of water goes south it mixes with the cold Antarctic water and then redistributes into other parts of the world (Fig 1). As high latitude warming and ocean refreshing reduce water density, North Atlantic Deep Water (NADW) formation can be prohibited.
Based on model prediction, NADW ventilation is a robust feature in the present interglacial age. Only modest NADW variability has been found during interglacial ages compared to colder glacial ages. We are far from reaching a threshold over which the NADW formation could be stopped. However, large but short transient time may be possible even during a stable NADW forming period, such as an abrupt decadal change within a generally stable millennial-scale period. In these cases, the stability threshold could be crossed and the North Atlantic reaches a steady state wherein no NADW formation happens.

Method

Fig 2. Locations of core sites.

Fig 2. Locations of core sites.

Sediment cores from the Eirik Drift were used to study centennial-scale variability in NADW over the warm interval of the last interglacial period (Fig 2). This period of time helps to understand impacts from key features on NADW including warming, ocean refreshing and the retreat of the North Atlantic ice sheet.

This site is great in that it has the perfect sedimentation rate allowing a long term study of water properties flowing over. It is located in the North Atlantic near the Arctic, where water from Nordic Seas flows.

Fig 3. Deep-water property changes during the last interglacial time.

Fig 3. Deep-water property changes during the last interglacial time.

 

Geographic patterns seen in the fossil records of planktonic forams can be used to reconstruct ancient ocean currents. We use carbon isotopic composition values to deduce values for bottom water. It is an excellent method to distinguish high NADW and low  southern ocean sourced bottom water (SSW). Interestingly, on a short time scale there were several sharp decreases in  (Fig 3).

Fig 4. Proxy records spanning the LIGn(116.1 to 128.0 ky) section of core.

Fig 4. Proxy records spanning the LIGn(116.1 to 128.0 ky) section of core.

Results

These decreases suggest a relationship with freshwater forcing. The minimum value at 124 ky B.P.(before the present) is consistent with reduced NADW production. SSW came in as a compensation for reduced NADW could explain the small value. Furthermore, the increasing anomalies at early stage around 124 ky B.P. shows some links to glacial melting and freshwater input. During forams calcification (Calcium Carbonate formation) some Barium may enter the shell as well due to its similarity as Calcium. By studying the Barium to Calcium ratio we can get properties of the bottom water when shells are formed. Glacial melting water is rich in Barium and causes a higher Ba/Ca value. Around 12 ky B.P. the Ba/Ca value reaches peaks indicating inputs from icebergs (Fig 4). Further supporting a role for freshwater triggering the deep water anomalies, a flood event happened early in the period of study is followed immediately by the largest and longest NADW reduction of the period.

 

Implications

Results from this study call for a reevaluation for the notion that NADW formation is rather stable and vigorous during interglacial time. Sharp changes in short period of time could lead to disturbance to NADW formation, resulting in a change in global heat transport by ocean currents. This causes widespread and long-lasting impacts on our climate.

Caoxin Sun
Caoxin is a graduate student in the Graduate School of Oceanography at the University of Rhode Island. Her research interest lies in persistent organic pollutants in the environment. When she is not doing research she likes to create new cuisines.

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