Ocean-land response to freshwater forcing in the North Atlantic: Using the 8.2kyr Event to understand future climate change.

Overview

The transformation of warm and salty surface waters in the North Atlantic into cold and dense deep waters, plays a key role in regional and global climate change. This process, referred to as the Atlantic Meridional Overturning Circulation (AMOC), is crucial for the re-distribution of heat, nutrients and gasses across the globe. For example, in the North Atlantic it contributes towards the northward transport of heat to higher latitudes, which helps maintain the relatively mild climate in NW Europe.

Over the last 60 years, melting of Greenland and Arctic ice has increased the flux of freshwater reaching the North Atlantic [Bamber et al., 2018]. Controversy remains on whether this continued increase in freshwater fluxes will result in a future collapse or slowdown of the AMOC [Stouffer et al., 2006], or if it may already be underway [Thornalley et al., 2018]. This AMOC weakening would initially cool the northern hemisphere and particularly the regions in and around Greenland [Rahmstorf et al., 2002].

The outbursts of glacial lakes Agassiz and Ojibway around 8200 years ago provides the best known analogue to study the response of the AMOC to freshwater forcing and its potential impacts on the North Atlantic and global climate in the near future. The freshwater input from glacial lakes 8200 years ago, has been hypothesized to have severely reduced the production of deepwater formation producing a regional and even global cooling [Rohling, and Pälike 2005]. This included the temporary regrowth of the Greenland Ice Sheet in response to regional cooling (Long et al., 2006). However, there is sparse and often conflicting evidence for a weakening of the AMOC and its impact on Greenland temperatures. This interdisciplinary project aims to investigate the AMOC collapse and its climatic and cryospheric consequences during this event using a suite of terrestrial, shelf and marine sediment archives from around Greenland.

Surface and deep water reconstructions from a decadally resolved marine sediment core in the Labrador Sea [Moffa-Sanchez et al., 2014, 2015] will be used to study the surface and deep ocean response in this key deepwater formation region across the 8.2kyr event. Onshore lake cores will be used to establish pan ice sheet and terrestrial system response to regional cooling [Long et al., 2006; Woodroffe et al., 2014].

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Image Captions

Photo taken from the Greenland coast (S. Woodroffe)

Methodology

The student will use a suite of paleoclimatological proxies on the different archives and generate climate reconstructions of sea surface temperature, sea-ice and ocean current flow speed. In the marine reconstructions these will include foraminiferal counts and geochemical analyses of foraminiferal carbonate and organic compounds. The deep water flow speed records will be produced by using grain size analysis. For terrestrial environmental reconstructions multi proxy analysis on lake sediments will be carried out and will include sediment properties and microfossil assemblages.

Project Timeline

Year 1

Review of existing literature relating to the 8.2kyr event, the AMOC and its response to freshwater forcing and the North Atlantic climate. Laboratory training for the sampling and processing of marine and lake sediment samples for micropaleontological identification and grain size analysis.

Year 2

Preparation and analysis of organic and inorganic chemistry including oxygen isotopes, organic compounds (IP25 and C37). Data analysis and interpretation of the datasets. Presentation of the results at the 14th International Conference in Paleoceanography (Bergen, September 2022). Production of paper I.

Year 3

Continued data analysis and interpretation and comparison between the ocean-land records. Production of paper II.

Year 3.5

Interpretation of the datasets, including comparison between the ocean-terrestrial records and comparison to other published data. Presentation at EGU (Vienna, May 2024). Production of paper III

Training
& Skills

This project will provide the student with a wide range of skills in climate reconstructions using proxies from ocean and terrestrial archives, including geochemical, micropaleontological and physical proxies. The student will be trained on micropaleontological indentification, and geochemical analysis in marine and lake sediments. In addition, the student will receive training on core chronology development using different mathematical approaches. Participation on a research cruise will be sought and opportunities for data and climate model integration may be pursued. The student will also benefit from the research environment at the Department of Geography in Durham by becoming part of the Ice-Sheet and Sea Level group.

References & further reading

Bamber, Tedstone, King, Howat, Enderlin, van den Broeke, and Noel (2018), Land Ice Freshwater Budget of the Arctic and North Atlantic Oceans: 1. Data, Methods, and Results, 123(3), 1827-1837.
Long, A.J., Roberts, D.H., Dawson, S., 2006. Early Holocene history of the West Greenland Ice Sheet and the GH-8.2 event. Quaternary Sci Rev 25, 904-922.
Moffa-Sanchez, Hall, Thornalley, Barker, and Stewart (2015), Changes in the strength of the Nordic Seas Overflows over the past 3000 years, Quaternary Science Reviews, 123, 134-143.
Moffa-Sanchez, Hall, Barker, Thornalley, and Yashayaev (2014), Surface changes in the eastern Labrador Sea around the onset of the Little Ice Age, 29(3), 160-175.
Rahmstorf, S., 2002: Ocean circulation and climate during the past 120, 000 years. Nature, 419, 207-214.
Rohling, and Palike (2005), Centennial-scale climate cooling with a sudden cold event around 8,200 years ago, Nature, 434(7036), 975-979.
Thornalley, et al. (2018), Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years, Nature, 556(7700), 227-230.
Woodroffe SA, Long AJ, Lecavalier BS, et al. Using relative sea-level data to constrain the deglacial and Holocene history of southern Greenland. Quat Sci Rev. 2014;92:345-56.

Further Information

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