UCL Earth Sciences


Zarina Hewett

"It is possible that the North Atlantic will reach a ‘tipping point’ induced by freshwater entering the surface ocean from the melting of Arctic ice and rainfall leading to abrupt cold climate event."

PhD project title:

North Atlantic circulation during past abrupt climate events in the Holocene.

 Project description:

The North Atlantic is an important region for ocean circulation and, by extension, global climate. For example, Europe’s relatively temperate climate depends on the northward transport of warm surface waters in the North Atlantic.  As these warm waters enter the high latitudes of the North Atlantic and the Nordic Seas, they lose their heat to the atmosphere and form dense waters that sink into the ocean interior. As these dense waters circulate, they ventilate the deep ocean and, crucially, they form an important component of the North Atlantic Overturning Circulation (AMOC), known as North Atlantic Deep Water (NADW). There is some concern that AMOC will weaken as a result of anthropogenic climate change, and some observational evidence has suggested that this might already be happening. It is also possible that the North Atlantic will reach a ‘tipping point’ induced by freshwater entering the surface ocean via a combination of the melting of arctic ice and increased rainfall, leading to an abrupt and widespread cold climate event. Similar abrupt climate events have happened in the past and have been recorded in natural archives such as lake sediments, speleothems, and the Greenland Ice cores. Some of the events recorded in these archives have also been related to changes recorded in deep sea archives in the North Atlantic, though the extent to which the North Atlantic plays a role in these events, and the mechanisms involved, are not fully understood.
My project will focus on the North Atlantic’s role in some of the key abrupt climate events of the Holocene (the last ~11700 years), and whether changes to NADW production or circulation during these events. In particular, I will be analysing seafloor sediment archives to study deep ocean circulation changes. Key water masses (such as NADW and its components) will be traced using properties such as the neodymium isotopic content, as well as stable isotope ratios δ13C and δ18O. The circulation vigour of these water masses will also be traced by the measurement of the mean size of the sortable silt (10μm-63μm) fraction. Combined, these palaeoceanographic tracers will help constrain the role of AMOC in past periods of abrupt climate change, which are increasingly likely under current climate projections.