Neodymium isotopes in benthic foraminifera as robust tracers of ocean circulation

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences


The Poles receive much less heat from the Sun than the Equator. This results in an Equator-Pole temperature gradient that causes convection in the two fluid reservoirs at the surface of the Earth - the atmosphere and the ocean, transporting heat away from the Equator and towards the Poles. The Atlantic portion of the oceanic convection system is particularly important. Here, the Gulf Stream carries warm, salty water from low latitudes to the North Atlantic. There, it circulates in the cold sub-polar NE Atlantic surface ocean, cooling and losing heat to the atmosphere. This warming of the atmosphere by the surface ocean is very important to the temperate climate of Britain. As the salty waters of the Gulf Stream cool they become dense (salty and cold water is denser than fresher, warm water) and this results in sinking of this water from the surface. Several sinking water masses coalesce to form North Atlantic Deep Water (NADW), a cold and relatively saline water mass, which then flows southward at depth and forms the return arm of the convection cell. Scientists have devised various means to monitor the past environmental conditions on the Earth, principally using information trapped in deep-sea sediments. These records have established that the Earth has undergone dramatic swings in its climate over the past 2-3 Myr, from relatively warm states like the present-day 'interglacial' period to much colder conditions during 'glacial' periods, when much of North America and Europe were covered by large ice-caps. It is also reasonably well-established that these swings in climate were predominantly caused by small changes in the way the Earth orbits the Sun - thus changing the seasonal distribution of heat supply to the Earth. However, within and towards the ends of the glacial periods, there are very rapid changes in climate (at 10-1000 year timescales) that are too fast to be caused by changes in the orbit of the Earth. These rapid changes are particularly striking in records of North Atlantic climate and may be related to variations in strength of the Atlantic oceanic convection system, and to the efficiency with which it transports heat northwards. These variations are thought to be controlled by changes in the rate of freshwater input to the North Atlantic - from melting of ice or from rivers - which freshens the surface ocean, lowers its density and prevents surface waters from sinking. This process also weakens the northward flow of warm surface waters and has been the subject of much public interest recently in the light of the possibility that increased river discharge and/or melting of the Greenland ice-cap could dramatically freshen the surface North Atlantic. Our aim in this proposal is to develop a new tool to investigate the behaviour of the circulation during rapid climate changes. Models show that during times when NADW was weak, the deep Atlantic should be filled instead by water derived from the southern hemisphere. The main traditional tracer of this process - stable carbon isotopes - are ambiguous because they are dominated by changes in the way carbon is partitioned between various reservoirs on the Earth and are compromised by biological artefacts. Our focus here is a more newly-developed tracer, neodymium (Nd) isotopes. Our aim is to investigate a more robust substrate (benthic foraminifera) for recording Nd isotopes in the deep ocean that those heretofore available. We also aim to resolve ambiguities pertaining to the application of Nd isotopes to the study of the intensity of deep water export from the North Atlantic by building a record of the evolution of neodymium isotope compositions in the deep North Atlantic. This record can then be used as a template for the interpretation of Southern Ocean Nd isotopes in terms of either greater water export from the deep North Atlantic or changes in the Nd isotope composition of the North Atlantic itself.