Coupled radiocarbon and neodymium isotopes: Improved reconstructions of deep-ocean circulation change

Climate change resulting from the introduction of greenhouse gases (e.g. carbon dioxide, CO2) into the atmosphere is one of the most significant and pressing challenges facing mankind in the 21st century and beyond. For years, scientists have been developing models of the ocean-atmosphere system, with the aim of predicting how increased CO2 emissions will affect our climate. Before future climatic behaviour can be predicted with any confidence, however, it is necessary to investigate not only how the climate has behaved in the past but also how CO2 has moved between the various natural reservoirs. Using information primarily derived from marine sediments, climate scientists have ascertained that for the last few million years the Earth's climate has undergone dramatic changes, from warm conditions similar to the present day to 'glacial' conditions, when much of the northern hemisphere was covered with ice. These glacial-interglacial cycles are brought about through external forcing by variations in incident solar radiation combined with internal feedbacks due to changes in the atmospheric concentration of the greenhouse gas CO2. Although CO2 is known to be important for past climate change, we still do not know exactly how or why CO2 levels have changed as they have. Climate scientists use radiocarbon (14C), which is produced in the upper atmosphere by cosmic radiation, to understand how and when CO2 has been transferred between the deep ocean, which is the Earth's largest reservoir of CO2 on millennial timescales, and the atmosphere. As a proxy for deep ocean ventilation 14C is incredibly useful but its interpretation is challenging. The utility of this valuable proxy can be greatly enhanced by a suitable 'dye tracer' that can reveal the sourcing and mixing of deep-water masses, both of which can impact the interpretation of 14C data. Here we propose to employ a newly-developed and potentially very powerful tracer of water mass sourcing and circulation, neodymium (Nd) isotopes. A recent pioneering study has demonstrated that benthic foraminifera are a useful archive of past seawater Nd isotope composition, and that this represents a very promising avenue for further development. We aim to conduct the necessary research to fully demonstrate just how effective this tracer can be, particularly when combined with 14C. An initial investigation will be followed by two case studies that will combine this exciting new tracer with 14C data from two deep-sea sediment cores. We aim to use these data to understand how 14C (and hence CO2) has moved between the atmosphere and the deep oceans over the last 20,000 years, and how this has been influenced by deep ocean circulatory patterns.