Rapid climate change in the glacial North Atlantic : new insights from geochemical records of temperature, CO₂, redox and circulation

Abstract

Rapid climate change events of the last glacial period are a prime example of the climate system’s ability to flip quickly between different states. These are associated with abrupt shifts in temperature, productivity and CO₂ and are commonly linked with major changes in circulation and sea-ice coverage in the high latitude North Atlantic Ocean. However, despite what is an increasingly well-established paradigm, major questions remain about the links between circulation, temperature, ice-sheet stability and CO₂ storage. This thesis presents a collection of new, high resolution records from Northeast Atlantic core ODP 980, which develop our understanding of the region’s role in rapid climate change.

Analysis of two robust paleo-temperature proxies (percentage abundance and Mg/Ca of Neogloboquadrina pachyderma) reveals the first marine evidence that Heinrich stadials were intervals of extreme seasonality, characterised by very cold winters and increasingly warm summers. These rising temperatures which are attributed to weak Atlantic Meridional Overturning circulation (AMOC), climbing atmospheric CO₂ and the insulating effect of sea ice, support theories for ice-sheet destabilisation in an otherwise cold climate. High resolution boron isotope data also resolve major perturbations of CO₂ in the surface ocean at climate transitions. These show that the North Atlantic CO₂ sink, which persists during stadial periods, weakened dramatically at interstadial onset, likely maintaining elevated atmospheric CO₂ and uncovering a new role for the region in rapid climate change.

Recent studies using authigenic coatings on foraminifera, have employed redox sensitive elements to reconstruct deep water oxygen concentrations. However, data from different oceanic settings, combined with a geochemical model, highlight the sensitivity of element enrichment to changes in surface productivity. The influence of sediment composition is also examined with regards to authigenic Nd/Mn ratios, which show exciting promise as a new indicator of AMOC strength. Together these authigenic data offer new opportunities for paleoclimate reconstruction.