Professor Luke Skinner

Earth System Dynamics, Ocean Circulation and Biogeochemical cycling

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Zonally averaged ocean-atmosphere radiocarbon offsets (B-Atm) for the Last Glacial Maximum and modern seawater (Key et al., 2004), showing a clear reduction in 'ventilation' of the glacial ocean (see Skinner et al., Clim Past, 2023).  

My research looks at the role of the ocean circulation in past climate change, including in particular its impact on the hydrological- and biogeochemical cycles.  Ultimately this work aims to provide a geological perspective on the climate system and its dynamics on relatively short time-scales (10-10⁴ yrs); time-scales that are relevant to past and future human activities. I use a range of geochemical, sedimentological and micropalaeontological methods to infer past hydrographic changes (including stable and radiogenic isotope and trace element analyses).  Ongoing research projects include investigations of: 1) the causes and consequences of abrupt oceanographic and climate change on the southern Iberian Margin during the late Pleistocene; 2) the deep ocean’s temperature, oxygenation and carbonate ion response to rapid climate change during MIS 3 and MIS 6 (and its link to atmospheric CO₂); 3) laser-ablation analysis techniques to understand rare earth element and redox sensitive element cycling; 4) the role of the ocean circulation in glacial-interglacial CO₂ change and the evolution of the marine radiocarbon inventory across the last deglaciation; and 5) the hydraulics of the last Lake Agassiz ‘super-flood’. 

Some aspects of my research are outlined below. More details can be found at this webpage.

Glacial-interglacial climate change

The dominant mode of climate variabiity during the Quaternary/Pleistocene has been characterised by ~100,000 year oscillations between glacial- and interglacial climate states.  It seems clear that these climate cycles have been paced by changes in the seasonality of solar radiation (i.e. insolation), but it is also clear that this pacing was made possible, and was amplified by strong positive feedbacks internal to the climate system.  Albedo- (ice sheet) and carbon cycle (i.e. carbon dioxide) feedbacks appear to have been foremost amongst the contributing feedbacks.  Despite over a century of research, it remains unclear exactly how these feedbacks were triggered and coordinated by insolation changes.  Ongoing research is seeking to address just this question, and to use the ‘zoo’ of past glacial and interglacial climate states as a natural laboratory in which to investigate the context dependence of climate feedbacks, which ultimately must underpin the non-linearity that Quaternary glacial-interglacial dynamics exemplify.

Abrupt climate change

The recovery of very high-resolution records of climate variability from the polar ice-sheets, lakes, speleothems and high sedimentation-rate marine sediments has fundamentally changed our perspective on global climate change by revealing the existence of very rapid and intense climate changes in the past. Such records serve to underline the fact that significant climatic transitions may arise from the interaction of parameters that are completely internal to the climate system, emphasizing the importance of feedbacks in the climate system.  One key aspect of the abrupt climate variability recorded during past glacial periods is its ‘bi-polar’ pattern, with abrupt climate swings in the North Atlantic region coincident with more gradual and subdued changes over Antarctica. The canonical explanation for this inter-hemispheric coupling is the ‘bipolar seesaw’ mechanism, whereby changes in the strength of the Atlantic overturning circulation result in changes in the interhemispheric heat transport. A sudden reduction of the overturning circulation in the North Atlantic (triggered by a massive iceberg/freshwater release for example) is thus thought to result in a reduction in the northward heat transport to the Northern Hemisphere and a concomitant increase in the southward heat transport to the Southern Hemisphere. The transport of this ‘excess’ heat across the Southern Ocean to Antarctica is presumed to take some time, hence the slower response of Antarctica. The bipolar seesaw mechanism is a neat and highly explanatory theory. It builds on ideas of hysteresis and bi-stabilty in the overturning circulation. However, much remains to be learned about how well this theory fits with reality, what it tells us about the stability of the ocean circulation, and how its behaviour might depend on background climate conditions and/or the character of forcing applied. Current research is looking at the response of the tropical- and North Atlantic, and the Southern Ocean, to these abrupt climate swings, as well as the existence and character of rapid climate change during different climatic regimes, including interglacial (warm) climates like today’s.

Radiocarbon and past carbon cycle change

By measuring the radiocarbon age of bottom-dwelling foraminifera, and comparing this with the radiocarbon content of the contemporary atmosphere, it is possible to infer past changes in the exchange of carbon dioxide between the ocean and the atmosphere. Radiocarbon measurements from the deep North Atlantic have underlined the role of the overturning circulation in effecting rapid and intense changes in regional climate and carbon cycling across the Bolling-Allerod and the Younger Dryas events  (~11-15,000 years BP), and further data from around the globe have confirmed an increase in the mean residence time of carbon dioxide dissolved in the deep glacial ocean. Ongoing work is seeking to combine these observations with auxiliary proxies that may help to quantify the impact on the marine respired carbon inventory, and therefore atmospheric CO2.

The Marine Isotope Stage stratigraphy

On multi-millennial timescales the marine oxygen isotope record may provide a powerful stratigraphic reference of global relevance. However, given the variability of temperature and water-δ¹⁸O in the world's oceans (the distribution of which is controlled by the ocean circulation and hence climate), it need not be globally synchronous on 'sub-orbital' timescales. Deep-water temperature estimates across glacial Terminations, in different ocean basins, indicate that the development of our stratigraphic systems at the high temporal resolution that is now necessary is inherently bound up with the very climate changes that we use those stratigraphic systems to study. This presents a serious challenge that will have to be met if we are to make full use of the globally distributed high-resolution records that now exist, and if we are to eventually solve the 'mystery' of insolation- and carbon dioxide forcing on long-term climate evolution.

The last Lake Agassiz superfood, and the '8.2 kyr event'

Late Quaternary deposits in the Moose River Basin (James Bay Lowlands) area of northern Ontario (Canada) consist of a succession of glacial tills with interceding interglacial deposits, which culminate in a sequence that corresponds to the last glacial cycle. At the top pf this sequence is a ‘deglacial succession’ that records the demise of a vast pro-glacial lake (roughly the size of England) that had pooled against the dwindling Laurentide ice-sheet.  Ongoing work is targeting reconstructions of the hydraulic and sedimentary conditions that led to the deposition of the ‘flood deposits’ , as well as the occurrence of sudden changes in the provenance of siliciclastics incorporated into these deposits.