I am fundamentally interested in how and why the Earth’s climate has changed through time. The geologic record is full of materials that allow us to probe how climate has changed through time.
Answering why climate has changed through time is a much more difficult question because it requires that we not only identify what changes occurred in different components of the climate system (e.g. ice sheets, biosphere, oceans, land surface), but when they occurred relative to each other. By understanding the sequence of events associated with a climatic change, we can begin to isolate what driving mechanisms were responsible for the changes that have occurred in the past and also gain a better sense of how the climate system works as a whole.
Because oceans cover approximately 70 percent of the Earth’s surface, they are a good place to start in terms of characterizing Earth’s climate. Furthermore, large quantities of sediment continuously accumulate on the seafloor, providing an extensive and, importantly, nearly continuous record of the climatic changes that have occurred during recent Earth history. In fact, most of what we know about Earth’s climatic history for the past ~70 million years comes from studying ocean sediments. Using ocean sediment cores and the remnants of ocean organisms that are preserved in them, I produce quantitative estimates of past changes in ocean surface temperature and qualitative reconstructions of past ocean biologic productivity.
My work focuses on the study of climatic change over the past 5 million years. This interval witnesses the last major climate transition in Earth history in which the Earth moved from a relatively warm and stable state with little or no northern hemisphere ice during the Pliocene period, to a colder, more variable state in which large ice sheets expanded across continents in the northern hemisphere, the Pleistocene. My interests in the so called Plio-Pleistocene are threefold. First, because this transition is the last big change the Earth’s climate system has experienced, it offers an opportunity to examine the dynamics of Earth’s climate as it moves between warm and cold states, providing insight into not only the nature and variability of climate in those two endmember states, but also into the natural mechanisms that drive climatic change. Second, the Pliocene has been used as an analog for future warm climate scenarios. The Intergovernmental Panel on Climate Change, a group of scientists who have been asked by the UN to formally assess future changes in Earth’s climate and the role of human activities in changing the climate, projects that global mean annual temperature in the year 2100 will be 3ºC warmer than modern. The last time in Earth history global mean annual temperature was 3ºC warmer than modern was during the Pliocene. Lastly, during this interval the human lineage evolved in east Africa. I am interested in the role climate may have played in those biological changes.
While there are numerous climate records that span the Plio-Pleistocene transition, my contribution has been to apply a technique (alkenone paleothermometry), that had previously been applied primarily to studying more recent climatic changes, to this older transition. Alkenone paleothermometry involves using lipid (fat) compounds, alkenones, that are produced by a few species of ocean surface dwelling algae who make these compounds in a way that is sensitive to temperature. These organisms make more of one kind of fat and less of another when the ocean waters they live in are warm, and visa versa when the waters they live in are cold. By measuring the relative abundance of alkenones preserved in the different layers of sediment that accumulate on the ocean floor (oldest on bottom, youngest on top), I can estimate what the past temperature at the ocean surface was. By measuring the amount of these lipids present in the sediments, I can qualitatively measure past changes in ocean surface productivity. The climate records I have produced and that my students and I are currently producing are novel and thus have been of great interest within the Pliocene climate community, as they allow new and exciting insights into understanding the Plio-Pleistocene climate transition.