Paleoclimatological Reality Check: Multi-Proxy Comparisons

In addition to alkenone paleothermometry many other paleoclimate indicators, so called proxies, have been used to characterize past climate. Many of these proxies make use of the shells of different kinds of tiny plants and animals that live in the ocean. By characterizing the distribution of these organisms relative to modern distributions or by examining the geochemical composition of their shells, scientists are able to produce estimates of past temperature, sea ice extent, productivity, continental ice volume, salinity, etc. How reliable are these paleoclimate proxies? Since it is impossible to travel back in time to compare the paleoclimate estimates produced by these proxies with actual conditions at some time in the past, paleoclimatologists employ what are called multi-proxy studies, in which multiple different proxies that are derived from different organisms but purportedly yield the same kinds of paleoclimate estimates are employed to study the same exact time period in the same exact location. If the records derived from these proxies, yield the same estimate of what the past climate state was like, that provides validation for these paleoclimate indicators. If, however, the results yield different answers, then we need to further explore what might give rise to the inconsistency in the estimates.

I am involved in a number of collaborations that employ this multi-proxy approach. In a study recently published in Paleoceanography, which was lead by Dr. Marci Robinson and Dr. Harry Dowsett from the US Geologic Survey, we compared estimates from three different surface temperature proxies from North Atlantic Pliocene sediments and found that the three techniques produced very similar results. In a collaboration with Ph.D. student Beth Caissie and her advisor Dr. Julie Brigham-Grette from the University of Massachusetts, Amherst, we examined the history of sea ice in the North Pacific during the last glacial period. Beth’s data characterizing the distribution of sea ice-loving organisms were corroborated by the alkenone productivity and temperature records I produced from the same sediments. Our results were published in Paleoceanography in 2010. To explore the robustness of the qualitative productivity records I generate, I have also collaborated with Dr. Clara Bolton formerly a Ph.D. student of Dr. Paul Wilson and Dr. Samantha Gibbs at the University of Southampton in the UK. Clara examined past change in ocean productivity by looking at the distribution of the shells of primary producers that are preserved on the sea floor. Her results indicate remarkable similarities between my alkenone-derived records and her floral records, which provides strong validation for the use of our alkenone-derived proxy for qualitatively estimating past ocean productivity. Our results have been published in both Paleoceanography and Earth and Planetary Science Letters.