Research

Research in the Petersen Group (SCIPP Lab) focuses on reconstructing past climate using geochemical tools, in particular, stable and clumped isotopes of carbonate fossils. We have ongoing projects focusing on reconstructing climate in the Cretaceous, Miocene, Pleistocene and other time periods, looking at how climate has changed on million-year down to sub-annual timescales.

Here are some ongoing topics of focus:

Interglacial Climate in the Atlantic and Caribbean

The Last Interglacial (LIG) interval (~125,000 years ago) is the most recent time that global climate was warmer than today. Understanding how much warmer it was, and the spatial pattern of that warming, is relevant for our future under anthropogenic global warming. In particular, how much warmer was it in the western Atlantic, along the U.S. East Coast, and in the Caribbean? Our group has a funded NSF grant to answer this question by studying the geochemistry of fossil shells from Bermuda and multiple sites in the Carribbean and along the U.S. East Coast. This project is in collaboration with Dr. Kacey Lohmann (UMich) and Dr. Ian Winkelstern (GVSU).

[For prospective students: this project involves field work to exciting (tropical!) locations, followed by lab work to analyze fossils and water samples we’ve collected.]

Related papers:

Cretaceous-Paleogene Mass Extinction

Roughly 250 kyr before the end of the Cretaceous, the Deccan Traps in India began erupting an enormous amount of lava and spewing CO2 and other volatiles into the atmosphere and warming the planet. This large, but slow-developing environmental change was punctuated by the Chicxulub meteorite impact, which dealt a devastating blow to life on earth. In total, the end-Cretaceous mass extinction wiped out ~75% of species. Debate continues about the relative contribution of the Deccan Traps vs. the Chicxulub meteorite to the overall extinction.

In the Petersen group, we are working on tracking climate change leading up to the Cretaceous-Paleogene boundary, especially warming resulting from Deccan Traps CO2, looking at sites both near and far from the volcano. We have an NSF funded project in collaboration with a group of paleontologists to reconstruct ocean conditions in the Gulf Coastal Plain region across this boundary in support of ecological modeling studies. We also continue to study other new sites globally, such as the type section of the Maastrichtian interval in Maastricht, Netherlands.

[For prospective students: this project involves optional field work in the Gulf Coast region, as well as lots of lab work to analyze fossils we and our collaborators have collected in the field, or gathered from museum collections.]

Related papers:

Paleo-Seasonality

Political and societal discussions of modern climate change focus primarily on how global mean annual temperatures are increasing from year to year. However, for most organisms, the day-to-day and season-to-season experience, not the long-term average climate state, determines their survival or extinction, their success or failure. Relatively few paleoclimate archives have temporal resolution sufficient to identify seasonal-scale temperature changes or the longevity to preserve this information from ancient greenhouse climate intervals.

Mollusk shells represent a unique archive that is both capable of seasonal resolution and is preserved into the deep past. Mollusks precipitate their calcium carbonate shells in annual, monthly, and even daily growth increments, and their skeletons can survive over 100s of millions of years, given the right conditions. We use sclerochronology (sub-annual isotopic sampling of mollusk shells) to reconstruct past seasonal variations in temperature and δ18Oseawater. We have ongoing projects calibrating this approach using modern bivalves and gastropods, as well as paleoclimate applications using fossil shells from multiple time periods.

We recently collaborated with an international team to reconstruct tropical  temperature seasonality in during the Mid-Miocene Climatic Optimum (MMCO, ~15-17Ma). We are continuing this work with a modern calibration study sampling fast-growing, shallow-dwelling marine gastropods (Turritella sp.) and relating their sub-annual isotopic records to local tempeature seasonality and precipitation variance (diff. btw. wettest and driest months).

The Pliocene is often thought of as a good analog for future climate due to elevated CO2 levels similar to end-century expectations. We are working with Dr. Andy Johnson (Derby Univ, UK) to study scallop shells from Virginia, the Carolinas, and Florida from the warm Pliocene period. We sample shells at both highest and lowest δ18Ocarb values to assess past seasonal ranges in sea surface temperatures.

[For prospective students: We are pioneering new methods to reveal subannual climate that can be applied to any time period, and many fossil mollusk taxa. New directions galore!]

Related papers:

Cretaceous Hothouse Climates

Global temperatures during the Cretaceous period (66-144 Ma) were 6-14°C warmer than today [Barron, 1983] and atmospheric CO2 levels were 400-1500 ppm [Hong and Lee, 2012]. Therefore, this period can act as a climate analog for temperatures and CO2 levels we are expected to reach in the coming 1-2 centuries. The presence of crocodiles and palm trees in Canada [Manchester et al., 2010] and forests on the Antarctic Peninsula [Francis & Poole, 2002] suggest the poles were much warmer than today, but data is murkier when it comes to tropical and mid-latitude warming. Climate reconstructions disagree on the magnitude and spatial pattern of the warming [Hay, 2008 and references therein]. Was the equator-to-pole temperature gradient similar to today? Flatter? Was it warmer everywhere? Or warmer at the poles but similar to modern in the tropics?

We are applying the clumped isotope paleothermometer to revisit the question of equator-to-pole temperature gradient in the Cretaceous period. We are first gathering Maastrichtian-aged bivalve and gastropod shell fossils from as many locations around the world as possible to construct a time slice of the global temperature pattern for the very end of the Cretaceous. Our next target is the Cenomanian/Turonian interval of peak greenhouse conditions.

Technical Advancements in Clumped Isotope Paleothermometry

We are constantly trying to improve the clumped isotope measurement and data processing techniques we use. This includes things like developing methods to reduce the sample size requirements for a single measurement or documenting isotopic fractionations induced by having a Porapak trap that is too cold. We have ongoing projects looking at the effects of [SO4–] levels on D47 (w/Itay Halevy), and the impacts of different types of contaminants on the D47 and D48 signal (w/Ben Passey).

We recently completed an 11-lab compilation and reprocessing of published clumped isotope calibration data using updated isotopic parameters (R18_VSMOW, R17_VSMOW, R13_VPDB, and lambda). This effort spawned the ‘ClumpDB’, a permanent data archive for clumped isotope data, for which we developed a universal template. This will hopefully facilitate future clumped isotope data compilation studies.

With the benefit of having multiple different models of clumped-capable mass spectrometers (ThermoMAT253 and Nu Perspective+NuCarb in our lab as well as Nu Perspective + “Passey device” in the IsoPaleo Lab upstairs), we plan future inter-instrument comparisons. We also welcome inter-lab comparisons.

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