Research Interests

My research interests focus on understanding the role of the ocean, and in particular the role of marine carbonate chemistry in global climate change. As I was originally trained as a (marine) biologist, my way of approaching paleoceanographic questions often includes a biological component. Coretop observations on sub-fossil specimens and culture experiments with living marine calcifiers form an important aspect of my research and help me to better understand the proxies used for paleoreconstructions. I am specifically interested in estimating past seawater-pH and atmospheric CO2.

Reconstructing paleo-seawater acidity and paleobarometry (i.e. paleo-CO2)

Boron isotopes in marine carbonates have the potential to provide us with information about past ocean carbonate chemistry, as the boron isotopic composition of marine carbonates is primarily controlled by the pH of seawater. When combined with a second parameter of the marine carbonate system, surface ocean PCO2 can be calculated from this proxy (see e-book on Boron Proxies in Paleoceanography and Paleoclimatology for a detailed explanation of the proxy and procedures). Application of the boron isotope pH proxy to the late Pleistocene glacial cycles has led to a convincing estimation of surface ocean pH and atmospheric CO2 as recorded in ice cores (Hönisch & Hemming, 2005), and laid the foundation for reconstructing paleo-acidity and paleobarometry millions of years back in time. Reconstructions of past periods of elevated atmospheric CO2 help scientists to gain a better understanding of the linkage between atmospheric CO2 and global climate, and the study of past acidification events such as the Paleocene hyperthermals allows us to gauge the environmental and ecological consequences of rapid CO2 emissions similar to the Anthropocene. I am also leading a team of experts on terrestrial and marine paleo-CO2 proxies to create a paleo-CO2 database. For more information on paleo-CO2, please see our website paleo-CO2.org.

Current projects:

1. •Creating A Scientifically Rigorous and Accessible CO2 ‘Keeling Curve’ for Geologic Time; co-PIs: Vicki Ferrini and Pratigya Polissar
   

2. •Validating boron isotope systematics in extinct planktic foraminifera and reconstructing Cenozoic CO2; co-PI: Andy Ridgwell  
   

3. •Reconstructing ocean acidification and environmental changes across Eocene Thermal Maximum 2 - An Eocene perspective on future recovery rates of climate and ocean chemistry; co-PIs: Jim Zachos, Richard Zeebe
   

4. •Thermohaline Circulation and Deep Ocean Carbonate Chemistry across the Mid-Pleistocene Transition; PI and co-PIs: Leo Pena, Steve Goldstein, Maureen Raymo
   

5. •Reconstructing Arctic Ocean acidification during past interglacials from boron isotopes in benthic foraminifera; co-PI: Jesse Farmer
   

Culture experiments with living planktic foraminifers

 

Boron isotopes alone provide us with only one parameter (i.e. pH) of the marine carbonate system. For accurately translating boron isotope data into pH and subsequently calculation of other parameters of the carbonate system such as aqueous PCO2, we need additional information on temperature, salinity and a second carbonate parameter such as alkalinity. I therefore often complement my data with temperature estimates from Mg/Ca ratios and salinity estimates from oxygen isotopes. Understanding the limitations of these auxiliary proxies is just as important as validating the boron isotope proxy itself. Because ocean temperature, carbonate chemistry and salinity often change in parallel, the relative effects of these parameters on a proxy can best be studied in laboratory culture experiments, where single parameters can be studied in isolation. These experiments are typically carried out on Catalina Island, where the subtropical to temperate species Orbulina universa and Globigerina bulloides occur, or in Puerto Rico, where tropical conditions allow the study of Globigerinoides sacculifer and G. ruber. While testing fundamental controls on proxies under modern seawater conditions remains an important component of this work, our most recent experiments also explore the effects of changing seawater elemental composition on geochemical proxies, and we find that proxy sensitivities in, e.g., simulated Paleocene seawater change significantly compared to modern proxy calibrations (e.g., Haynes et al. 2019, Holland et al., in revision).

Current projects:

1. •Testing geochemical proxy relationships under variable paleo-seawater chemical compositions