Research Interests

My research interests are focused 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 pCO2.

Paleo seawater-pH

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. 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 pCO2 as recorded in ice cores (Hönisch and Hemming, 2005), and later studies estimated atmospheric CO2 back into the Pliocene (Hönisch et al. 2009, Bartoli et al. 2011). My current work focuses on the Paleocene-Eocene Thermal Maximum (PETM), where we estimate the magnitude of surface ocean acidification was similar to the extent of anthropogenic acidification projected for the end of the 21st century (Penman et al., 2014).

Current projects:

Establishing The Magnitude Of Sea-Surface Acidification During the Paleocene-Eocene Thermal Maximum; co-PIs: Jim Zachos, Richard Zeebe

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

Boron isotope measurements on Arctic ocean benthic foraminifers; co-PI: Jesse Farmer

Proxy validation

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 values and subsequently for calculating other parameters of the carbonate system such as aqueous PCO2, we need additional information on temperature, salinity and a second carbonate parameter such as carbonate ion concentration or 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 unison, the relative effect 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. In 2006 our culturing work in Puerto Rico was filmed by a TV crew and the segment on foraminifer culturing can now be viewed online at Exploring Time TV special.

Current projects:

Testing geochemical proxy relationships under variable paleo-seawater chemical compositions, collaborators: GRA Laura Haynes, Steve Eggins

Calibration and application of the boron isotope seawater-pH indicator in deep-water corals; collaborators: GRA Jesse Farmer, Laura Robinson, Tessa Hill

Experimental Evaluation of the pH-Dependence of Boron-Adsorption onto Clay Minerals and Implications for the secular evolution of seawater d11B and [B]; PI: Nina Ruprecht

Calcification and shell chemistry response of Southern Ocean planktic foraminifers to ocean acidification and changing climates, PI: Steve Eggins