Overview. The overarching objective of my research is

• to describe, understand, and quantify the distribution of sedimentary particles and of tracers, such as specific biomarkers or isotopes;
• to establish their mechanistic relation to processes of interest, such as ocean currents, surface ocean conditions, or marine biogeochemical cycles;
• and to determine their variability through time, with a specific focus on the last glacial-interglacial cycle.

Specific long-term questions that guide my research are: How did the tropical Pacific influence global climate change in the past? To what extent did the marine nitrogen cycle cause the glacial-interglacial swings in atmospheric CO₂ concentrations? How can we better quantify past sea surface temperatures and salinities?

The tropical Pacific is a dynamic part of glacial-interglacial climate change. As one example, the El Niño Southern Oscillation (ENSO) instabilities in the tropical Pacific ultimately alter global climate today, and it thus has the potential to play a lead role in driving global climate change on longer time scales as well. Our research aims to understand the intricate linkages between tropical Pacific and global climate change on glacial-interglacial and centennial- to millennial-scale time scales, and to establish the phasing between, and causative linkages of, climate change associated with Southern Ocean and N. Atlantic dynamics.

The nitrogen cycle plays a central role in the biogeochemistry of the ocean and is intimately linked with global climate change on interannual to glacial-interglacial timescales, with causative linkages in both directions. Thus, whereas increases of the oceanic N inventory during the last glacial period have long been proposed to strengthen the marine “biological pump” and thereby to lower atmospheric CO2 concentrations, other evidence points to an immediate response of water column denitrification, for instance, to regional and global climate and ocean changes. In turn, variations in the extent and intensity of water column denitrification affect atmospheric concentrations of N2O, a potent greenhouse gas. We study nitrogen isotopes (d15N) in the modern ocean and as recorded in the sedimentary record in order to learn about the dynamics of the marine nitrogen cycle throughout the Quaternary. (see also http://www.pages-igbp.org/workinggroups/nicopp)

In the process of reconstructing past ocean dynamics, paleoceanographers have to continuously advance their understanding of the proxy records themselves, and how measurable descriptors relate to the environmental parameter of interest. Our research contributes to fine-tuning the appreciation of the proxy records themselves. For instance, we study how alkenone unsaturation relates to the growth temperature of a particular group of phytoplankton that synthesize these organic molecules, or to other non-thermal factors. We also aim to advance the understanding of nitrogen isotope records of past changes in the marine N cycle.