Visiting Assistant Professor of Geology

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Current Projects

Studies of nitrogen cycling in modern marine environments
The largest sink of fixed nitrogen in the oceans is denitrification in marine sediments (denitrification here defined as a conversion of biologically available forms of N to N2). By measuring concentrations and  δ15N (and δ18O) of various N species (NO3-, NO2-, NH4+, N2O, N2) and closely collaborating with a group of microbiologists, molecular biologists and ecologists, we have been able to identify and characterize novel biological pathways (see our recent paper here).

Studies of nitrogen cycle on the glacial-interglacial time scales using deep sea corals
Reconstructing changes in the N (nitrogen) cycle on a geological time scales provides insights into the links between biological productivity and past climate. The main tool used in the studies of N cycle through time is the N isotope composition of preserved Particulate Organic Nitrogen (δ15N-PON). However, δ15N of sedimentary PON is often subject to modification during post-burial diagenesis. To add to the arsenal of proxies used in studies of the N cycle through time, in collaboration with the research groups of Daniel Sigman and Branwen Williams, I have been investigating the δ15N-PON bound within the carbonate skeletons of deep-sea corals.  Deep sea corals have been increasingly used in paleoceanographic studies, serving as archives for geochemical proxies used to decipher the past oceanic conditions. Currently underway is a calibration study of the coral-bound δ15N-PON recently funded by the NSF-MGG program.

Factors controlling primary production in high and low latitude ecosystems
BERING SEA
Eastern Bering Sea shelf hosts the world’s richestsubarctic ecosystem, which presently supports over 50% of US commercial fisheries. The biological communities in this region evolved in adaptation to a strongly seasonal sea-ice dynamic, which is currently rapidly changing in response to the global warming. Thus, understanding of how the environmental changes affect the carbon flow through this biological system is a vitally important societal issue. Using the dual biological O2 tracer approach (simultaneous measurements of O2/Ar supersaturation and Oxygen Triple Isotope (OTI) composition of O2), I have been working on quantifying Net Community Production (NCP) Gross Photosynthetic Production (GPP) fluxes, and the NCP/GPP ratios, which are the measure of the ecosystem export efficiency. Our study, conducted in the context of an NSF-supported multi-PI field campaign, has demonstrated and quantified the direct links between the extent and duration of winter ice cover and the export efficiency of the community of primary producers (Prokopenko et al., in review)

EASTERN TROPICAL SOUTH PACIFIC
Along with the colleagues from USC (Doug Capone and Will Berelson) and from Florida State University (Angie Knapp), we have been studying biogeochemistry of Eastern Tropical South Pacific (ETSP), a high-nutrient,low chlorophyll region, which acts as a strong oceanic source of CO2 to the atmosphere, and is also hypothesized to harbor a nutrient stoichiometry favorable for the tight spatial coupling between denitrification and N2 fixation. Based on continuous underway surveys of O2/Ar supersaturation within the surface mixed layer, as well as O2/Ar and OTI profiles, we have mapped the spatial distribution of the NCP and GPP fluxes in the transition zone from Peru-Chile coastal upwelling to the South Pacific Gyre. The results of this work have elucidated the links between selected environmental parameters (e.g. nutrient stoichiometry, regional and large scale ocean circulation patterns) and the rates of biological carbon cycling (Prokopenko et al., in prep).

SOUTHERN CALIFORNIA UPWELLING : Up.R.i.S.E.E. (Upwelling Regime in situ Ecosystem Efficiency) STUDY
While the O2 dual tracer approach of quantifying net and gross production terms is one of the few methods currently available for the in situ studies of biological productivity, it relies on the construction of the mass balance of the O2 fluxes, including physically driven fluxes. In regions of upwelling, which are the important players in marine organic carbon cycling, application of this technique has been severely limited due to poorly constrained large vertical transport term in O2 mass balance. Together with colleagues at USC, Doug Hammond and Willie Haskell, we are currently conducting a study of a coastal upwelling, recently funded through an OCE-NSF. The study is designed to investigate the links between the net and gross primary production and the upwelling dynamic in the Southern California Bight by combining the O2/Ar and OTI approach and 234Th-based estimates of organic carbon export, with a detailed investigation of upwelling rates (using 7Be isotopes) and nutrient stoichiometry.