Project: US GEOTRACES Pacific: Measuring the d13C-DIC distribution and quantifying the impact of organic matter export on d13C, nutrients and trace metals

Text from the NSF award abstract:

The international GEOTRACES program has two major goals: (1) to determine global ocean distributions of selected trace elements and isotopes and evaluate the sources, sinks, and internal cycling of these species to characterize more completely the physical, chemical and biological processes regulating their distributions; and (2) to understand the processes that control the concentrations of geochemical species used for proxies of the past environment. Because the cycling of trace elements and isotopes in the sea is intimately connected with both the organic and the inorganic biogeochemical transformations of carbon, the achievement of the goals of GEOTRACES would be very difficult if not impossible unless synoptic measurements are also made on the organic and inorganic marine carbon system.

In this project, researchers at the University of Washington will participate in the U.S.GEOTRACES Pacific campaign in 2013 to characterize the regional ocean carbon system. They will use two approaches. First, they will measure the depth distribution of the 13C/12C stable carbon isotopic ratio of the dissolved inorganic carbon (del13C), which has been designated in the international GEOTRACES Science Plan as a key parameter and has the highest measurement priority. Second, they will estimate the rate of organic matter (OM) export from the surface layer based on dissolved O2/Ar gas ratios. Measurements of del13C will be made at about 22 stations for a total of ~525 samples. Additionally, they will measure the dissolved O2/Ar gas ratio at ~5 km spatial resolution in the surface layer using an underway measurement method that has been used extensively in other oceanic regions.

The cruise track will cross a wide range of biological productivity regimes from the coastal upwelling zone off Peru to the oligotrophic gyre surrounding Tahiti. The proposed high resolution O2/Ar-based OM export rates should clearly detect the expected offshore decrease in OM export and locate transition zones or fronts in productivity. The expected large offshore productivity gradient will provide the opportunity to determine the impact of OM export from the surface layer and OM degradation at depth on upper ocean depth distribution of del13C, trace elements (TEs), O2 and nutrients along the section.

The initial del13C and TE measurements during GEOTRACES illustrate the potential to improve our understanding of the processes that control del13C, TEs and nutrient distributions in the modern ocean and, as a result, improve the utility of del13C and TEs as tracers of past changes in the ocean circulation and carbon cycling, which is a long term goal of this research.

Broader Impacts. The data and research results will be broadly distributed to the international oceanographic and paleoclimate communities and incorporated into the PI's graduate and undergraduate teaching curricula. Additionally, there will be active undergraduate participation (5 students currently) in preparing samples for del13C analyses.