Collaborative research: US GEOTRACES PMT: Measuring the d13C-DIC distribution and estimating organic matter export rates

University Of Washington, Seattle WA

Investigators

Abstract

The concentration of carbon dioxide gas (CO2) in the ocean depends in part on the rate of photosynthesis in the ocean because CO2 is consumed by microscopic plants in surface ocean (phytoplankton). The photosynthesis reaction can be written as CO2 + H2O + light energy -> CH2O (sugar) + O2. During photosynthesis, the plankton preferentially consume CO2 molecules comprised of 12C atoms and less so CO2 molecules comprised of 13C atoms. Measuring the ratio of 13CO2 to 12CO2 dissolved in seawater provides a means to estimate the photosynthesis rate. Below the surface layer where photosynthesis occurs (the photic layer), bacteria use sinking plankton material for food and energy via a process called respiration (which is the reverse of the photosynthesis reaction). Respiration produces CO2 in the deeper layers of the ocean. The goal of our research is to measure the concentration of 13CO2 and 12CO2 in seawater from the surface to the bottom of the Pacific Ocean along a cruise path that starts in Seward, Alaska and goes southward to Tahiti. We will use these measurements to understand how photosynthesis, respiration and mixing in the ocean control the distributions of 13CO2 and 12CO2. The broader impact of our research is to determine how photosynthesis, respiration and mixing in the ocean affect the transfer rate of CO2 from the atmosphere to the ocean. This CO2 transfer rate is important because it affects the concentration of CO2, a greenhouse gas, in the atmosphere and the increasing acidity of the ocean. Other broader impacts include incorporating results from this study into classes taught by the researcher. One undergraduate student from the University of Washington would be supported and trained as part of this project. The specific goal of our research is to measure the depth distribution of the 13C/12C of the dissolved inorganic carbon (DIC) during the GEOTRACES PMT cruise (Seward, AK to Tahiti) in the N. Pacific Ocean. We intend to collect and measure d13C-DIC on ~850 seawater samples. The PMT section will cross a wide range of biological productive regimes from the oligotrophic gyres near Tahiti and Hawaii to the more productive HNLC regions in the equatorial and subarctic oceans. The expected meridional variations in productivity should provide the opportunity to determine the impact of organic matter (OM) export and subsequent degradation at depth on upper ocean distribution of d13C-IC, trace elements (TEs), and nutrients along the PMT section. We intend to determine how the meridional variations in productivity and proximity to outcropping water masses affect the distributions of d13C, PO4, NO3 and bioactive TEs (like Cadmium) throughout the thermocline and deep sea along the PMT cruise track. Improving our understanding of the processes that control d13C, TEs and nutrient distributions in the modern ocean will improve our ability to use d13C and TEs measurements in the sedimentary record as tracers of past changes in the ocean circulation and carbon cycling. From a broader impact perspective, our address research addresses two basic goals of the GEOTRACES program. First, 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. Second, to understand the processes that control the concentrations of geochemical species used for proxies of the ocean's past environment which will benefit the paleoceanography community.