Collaborative Research: CarboMODE: Carbon Dioxide Dynamics in Mode Water of the North Atlantic Ocean

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The formation of mode waters, like Eighteen Degree Water (EDW) in the North Atlantic Ocean, is important for driving ocean circulation, ventilating and transferring biogeochemical properties to the ocean interior. Recent studies suggest that EDW plays an important role in setting the nutrient reservoir of the subtropical gyre [Jenkins and Doney, 2003; Doney and Jenkins, 2004; Palter et al., 2005], with significant implications for nutrient and carbon dynamics, and productivity in the subtropical gyre of the North Atlantic. In addition, EDW has a potentially important role in the ocean uptake and decadal variability of atmospheric CO2 [Bates et al., 2002]. In this study, researchers at the Bermuda Biological Station for Research (BBSR) and the Woods Hole Oceanographic Institution (WHOI) hope to achieve a better quantitative and mechanistic understanding of the CO2 dynamics in EDW. The work leverages the 2006-2007 field program and improved understanding about the physics of EDW that an NSF sponsored field project, CLImate MOde water Dynamics Experiment (CLIMODE) will gain. The main question posed in CarboMODE is "What is the oceanic uptake and fate of CO2 in EDW in the North Atlantic Ocean?" From this general question, more specific questions are raised, including: (1) What is the air-sea CO2 flux during wintertime EDW formation? (2) What are the relative contributions from vertical/lateral mixing, advection/stirring, air-sea CO2 gas exchange and biological depletion of CO2 due to net community production during EDW formation that influence the DIC properties of EDW? (3) What is the dissolved inorganic carbon (DIC) content of EDW upon subduction (injection) into the subtropical gyre and what is the overall flux? (4) How does the formation of EDW impact the subsurface inorganic carbon reservoir and air-sea CO2 fluxes in the subtropical gyre of the North Atlantic Ocean? (5) What is the fate of inorganic carbon in EDW as it advects away from the region of formation and how does subsurface remineralization contribute to the DIC content of EDW? In addressing these questions, the investigators propose will collect inorganic carbon data in 2007 as part of the CLIMODE project. Their contribution to the CLIMODE (and CarboMODE) project will be measurements of DIC, Total Alkalinity (TA) and underway pCO2 (i.e., seawater and air pCO2). Although focused on physics, the observational and modeling program framed by CLIMODE's questions and hypotheses fortuitously provide a timely and unique opportunity to address questions raised about CO2 dynamics (and related issues concerning nutrient and dissolved oxygen dynamics). Synthesis and modeling of several different datasets, including the 2007 CLIMODE field surveys of EDW, CO2 data collected from a 2006 CLIMODE cruise, a 4 day northward extension of the BATS Bermuda-Puerto Rico annual transect, and surface seawater pCO2 (and DIC and alkalinity) data collected twice a week in the region of EDW formation from the Volunteer Observing Ship (VOS) MV Oleander (funded by NOAA COSP), form the nucleus for addressing relevant CarboMODE questions. In terms of broader impacts, this study will provide fundamental data on the oceanic sink of CO2 into North Atlantic EDW layer and will contribute to carbon cycle studies coordinated under a variety of science programs and implementation structures (e.g., NACP, CCSP, OCCC, SOLAR-IMBER) that aim to establish accurate estimates of oceanic CO2 sources and sinks, and the underlying mechanisms that regulate it. The project will support one full time post-doctoral fellow at BBSR. Scientific understanding and research products from the research will be used in undergraduate and graduate educational activities at BBSR, and contribute to the research and technical skills of two research technicians, and one undergraduate student from WHOI.