Award: OCE-1235024

Collaborative Research: Seawater Inorganic and Organic Carbon Measurements for the US GEOTRACES Eastern Pacific Zonal Transect Dissolved organic carbon (DOC) represents the largest pool of reduced carbon in the ocean. DOC is produced as a result of myriad foodweb processes. Most of the DOC production is consumed rapidly by microbial processes. However, a portion of the DOC escapes microbial removal and accumulates in the oceanic water column. This accumulated DOC has varying turnover rates but DOC can persist for millennia and thus has important roles with regard to carbon storage. Ocean biogeochemists are interested in assessing the inventory of DOC (currently estimated at 662 petagrams (10^15 g)) and its distribution in the surface and deep waters of the global ocean. Oceanographers are also interested in assessing the sources and sinks of DOC. The deep ocean DOC is generally characterized by low concentrations that are very recalcitrant to biological removal. Ocean transect allow biogeochemists to observe where in the deep ocean water masses move DOC and anomalies in the concentration point to sources or sinks of DOC. This project was a collaborative project between Craig Carlson (UCSB) and Nicholas Bates (BIOS). The major objective of this collaborative effort was to assess the inventories and distributions of dissolved inorganic and DOC as part of the the U.S. GEOTRACES Eastern Pacific Zonal Transect. This GEOTRACES section will provide data between Peru and Tahiti in the eastern Pacific Ocean. The major objectives of CarlsonÆs component were to 1) To produce high resolution DOC measurements to assess vertical and horizontal gradients in the organic carbon field along the Peru – Tahiti section. 2) To assess variability of DOC within the hydrothermal plume of the East Pacific Rise and assess the partitioning of organic matter between DOC and POC along gradients within the plume. Typical of bulk DOC profiles concentrations were greatest at within the surface 100 m and decreased over depth. Surface concentrations were lowest by the coast (55 µmol/ l)which is suggestive of recently upwelled water and greatest between 128° - 136° west (>70 µmol / L). DOC concentration decreased significantly and were maintained at concentrations below 40 µmol /L below 1000m. There was significant variability observed in the deep water some of which is consistent with water mass variability, for example deep DOC concentrations were relatively greater in the Antarctic bottom water in the western part of the transect. Along the transect DOC concentrations were variable between 35 - 39 µmol / L with pockets of low DOC throughout the transect. The high resolution sampling just above the hydrothermal vent system near 113°W revealed a coherent feature of low DOC concentrations of ≤ 35µmol DOC. Greater than 25 individual samples around at the hydrothermal feature provides evidence of a DOC removal process. It remains unclear if this is a result of enhanced biological removal or abiotic removal. The broader impact of this study will be the use of the data will by the GEOTRACES science team as well as the greater oceanographic community. The data will help in the for synthesis and further interpretation of the controls of trace elements and isotope. This transect also adds to the valuable global ocean DOC data base filling in a portion of the ocean where DOC measurments are rare. Those synthesis efforts are currently underway. Last Modified: 04/29/2016 Submitted by: Craig A Carlson