Marine particulate matter in the twilight zone: Insights on iron cycling and remineralization of particulate organic carbon in the ocean

The marine biogeochemical cycling of iron and carbon in high latitude, iron-limited waters of the Pacific (the Subarctic North Pacific and the Southern Ocean) are investigated using marine particulate matter. Marine particulates were collected using large volume in-situ filtration from the euphotic and twilight zones (upper 1000 m) of the ocean. A combination of chemical approaches (Inductively Coupled Plasma-Mass Spectrometry, synchrotron X-Ray Fluorescence, synchrotron X-ray Absorption Spectroscopy) and imaging techniques (digital imaging, Scanning Electron Microscopy, and Scanning Transmission X-ray Microscopy) spanning bulk and macroscopic to molecular and sub-micron of scales were used on these samples to understand the biogeochemical cycling of iron and carbon.; Evidence is presented supporting the hypothesis that lateral supply of particulate iron from the continental margin off the Aleutian Islands in the winter supported an observed biological bloom at Ocean Station Papa (OSP) in the Subarctic Pacific. Synchrotron x-ray analysis was used to describe the physical form, chemistry, and depth distributions of iron in size fractionated particulate matter samples, revealing discrete micron-sized iron-rich hotspots that were ubiquitous in the upper 200 m at OSP, more than 900 km from the closest coast. The specifics of the chemistry and depth profiles of the Fe hotspots traced them to the continental margins.; Examination of the chemistry and distribution of large (>51 mum) particles collected from two locations in the Southern Ocean during the Southern Ocean Iron Experiment (SOFeX) in 2002 provided insight on the factors determining the vertical distribution of POC, and the consequences of iron addition for this distribution. A striking difference in the shapes of particulate organic carbon (POC) profiles was observed for samples from the Subantarctic (55°S) and the Antarctic (66°S). Differences in particle settling rates due to water viscosity and particle excess density differences between 55°S and 66°S were negligible and could not explain the differences in POC concentrations, pointing to the importance of particle loss rates from grazing and remineralization as determining factors. The observation of high surface biomass was correlated with low POC at depth. The existence and importance of High Biomass Low Export (HBLE) regions is proposed.