Showing papers by "Tim DeVries published in 2016"

Deep ocean nutrients imply large latitudinal variation in particle transfer efficiency

Abstract: The “transfer efficiency” of sinking organic particles through the mesopelagic zone and into the deep ocean is a critical determinant of the atmosphere−ocean partition of carbon dioxide (CO2). Our ability to detect large-scale spatial variations in transfer efficiency is limited by the scarcity and uncertainties of particle flux data. Here we reconstruct deep ocean particle fluxes by diagnosing the rate of nutrient accumulation along transport pathways in a data-constrained ocean circulation model. Combined with estimates of organic matter export from the surface, these diagnosed fluxes reveal a global pattern of transfer efficiency to 1,000 m that is high (∼25%) at high latitudes and low (∼5%) in subtropical gyres, with intermediate values in the tropics. This pattern is well correlated with spatial variations in phytoplankton community structure and the export of ballast minerals, which control the size and density of sinking particles. These findings accentuate the importance of high-latitude oceans in sequestering carbon over long timescales, and highlight potential impacts on remineralization depth as phytoplankton communities respond to a warming climate.

Constraints on the global marine iron cycle from a simple inverse model

Abstract: A simple model of the global marine iron cycle is used to constrain the sources, sinks, and biological cycling of iron. The iron model is embedded in a data-assimilated steady state circulation, with biological cycling driven by a prescribed, data-constrained phosphate cycle. Biogeochemical parameters are determined by minimizing a suitably weighted quadratic mismatch with available dissolved iron (dFe) observations, including GEOTRACES transects. Because the effective iron sources and sinks overlap, current dFe observations cannot constrain sources and sinks independently. We therefore determine a family of optimal solutions for a range of the aeolian source strength σA from 0.3 to 6.1 Gmol/yr. We find that the dFe observations constrain the maximum Fe:P uptake ratio R0 to be proportional to σA, with a range that spans most available measurements. Thus, with either R0 or σA specified, a unique solution is determined. Global inventories of total and free iron are well constrained at (7.4 ± 0.2) × 1011 and (1.39 ± 0.05) × 1010 mol Fe, respectively. The dFe distributions are very similar across the family of solutions, with iron limitation in the known high-nutrient low-chlorophyll regions. Hydrothermal source strength ranges from 0.57 to 0.73 Gmol/yr and does not vary systematically with σA suggesting that the hydrothermal and aeolian parts of the iron cycle are largely decoupled. The hydrothermal dFe anomaly in the euphotic zone is ∼10% and concentrated in subpolar regions of iron limitation. Enhanced ligand concentrations in old waters and in hydrothermal plumes are necessary to capture key features of the dFe observations.