Traditionally, export of organic carbon from the surface layer of the ocean has been measured and modeled in terms of the sinking flux of particulate organic carbon (POC). But, a large fraction of the organic carbon pool consists of small particles that do not sink at a significant rate. Analysis of backscatter, fluorescence, temperature and salinity in glider profiles obtained from the 2008 North Atlantic Bloom (NAB08) study reveals that small, neutrally buoyant phytoplankton cells are transported downward along sloping isopycnal surfaces in intrusions that show distinctly different water mass characteristics from the surrounding water. Such features are also seen in a high-resolution, three-dimensional model.
In this project, a research team at the Woods Hole Oceanographic Institution will explore eddy-driven subduction as a contributory mechanism to the biological pump of the North Atlantic. Unlike the sinking of large particles or large-scale subduction on seasonal time scales, the contribution in this case, is due to downwelling or subduction associated with eddies and fronts on scales of 1-10 km, not usually resolved in global carbon cycle models. Specifically, the team will: (1) characterize the mechanism for eddy-driven subduction and relate it to the flow field, via vorticity, density stratification and frontal gradients, wind, and heat fluxes; (2) trace the pathways for vertical flux, identify the age and origin of features that convey the POC from the mixed layer to depth, ascertain the ubiquity of these features, their frequency in space and time, and the depth to which they transport carbon; and (3) quantify the flux of POC through eddy-driven subduction, relate it to the surface POC distribution and concentration and to time scales of remineralization, subduction and sinking, and then compare this POC flux to estimates for export of large POC during the North Atlantic bloom. These goals will be achieved through the detailed analysis of the NAB08 data set in conjunction with a three-dimensional process based modeling study that will help to examine the mechanisms and relate the transport of POC to the characteristics of the dynamics and surface POC.
Broader Impacts: This work will address an important question with broad potential implications for carbon export (including export of dissolved organic matter), air-sea exchange of CO2, oxygen budgets, and ventilation of the thermocline on short time scales. It will support a postdoctoral investigator and host a visiting student from Hamburg University for a period of 6 months through the WHOI Guest Student program. Both the PI and CoPI will continue to be involved in outreach activities through local museums and avenues for teacher training.
Intellectual Merit This study showed that a dynamic eddying flow field in the upper ocean subducts surface water with high concentrations of non-sinking particulate organic carbon (POC). Autonomous observations made by gliders during the North Atlantic spring bloom show numerous instances of elevated POC, chlorophyll, oxygen and anomalous temperature/salinity signatures at depths of 100-350~m. High-resolution modeling reveals that three-dimensional intrusions of surface water descend as coherent, 1-10 km scale, filamentous features, often along the perimeter of eddies. Such a submesoscale eddy-driven flux of POC can contribute as much as half of the total springtime export of POC from the upper 100~m of the highly productive subpolar oceans with deep, seasonal mixed layers. A new mathematical model was developed for the vertical profile of the sinking flux of particulate organic carbon. Broader Impacts This study helped to identify and quantify a mechanism for the export of particulate organic carbon that is presently unaccounted in global carbon cycle models. A scaling estimate was derived for this flux. It was shown to be a significant mechanism for export in regions with intense spring phytoplankton blooms. A postdoctoral scientist was supported in this study. She will begin as Assistant Professor in Oceanography at URI next month. Three students, including 2 undergraduates and one masters student were mentored.
