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New approaches for understanding oceanic carbon uptake

Final Report Summary - CARBON EXPORT (New approaches for understanding oceanic carbon uptake)

A critical gap in conceptual models of global ocean carbon biogeochemistry is in understanding the mechanisms that facilitate carbon export (and hence oceanic CO2 uptake) from marine surface waters. Several studies have suggested that the magnitude of the biological pump is highly dependent on episodic processes that can be both physical (e.g. nutrient injection) and biological (e.g. diatom senescence) in nature. Nitrogen fixation, the uptake and consumption of N2 by certain organisms, plays a now well-recognised role in enhancing primary production and export in oligotrophic regions of the subtropical and tropical oceans. Recent evidence further suggests that N2 fixation may be even more globally significant, perhaps commonly occurring in regions of the surface ocean proximate to zones of intense subsurface denitrification.

In this study, we examined how the interactions between denitrification and N2 fixation enhance primary and export production in one such area, the Gulf of California (GoCal) and adjacent waters of the eastern subtropical North Pacific (ETNP). This oceanographic region was chosen as it provides an excellent field laboratory in which to study these processes due to its rich biological productivity and the predominance of denitrified intermediate waters. Water and suspended and sinking particles were sampled at high vertical resolution during a month-long research cruise in August 2008.

Although still preliminary, our results suggest a number of important findings from this research. High rates of N2 fixation were observed within the GoCal (Stations 2 and 8), coincident with relatively high sea surface temperatures and enhanced chlorophyll. Identified N2 fixing organisms were typical of the region and included Trichodesmium and the endosymbiont Richelia contained within diatoms of two genera (Hemialus and Rhizosolenia). A second region of high nitrogen fixation rates was located outside of the Gulf (Station 11), associated with colder temperatures and more unique unicellular Group A diazotrophs. Surprisingly, this latter cold water station was where nitrogen fixation rates accounted for the largest proportion of fixed carbon production (about 10 %) measured throughout the program.

Nitrogen fixation mediated particle formation, remineralisation, and export were measured using a combination of sediment traps deployed at 100 m and two naturally occurring, short-lived radioisotopes, 234Th (t1/2 = 24.1 days) and 210Po (t1/2 = 138 days). Results suggest that most particle formation and export occurred over the upper 100 m. Sediment trap derived particulate carbon fluxes were similar to those determined using water column 234Th deficits coupled with sediment trap carbon/234Th ratios. Highest particulate carbon and nitrogen fluxes were associated with high abundances of diatoms, phenomena typical of many oceanic regimes. Surprisingly however, a secondary of region of high particle export was associated with high abundances of the more unique unicellular Group A. This suggests that both diatom and diazotrophic dominated plankton assemblages enhance carbon and nitrogen export from surface waters. Closer inspection of the export efficiency (carbon fixed versus carbon exported) further suggests that that diazatroph mediated export was significantly more efficient in the amount of carbon transported to depth (50 % versus 10-15 % for diatoms). Therefore, understanding how specific plankton communities impact both the magnitude and the efficiency of particle export is of paramount importance for elucidating carbon sequestration in these systems.

Fine scale depth measurements of both radioisotope tracers coupled with biomarkers of diazatrophy should help to pinpoint where in the water column particle formation and remineralisation has occurred as well as abiotic versus potentially species specific biotic control mechanisms of particle export. Samples are currently being analyzed for species.

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