Archive for shelf system

The role of nutrient trapping in promoting shelf hypoxia in the southern Benguela upwelling system

Posted by mmaheigan 
· Thursday, September 3rd, 2020 

The southern Benguela upwelling system (SBUS) off southwest Africa is an exceptionally fertile ocean region that supports valuable commercial fisheries. The productivity of this system derives from the upwelling of nutrient-rich Subantarctic Mode Water, and from the concurrent entrainment of nutrients regenerated proximately on the expansive continental shelf. The SBUS is prone to severe seasonal hypoxic events that decimate regional fisheries, occurrences of which are inextricably linked to the inherent nutrient dynamics. In a study recently published in JGR Oceans, the authors sought to understand the mechanisms sustaining elevated concentrations and seasonally-variable distributions of nutrients in the SBUS, in relation to the subsurface oxygen content. Inter-seasonal measurements of nutrients and nitrate isotope ratios across the SBUS in 2017 revealed that upwards of 48% (summer) and 63% (winter) of the on‐shelf nutrients derived from regeneration in situ.  The severity of hypoxia at the shelf bottom, in turn, correlated with the incidence of regenerated nutrients. The accrual of nutrients at the shelf bottom appears to be aided by hydrographic fronts that restrict offshore transport, trapping regenerated nutrients on the SBUS shelf and increasing the pool of nutrients available for upwelling – ultimately contributing to hypoxic events. This study underscores the need – if we are to develop a mechanistic and predictive understanding of hypoxia in the SBUS and elsewhere – to elucidate the role of shelf circulation in promoting the accrual of regenerated nutrients on the continental shelf. The next step is to combine new and existing observations with quantitative simulations to further interrogate the coupled physical-biogeochemical mechanisms that modulate the intensity of hypoxia.

Figure caption: Schematic of proposed nutrient-trapping mechanism: Deep nutrient-rich Subantarctic Mode Water (SAMW) acquires more nutrients as it passes over the shelf sediments from the regeneration of exported particulate organic material (POM). The production of this POM is fueled by nutrients stripped from the surface waters advecting back off-shore. The thickness of the arrows represents nutrient concentrations. Triangles indicate the positions of the Shelf Break Front (SBF) and Columbine Front (CF), coincident with an observed subduction of the Ekman layer and downwelling at the inner front boundary.

Authors
Raquel Flynn (University of Cape Town)
Julie Granger (University of Connecticut)
Jennifer Veitch (South African Environmental Observation Network)
Samantha Siedlecki (University of Connecticut)
Jessica Burger (University of Cape Town)
Keshnee Pillay (South Africa Department of Environment, Forestry and Fisheries)
Sarah Fawcett (University of Cape Town)

The Ross Sea deep microbial community’s role in sequestering CO2

Posted by mmaheigan 
· Thursday, November 9th, 2017 

Antarctic shelf systems generate the densest waters in the world. These shelf waters are the building blocks of Antarctic Bottom Water, the ocean’s abyssal water mass. These bottom waters have the potential to sequester carbon out of the atmosphere for millennia. One such form of marine carbon is dissolved organic carbon (DOC). DOC is produced in the surface ocean via primary production and is the global ocean’s largest standing stock of reduced carbon.

In a recent study, Bercovici et al (2017) used hydrographic and biogeochemical measurements to assess the mechanism that brings DOC into the shelf waters of the Ross Sea, the shelf system in the Pacific sector of Antarctica. These mechanisms include sinking particles, brine rejection caused by katabatic winds in the Terra Nova Bay polynya, and vertical mixing. This study revealed that DOC is primarily introduced into the deeper shelf waters via convective overturning and deep vertical mixing upon the onset of austral winter. Substantial DOC enrichment of shelf waters suggests that this carbon is exported off the shelf into Antarctic Bottom Water. However, this study finds much of the excess Ross Sea shelf DOC is actually consumed and remineralized to CO2 by deep microbial communities at the slope of the Ross Sea shelf, ultimately sequestering this carbon into the ocean’s interior.

Physical and biological processes have the potential to introduce carbon into the dense shelf waters (blue) in the Ross Sea. Incoming waters (yellow) are modified from the Southern Ocean’s circumpolar waters. At the onset of winter, cooler temperatures and katabatic winds cause brine rejection. The rejection of brine, sinking particles and vertical mixing are all potential mechanisms for bringing DOC to the dense shelf waters. At the shelf slope, outflowing shelf waters ultimately contribute to Antarctic Bottom Water formation. This research furthers our understanding of global carbon cycling through demonstrating that Antarctic shelf systems have the potential to sequester organic carbon into the abyssal ocean.

Authors:
Sarah K. Bercovici (Rosenstiel School of Marine and Atmospheric Science, University of Miami)
Bruce A. Huber (Lamont Doherty Earth Observatory, Columbia University)
Hans B. Dejong (Stanford University)
Robert B. Dunbar (Stanford University)
Dennis A. Hansell (Rosenstiel School of Marine and Atmospheric Science, University of Miami)

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