The role of topography in cross-shelf-break tracer exchange

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The rotation of the Earth, and its Coriolis force, limit the exchange of water between the coastal ocean and the deep ocean. This limited exchange is often illustrated by significant changes in water properties, such as colour, near the location of the shelf-break. Various processes can break these dynamical constraints and allow exchange flow. One of these is caused by small scale topography such as underwater canyons. In previous research, we have determined the amount and depth of water which flows onto the shelf through submarine canyons. The enhanced advection through canyons is significant but more importantly it comes from deeper depths than occurs in the absence of canyons. In this grant we propose to quantify the amount of specific, important tracers that are transported through submarine canyons. These tracers include nitrate, dissolved inorganic carbon, and water containing low oxygen. Nitrate and inorganic carbon increase with depth in the ocean and oxygen decreases in the North Pacific to depths of about 500 m. Advection onto the shelf from deeper in the water column thus implies more nitrate and carbon and less oxygen. Nitrate is a limiting nutrient for phytoplankton growth, the one-celled photosynthetic organisms that form the bottom of the ocean food chain. Quantifying its flux is necessary to determine changes food-availability to higher trophic levels and through to fish. Advection of low oxygen water contributes to anoxia which causes benthic die-offs (so called ``dead zones''). Even though much of the oxygen utilization is due to shelf processes, if the water advected onto the shelf is initially low in oxygen, the final minimum oxygen levels reached will be lower. Dissolved inorganic carbon advected onto the shelf can be upwelled and increase the flux of carbon dioxide to the atmosphere. Quantifying the amount of flux through this pathway is part of determining the coastal ocean's role in the carbon cycle.