Freund, Madeleine (2020) Dispersion of a Tracer in the Eastern Tropical South Pacific - an Investigation of Interactions from the Benthic Boundary Layer to the Ocean Interior. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 102 pp.

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Abstract

Wind and buoyancy driven oceanic circulation and turbulent diffusion distribute heat, salt, and other dissolved constituents such as oxygen, carbon dioxide, or nutrients in the ocean. However, natural variability of physical processes, and the nutrient fluxes are complex, highly dynamic and difficult to observe directly. Especially, how the physical processes affect overall biogeochemical tracers is still an important subject of research. The region of interest is the Eastern Tropical South Pacific where anoxic conditions on the continental shelf off Peru and Chile lead to a release of nutrients from the sediments into the water directly above. The fate of these nutrients is not well understood, yet. This thesis summarizes the studies inferring the spread of an artificial dissolved constituent, a tracer potentially representing such nutrients, using a so called tracer release experiment performed to quantify the benthic pelagic exchanges in the Peruvian oxygen minimum zone. This provides the physical basis to identify and quantify pathways potentially used by the nutrients. The Peru-Chile undercurrent and mesoscale eddy activity are found to dominate the lateral tracer dispersion and are responsible for a fast and wide tracer spread to the south and in the offshore ocean. The tracer transport velocity in the Peru-Chile undercurrent near by Callao (∼12°S) is 3-5 cm s−1 and 7-17 cm s−1 southward from Paracas headland (14-17.5°S). Anticyclonic eddies (mainly the subsurface ones) capture 21-54% of the tracer in their cores which reaches the offshore ocean. This way the eddies denote the connection between the Peruvian shelf and the offshore ocean, but the Peru-Chile undercurrent acts first and place the tracer into the eddy generation hot spot. In the vertical, there is an upward tracer motion. It happens basically diapycnal, i.e. across density surfaces, and thus is accompanied by a lightening of tracer tagged-water. In the process, the tracer crawls upward the shelf along the slope within the bottom boundary water layer. Thus the theory by Ferrari et al. (2016); Holmes et al. (2019) seems to sufficiently explain the observed behavior. Overall 10-30±10% of the tracer rise, even high enough to reach the surface waters. In contrast, the open ocean diapycnal diffusivity coefficient is (1.4 ± 0.4)·10−5 m2 s−1 , representative for calm open ocean regimes. Thus it cannot account for the upward motion but fills in a geographic gap of such values. With this physical basis it is possible to conclude on the nutrient constrains. Typical nutrient concentration pattern over the Peruvian shelf not only waft back and forth or recirculate locally but need to be permanent reestablished by a source, namely their release from the sediments, as the substances in the water column are permanently transported away. The upward motion is especially important as it shows a pathway into the euphotic surface waters provided by ocean physics. Thus nutrients released from the shelf sediments could potentially be used for primary production in these realms.

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