Christelle Claude

Estimation of air-water gas exchange coefficient in a shallow lagoon based on 222Rn mass balance.

The radon-222 mass balance is now commonly used to quantify water fluxes due to Submarine Groundwater Discharge (SGD) in coastal areas. One of the main loss terms of this mass balance, the radon evasion to the atmosphere, is based on empirical equations. This term is generally estimated using one among the many empirical equations describing the gas transfer velocity as a function of wind speed that have been proposed in the literature. These equations were, however, mainly obtained from areas of deep water and may be less appropriate for shallow areas. Here, we calculate the radon mass balance for a windy shallow coastal lagoon (mean depth of 6m and surface area of 1.55*10(8) m(2)) and use these data to estimate the radon loss to the atmosphere and the corresponding gas transfer velocity. We present new equations, adapted to our shallow water body, to express the gas transfer velocity as a function of wind speed at 10 m height (wind range from 2 to 12.5 m/s). When compared with those from the literature, these equations fit particularly well with the one of Kremer et al. (2003). Finally, we emphasize that some gas transfer exchange may always occur, even for conditions without wind.

Chapter Fifteen Isotopic Tracers of Water Masses and Deep Currents

Publisher Summary This chapter focuses on the isotopic tracers of water masses and deep currents, and reviews the applications of “radiogenic” isotopes in chemical oceanography and of their potential use as tracers for paleoceanographic research on timescales going back to ∼60 Ma. Radiogenic isotopes are introduced into the ocean as weathering and erosional products of the Earths crust, transported via aeolian, riverine or hydrothermal circulations. The isotopic composition of the corresponding elements, that is, the relative abundance of their radiogenic isotopes, depends upon the geological history of bedrock sources. This isotopic composition is preserved and remains normally unaltered during biogeochemical reactions in the oceanic water column. The relative abundance of radiogenic isotopes in the ocean may thus be seen as an isotopic “signature” of chemical element sources and of their subsequent transport by oceanic currents. While these elements have mostly low solubility and are highly reactive, their isotopic composition depends only on mixing between different source-rocks, and behave as conservative tracers of carrier water masses in the ocean. Unlike other conservative tracers of oceanic circulation, radiogenic isotopes have the immense advantage of being trapped in solid phases precipitated in chemical equilibrium with seawater. Thus, they have the ability to record some aspects of past ocean circulation properties. The chapter illustrates various examples of different isotopic tracers and different case studies, and focuses specifically on the stable radiogenic isotopes.