Peter Holtermann

The Baltic Sea Tracer Release Experiment: 1. Mixing rates

In this study, results from the Baltic Sea Tracer Release Experiment (BATRE) are described, in which deep water mixing rates and mixing processes in the central Baltic Sea were investigated. In September 2007, an inert tracer gas (CF3SF5) was injected at approximately 200 m depth in the Gotland Basin, and the subsequent spreading of the tracer was observed during six surveys until February 2009. These data describe the diapycnal and lateral mixing during a stagnation period without any significant deep water renewal due to inflow events. As one of the main results, vertical mixing rates were found to dramatically increase after the tracer had reached the lateral boundaries of the basin, suggesting boundary mixing as the key process for basin-scale vertical mixing. Basin-scale vertical diffusivities were of the order of 10−5 m2 s−1 (about 1 order of magnitude larger than interior diffusivities) with evidence for a seasonal and vertical variability. In contrast to tracer experiments in the open ocean, the basin geometry (hypsography) was found to have a crucial impact on the vertical tracer spreading. The e-folding time scale for deep water renewal due to mixing was slightly less than 2 years, the time scale for the lateral homogenization of the tracer patch was of the order of a few months. Key Points: Mixing rates in the Gotland Basin are dominated by boundary mixing processes; The time scale for Gotland Basin deep water renewal is approximately 2 years; Mixing rates determined from the tracer CF3SF5

Dense bottom gravity currents and their impact on pelagic methanotrophy at oxic/anoxic transition zones

We show that inflows of oxygenated waters into sulfidic layers have a strong impact on biogeochemical transformation at oxic/anoxic transition zones. Taking the pelagic methane dynamics in the Gotland Basin as an example, we performed our studies when one of the largest inflows ever recorded entered the Baltic Sea in March 2015. An inflowing gravity current transported oxic waters into the sulfidic deep layers and freshly generated a near-bottom secondary redox interface. At the upper slope, where the inflowing water masses were vigorously turbulent and the main and secondary redox interfaces in close contact to each other, methane oxidation rates inside the transition zone were found to be higher compared to the weakly turbulent basin interior. At the main redox interface in the basin center, lateral intrusions of oxygenated waters into intermediate water depth may have stimulated the growth of the methanotrophic community and their activity.

Hunting a New Ocean Tracer

A useful method to obtain integrated estimates of vertical mixing in the ocean over a long period of time and a large area is the release of a tracer. Recent large-scale tracer release experiments conducted in the Southern Ocean, such as the Diapycnal and Isopycnal Mixing Experiment (DIMES [see Gille et al., 2007]), and in the equatorial Atlantic will rely on a new tracer chemical called trifluoromethyl sulfur pentafluoride (SF5CF3), which is likely to become a standard for future experiments. Here we report results from the first injection of pure SF5CF3 into the ocean, which was carried out in a deep basin of the Baltic Sea. Using the Baltic Sea as a natural laboratory for the investigation of physical mixing processes, this pilot study aims at improving our understanding of one of the most puzzling mixing properties in stratified ocean basins: Almost independent of the basins size, the basin-scale vertical mixing rates exceed the rates inferred from local turbulence measurements in the basin center by approximately 1 order of magnitude [see, e.g., Ledwell and Bratkovich, 1995].

Diffusive Convection under Rapidly Varying Conditions

AbstractIn most observations of diffusive convection in the ocean and in lakes, the characteristic diffusive staircases evolve over long time scales under quasi-stationary background conditions. In...