Biogeochemical transformations and fluxes in redox-stratified environments: from the shallow coastal ocean to the deep subsurface. Geologica Ultraiectina (291)

Abstract

Biogeochemical activity is often concentrated at interfaces where the supplies of bioessential elements and energy converge. Processes in these environments control to a large extent the exchange fluxes between the biosphere, atmosphere, hydrosphere and geosphere and, therefore, play an important role in global biogeochemical cycles. In this study, reactive transport modeling is used to advance the mechanistic understanding and quantitative analysis of the complex network of transformations and fluxes that control the biogeochemical dynamics in two important interfacial areas: the land-ocean transition zone and deeply buried marine sediments. Major goals are to unravel the relative importance of system processes, external forcings and boundary conditions in the overall functioning of the biogeochemical system, to identify and quantify the controlling parameters, and, in a more general sense, to illustrate the diagnostic, prognostic and integrative uses of RTMs in the field of biogeochemistry. Simulation results show that an assessment of the relative importance of system processes, external forcings and boundary conditions in the overall functioning of the biogeochemical system inevitably requires an integration of the full suite of interacting physical, biological and chemical processes that control the coupled transformations. Biogeochemical dynamics have to be evaluated in the context of the physical environment. Simulation results illustrate that the synchrony of physical and biogeochemical events and the time scales at which they interact have a significant influence on the system's functioning. Particularly along the highly dynamic land-ocean continuum, the hydrodynamic processes sustain a permanent hydrodynamic stress on the entire system, which imposes first-order constraints on the distributions of chemical species and organisms. In addition, the RTMs ideally complement field observations, since they allow investigating dynamics over spatial and temporal scales that are not readily accessible with observations. The integration of local observational data and reactive transport modeling results in a comprehensive, synoptic view of the biogeochemical dynamics along the entire land-ocean continuum. In addition, results showed that the quantification of element fluxes is hardly achievable by observations alone because the oscillatory tidal flux is generally several orders of magnitude larger than the biogeochemically-relevant residual flux. For deeply buried sediments, reactive-transport modeling helped hindcast the diagenetic history that created the observed distribution of authigenic barites and it provided a detailed, quantitative understanding of the biogeochemical dynamics over geological timescales (100 Myrs). In addition, the RTMs allow inferring information about unmeasurable or expensively measured properties from more readily measured variables that are related to the variable of interest. They prove useful to infer the rate of biogeochemical transformation rates from available porewater depth profiles and, therefore, help to advance a quantitative understanding of the deep biosphere in deeply buried sediments. The mechanistic understanding gained through these detailed studies of different compartments of the earth system can be used to develop a common, comprehensive and conceptual approach that will facilitate the extrapolation of this knowledge to the global scale. It can guide meaningful simplifications of the complex biogeochemical cycling in different environments and thus provides an important basis for the construction of global biogeochemical models.