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Abstract:
Fluxes between the ocean waters and the sediments are key regulation processes for the marine biogeochemical cycles and, thus, their quantification is of crucial importance. At this transition it is ultimately determined how much of a primary particulate signal is preserved or mineralized and hence recycled. Our review summarizes two major approaches how to use spatial information obtained from surface sediments: (1) In the first part we summarize the state-of-the-art regarding the use of biogenic barium as a proxy for primary productivity. We discuss the possibilities and limitations of this approach mainly based on the results of a recent study in the South Atlantic. The general outcome of this study was that the spatial pattern of primary productivity can well be traced back by calculating (sub-)recent accumulation rates of biogenic barium and applying available and newly formulated empirical equations. Most of those equations, however, fail to give the really observed magnitude of today’s productivity values. The main reasons for this are mostly the uncertainty of the Corg/Babio depth relation, which differs between distinct ocean regions, dynamic sedimentary processes at ocean margins combined with badly constrained values of terrigeneous barium input, and the effect of barite dissolution due to subsequent anoxic diagenesis. To improve the quality of prognoses for past productivity multi proxy approaches are recommended to bypass the uncertainty in predictions from a single proxy. (2) The more extensive second part is based on the large amount of studies that aimed at the quantification of benthic fluxes of nutrients and oxygen, which are good measures for the amount of reactive particulate material being mineralized at the seafloor and thus returned into the marine cycle. Those results enabled us to give profound calculations of the benthic oxygen consumption and the release of nitrate, phosphate, and silicate at the seafloor of the South Atlantic and give upscaled estimates for the global area of the sea floor. Additionally, we discuss more detailed studies focusing on control parameters for benthic fluxes like primary production and lateral advection along the ocean margins off Southwest Africa and Argentina. A very conspicuous result was obtained by calculating mass balances for biogenic opal in those regions indicating a dramatic underestimation of accumulation fluxes of opal by “conservative” methods, which is believed to be of global significance. The last section mainly focuses on the effect of benthic mineralization on the dissolution of calcium carbonate even above the chemical lysocline. This process is in discussion since more than two decades. A number of studies have been performed, mainly using in situ devices, to determine CaCO3 dissolution. We summarize and discuss the results obtained from the South Atlantic and use a recently developed empirical algorithm to show the worldwide distribution of supralysoclinal CaCO3 dissolution fluxes in marine surface sediments and give an estimate of their total amount. Finally, a table for benthic fluxes of major constituents is provided on ocean wide and global scales.
