THE PHOSPHORUS CYCLE, PHOSPHOGENESIS AND MARINE PHOSPHATE-RICH DEPOSITS

Phosphorus (in the form of phosphate) is an essential nutrient and ene rgy carrier on many different levels of life, and a key element in med iating between living and lifeless parts of the biosphere. One of the most important aspects of the phosphorus cycle is its vital role in go verning productivity, thereby interacting with the exogenic part of th e carbon cycle, which, in turn, is important in regulating Earth's cli mate. Phosphorus is a prime element to be traced in Earth's history, b ecause it allows for the reconstruction of long-term feedback mechanis ms between climate, environment and ecology, and of global change as s uch. Marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle, because, in the case of ubiquit y, their origin may result from a general acceleration of the global p hosphorus cycle. Sources of sedimentary phosphate are microbial breakd own of buried organic matter and redox-driven phosphate desorption fro m iron and manganese oxyhydroxides. Dissolved sea-water phosphate repr esents an additional source which may become important in the formatio n of phosphatic hardgrounds. The main locus of phosphogenesis is near the sediment-water interface, but phosphogenesis also occurs at greate r sediment depths. Current-induced winnowing and transport processes a long the sea floor concentrate phosphate precipitates into deposits, w hich exhibit internal stratification patterns typical for the prevaili ng hydraulic energy regime. In a sequence-stratigraphic context, phosp hate deposits preferentially occur along marine or maximum flooding su rfaces. Consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits. (Bio-)chemical weatheri ng on continents represents the most significant source of bioavailabl e phosphorus. This implies that long-term changes in marine phosphorus levels - and with these changes in marine ecology, productivity rates and ratios of exported carbonate carbon and organic carbon - are a re sponse to changes in continental weathering rates. A compilation of ma rine sedimentary phosphorus burial rates for the last 160 Myr suggests that natural variations have occurred that span one order of magnitud e. For the late Jurassic, Cretaceous and most of the Paleogene, the ph osphorus cycle appears to have been accelerated in times of climate wa rming, which was most likely due to the spreading of zones of humid cl imate and more intense continental weathering. In the Neogene, the pho sphorus cycle appears to have responded to changes in glacially induce d weathering. This suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated wit h caution. Integrated analyses of the sedimentary and biogeochemical c ontext of phosphorite occurrences may help to identify paleoenvironmen tal conditions, as well as to improve our understanding of periods of enhanced phosphate accumulation, periods which were usually characteri zed by steep gradients in the development of climate and environment. With regard to the complexity of feedback mechanisms between the phosp horus cycle and the biosphere, the present-day input rates of phosphat e into the world's oceans should be of great concern. They are more th an doubled by anthropogenic means and affect ecological systems on a r apidly increasing scale.