THE PHOSPHORUS CYCLE, PHOSPHOGENESIS AND MARINE PHOSPHATE-RICH DEPOSITS

Abstract

Phosphorus (in the form of phosphate) is an essential nutrient and energy carrier on many different levels of life, and a key element in mediating between living and lifeless parts of the biosphere. One of the most important aspects of the phosphorus cycle is its vital role in governing productivity, thereby interacting with the exogenic part of the carbon cycle, which, in turn, is important in regulating Earth's climate.Phosphorus is a prime element to be traced in Earth's history, because it allows for the reconstruction of long-term feedback mechanisms between climate, environment and ecology, and of global change as such. Marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle, because, in the case of ubiquity, their origin may result from a general acceleration of the global phosphorus cycle. Sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox-driven phosphate desorption from iron and manganese oxyhydroxides. Dissolved sea-water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds. The main locus of phosphogenesis is near the sediment-water interface, but phosphogenesis also occurs at greater sediment depths. Current-induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits, which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime. In a sequence-stratigraphic context, phosphate deposits preferentially occur along marine or maximum flooding surfaces. Consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits.(Bio-)chemical weathering on continents represents the most significant source of bioavailable 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 response to changes in continental weathering rates. A compilation of marine sedimentary phosphorus burial rates for the last 160 Myr suggests that natural variations have occurred that span one order of magnitude. For the late Jurassic, Cretaceous and most of the Paleogene, the phosphorus cycle appears to have been accelerated in times of climate warming, which was most likely due to the spreading of zones of humid climate and more intense continental weathering. In the Neogene, the phosphorus cycle appears to have responded to changes in glacially induced weathering. This suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution. Integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions, as well as to improve our understanding of periods of enhanced phosphate accumulation, periods which were usually characterized by steep gradients in the development of climate and environment.With regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere, the present-day input rates of phosphate into the world's oceans should be of great concern. They are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale.