THE GLOBAL PHOSPHORUS CYCLE

Phosphorus (P) is a limiting nutrient for terrestrial biological productivity, and thus it commonly plays a key role in net carbon uptake in terrestrial ecosystems. Unlike nitrogen (another limiting nutrient but one with an abundant atmospheric pool), the availability of “new” P in ecosystems is restricted by the rate of release of this element during soil weathering. Because of the limitations of P availability, P is generally recycled to various extents in ecosystems depending on climate, soil type, and ecosystem level. The weathering of P from the terrestrial system and transport by rivers is the only appreciable source of P to the oceans. On longer time scales, this supply of P also limits the total amount of primary production in the ocean.

We focus here on two related aspects of the global P cycle: the release of P from landscapes and the role of margins in the modulating P delivery to the ocean since the Last Glacial Maximum. Detailed geochemical analysis of P in lake-sediment cores reveal large changes in terrestrial P cycling on these time scales. P is transformed from a mineralized form to more bioavailable forms with increased soil development and landscape stability in the catchments. This process occurs relatively rapidly (i.e., within 3000-5000 yr) and can be reset quickly; the degree of transformation depends on local climate and the starting point of the soil system. The release of solid-phase P from these landscapes follows modeled patterns of high rates during the initial stages of soil development and low rates upon soil and landscape stabilization. These records point to significant variability in the terrestrial P mass balance on glacial time scales. Large changes in marginal sediment dynamics also occur on these time scales, driven by eustatic sea level rise. These changes are reflected in P input/throughput/output along margin systems, and were explored in several settings. When explored from a mass balance perspective, the margins become a huge variable filter modulating the delivery of P to the open ocean and driving non-steady state conditions for the oceanic P cycle. Thus, the oceanic record of P accumulation on glacial timescales has to be considered in light of both changes in weathering and changes in marginal burial.