Abstract

Past variations in the terrestrial cycle of phosphorus (P), a biolimiting nutrient, are poorly understood. Detailed geochemical analysis of P in lake-sediment cores spanning the late Pleistocene to the present from the western Appalachian Plateau, the Coast Mountains of British Columbia, and the San Bernardino Mountains of southern California reveal large changes in terrestrial P cycling. 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. Changes in weathering style were confirmed for the southern California catchment by analysis of Ge/Si ratios of lake diatoms, which revealed substantially higher values during an interval of high P release and low values at present. These records point to significant variability in the terrestrial P mass balance on glacial time scales, a feature that needs to be incorporated into models of nutrient and carbon cycling in the past.

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