Using geochronology to reconstruct the evolution of particulate phosphorus inputs during the past century in the Papeete Lagoon (French Polynesia)

During the past century, the sediment deposition rate has been assessed by measuring excess Pb-210 activity in a sediment core sampled in the harbour area of the Papeete Lagoon (Tahiti, French Polynesia). Dated sediment samples were analysed following a sequential extraction method to quantify five sedimentary phosphorus pools: loosely sorbed or exchangeable phosphorus, ferric iron-bound phosphorus, authigenic calcium carbonate associated phosphorus, detrital apatite and detritic inorganic phosphorus, acid organic phosphorus. Results from sediment dating showed the core to cover the period from 1865 to 1995, with a sharp change around 1957 corresponding to a doubling in sediment deposition rates. From the bottom to the top of the core, concentrations decreased for authigenic calcium carbonate-associated phosphorus, were almost constant for exchangeable phosphorus, and increased significantly for iron-bound phosphorus, detritic inorganic phosphorus and organic phosphorus. As a consequence, the calculated phosphorus total accumulation rate was multiplied by four from top to bottom. Those trends were related to modifications in the relative importance of the five phosphorus reservoirs, especially for organic phosphorus, which contribution increased to 30% of total phosphorus in recent sediments. From the sequential extraction approach it is possible to estimate that phosphorus with a terrestrial origin represented 54% of total phosphorus in recent sediment, while it only represented 30% at the beginning of the century. Present particulate phosphorus inputs were also assessed from a sediment trap approach and a budget of phosphorus burial efficiency is proposed.