Geochemical speciation and spatial distributions of phosphorus in surface sediments from the basin of the Marcus-Wake seamounts in the western Pacific Ocean

The concentrations of five forms of phosphorus (P) including exchangeable or loosely adsorbed P (Ex-P), Fe-bound P (Fe-P), authigenic P (Auth-P), detrital P (Det-P), and organic P (Org-P) from the basin among the Marcus-Wake seamounts (19.4°–24°N, 156.5°–161.5°E) in the western Pacific Ocean were quantified using a sequential extraction method (SEDEX) to investigate the distribution and sources of different P species. Concentrations of total P (TP) varied from 14.0 μmol/g to 44.1 μmol/g, with an average of (32.4±7.7) μmol/g. Inorganic phosphorus, which was the major chemical form of sedimentary P, ranged from 12.6 μmol/g to 40.6 μmol/g, while the concentration of Org-P varied between 1.38 μmol/g and 5.18 μmol/g, accounting for 83.4%–93.4% and 6.6%–16.6% of the TP, respectively. The relative proportions of the five P species followed the order of Det-P>Auth-P>Org-P>Fe-P>Ex-P. On average, Det-P was the major P sink resulted from the atmospheric input and accounted for approximately 58.9%±12.4% of the TP. Auth-P and Org-P comprised 22.8%±11.4% and 11.5%±3.0% of the TP, respectively, while Fe-P accounted for 5.1%±2.6%. Lastly, Ex-P comprised 1.6%±0.3% of the TP. Org-P exhibited a negative correlation with Fe-P and Auth-P, while Fe-P showed a positive correlation with Auth-P. This indicated that the formation of Fe-P and Auth-P was at the expense of the regeneration or remineralization of Org-P during early diagenesis. High concentrations of Det-P and Auth-P as well as a low ratio of total organic C to reactive P (TOC/Rea-P) suggested that the aeolian input may play a significant role in sedimentary P budget in the study area. Additionally, well-oxygenated bottom water and low sedimentation rate could be responsible for the low TOC/Org-P ratio in the sediment.