Burial of fish bones, which consist primarily of hydroxyapatite, has been recognized as a mechanism to remove reactive phosphorus from the oceans. In this study, a new method is presented, which differentiates P associated with biogenous apatite (P_fish) from other P fractions. The method, consisting of a sequential chemical extraction with 2 M NH₄Cl, has been successfully tested on standard materials. It enables us, for the first time, to quantify fish debris in sediment records and to assess their importance for the marine phosphorus cycle.

The NH₄Cl extraction has been applied to sediment samples from the Arabian Sea. Preservation of fish debris is significantly higher in sediments located above 1,200 m water depth than it is for deeper sediments. The distribution of P_fish contents in surface sediments is predominantly governed by the extent of fish debris regeneration, which is related to differences in water depth and sedimentation rates. In addition, a good correlation between high sedimentary P_fish contents and low oxygen bottom water concentrations suggests that the presence of the intense oxygen minimum zone may account for the enhanced preservation of fish debris in continental slope sediments.