Francolite authigenesis in California continental slope sediments and its implications for the marine P cycle

Abstract

Authigenesis of francolite (carbonate fluorapatite) in continental slope sediments may represent the most important oceanic sink for reactive P. We infer from sedimentary P speciation and pore-fluid fluoride concentrations that francolite currently forms in Holocene sediments on the California continental slope. Authigenic-P accumulation rates range from 0.006 to 0.089 mu mol/cm(2)/yr, and dissolved fluoride uptake rates range from 0.004 to 0.039 mu mol/cm(2)/yr. These two independent parameters correlate positively with each other among all sites (R(2) = 0.77), and the slope of their linear relationship gives a P/F molar ratio of 2.4, a typical value for authigenic francolite. Our results imply that active francolite authigenesis sequesters nearly 30% of the remobilized organic and Fe-bound P at shallow burial depths in California continental slope sediments. Assuming that similar processes occur globally, we estimate that francolite authigenesis in near-surface, continental-slope sediments may account for a burial flux of 1.4 +/- 1.0 x 10(10) mol P/yr, or 8 to 48% of the global input flux of reactive P to the ocean. (C) 1999 Elsevier Science Ltd.