RATES OF FORMATION OF MODERN PHOSPHORITE OFF WESTERN MEXICO

Phosphorite deposition represents a significant but poorly constrained sink in the marine P cycle. This study establishes conclusively that phosphorite forms today throughout a widespread region along the upper continental slope off southern Baja California, Mexico. In this regio n, O2-depleted (5-50 muM) bottomwater overlies the seafloor, and a nar row range of suboxic to anoxic redox conditions prevail in the modern (0-18,000 yr old), near-surface (0-50 cm), organic-C-rich (2-10 wt%) s ediment. Interstitial gradients of total alkalinity (TA) and inferred effects of organic-matter decay correlate directly with sedimentation rates calculated from C-14 dating. Other porewater and solid-phase dat a indicate that francolite currently precipitates authigenically at ea ch of five study sites. Moderately high concentrations (20-100 muM) of dissolved inorganic phosphate (DIP) produced in the upper 5 cm of sed iment sustain francolite growth at slightly greater depths, while the necessary F- diffuses into the sediment from above. Neither organic-ma tter decay nor fish-bone dissolution seem capable of producing the ste ep, near-surface gradients of DIP measured in this study. At some site s, DIP production coincides spatially with the reversible production o f dissolved Fe2+, but thorough evaluation of the possible link between these processes requires further study. The mechanism responsible for the near-surface enrichment of interstitial DIP thus remains unidenti fied. Box cores from each site contain an unreworked sequence of up to six thin (1-3 cm), discrete, francolite-rich horizons, several of whi ch qualify as true phosphorite (greater-than-or-equal-to 50 wt% franco lite). Estimated diffusional burial fluxes of inorganic P and F range respectively from 2-40 mug cm-2 y-1 and 4-12 mug cm-2 y-1, or the equi valent in francolite of 16-260 mug cm-2 y-1. These present-day accumul ation rates lie within the range of those determined for several ancie nt phosphorite deposits and for the extensively studied region of mode rn phosphorite deposition off Peru.