Rates of formation of modern phosphorite off western Mexico

Abstract 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 region, O 2 -depleted (5–50 μM) bottomwater overlies the seafloor, and a narrow 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%) sediment. Interstitial gradients of total alkalinity (TA) and inferred effects of organic-matter decay correlate directly with sedimentation rates calculated from 14 C dating. Other porewater and solid-phase data indicate that francolite currently precipitates authigenically at each of five study sites. Moderately high concentrations (20–100 μM) of dissolved inorganic phosphate (DIP) produced in the upper 5 cm of sediment sustain francolite growth at slightly greater depths, while the necessary F − diffuses into the sediment from above. Neither organic-matter decay nor fish-bone dissolution seem capable of producing the steep, near-surface gradients of DIP measured in this study. At some sites, DIP production coincides spatially with the reversible production of dissolved Fe 2+ , 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 unidentified. Box cores from each site contain an unreworked sequence of up to six thin (1–3 cm), discrete, francolite-rich horizons, several of which qualify as true phosphorite (≥ 50 wt% francolite). Estimated diffusional burial fluxes of inorganic P and F range respectively from 2–40 μg cm −2 y −1 and 4–12 μg cm −2 y −1 , or the equivalent in francolite of 16–260 μg cm −2 y −1 . These present-day accumulation rates lie within the range of those determined for several ancient phosphorite deposits and for the extensively studied region of modern phosphorite deposition off Peru.