Rates of formation of modern phosphorite off western Mexico

Details Rates of formation of modern phosphorite off western Mexico Rates of formation of modern phosphorite off western Mexico

Nov 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.5001s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 22, pp.5001-5010

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4

Scientific paper

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 M) 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 M) 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 g cm -2 y -1 and 4-12 g cm -2 y -1 , or the equivalent in francolite of 16-260 g 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.

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