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Phosphorites on the Peru shelf occur mainly as dark and dense conglomerate (“Diphosphates) and occasionally as soft, friable (“F”-phosphatcs) protocrusts, which appear as laterally extensive, centimeter–thick plates of bored, pbosphatized sediment. Several undisturbed and well–documented samples of these protocrusts were collected by box coring and submersible operations during an expedition of the R/N Seaward Johnson in 1992. Three of these crusts were analyzed in layer-by-layer fashion for U-series isotopes, while two were analyzed for AMS-14C. and stable carbon isotopes. Unlike the D-phosphates, which may show a progressive decrease in their δ13C values (from 0 to –5%c) with a co-linear increase in structural CO2 contents (from 2–6 wt. %), the protocrusts show a more restricted range with most values from 0 to –2%,δ13C and 4.2-4.7 wt.% CO2, indicative of their precipitation directly on or slightly below the seafloor.

U-series disequilibrium results (230Th/234U, 231Pa/235U) indicate that the protocrusts are very young. They are so young, in fact, that corrections for“common” 230Th and 231Pa (that is, those amounts not associated with decay of the incipient uranium) are very important, and different interpretations (of growth direction, for example) are possible depending upon which assumptions are applied. AMS 14C results are very consistent with the U-series ages in the topmost, highest-phosphate portion of these crusts. A few millimeters into the less phosphatic portions of the crusts, uncorrected 230Th and 231Pa ages diverge from the l4C ages, probably because of contamination of these daughter nuclides from associated sediment grains. An isochron approach confirms that there is a minimal age difference between the upper and lower layers of the protocrusts. All protocrusts display a distinct trend of decreasing phosphate content with depth into the crust. Based on these observations, we propose a growth model that suggests that phosphatic protocrusts grow upward at rates of 2–9 mm/ky in response to downward diffusion of phosphate from an interstitial phosphate pore water spike just below the sediment-seawater interface. Later exhumation of protocrusts resulted in erosion into the more common and more complicated phosphatic hardgrounds, conglomerates, and nodules found scattered throughout the sediments of the Peru shelf upwelling zone.

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