31. DEPTH TRENDS IN PHOSPHORUS DISTRIBUTION AND C:N:P RATIOS OF ORGANIC MATTER IN AMAZON FAN SEDIMENTS: INDICES OF ORGANIC MATTER SOURCE AND BURIAL HISTORY1K.C. Ruttenberg2 and M.A. Goņi3 |
ABSTRACTSolid-phase total, inorganic, and organic phosphorus (TP, IP, and OP) concentrations and C:N:P ratios of sediment organic matter from Sites 932 and 942 on the Amazon Fan, collected during Ocean Drilling Program Leg 155, are presented here. Depth trends in phosphorus distribution are consistent with lithostratigraphic evidence for variations in the supply of terrestrially derived detritus, display evidence of remineralization of OP with depth, and are suggestive of reincorporation of remineralized OP by secondary authigenic phosphate minerals. The diagenetic transformation of phosphorus suggested by the solid-phase data is supported by pore-water data. Variations in elemental C:N:P ratios with depth are interpreted as resulting predominantly from changing proportions of terrestrially derived vs. marine-derived organic matter. The timing of these shifts is consistent with those predicted on the basis of lithostratigraphic evidence, and other organic geochemical data, which suggest that these ratios may be used as indicators of organic-matter source. This conclusion is supported by the systematic correlation of organic matter-derived C:N:P elemental ratios with the carbon isotopic composition of organic matter from these sediments. Taken together, the lithostratigraphic, elemental ratio, and carbon isotopic composition data, are consistent with shifts between the accumulation of dominantly terrestrially derived detritus on the Amazon Fan during times of low sea-level stand, giving way to dominantly hemipelagic sedimentation during times of high sea-level stand. The oxygen isotope stratigraphy developed for Sites 932 and 942 is consistent with the timing of sea-level change inferred from the C:N:P elemental ratios and carbon isotopic signatures of organic matter in these sediments. |
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