Graduate Thesis Or Dissertation
 

Sedimentary organic matter : distribution and alteration processes in the coastal upwelling region off Peru

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  • Recent organic-rich deposits on the Peru continental margin occur along the upper slope (100-450 m) between about ll°S and 16°S and along the lower continental slope (>2000 m). The upper slope deposits reflect high biological production in response to coastal upwelling, but preservation of organic matter is enhanced by the low oxygen content of bottom waters and rapid sedimentation, as well. The lower slope deposits record both subsequent downslope redistribution and primary input. In the temporal record of the Peru upper slope, a series of onlapping sediment sequences are separated by hiatuses. Currents impinging on the slope have caused the hiatuses. However, the timing cf the hiatuses and changes in bulk sediment and organic carbon accumulation rates during periods for which there is an extensive sediment record, may be attributed to global climatic and oceanographic variations throughout the Late Quaternary. A smaller scale temporal picture of the fluctuations in organic matter cycling and accumulation at three upper slope sites (~11°S) is reflected in the chemical and isotopic signature of 26 insoluble and non-hydzolyzable proto-kerogens, which constitute the major organic fraction (>60%) of the Peru sediments. Proto-kerogen 0/C and H/C ratios were found to be low during cold Holocene climatic periods and to increase during warm periods. These variations are attributed to the degree of aerobic biodegradation prior to burial, and, interpreted to indicate a more depleted oxygen minimum during Holocene Neoglacial periods. ẟ¹³c values were characteristic of organic matter derived from marine plankton (-21.3 to -23.2). ẟ¹⁵N values were anomalously low (-0.5 to +6.2) and may be due to an admixture of ¹⁴N-enriched mesopelagic or benthic biomass with detrital organic matter prior to burial. An important source of benthic bioinass in the Peru coastal upwelling region is the chemolithotrophic sulfide bacterium, Thioploca. The "sheaths" of these bacteria were seen (by SEN-EDAX) to have a significant effect on the microstructure of certain highly porous upper slope surface sediments, and to provide labile substrates for sulfate reduction. In dawncore sediments the microstructure appeared dominated by organo-xnineral aggregates, and these probably cause higher than expected undrained shear strengths and apparent overconsolidation. Finally, the partitioning of organic carbon fluxes at two sites on the Peru margin was studied to determine the relative importance of pre-burial benthic utilization and early diagenesis as limiting factors for the burial of sedimentary organic matter. These sites were also compared to three SW Pacific-Antarctic stations where bottom sediments are oxic and nitrification is the dominate diagenetic reaction consuing organic matter. The results showed that 35- 85% of the total near-bottom organic carbon flux is utilized before burial, and that this percentage is not a priori higher in oxic environiftents. Rate constants for the regeneration of organic carbon from oxygenated sediments are intepreted to reflect bioturbation. in addition to the catabolic processes of microbially mediated diagenesis.
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