Ocean Drilling Program, United States

A high-resolution elemental, isotopic, and molecular record of organic matter was established for the uppermost sediments of Hole 1229E (0-700 cm below seafloor), drilled during Ocean Drilling Program (ODP) Leg 201 on the Peruvian margin. Diagnostic parameters of organic matter (OM) such as the atomic C (sub org) over total nitrogen ratio (C/N), the organic carbon isotopic signatures (delta 13C (sub org) ), and ligninderived biomarker concentrations (Sigma 8) all suggest that terrigenous OM influx was more important during glacial than interglacial periods on the Peruvian shelf. In contrast to other shelf environments, however, fluctuations in lignin ratios ([Ad/Al]v, S/V, and C/V) across the Pleistocene-Holocene transition point to shifting composition in terrigenous OM sources from coarse and undegraded woody materials during the Last Glacial Maximum (LGM) to much more altered and finer soft tissue materials during the Holocene. In addition, the total OM content preserved in the Peruvian shelf sediments was lower during the LGM. This observation is in agreement with prior work in the region and may be explained by a migration of the upwelling cells toward deeper sections of the shelf during glacial lower sea level stances. The potential decrease in upwelling strength at ODP Site 1229 and other similar shallow sites of the region may thus have been responsible for lower watercolumn productivity and decreased preservation potential (decrease in oxygen minimum conditions) of settling organic matter. Finally, during the Holocene, the shifts in Corg concentrations, (C/N) ratios, and delta (super 13) C (sub org) signatures are not entirely controlled by terrigenous OM inputs in this system. Apparent terrigenous signatures during that period seem to be derived from diagenetic processes in addition to potential source changes.