350 ka Organic δ13C record of the monsoon variability on the Oman continental margin, Arabian Sea

Abstract

The stable isotope compositions of sedimentary organic carbon and content of organic carbon for sediment cores recovered at two sites (sites 724C and 725C) during Ocean Drilling Program (ODP) Leg. 117 on the Oman continental margin are used to document variability of the monsoon winds for the past 350 ka. Although both sites have a mean δ13C value of -20.1%o, three zones depleted in 13C are observable at site 724C during isotope stages 3, 8 and 10, while only one zone is recognizable at site 725C. Increased coastal upwelling during isotope stage 3 owing to intense SW monsoon winds resulted in higher concentration of CO2 in the water column causing the formation of organic matter that was depleted in 13C. The other two zones deposited during oxygen isotope stages 8 and 10, which are also characterized by low values of organic carbon, nitrogen and C/N ratios, could be attributed to the dilution by terrestrial material derived from paleosol transported by northwesterlies. Because of utilization of 13C enriched dissolved CO2 during the last glacial maximum Holocene sedimentary organic materials are depleted in 13C relative to the former. The content of residual organic carbon (ROC) is higher at site 724C (with an average of 2.3 ±1.2%) relative to site 725C, which averages to 0.9 ±0.4% probably because of differences in the degree of preservation. Organic material deposited at site 725C has undergone more degradation relative to site 724C as reflected by a systematic downcore decrease in 13C resulting from a loss of 13C enriched organic compounds. Owing to lack of good chronology at site 725C, a zone that is characterized by low δ13C values it could not be correlated with the other three zones observed at Site 724C.