350 ka Organic delta C-13 record of the monsoon variability on the Oman continental margin, Arabian Sea

The stable isotope compositions of sedimentary organic carbon and content o f 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 3 50ka, Although both sites have a mean delta C-13 value of -20.1 parts per t housand, three zones depleted in C-13 are observable at site 724C during is otope 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 mon soon winds resulted in higher concentration of CO2 in the water column caus ing the formation of organic matter that was depleted in C-13. The other tw o zones deposited during oxygen isotope stages 8 and 10, which are also cha racterized 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 C-13 enriched di ssolved CO2 during the last glacial maximum Holocene sedimentary organic ma terials 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 725C as reflecte d by a systematic downcore decrease in C-13 resulting from a loss of C-13 e nriched organic compounds. Owing to lack of good chronology at site 725C, a zone that is characterized by low delta C-13 values it could not be correl ated with the other three zones observed at Site 724C.