Abstract:
Rock magnetic concentration, grain size and mineralogy parameters together with organic carbon, calcium carbonate, redox-sensitive elements, δ18O of Globigerinoides ruber and radiocarbon dating were carried out on a 445 cm long sediment core collected at 1380 m depth off Mangalore, southwestern margin of India. The top 290 cm sediments of the core correspond to the last 18 kaBP. The δ18O and magnetic records exhibit major events at ∼ 16 kaBP, 14.5 kaBP, 11.5 kaBP and 9.8/8.6 kaBP related to start and intensity of the summer monsoon and climate change, and are synchronous with that of the western Arabian Sea and North Atlantic. The sediments with high magnetic susceptibility correlate with high sedimentation rates. The sediments are dominated by fine-grained magnetite, but intervals of 1.2–3.8 kaBP and 10–13.5 kaBP were subjected to diagenetic changes, resulting in the dissolution of fine-grained magnetites and enrichment of redox-sensitive trace elements (Cu, Ni, Zn, V, Mo and U). The sediments between 290 cm and 445 cm correspond to 18–27 kaBP and are characterized by distinct decrease in magnetic concentration, grain size and mineralogy parameters, high organic carbon, low concentrations of redox-sensitive trace elements and abundant pyritized tubules. The reductive diagenetic conditions indicated by rock magnetic properties are in contrast with the weak sub-oxic conditions revealed by low concentrations of trace elements in the sediments. The seasonal organic matter flux produced during the winter monsoon and moderate sedimentation rates favoured reductive diagenesis in the sediments at and below the last glacial maximum (LGM). Intermittent bioturbation, however, allowed oxidants to penetrate into the sediments, remobilized redox-sensitive trace elements into the water column and modified the primary geochemical signal of the sedimentary environment.
