Mid to Late Holocene Reconstruction of the Southwest Monsoonal Shifts Based on a Marine Sediment Core, off the Landfall Island, Bay of Bengal

Abstract

Undisturbed marine sediment cores raised from the oceans hold continuous records of sedimentation and palaeoenvironmental changes, hence it is suitable material for reconstructing past climate fluctuations. In the present study a marine sediment core about 124 cm raised from a depth of 250 m, near the Landfall Island, North Andaman, Bay of Bengal (BOB) was used to reconstruct the south west monsoonal shifts over the Indian sub-continent and BOB during the mid to late Holocene by using various proxies such as clay mineralogy, oxygen (δ18O) and carbon (δ13C) stable isotopes, and nanoplankton. Five organic carbon sediment samples were radiocarbon dated. The calibrated radiocarbon date ranges in age from (6078 to 1658 years BP) that is from the mid to late Holocene period. The texture analysis indicates that the sediments are predominantly clayey silt in nature. Smectite, illite, kaolinite and chlorite were the clay minerals present in the sediment core in which smectite and illite are dominant. The high smectite content reflects the weathering product of mafic rock contributed from the nearby island whereas illite is predominantly coming from the rivers such as Irrawaddy, Salween and Sittang rivers from Myanmar. Occurrence of nanoplankton such as Gephyrocapsa oceanica, Emiliania huxleyi, and Ascidian spicules in the calcareous nanoplankton assemblage suggest a mixed source also from the sedimentary rocks of the Mio-Pliocene age. The sediment core reveals layers of coarser sand flux since ~6500–6000 years BP and ~3300 years BP that reflects a strengthened South West Monsoon (SWM) in an overall weakening of the SWM from the middle Holocene (6000 yrs BP) to the late Holocene period (2000 years BP). This is also inferred from the high smectite, high C/I ratio and low K/C ratio. Within this period, an intense weakening of the SWM is noticed at ~4400–4200 years BP (Sub—Boreal Optimum), which is also supported by the δ18O data of G. ruber becoming more positive from −3.39 to −2.33‰. A major wet phase was found reaching its maximum around 3400–3200 years BP and amelioration in climate ~2000 years BP with a warm dry phase ~2200–1800 years BP (Roman Warm Period) followed by the Medieval Warm Period (1000–800 years BP).