Environmental magnetic records of the sediments from the Andaman Sea: Evidence for late Quaternary changes in provenance and monsoon

Abstract

Two AMS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> C dated gravity cores from the northern and southern Andaman Sea have been investigated using the environmental magnetic properties of the bulk sediments. The cores exhibit significant variations in environmental magnetic parameters (χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lf</inf> , χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fd</inf> %, χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ARM</inf> , SIRM, HIRM and various inter parametric ratios) from the last glacial maximum (LGM) to the Holocene. During the last glacial period (26-11.6 ka), the southern Andaman Sea sediments showed relatively lower magnetite content compared to the northern Andaman Sea. This reflects the decline in sediment contribution from Himalayan Rivers to the Andaman Sea, which is most likely constrained by the combined effect of the weaker Indian summer monsoon (ISM) and the large extent of glaciations. During this period, peninsular Myanmar Rivers (Irrawaddy, Salween and Sittang) were the dominant contributors of sediments to the Andaman Sea. The Holocene sediments are characterized by lower magnetite content as seen through a decline in values of concentration-dependent magnetite parameters (χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lf</inf> , χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ARM</inf> , SIRM). This period is characterized by lesser input of magnetically finer grains as indicated by lower values for magnetic grain size proxies (χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ARM</inf> /χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lf</inf> and χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ARM</inf> /SIRM). This suggests that the source area has witnessed a strong weathering environment during Holocene. The southern Andaman Sea on the other hand has less weathered sediments which is clearly reflected by the presence of higher magnetite content and coarser magnetic particles. This observation provides vital clues on the differential monsoonal sensitivity of the source area of the sediments and can be reconciled with the different weathering characteristics during Holocene and LGM periods. The observed variations in the magnetic records during LGM indicate the sediments are mainly delivered by Myanmar Rivers with dominant hematite mineralogy, and the weathering was weak. Whereas, the northern Andaman Basin sediments are characterized predominantly by magnetite mineralogy, with a Himalayan source. During weaker (stronger) ISM, the contribution of suspended and dissolved sediments may be low (high) from the fluvial sources in the LGM (Holocene). The observed high values of smectite, low values of illite and high values of magnetic susceptibility of the northern core also corroborates the nature of provenance and the sources of sedimentation as peninsular Myanmar Rivers. Magnetic grain size parameters of the northern Andaman Sea display coarse-grained magnetic minerals and hematite mineralogy, whereas the southern part shows a fine-grained magnetite dominant environment. The moderate χfd in the deep-sea sediments of the Andaman Sea indicate a mixture of super paramagnetic and coarse magnetic minerals. The inter-parametric ratios (χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ARM</inf> /SIRM and χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ARM</inf> /χ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lf</inf> ) also suggest a coarser magnetic grain size for the northern Andaman Sea than the southern part. Overall, the present study highlights the potential of using magnetic methods as a proxy for paleo-precipitation variations in the Myanmar and Indo-Burman watersheds contributing sediments to the Andaman Sea.