Magnetic properties of surface sediments in a closed alkaline volcanic lake: A case study from Lake Batur, Bali, Indonesia

Lake Batur is a closed volcanic lake with alkaline water, located on the island of Bali, Indonesia. This study of Lake Batur presents a comprehensive rock magnetism analysis of the sediments and a geochemical examination of the water. Rock magnetism analysis revealed magnetite or titanomagnetite as the dominant magnetic minerals in the sediments. Other magnetic minerals identified include titanomaghemite and magnesioferrite or copper ferrite. The domain state of the magnetic minerals in the samples is a combination of pseudo-single domain (PSD) and single domain (SD). Magnetic susceptibility was found to be high in the northern part of the lake, caused by more intense volcanic rock weathering and anthropogenic activities. Hydrogeochemical analysis shows that the lake water is of the sodium chloride type, with the dominant water source falling within the steam-heated water category, and has high concentrations of Na + , HCO 3 - , Cl - , and SO 4 2− ions. In this lake, higher concentrations of sulfate ions were found compared to bicarbonate. This finding shows unusual characteristics for a volcanic-alkaline lake. The pH > 9 in lake water is caused by the high proportion of Ca 2+ , Mg 2+ , and HCO 3 − , and the presence of a carbonate layer beneath the caldera. The similarity of magnetic properties and mineralogy between the sediments and the surrounding rocks suggests that weathering of caldera rocks is the primary source of the magnetic minerals in the lake, there is no influence of alkaline water in the lake on the magnetic properties of the sediments, and there is no evidence of mineral diagenesis. These findings suggest that the magnetic properties of sediments in closed alkaline lakes, such as Lake Batur, can provide valuable data for paleomagnetic studies because they are unaffected by diagenesis.