South Asian monsoon response to the ~74 ka Toba super-eruption revealed by µm-scale imaging on Arabian Sea sediment

As the largest volcanic eruption in the Quaternary period, the ~74 ka Toba super eruption’s impact on the global heat budget and monsoon systems has been prominently debated. Despite particular focus on its consequences in India as one of the key regions of early modern human dispersal — ranging from catastrophic to minimal — high-resolution proxy evidence from geological archives that empirically support the claims has been scarce. In this study, we trace pre- and post-Toba monsoonal dynamics from a finely laminated sedimentary section from the Arabian Sea that brackets Toba tephra as an event marker of the eruption. The sediment core SO130-289KL was retrieved from the northeastern margin of the Arabian Sea outside of the upwelling zone (Sindh continental margin), at a water depth of 571 m, which today lies within the oxygen minimum zone (OMZ). The site is sensitive to both monsoon seasons, as the South Asian summer monsoon controls sedimentary dynamics, whereas the wind strength of the winter monsoon primarily influences the sea surface temperatures (SSTs). Thus, the sediment core sensitively records the evolution of South Asian summer and winter monsoons. In order to reconstruct the regional climatic response to Toba at near-annual resolution, we produced time-series data of elemental and SST variations using µm-scale measurements (100–200 µm resolution) by x-ray fluorescence (µXRF) scanning and mass spectrometry imaging (MSI) techniques, respectively. The µXRF elemental data trace terrestrial components primarily sourced by runoff from the summer monsoon, which are complemented by the glycerol dialkyl glycerol tetraether-based SST calculations from MSI that are affected by OMZ intensity. On the other hand, the alkenone measurements from MSI more sensitively trace SST variations that are primarily governed by the winter monsoon. Supplemented by conventional biomarker and stable hydrogen and carbon isotope measurements, which trace precipitation and vegetation dynamics over the Indus River catchment, respectively, our multi-proxy data contribute to a better understanding of the impact that the Toba eruption had on the regional climate, environment, and eventually, contemporaneous humans.