Toward a New Assessment of the Environmental Impacts of the Younger Toba Super Eruption
Abstract:The Toba super eruption, which occurred at MIS 5a-4 transition in northern Sumatra, is considered as the word largest known explosive volcanic eruption. Ash preserved in marine and terrestrial sediments shows that the dispersal of the Younger Toba Tuff (YTT) extended to 140S in the southern hemisphere, westward as far as 640E, and northeastwards ~1800 km into the South China Sea. The impact of this explosive event on the global and regional climate and the environment in general remains speculative based on models. Our study was undertaken to directly evaluate changes occurred in climate and environment after the eruption.
The YTT layer bearing, top two sections of NGHP 19B 2H sediment core, collected from northern Bay of Bengal, was studied to understand the impacts,. Approximate sedimentation rate of the site during the period was 10 cmkyr-1, Diffuse color reflectance (DSR) spectra, chemical composition of the bulk sediment (XRF), Mg/Ca and δ18O of Globigerinoides ruber were measured at 0.5-2 cm resolutions. Down core variability in Ti/Ca and Fe/Ca ratios and a* and b* color coordinates of sediments, which are proxies for terrestrial input vs productivity, together with SST, calculated with Mg/Ca and δ18O corrected for temperature and ice volume (δ18Osw), suggest climate and environmental changes occurred.
Since the Toba event occurred at the beginning of MIS 4 glacial period, only the deviations from the cooling related trend in the proxy records were taken in to account. Decrease in Ti/Ca and Fe/Ca ratios by about 25% after the event indicates either decrease in terrestrial influx due to decrease in precipitation or increased productivity leading to enhanced Ca burial or both. Decreased a* and b* (about 12%) indicates shifting sediment color towards blue and green indicating higher productivity. However, 0.5°C decrease of SST derived from Mg/Ca of G. ruber and about 0.34 (‰) increase of δ18Osw, which indicates saltier conditions, are within the error margins. Therefore, it is reasonable to interpret that there was a short-lived increase in productivity due to increase in nutrient supply. A decrease in rainfall intensity could also be plausible. Future work will test the reaction of the continental ecosystem to the Toba eruption.