Impacts of the Toba super-eruption on the temperature and pH of the Andaman Sea
Résumé
The Toba volcano super-eruption occurred about 74,000 years ago on the island of Sumatra, during the transition between interglacial Marine Isotope Stage (MIS) 5 and glacial MIS 4. This eruption, which led to the deposition of the so-called Youngest Toba Tuff (YTT), is currently described as the largest volcanic eruption of the Quaternary. However, its impact on climate is widely debated and its effects on the ocean remain poorly understood.The aim of this work is to estimate the impact of YTT on surface and thermocline oceanic temperatures and pH of the Andaman Sea. To do so, we measured Mg/Ca (paleothermometer) and d11B (pH proxy) on monospecific samples of two planktonic foraminiferal species picked from core BAR94-25, located 600 km to the Toba caldera : the surface-dwelling Globigerinoides ruber and the thermocline-dwelling Pulleniatina obliquiloculata. Mg/Ca and d11B data obtained on these two species make it possible to reconstruct temperature and pH variations as well as changes in their vertical gradients in response to upper ocean dynamics and potential disturbances linked to the Toba super-eruption.We selected the interval from 258 to 355 cm, corresponding to an age between 57 and 82 ka. In this interval, several Toba tephra/cryptotephra layers are identified, which correspond to distinct eruptive events. Our results suggest almost 2°C of abrupt sea surface cooling across the main YTT deposit, that cannot be explained by the MIS 5-4 transition alone. Thermocline seawater temperature is less impacted by the eruption than surface waters (cooling of 1°C). This cooling event correlates well with a decrease in the pH of the Andaman Sea surface, that we interpret as reflecting the acidifying impact of Toba sulfur emissions.