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 oceanic temperatures and pH of the Andaman Sea. To do so, we measured Mg/Ca (paleothermometer) and δ11B (pH proxy) on monospecific samples of surface-dwelling planktonic foraminifera Globigerinoides ruber picked from core BAR94-25, located 600 km to the Toba caldera. Mg/Ca and δ11B data obtained make it possible to see potential temperature and pH disturbances linked to the Toba super-eruption. The δ11B measurements were especially achieved with a new protocol combining an automated and miniaturized direct injection system, the μ-dDIHEN, for the injection of small volumes to MC-ICP-MS and a boron extraction from the carbonate matrix through the fast handling and low blanks microsublimation method [1]. 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. 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.Ref: [1] Buisson, M., Louvat, P., Thaler, C. & Rollion- Bard, C. High precision MC-ICP-MS measurements of 11B/10B ratios from ng amounts of boron in carbonate samples using microsublimation and direct injection (μ- dDIHEN). J. Anal. At. Spectrom. 36, 2116–2131 (2021).