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Atmospheric CO2variability on the glacial–interglacial (G–IG) timescale reflects a balance between oceanic and terrestrial processes involving carbon uptake and release. The Southern Ocean CO2uptake is considered as an important modulator for the G–IG atmospheric CO2variability, while the role of tropical ocean ventilation remains enigmatic. We present critical evidence for CO2ventilation from the tropical Indian Ocean through the reconstruction of the Arabian Sea‐surface pCO2for the past ∼136 ka utilizing boron isotope (δ11B) record of planktic foraminifera, Globigerinoides ruber. Our site in the Arabian Sea presently acts as a significant source of CO2. The reconstructed ΔpCO2(ΔpCO2= pCO2 Seawater− pCO2 Atmosphere) record shows an enhanced CO2degassing up to ∼50 ppm during the major cooling events, such as the Last Glacial Maximum, Younger Dryas, and Heinrich‐Stadials. Our investigation based on multiproxy records of sea‐surface temperature, salinity, and productivity suggests that the northward invasion and shoaling of southern source CO2‐rich water, coupled with stronger upwelling, resulted in CO2degassing during these cold intervals. This finding is in align with the tropical Atlantic which also demonstrated an enhanced CO2degassing during the cold intervals; however, most of the upwelled CO2was consumed as the water moved away from the upwelling sites. Therefore, our finding, when considered alongside tropical Atlantic records, suggests that tropical oceans played a minor role in reducing atmospheric CO2levels during the cold intervals of the last glacial cycle, supporting the prevailing hypothesis. Foraminifera δ11B‐based CO2record of Arabian Sea surface for the past ∼136 kaEnhanced CO2degassing during the cold climate events, that is, Heinrich‐Stadials, Younger Dryas, and Last Glacial MaximumThis degassing was caused by more export of southern‐sourced water and its upwelling Foraminifera δ11B‐based CO2record of Arabian Sea surface for the past ∼136 ka Enhanced CO2degassing during the cold climate events, that is, Heinrich‐Stadials, Younger Dryas, and Last Glacial Maximum This degassing was caused by more export of southern‐sourced water and its upwelling

Atmospheric CO2rise in post‐industrial era has resulted in decline in surface ocean pH, commonly known as “ocean acidification (OA),” which has become a threat to marine calcifiers. Instrumental records of ocean pH and its reconstruction utilizing boron isotope (δ11B) composition of corals demonstrate a long‐term OA trend characterized by large spatio‐temporal variability in both Pacific and Atlantic oceans. However, no such record exists to elucidate long‐term OA trend of the Indian Ocean. We report the first sub‐annually resolved pH record (1990–2013) from the Arabian Sea based on δ11B measurements on Poritescoral from Lakshadweep coral reefs. This pH record is characterized by large variability ranging from 7.93 to 8.65 with no long‐term discernable trend. The long‐term declining trend expected from the ∼50 ppm increase in atmospheric CO2during the coral growth interval appears to be obscured by large surface pH variability in the Arabian Sea. Our investigation reveals that physical oceanographic processes for example, upwelling, downwelling and convective mixing modulated by El Niño‐Southern Oscillation (ENSO) largely control surface pH variability and masked expected long‐term OA trend resulting from anthropogenic CO2rise. Combining the model‐based predictions of increase in frequency and amplitude of ENSO events in a future warming scenario and the observed ENSO dependency of surface water pH, we predict more frequent and large pH variability (“pH extremes”) in this region. Such pH extremes and their occurrences might be critical for the resilience and adaptability of corals and other calcifiers in Arabian Sea and other similar oceanic settings elsewhere. Increase in the atmospheric CO2concentration since the industrial revolution (∼1850) has resulted in decrease in ocean pH, known as ocean acidification (OA). Only limited number of pH records are avalable to assess the impacts of OA on marine ecosystems. The available pH records from the Pacific and Atlantic oceans, based on both instrumental observations and coral boron isotope records, demonstrate a long‐term declining trend with significant internal variability. However, no such records are available from the Indian Ocean. Here, we provide the first seasonally resolved record of Indian Ocean pH for a 23 year period (1990–2013) based on boron isotope study of corals collected from the Lakshadweep, Arabian Sea. pH variability in the Arabian Sea is domiantly controlled by ENSO modulated oceanographic processes such as upwelling and mixing. We did not observe any long‐term declining trend corresponding to the ∼50 ppm rise in atmospheric CO2during the studied interval. The OA trend is possibly obscured by large surface pH variability in the Arabian Sea. Based on our observation and model based simulation showing an increase in ENSO frequency and amplitude for future scenarios, we expect that pH extremes are likely to increase, which is critical for resilience and adaptability of calcifying marine organisms. We provide the first coral δ11B‐pH record (1990–2013) from the Indian Ocean at sub‐annual resolutionLong‐term ocean acidification trend expected from atmospheric CO2rise is obscured by large surface pH variability in the Arabian SeaPhysical oceanographic processes modulated by El Niño‐Southern Oscillation control large inter‐annual surface pH variability in the Arabian Sea We provide the first coral δ11B‐pH record (1990–2013) from the Indian Ocean at sub‐annual resolution Long‐term ocean acidification trend expected from atmospheric CO2rise is obscured by large surface pH variability in the Arabian Sea Physical oceanographic processes modulated by El Niño‐Southern Oscillation control large inter‐annual surface pH variability in the Arabian Sea