Surface pH record (1990–2013) of the Arabian Sea from boron isotopes of Lakshadweep corals—trend, variability, and control


Atmospheric CO2 rise 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 Porites coral 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 CO2 during 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 CO2 rise. 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.

Plain Language Summary

Increase in the atmospheric CO2 concentration 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 CO2 during 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.

Tarique M., Rahaman W., Fousiya A. A., Lathika N., Thamban M., Achyuthan H. & Misra S., 2021. Surface pH record (1990–2013) of the Arabian Sea from boron isotopes of Lakshadweep corals—trend, variability, and control. Journal of Geophysical Research: Biogeosciences 126: e2020JG006122. doi: 10.1029/2020JG006122. Article (subscription required).

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