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.

中文翻译：

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来自 Lakshadweep 珊瑚硼同位素的阿拉伯海表面 pH 值记录（1990-2013）——趋势、变异性和控制

后工业时代大气 CO ₂上升导致表层海洋 pH 值下降，俗称“海洋酸化 (OA)”，已成为对海洋钙化物的威胁。海洋 pH 值的仪器记录及其利用珊瑚的硼同位素 (δ ¹¹ B) 组成的重建显示出长期 OA 趋势，其特征是太平洋和大西洋的时空变化很大。然而，没有这样的记录来阐明印度洋的长期 OA 趋势。我们报告了阿拉伯海的第一个分年度解析的 pH 值记录（1990-2013），基于对Porites 的δ ¹¹ B 测量来自 Lakshadweep 珊瑚礁的珊瑚。该 pH 值记录的特点是在 7.93 到 8.65 之间存在较大的变异性，没有长期可辨别的趋势。在珊瑚生长间隔期间大气 CO ₂增加约 50 ppm 所预期的长期下降趋势似乎被阿拉伯海的大表面 pH 变化所掩盖。我们的调查表明，物理海洋学过程，例如厄尔尼诺-南方涛动 (ENSO) 调制的上升流、下降流和对流混合在很大程度上控制了地表 pH 值的变化，并掩盖了人为 CO ₂导致的预期长期 OA 趋势上升。结合基于模型的对未来变暖情景中 ENSO 事件频率和幅度增加的预测以及观察到的 ENSO 对地表水 pH 值的依赖性，我们预测该地区更频繁和更大的 pH 值变化（“pH 极端值”）。这种极端的 pH 值及其发生可能对阿拉伯海和其他其他类似海洋环境中的珊瑚和其他钙化物的弹性和适应性至关重要。