Abrupt upwelling and CO outgassing episodes in the north-eastern Arabian Sea since mid-Holocene.

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO due to complex earth processes has been a major challenge for climate scientists in the past few decades. Recent developments in the boron isotope (δB) based seawater pH and pCO (or pCO) proxy have been pivotal in understanding the various oceanic processes involved in air-sea CO exchange. Here we present the first foraminifera-based δB record from the north-eastern Arabian Sea (NEAS) covering the mid-late Holocene (~ 8-1 ka). Our record suggests that the region was overall a moderate to strong CO sink during the last 7.7 kyr. The region behaved as a significant CO source during two short intervals around 5.5-4 ka and 2.8-2.5 ka. The decreased pH and increased CO outgassing during those abrupt episodes are associated with the increased upwelling in the area. The upwelled waters may have increased the nutrient content of the surface water through either increased supply or weaker export production. This new dataset from the coastal NEAS suggests that, as a potential result of changes in the strength of the El-Nino Southern Oscillation, the region experienced short episodes of high CO outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years.