Abrupt upwelling and CO2 outgassing episodes in the north-eastern Arabian Sea since mid-Holocene

Syed Azharuddin,Pawan Govil,Ravi Mishra,Mayank Shekhar,Thomas B. Chalk,Gavin L. Foster

doi:10.1038/s41598-022-07774-4

Abstract

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO2 due to complex earth processes has been a major challenge for climate scientists in the past few decades. Recent developments in the boron isotope (δ11B) based seawater pH and pCO2 (or pCO2sw) proxy have been pivotal in understanding the various oceanic processes involved in air-sea CO2 exchange. Here we present the first foraminifera-based δ11B 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 CO2 sink during the last 7.7 kyr. The region behaved as a significant CO2 source during two short intervals around 5.5–4 ka and 2.8–2.5 ka. The decreased pH and increased CO2 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 CO2 outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years.

Highlights

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO2 due to complex earth processes has been a major challenge for climate scientists in the past few decades
The northern and north-eastern parts of Arabian Sea experience comparatively high productivity during the winter season[21,22], this is due to the combined effect of northeast monsoon (NEM) influenced winter mixing[23–25] as well as the nutrient-rich water advected from Bay of Bengal via the south-eastern Arabian Sea (West Indian Coastal Current or WICC)
This can be observed in the monthly salinity record of nearby location (21.5°N; 68.5°E) which shows an extreme Sea Surface Salinity (SSS) decrease around October (Supplementary Fig. 1)

Summary

Introduction

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO2 due to complex earth processes has been a major challenge for climate scientists in the past few decades. 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 CO2 outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years. Studies related to the quantification of monsoon-controlled export flux to the interior parts of Arabian Sea suggests that the carbon-captured by sediments during upwelling is a potential sink of ~ 820 mmol C/m2/yr[35] By virtue of such air-sea interaction, several modelling studies suggest that the inter-annual SST variability in the Indian Ocean is linked to internal oceanic cycling as well as the external atmospheric forcing[36–39]. These existing teleconnections are yet to be explored in past climate records

Methods

Results

Conclusion