Salinity stratification controlled productivity variation over 300 ky in the Bay of Bengal

R Da Silva,T Mapder,B Sawant,A Mazumdar,P Mahalakshmi,A Peketi,B G Naik,S K Molletti,A Shaji,M A Carvalho,R K Joshi

doi:10.1038/s41598-017-14781-3

Abstract

The unique hydrographic setting of the Bay of Bengal (BoB) makes it an ideal tropical marine system to study the influence of regional and global forcings on productivity and [CO2aq] through the late quaternary. Enormous fresh water flux into the BoB and consequent salinity stratification significantly weaken the convective mixing and wind driven processes which are commonly responsible for transport of nutrients to the euphotic zone driving primary productivity. Here we present a high resolution organic carbon-CaCO3 MAR and δ13CTOC records for the last 300 ky from the BoB. The results show significant productivity variation at marine isotope sub-stages and millennial timescales. Colder sub-stages and stadials (Dansgard-Oeschger cycle) show a boost in productivity which may be attributed to thinning of low salinity cap, thereby facilitating efficient nutrient transport across the euphotic zone by the combination of wind driven processes (entrainment and upwelling), convective mixing and cold core eddies. The [CO2aq] was a net result of global pCO2 variation and regional processes. Our long term high-resolution data indicates a possibility of marked change in productivity/biogeochemistry of BOB in the future due to global warming, thus affecting the coastal economy.

Highlights

The unique hydrographic setting of the Bay of Bengal (BoB) makes it an ideal tropical marine system to study the influence of regional and global forcings on productivity and [CO2aq] through the late quaternary
Owing to these enigmatic oceanographic characteristics, physical processes like convective mixing and wind driven processes which are responsible for the transport of nutrients across the euphotic zone leading to primary productivity are significantly weaker in BoB compared to that of Arabian Sea[8]
Total inorganic carbon (TIC) concentrations range from 0.01% to 5.6 wt% (Supplementary Table 2)

Summary

Introduction

The unique hydrographic setting of the Bay of Bengal (BoB) makes it an ideal tropical marine system to study the influence of regional and global forcings on productivity and [CO2aq] through the late quaternary. The salinity stratification affects the vertical distribution of heat in the near surface layers of BoB and can influence processes such as the active–break cycles of summer monsoons and development of regional tropical cyclones[7]. Owing to these enigmatic oceanographic characteristics, physical processes like convective mixing and wind driven processes (upwelling and nutrient entrainment) which are responsible for the transport of nutrients across the euphotic zone leading to primary productivity are significantly weaker in BoB compared to that of Arabian Sea[8]. Understanding the natural variability of the paleoceanic proxies at high temporal resolution will allow more accurate modeling of the consequences of future warming

Methods

Results

Conclusion