Variability of subsurface denitrification and surface productivity in the coastal eastern Arabian Sea over the past seven centuries

Abstract

Temporal variations over the last ~700 years in surface productivity and subsurface denitrification in the coastal eastern Arabian Sea (off Goa), a hydrographically unique environment that experiences seasonal near-bottom anoxia, have been investigated using multiple geochemical and isotopic proxies. The results suggest that surface productivity was much lower than it is today during all major solar minima during this period, but subsurface denitrification appears to have weakened substantially only during ~AD 1650 to 1750, within the period of the `Little Ice Age' (LIA; from ~AD 1500 to 1750), most probably a result of a less vigorous upwelling associated with a decreased southwest monsoon strength. The proxy data indicate that the productivity during the last ~150 years (the Anthropocene) has been higher than ever before in the past 700 years, and water column measurements demonstrate that the region currently experiences extremely intense denitrification; yet, the nitrogen isotopic ratio in sedimentary organic matter (δ15N) exhibits a moderately declining trend during this period. This is ascribed to dilution by terrestrial sources and/or to small isotopic effect associated with vigorous denitrification that removes all nitrate, with possibly additional inputs of light nitrogen through fixation by the diazotrophs that is probably tightly coupled to denitrification. Thus, our results reveal limitation of sedimentary δ 15N as a proxy for palaeodenitrification despite excellent preservation of organic matter in coastal sediments.