Natural isotopic composition of particulate organic nitrogen in the eastern and central Arabian Sea

Abstract

We report measurements of isotopic composition of nitrogen (﷿d 15 N) of particulate organic nitrogen (PON) at eighteen sampling locations in the north-eastern and central Arabian Sea during the waning phase of the summer monsoon. This season is ideal to detect signatures of upwelling and denitrification in the Arabian Sea. Our observations indicate significant spatial variability in the﷿d 15 N as a result of the upper ocean response to the monsoonal forcing that triggers upwelling and thus supplies sub surface nitrate to the surface. An increasing (decreasing) trend has been found in the ﷿d 15 N (PON) from the upwelling zone to the open ocean, which tracks the progressive utilization of advected nitrate from the coast. Monsoondriven mixing and supply of nitrate and/or PON with lower﷿d 15 N from the west through advection are mainly confined to the upper 50 m, however, at some locations advection imprints its signature in the deeper layers as well. The results have implications to the interpretation of sedimentary﷿d 15 N record from this region and deciphering the role of monsoonal forcing over the nitrogen utilization by plankton.