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.

Historical records of coastal eutrophication-induced hypoxia

“…the last four decades), corresponding to a period of record high productivity. An increase in the Cu/Al ratio was also recorded during the same period (Agnihotri et al, 2008).…”

“…This implies prevalence of anoxic conditions in sediments throughout the depositional period. In two recent studies on the Indian shelf off Goa, based on three sediment cores from the zone of present seasonal anoxia and covering the last ∼200, 250 and 700 years, the V/Al ratio varied between 1 and 14×10 −4 (D 'Souza, 2007;Agnihotri et al, 2008). While this range is less than the average crustal value (16.4×10 −4 ; Taylor, 1964), the highest ratios were generally recorded in the upper portions of the cores (i.e.…”

“…In Bunnefjord (the innermost part of the Oslofjord), TOC concentrations were highest in sediment layers corresponding to the period of most intense anthropogenic eutrophication (Pinturier-Geiss et al, 2002). Along the west coast of India off Goa, large upcore increases in TOC and TN observed in the sediments of the inner-and mid-shelf since ∼1950 have been attributed to enhanced biological production fuelled by a combination of intensified summer monsoon-driven upwelling and a greater runoff of nutrients from land (D'Souza, 2007;Agnihotri et al, 2008;Kurian et al, 2009). Similar increases in organic carbon accumulation rates in cores from the southern Baltic Sea are likewise believed to reflect increased input of organic carbon resulting from eutrophication (Andrén, 1999;Andrén et al, 1999;Struck et al, 2000).…”

Comparative organic geochemistry of Indian margin (Arabian Sea) sediments: estuary to continental slope

“…Cowie et al, 1999;Schulte et al, 2000;Jeffreys et al, 2009) and for the Indian margin (e.g. Calvert et al, 1995;Agnihotri et al, 2008;Kurian et al, 2013). The damming of the Indus in the 1970s dramatically reduced sediment export (e.g.…”

Decadal Variations in Oceanic Properties of the Arabian Sea Water Column since Geosecs

“…In addition, Stramma et al (2010) reported that large areas of the subsurface tropical oceans are undergoing decreasing oxygen levels during the past few decades. Following the same ramp, evidence from recent sedimentary records reveals enhancement of surface biological productivity on both the western and eastern sides of the Arabian Sea (Anderson et al 2002;Agnihotri et al 2008). A plausible enhancement of denitrification and consequent increased production of N 2 O has been projected in an anthropogenic climate change scenario by Naqvi et al (2000).…”