Organic matter distribution pattern in Arabian Sea: Palynofacies analysis from the surface sediments off Karwar coast (west coast of India)

Abstract

Central Arabian Sea region, situated off the Karwar coast, is characterized by intense mid depth (~120-1200 m) oxygen minima zone and shows preservation and accumulation of relatively high organic matter content. Palynofacies analysis was carried out with a view to understand the organic matter production, preservation and degradation in the surface sediments of Arabian Sea from 15 m-2750 m depth off Karwar [14.47-14.40 ° N -70.77-74.25 ° E transect]. Palynofacies analysis, which involves qualitative and quantitative estimation of terrestrial and marine organic matter is a useful tool to decipher and assess paleoenvironmental changes in various water depth zones from shelf-slope region off the Karwar coast. There is a marked change in the palynofacies characteristics of the organic matter recovered from various depth zones. High SW monsoonal activity over Karwar coast results in increased runoff and nutrient loading in the coastal waters. This enhances primary productivity in the inner shelf region. Organic walled dinoflagellate cysts as important constituent of primary productivity, predominate under such conditions, being converted into Amorphous organic matter (AOM) as a result of degradation under low oxygen environment. Hence, in the present study high AOM is used as proxy for the low oxygen content at sediment-water interface. It also provides evidence of high primary productivity in the photic zone. Study further reveals that terrestrially derived charcoal and woody plant tissue resistant to degradation, are transported to continental slope regions at greater depths. Occurrence of a large proportion of well preserved labile organic matter (exoskeleton fragments of planktonic crustaceans) and AOM in the mid-outer slope surface sediments indicate enhanced primary productivity and high rate of burial efficiency making these areas, characteristic of low oxygen. The study shows that the Karwar coast margin is highly productive as a result of runoff related nutrient loading and is the primary cause for oxygen minima conditions.