Mid-Holocene intensification of the oxygen minimum zone in the northeastern Arabian Sea

Abstract

• The Arabian Sea has an intense oxygen minimum zone at intermediate water depths. • Lower sea level and weak OMZ conditions in the study area existed during the early Holocene . • The rise in sea level and increased surface water productivity intensified OMZ during the middle Holocene . • The enhanced sub-surface denitrification in the OMZ increases the nitrous oxide emission. The Arabian Sea is characterized by a strong Oxygen Minimum Zone (OMZ) bearing sub-oxic conditions at the intermediate water depth. We analyzed a sediment core near the upper margin of OMZ (174 m water depth) from offshore Saurashtra, northeastern (NE) Arabian Sea to reconstruct multi-proxy biogeochemical response in the area during the Early-Middle Holocene (∼10–4 ka before present). The results indicate lower foraminiferal productivity (both benthic and planktic) and weak sub-surface denitrification causing mild OMZ conditions at the study site during the early Holocene (∼10–8 ka). Subsequently, an increased foraminiferal productivity and sub-surface (both the water column and sediment) denitrification in the area led to intensified OMZ conditions during the mid-Holocene (after ∼8 ka). Our results corroborate other denitrification records from the Arabian Sea, suggesting the large-scale changes in Arabian Sea OMZ during the mid-Holocene. We suggest that the increased nutrient supply with significant freshwater input may have enhanced the primary productivity and further resulted in the intensified OMZ and escalated outgassing of N 2 O from the area during the mid-Holocene.