Eolian versus fluvial supply to the northern Arabian Sea during the Holocene based on Nd isotope and geochemical records

Abstract

The north-eastern Arabian Sea (NE-AS) comes under a strong influence of land–ocean–climate interactions and regulates biogeochemical processes through the supply of huge amounts of dissolved and particulate materials and nutrients via eolian and fluvial supply. These processes underwent dramatic changes in the coastal regions due to sea-level rise and climate change during the Holocene; however, their relative roles remain elusive. The NE-AS receives large amounts of dissolved and particulate fluxes, and therefore, reconstruction of the past surface water Nd isotope composition (ε_Nd) and tracing the provenance of sediment using detrital ε_Nd and geochemical records would enable us to assess the role of various processes controlling these fluxes to the northern Arabian Sea. In this study, we have generated authigenic and detrital ε_Nd records and geochemical records in a sediment core from the coastal region of the NE-AS, offshore Saurashtra. We found that the authigenic ε_Nd profile closely followed the Holocene sea-level records; early Holocene less radiogenic values (∼ −8) were sharply shifted to more radiogenic values (∼ −5.5) during the mid-Holocene (6–7 ka) and thereafter remained stable, close to the modern surface water ε_Nd value. The detrital ε_Nd record broadly followed the authigenic ε_Nd record, however, they differ in magnitude. The geochemical records based on major and trace elemental abundances show a similar trend to the authigenic ε_Nd record and concomitant changes with the Holocene sea-level. Our investigation reveals that lower sea-level stand combined with a stronger monsoon during the early Holocene resulted in enhanced fluvial weathering fluxes from the west-flowing rivers and contributed to less radiogenic Nd. This situation changed dramatically during the mid-Holocene due to the weakening of the south-west monsoon and rapid sea level rise, which caused enhanced influence of open ocean water characterised by more radiogenic ε_Nd (∼6) derived from the dissolution of dust from Arabia and African desserts. This dramatic shift in ε_Nd profile indicates the enhanced influence of eolian over the fluvial supply of chemical weathering and erosion fluxes during the mid-Holocene. This observation is also consistent with the higher sedimentation rates with more radiogenic detrital supply. The finding of enhanced influence of eolian over fluvial mode of weathering and erosional inputs to the northern Arabian Sea has important implications for past nutrient supply (fluxes and compositions) and its impact on biogeochemical processes in the Arabian Sea.