Provenance tracing of long-range transported dust over the Northeastern Arabian Sea during the southwest monsoon

Abstract

One of the major pathways for dust supply to surface water of the Arabian Sea is via wet deposition, which is significant during southwest Monsoon (June-September). Aeolian dust supplies macro and micronutrients, which can impact on several marine biogeochemical processes. Here we report, major elements and rare earth elements (REEs) content as well as mineralogical and Sr-Nd isotopic composition of particulate collected from wet deposition (WDP) during the southwest monsoon period over the Northeastern Arabian Sea (NEAS) (Goa; 15.4° N, 73.8° E) in 2013. The collected WDP samples are classified into three categories based on changing wind regime, namely: (1) beginning of monsoon (BM; June); (2) mid of monsoon (MM; July-August) and (3) end of monsoon (EM; September). Our result highlights a significant temporal variability in REEs pattern showing a decreasing trend of Lan/Ybn and relatively higher Eu*/Eu anomaly during mid of monsoon compared to beginning and end of monsoon period. High palygorskite (P) with moderate smectite (S) is observed for beginning of monsoon and majority of mid monsoon samples, however, high smectite with moderate palygorskite and negligible palygorskite and smectite abundance is observed for few of mid monsoon and end of monsoon samples, respectively, indicating change in dust sources during the southwest monsoon period. We also observed temporal variability in 87Sr/86Sr (0.7090–0.7193) and εNd (−2.3 to −9.1) isotopic compositions wherein less radiogenic 87Sr/86Sr and relatively more positive εNd is exhibited by mid of monsoon samples compared to those observed in beginning and end of monsoon collections. The seasonal variability in geochemical tracers are related to changes in the dust source areas which are identified as (1) the Arabian Peninsula contributing significantly in beginning of monsoon and majority of mid monsoon samples; (2) Northeastern Africa contributing to some of the mid monsoon samples and (3) Southwest Asia and Thar Desert in end of monsoon samples. These source identification using geochemical tracers are corroborated by the satellite images (CALIPSO and aerosol index maps) and back-trajectory analyses.