Geochemical tracing of synoptic scale modern dust transport over the Northeast Arabian Sea during the southwest monsoon

During the Southwest monsoon (SWM), aeolian dust is mainly supplied via wet deposition over the north-eastern Arabian Sea (NEAS). To understand their impact on the biogeochemistry of the Arabian Sea, it is important to identify their sources and characteristics. In this context, wet deposit particulate (WDP) samples were collected at a coastal station (Goa; 15.4° N, 73.8° E) in the NEAS during the SWM for three years. These samples were used to characterize and identify mineral dust sources using mineralogical, elemental, and isotopic (Sr and Nd) signatures. The WDP samples were classified as Beginning of Monsoon (BM, June samples), Mid Monsoon (MM, July-August samples) and End of Monsoon (EM, September samples). Clay mineralogical composition indicate high palygorskite content during BM, which subsequently found to decrease in MM, and almost negligible in EM. However, smectite is highest during MM, with moderate presence of palygorskite during this period. The considerable variation in the relative percentages of clay minerals suggests significant temporal variability in dust sources which is further corroborated by the radiogenic isotopic composition. A strong seasonality in the isotopic composition is observed with ⁸⁷Sr/⁸⁶Sr ratio being relatively less radiogenic during MM than the BM and highly radiogenic at the EM. Whereas Ɛ_Nd values show an opposite trend to ⁸⁷Sr/⁸⁶Sr ratios throughout the monsoon, with more radiogenic Ɛ_Nd in the MM, and less radiogenic at the EM. End member mixing plot indicate dominant contribution of dust from the Arabian Peninsula (ARB) and Northeast African (NEA) sources during BM and MM, while a shift towards the Thar desert and Southwest Asian (SWA) sources at the EM. Trace elements associated with different sources were quantified and suggest high Fe concentration is associated with NEA dust sources, despite of ARB is the major supplier of aeolian dust to the Arabian Sea.

Geochemical characterization of modern aeolian dust over the Northeastern Arabian Sea: Implication for dust transport in the Arabian Sea

The Arabian Sea, one of the most biologically productive oceanic regions, receive significant amount of nutrients through aeolian dust deposition which have potential to modulate surface water biogeochemical processes. Identification of dust sources is important to assess role of dust deposition to surface water biogeochemistry. Here we report geochemical characteristics (major elements and trace metals composition, clay mineralogy and radiogenic isotopes of Sr and Nd) of aeolian dust, derived from dust storms in Middle East and South West Asia, to decipher its sources and possible transport pathways over the Arabian Sea. Twelve bulk aerosol samples were collected, during dusty period, at a coastal station (Goa; 15.4^o N, 73.8^o E) located in the Northeastern Arabian Sea, spanning over three years in different seasons. A preliminary estimation of sources for dust storms was done using satellite imagery and air mass back trajectory analyses. Major element and trace metal compositions of storm derived dust samples indicate insignificant contribution from anthropogenic sources. Illite and chlorite are major clay minerals found in all dust samples, however, high palygorskite content were observed for dust sourced from the Arabian Peninsula. A more uniform Sr-Nd signature is found for those derived from Arabia, however, highly variable Sr-Nd isotope signatures were observed for those derived from the South West Asia (Iran and Makran coast). The reported comprehensive data set on geochemical characteristics of mineral dust, is the first attempt that provides important information to understand present and past dust emission and deposition over the Arabian Sea.

Difference in attenuation among Mn, As, and Fe in riverbed sediments

We report, for the first time, a detailed study at river water and hyporheic zone systems through collection and analyses of shallow sediments and selected source rocks, pore water, and river water from forty-two locations at the Chianan Plain (CP), SW Taiwan. The study was focused to understand the possible changes in the river water and sediment chemistry as a consequence of high arsenic (mean±SD=71.28±16.24μg/L, n=46) groundwater discharge to three major rivers in the plain. The study shows, except few locations, As concentration in river sediments corresponds to average As concentration in soil and upper crustal abundance and of source rock. Sequential extraction indicates that As is mostly bound to FeOOH. No enrichment of arsenic in river sediments or depletion of aqueous As and iron in pore water was observed down to the maximum sampling depth of 1.7m although manganese is enriched in sediments. Dissolved As concentrations in the river sediments are much lower compared to the hotspots in the CP aquifers. This suggests that no As attenuation processes are active or they cannot be detected in this zone. Mn precipitates at higher redox level compared to Fe and As and thus attenuates in the studied zone.