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 northeast 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 ƐNd values show an opposite trend to 87Sr/86Sr ratios throughout the monsoon, with more radiogenic ƐNd 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 ARB being major supplier of aeolian dust to the Arabian Sea.

Modern-like deep water circulation in Indian Ocean caused by Central American Seaway closure.

Global overturning circulation underwent significant changes in the late Miocene, driven by tectonic forcing, and impacted the global climate. Prevailing hypotheses related to the late Miocene deep water circulation (DWC) changes driven by the closure of the Central American Seaways (CAS) and its widespread impact remains untested due to the paucity of suitable records away from the CAS region. Here, we test the hypothesis of the large-scale circulation changes by providing a high-resolution record of DWC since the late Miocene (11.3 to ~2 Ma) from the north-western Indian Ocean. Our investigation reveals a progressive shift from Pacific-dominated DWC before ~9.0 Ma to the onset of a modern-like DWC system in the Indian Ocean comprising of Antarctic bottom water and northern component water during the Miocene-Pliocene transition (~6 Ma) caused by progressive shoaling of the CAS and suggests its widespread impact.

Tracing impact of El Niño Southern Oscillation on coastal hydrology using coral 87Sr/86Sr record from Lakshadweep, South-Eastern Arabian Sea.

El Niño Southern Oscillation (ENSO) is one of the dominant climate modes influencing global precipitation and temperature. ENSO has a large impact on the monsoonal precipitations over the Indian subcontinent and thereby influences hydrological conditions. Due to dearth of long-term instrumental records of the hydrological parameters on sufficient spatial resolution, it is difficult to assess the impact of ENSO on regional hydrology. Though several geochemical proxies have been used to reconstruct past ENSO events through tracing the changes in past hydrological and climatic parameters, however, such reconstructions are often complicated by the influence of multiple processes and/or factors and their nonlinear relation with the proxy records. In this study, Sr isotope composition (87Sr/86Sr) was measured in Porites coral from the Lakshadweep, south-eastern Arabian Sea to reconstruct past ENSO events and to trace its regional hydrological impacts. The high precision measurements of 87Sr/86Sr in Lakshadweep coral show resolvable variations ranging from 0.709080 to 0.709210. The 87Sr/86Sr record shows an inverse relation with Niño 3.4 record; lower values matched with El Niño years and higher values with La Niña years. Our investigation reveals that ENSO driven precipitation changes impacted submarine groundwater discharge (SGD) to the Minicoy Atoll and resulted in 87Sr/86Sr variations of the Minicoy Atoll water. Therefore, deviation from the average seawater 87Sr/86Sr value can be quantified in terms of SGD contribution to the Minicoy Atoll. Our estimates based on binary mixing between seawater and SGD 87Sr/86Sr suggest a significant supply of SGD, maximum up to ~20 % of the total volume of the Minicoy Atoll during La Niña years due to higher rainfall compared to El Niño years. This finding highlights potential application of coral 87Sr/86Sr record as an alternate proxy to reconstruct past ENSO events and to trace its quantitative impact on regional hydrology, chemical and nutrient fluxes to coastal oceans via SGD.

Sources, distribution and biogeochemical cycling of dissolved trace elements in the coastal lakes of Larsemann Hills, East Antarctica.

Coastal lakes in Antarctica receive an enormous amount of ions and trace elements (TEs) during the austral summer. Some of these TEs and ions are utilised as essential nutrients in primary productivity. In the present study, selected dissolved TEs (Ba, Mn, Cu, Co, Cd, Mo and U) along with dissolved organic carbon (DOC) and Chlorophyll-a were studied in ten coastal lakes of the Larsemann Hills, East Antarctica to decipher their (TEs) sources, understand geochemical behaviour and assess their role on nutrient dynamics. Dissolved concentrations of these TEs are in sub-nanomolar range; almost an order of magnitude lower than the average seawater and global river concentrations. Sea-salt spray and chemical weathering in the catchments of these lakes are dominant sources for these TEs and ions. Though most of the Antarctic lakes have been reported for their oligotrophic character, however, a significant amount of DOC and Chlorophyll-a, and occurrence of algal mats in some of the LH lakes indicate seasonal (austral summer) productivity with the availability of sunlight and nutrients. Our investigation reveals that phosphate (PO43-) and Mo act as limiting nutrients because of their lower concentrations in the water column. Dissolved Cu plays an important role in bacterial-induced organic matter decompositions and release of organic carbon to lake water. We also found Ba excess (non-terrigenous) in the lake and catchment sediments varying from 26 to 63%. The higher Baexcess in the catchment sediments could be due to significant removal of dissolved Ba during the solute transport and later supplied to these lakes. The geochemical data sets presented in this study were found at a natural background level and therefore, would be useful for comparison with other global aquatic environments. Findings of the present study improve our understanding about the biogeochemical cycling of trace elements and their critical role in oligotrophic lakes of Antarctica.

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; 15.4o N, 73.8o 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.

Increased influence of ENSO on Antarctic temperature since the Industrial Era.

Under the influence of recent global warming, modulation of frequencies and amplitude of El Niño-Southern Oscillation (ENSO) and its impacts on global climate have become great concerns to the global community. Antarctic climate is sensitive to these changes owing to tropical and Southern Hemispheric (SH) teleconnections. Antarctic surface air temperature (SAT) reconstructed approximately for the past five centuries (~1533 to 1993 CE) based on multiple oxygen isotope (δ18O) records of ice cores from East and West Antarctica show dominant oscillations in ENSO and Pacific Decadal Oscillation (PDO) frequency bands. Further, variance of the East Antarctica (EA) temperature record shows significant increasing trend at ENSO band and decreasing trend at PDO band since the industrial era (~1850 CE). This observation is consistent with the earlier report of increasing ENSO activity, reconstructed based on tropical-subtropical tree ring records. ENSO influence in the SH high-latitude is known to be characterized by Pacific South American (PSA) pattern reflected in the atmospheric pressure fields. Our investigation of greenhouse gas (GHG) forced model simulation results show an increasing trend in PSA activity since the industrial era. Thus, we suggest ENSO activity and its influence on Antarctic temperature are increasing in response to increasing radiative GHG forcing since the industrial era.