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

Anthropogenic Lead (Pb) deposition history of the western Indian Ocean from coral-based Pb/Ca ratio and Pb isotope records

Despite the rapid industrial growth and urban expansion along the coastline of the Western Indian Ocean, knowledge of both historical and current levels of anthropogenic lead (Pb) contamination, as well as its impact on the biosphere, remains limited compared to other industrialized regions. We present a twenty-four year long coralline record (1989-2013) of Pb/Ca ratio and Pb isotopes from the Lakshadweep coral reef in the Western Indian Ocean. This new record provides critical insight into source(s), possible transport pathways, and temporal trends in Pb deposition during the studied interval. The long-term trend in the surface seawater Pb concentration ([Pb]SW), reconstructed from the coralline Pb/Ca record, reveals almost doubling in [Pb]SW from ~50 pmol/kg in the year 1990 to ~107 pmol/kg in the year 2013. Bayesian mixing model calculations reveal that among the potential Pb polluting sources to this region, anthropogenic aerosol from the hinterland of the continents was the dominant contributor of Pb (23-89 %). A compilation of available Pb records from the Indian Ocean reveals that Pb isotope distribution patterns in the western and central equatorial Indian Oceans are distinctly different from those observed in the eastern Indian Ocean. The western Indian Ocean records exhibit lower Pb isotope ratios (206Pb/207Pb and 208Pb/207Pb) compared to the East Indian Ocean, suggesting a greater influence of anthropogenic Pb on seawater concentration. These findings highlight the spatio-temporally spread of anthropogenic Pb pollution and its potential impact on the biosphere in the Indian Ocean and therefore emphasize the urgent need for region-specific environmental management strategies. PLAIN LANGUAGE SUMMARY: This study reconstructs the history of lead (Pb) pollution in the Western Indian Ocean. We analyzed a specimen of coral, collected from Lakshadweep, to create a 24-year-long (years 1989 to 2013) for Pb concentration and isotopic composition of seawater in the Western Indian Ocean. Using the coralline Pb/Ca ratio and Pb isotope data, we have reconstructed surface ocean Pb concentration ([PbSW]) and isotopic composition to understand the sources, transport pathways, and temporal depositional trends over the western Indian Ocean during the past two decades. This reconstruction of [PbSW] reveals a doubling from ~50 pmol/kg in the year 1990 to ~107 pmol/kg in the year 2013. Our investigations to fingerprint the Pb source(s) to our study area reveal that majority of the anthropogenic Pb has been contributed by aerosol deposition sourced from the hinterland of the surrounding continents. Our investigation also revealed that the western Indian Ocean is more contaminated by anthropogenic Pb compared to the eastern Indian Ocean. These findings highlight the need for region-specific monitoring efforts in the Indian Ocean as well as the formulation of environmental strategies to mitigate the impact of Pb pollution.

Suspended particulate matter in the Gulf of Oman: Spatial variations in concentration, bulk composition, and particulate metals controlled by physical and biogeochemical processes

The present work aims to assess the biogeochemical and physical sources of variation in the spatial distribution of suspended particulate matter (SPM), its major biotic and abiotic components, particulate metals, and the Redfield (N:P) stoichiometry of particles in a poorly understood basin of the Gulf of Oman. Particulate samples were collected in February 2022 from the Gulf of Oman aboard the R/V Persian Gulf Explorer, revealing surface SPM concentrations ranging from 140 to 1145 μg/l. The elemental composition of crustal-type elements in the surface offshore region confirmed the input of lithogenic components by aeolian dust from the surrounding deserts. The highest mid-depth SPM levels, with remarkable contribution from CaCO3, were observed at the western shelf edge at 100-300 m depth, supported by the Persian Gulf outflow. Conversely, mid-depth maxima with elevated concentrations of terrigenous elements were observed in the eastern edge, emanating from sediment resuspension and lateral transport under eddy-topography interaction. Organic matter is the most important phase, followed by biogenic silica from the basin-wide winter bloom of diatoms. While signs of CaCO3 dissolution are evident at depths >500 m, the oxidative precipitation of Mn (II) in the upper boundary of the oxygen minimum zone leads to the appearance of perceptible maxima in the vertical profile of particulate MnO2. Seasonal variations in the organic N:P ratio, from summer to winter, at the western station were linked to shifts in phytoplankton assemblage structure, transitioning from cyanobacteria dominance in summer to chain-forming diatoms in winter. The particulate pool of biologically important trace metals was dominated by a non-crustal fraction with enrichment factor followed a descending order: Cd > Mo > Pb > Zn > Ni in surface offshore samples. Metal/P ratios comparison with some previous data from the open ocean SPM and lab cultures of phytoplankton reveals that the Zn/P ratio is significantly exceeded in cultured communities, whereas the Cd/P ratio reflected the consistent demand in the Gulf of Oman compared to reported lab culture requirements.

Identification and physico-chemical characterization of microplastics in marine aerosols over the northeast Arabian Sea

Microplastics (MPs) in the atmosphere can undergo long-range transport from emission regions to pristine terrestrial and oceanic ecosystems. Due to their inherent toxic and hazardous characteristics, MPs pose serious risks to both human well-being and the equilibrium of ecosystem. The present study outlines the comprehensive characterization, spanning physical and chemical attributes of MPs associated with atmospheric aerosols. Total suspended particulates (TSPs) were collected on a quartz fibre filter by operating a high-volume sampler for 24 h during distinct years (March, 2016 and November, 2020) at a coastal location in the northeast Arabian Sea. Subsequent to the sampling, a series of techniques were applied including density separation. The assessment and scrutiny of the MPs was carried out using stereo-zoom microscopy with supplementary validation using advanced fluorescence microscopy for enhanced precision in identification. Our comparative assessment suggests peroxide treatment followed by density separation could be a robust procedure for the definitive identification and characterization of MPs in the atmosphere. Average total abundance of MPs was found to be 1.30 ± 0.14 n/m3 in 2016 and 1.46 ± 0.12 n/m3 in 2020 with fibres, fragments and films having similar relative contributions (41 %, 31 %, 28 % in 2016 and 40 %, 35 %, 25 % in 2020). Fibres were found to be dominant morphotype followed by fragments and films over the coastal region of the Arabian Sea. In order to unravel the detailed chemical nature of these MPs, spectral analysis using μ-FTIR was carried out. The outcome of the analysis showed prevailing polymers as polyvinyl chloride and polymethyl methacrylate (50545 %) as dominant polymers followed by polyester (15 %), styrene butyl methacrylate (11 %), and polyacetal (9 %). MPs present in the vicinity of the Arabian Sea have potential to supply nutrients and toxicants, consequently can contribute to the modulation of the surface water biogeochemical processes.

Habitat sensitivity in the West African coastal area: inferences and implications for regional adaptations to climate change and ocean acidification

This study focuses on assessing coastal vulnerability and habitat sensitivity along the West African coast by delineating hotspots based on surface temperature, pH, chlorophyll-a, particulate organic carbon, and carbonate concentrations between 2018 and 2023 depending on data availability. Initial exploration of these variables revealed two distinct focal points i.e., the Togo-Nigerian coastal stretch and the stretch from Sierra Leone to Mauritania. Lower pH trends (acidification) in surface waters were observed off the West African coast, particularly in areas around the south-south Niger Delta in Nigeria and the coastal regions of Guinea and Guinea Bissau. Sea surface temperature analysis revealed highest temperatures (27-30°C) within Nigeria to Guinea coastal stretch, intermediate temperatures (24-27°C) within the Guinea Bissau and Senegal coastal stretch, and the lowest temperatures off the coast of Mauritania. Furthermore, correlation analysis between sea surface temperature and calcite concentration in the Mauritania-Senegal hotspot, as well as between overland runoff and particulate organic carbon in the Togo-Nigeria hotspot, revealed strong positive associations (r>0.60) and considerable predictive variability (R2 ≈ 0.40). From the habitat sensitivity analysis, certain regions, including Cape Verde, Côte d'Ivoire, Nigeria, Senegal, and Sierra Leone, exhibited high sensitivity due to environmental challenges and strong human dependence on coastal resources. Conversely, Gambia, Guinea, Guinea-Bissau, Liberia, and Togo displayed lower sensitivity, influenced by geographical-related factors (e.g. coastal layout, topography, etc.) and current levels of economic development (relatively lower industrialization levels). Regional pH variations in West African coastal waters have profound implications for ecosystems, fisheries, and communities. Addressing these challenges requires collaborative regional policies to safeguard shared marine resources. These findings underscore the link between ecosystem health, socioeconomics, and the need for integrated coastal management and ongoing research to support effective conservation.

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.

On the net primary productivity over the Arabian Sea due to the reduction in mineral dust deposition

The dust plume tracks from the Middle East and Eastern Africa to the Indian subcontinent have an impact on the atmospheric and ocean biogeochemistry of the Arabian Sea (AS). Here, we present the impact of dust on net primary productivity (NPP) over the AS using satellite-based observation and model simulation. Seasonal episodes and long-term trends in dust optical depth (DOD), dust mass flux (DMF) and dust deposition flux (DDF) from 2007 to 2020 are quantified. Nearly 32% of the total dust is advected to the AS during transport (maximum in JJA; DMF ~ 33.1 Tg year⁻¹ ~ 56% of annual and DDF ~ 5.5 Tg year⁻¹ ~ 63% of annual). Over the last one and half decades, there has been a statistically significant decreasing trend in DOD, associated with precipitation, enhanced vegetation index and surface soil moisture over the landmasses in the proximity of the AS. Similarly, the depletion in DDF suppresses the NPP over different regions of the AS, especially over the central AS, where the oceanic supply of nutrients is limited.

Long-Term Variability of Dust Events in Southwestern Iran and Its Relationship with the Drought

Dust storms represent a major environmental challenge in the Middle East. The southwest part of Iran is highly affected by dust events transported from neighboring desert regions, mostly from the Iraqi plains and Saudi Arabia, as well as from local dust storms. This study analyzes the spatio-temporal distribution of dust days at five meteorological stations located in southwestern Iran covering a period of 22 years (from 1997 to 2018). Dust codes (06, 07, 30 to 35) from meteorological observations are analyzed at each station, indicating that 84% of the dust events are not of local origin. The average number of dust days maximizes in June and July (188 and 193, respectively), while the dust activity weakens after August. The dust events exhibit large inter-annual variability, with statistically significant increasing trends in all of five stations. Spatial distributions of the aerosol optical depth (AOD), dust loading, and surface dust concentrations from a moderate resolution imaging spectroradiometer (MODIS) and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) retrievals reveal high dust accumulation over southwest Iran and surrounding regions. Furthermore, the spatial distribution of the (MODIS)-AOD trend (%) over southwest Iran indicates a large spatial heterogeneity during 2000–2018 with trends ranging mostly between −9% and 9% (not statistically significant). 2009 was the most active dust year, followed by 2011 and 2008, due to prolonged drought conditions in the fertile crescent and the enhanced dust emissions in the Iraqi plains during this period. In these years, the AOD was much higher than the 19-year average (2000 to 2018), while July 2009 was the dustiest month with about 25–30 dust days in each station. The years with highest dust activity were associated with less precipitation, negative anomalies of the vegetation health index (VHI) and normalized difference vegetation index (NDVI) over the Iraqi plains and southwest Iran, and favorable meteorological dynamics triggering stronger winds.

Hf-Nd-Sr Isotopic Composition of the Tibetan Plateau Dust as a Fingerprint for Regional to Hemispherical Transport

Large areas of arid regions in the Tibetan Plateau (TP) are undergoing desertification and subsequent aeolian emission and transport. The contribution of TP soils to the atmospheric aerosol burden in Asia and elsewhere is not known. Here, we use Hf, Nd, and Sr isotopes to distinguish the TP from other Asian dust-producing regions and compare the signatures to sediments in major dust sink regions. We found that the Hf-Nd-Sr isotopes of TP soils showed unique spatial signatures. From north to south, ⁸⁷Sr/⁸⁶Sr ratios gradually increased, while ε_Nd and ε_Hf values gradually decreased; from west to east, ⁸⁷Sr/⁸⁶Sr and ε_Hf gradually increased, while ε_Nd changed indistinctly. The Hf-Nd-Sr isotopic compositions of TP soils were controlled by four geographic isotope regions: the northern, southern, western, and eastern TP. Compared with Asian large deserts, the TP showed a unique isotopic composition, which together exhibited a significant spatial change across Asia. Compared to dust isotopes in prominent sink areas, we found that the TP is an important dust source to eastern TP glaciers, the Chinese Loess Plateau, South China Sea, Japan, and Greenland. This study provides clear isotopic evidence that the TP is a major aeolian contributor in the Northern Hemisphere and may have important implications for the global aeolian cycle.

Atmospheric Dynamics and Numerical Simulations of Six Frontal Dust Storms in the Middle East Region

This study analyzes six frontal dust storms in the Middle East during the cold period (October–March), aiming to examine the atmospheric circulation patterns and force dynamics that triggered the fronts and the associated (pre- or post-frontal) dust storms. Cold troughs mostly located over Turkey, Syria and north Iraq played a major role in the front propagation at the surface, while cyclonic conditions and strong winds facilitated the dust storms. The presence of an upper-atmosphere (300 hPa) sub-tropical jet stream traversing from Egypt to Iran constitutes also a dynamic force accompanying the frontal dust storms. Moderate-Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations are used to monitor the spatial and vertical extent of the dust storms, while model (Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), Copernicus Atmospheric Monitoring Service (CAMS), Regional Climate Model-4 (RegCM4)) simulations are also analyzed. The WRF-Chem outputs were in better agreement with the MODIS observations compared to those of CAMS and RegCM4. The fronts were identified by WRF-Chem simulations via gradients in the potential temperature and sudden changes of wind direction in vertical cross-sections. Overall, the uncertainties in the simulations and the remarkable differences between the model outputs indicate that modelling of dust storms in the Middle East is really challenging due to the complex terrain, incorrect representation of the dust sources and soil/surface characteristics, and uncertainties in simulating the wind speed/direction and meteorological dynamics. Given the potential threat by dust storms, more attention should be directed to the dust model development in this region.

Baseline marine investigation and impact of fish farming on the marine environment in Jebel Ali, Dubai, United Arab Emirates

Scattered seashells were observed on the seabed with no marine corals. The baseline studies indicate that biodiversity decreased from the northeast to southwest direction. The dominant groups of phytoplankton were diatoms followed by dinoflagellates, with insignificant vertical variations in species composition and population due to shallow water. The benthic diversity over the majority of the study area was relatively low compared with other nearshore areas in the region. All subtidal habitats showed evidence of disturbance to varying degrees, with no fish species recorded at these locations. The soft sediment habitat was found to cover much of the area footprint, and faunal diversity was very low. Fish diversity and abundance were equally poor with only a few demersal species recorded. No evidence of coral colonization was recorded although the presence of a low-profile, encrusting species was recorded in close proximity, to the east of the study area. Mangrove, coral, and seagrass were absent in the study area and its immediate vicinity. Modelling of waste plume suggested that the harbor water is fairly well-mixed, and the dispersion of ammonia attenuates with distance.