Isotopic and trace element record of changing metal source contributions to tropical freshwater Lake Naivasha (Kenya).

Lake Naivasha, Kenya's second-largest freshwater body is a wetland of international ecological importance and currently subjected to unprecedented anthropogenic influence. The study aims to chronologically reconstruct the main human activities and background weathering reactions that govern metal mobilizations into the lake and their potentially adverse effects on its ecological status. We combine extensive geochemical analyses (major, trace elements, Zn-Pb isotope ratios) in a dated lake sediment record and catchment rocks with remote sensing techniques. Downcore geochemical variations reflect natural ecosystem destabilizations occurring as early as the first half of the 20th century. These coincide with changes towards less radiogenic Pb-isotope values which persist towards the top of the core (206Pb/207Pb = 1.243 at core base ~1843, to 206Pb/207Pb = 1.225 at ~1978). We interpret the land-clearance for agricultural purposes on the Aberdare Range and documented early aviation activities as vectors of this early Pb-isotope excursion. The overlapping Pb-isotope signatures between sediment sources and anthropogenic contributions challenges a straightforward deconvolution of the two. Our conservative model calculations suggest, nevertheless, that an addition of up to ~1.8 % Pb-gasoline influx to the total Pb flux, peaking in the 1980s is able to explain the Pb distribution trend. Homogeneous Zn-isotope compositions in sediments deposited until ~1970s (δ66/64Zn = 0.216-0.225 ‰) do not follow major hydro-climatic events or anthropogenic forcing but reflect lake-specific natural cycling. Subsequent higher variations to both heavier (up to δ66/64Zn = 0.242 ±â€¯0.005 ‰) and lighter (down to δ66/64Zn = 0.184 ±â€¯0.003 ‰) Zn-isotope values are contemporaneous with intensification of large-scale horticultural industry in the catchment. Together with supporting indicators, the lighter Zn-isotope compositions in youngest analysed sediments (21st century) are attributable to increased biological productivity (algal blooms) and ongoing lake eutrophication. Our study demonstrates the applicability of the heavy metal isotope tool to reconstruct human influences on lake environments with complex geological settings such as the East African Rift System.

Assessment of atmospheric pollution by potentially toxic elements in the urban areas of the Riotinto mining district.

Ore mineralizations in bedrock and their exploitation may have a negative impact on air quality of surrounding urban areas and, subsequently, on human health. This study uses lichens as bioindicators of atmospheric pollution to evaluate the spatial distribution of potentially toxic elements (PTEs) in the towns close to the massive sulfide deposits of the Iberian Pyrite Belt (IPB) in SW Spain. Altogether 89 native lichen samples of Xanthoria parietina were collected from the mining towns, control towns out of the reach of the mining activity, as well as from distal sampling sites. The samples were analyzed for 29 elements after acid digestion. The concentrations for Co, Ni, Cu, Zn, As, Rb, Mo, Cd, Sn, Sb, Cs, Ba, W, Tl, Pb, S, and Fe are significantly higher in the mining towns in comparison to the control towns. The ore mineral-associated PTEs, including Cu, Zn, As, Ba, and Pb, exhibit extreme concentrations in the urban areas close to the mining activity, and particularly in the small settlement of La Dehesa next to the mineral processing plant and the tailings pond. The distal samples confirm the decrease in the concentrations of all PTEs, and these samples present similar values as in the control areas. The results, point at increased bioaccumulation of PTEs in the lichen thalli of the adjacent urban areas, suggesting that the air quality of the adjacent urban areas is locally impacted by the massive polymetallic sulfide deposits which is enhanced by the mining activity. Therefore, monitoring the urban air quality is recommended.

Combined zinc-lead isotope and trace-metal assessment of recent atmospheric pollution sources recorded in Irish peatlands.

Atmospherically-fed Earth surface archives such as ombrotrophic peatlands, lake sediments, and ice consistently show an upward increase in Zn concentrations of hitherto unclear origin. Here, we present a combined stable Zn isotope and trace element (Zn, Cd, Ni, Cu, Cr, V, Ta, Pb) dataset for a historically polluted, near-urban bog (Liffey Head) from the east coast of Ireland. This peat record is compared to an archive from a rural site at the west coast of Ireland (Brackloon Wood). Both archives show a clear near-surface increase in Zn deposition, accompanied by periodic deposition in Cr, Ni, Mo, and V suggesting a co-genetic origin of these elements. In the Liffey Head site, biologic upward distillation of nutrients can be excluded as the origin of the elemental enrichments. The differences in the excess metal ratios between the two sites (e.g., Zn/Cd of 426-1564, east, and 77-106, west) are attributed to a higher contribution from traffic emissions (diesel, petrol) and oil-burning at the near-urban site, and dominant atmospheric influence from solid fossil fuel combustion emissions (e.g., mixed fuel, coal and wood) at the rural site. The Zn isotope composition in the historically-polluted Liffey Head bog evolved from δ66/64ZnJMC-Lyon values of 0.72 ±â€¯0.03‰ in the peat accumulated during the 19th century to lighter ratios (0.18 ±â€¯0.03‰) towards the top of the monolith (i.e., recent). Zinc-isotope ratios are positively correlated with excess metal/Cd ratios and also with 206Pb/207Pb, collectively fingerprinting the gradual change from a mining-dominated to a traffic-dominated atmospheric pollution at the east coast over the past century. A prevalent input, interpreted to represent combustion emissions from diesel engines, is observed for the past 15 years. Combined with trace elements and radiogenic Pb isotopes, the information obtained with the Zn isotope systematics adds towards an in-depth characterisation of the pollution signals.

Elemental and isotopic behaviour of Zn in Deccan basalt weathering profiles: Chemical weathering from bedrock to laterite and links to Zn deficiency in tropical soils.

Zinc (Zn) is a micronutrient for organisms and essential for plant growth, therefore knowledge of its elemental cycling in the surface environment is important regarding wider aspects of human nutrition and health. To explore the nature of Zn cycling, we compared its weathering behaviour in a sub-recent regolith versus an ancient laterite profile of the Deccan Traps, India - an area of known soil Zn deficiency. We demonstrate that progressive breakdown of primary minerals and the associated formation of phyllosilicates and iron oxides leads to a depletion in Zn, ultimately resulting in a loss of 80% in lateritic residues. This residue is mainly composed of resistant iron oxides and hydroxides ultimately delivering insufficient amounts of bio-available Zn. Moreover, (sub)-tropical weathering in regions experiencing extended tectonic quiescence (e.g., cratons) further enhance the development of old and deep soil profiles that become deficient in Zn. This situation is clearly revealed by the spatial correlation of the global distribution of laterites, cratons (Africa, India, South America and Australia) and known regions of Zn deficient soils that result in health problems for humans whose diet is derived from such land. We also investigate whether this elemental depletion of Zn is accompanied by isotope fractionation. In the saprolitic horizons of both weathering profiles, compositions of δ66ZnJMC-Lyon lie within the "crustal average" of +0.27±0.07‰ δ66ZnJMC-Lyon. By contrast, soil horizons enriched in secondary oxides show lighter isotope compositions. The isotopic signature of Zn (Δ66Znsample-protolith up to ~ -0.65‰) during the formation of the ferruginous-lateritic weathering profile likely resulted from a combination of biotically- and kinetically-controlled sorption reactions on Fe-oxyhydroxides. Our findings suggest that oxide rich soil types/horizons in (sub)-tropical regions likely exert a control on riverine Zn isotope compositions such that these become heavier than the crustal average. This isotopic behaviour invites a broader study of global soils to test whether light isotope composition alone could serve as an indicator for reduced bioavailability of Zn.