Ultra-trace element characterization of the central Ottawa River basin using a rapid, flexible, and low-volume ICP-MS method

Ultra-trace (<1 ng g-1) rare earth elements and yttrium (REE+Y) and high field strength element (HFSE) geochemistry of freshwater can constrain element sources, aqueous processes in hydrologic catchments, and the signature of dissolved terrestrial fluxes to the oceans. This study details an adapted method capable of quantifying ≥38 elements (including all REE+Y, Nb, Ta, Zr, Hf, Mo, W, Th, U) with minimal sample preparation in natural water aliquots as low as ≤2 mL. The method precision and accuracy are demonstrated using measurement of the National Research Council – Conseil national de recherches Canada (NRC-CNRC) river water certified reference material (CRM) SLRS-6 sampled from the Ottawa River (OR). Data from SLRS CRM are compared to those of new, filtered (HREE-enriched REE+Y patterns, small natural positive Y and Gd anomalies, and negative Eu and Ce anomalies. These REE+Y features are coherent downstream in the OR apart from amplification of Eu and Ce anomalies during REE removal/dilution. The OR samples capture a downstream decrease in sparingly soluble HFSE (Th, Nb, Ta, Zr, Hf), presumably related to their colloid-particulate removal from the dissolved load, accompanied by crustal Zr/Hf (32.5 ± 5.1) and supercrustal Nb/Ta (25.1 ± 7.7) ratios. Subcrustal Th/U (0.17-0.96) and supercrustal Mo/W (12.0-74.5) ratios in all ORB waters indicate preferential release and aqueous solubility of U>Th and Mo>W, with the latter attributed primarily to preferential W adsorption on soil or upstream aquatic (oxy)(hydr)oxide surfaces

Trace element and Pb isotope fingerprinting of atmospheric pollution sources: A case study from the east coast of Ireland

Unravelling inputs of multiple air pollution sources and reconstructing their historic contribution can be a difficult task. Here, new trace metal concentrations and Pb isotope data were combined for a radionuclide (210Pb-241Am) dated peat core from the Liffey Head bog (LHB) in eastern Ireland in order to reconstruct how different sources contributed to the atmospheric pollution over the past century. Highest enrichments in the heavy metals Pb, Cu, Ag, Sn, and Sb, together with a Pb isotope composition (206Pb/204Pb: 18.351 ± 0.013; 206Pb/207Pb: 1.174 ± 0.012) close to that of the Wicklow mineralisation demonstrates significant aerial influx of heavy metals from local mining and smelting activities during the 19th century until ca. 1940s. A dramatic compositional shift defined by elevated Co, Cr, Ni, Mo, Zn, and V enrichments and a sharp transition towards unradiogenic 206Pb values (206Pb/204Pb: 18.271 ± 0.013–17.678 ± 0.006; 206Pb/207Pb: 1.170 ± 0.012–1.135 ± 0.007) is documented from the 1940s until ca. 2000. These are attributed to the atmospheric impact of fossil fuels and especially leaded petrol, modelled to have contributed between 6 and 78% to the total Pb pollution at this site. The subsequent turn to a more radiogenic Pb isotope signature since 2000 in Ireland is clearly documented in the investigated archive (206Pb/204Pb: 17.930 ± 0.006; 206Pb/207Pb: 1.148 ± 0.007) and reflects the abolishment of leaded petrol. However, there remains a persisting and even increasing pollution in Ni, Mo, Cu, and especially Zn, collectively originating from countrywide use of fossil fuels(peat, coal, heating oil, and unleaded vehicle fuels) for domestic and industrial purposes. This illustrates the continued anthropogenic influence on important natural archives such as bogs in Ireland despite the phase-out of leaded petrol

Silicon and chromium stable isotopic systematics during basalt weathering and lateritisation: A comparison of variably weathered basalt profiles in the Deccan Traps, India

Global biomass production is fundamentally affected by the hydrological cycling of elements at the Earth's surface. Continental weathering processes are the major source for most bio-essential elements in marine environments and therefore affect primary productivity. In addition, critical zone biomass depends on energy and chemical exchange reactions in weathering profiles. The latter reservoirs are in turn influenced by different climatic conditions that control weathering and pore water parameters like pH and Eh, which regulate mineral break down rates and dictate the mobility and mass flux of elements. Two Deccan Traps basalt weathering profiles of contrasting age and alteration intensity provide a natural laboratory for investigating the effects of rock alteration on Si and Cr and their isotopic. systematics. The Quaternary Chhindwara profile has progressed to a moderate degree of alteration (saprolite), while the Paleogene Bidar example displays an extremely altered laterite. The Chhindwara saprolite profile shows a near uniform Cr and Si concentration and isotopic composition, whereas the Bidar laterite profile is characterised by an intense loss of Si, a large enrichment of Cr within the most altered uppermost levels, and a wide range of Cr stable isotope ratios (-0.85 to 0.36 parts per thousand delta Cr-53/52). A co-variation between Si and Cr isotopes, as well as their co-variation with iron content, provides empirical evidence that iron redistribution within the profile has a large effect on Cr mobility and Si isotopic fractionation. Therefore, it is concluded that iron oxides exert a primary control over the isotopic composition of both Cr and Si in pore waters of laterites. Since laterite formation is promoted by tropical climates, the results of this study provide new evidence to suggest that the hydrological Cr and Si fluxes originating from continental weathering have changed in accordance with large-scale, deep time climate variation and continental plate configuration. An increased flux of Si and greater magnitude of Cr mobility and isotopic fractionation are possibly amplified under CO2-rich, greenhouse episodes and/or when large landmasses were tectonically arranged at near equatorial latitudes

Oxidative mafic rock weathering through geological time

THESIS 10939Oxidative chemical weathering requires the presence of free oxygen in the Earth\u27s atmosphere. Reconstructing the terrestrial oxidation history in the ancient rock record has helped to detect the intrinsic links between atmospheric composition and biological evolution of oxygenic photosynthesis. This thesis contributes to this theme via the study of oxidative chemical weathering at different stages of Earth\u27s history and at different weathering intensities. It presents detailed geochemical investigations of three profiles developed on mafic rock. Laterite (Bidar) and saprolite (Chhindwara) profiles hosted on Deccan Traps basalt (India) capture weathering in the Phanerozoic, while the ca. 1.85 Ga Flin Flon paleosol (Canada) hosted on greenstone captures weathering after the Great Oxidation Event (GOE, ca. 2.4-2.3 Ga)

Unusually heavy stable Mo isotope signatures of the Ottawa River: Causes and implications for global riverine Mo fluxes

The accurate use of Mo isotope mass balance modelling of ancient oceans relies on the assumption that the δ98Mo of modern riverine inputs represents a reasonable estimate of the past. A growing number of studies of global rivers have demonstrated significant variation in δ98Mo from the bedrock sources of Mo. The Ottawa River, Canada, was previously identified as having an anomalously heavy Mo isotope composition, with a δ98Mo signature close to seawater (2.3‰), for a seemingly pristine natural river. To further explore this unusual signature, we collected and filtered 29 water samples from the Ottawa River, tributaries, and small lakes, and analysed them for Mo isotopes as well as major and trace elements. Here, we fully document heavy δ98Mo signatures throughout the Ottawa River and its absence in surrounding areas of the wider Ottawa River basin (ORB). Our results reveal a progressive upstream increase in both Mo concentration and δ98Mo signatures in the Ottawa River towards values even heavier than seawater, up to 3.13‰ – the heaviest Mo isotopic signature of river water measured to date. In contrast, the tributaries and lakes display far lighter and more consistent δ98Mo signatures within the range previously found in other rivers. Weathering of an isotopically heavy bedrock source, fractionation during weathering and retention of light isotopes in soils have all been proposed as sources of heavy δ98Mo in rivers; however, none of these mechanisms can satisfactorily explain our new observations. Colloidal and particulate processes that remove elements downstream, as inferred from some trace element proxies, also cannot explain the decreasing δ98Mo, since the preferential removal of light Mo isotopes is predicted from these processes. Similarly, the downstream trends show no apparent relationships with constructed dams or known potential industrial sources. Therefore, our findings from the Ottawa River are best explained as the dilution of a yet unidentified point source of heavy Mo upstream of sampling, or a significant permanent sink for light isotopes existing only in the upper reaches of the catchment. In both cases, anthropogenic contribution from a large mining district in the headwaters of the river must be considered and should be explored further. Fractionation of Mo in waste rock storage facilities have been previously identified and may provide an unnatural sink for isotopically light Mo through the Rayleigh-type fractionation of dissolved Mo on oxyhydr(oxide) mineral surfaces. The implied anthropogenic alteration of the natural Mo cycle highlights the significant and wide-reaching effects of unnatural point sources of Mo on the cumulative δ98Mo signatures of the catchment, and emphasises the necessity for detailed geochemical screening of anomalous river water isotope signatures before natural isotope compositions are inferred

New insights into Paleoproterozoic surficial conditions revealed by 1.85 Ga corestone-rich saprolith

Spheroidally weathered corestones, which are remnant pieces of bedrock surrounded by progressively weathered saprolite, preserve an ideal, small-scale natural interface for examining incipient to intermediate weathering reactions. Ancient corestones are preserved in some Precambrian paleosols but have remained surprisingly understudied. Here detailed mineral-chemical trends are examined across corestone-saprolith interfaces in ca. 1.85 Ga dolerite-hosted paleosol from the Flin Flon-Creighton area (Manitoba and Saskatchewan, Canada). The study presents the first comprehensive paleo-redox tracer suite (Fe-Mn-Mo-U-V-Cr-Ce) for the classic Flin Flon paleosol, which formed during a crucial period in the Paleoproterozoic for which marine sedimentary archives infer a return to low oxygen levels after the Great Oxidation Event (GOE). The textural and mineralogical progression across corestone-saprolith interfaces documented with petrography and scanning electron microscope-mineral liberation analysis (SEM-MLA) show many features (e.g., development of weathering rindlets and solution channels) strikingly reminiscent of modern mafic rock-hosted saprolite, despite later overprint. Albite-dominated cores preferentially preserving carbonate and sulfide (pyrite, chalcopyrite) are progressively altered outwards to a finer-grained saprolith rich in chlorite, illite, and muscovite, with embaying rindlets bearing cryptocrystalline hematite-quartz-illite. These mineral-textural observations guided sub-sampling for bulk Fe(II) and solution ICP-MS ultra-trace and major element analysis, and mineral-scale LA-ICP-MS analysis. Many high-field-strength elements (Al, Ti, Zr, Nb, Hf, Ta, and Th), remained immobile across the interface and ascertain the homogeneity of the parent dolerite. Progressive weathering from corestone to saprolith was quantified with chemical index of alteration minus K (CIA-K) values that fall into three zones (incipient (1): 45–55; modest (2): 55–65; and moderate (3): 65–75). Mass balance and spatial geochemical analysis revealed the following features: outward migration of Fe(II) and Mn from corestone to saprolith, with partial oxidation of Fe(II) to Fe(III) in rindlets; minimal REE mobility with no pronounced Ce anomalies, but well-developed, unidirectional Y/Ho fractionation; significant Cr and V mobility from corestones outwards with enrichment in saprolith; minimal dm-scale cycling and limited loss of Mo and U from saprolith; and post-depositional enrichment of K, Rb, Cs, Tl, Ba, Be, and W. Using semi-quantitative LA-ICP-MS elemental maps, it was possible to contextualize the mineralogical controls on chemical weathering reactions. Iron, Mg, and Mn are coupled within chlorite (representing former pedogenic phyllosilicates), which also scavenged Cr. The REEs are predominantly hosted by apatite and titanite. The Ti-phases ilmenite and titanite are the predominant U- and Mo-bearing hosts. Collectively, the insights from the Flin Flon paleosol converge with those from other ca. 1.90–1.85 Ga terrestrial and marine deposits in inferring an oxygen-limited atmosphere capable of efficiently oxidizing Fe and S, but not Mn, in terrestrial environments. The Cr and V distributions are most consistent with small-scale solubilization and redeposition that appears to be linked to the weatherability of protolith minerals and locally-generated acid- and/or ligand-rich conditions rather than oxidation. Although normally sensitive indicators of oxidative weathering, the inhibited release of Mo and U from the corestone was instead primarily dictated by the weathering resistance of their host minerals. Oxygen-limited weathering in saprolite supplied sulfate to the oceans, but the continental flux of redox-sensitive trace elements from this zone as well as more weathered substrates was limited, ultimately contributing to sustaining a largely redox-dynamic, weakly buffered, and nutrient-limited ocean.</p