Evidence for the onset of mining activities during the 13th century in Poland using lead isotopes from lake sediment cores

Efforts to study how human activities have influenced the environment since the end of the Roman period to present day are lacking for North Central Europe. Here, we present new lead (Pb) isotope data determined from two sediment cores collected from ancient lakes spanning the last 1,500 years, located in the Kuyavian-Pomeranian Voivodeship, Poland. Study sites at Radzyń Chełmiński and Rywałd were used to differentiate Pb sources. Radzyń Chełmiński is located in the vicinity of a late Medieval Teutonic Order castle and town, while Rywałd is situated within a relatively pristine area until the 19th century when it became used for agricultural purpose. Core samples were analyzed for Pb concentration and isotopes (206Pb, 207Pb, 208Pb). Bayesian modelling was used to isolate the anthropogenic signal at each site over time. For both sites, Pb enrichment factors relative to titanium (Ti) and upper continental crust values range from 13 to 159. Lead isotopic ratios range from background, pre-anthropogenic local values (206Pb/207Pb =1.31±0.03‰, 208Pb/206Pb = 1.97±0.04‰) to anthropogenic values (SW Poland coal, ore, slag 206Pb/207Pb = 1.17±0.01‰, 208Pb/206Pb = 2.09±0.01‰). Modelled anthropogenic contribution varies greatly over time, ranging from 14 to 100%. At Radzyń Chełmiński, modeled anthropogenic Pb contribution and measured Pb concentration follow similar trends. However, at Rywałd, from around A.D.1000 to A.D. 1400 these profiles diverge significantly. Our new insights highlight different sources of Pb from the 12th century to present day: (1) short range agricultural activities from the town, and (2) long range mining activities. Additionally, prior to the 12th century, our data suggest continental anthropogenic activity possibly favored by a warmer climate

Does pyrite act as an important host for molybdenum in modern and ancient euxinic sediments?

Molybdenum (Mo) is a popular paleoproxy for tracking the spatiotemporal pattern of euxinic (anoxic and sulfidic) conditions in the ancient ocean, yet surprisingly little is known about the processes leading to its fixation under sulfidic conditions. Pyrite has been proposed to be the main host phase for Mo sequestration. To clarify the role played by pyrite, and thus to refine the utility of this paleoproxy, modern and ancient samples from six different study sites were analyzed, all representing euxinic conditions, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Although pyrite often shows substantial enrichments relative to average crust and even matrix samples of similar size, our results show that most of the Mo in euxinic muds and shales is found in the non-pyrite matrix (80–100%) and not in the pyrite grains (0–20%)—simply because the volume of matrix dominates the bulk sediments/rocks. A relationship between the percent of Mo hosted by pyrite and the sulfur isotope composition of that pyrite is observed and can be linked to post-depositional alteration. Specifically, the oldest, typically most altered samples, show the highest δ^(34)S values because of limited sulfate availability at the time of their formation in the early ocean. In these old samples, the relatively small amount of Mo sequestered initially within pyrite is more likely to have been released to the matrix during the strong recrystallization overprints that these rocks have disproportionately suffered. Despite the universal importance of appreciable H_2S availability during Mo uptake, we conclude that pyrite should be viewed as a nontrivial sink for Mo but clearly not the primary host in most euxinic shales and rather suggest that other burial pathways should be emphasized in future studies of the mechanisms of Mo sequestration in such settings

Molybdenum geochemistry in a seasonally dysoxic Mo-limited lacustrine ecosystem

Lakes are important for storage of the essential micronutrient molybdenum (Mo) during its transfer from the continents to the oceans, but little is known about the major sources and sinks for Mo in lacustrine ecosystems. We studied Mo cycling in Castle Lake, a small subalpine lake in the Klamath-Siskiyou Mountains of Northern California underlain primarily by mafic and ultramafic rocks where primary productivity is limited by Mo bioavailability. The deeper water of the lake becomes dysoxic (9–90 μM dissolved oxygen) during the summer. This study was undertaken to identify the sources of Mo to Castle Lake and establish a Mo budget. We measured Mo concentrations in a suite of bulk solids (lake sediments, soils and bedrock) and aqueous samples (sediment porewaters, soil runoff, spring waters, snow and ice) from Castle Lake and its watershed. Lake sediments have elevated Mo (7–36 ppm) compared to soils and bedrock (0.2–2 ppm) and Mo/Al values were nearly two orders of magnitude higher than the crustal abundance. Sediment porewaters had higher Mo (4–15 nM) than lake water (2–4 nM), soil runoff (0.1–6.2 nM), snowmelt (⩽0.1 nM), lake ice (0.3–2.2 nM) and local spring waters (0.03–2.72 nM). Bulk lake sediments had negative δ^(98/95)Mo values, ranging from −0.5 to −1.0‰ (±0.1). We used the numerical model PROFILE to estimate the net reaction rate of Mo in the porewater. Model calculations ruled out diagenesis as a source of Mo to lake sediments; diagenetic Mo always represented ⩽5% of the total Mo content in sediment. We also ruled out dissolved Mo inputs from groundwater and watershed inflow as important sources of Mo. Two whole-lake experimental Mo additions in the 1960’s could have contributed a sizeable amount of Mo to the lake sediments, but only over a short period. Atmospheric deposition of anthropogenic Mo from extensive copper smelting that occurred south of Castle Lake from 1896 to 1919 and from major Californian urban centers today were negligible Mo sources. Mo flux from the sediments (0.4–0.5 nmol cm^(−2) yr^(−1)) was roughly equal to Mo fluxes from surface inflow and outflow, whereas Mo burial fluxes were significantly higher (11.5 nmol cm^(−2) yr^(−1)). Because dissolved Mo fluxes were minimal, and atmospheric Mo deposition was estimated to be a minor source of Mo (<1 nmol cm^(−2) yr^(−1)), the largest source of Mo is non-detrital particulate matter (∼12 nmol cm^(−2) yr^(−1)), likely a mixture of organic matter and Fe–Mn oxyhydroxides as supported by Mo isotopic data

Molybdenum speciation as a paleo-redox proxy: A case study from Late Cretaceous Western Interior Seaway black shales

Molybdenum (Mo) geochemistry is widely used to reconstruct ocean oxygenation throughout Earth’s history. However, gaps in our fundamental knowledge of Mo burial within sediments hinder the utility of Mo as a proxy for paleo-redox reconstruction. To improve our understanding of Mo burial pathways and sedimentary Mo speciation, we present combined geochemical analysis and X-ray absorption fine structure spectroscopy from black shales deposited in the North American Western Interior Seaway (WIS) during the Late Cretaceous. Molybdenum was present in multiple phases during deposition of the black shales, probably as a combination of Mo(IV) and Mo(VI) species with no more than 54% of Mo associated with Mo-sulfide phases. These Mo molecular geochemistry results indicate that sediments were not deposited under permanent euxinic conditions, but rather weakly euxinic, periodically euxinic, or anoxic conditions (with sulfide restricted to pore waters). This is the case even in samples where other redox indicators would indicate strongly euxinic conditions. Additionally, differences in Mo speciation between our two localities highlight variability in the intensity of oxygen limitation within the WIS, providing a more nuanced redox reconstruction from sections deposited in conditions between weakly and strongly euxinic, suggesting that Mo molecular geochemistry may allow for more detailed spatial reconstructions of past stratified anoxic basins and oxygen minimum zones

Integrating Empirically Dissolved Organic Matter Quality for WHAM VI using the DOM Optical Properties: A Case Study of Cu–Al–DOM Interactions

Metal speciation is important for understanding the toxicity of metals in aquatic systems, and can be predicted for mixtures of metals in presence of dissolved organic matter (DOM) with thermodynamic models such as WHAM VI. The influence of the DOM source (quality) has been demonstrated, but is presently neglected in predicting Cu activity (WHAM VI). Here we determined the effect of aluminum (Al) competition on copper (Cu) complexation for four different DOMs, from a high-colored DOM (more humic) to a low-colored DOM (less humic). In presence of Al, free Cu activities (defined as free ion activity) increased, consistent with competition between Cu and Al for the same binding sites on all DOM. The apparent competition decreased with increasing DOM color. Equilibrium modeling of Cu speciation with WHAM VI explained 49% of the variance in measured Cu activity. When modified to integrate DOM quality using a new empirical coefficient <i>F</i> related to DOM optical properties, Cu activities predicted from WHAM VI were significantly improved to about 80% of the observed variance explained. The effects of Al on Cu activity were well predicted by WHAM VI. Subsequently, we compared the relative effects of DOM concentration, DOM quality, and Al competition with other determinants of Cu activity represented in legislation and scientific literature (pH and hardness), and qualitatively ranked them by their influence on Cu activity for normal ranges encountered in fresh waters using WHAM VI. Our experimental results indicate that DOM quality is an important variable that should be included in predictive models of ion speciation (WHAM VI) and eco-toxicological models such as the biotic ligand model (BLM)

Effect of thermal maturity on remobilization of molybdenum in black shales

Molybdenum (Mo) concentrations in sedimentary records have been widely used as a method to assess paleo-redox conditions prevailing in the ancient oceans. However, the potential effects of post-depositional processes, such as thermal maturity and burial diagenesis, on Mo concentrations in organic-rich shales have not been addressed, compromising its use as a redox proxy. This study investigates the distribution and speciation of Mo at various thermal maturities in the Upper Ordovician Utica Shale from southern Quebec, Canada. Samples display maturities ranging from the peak oil window (VRo∼1%) to the dry gas zone (VRo∼2%). While our data show a significant correlation between total organic carbon (TOC) and Mo (R2=0.40, n=28, P30 ppm). Our results show the presence of two Mo species: molybdenite Mo(IV)S2 (39±5%) and Mo(VI)-Organic Matter (61±5%). This new evidence suggests that at higher thermal maturities, TSR causes sulfate reduction coupled with oxidation of organic matter (OM). This process is associated with H2S generation and pyrite formation and recrystallization. This in turn leads to the remobilization of Mo and co-precipitation of molybdenite with TSR-derived carbonates in the porous intervals. This could lead to alteration of the initial sedimentary signature of Mo in the affected intervals, hence challenging its use as a paleo-redox proxy in overmature black shales