Quantifying seawater exchange rates in the Eocene Arctic Basin using osmium isotopes

The closure of seaways that connected the Arctic Ocean to the global ocean during the early Paleogene led to severe hydrographic restriction. We present new osmium isotope data from organic-rich sediments deposited in the central Arctic Ocean during the Early–Middle Eocene. The new data show that the long term isotopic composition of osmium in Arctic seawater began to diverge from that of the global ocean at ∼54 Ma, after the Eocene Thermal Maximum 2 hyperthermal event. This divergence was probably caused by the gradual closure of seaways connecting the Arctic Ocean to the global ocean. The Os data are used to calculate water exchange rates between the Arctic and surrounding oceans and to calculate Arctic Ocean salinity during the Early Eocene. The results show that the development of severe, long term Arctic Basin restriction after ∼54 Ma occurred as open ocean seawater input decreased below ∼0.01 Sv, resulting in a mean basin salinity between 8–16 PSU, depending on model assumptions

Evaluating δ^238/235U records in mudrocks as a proxy for ocean oxygenation during the Early Eocene

It is generally established that Early Eocene climate was characterised by persistent warmth punctuated by abrupt global warming events that were associated with perturbations in the global carbon cycle. The distribution of O2 in the oceans would have been profoundly affected but the timing and extent of any fluctuations in global ocean oxygenation during these events are still poorly constrained. Records of seawater Mo isotope compositions derived from marine sediments from the Arctic Ocean suggest that euxinic areas were slightly more widespread in the Early Eocene ocean than they are at present [1,2]. Here, we present δ238/235U data from the same location. By comparing these new U isotope data with complementary δ98/95Mo data, we assess their capacity to provide additional constraints on the variations of global ocean oxygenation during this period. The residence times of Mo and U in the oceans differ from one another, and their isotope fractionations display different sensitivities to dissolved oxygen concentrations. By combining information from both isotope systems, we should therefore be able to better constrain the onset and the severity of the episodes of seawater anoxia during the Eocene, and to improve our understanding of the processes that control ocean oxygenation. Our analyses of Arctic Ocean mudrocks show that where δ98/95Mo likely records a global redox signal, δ238/235U has more complicated variations that appear to be related to changes in the nature and/or the flux of detrital material. Our observations highlight the importance of quantifying continental inputs and local sedimentation rates, as well as understanding the processes controlling U isotope fractionation, before δ238/235U data can be used reliably for paleo-environmental reconstructions. [1] Dickson, Cohen & Coe (2012), Geology 40-7, 639-642. [2] Dickson & Cohen (2012), Paleoceanography 27, PA3230

Origin of calcium isotope fractionation in river waters: evidence from the Strengbach catchment, France

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Temporal variations of chemical weathering fluxes in boreal rivers under permafrost conditions. Example of the Nizhnaya Tunguska watershed (Central Siberia)

Boreal regions will be very sensitive to global warming which should induce an important reduction of the permafrost area, and also probably a significant modification of the hydrological regime of these high latitude regions. In permafrost areas the hydrological characteristics of rivers are indeed very specific with the occurrence of three highly contrasted periods: a very low water period from October to May, a spring flood in May/June, and a relatively high water period in summer, from June to the end of September [1]. However, previous geochemical studies interested in boreal systems do not take these temporal variations into account. In this work, we propose to characterize the temporal variability of dissolved chemical fluxes carried by boreal rivers under permafrost conditions. For this purpose, two rivers draining the South of the basaltic plateau of Putorana in Central Siberia (Kochechumo and Nizhnaya Tunguska) were sampled along the year and an extended spatial sampling of the watershed was carried out during the summer. The dissolved load of these water samples were analysed for major and trace element concentrations as well as for strontium and uranium isotopic compositions On the basis of element concentration variations, three periods can be marked out, matching the three hydrological periods. Variations of concentration ratios as well as variations of Sr isotope ratios show however that annual concentration pattern cannot be explained solely by dilution processes but have to involve the contribution of different sources. Thus, the significant increase of aluminium and iron concentrations when the spring flood discharge occurs is certainly linked to the presence of colloidal substances, most likely originating from upper soil horizons during the period of snow melting. Temporal variations of (234U/238U) activity ratios are also observed in the dissolved load of the two rivers, with higher values in winter (>2) than in spring and summer (from 1.2 to 1.5). We propose that in winter, when all surface waters are frozen, the only contribution to the riverine water flux would come from deep underground reservoirs having high rock/water ratios and long periods of interaction thus producing high uranium activity ratios [2]. In summer, the contribution of surface waters, flowing over the permafrost in the active layer (suprapermafrost flow), would be predominant and thus constitute the main chemical flux carried by these rivers. Overall, permafrost regions represent very specific hydrogeochemical systems compared to tropical and temperate systems with two different fluxes over the year : a deep water flux in winter and a predominant surface water flux in spring and summer. [1] Pokrovsky O., Schott J., Kudryatvtzev D.I., Dupré B. (2002). Basalt weathering in Central Siberia under permafrost conditions.Geochim. Cosmochim. Acta 69, 5659-5680. [2] Durand S., Chabaux F., Rhis S., Duringer P., Elsass P. (2005). U isotope ratios as tracers of groundwater inputs into surface waters : Example of the Upper Rhine hydrosystem. Chem. Geol. 220, 1-19

Protein carbonylation during natural leaf senescence in winter wheat, as probed by fluorescein-5-thiosemicarbazide

Leaf senescence is characterised by a massive degradation of proteins in order to recycle nitrogen to other parts of the plant, such as younger leaves or developing grain/seed. Protein degradation during leaf senescence is a highly regulated process and it is suggested that proteins to be degraded are marked by an oxidative modification (carbonylation) that makes them more susceptible to proteolysis. However, there is as yet no evidence of an increase in protein carbonylation level during natural leaf senescence. The aim of our study was thus to monitor protein carbonylation level during the process of natural senescence in the flag leaf of field-grown winter wheat plants. For this purpose, we adapted a fluorescence-based method using fluorescein-5-thiosemicarbazide (FTC) as a probe for detecting protein carbonyl derivatives. As used for the first time on plant material, this method allowed the detection of both quantitative and qualitative modifications in protein carbonyl levels during the last stages of wheat flag leaf development. The method described herein represents a convenient, sensitive and reproducible alternative to the commonly used 2,4-dinitrophenylhydrazine (DNPH)-based method. In addition, our analysis revealed changes in protein carbonylation level during leaf development that were associated with qualitative changes in protein abundance and carbonylation profiles. In the senescing flag leaf, protein carbonylation increased concomitantly with a stimulation of endoproteolytic activity and a decrease in protein content, which supports the suggested relationship between protein oxidation and proteolysis during natural leaf senescenc

Sources of colloidal and dissolved loads over the hydrological cycle in Siberian rivers

High latitude permafrost-dominated areas such as Central Siberia present an atypical hydrological cycle punctuated by an important spring flood resulting from snow melting. However, chemical variations in river waters associated to these hydrological variations are rarely taken into account in environmental geochemistry studies. This study aims to work out how the highly contrasted hydrological cycle and the presence of permafrost influence the temporal pattern of chemical element migration. For this purpose, major and trace element concentrations as well as Sr and U isotopic ratios were analyzed in the dissolved load of two Siberian rivers regularly sampled over two hydrological cycles (2005-2007), Nizhniya Tunguska and Kochchumo rivers, following the classical approaches used in the lab (e.g. [1]). Our results highlight that DOC and traditionnaly insoluble elements such as Al and Fe, but also REE and Th, are mobilized as a major colloidal flux at the spring flood and to a lesser extent during the summer period. The data show the occurrence of two colloid sources, successively involved over time: during the spring flood, the main source of colloids is the uppermost organic soil horizon whereas later in summer colloids come mainly from deeper soil compartments. Similarly, the results point out that the dissolved load of these rivers have to be explained in terms of mixing between deep underground brines that dominate during winter and a summer suprapermafrost flow draining deep soils, with a minor contribution of shallow soil layers during spring flood. It is critical to consider these temporal variations in the intensity and in the nature of dissolved geochemical flux in order to establish reliable chemical budgets and to evaluate weathering rates in boreal regions

Origin of chemical fluxes carried by boreal rivers : evidence from major, and trace element, U and Sr isotope data in two Siberian rivers

High latitude regions are characterized by very contrasted hydrological periods, marked by (a) a very low water flow during cold period (October-May), (b) an intense spring flood in May/June and (c) an intermediate to high water flow in summer (June– September). We propose here to quantify the intensity of geochemical fluxes associated to each of these hydrological periods and to constrain their origin. For this purpose, we analysed the temporal variations of the geochemical composition of water samples (filtered at 0.22µm) collected at the outlet of the Kochechumo and the Nizhnaya Tunguska rivers in Central Siberia (Russia). These analyses, performed over two hydrological cycles (2006-2008), were completed by a study of a smaller experimental watershed within the Kochechumo watershed. Our results combining major and trace element data together with Sr and U isotope ratios show that the melting flood in May results in the input of specific insoluble and soluble element fluxes in river waters. The mobilization of organic and inorganic colloids (from surface soil horizons) accounts for the insoluble element input during the flood period. The source of the dissolved element flux is clearly distinct from the source of winter waters and also originates from the uppermost horizons of the soil-permafrost system, with slight modification during the melting flood. Indeed, melting snow and leached litter appear to be the main chemical source at the beginning of the flood event whereas the suprapermafrost flow, more affected by water-rock interactions, dominates afterwards. This latter flux also dominates the chemical composition of summer river waters. During the winter, we observe in rivers the predominance of deep underground waters possibly affected by mineral precipitation or dissolution processes

The impact of water-rock interaction and vegetation on calcium isotope fractionation in soil- and stream waters of a small, forested catchment (the Strengbach case)

This study aims to constrain the factors controlling the calcium isotopic compositions in surface waters, especially the respective role of vegetation and water-rock interactions on Ca isotope fractionation in a continental forested ecosystem. The approach is to follow changes in space and time of the isotopic composition and concentration of Ca along its pathway through the hydro-geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls, percolating soil solutions and springs. The study is focused on the Strengbach catchment, a small forested watershed located in the northeast of France in the Vosges mountains. The δ44/40Ca values of springs, brooks and stream waters from the catchment are comparable to those of continental rivers and fluctuate between 0.17 and 0.87‰. Soil solutions, however, are significantly depleted in lighter isotopes (δ44/40Ca: 1.00-1.47‰), whereas vegetation is strongly enriched (δ44/40Ca: -0.48‰ to +0.19‰). These results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions, with depletion in "light" calcium in the soil solutions from deeper parts of the soil compartments due to preferential 40Ca uptake by the plants rootsystem. However, mass balance calculations require the contribution of an additional Ca flux into the soil solutions most probably associated with water-rock interactions. The stream waters are marked by a seasonal variation of their δ44/40Ca, with low δ44/40Ca in winter and high δ44/40Ca in spring, summer and autumn. For some springs, nourishing the streamlet, a decrease of the δ44/40Ca value is observed when the discharge of the spring increases, with, in addition, a clear covariation between the δ44/40Ca and corresponding H4SiO4 concentrations: high δ44/40Ca values and low H4SiO4 concentrations at high discharge; low δ44/40Ca values and high H4SiO4 concentrations at low discharge. These data imply that during dry periods and low water flow rate the source waters carry a Ca isotopic signature from alteration of soil minerals, whereas during wet periods and high flow rates admixture of significant quantities of 40Ca depleted waters (vegetation induced signal) from uppermost soil horizons controls the isotopic composition of the source waters. This study clearly emphasizes the potential of Ca isotopes as tracers of biogeochemical processes at the water-rock-vegetation interface in a small forested catchment