A model for inferring dissolved organic carbon (DOC) in lakewater from visible-near-infrared spectroscopy (VNIRS) measures in lake sediment

We developed an inference model to infer dissolved organic carbon (DOC) in lakewater from lake sediments using visible-near-infrared spectroscopy (VNIRS). The inference model used surface sediment samples collected from 160 Arctic Canada lakes, covering broad latitudinal (60–83°N), longitudinal (71–138°W) and environmental gradients, with a DOC range of 0.6–39.6 mg L⁻¹. The model was applied to Holocene lake sediment cores from Sweden and Canada and our inferences are compared to results from previous multiproxy paleolimnological investigations at these two sites. The inferred Swedish and Canadian DOC profiles are compared, respectively, to inferences from a Swedish-based VNIRS-total organic carbon (TOC) model and a Canadian-based diatom-inferred (Di-DOC) model from the same sediment records. The 5-component Partial Least Squares (PLS) model yields a cross-validated (CV) R_CV² R_{CV}^{2}  = 0.61 and a root mean squared error of prediction (RMSEP _CV ) = 4.4 mg L⁻¹ (11% of DOC gradient). The trends inferred for the two lakes were remarkably similar to the VNIRS-TOC and the Di-DOC inferred profiles and consistent with the other paleolimnological proxies, although absolute values differed. Differences in the calibration set gradients and lack of analogous VNIRS signatures in the modern datasets may explain this discrepancy. Our results corroborate previous geographically independent studies on the potential of using VNIRS to reconstruct past trends in lakewater DOC concentrations rapidly.     

Sub-recent nitrogen-isotope trends in sediments from Skagerrak (North Sea) and Kattegat: Changes in N-budgets and N-sources?

We determined ¹⁵N/¹⁴N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand (δ¹⁵N = 5‰) and the imprint of river-borne nitrogen input into the SE North Sea (δ¹⁵N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ¹⁵N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ¹⁵N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea.     

The effects of meteorite impacts on the availability of bioessential elements for endolithic organisms

Abstract– Meteorite impacts, one of the most ubiquitous processes in the solar system, have the ability to destroy as well as create habitats for life. The impact process can increase the translucency and porosity of the target substrate, as well as mobilize biologically relevant elements within the substrate. For endolithic organisms, this process has important implications, especially in extreme environments where they are forced to seek refuge in the interior of rocks. Here, we show that unshocked target rocks and rocks that have experienced pressures up to about 80 GPa from the Haughton impact structure, Devon Island, Canada, possess a small, but discernible change in bulk chemistry within the major oxide analysis. However, changes in the distribution of elements did occur with increasing shock level for both the sedimentary and crystalline target. Both the crystalline and sedimentary target rocks contain significant amounts of glasses at higher shock levels (up to about 95% by volume), which would improve the availability of these elements to potential microbial endoliths as glasses are more easily dissolved by organic acids. The implication that impact events do not impoverish their capacity to serve as a “substrate” through volatilization is important with respect to analogous impact structures on Mars. After the deleterious effects of the direct meteorite impact, any microorganisms on Mars would have benefited from the input of heat, the mobilization of a possible frozen groundwater system, as well as increased translucency, porosity, and trace nutrient availability of the target substrate.     

Post‐European settlement response gradients of river sensitivity and recovery across the upper Hunter catchment,Australia

Most analyses of river adjustment have focused on parts of catchments where metamorphosis has occurred. This provides a non‐representative view of river responses to human‐disturbance. Although many rivers have been subjected to systematic land‐use change and disturbance, significant variability is evident in the form, extent and consequences of adjustment. This study documents the catchment‐wide distribution of river sensitivity and adjustment in the upper Hunter catchment, New South Wales, Australia in the period since European settlement. The spatial distribution and timing of lateral, vertical and wholesale river adjustments are used to assess river sensitivity to change. The type and pattern of rivers, influenced largely by valley setting, have induced a fragmented pattern of river adjustment in the upper Hunter catchment. Adjustments have been largely non‐uniform and localized, reflecting the predominance of bedrock‐controlled rivers which have limited capacity to adjust and are resilient to change. Less than 20% of river courses have experienced metamorphosis. Phases of reach‐scale geomorphic adjustment to human disturbance are characterized as a gradient of primary, secondary and tertiary responses. In general terms, primary responses such as cutoffs or straightening were followed by secondary responses such as channel expansion. These secondary responses occurred between 50–70 years after initial disturbance. A subsequent tertiary phase of river recovery, denoted as a transition from predominantly erosional to predominantly depositional geomorphic processes such as channel contraction, occurred around 70–120 years after initial disturbance. Such responses are ongoing across much of the upper Hunter catchment. Copyright © 2009 John Wiley & Sons, Ltd.     

Sustainability of groundwater in Mali,West Africa

The following paper describes the goals and some preliminary work in the Bani sustainability study, an ongoing project in Mali, West Africa. Rural communities in Mali are increasingly relying on hand-pumps, which tap groundwater resources, as a means of obtaining potable water. The long-term sustainable yield of groundwater resources is not known but can be evaluated in sustainability study. In 2005, a groundwater sustainability study was established along the Bani River of Mali. The Bani study collected groundwater levels that were used in a conceptual groundwater flow model—the Bani model—to develop an understanding of current aquifer conditions and to make limited predictions of sustainability under various future scenarios. The Bani model showed the climatic parameters of recharge (derived from precipitation) and evapotranspiration to influence simulated groundwater levels and groundwater volume available, while increased pumping rates, due to population growth, showed little effect. When considered in the context of the actual Bani sustainability study area, the change in groundwater levels resulting from climatic parameters may have negative implications, especially during several consecutive years of decreased precipitation, such as drought, or if downward trends anticipated for precipitation continue.     

Nitrogen cycling in the German Bight (SE North Sea) — Clues from modelling stable nitrogen isotopes

Nitrogen isotope values (δ¹⁵N) of surface sediments in the German Bight of the North Sea exhibit a significant gradient from values of 5–6‰ of the open shelf sea to values above 11‰ in the German Bight. This signal has been attributed to high reactive N (N_r) loading enriched in ¹⁵N from rivers and the atmosphere. To better understand the processes that determine the intensity and spatial distribution of δ¹⁵N anomalies in surface sediments, and to explore their usefulness for reconstructions of pristine N-input from rivers, we modeled the cycling of the stable isotopes ¹⁴N and ¹⁵N in reactive nitrogen through the ecosystem of the central and southern North Sea (50.9–57.3°N, 3.4°W−9.2°E) for the year 1995. The 3D-ecosystem model ECOHAM amended with an isotope-tracking module was validated by δ¹⁵N data of surface sediments within the model domain. A typical marine value (δ¹⁵N_nitrate=5‰) was prescribed for nitrate advected into the model domain at the seaside boundaries, whereas δ¹⁵N_nitrate of river inputs were those measured bi-monthly over 1 year; δ¹⁵N values of atmospheric deposition were set to 6‰ and 7‰ for NO_x and NH_y, respectively. The simulated δ¹⁵N values of different nitrogen compounds in the German Bight strongly depend on the mass transfers in the ecosystem. These fluxes, summarized in a nitrogen budget for 1995, give an estimate of the impacts of hydrodynamical and hydrological boundary conditions, and internal biogeochemical transformations on the nitrogen budget of the bight.     

Experimental study of polybaric REE partitioning between olivine, pyroxene and melt of the Yamato 980459 composition: Insights into the petrogenesis of depleted shergottites

A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (D_REE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt D_REE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε¹⁴³Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε¹⁸²W, ε¹⁴²Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε¹⁴³Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.     

Characterisation of dispersed organic matter from lower Palaeozoic metasedimentary rocks by organic petrography, X-ray diffraction and micro-Raman spectroscopy analyses

Dispersed organic matter (DOM) concentrates of C-rich rocks from areas with different metamorphic characteristics were studied by organic petrography, X-ray diffraction (XRD) and micro-Raman spectroscopy analysis. The concentrates contain several types of DOM with different morphologies, reflectance, X-ray diffraction and micro-Raman characteristics.Four different morphological types were identified in the two studied areas. The different types have different distributions and their reflectance is quite variable in the four types, with a dominance of the highest reflectance values in the DOM from the area with the highest metamorphic grade.XRD analyses of samples from the areas reveal the presence of fine graphite together with non-graphitised carbons.The Raman spectral profiles show the usual bands G (1582 cm⁻¹) and D1 (1350 cm⁻¹), on the first-order Raman spectrum, and S1 (2700 cm⁻¹) on the second-order spectrum. Additional weaker bands, D2 (1620 cm⁻¹), and more rarely D3 (1500 cm⁻¹) and S2 (2900 cm⁻¹), are present. These are characteristic for disordered carbons in the different types of DOM and in both studied areas. However, the Raman parameters (D1/G intensity area ratio and the frequency and width of G band) indicate variable degrees of organisation in all DOM types.The existence of different types of DOM with different degrees of ordering in the same lithologies and metamorphic grade seems to be related to different organic precursors, as they are graphitised to different extents under the same metamorphic conditions. However, in the same lithologies and metamorphic grade, the existence of various stages of graphitisation within the same type of DOM can only be explained though the interaction of DOM with the metamorphic fluids present in the rocks. The ordering graphitisation process may be due to the existence of metamorphic fluid circulation events with a variety of compositions.