Isotopic Composition Of Sediments Research Articles

Provenance characteristics of Mesoproterozoic metasedimentary rocks from Telemark, South Norway: a Nd-isotope mass-balance model

Abstract Whole-rock Nd isotope data on stratigraphically constrained samples of metasedimentary rocks from the ca. 1.50 to ca. 1.12 Ga Telemark supracrustal sequence in central South Norway define a linear array with a meaningless, late Archean apparent age; this is a mixing-line defined by two distinct endmembers, characterised by 147 Sm / 144 Nd=ca. 0.09 and ca. 0.13; and present-day 143 Nd / 144 Nd=0.5116 and 0.5122, respectively. From published SIMS and new LAM-ICPMS U–Pb ages of detrital zircon populations, ca. 1.8 Ga, ca. 1.5 Ga and ca. 1.15 Ga source components can be identified. The initial Nd isotope composition of sediments varies with stratigraphic level and geographic position within the basins, reflecting competition between juvenile volcanic sources and Paleoproterozoic granitoids throughout the ca. 400 Ma depositional period of the sediments. Given adequate external constraints, a normal whole-rock Sm–Nd analysis of a (meta)sedimentary rock can in principle be used to determine the relative contributions of up to four provenance components. The REE concentrations in the sedimentary rocks are controlled by source characteristics and differentiation processes acting during transport and deposition, which for most units have not affected the Sm/Nd ratio of the sediment. Initial Nd isotopic compositions and model ages are inherited from the source components, and are not significantly controlled by syn- or post-depositional differentiation processes. A (semi)quantitative model of sedimentary mass balance indicates that variations in volcanogenic input are erratic in the ca. 1.4 Ga Vindeggen group and the 1.15 Ga Lifjell group (both previously assigned to the obsolete Seljord group), reflecting changes in provenance with time as well as laterally. During deposition of the 1.15 Ga and younger Oftefjell and Hoydalsmo groups (parts of the obsolete Bandak group), the accumulated volcanogenic input increased with time, as a result of Sveconorwegian volcanism. The present findings support the presence of Paleoproterozoic (1.7–1.9 Ga) source terranes in central South Norway.

Tracing the distribution of erosion in the Brahmaputra watershed from isotopic compositions of stream sediments

Bank sediments and suspended loads of the Brahmaputra River and its important tributaries were collected from the Himalayan front to Bangladesh along with most of the important tributaries. Chemical and isotopic compositions of the sediments are used to trace sediment provenance and to understand erosion patterns in the basin. Overall isotopic compositions range from 0.7053 to 0.8250 for Sr and ϵNd from −20.5 to −6.9. This large range derives from the variable proportions of sediments from Himalayan formations with high Sr isotopic ratios and low ϵNd, and Transhimalayan plutonic belt with lower Sr isotopic ratios and higher ϵNd. The latter are exposed to erosion in the Tsangpo and in the eastern tributary drainages. Overall erosion of the Himalayan rocks is dominant, representing ca 70% of the detrital influx. Compositions of the Brahmaputra main channel are rather stable between 0.7177 and 0.7284 for Sr and between −14.4 and −12.5 for ϵNd throughout its course in the plain from the Siang-Tsangpo at the foot of the Himalayan range down to the delta. This stability, despite the input of large Himalayan rivers suggests that the Siang-Tsangpo River represents the major source of sediment to the whole Brahmaputra. Geochemical budget implies that erosion of the Namche Barwa zone represents about 45% of the total flux at its outflow before confluence with the Ganga from only 20% of the mountain area. Higher erosion rates in the eastern syntaxis compared to the other Himalayan ranges is related to the rapid exhumation rates of this region, possibly triggered by higher precipitation over the far-eastern Himalaya and the high incision potential of the Tsangpo River due to its very high water discharge.

The correlation of inorganic C, O isotopic values for Lake Chenghai sediments and its environmental implications

As one of the lakes on the Yunnan-Guizhou plateau, Lake Chenghai, which is a typical closed lake with the precipitation accounting for one-third or more of the annual water input, has a high total salinity (almost like a saline lake). The inorganic C, O isotopic composition of lake sediments bears much sensitive information about environmental change in the catchment, while their correlations revealed the hydrological conditions under which the lake was closed. Their compositional variations are controlled by temperature, precipitation, photosynthesis, dissolving equilibrium of the carbonate system and hydrological condition. According to our research on inorganic C, O isotopic composition of Lake Chenghai sediments, we investigated the environmental change of this catchment several decades ago. The results showed that Lake Chenghai has kept good hydrological closing conditions in the past several decades, as indicated by the good correlation of inorganic C, O isotopic composition of sediments; and that the environmental change in this catchment shows a tendency of periodical evolution on a 10−11-years scale, although the signal noise is relatively high at the bottom of the sediment core. And we also can extend C, O isotopes, a sensitive environmental indicator, to nearly saline lake environments with a high degree of mineralization.

PCB Levels in Laminated Coastal Sediments of the Baltic Sea along Gradients of Eutrophication Revealed by Stable Isotopes (δ15N, δ13C)

Abstract A systematic investigation of PCB in coastal sediments of the Baltic Sea was initiated in 1996 by retrieving dated subsurface layers (1992–1994) from 88 sites in 19 sheltered archipelago areas, where laminated sediments were found to be common. PCBs7 (HELCOM's standard sum of 7 congeners, ≈1/6 of total PCB) was quantified, as well as the abundance and isotopic composition of nutrients (N, C), in order to trace effects of eutrophication and potential influences on the turnover of PCB. In most of the areas, the median of PCBs7/C was within 70 to 120 ng g−1 C. This is similar to recent values in regional offshore sediments, and thus consistent with a rapid exchange between coastal and open waters. Elevated PCBs7 levels up to around 1000 ng g−1 C were found within 50 km from Stockholm. Here, but not in other areas, PCBs7/C increased linearly with the degree of eutrophication. This was evident from the isotopic composition of sediment N and C, reflecting local eutrophication as well as its link to fre...

Monitoring detrital input and resuspension effects on sediment trap material using mineralogy and stable isotopes ( δ18O and δ13C): the case of Lake Neuchâtel (Switzerland)

Isotopic analyses on bulk carbonates are considered a useful tool for palaeoclimatic reconstruction assuming calcite precipitation occurring at oxygen isotope equilibrium with local water and detrital carbonate input being absent or insignificant. We present results from Lake Neuchâtel (western Switzerland) that demonstrate equilibrium precipitation of calcite, except during high productivity periods, and the presence of detrital and resuspended calcite. Mineralogy, geochemistry and stable isotope values of Lake Neuchâtel trap sediments and adjacent rivers suspension were studied. Mineralogy of suspended matter in the major inflowing rivers documents an important contribution of detrital carbonates, predominantly calcite with minor amounts of dolomite and ankerite. Using mineralogical data, the quantity of allochthonous calcite can be estimated by comparing the ratio ankerite + dolomite/calcite + ankerite + dolomite in the inflowing rivers and in the traps. Material taken from sediment traps shows an evolution from practically pure endogenic calcite in summer (10–20% detrital material) to higher percentages of detrital material in winter (up to 20–40%). Reflecting these mineralogical variations, δ 13C and δ 18O values of calcite from sediment traps are more negative in summer than in winter times. Since no significant variations in isotopic composition of lake water were detected over one year, factors controlling oxygen isotopic composition of calcite in sediment traps are the precipitation temperature, and the percentage of resuspended and detrital calcite. Samples taken close to the river inflow generally have higher δ values than the others, confirming detrital influence. SEM and isotopic studies on different size fractions (<2, 2–6, 6–20, 20–60, >60 μm) of winter and summer samples allowed the recognition of resuspension and to separate new endogenic calcite from detrital calcite. Fractions >60 and <2 μm have the highest percentage of detritus. Fractions 2–6 and 6–20 μm are typical for the new endogenic calcite in summer, as given by calculations assuming isotopic equilibrium with local water. In winter such fractions show similar values than in summer, indicating resuspension. Using the isotopic composition of sediment traps material and of different size fractions, as well as the isotopic composition of lake water, the water temperature measurements and mineralogy, we re-evaluated the bulk carbonate potential for palaeoclimatic reconstruction in the presence of detrital and re-suspended calcite. This re-evaluation leads to the following conclusion: (1) the endogenic signal can be amplified by applying a particle-size separation, once the size of endogenic calcite is known from SEM study; (2) resuspended calcite does not alter the endogenic signal, but it lowers the time resolution; (3) detrital input decreases at increasing distances from the source, and it modifies the isotopic signal only when very abundant; (4) influence of detrital calcite on bulk sediment isotopic composition can be calculated.

Lead isotopes as tracers of lead pollution from various sources: an example from northern Sweden

Abstract The surface sediments in the central Bothnian Bay are contaminated with anthropogenic Pb. Preindustrial concentrations range from 9 to 43 ppm, but the surface sediments contain up to 158 ppm Pb. Possible sources of the Pb pollution are anthropogenic atmospheric Pb, where the main Pb source is combustion of gasoline, and ore from the Laisvall mine which is processed in the smelter Ronnskars-verken on the coast of Bothnian Bay. Because the Pb isotope composition of these possible sources are known, an attempt was made to trace the origin of the anthropogenic Pb in Bothnian Bay by studying the Pb isotope composition of sediments. The results show unequivocally that Laisvall Pb is not the source of the pollution. The enrichment of Pb in surface sediments is caused by atmospheric deposition of Pb additives from gasoline. Lead emitted from Ronnskarsverken is deposited relatively close to the smelter, and has not reached central Bothnian Bay in traceable amounts.

Simulation of the Diagenesis of Carbon, Sulfur, and Dissolved Oxygen in Salt Marsh Sediments

A steady—state numerical model has been developed for simulating vertical profiles of the concentrations of organic matter, pyritic sulfur, dissolved oxygen, and the carbon isotope composition of organic matter in marsh sediments.In the model organic matter enters the sediment via sedimentation, belowground production of roots, and chemoautotrophic fixation of interstitial CO2 associated with pyrite oxidation. Pyrite is formed by sulfate reduction and consumed by oxidation with dissolved oxygen in the interstitial water. Exchanges of organic matter, carbon isotopes, pyrite, and dissolved oxygen between the sediment and surface environment occur via fiddler crab bioturbation. Aeration of the sediment is caused by diffusion of oxygen into the interstitial water from air cavities assumed to be present in roots and in desaturated sediment pores formed in the upper part of the sediment by drainage and/or evapotranspiration. Sensitivity experiments with the model suggest that the accumulation of pyrite and organic matter in marsh sediments is governed in large part by the turnover time of roots and by the mean diameter of roots. The isotopic composition of the sediment was most sensitive to the rate of belowground production and to a lesser extent to the intensity of fiddler crab bioturbation. The model also indicated that fiddler crab burrowing can account for the observed isotope composition of creekbank sediments but not back (mid) marsh sediment. In both back and creekbank marshes, intense aeration of the sediment by roots is required to prevent the buildup of pyrite to unrealistically high concentrations at depth.

Elemental and isotopic variations in subduction related basalts: evidence for a three component model

Subduction related basalts display wide ranges in large ion lithophile element ratios (e.g., Rb/Ba and Rb/ Sr) which are unlikely to result from mixing, but suggest a role for small degree partial melting of a relatively Rb-poor mantle wedge source. However, these variations do not correlate with other trace element criteria, such as the depletions of high field strength elements (HFSE) and light rare earth elements (LREE) relative to the LILE, which characterise subduction related magmatism. Integration of radiogenic isotope and trace element data demonstrates that the elemental enrichment cannot be simply related to two component mixtures inferred from isotopic variations. Thus a minimum of three components is required to describe the geochemistry of subduction zone basalts. Two are subduction related: high Sr/Nd material is derived from the dehydration of subducted basaltic ocean crust, and a low Sr/Nd component is thought to be from subducted terrigenous sediment. The third component is in the mantle wedge, it is usually similar to the source of MORB, particularly in its isotopic composition. However, in some cases, notably continental areas, more enriched mantle wedge material with relatively high 87Sr/86Sr, low 143Nd/144Nd and elevated incompatible trace element contents may be involved Mixing of these three components is capable of producing both the entire range of Sr, Nd and Pb isotope signatures observed in destructive margin basalts, and their distinctive trace element compositions. The isotope differences between Atlantic and Pacific island arc basalts are attributed to the isotope compositions of sediments in the two oceans.

Sediment subduction and magma genesis in the Lesser Antilles: Isotopic and trace element constraints

We present analyses of Pb, Sr, and Nd isotope ratios and of K, Rb, Cs, Sr, Ba, U, Th, Pb, and rare earth elements (REE) of a suite of volcanic rocks from the Lesser Antilles island arc, including representatives from all the major islands. An unusually wide range of compositions is present. 87Sr/86Sr and 143Nd/144Nd overlap extensively with those of oceanic islands and indicate slightly more “enriched” source signatures than most island arcs. Pb isotope ratios range to extremely radiogenic values (206pb/204pb = 19.75, 207pb/204pb = 15.83) and extend significantly beyond the oceanic basalt field. REE patterns range from slightly light‐rare‐earth depleted to strongly light‐rare‐earth enriched; several exhibit negative Ce anomalies. The Pb isotopic compositions are more radiogenic than those of most marine sediments but are similar to isotope compositions of sediments in front of the arc. This, and other evidence, such as high Cs/Rb ratios and a southward increase in Pb isotope ratios in both arc volcanics and sediments in front of the arc, is regarded as strong evidence for a sediment component in arc magmas. Metasedimentary xenoliths in arc magmas and other evidence indicate that sediment can be a high‐level contaminant of ascending magmas. However, the ubiquitous nature of the “sedimentary” isotopic and trace element signature in arc magmas, including primitive and peridotite‐xenolith‐bearing ones, indicates subducted sediment must also be a component of arc magma sources. Mixing calculations show the isotope geochemistry of most arc magmas is readily explained if their sources are mixtures of depleted mantle and 3% or less subducted sediment. On average, subduction of 90 m of sediment would be required to explain the isotopic compositions of arc magmas. Simple binary mixing models also explain the REE patterns of low‐K basalts but fail to explain all the excess enrichment of the alkalis and alkaline earths relative to the REE. The excess abundance of these elements is probably due to their enrichment (relative to the REE) in an aqueous fluid generated by dehydration of the subducting sediment and basalt. This fluid mixes with (metasomatizes) overlying depleted mantle, which melts to produce the arc magmas.

Reconstructing Pleistocene Climatic Change from the Oxygen Isotope Composition of Sediments: A Review

The isotope composition of ocean sediments is the chief data source for Pleistocene climatic changes. It is generally believed that the 18O/16O ratio of a sample indicates the global total of glacial ice at the time the sample was deposited. This is roughly correct, but numerous complicating factors limit the accuracy of the isotope proxies.

Reconstructing Pleistocene Climatic Change from the Oxygen Isotope Composition of Sediments: A Review

The isotope composition of ocean sediments is the chief data source for Pleistocene climatic changes. It is generally believed that the18O/16O ratio of a sample indicates the global total of glacial ice at the time the sample was deposited. This is roughly correct, but numerous complicating factors limit the accuracy of the isotope proxies.