Low Isotope Values Research Articles

Carbonate horizons, paleosols, and lake flooding cycles: Beds I and II of Olduvai Gorge, Tanzania

This study documents the petrology and stable isotope geochemistry of carbonates from six horizons from Beds I and II of Olduvai Gorge, Tanzania. The studied succession, immediately below and above Tuff IF, consists of interbedded waxy and earthy claystones with discrete carbonate horizons and thin sandstones. The succession was deposited in response to repeated flooding and withdrawal of a saline-alkaline lake. The carbonates and their overlying disconformities are important because they help define the surfaces on which hominin activity took place and allow very high-resolution correlation of geographically separated levels of hominin exploitation.The range of different carbonates includes unambiguous land-surface and pedogenic features including calcified rootmat horizons, rhizocretions, and micritic nodules, together with less determinate sparry calcite nodules. Stellate nodules are interpreted as pseudomorphs after sulfate-roses. The carbonate nodules are synsedimentary features, truncated by fluvial and other erosional surfaces. The isotopic composition of the carbonates is variable with δ18O ranging from −7.0‰ to −4.3‰, and δ13C from −8.5‰ to −1.6‰. A covariant increase in δ13C and δ18O repeats in each carbonate horizon and in individual nodules (inner to outer layers): it reflects the evolution of synsedimentary groundwaters. At times of low lake level, the carbonates started to precipitate from meteoric waters with low isotopic values and continued to form as lake levels rose and the waters became increasingly saline. Some of the samples have a last-stage cement of strontium rich dolomite, which supports late-stage flooding by the saline-alkaline lake. Previous studies of carbonate horizons from Olduvai have interpreted carbon isotope values in terms of changes in C3 and C4 plants that colonized the land surface. This study demonstrates that in some instances the isotope values from carbonates deposited in these lake marginal settings reflect changes in hydrology rather than vegetation.

Tree-ring δ15N values to infer air quality changes at regional scale

In this study, tree-ring concentrations and stable isotope ratios of nitrogen in red spruce trees growing near Québec City (Canada) were used as records of environmental changes for the period between 1880–81 and 2007. On one hand, the trends in nitrogen concentrations of red spruce trees are strongly influenced by the heartwood–sapwood transition zone. Such physiologically-influenced pattern suggests that tree-ring nitrogen concentrations are not representative of soil or air nitrogen variations. On the other hand, the δ15N trends of the investigated trees do not show any sign of influence from the heartwood-sapwood boundary, meaning that these isotopic values can be representative of environmental conditions. The interannual variations of the δ15N values were correlated with the total annual precipitation and the measured amounts of regional nitrogen oxide emissions for the 1990–2007 period. The long-term δ15N values show a decrease of 1.8‰ between 1956–57 and 2007. This decreasing trend is statistically linked to the increasing number of registered cars in the Quebec Province, which produce N emissions characterized by low isotopic values. We suggest that this long-term trend could have been caused by a change to a dominant source of N emissions with low δ15N values, and/or by the microbiological processes, such as the release of N from mycorrhizas under N-limited environment. Importantly, we demonstrate that this clear isotopic pattern is matching that of other species (pine and beech trees) growing under different ecological conditions, at 260km from our study site. Therefore, we suggest that nitrogen oxides emitted by car exhausts, themselves linked to the amount of transport-related N deposition at the provincial scale, could have caused the observed trends and have participated to the acidification of soil. This investigation allows suggesting that the δ15N values of trees have the potential to reflect perturbations of the soil and air nitrogen dynamics caused by anthropogenic N emissions at a large scale.

Eclipse Ice Core Accumulation and Stable Isotope Variability as an Indicator of North Pacific Climate

Abstract The high accumulation rate and negligible amount of melt at Eclipse Icefield (3017 m) in the Saint Elias Range of Yukon, Canada, allows for the preservation of a high-resolution isotopic and glaciochemical records valuable for reconstruction of climatic variables. Each of the three Eclipse ice cores have a well-constrained depth–age scale with dozens of reference horizons over the twentieth century that permits an exceptional level of confidence in the results of the current calibration exercise. Stacked time series of accumulation and stable isotopes were divided into cold and warm seasons and seasons of extreme high and extreme low accumulation and stable isotope values (eight groups). For each group, season-averaged composites of 500-hPa geopotential height grids, and the individual seasons that constitute them, were analyzed to elucidate common anomalous flow patterns. This analysis shows that the most fractionated isotopes and lowest accumulation cold seasons reflect a more zonal height pattern in the North Pacific associated with negative Pacific–North American (PNA) and Pacific decadal oscillation (PDO) indices. Conversely, the least fractionated isotopes and highest accumulation cold seasons are associated with a positive PNA pattern. Although only a maximum of approximately 20% of the total number of accumulation and stable isotope seasons exhibit a relatively consistent relationship with 500-hPa geopotential height patterns, these results support the hypothesis that the most extreme accumulation and extreme isotope cold-season values in the Saint Elias Mountains are related to consistent atmospheric circulation and oceanic sea surface temperature patterns.

Multi-isotope analysis of the population of the lost medieval village of Auldhame, East Lothian, Scotland

This study is one of only a handful to combine strontium, oxygen, carbon, nitrogen and sulphur isotope data for medieval human remains, in this case from individuals buried in a cemetery in the remote Scottish coastal village of Auldhame, which was abandoned in the 17th century AD. The strontium and oxygen isotope analysis of tooth enamel suggests that the group predominantly comprised a local, static population and thus this allows the examination of the dietary habits of a remote coastal community. The combination of relatively high nitrogen isotope values with relatively low carbon isotope values within bone collagen suggests little marine protein in the diet, which is unusual given the coastal location. The community may have been consuming some freshwater fish or omnivores (pigs fed on animal diets), but also we suggest that the combination of isotope values could be explained by soil improvement methods. Some evidence for soil deepening at the site, and by association manuring, suggests that the consumption of cereals was important to the diet, and may explain the high nitrogen values found in combination with terrestrial carbon isotope values. This combination of dietary isotopes has previously been suggested to be unusual for the medieval period, but we propose it is perhaps more common than originally conceived. As there are few previous multi-isotope studies from Scottish medieval assemblages on this scale, the study provides an opportunity to construct a picture of medieval and early post-medieval life in rural Scotland.

Bitumen biomarkers in the Mid-Proterozoic Ilímaussaq intrusion, Southwest Greenland – A challenge to the mantle gas theory

GC and GC/MS/MS analysis on rock extracts has shown that the bitumen in the peralkaline Ilímaussaq intrusion, previously assumed to be abiogenic, is biotic in origin. A biotic origin is in accordance with previously published stable carbon isotopic data on bituminous matter in the rocks. The biomarker distribution in the bitumen, including the less common bicadinanes, resembles that of oil seeps on the central West Greenland coast 2200 km farther north, whose source rocks and migration history are relatively well established. We use a recent re-construction of the subsidence and later exhumation of the West Greenland coastal region during the Mesozoic-Cenozoic ( Japsen et al., 2006a, b) to anticipate that hydrocarbons migrated from deeper parts of the basin offshore west of Greenland. The rocks of Ilímaussaq were probably more deeply invaded than the surrounding granites due to their higher proportion of corroded minerals, which may explain why bitumen has not been observed elsewhere in the area. Hydrocarbon gases (C1–C5) present in fluid inclusions were also analysed, after having been released by treatment with hydrochloric acid that resulted in an almost complete disintegration of the Ilímaussaq intrusion rocks. The acid extraction method proved generally more efficient than the crushing procedure applied by others, but gave similar results for the chemical composition of the gas (CH 4: 88–97%) and isotopic ratios (δ 13 C CH 4 : −1.6 to −5.0‰; δ 13 C C 2 H 6 : −9.2 to −12.5‰), with the exception of hydrocarbons hosted in quartz, which showed significantly lower isotopic values for methane ( Graser et al., 2008). Previous researchers have suggested an abiotic origin for these hydrocarbon gases, but we suggest a biotic origin for the majority of them, not just those in quartz, assuming that the isotopic ratio of the constituents have changed due to loss of gas by diffusion. The assumption of gas loss via diffusion is supported by published studies on micro-fissures in minerals typical of the Ilímaussaq and field investigations showing diffusive loss of gas from the peralkaline Khibina and Lovozero massifs on the Kola Peninsula, Russia, which are, in many respects of mineralogy and hydrocarbon content, similar to the Ilímaussaq intrusion. Both the hydrocarbon gases and bitumen in the Khibina and Lovozero massifs have been cited as prime examples of a deep mantle source, although the carbon isotopic ratio of the bitumen clearly pointed to an organic origin. The trends in carbon isotopic ratio of methane released with time from freshly exposed rocks also supports our hypothesis of 13C enrichment of the methane remaining within the rock. Thus, there is good evidence that the hydrocarbons in the Kola alkaline massifs are mostly biotic in origin, in which case the probability of finding economic hydrocarbon accumulations from a deep mantle source seems exceedingly small.

Speleothems in the dry Cave Parts of the Gamslöcher-Kolowrat Cave, Untersberg near Salzburg (Austria)

New, remarkably dry parts of the Gamslöcher-Kolowrat Cave at 728 to 853 m depth have been explored in the Untersberg near Salzburg in Austria. This region is called the Desert, its greatest cavity is called the White Hall. The new cave part is characterized by various white speleothems. The predominant ones are snow-like calcite powder with an extremely low density, and fine gypsum needles. Gypsum also occurs in the form of balls stuck to vertical walls. In addition, fluorescent hydromagnesite crusts, Mg5(CO3)4(OH)2·4H2O, as well as the sodium sulfate mineral mirabilite were identified in this part of the cave. Mirabilite and gypsum needles differ from the gypsum balls in their isotopic sulfur signature (δ34S of -16.9‰ and -18.4‰ vs. +2.9‰). The unusually low sulfur isotopic compositional values are tentatively explained by a source of bacteriogenetic sulfur from sulfides.

Seasonal variations in planktonic foraminiferal flux and the chemical properties of their shells in the southern South China Sea

Results from sediment trap experiments conducted in the southern South China Sea from May 2004 to March 2006 revealed significant monsoon-induced seasonal variations in flux and shell geochemistry of planktonic foraminifera. The total and species-specific fluxes showed bimodal pattern, such as those of Globigerinoides ruber, Globigerinoides sacculifer, Neogloboquadrina dutertrei, Globigerinita glutinata, and Globigerina bulloides. Their high values occurred in the prevailing periods of the northeast and southwest monsoons, and the low ones appeared between the monsoons. Pulleniatina obliquiloculata had high flux rates mainly during northeast monsoon, with exceptional appearance in August 2004. These fluxes changed largely in accord with those of total particle matter and organic carbon, following chlorophyll concentration and wind force. It is inferred that the biogenic particle fluxes are controlled essentially by primary productivity under the influence of East Asian monsoon in the southern SCS. Shell stable oxygen isotope and Mg/Ca data correspond with seasonal variation of sea surface temperature. Shell δ 18O values are affected primarily by sea water temperature, and the δ 18O changes of different-depth dwelling species indicate upper sea water temperature gradient. Besides, the low carbon isotope values occurred in the periods of East Asian monsoon in general, whereas the high ones between the monsoons. The pattern is in contrary to chlorophyll concentration change, which indicates that the variation of the carbon isotope could probably reflect the change of sea surface productivity.

Origin of anorogenic ‘lamproite-like’ potassic lavas from the Denizli region in Western Anatolia Extensional Province, Turkey

The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low 87Sr/86Sr, 0.703523–0.703757; high 143Nd/144Nd, 0.512708–0.512784; high 206Pb/204Pb, 19.079–19.227, 207Pb/204Pb, 15.635–15.682, 208Pb/204Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. LaN = 319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling.

Formation of secondary carbonates and native sulphur in sulphate-rich Messinian strata, Sicily

Abstract Microbially formed authigenic carbonates accompanied by native sulphur are present in the ‘Calcare Solfifero’ below a thick succession of gypsum deposited during the Messinian salinity crisis in Sicily. We sampled these carbonates and associated sulphur in five former sulphur mines to subject them to a detailed petrographic and geochemical study in order to explore their different modes of formation. Native sulphur formed in conjunction with microbial sulphate reduction, which is reflected in its depletion in 34S (δ34S values as low as − 2‰ vs. V-CDT) and an enrichment of 34S in the residual sulphate (δ34S values as high as + 61‰). The oxidation of organic matter by sulphate reduction increased alkalinity, inducing precipitation of secondary carbonate minerals. A set of authigenic limestones lacking sulphate minerals, but characterized by pseudomorphs after gypsum and high δ18O values (as high as + 9‰ vs. V-PDB) reflects syngenetic mineral formation within evaporitic settings. Low δ13C values (as low as − 52‰ vs. V-PDB) reveal that these carbonate phases were formed by microbial sulphate reduction coupled to the oxidation of biogenic methane. Another set of authigenic carbonates that replaced sulphate minerals is typified by low δ18O values (as low as − 4‰). These carbonates formed epigenetically during later diagenesis following compaction. Dissolution of gypsum or anhydrite by meteoric waters delivered the sulphate for microbial sulphate reduction. Low carbon isotope values of these carbonates (− 29 to − 5‰) indicate that carbonate was derived from the oxidation of crude oil and possibly minor methane, partly involving different degrees of admixture of dissolved carbonate from other sources. Although the studied rocks with their vast amounts of secondary carbonate minerals and sulphur seem to indicate a similar genesis at first glance – having formed by biogeochemical transformations of sulphate and hydrocarbons – this study reveals that these processes can occur at different times in variable geological environments.

High Contribution of Recalcitrant Organic Matter to DOC in a Japanese Oligotrophic Lake Revealed by 14C Measurements

Carbon isotopes (14C and 13C) of dissolved organic carbon (DOC) in a Japanese oligotrophic lake (Lake Towada) were measured to study the origin and cycling of dissolved organic matter (DOM) in Lake Towada. Lake water samples were collected at 3 depths (0, 30, and 80 or 85 m) during 4 months (April, June, August, and October) in 2006. 14C measurements of DOC were performed by accelerator mass spectrometry (AMS) at the National Institute for Environmental Studies (NIES-TERRA) in Japan. Δ14C and δ13C values of DOC in Lake Towada showed light carbon isotopic values ranging from –750 to −514‰ and –29.0 to –27.8‰, respectively. These values are similar to those of humic substances reported. The very low carbon isotopic values of DOC in Lake Towada suggest a very small contribution of DOC derived from fresh phytoplankton to the lake DOC. There is an extremely high linear relationship between the Δ14C and δ13C of DOC in Lake Towada when all data points are plotted (r2 =0.818, p < 0.01), suggesting that the DOC in Lake Towada has 2 specific sources contributing heavy and light carbon isotopes. Although the freshly produced DOC of phytoplankton origin can be decomposed easily, the variation in the autochthonous DOC should influence the carbon isotopic values of DOC in Lake Towada.

Sulphur isotope evidence for freshwater fish consumption: a case study from the Danube Gorges, SE Europe

To explore the use of sulphur isotopes as an indicator of the consumption of freshwater fish, we undertook sulphur isotope analysis on bone collagen extracted from humans and animals from five archaeological sites from the Danube Gorges region dating from the Mesolithic to the middle Neolithic periods. The results show a difference in the sulphur isotope values between freshwater and terrestrial ecosystems of 8.7‰. To reconstruct human diets, bone collagen from 24 individuals was analysed for carbon, nitrogen and sulphur isotopic values. The nitrogen isotope ratios ranged from 10.3 to 16.5‰ and the carbon isotope ratios ranged from −20.8 to −18.3‰. Low nitrogen isotope values were found for individuals with low sulphur isotope ratios reflecting the low sulphur isotopic values of the terrestrial animals. The highest human nitrogen isotope values coincided with higher sulphur isotope ratios, which are related to the higher sulphur isotope values of the freshwater fish. Intermediate human sulphur isotopic ratios between these two extremes showed mixed diets of both terrestrial and freshwater resources.

Radiocarbon measurements of dissolved organic carbon in sewage-treatment-plant effluent and domestic sewage

Abstract In an attempt to better characterize dissolved organic carbon (DOC) in several specific sources to Lake Kasumigaura, such as sewage-treatment-plant effluent (STPE), domestic sewage (DS) and forest stream (FS), we analyzed radiocarbon (14C) and stable carbon isotopic compositions (13C) of the DOCs. The measurements of 14C for DOC were performed by an accelerator mass spectrometer (AMS) at the National Institute for Environmental Studies (NIES-TERRA) in Japan. The Δ14C and δ13C values of the DOCs in several sources to Lake Kasumigaura, have low carbon isotopic values, ranging from −470‰ to −79‰ and from −27.9‰ to −24.2‰, respectively. These carbon isotopic values are substantially different from those of Lake Kasumigaura. These results imply different origins for the DOC in Lake Kasumigaura. The 14C and 13C analyses of DOC led to a useful classification for DOCs in Lake Kasumigaura, Japan.

Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China

Abstract Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ13C values are found in the Tarim and Qaidam basins (δ13C1: − 19.0 to − 29.9‰; δ13C2: − 18.8 to − 27.1‰), and those with lowest δ13C values occur in the Turpan-Harmi and Junggar basins (δ13C1: − 40.1 to − 44.0‰; δ13C2: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ13C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world.

The significance and lag-time of deep through flow: an example from a small, ephemeral catchment with contrasting soil types in the Adelaide Hills, South Australia

Abstract. The importance of deep soil-regolith through flow in a small (3.4 km2) ephemeral catchment in the Adelaide Hills of South Australia was investigated by detailed hydrochemical analysis of soil water and stream flow during autumn and early winter rains. In this Mediterranean climate with strong summer moisture deficits, several significant rainfalls are required to generate soil through flow and stream flow [in ephemeral streams]. During autumn 2007, a large (127 mm) drought-breaking rain occurred in April followed by significant May rains; most of this April and May precipitation occurred prior to the initiation of stream flow in late May. These early events, especially the 127 mm April event, had low stable water isotope values compared with later rains during June and July and average winter precipitation. Thus, this large early autumn rain event with low isotopic values (δ18O, δD) provided an excellent natural tracer. During later June and July rainfall events, daily stream and soil water samples were collected and analysed. Results from major and trace elements, water isotopes (δ18O, δD), and dissolved organic carbon analysis clearly demonstrate that a large component of this early April and May rain was stored and later pushed out of deep soil and regolith zones. This pre-event water was identified in the stream as well as identified in deep soil horizons due to its different isotopic signature which contrasted sharply with the June–July event water. Based on this data, the soil-regolith hydrologic system for this catchment has been re-thought. The catchment area consists of about 60% sandy and 40% clayey soils. Regolith flow in the sandy soil system and not the clayey soil system is now thought to dominate the deep subsurface flow in this catchment. The clayey texture contrast soils had rapid response to rain events and saturation excess overland flow. The sandy soils had delayed soil through flow and infiltration excess overland flow. A pulse of macropore through flow was observed in the sandy soils three days after the rainfall event largely ended. The macropore water was a mixture of pre-event and event water, demonstrating the lag-time and mixing of the water masses in the sandy soil system. By contrast, the clayey soil horizons were not dominated by pre-event water, demonstrating the quicker response and shallow through flow of the clayey soil system. Thus, the sandy terrain has a greater vadose zone storage and greater lag time of through flow than the clayey terrain.

Carbon and Oxygen Isotopic Composition of Surface‐Sediment Carbonate in Bosten Lake (Xinjiang, China) and its Controlling Factors

Abstract:Bosten Lake is a mid‐latitude lake with water mainly supplied by melting ice and snow in the Tianshan Mountains. The depositional environment of the lake is spatially not uniform due to the proximity of the major inlet and the single outlet in the western part of the lake. The analytical results show that the carbon and oxygen isotopic composition of recent lake sediments is related to this specific lacustrine depositional environment and to the resulting carbonate mineralogy. In the southwestern lake region between the Kaidu River inlet and the Kongqi River outlet, carbon isotope composition (δ13C) values of the carbonate sediment (−1‰ to −2‰) have no relation to the oxygen isotope composition of the carbonate (δ18O) values (−7‰ to –8‰), with both isotopes showing a low variability. The carbonate content is low (<20%). Carbonate minerals analyzed by X‐ray diffraction are mainly composed of calcite, while aragonite was not recorded. The salinity of the lake water is low in the estuary region as a result of the Kaidu River inflow. In comparison, the carbon and oxygen isotope values are higher in the middle and eastern parts of the lake, with δ13C values between approximately +0.5‰ and +3‰, and δ18O values between −1‰ and −5‰. There is a moderate correlation between the stable oxygen and carbon isotopes, with a coefficient of correlation r of approximately 0.63. This implies that the lake water has a relatively short residence time. Carbonate minerals constitute calcite and aragonite in the middle and eastern region of the lake. Aragonite and Mg–calcite are formed at higher lake water salinity and temperatures, and larger evaporation effects. More saline lake water in the middle and eastern region of the lake and the enhanced isotopic equilibrium between water and atmospheric CO2 cause the correlating carbon and oxygen isotope values determined for aragonite and Mg–calcite. Evaporation and biological processes are the main reasons for the salinity and carbonate mineralogy influence of the surface‐sediment carbonate in Bosten Lake. The lake water residence time and the CO2 exchange between the atmosphere and the water body control the carbon and oxygen isotope composition of the carbonate sediment. In addition, organic matter pollution and decomposition result in the abnormally low carbon isotope values of the lake surface‐sediment carbonate.

Eclogite xenoliths from Kimberley, South Africa — A case study of mantle metasomatism in eclogites

Eclogite xenoliths from the Kimberley diamond mine dumps were studied for major, trace elements and oxygen isotopes. The suite consists of typical garnet and clinopyroxene rocks as well as kyanite-eclogites and olivine- or orthopyroxene-bearing samples. All samples have Al[6]/Al[4] ratios > 2 which indicate high equilibration pressures, i.e. in the eclogite stability field as opposed to pyroxenites. Most samples are modally and cryptically metasomatized and contain up to 10 vol.% phlogopite plus ilmenite and rutile. Trace element geochemical characteristics of phlogopite and Ti-oxides are similar to those described for metasomatized peridotite xenoliths from the Kaapvaal craton, and show that the eclogite xenoliths experienced the same metasomatic event. Oxygen isotopic values of the phlogopites plot with those for pristine mantle, and are taken as evidence that the metasomatic agent did not have an exotic oxygen isotopic composition. In contrast, the eclogitic garnets are significantly lighter in oxygen isotopes than pristine mantle. These values cannot have been caused by the metasomatism that produced phlogopite, but are a preserved feature of the eclogite protoliths. The occurrence of kyanite is a strong indication for a low pressure origin of the protoliths, This, together with low oxygen isotopic values, a range of Mg-numbers of 59.5 to 88.8 and overall high trace element abundances compared to other eclogite xenolith suites, is taken as evidence that the protoliths of the Kimberley eclogites were seawater-altered oceanic cumulates which were metamorphosed in eclogite facies conditions and later overprinted by a mantle-derived metasomatic event.

Isotopic features of rivers and groundwater of the Parma Province (Northern Italy) and their relationships with precipitation

The Enza, Parma, Baganza, Taro and Po rivers and several waters from aquifers of the area of the Parma town (northern Italy) have been investigated for oxygen and hydrogen isotopes. Enza, Parma, Baganza and Taro differ from Po in having higher isotope ratios and the Taro River exhibits an isotope ratio higher than those of Enza, Parma and Baganza. The low values of Po are due to the dominant contribution of waters coming from glaciers and precipitation occurring along the Alps. The high values of the Taro River are probably due to the fact that its catchment basin extends close to the Tyrrhenian sea and is well opened to perturbations coming from the west. The isotopic values of the present day precipitations occurring along the northern Apennine piedmont area are lower than in rivers. On the contrary, the rivers resemble waters issuing from springs located close the Apennine ridge; the isotopic values of the last waters probably approach those in the precipitation occurring at present along the Apennine ridge. Reasonably, the low isotope values recorded in precipitation of the Apennines piedmont area are due to cold air masses invading the Po plain from N or NE rather than only to the change in the regime of precipitation (higher amount in cold periods, lower amount in the hot periods). Waters of the aquifer of the Po plain have been investigated in the Priorato, Parola and Parma fields (Parma Province). The aquifers are largely fed by the surface hydrographic system: their waters have constant isotopic composition and resemble waters of the Apennines rivers. They are also isotopically similar to the average precipitation occurring from 1995 to 2006, but differ largely from recent (2003–2005) precipitation. Minor isotopic variations recorded in a well of the Priorato field suggest that this well is partly fed by local precipitation.

Recent trends in accumulation rate, elemental and isotopic composition of organic matter in the Magu bay, Lake Victoria

Stable isotope composition of organic carbon (OC), 14C data, OC and nitrogen content of recently deposited sediments in Magu Bay of Speke Gulf, Lake Victoria are used to document accumulation rates, sources and distribution of organic matter in the bay. The stable isotope values of both organic carbon and nitrogen decrease towards offshore, and this trend is associated with increase in the contents of organic carbon and nitrogen. The isotope results show that the organic material deposited in the bay are generally derived from C4 type of plants and are not transported far from the river mouth. Also the results show that preservation of organic matter is high in open water probably due to high primary productivity owing to high concentration of nutrients and low in turbidity. Low isotope values in offshore sites indicate that the available stock of nutrients far exceeds the demand. Sedimentation rates in the bay are not high and range from 5 to 54 cm/ka. This could be attributed to trapping efficiency by shoreline aquatic macrophyte. Furthermore, the mean accumulation rates of organic carbon and nitrogen for the bay, range from 6.92 to 57.25 gC/m2/yr and 0.51 to 4.37 gN/m2/yr, respectively, and show that Magu Bay is currently experiencing eutrophication. Tanzania Journal of Science Vol. 31 (1) 2005: pp. 23-34

Nonsulfide zinc deposits in the Silesia–Cracow district, Southern Poland

The Silesia–Cracow district in Poland has been one of the world’s principal sources of zinc from nonsulfide zinc ore (Polish: galman). The still remaining nonsulfide ore resources can be estimated at 57 Mt at 5.6% Zn and 1.4% Pb. Nonsulfide mineralization is mainly hosted by Lower Muschelkalk (Triassic) limestone and is associated with different generations of the hydrothermal ore-bearing dolomite (OBD I, II, III). A fundamental ore control is believed to have been exerted by the basement faults, which were repeatedly reactivated during the Alpine tectonic cycle, leading to the formation of horst-and-graben structures: these dislocations may have caused short periods of emersion and the circulation of meteoric waters during the Cenozoic. Nonsulfide ores show a wide range of morphological characteristics and textures. They occur as earthy masses, crystalline aggregates, and concretions in cavities. Breccia and replacement textures are also very common. The most important mineral phases are: smithsonite, Fe–smithsonite, Zn–dolomite, goethite, and Fe–Mn(hydr)oxides. Minor hemimorphite and hydrozincite have also been detected. Two distinct nonsulfide ore types occur: the predominant red galman and the rare white galman. In the white galman, Fe–smithsonite and Zn–dolomite are particularly abundant. This ore type is commonly considered as a peripheral hydrothermal alteration product related to the same fluids that precipitated both the OBD II–III and the sulfides. In contrast, a supergene origin is commonly assumed for the red galman. Evidence of the petrographic and mineralogical difference between white and red galman is also found in stable isotope data. Smithsonite from red galman shows a limited range of δ13CVPDB values (−10.1 to −11.4‰), and δ18OVSMOW values (25.3‰ to 28.5‰, mean 26.8 ± 0.3‰). The uniform and low carbon isotope values of red galman smithsonite are unusual for supergene carbonate-hosted deposits and indicate the predominance of a single organic carbon source. Smithsonite from white galman has a more variable, slightly more positive carbon isotope (−2.9‰ to −7.4‰), but broadly similar oxygen isotope composition (26.8‰ to 28.9‰). The relationship of the white galman ore with the hydrothermal system responsible for OBD II and sulfide generation is still uncertain. The most important paleoweathering events took place in both Lower and Upper Silesia during Late Cretaceous up to Paleogene and early Neogene time. During this period, several short-lasting emersions and intense weathering episodes facilitated the formation of sinkholes in the Triassic carbonate rocks and the oxidation of sulfide orebodies through percolating meteoric waters. These phenomena may have lasted until the Middle Miocene.

Paleo- to Mesoarchean basement recycling and terrane definition in the Northeastern Superior Province, Québec, Canada

The Northeastern Superior Province (NESP) is a 500 km × 700 km segment of plutonic Archean crust generated by 1.2 Ga of tectonomagmatic activity. The NESP exhibits the classical plutonic series of Archean terranes, low-K tonalite–trondhjemite–granodiorite (TTG) and high-K granodiorite–granite–monzogranite (GGM) with unusually large volumes of Px-TTGs (mostly enderbites and subordinate charnockites). Volcanic rocks constitute <10% of the NESP and are dominated by magnesian basalt-komatiite assemblages and bimodal mafic tholeiitic and calc-alkaline felsic tuffs. Mapping, U/Pb geochronological, and Nd isotopic data suggest the presence of two terranes with distinct magmatic histories: the Hudson Bay to the SW, and the Rivière Arnaud to the NE. These terranes are separated by a metasedimentary basin, which may be a paleosuture. The Hudson Bay terrane is a Paleo- to Mesoarchean TTG-greenstone craton (3.8–2.9 Ga), intruded by 2.76–2.74 Ga low-K TTGs and younger 2.72–2.69 Ga high-K GGM and Px-TTGs that have low Nd isotopic values (ɛNd T = +0.45 to −5.03). Mafic to felsic volcanic rocks and mafic to ultramafic plutonic rocks <2.76 Ga also have enriched ɛNd T = +0.40 to −6.90. These radiogenic signatures (as well as inherited zircon cores) record recycling of Mesoarchean (3.8–2.9 Ga) crust, either through direct melting, or by coupled assimilation-fractional crystallization processes. In comparison, the Rivière Arnaud terrane is underlain by younger (<3.0 Ga), more juvenile TTG crust, that is intruded by plutons of low-K TTGs (2.79–2.76 Ga), high-K GGM (2.73–2.72 Ga), and Px-TTGs (2.74–2.72 Ga). Most of these rocks have positive to moderately negative ɛNd T values (+2.34 to −0.67), indicating either a juvenile character, or recycling of fairly juvenile Late Mesoarchean-Early Neoarchean crust. Overall, geochemical and isotopic data of the Late Neoarchean (<2.76 Ga) low-K TTG and Px-TTG series of the NE Superior Province are not consistent with partial melting solely of juvenile basaltic crust in an oceanic setting. The data are most consistent with remelting of a composite crust constituted of older, isotopically enriched mafic–ultramafic rocks and more juvenile metabasaltic rocks. The basaltic component may have been introduced either by crustal underplating, intra-plating or foundering of supracrustal belts. The mafic melts injected into the maturing cratonic nuclei assimilated the older rocks by an AFC-type process and provided heat. The isotopic and geochemical data imply that the high-K GGM series melts were produced by melting at shallower depth of an older composite TTG crust, with less involvement of the metabasaltic component.