ALBERT

Beschreibung: We describe a new compilation of radiocarbon age measurements performed by the NERC Radiocarbon Laboratory that is freely available to access over the World Wide Web. The database contains 1000 14C measurements performed using the liquid scintillation counting method between 1996 and 2005, and further results will be added as the information is compiled. Contextual information including sampling location and the nature of sample material is provided, alongside 14C age results and publications codes. Hypertext links provide access to the original 14C age report associated with the samples, providing additional details. The 14C measurements were originally performed for earth and environmental science NERC projects and are therefore likely to be most relevant to the Quaternary research community.

Beschreibung: A method for collecting an isotopically representative sample of CO2 from an air stream using a zeolite molecular sieve is described. A robust sampling system was designed and developed for use in the field that includes reusable molecular sieve cartridges, a lightweight pump, and a portable infrared gas analyzer (IRGA). The system was tested using international isotopic standards (13C and 14C). Results showed that CO2 could be trapped and recovered for both δ13C and 14C analysis by isotope ratio mass spectrometry (IRMS) and accelerator mass spectrometry (AMS), respectively, without any contamination, fractionation, or memory effect. The system was primarily designed for use in carbon isotope studies of ecosystem respiration, with potential for use in other applications that require CO2 collection from air.

Beschreibung: As part of a study investigating the carbon balance of a blanket bog, we made an assessment of the spatial variation of radiocarbon concentrations in the surface layers of a small area of peatland in the north of England. The peat depth at which bomb-14C content was the highest varied considerably between cores sampled from across the site. At several sampling locations, 14C levels 〉100% Modern were confined to the surface 8 cm, whereas bomb 14C was evident at 1 site, located only meters away, to a depth of at least 12–16 cm. Using the layer where 14C levels first exceeded 100% Modern as a chronological reference layer, we estimated the carbon accumulation rate over the last 50 yr for the surface peat at each site (range ∼20 to ∼125 g C m2 yr-1). Our results show that although carbon accumulation over the last 50 yr was similar across the site, variation in the depth to which bomb 14C was evident implied considerable variation in the vertical peat growth rate.

Beschreibung: The 150-m-thick Kuetsjärvi Sedimentary Formation (KSF) from the Pechenga Greenstone Belt, NW Russia, is one of the key formations in the study of a positive δ13Ccarb excursion occurring globally in the Palaeoproterozoic. The KSF formed in an intracratonic rift setting and is sandwiched between two, 2-km-thick subaerially erupted volcanic units. The KSF was previously interpreted as shallow marine, but new data reported here indicate that it is a non-marine unit deposited on a deeply subaerially weathered surface mantling the underlying volcanic rocks. The lowermost part of the KSF represents an alluvial–fluvial plain, followed by a laterally and vertically variable succession of variegated to mottled fine-grained siliciclastic rocks and ‘red beds‘, dolostones containing stromatolite sheets, hydrothermal travertine deposits and abundant desiccation features (e.g.tepees, surfical silicified crusts and dissolution cavities), including probable pseudomorphed evaporites. Measured S and Corg concentrations for the carbonate and siliciclastic rocks are low. Combined, these features indicate that the carbonate rocks of the KSF accumulated in a shallow lacustrine setting. Major types of carbonate facies were formed by: (1) biologically-induced precipitation; (2) evaporitic removal of CO2 in a closed lake environment; and (3) chemical precipitation from thermal springs. Apparently, none of these carbonate facies was in full isotopic equilibrium with atmospheric CO2. This interpretation shows the importance of taking into account the interplay between global and local depositional factors when interpreting the isotopic signature of the KSF dolostones and its implication for the Palaeoproterozoic carbon isotope excursion

Beschreibung: It is now generally accepted that British Tertiary granites contain crustal and mantle components. Genesis principally by differentiation of crustally contaminated basaltic magmas is widely held and silicic melts with some remarkable trace element similarities were generated within different upper crust along the St Kilda/Skye - Carlingford zone.New whole-rock (and mineral) O isotope data for the southern sector of the province (N Arran, Ailsa Craig, Mourne Mountains, Slieve Gullion, etc) reveal that δ18O lies in the range +5·1 to +9·7‰ for most of the analysed granites, meteoric water-rock interaction having been in general less intensive than at Skye and Mull. Nevertheless, highly 18O-depleted country rocks (with δ18O

Beschreibung: A comprehensive study of the ∼2200-Ma-old Kuetsjärvi Sedimentary Formation (KSF), NW Russia, was undertaken to contribute to our understanding of palaeoenvironments associated with the global perturbation of the carbon cycle between 2330 and 2060 Ma. Closely spaced drill core samples (n=95) were obtained from a 150-m-thick unit deposited in rift-bound fluvial-deltaic and shallow-water lacustrine settings with a short-term invasion of sea water. Apart from a very few de-dolomitised samples, all other carbonate lithologies are represented by Corg-free, S-poor, quartz-rich dolostones, stromatolites and travertines which have high Sr concentrations (51–1069 ppm) and low Mn/Sr ratios (2·9 ± 2·1). The carbonate succession, excluding travertines, shows high δ13C (+7·5 ± 0·6‰, n=95) with a limited variation (+5·8 to +8·9‰). Fluctuating δ18O values (10·8–20·4‰) were overprinted during diagenesis, regional greenschist-grade and later retrograde metamorphism. Several short-term stratigraphic excursions of δ13C were apparently governed by evaporation and CO2 degassing combined with pulses of12C-rich hydrothermal waters precipitating travertines. However, the 13C-rich nature of the dolostones reflects the global isotopic signal, which was modified in a shallow water lacustrine environment by evaporation, enhanced uptake of 12C by cyanobacteria, and pene-contemporaneous oxidation and loss of organic material. The best proxies to δ13C and 87Sr/86 Sr of coeval sea water recorded in the KSF dolostones are likely to be around +5–6‰ and 0·70406, respectively. The study of the KSF has shown that circumspection is necessary when attempting to model the behaviour and evolution of the global C-cycle in Deep Time. Models which purport to explain global oceanic–atmospheric evolution without first adequately accounting for the possibility that many Precambrian carbonate deposits might be non-marine, or at least influenced by non-marine fluids, should be viewed with caution

Beschreibung: There is a general consensus that the global chemistry of ocean water has not changed markedly during the Phanerozoic. Nevertheless, significant changes have occurred in the geochemical cycles of some elements and patterns of change have been reconstructed, in various forms, through consideration of the isotope ratios 13C/12C, 34S/32S, 87Sr/86Sr and 143Nd/144Nd. There have also been attempts to constrain variations in the isotopic composition of sea water itself through measurements of D/H and 18O/16O, the latter both directly and indirectly. Dissolved constituents in seawater display secular changes in isotopic composition as a consequence of quite different driving mechanisms. δ13C and δ34S variations are broadly correlated and linked by carbon and sulphur exogenic cycle interaction through redox reactions (the “free oxygen cycle”). The 87Sr/86Sr trend is determined by the balance among different Sr inputs to the oceanic pool, which vary in their isotopic composition (limestones, old granitic material and young basaltic material). Neodymium isotope variations are not globally synchronous. Changes in 143Nd/144Nd are influenced by local erosion products from continental landmasses and can therefore be different for coexisting palaeocean basins.

Beschreibung: Stable isotope data from the East Kirkton succession are used to elucidate the extent of hot-spring influence in the palaeoenvironment by constraining conditions of deposition of the silica and the formation of sulphides.Petrographically silica occurs as chert laminae thought to be primary, and as patchy chert considered as replacive. No evidence for biogenic silica was observed. For 20 silica samples δ18O was measured for structural oxygen and δD for bound water. δ18O(SMOW) varied between +21 and +27‰ with no sample groupings related to petrography. The range in δD(SMOW) was from −50 to −90‰ with lower values characterising replacive or altered silica; water contents of both petrographic groups were similar. A plot of δ18O versus δD for the laminated primary silica defines a grouping about the line defined by Scottish agates (Fallick et al. 1985). This suggests for the unaltered silica a formation temperature of about 60°C and a fluid containing a strong component of meteoric water. The data imply a Lower Carboniferous meteoric water δ18O composition of −3‰ and δD of −15‰, consistent with the known palaeolatitude.The only sulphide observed was pyrite; 34 samples were selected from a wide variety of lithological and textural occurrences. δ34S(CDT) ranges widely and continuously between +8 and −34‰ with no strong mode. The sulphur appears to be derived from several sources, and pyrite formation from a variety of conditions as indicated by such wide ranging data, but for the samples with the lowest δ34S the involvement of bacteria in sulphate reduction is inferred.

Beschreibung: Stable isotopic ratios (mainly 18O/16O, but also D/H) have been measured for the three most important types of late-Hercynian granites, and their hosts, in the western area of the Central Iberian Massif (CIM), Spain. These granites are amphibole-bearing biotite granites, biotite granites and cordierite-bearing biotite granites. No intrusive relationships have been observed among them; the contact of each granite with the others is always gradational. Host-rocks are Precambrian/Cambrian metasediments, ranging from low-grade schists to migmatites (nebulites).Whole-rock δ18OSMOW values are as follows: amphibole-bearing biotite granites 8·9 ± 0·58% (1σ, n = 17); biotite granites 9·0 ± 0·35% (1σ, n = 11); cordierite-bearing biotite granites 9·6 ± 0·24% (1σ, n = 21). δ18O values for nebulites, into which some of these granites were emplaced, are significantly higher, at 11·1 ± 0·58‰ (1σ, n = 13). The Precambrian to Cambrian shales gave an average value of δ18O = 11·9 ± l·23‰ (lδ, n = 5). Whole-rock oxygen isotope ratios indicate that the origin of the granites was in neither purely sedimentary/metasedimentary rocks nor pristine mantle melts. δ18O values close to 9·0‰ require a crustal protolith, having an important recycled component.Oxygen isotope results are compatible with the cordierite-bearing granites being generated by assimilation of nebulite-like material by a biotite granite magma. However, 18O/16O of mineral separates obtained from the three different granites and the nebulite indicate that isotopic equilibrium, if ever reached, has not been preserved. The modified isotopic equilibrium is attributed to fluid activity, but mineral-pair δ-δ plots suggest that the granite system behaved as a closed system, and that the fluid was deuteric (magmatic) in origin. This implies that if assimilation did happen, it occurred at a temperature higher than the closure temperature of the different minerals to isotopic exchange. In a δ18O vs δD plot, hornblende and biotite separates from the granites plot within the igneous field. A simple mesocrustal anatectic origin for the peraluminous late Hercynian granites of the western area of the CIM is difficult to sustain on the basis of the stable isotope data, consistent with other field, petrographic and geochemical evidence. Cordierite in the cordierite-bearing granites is not “restitic” from a deep source area, but rather is xenocrystic from the high-grade metamorphic country rock (nebulites).