Paul F. Dennis

The stable isotope record of environmental and climatic signals in modern terrestrial microbial carbonates from Europe

Stable carbon and oxygen isotope data from over 80 samples of Recent freshwater microbial carbonates from western Europe, confirm that these deposits record environmental and climatic information. Our sample area tested whether recent microbial carbonates record environmental signals over large regions with differing δ18O compositions for rainfall (δw), particularly in the Alps where δw is influenced by lower condensation temperatures caused by the orographic effect. Microbial crusts in Alpine areas are clearly distinguished, i.e., have isotopically lower δ18O values by up to 4‰, from those forming in lowland areas on the east side of the mountains. Bavarian lakes and rivers which receive runoff from the Alps also have δ18O compositions that reflect the Alpine meteoric water input. Microbial crusts in the higher Alpine sites have δ13C values around −4‰, which are between 2 and 6‰ higher than values from lowland sites. This difference probably reflects a smaller soil-zone carbon component in the mountain sites where soils are thin, poorly vegetated, or absent. Oxygen and carbon isotope values do not vary significantly between different types of microbial precipitate (e.g., undifferentiated crusts, Rivularia colonies, moss tufa, etc.) at a site. The oxygen isotope compositions of freshwater lacustrine mussel shell aragonite and associated (sometimes shell encrusting) microbial carbonates differ by <1‰, and both are probably close to equilibrium isotopic values. However, δ13C values in mussel shell aragonite are consistently lower, typically by 5‰, than the associated microbial carbonate value. This suggests that the δ13C of the microbial carbonate is affected by the microenvironmental photosynthetic processes of the microbes. These microenvironmental effects are only evident at lake shore sites where water flushing rates are low. These data suggest that selected ancient microbial carbonates may contain clear records of palaeoclimate (particularly relative changes in temperature) and environmental change. Pools behind fluvial barrage tufas are probably the best targets for reasonably continuous, long, dateable records with isotopic conditions least affected by microenvironmental processes.

A dual isotope approach to identify denitrification in groundwater at a river-bank infiltration site

The identification of denitrification in the Torgau sand and gravel aquifer, Germany, was carried out by a dual isotope method of measuring both the delta 15N and delta 18O in NO3-. Samples were prepared by an anion exchange resin method (Silva et al., J. Hydrol. 228 (2000) 22) with a modification to the AgNO3-drying process from a freeze-drying to an oven-drying method. The occurrence of denitrification in the aquifer was confirmed by comparing the reduction of dissolved oxygen, dissolved organic carbon and NO3- concentrations with the dual isotope signatures. High nitrate concentrations were associated with low delta 15N and delta 18O values, and vice versa. The denitrification accords with a Rayleigh equation with calculated enrichment factors of epsilon = -13.62@1000 for delta 15N and epsilon = -9.80@1000 for delta 18O. The slope of the straight-line relationship between the delta 15N and delta 18O data demonstrated that the enrichment of the heavy nitrogen isotope was higher by a factor of 1.3 compared with the heavy oxygen isotope. It is concluded that the identification of this factor is a useful means for confirming denitrification in future groundwater studies.

Possible Microbial Effects on Stable Carbon Isotopes in Hot-spring Travertines

Recent and older (early Holocene to Pleistocene) hot-spring travertine carbonates from central Italy have two distinctive macrofabrics, crystalline crusts and shrubs. Crystalline crusts are laminated slope deposits that formed abiotically following CO 2 degassing from spring water. The formation of shrub travertine--irregular, dendritic precipitates from pool environments--is controversial and has been attributed to both abiotic and microbial processes. Oxygen isotope variation in our travertines can be explained by abiotic processes, mainly CO 2 degassing. In contrast, our carbon isotope data cannot be wholly explained by abiotic CO 2 degassing invoked in earlier studies. Because photosynthesis is known to preferentially remove 12 C, leaving ambient waters enriched in 13 C, this fractionation should be recorded in delta 13 C values of microbially influenced travertine. Our shrub carbon isotope values are between 0.5 per thousand and 6.0 per thousand larger than values for associated abiotic precipitates, and this difference is probably caused by microbial activity. These isotope data support previous visual evidence that some shrub fabrics are microbially influenced.

Comparisons of aerosol nitrogen isotopic composition at two polluted coastal sites

Abstract Atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters. Stable nitrogen isotope data can provide important information regarding the sources and processing of atmospheric fixed-nitrogen species and is thus important in controlling eutrophication. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland and also aboard the RRS Challenger in the Eastern Atlantic Ocean. Aerosol concentrations of ammonium and nitrate were determined prior to δ15N isotopic analysis. For both species a significant difference in mean isotopic composition was seen between samples from Weybourne ( +6±6‰ for ammonium and +7±6‰ for nitrate) and Mace Head and RRS Challenger campaigns ( −9±8‰ for ammonium and −1±3‰ for nitrate). At each site a strong dependence of isotopic composition on the geographical origin of the sampled air mass was also observed. For aerosol ammonium, marine and terrestrially dominated samples were found to be isotopically distinct, perhaps reflecting the presence of oceanic sources of ammonia in addition to anthropogenic or natural terrestrial sources. Further distinctions were made within terrestrially dominated samples, possibly indicative of different types of animal husbandry regimes or other forms of anthropogenic activity. For aerosol nitrate, there was found to be generally less variation between samples at each site, although at Weybourne a significant difference was observed between the mean isotopic composition of samples originating from the northern UK ( +15±3‰ ) and that of those originating from the southern UK (+10±3‰) , suggesting that aerosol δ15N data might possibly facilitate source apportionment between NOx emissions from power stations and those from vehicle exhausts. The nitrate data also appeared to show seasonality with higher concentrations and lower δ15N values seen in the summer.

Factors affecting denitrification during infiltration of river water into a sand and gravel aquifer in Saxony, Germany

Abstract River infiltration into a sand and gravel aquifer was investigated to assess the importance of denitrification in maintaining low-NO − 3 groundwater supplies. Samples from the River Elbe and groundwater sampling points along a section of the aquifer were analysed for dissolved organic carbon, major ions and the 15 N 14 N isotopic ratio of dissolved NO − 3 . Input of NO − 3 to the aquifer is influenced by seasonal, temperature-dependent denitrification in the river bed sediments. Along an upper flowpath in the aquifer from the River Elbe to a sampling point at a distance of 55 m, the median NO − 3 concentration decreased by 4.8 mg litre −1 and the δ 15 N composition increased by +9.0‰, consistent with denitrification. Similar isotopic enrichment was demonstrated in a laboratory column experiment with a reduction in NO − 3 of 10.5 mg litre −1 for an increase in δ 15 N of +9.8‰, yielding an isotopic enrichment factor of −14.6‰. A mass balance for denitrification shows that oxidizable organic carbon required for denitrification is derived from both the infiltrating river water and solid organic matter fixed in the river bed sediments and aquifer material.

The recovery and isotopic measurement of water from fluid inclusions in speleothems

The isotopic composition of speleothems is a useful palaeoclimatic indicator, but its value would be enhanced if information on the composition of the parent dripwaters could be recovered from fluid inclusions in the speleothem calcite. To develop a robust method for extracting and measuring oxygen and hydrogen isotopic composition of fluid inclusions we have used anhydrous Iceland Spar and microlitre glass capillaries of water as an analogue system. Crushing the capillary and calcite together in a high vacuum cell we have investigated the adsorbtive and isotopic behaviour of water when exposed to clean fracture surfaces. Significant water adsorption occurs at room temperature, accompanied by large negative isotopic shifts of both oxygen and hydrogen in the recovered foe water at H2O/CaCO3 mass ratios C for 60 min, totally desorbs the water and allows retrieval of the correct isotopic composition. Application of these methods to a British Late Holocene speleothem yields delta O-18 and delta H-2 compositions for the inclusion water which are closely comparable with the modern cave dripwaters and local precipitation. The results show that isotopic compositions can be recovered from inclusion samples of L (equivalent to approximately Ig of calcite) with precisions that are useful for palaeoclimatic research, +/-0.4% for delta O-18 and +/-3% for delta H-2. Greater precision than this will require replicate analysis for each speleothem growth increment. Copyright (C) 2001 Elsevier Science Ltd.

The oxygen isotope composition of water masses in the northern North Atlantic

The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, 18O) is reported for seawater samples collected from seven full-depth CTD casts in the northern North Atlantic between 20° and 41°W, 52° and 60°N. Water masses in the study region are distinguished by their 18O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of 18O and salinity (S) lie on an evaporation–precipitation line with slope of 0.6 in 18O–S space. Surface isotopic values become progressively lighter to the west of the region due to the addition of 18O-depleted precipitation. This appears to be mainly the meteoric water outflow from the Arctic rather than local precipitation. Surface samples near the southwest of the survey area (close to the Charlie Gibbs Fracture Zone) show a deviation in 18O–S space from the precipitation mixing line due to the influence of sea ice meltwater. We speculate that this is the effect of the sea ice meltwater efflux from the Labrador Sea. Subpolar Mode Water (SPMW) is modified en route to the Labrador Sea where it forms Labrador Sea Water (LSW). LSW lies to the right (saline) side of the precipitation mixing line, indicating that there is a positive net sea ice formation from its source waters. We estimate that a sea ice deficit of ?250 km3 is incorporated annually into LSW. This ice forms further north from the Labrador Sea, but its effect is transferred to the Labrador Sea via, e.g. the East Greenland Current. East Greenland Current waters are relatively fresh due to dilution with a large amount of meteoric water, but also contain waters that have had a significant amount of sea ice formed from them. The Northeast Atlantic Deep Water (NEADW, 18O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, 18O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in 18O–salinity space, whereas NWABW lies between NEADW and LSW on 18O–salinity plots. The offset of NWABW relative to the North Atlantic precipitation mixing line is partially due to entrainment of LSW by the Denmark Strait overflow water during its overflow of the Denmark Strait sill. In the eastern basin, lower deep water (LDW, modified Antarctic bottom water) is identified as far north as 55°N. This LDW has 18O of 0.13‰, making it quite distinct from NEADW. It is also warmer than NWABW, despite having a similar isotopic composition to this latter water mass.

Ice core evidence for significant 100‐year regional warming on the Antarctic Peninsula

We present a new 150-year, high-resolution, stable isotope record (delta O-18) from the Gomez ice core, drilled on the data sparse south western Antarctic Peninsula, revealing a similar to 2.7 degrees C rise in surface temperatures since the 1950s. The record is highly correlated with satellite-derived temperature reconstructions and instrumental records from Faraday station on the north west coast, thus making it a robust proxy for local and regional temperatures since the 1850s. We conclude that the exceptional 50-year warming, previously only observed in the northern Peninsula, is not just a local phenomena but part of a statistically significant 100-year regional warming trend that began around 1900. A suite of coupled climate models are employed to demonstrate that the 50 and 100 year temperature trends are outside of the expected range of variability from pre-industrial control runs, indicating that the warming is likely the result of external climate forcing. Citation: Thomas, E. R., P. F. Dennis, T. J. Bracegirdle, and C. Franzke (2009), Ice core evidence for significant 100-year regional warming on the Antarctic Peninsula, Geophys. Res. Lett., 36, L20704, doi: 10.1029/2009GL040104.

Nitrogen isotope hydrochemistry and denitrification within the Chalk aquifer system of north Norfolk, UK

Although there has been much debate over the potential health risks of nitrate in drinking water, there is a real issue of the costs associated with removing nitrate from drinking water supplies. In the Chalk aquifer system of north-east Norfolk, modern contaminants of a mainly agricultural origin produce high levels of nitrate (>15 mg/l NO3-N) in the unconfined valley areas, whereas in confined regions the levels of nitrate are low and commonly below detection limits (<0.04 mg/l NO3-N). To understand the source and fate of nitrate within this aquifer system, a detailed hydrochemical sampling programme has been completed in the River Bure catchment. Nitrogen isotope values (δ15N) for nitrate within the unconfined and semi-confined zones range between +4‰ and +10‰, characteristic of nitrified soil organic nitrogen. However, many Chalk groundwaters possess high N2/Ar ratios (39–72) indicating a significant contribution to dissolved N2 from denitrification. Denitrification is believed to be occurring within the overlying glacial deposits, providing a mechanism for naturally improving groundwater quality. δ15N values of low-nitrate groundwaters from the confined zone are isotopically light (−3‰ to +4‰), inconsistent with an origin from denitrification: it is suggested that these waters have a pre-anthropogenic nitrate signature.

Concentration of carbon dioxide in the Late Cretaceous atmosphere

Stable carbon isotope data from Late Cretaceous (Maastrichtian) palaeosols in India are used to estimate the concentration of carbon dioxide in the Late Cretaceous atmosphere. We show that the Maastrichtian atmosphere is unlikely to have contained more than about 1300 ppm by volume of CO2.This value agrees with an independently modeled value of CO2 in the Late Cretaceous atmosphere. A low concentration of the greenhouse gas carbon dioxide in the Maastrichtian atmosphere (relative to concentrations in the earlier Cretaceous) is consistent with palaeotemperature information from terrestrial plant and marine fossils, which suggest that the global climate cooled toward the end of the Cretaceous Period.