Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: VII. Carbonate isotope geochemistry as a record of riverine runoff

Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a ¹⁸O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a ¹⁸O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the ¹⁸O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the ¹⁸O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell ¹⁸O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell ¹⁸O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the ¹³C of shells against predicted values based solely on the ¹³C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle.     

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: VI. Geochemical indicators

Geochemical signals of bulk sedimentary organic matter from three cores from Lake Tanganyika provided information about both internal processes and terrestrial inputs to the lake. Indications of land use change were detected in the geochemical records of the watersheds, and the timing of these changes was consistent with historical records of population demographics. While C:N ratios suggested that the distance from shore might be important in influencing the relative amount of allochthonous vs. autochthonous material, all cores were dominated by autochthonous organic matter. In general, nitrogen isotopes were more positive at disturbed sites, indicating inputs of enriched soil nitrate that was subsequently taken up by phytoplankton. In contrast, carbon isotopes did not reflect land use patterns, and a post-1950s decline in carbon isotope ratios found in all cores may indicate a lake-wide decrease in productivity. These interpretations were consistent with pollen and climate records.     

Limnic Responses to Increased Effective Humidity during the 8200 cal. yr BP Cooling Event in Southern Sweden

Highly resolved sediment stratigraphic and stable isotope records from a lake in south-central Sweden give evidence of relatively humid summers during the wide-spread cold event at c. 8200 cal. BP. A transient lake-level rise led to increased catchment erosion followed by enhanced phytoplankton production and disturbance of the Chara-dominated algal community. An associated increase in the deposition of organic material resulted in hypolimnetic oxygen deficit and methane formation as inferred from elevated sulphur content of the sediments and strong depletion in ¹³C of bulk organic material. In combination with dry and cold winter conditions as revealed by other proxy records, these data suggest the development of a strongly seasonal climate in northwestern Europe in response to altered atmospheric circulation at this stage. Enhanced zonal circulation with frequent cyclonic activity and increased effective humidity probably characterized the summer climate of southern and central Scandinavia, consistent with conditions inferred for continental Europe. In contrast, the winter climate was dominated by blocking high-pressure circulation, giving rise to cold and dry conditions.     

Diffuse CO<Subscript>2</Subscript> degassing at Vesuvio,Italy

At Vesuvio, a significant fraction of the rising hydrothermal–volcanic fluids is subjected to a condensation and separation process producing a CO₂–rich gas phase, mainly expulsed through soil diffuse degassing from well defined areas called diffuse degassing structures (DDS), and a liquid phase that flows towards the outer part of the volcanic cone. A large amount of thermal energy is associated with the steam condensation process and subsequent cooling of the liquid phase. The total amount of volcanic–hydrothermal CO₂ discharged through diffuse degassing has been computed through a sequential Gaussian simulation (sGs) approach based on several hundred accumulation chamber measurements and, at the time of the survey, amounted to 151 t d^–1. The steam associated with the CO₂ output, computed assuming that the original H₂O/CO₂ ratio of hydrothermal fluids is preserved in fumarolic effluents, is 553 t d^–1, and the energy produced by the steam condensation and cooling of the liquid phase is 1.47×10¹² J d^–1 (17 MW). The location of the CO₂ and temperature anomalies show that most of the gas is discharged from the inner part of the crater and suggests that crater morphology and local stratigraphy exert strong control on CO₂ degassing and subsurface steam condensation. The amounts of gas and energy released by Vesuvio are comparable to those released by other volcanic degassing areas of the world and their estimates, through periodic surveys of soil CO₂ flux, can constitute a useful tool to monitor volcanic activity.Editorial responsibility: H. Shinohara     

12,000-Year, preliminary results of the stable nitrogen and carbon isotope record from the Empakai Crater lake sediments, Northern Tanzania

The stable isotope compositions of organic carbon and nitrogen, the contents of organic carbon and nitrogen and C/N ratios for two cores recovered from the Empakai Crater at water depths of 11 and 20 m are used to document climatic changes in northern Tanzania. Eight ¹⁴C AMS dates determined on total organic matter (OM) indicate that the sedimentation rate in this lake is about 30 cm/ka for the late Pleistocene to early Holocene period. There are differences in the δ¹³C values of organic carbon between the two cores, which may be a result of differences in location from the present shoreline and of different water depths. In the deeper-water core the δ¹³C values show a general downcore decrease to the base of the core with a sharp change to lower values of about 4‰ at a depth of 100 cm (8.7 ka). The general trend of downcore decrease in ¹³C values can be attributed either to a systematic decrease in the relative proportion of C₄ type of OM, owing to an increase in precipitation and change in vegetation cover from grassland to forest, or to utilization of isotopically enriched carbon during photosynthesis. The δ¹⁵N values show a general downcore increase with again a sharp change of about 5‰ to lower values at about 8.7 ka. A sharp change of about 5‰ and 4‰ to more depleted values at a depth of 100 cm of both ¹⁵N and ¹³C, respectively, suggests either hiatus or abrupt change in climatic condition from wetter conditions to drier conditions. There is enhanced preservation of OM in the lake as depicted by high mean values of organic carbon and nitrogen at both sites.     

Carbon isotope stratigraphy of Lochkovian to Eifelian limestones from the Devonian of central and southern Europe

Lower to Middle Devonian carbonates of the Prague Syncline, the Carnic Alps, the Montagne Noire, and the Cantabrian Mountains were investigated for δ¹³C_carb and δ¹³C_org. These values were measured on bulk rocks, selected components and cements. Many carbonates exhibit primary marine values, but some are altered by diagenesis. A δ¹³C curve can be presented for the latest Pridolian to Emsian time interval. Several sharp or broad positive excursions are obvious in the woschmidti-postwoschmidti, sulcatus, kitabicus, Late serotinus, and kockelianus conodont zones. The excursion at the Silurian–Devonian boundary is known worldwide and therefore considered global in nature. Some of the others are described for the first time from central and southern Europe, and their global nature has to be verified by further investigations in other regions. Most excursions relate to and/or started during major regressions whereas sea-level highstands correspond to minimal δ¹³C values. Similar relationships between sea-level changes and δ¹³C have been observed from other early Palaeozoic intervals. The transgressive Chote? (?) and Ka?ák events are marked by positive isotope excursions, this type of combination is usually observed in late Palaeozoic to Cenozoic black shale events.     

潮汐湿地土壤碳矿化及其对电子受体响应研究进展

潮汐湿地是一种重要的湿地类型,受周期性变化的潮汐水位和盐分等特殊因子的影响,其土壤碳矿化过程亦具独特性,综述了潮汐湿地土壤碳矿化及其对电子受体响应的最新研究进展。结果表明,潮汐湿地土壤碳矿化除有氧碳矿化外,三价铁和硫酸盐还原过程主导的厌氧碳矿化也是土壤碳矿化的重要途径;潮汐湿地类型中的红树林土壤碳矿化速率高于盐沼土壤碳矿化速率,并均以二氧化碳为土壤碳矿化的主要产物;生境的差异使不同电子受体在土壤碳矿化中的作用有所不同,并受到电子受体和电子供体数量的调节;潮汐、盐分、生物干扰等是影响潮汐湿地土壤碳矿化过程的主要因子。     

Performance of various similarity functions for nondimensional wind and temperature profiles in the surface layer in stable conditions

The linear functions for non-dimensional wind and temperature profiles are commonly used to describe the surface layer fluxes in atmospheric models. However, their applicability is limited to smaller values of the stability parameter z/L (where z is the height above ground and L is the Obukhov length) i.e. z/L < 1.0. These linear functions have been modified (Webb 1970, Quart. J. Roy. Meteor. Soc. 96, 67–90; Clarke 1970, Quart. J. Roy. Meteor. Soc. 96, 91–114; Hicks 1976, Quart. J. Roy. Meteor. Soc. 102, 535–551; Beljaars and Holtslag 1991, J. Appl. Meteorol. 30, 327–341; Cheng and Brutsaert 2005, Boundary-Layer Meteorol. 114, 519–538) over the years for calculating fluxes when z/L > 1.0 under strongly stable conditions. In view of this, the objective of the present study is to analyze the performance of these similarity functions to compute surface fluxes in stable conditions.The meteorological observations from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) experiment are utilized for computing the surface fluxes in stable conditions. The computed fluxes are found to be reasonably close to those observed. The ratio of observed to computed fluxes reveals that the computed fluxes are close to the observations for all the similarity functions for z/L < 1.0 whereas the computed values show relatively a large scatter from observations for z/L > 1.0. The computed values of u and heat flux do not show significant differences from those observed at 99% confidence limit. The performance of all the similarity functions considered here is found to be comparable to each other in strongly stable conditions.     

Transnational linkages and the spillover of environment-efficiency into developing countries

Arguments about the “positive” influence of growing transnational linkages have typically focused on their role in diffusing environmentally superior innovations which help to raise countries’ environment-efficiency. The present article empirically tests these claims by examining whether developing countries’ linkages with more CO₂- and SO₂-efficient economies contribute to domestic improvements in CO₂- and SO₂-efficiency. Our large-N, statistical findings caution against some of the efficiency-oriented optimism voiced by supporters of globalization. Although imports ties with more pollution-efficient countries are found to spillover into improved domestic CO₂- and SO₂-efficiency, neither transnational linkages via exports, inward foreign direct investment (FDI) nor telephone calls appear to have any influence on domestic pollution-efficiency.