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A sequence of seven superimposed paleosols developed on eolian calcarenites and alluvium was sampled on the island of Lanzarote in order to examine the possibility of dating land snail shells by the U-series method, using a TIMS technique allowing measurement of U and Th isotopes in very small samples. In the lower six units, the fossil shells yielded D-allo/L-isoleucine (A/I) ratios of about 0.5 and apparent AMS 14C ages ranging from 41,000 to 34,000 yr B.P., indicating that most paleosols formed during a relatively short mid-Würm humid episode. The upper unit (paleosol 7) yielded more variable A/I ratios (ranging from 0.6 to 0.2) and a younger 14C age ∼27,000 yr B.P. Most samples contained enough U to allow the calculation of U-series ages, after correction for the presence of a detrital component. In samples containing a few tens of ppb of U (paleosols 1, 2, 3, 6, and 7), the ages are strongly dependent upon the model used for the correction. In samples containing more than 300 ppb of U (paleosols 4 and 5), concordant ages of ∼31,000 ± 1000 yr were obtained regardless of the correction model used. U uptake in these shells occurred during one single early diagenetic phase, soon after burial, since shells of modern snails do not contain any significant amount of U. The arid conditions subsequent to the mid-Würm humid episode have likely ensured since then a fair closure of the radioactive system.
Lago Verde is a fresh-water maar found on the lower slopes of San Martin volcano, at the Sierra de Los Tuxtlas, Mexico, currently the northernmost remnant of the tropical rain forest in America. 210Pb and 137Cs analyzed in a sediment core were used to reconstruct the historical fluxes of Ag, Cd, Cu, Pb, Hg and Zn to the site during the last ∼ 150 yr. The 210Pbxs-derived sediment accumulation rates, the magnetic susceptibility, C/N ratios and δ13C data evidenced background conditions at the lake until 1960s, when enhanced erosion related to the clearing of large forested areas at Los Tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material. Recent sediments from Lago Verde were found enriched by Pb (26-fold natural concentration level [NCLs]) and moderately enriched by Cd>Cu>Zn and Hg (6-, 5-, 4- and 4-fold corresponding NCLs, respectively). The fluxes of Cu, Hg, Pb and Zn have significantly increased since 1940s, with peak ratios of total modern to pre-industrial fluxes of 11, 11, 19 and 49, respectively. The lake occupies a relatively pristine, non-industrialized basin, and therefore the increased metal fluxes might be related to long-distance aeolian transport of trace metals.
Superimposed phases of stromatolite buildup are observed on the eastern margin of Lake Manyara and depict a paleoshoreline at about 20 m above modern lake level. Radiocarbon and Th/U measurements permitted the dating of the last two phases of stromatolite formation at ca. 90,000 yr and between 27,000 and 23,000 yr B.P., respectively. The Th/U chronology is based on the decay of a strong 230Th-excess (over 234U) inherited with the detrital particles cemented into the stromatolites. The various generations of stromatolites show comparable stable carbon and oxygen isotope contents and are located at the same paleolake stabilization levels. This indicates that stringent hydrological conditions are necessary for the development of the encrusting benthic microbial communities responsible for stromatolite formation. A comparison with similar stromatolitic units from the nearby Lake Natron-Lake Magadi basin shows that such conditions occurred during only a few of the late Quaternary humid episodes known in eastern Africa and that they are different in each basin. Stromatolites do not necessarily represent all high lake levels that Lake Manyara experienced during the late Pleistocene and Holocene.
Direct information on the isotopic composition and timing of the now-vanished Laurentide glacier can be found in subglacial calcite or in early diagenetic concretions from glaciolacustrine deposits. Th/U isotopic measurements on early diagenetic concretions frome Lake Deschaillons varves (St. Lawrence Lowland, Québec) date the early Wisconsinan major ice advance at ca. 80,000 yr (isotopic stage 5a/4 transition). Significant retreat occurred at ca. 46,000 yr (isotopic stage 3) as indicated by Th/U isochrones on early diagenetic concretions from Lake Gayhurst varves in the Appalachian foothills of Québec. During the maximum extension of the ice, which is dated at ca. 22,000 yr by Th/U measurements on subglacial calcite at Cantley (northern margin of the St. Lawrence Lowland), δ18O values (vs PDB) ranging from −16/−19‰ (surficial ice) to ca. −30‰ (basal ice) are inferred for the Laurentide ice isotopic composition. Superimposition of two phases of carbonate precipitation does not prevent the establishment of their respective age and isotopic composition through correlation of independent parameters and calculation of mixing lines.
During recent humid episodes, stromatolites were built along paleolake margins, some 60 m above the modern water level of Lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently covering pebbles and boulders eroded from the older ones. The youngest one yielded 14C ages ranging from approximately 12,000 to 10,000 yr B.P. Their δ13C values (≥2.6%) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO2, thereby lending credence to the reliability of the 14C. An initial 230Th/232Th ratio in the detrital component was determined by Th/U measurements on the 14C dated stromatolites. Using this value a 230Th/234U chronology for the older stromatolites was calculated. Ages of ≥240,000 and 135,000 ± 10,000 yr were obtained for the first and second generations, respectively. A humid episode apparently characterized eastern Africa during each glacial-interglacial transition. 18O and 13C measurements on stromatolites, when compared to values on modern waters and carbonates, provide paleohydrological information. Long residence time of the paleolake waters and less seasonally contrasted regimes are inferred.
Stable carbon isotope ratios of total organic matter were measured in two cores collected from the Kashiru peatbog in Burundi, Equatorial Africa. The record, which spans at least the last 40,000 yr, documents the C3-C4 biomass balance in the organic sediment. Among the major modern peat formers, most plants are C3 species and are characterized by δ13C values of -25.5 ± 2.3% (vs PDB). The C4 plants, which are characterized by higher δ13C values (-11.3 ± 0.7%) belong to the Gramineae (Miscanthidium sp.) and Cyperaceae families (Cyperus latifolius, C. papyrus, Pycreus nigricans). In the fossil record, δ13C values of total organic matter vary between -28 and -15% in response to the relative fluxes of C3 and C4 plants. Before 30,000 yr B.P., low δ13C values (-23.5 ± 1.1%) match high arboreal pollen contents. From 30,000 to 15,000 yr B.P., higher δ13C values (-17.6 ± 1.1%) correspond to a significant increase in percentages of grass pollen. During this episode, a short and sharp shift toward lighter carbon isotopic compositions at 21,000 yr B.P. is synchronous with higher input of arboreal pollen. From 15,000 to 12,000 yr B.P., the 13C content decreases (δ13C = -22.9 ± 1.4%). This shift, which cannot be explained by an increase in the arboreal vegetation, could be explained by the spreading of C3 Gramineae or C3 Cyperaceae. The interval from 12,000 to 7000 yr B.P. is poorly documented in these cores due to much lower organic matter accumulation. Low δ13C values (δ13C = -25.2 ± 1.3%) are observed from 7000 to 5000 yr B.P., when the pollen data show development of C3 mountain forest. The Late Holocene is characterized by a mixed C3-C4 organic matter accumulation (δ13C = -20.9 ± 1.6%). This study depicts a change in the dominant photosynthetic pathway among the herbaceous components, notably at the glacial-interglacial transition, when C3 plants were favored by increased water supply and/or higher atmospheric CO2 concentration.
A deep-sea sediment core covering the last 20 ka and located between the Polar and the Arctic fronts in the marginal ice zone (MIZ) of the central Fram Strait has been investigated for changes in paleoceanography and calcium carbonate preservation. The reconstruction is based on the distribution patterns of planktic foraminifera, mean shell weight and the degree of fragmentation of their shells, stable isotopes and other geochemical and sedimentological data. The results show that the planktic foraminifera shells are poorly preserved throughout most of the record. Only the intervals comprising the early Holocene from 10.8 to ~ 8 cal ka BP and the last 800 yr show improved preservation of CaCO3. The dissolution correlated with the extent of Arctic water and the associated marginal ice zone (MIZ) and high organic productivity. Dissolution of planktic foraminifera is generally high during the late deglaciation, mid and late Holocene prior to ~ 800 cal yr BP. The abundance of small subpolar species increases in the surface sediments dating from the last century, which could be interpreted as a large and significant surface water warming. However, this apparent high-magnitude warming seems to be overestimated due to preservation changes in the youngest sediments.
The metabolic response to a 150 or 400 g 13C-labelled pasta meal was studied for 8 h following rest or exercise at low or moderate workload (n 6). Following rest, the 400 g meal totally suppressed fat oxidation (v. 14.1 g following the 150 g meal) and a small amount of glucose was converted into fat (4.6 g), but fat oxidation remained high in subjects who had exercised following both the small (21.8 and 34.1 g) and large meal (14.1 and 32.3 g). Exogenous glucose oxidation was significantly higher in subjects who had remained at rest both following the small (67.6 g v. 60.4 and 51.3 g in subjects who exercised at low and moderate workloads) and large meal (152.2 v. 123.0 and 127.2 g). Endogenous glucose oxidation was similar in the three groups following the 150 g meal (42.3–58.0 g), but was significantly lower following the 400 g meal in subjects who had exercised at low workload (24.2 v. 72.2 g following rest; P<0.05), and was totally suppressed in those who had exercised at moderate workload. As a consequence, a larger positive glycogen balance was observed in subjects who exercised before the large meal (182.8–205.1 g v. 92.4 g following rest; P<0.05). Total fat oxidation calculated from 08.00 hours to 20.00 hours was similar in subjects who exercised at low and moderate workloads. These results indicate that: (1) de novo lipogenesis, which plays only a minor role for the disposal of an acute dietary carbohydrate load, is totally suppressed following exercise, even when a very large carbohydrate load is ingested; (2) the reduction in glycogen turnover as well as a preferential conversion of glucose into glycogen are responsible for the increase in glycogen stores following exercise; (3) for a similar energy expenditure, exercise at low workload for a longer period does not favour fat oxidation when the post-exercise period is taken into account.
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