Search

The approach commonly used to assess the potential for climate reconstruction is to use linear regressions to compare the isotopic signal stored in archives to instrumental climatic data sets. A new method is proposed that combines statistical and mechanistic approaches to extract climatic information from δ13C records in organic matter. Both a spatial and a temporal gradient of 13C discrimination in a moss species commonly found in temperate and tropical peat bogs are compared to meteorological records. The relevance of fossil and modern analogues to elucidate palaeoenvironment records are tested. It was found that the magnitude and, in some cases, the direction of the impact of temperature, humidity and CO2 atmospheric concentration on 13C discrimination depend on the calibration set considered. The use of a mechanistic model is shown to help greatly in specifying the joint influence of the climatic variables. [ABSTRACT FROM AUTHOR]

Despite the great potential of peat bogs as climatic archives, to date only few studies have focused on the climatic controls on cellulose isotopic composition in modern bog plants. This study attempts to calibrate plant–climate relationships by sampling a set of modern plant species (both vascular plants and mosses) and bog surface waters along an altitude transect in Switzerland. Isotopic analyses of water samples show that the δ18O-values of surface bog waters follow the trend of precipitation despite significant scatter in the data set. Detailed sampling of surface waters within one bog shows that δ18O-values vary widely and are closely related to the micro-topography of the bog surface. More enriched 18O/16O ratios in water samples collected from small raised hummocks than the ones collected from hollows are documented in both horizontal and vertical profiles. A δ18O–δD plot indicates that the process leading to the isotopic enrichment of the uppermost surface waters is evaporation, greater above Sphagnum covered hummocks than above open pools. To investigate the implications of such high variability of source water for plant α-cellulose δ18O-values, a detailed study of both surface water and α-cellulose δ18O-values within one site is conducted. The large δ18O variability observed in surface waters is found to be considerably smoothed in α-cellulose (by a factor of 5–10 depending on the plant species). This indicates that the water used by plant photosynthetic processes reflects the isotopic composition of the average annual precipitation. This points to a source water level for plants of a few decimeters where the variations are smaller than at the air–water interface. The response of the α-cellulose δ18O to the environmental gradient along the altitude transect varies considerably from species to species. For most of the species studied, the δ18O-values decrease with altitude, following the trends of δ18O-values in precipitation and in surface water samples. Some species, the cotton sedge Eriophorum vaginatum and the moss Sphagnum capillifolium, show statistically significant δ18O relationships to an altitude of −1.8‰/km and −2.9‰/km respectively. However, some other plant species, Calluna vulgaris, Vaccinium uliginosum, Andromeda poliflora, Carex pauciflora, Sphagnum cuspidatum and Sphagnum magellanicum, do not, or only partially, reflect changes in climatic parameters associated with an altitude increase. Furthermore, changes in relative humidity, which are not correlated with altitude, are found to explain a large part of the variability in δ18O-values for the sedge Carex pauciflora and the moss Sphagnum cuspidatum. Therefore, this study confirms the importance of species-specific studies when interpreting 18O/16O ratios of macrofossils along a fossil peat sequence as a record of past climate changes. Our study allows to extend the mechanistic model that isotopically links source water and cellulose to the physiological specificities of sedges and mosses. A comparison of the modeled and calculated net biological fractionation factors for Eriophorum vaginatum and Sphagnum capillifolium reveals that these two species appear to have a more homogeneous leaf reservoir than trees. [Copyright &y& Elsevier]

Abstract: Chronology of fast climatic changes during the last glacial period. The history of the glacial climate is punctuated by events occurring at the scale of a human life. They are characterised by temperature changes of large amplitude, simultaneously in Greenland and the North Atlantic. These events affected not only the surface hydrology, but also the deep circulation of this oceanic basin. A by-product of the obvious correspondence between Dansgaard–Oeschger and Heinrich in the polar ice and marine sediments is to allow, by correlation, the construction of a calendar chronology for the marine records. This chronostratigraphic approach was validated by means of radiocarbon dating of deep-sea sediments raised on the Iberian Margin. Our study also contributes to the international effort of calibration of the radiocarbon time scale by providing significant results in the interval between 33000 and 41000 years calendar BP. To cite this article : É. Bard et al., C. R. Palevol 5 (2006) . [Copyright &y& Elsevier]

We present a new set of 14C ages obtained by accelerator mass spectrometry (AMS) on planktonic foraminifera from a deep-sea core collected off the Iberian Margin (MD952042). This site, at 37°N, is distant from the high-latitude zones where 14C reservoir age is large and variable. Many independent proxies — alkenones, magnetic susceptibility, ice-rafted debris, foraminifera stable isotopes, abundances of foraminifera, pollen, and dinoflagellates — show abrupt changes correlative with Dansgaard-Oeschger and Heinrich events of the last glacial period. The good stratigraphic agreement of all proxies — from the fine to the coarse-size fractions — indicates that the foraminifera 14C ages are representative of the different sediment fractions. To obtain reliable 14C ages of foraminifera beyond 20,000 14C yr B.P., we leached the shells prior to carbonate hydrolysis and subsequent analysis. For a calendar age scale, we matched the Iberian Margin U37K′ profile with that of Greenland Summit δ18O. Both are proxies for temperature, which in models varies synchronously in the two areas. The match creates no spurious jumps in sedimentation rate and requires only a limited number of tie points. Except for ages older than 40,000 14C yr B.P., Greenland''s GISP2 and GRIP records yield similar calendars. The 14C and imported calendar ages of the Iberian Margin record are then compared to data — from lacustrine annual varves and from corals and speleothems dated by U–Th — previously used to extend the calibration beyond 20,000 14C yr B.P. The new record follows a smooth pattern between 23,000 and 50,000 cal yr B.P. We find good agreement with the previous data sets between 23,000 and 31,000 cal yr B.P. In the interval between 33,000 and 41,000 cal yr B.P., for which previous records disagree by up to 5000 cal yr, the Iberian Margin record closely follows the polynomial curve that was previously defined by an interpolation of the coral ages and runs between the Lake Suigetsu and the Bahamian speleothem data sets. [Copyright &y& Elsevier]

An accurate chronometer that covers the past 50,000 years is a fundamental tool for geology and archaeology. Accurate dating is essential to understand crucial time in human evolution. Accurate dating is also of paramount importance in fields such as geophysics, geochemistry, and paleoclimatology. The main chronometer for this time range is radiocarbon (14C), but its use is not free of perils. This is because the atmospheric 14C/12C ratio — the starting point of the radiocarbon clock — varies over time. To obtain accurate dates, these fluctuations must be accounted for with a calibration curve. Fossil trees have been used to produce a high-resolution tree-ring calibration, which has been extended with data from layered sediments and tropical corals.