Comments on : diet, physiology and ecology of fossil mammals as inferred from stable carbon and nitrogen isotope biogeochemistry: implications for Pleistocene bears

A detailed study of isotopic relationships in European Pleistocene ursid teeth have been presented by Bocherens et al. (1994). We agree with the resu1ts and broad conclusions derived from the stable carbon isotope relationships. These are findings that confirm the previous hypothesis relating to the diets of Ursus deningeri and Ursus spelaeu

Pleistocene paleoenvironmental evolution at continental middle latitude inferred from carbon and oxygen stable isotope analysis of ostracodes from the Guadix-Baza Basin (Granada, SE Spain)

A representative paleoenvironmental reconstruction of continental middle latitude from ca. 2my to the upper part of Middle Pleistocene (279±77ky) was obtained from the carbon and oxygen stable isotopes analyzed in ostracode shells (Cyprideis torosa) recovered in the Guadix-Baza Basin (SE Spain), an intramontaneous closed depression filled by alluvial and lacustrine sediments. This study was performed along a 356-m-thick composite section, dated previously by paleomagnetism and the amino acid racemization method. 013C and 0180 profiles reflected changes in temperature, the evaporationJinfill ratio in the water bodies and the amount of rain. 013C is also affected by changes in plant biomass: periods with high 013C and 0180 values are associated with warm and dry regimes, and with less vegetation, which, in some cases, coincide with the development of displacive gypsum crystals, whereas low 013C and 0180 values correlate with cold and humid episodes, which cause more vegetation biomass and, therefore, increasing the input of isotopically light carbon. Intermediate 0180 values are linked to temperate dry or humid episodes when they coincide with high or low 013C values, respectively. 86 paleoclimatic events were distinguished in the Pleistocene record from the 013C and 0180 profiles. From both the statistical analysis of the geochemical data and the geological observations, four Cold and Humid Long Periods (low 0180) and four Warm and Dry Long Periods (high 0180) were defined. This differs with respect to the paleoclimatological behavior established for the Northern Hemisphere where during cold periods (glacial), no water was available while permafrost conditions persisted, whereas in warm episodes (interglacial), higher precipitation rates occurred. Good correspondences between the Guadix-Baza Basin paleoclimatic record and a marine oxygen-isotope sequence, two continental cores and other long Mediterranean paleoenvironmental records (pollen sequences from Israel) were found, which suggested that climate changes in the Guadix-Baza Basin were in tune with global climatic changes

Scale and structure of time-averaging (age mixing) in terrestrial gastropod assemblages from Quaternary eolian deposits of the eastern Canary Islands

Quantitative estimates of time-averaging (age mixing) in gastropod shell accumulations from Quaternary (the late Pleistocene and Holocene) eolian deposits of Canary Islands were obtained by direct dating of individual gastropods obtained from exceptionally well-preserved dune and paleosol shell assemblages. A total of 203 shells of the gastropods Theba geminata and T. arinagae, representing 44 samples (= strati graphic horizons) from 14 sections, were dated using amino acid (isoleucine) epimerization ratios calibrated with 12 radiocarbon dates. Most samples reveal a substantial variation in shell age that exceeds the error that could be generated by dating imprecision, with the mean within-sample shell age range of 6670 years and the mean standard deviation of 2920 years. Even the most conservative approach (Monte Carlo simulations with a non-sequential Bonferroni correction) indicates that at least 25% of samples must have undergone substantial time-averaging (e.g., age variations within those samples cannot be explained by dating imprecision alone). Samples vary in shell age structure, including both left-skewed (17 out of 44) and right-skewed distributions (26 out of 44) as well as age distributions with a highly variable kurtosis. Dispersion and shape of age distributions of samples do not show any notable correlation with the stratigraphic age of samples, suggesting that the structure and scale of temporal mixing is time invariant. The statistically significant multi-millennial time-averaging observed here is consistent with previous studies of shell accumulations from various depositional settings and reinforces the importance of dating numerous specimens per horizon in geochrono logical studies. Unlike in the case of marine samples, typified by right-skewed age distributions (attributed to an exponential-like shell loss from older age classes), many of the samples analyzed here displayed leftskewed distributions, suggestive of different dynamics of age mixing in marine versus terrestrial shell accumulations