Land-sea Correlation Research Articles

Land-sea correlations in the Pleistocene based on isoleucine epimerization in non-marine molluscs

THE correlation between the oxygen isotope stratigraphy of the oceans and continental stratigraphic records is one of the main challenges for Quaternary research. Most continental classifications, however, are based on techniques and hypotheses that predate recent advances. Nevertheless, the deep-sea oxygen isotope global stratigraphic framework has been extended to the continents by means of some long sequences1; but these are exceptional and regional geology is unsatisfactorily classified for correlation with the oxygen isotope signal. Because successive geological events produced similar evidence (homotaxis), geochronometric dating is essential for correlation, but available methods are highly site-, regional- or sample-specific. Fortunately non-marine molluscs are ubiquitous in time and space, and here we use the time-dependent epimerization of L-isoleucine in these fossils to subdivide the Pleistocene of the British Isles, and to identify more events than recognized by the existing classification2. By calibrating the relative aminostratigraphic scale with independent dating methods we have set up a geochronology which is the basis for land—sea correlations back to oxygen isotope stage 15.

Tyrrhenian Sea tephrochronology of the oxygen isotope record for the past 60,000 years

A chronology of the marine oxygen isotope record of the past 60 ka is proposed by correlating eight marine ash-layers with terrestrial volcanic deposits, dated by the 14C and/or K Ar methods. Detailed oxygen isotope analyses were made on the planktonic foraminifera of three cores collected in the central Tyrrhenian Sea. The origin of the ash-layers was determined by major and trace elements analyses. Then the major pyroclastic terrestrial deposits were sampled, and analysed in order to provide land-sea correlations. Two ash-layers occurred during the warm event that followed the first step of the deglaciation, and which was dated at 12.6±0.5 ka B.P. Another ash-layer is present near the last glacial maximum that we have dated at 17.5±0.3 ka B.P. During isotopic stage 3, five ash-layers were recognized and dated, which allowed us to assign ages to the isotopically light oxygen peaks (warmer interstadials) at 31±1.5 ka and 33.4±1.6 ka (Denekamp), at 37.5±2 ka (Hengelo), and successively at 49±2 ka, 52.4±2.2 ka, 57±2.2 ka and 60.5±2.2 ka (Moershoofd complex).

The Cassignol technique for potassium—Argon dating, precision and accuracy: Examples from the Late Pleistocene to Recent volcanics from southern Italy

A particular KAr technique, the Cassignol technique, has been developed in order to date Upper Pleistocene and Holocene volcanic rocks. We describe here its principles and its technology. The limit of detectability of the radiogenic Ar portion corresponds to an error of less than 10 3 a for K-rich minerals and a few 10 3 a for basalts. The reliability of the results and the validity of the correction for atmospheric contamination have been checked by analysing historical lavas and by comparison with data obtained from radiocarbon and thermoluminescence dating methods. The results demonstrate that the technique is capable of achieving KAr dates as young as 2000 a with a few centuries accuracy. A precision of ± 1.5% is obtained for samples older than 10 5 a. Moreover, in rocks older than 10 7 a, the technique permits the accurate dating of minute amounts of pure separated mineral phases. A reconstruction of the recent volcano-tectonic evolution of the Naples area has been carried out. It allows us to establish a model for estimating volcanic hazards in the Phlegrean Fields. The dated terrestrial pyroclastic deposits have been recovered from cores collected in the central Tyrrhenian Sea. The land-sea correlations allow us to refine the chronology of the marine oxygen isotope records for the past 60,000 a.