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Absolute Dating (absolute + dating)
Selected AbstractsEffects of diagenesis on the astrochronological approach of defining stratigraphic boundaries in calcareous rhythmites: The Tortonian GSSPLETHAIA, Issue 4 2008HILDEGARD WESTPHAL Since the establishment of astrochronology, calcareous rhythmites are frequently used as the basis of high-resolution chronostratigraphy. In particular for the Neogene, calcareous rhythmites serve as stratotypes and for absolute dating of stratigraphic boundaries (Global Stratotype Sections and Points , GSSPs). However, the exact mechanisms responsible for the formation of the rhythmic intercalation of lithologies in such successions are complex and not easily reconstructed. To a large extent this is the effect of diagenetic modifications of the original sediment. Here, two examples of Neogene calcareous rhythmites are studied; the Monte dei Corvi section and the Monte Gibliscemi section. The first is the GSSP location of the Serravallian,Tortonian boundary, whereas the second is an auxiliary stratotype for the same boundary. During the past years, astrochronological approaches were applied to these successions to considerably increase time resolution compared to the elaborated biostratigraphic database. The present study focuses on micropetrographic, trace element and clay mineralogical methods in order to gain a better understanding of the genesis of the rhythmites. In the Monte Gibliscemi section, sediment parameters that are robust against diagenetic change clearly reflect primary differences, i.e. cyclical environmental changes. In contrast, no clear primary signal is determined for the Monte dei Corvi section on a couplet scale, leaving the origin of the rhythm ambiguous. This impedes the interpretation of the latter and the comparability between the two successions, and also compromises any bed-by-bed correlation between the two. The unclear origin of the rhythmites of Monte dei Corvi introduces uncertainty into the applicability of astrochronology to this succession. [source] An AFLP clock for absolute dating of shallow-time evolutionary history , too good to be true?MOLECULAR ECOLOGY, Issue 22 2009D. EHRICH A major drawback of Amplified Fragment Length Polymorphisms (AFLP) as genetic makers for phylogeographic studies is their lack of a temporal dimension. In a recent publication in Molecular Ecology, Kropf et al. (2009) proposed a molecular clock for AFLP. In this comment we evaluate the proposed approach both theoretically and empirically. A linear increase with time is a prerequisite to use a genetic distance as molecular clock. Testing the relationship between genetic distance and time in the data of Kropf et al. (2009) for linearity revealed that the relationship was in fact not linear for their pooled data, as well as for one of the three species analyzed. Also, the relationship was not linear in two new species, where divergence times could be inferred from macrofossils. When applying the proposed molecular clock to data from eight species, dates obtained were plausible in some cases, but very improbable in others. The suggested genetic distance was also influenced by intrapopulation genetic diversity, leading to a potential bias. In the future, investigations of AFLP mutation rates combined with phylogeographic modelling may contribute to adding a time scale to the understanding of AFLP data. [source] An AFLP clock for the absolute dating of shallow-time evolutionary history based on the intraspecific divergence of southwestern European alpine plant speciesMOLECULAR ECOLOGY, Issue 4 2009MATTHIAS KROPF Abstract The dating of recent events in the history of organisms needs divergence rates based on molecular fingerprint markers. Here, we used amplified fragment length polymorphisms (AFLPs) of three distantly related alpine plant species co-occurring in the Spanish Sierra Nevada, the Pyrenees and the southwestern Alps/Massif Central to establish divergence rates. Within each of these species (Gentiana alpina, Kernera saxatilis and Silene rupestris), we found that the degree of AFLP divergence (DN72) between mountain phylogroups was significantly correlated with their time of divergence (as inferred from palaeoclimatic/palynological data), indicating constant AFLP divergence rates. As these rates did not differ significantly among species, a regression analysis based on the pooled data was utilized to generate a general AFLP rate. The application of this latter rate to AFLP data from other herbaceous plant species (Minuartia biflora: Schönswetter et al. 2006; Nigella degenii: Comes et al. 2008) resulted in a plausible timing of the recolonization of the Svalbard Islands and the separation of populations from the Alps and Scandinavia (Minuartia), and of island population separation in the Aegean Archipelago (Nigella). Furthermore, the AFLP mutation rate obtained in our study is of the same magnitude as AFLP mutation rates published previously. The temporal limits of our AFLP rate, which is based on intraspecific vicariance events at shallow (i.e. late glacial/Early Holocene) time scales, remains to be tested. [source] Desert environments: landscapes and stratigraphyGEOLOGY TODAY, Issue 5 2009Peter G. Fookes It is common to think of hot deserts, i.e. hot arid or dry lands, as areas of little rain situated in the middle parts of the world, that are simply ,just there'. However, most of the world's deserts have a long geological history, sometimes of 50 million years or more and ways have been developing for some time now, particularly from geomorphological studies, of not only erecting the law of superposition of strata for the desert but also ,absolute' dating. The authors have often worked commercially in deserts world-wide but their recent experiences in the Oman have brought home to them the excellent work that has been going on in the last two or three decades in evaluating the geological history of deserts. The Oman experience is described in a feature in the next issue. [source] | ||||||||||