Orbital frequencies in the carbonate sedimentary record: distorted by diagenesis?

The most important archive of Earth’s climate change through geologic history is the sedimentary rock record. Rhythmic sedimentary alternations are usually interpreted as a consequence of periodic variations in the orbital parameters of the Earth. This interpretation enables the application of cyclostratigraphy as a very precise chronometer, when based on the assumption that orbital frequencies are faithfully recorded in the sedimentary archive. However, there are numerous uncertainties with the application of this concept. Particularly in carbonates, sediment properties such as mineralogical composition and fossil associations are severely altered during post-depositional alteration (diagenesis). We here point out that the assumption of a 1:1 recording of orbital signals in many cases is questionable for carbonate rhythmites. We use computer simulations to show the effect of diagenetic overprint on records of orbital signals in the carbonate record. Such orbital signals may be distorted in terms of frequency, amplitude, and phase by diagenetic processes, and cycles not present in the insolation record may emerge. This questions the routine use of carbonate rhythmites for chronostratigraphic datin

Abell 2384: the galaxy population of a cluster post-merger

We combine multi-object spectroscopy from the 2dF and EFOSC2 spectrographs with optical imaging of the inner 30'x30' of A2384 taken with the ESO Wide Field Imager. We carry out a kinematical analysis using the EMMIX algorithm and biweight statistics. We address the possible presence of cluster substructures with the Dressler-Shectman test. Cluster galaxies are investigated with respect to [OII] and H{\alpha} equivalent width. Galaxies covered by our optical imaging observations are additionally analysed in terms of colour, star formation rate and morphological descriptors such as Gini coefficient and M20 index. We study cluster galaxy properties as a function of clustercentric distance and investigate the distribution of various galaxy types in colour-magnitude and physical space. The Dressler-Shectman test reveals a substructure in the east of the 2dF field-of-view. We determine the mass ratio between the northern and southern subcluster to be 1.6:1. In accordance with other cluster studies, we find that a large fraction of the disk galaxies close to the cluster core show no detectable star formation. Probably these are systems which are quenched due to ram-pressure stripping. The sample of quenched disks populates the transition area between the blue cloud and the red sequence in colour-magnitude space. We also find a population of morphologically distorted galaxies in the central cluster region. The substructure in the east of A2384 might be a group of galaxies falling onto the main cluster. We speculate that our sample of quenched spirals represents an intermediate phase in the ram-pressure driven transformation of infalling field spirals into cluster S0s. This is motivated by their position in colour-magnitude space. The occurrence of morphologically distorted galaxies in the cluster core complies with the hypothesis of A2384 representing a post merger system.Comment: 14 pages, 18 figures, A&A accepte

New data on Austroalpine Liassic Ammonites from the Adnet Quarries and adjacent areas (Salzburg, Northern Calcareous Alps)

The biostratigraphic studies of ammonites from the Adnet quarries allow to propose a set of 11 horizons or levels for the Upper Austroalpine Late Hettangian–Sinemurian. The originality of the faunal assemblages [e.g. Adnethiceras adnethicus (HAUER), Gleviceras doris sensu PIA] and the discontinuity of the biostratigraphical sequence make local comparisons and correlations with NW Europe, the Middle Austroalpine and the Apennines rather difficult. By contrast, the Pliensbachian fauna from Wetzsteingraben presents a clear faunal homogeneity with the other studied regions of the Upper Austroalpine. Moreover, they well integrate with the Euroboreal and Tethyan standard biostratigraphical framework

Radiolarian faunal characteristics in Oligocene of the Kerguelen Plateau, Leg 183, Site 1138

Three sites from Ocean Drilling Program (ODP) Leg 183 (Kerguelen Plateau) have been analyzed to document faunal change in high-latitude radiolarians and to compare the faunal change to Eocene-Oligocene climatic deterioration. Radiolarians are not preserved in Eocene sediments. In Oligocene sediments, radiolarian preservation improves in a stepwise manner toward the Miocene. A total of 115 species were found in lower Oligocene samples from Site 1138; all are documented herein. Radiolarian preservation is presumably linked to productivity triggered by climatic cooling during the early Oligocene. Similar patterns of improving preservation through the Eocene/Oligocene boundary are documented from several Deep Sea Drilling Project and ODP sites in the Southern Ocean, indicating a general pattern. In contrast to the Southern Kerguelen Plateau, however, proxies for productivity are more divergent at Site 1138 (Central Kerguelen Plateau). Whereas carbonate dissolution, as indicated by poor preservation of foraminifers and common hiatuses, is very pronounced in the upper Eocene-lowermost Oligocene, the quality of radiolarian and diatom preservation does not significantly increase until the uppermost lower Oligocene. Multiple measures of radiolarian diversity in the Oligocene from Site 1138 closely parallel radiolarian preservation, indicating that preserved radiolarian diversity is controlled by productivity