English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Eighty samples of carbonate from Leg 47B were analyzed for O and C. Data show the isotopic profiles from Cretaceous upward are similar to those obtained in the Pacific and southern Atlantic oceans, especially for the EoceneOligocene transition. Isotope data, however, indicate differences in the paleoceanolographic setting. Isotope ratios were measured on bulk carbonate samples from Hole 398D (see values on Table 1), for comparative purposes with other DSDP legs (i.e., Legs 16 and 44) and for testing the data obtained on such materials for their value as paleoecological indicators. Foraminifers are the most common material used for isotope studies, and the comparison between planktonic and benthic species leads to useful information (Shackleton, 1967). However, the coccoliths which form the bulk of carbonate material in most samples also have been tested (Margolis et al., 1975) and have shown isotope ratios similar to those for planktonic foraminifers. In some cases, data obtained on bulk material also were presented (Douglas and Savin, 1973), which fit well with foraminiferal isotopic values. Eighty bulk carbonate samples from Leg 47B were analyzed, using classical techniques; reproducibility is about 0.1 per mil for O and somewhat better for C. When extraction of foraminifers was possible, their analysis was performed; those data will be analyzed by Vergnaud-Grazzini (this volume) to provide a comparison with bulk carbonate data. The isotope values for bulk carbonates are the result of several factors: 1) The composition of initial fossil material, either planktonic or benthic. The latter never constitutes an important fraction of the material. In all the material studied from Leg 47B, we observed that most of the figured calcite were coccoliths. For fresh material, it may be concluded that isotope analyses reflect the composition of the coccolith component. 2) Mixing of detrital material. This is easily verified by paleontologists and occurred in Cretaceous beds of Leg 47B (see Sigal, this volume). Biasing of the isotope record also may be of consequence when analysis is performed on selected fossils. 3) Dissolution and recrystallization. Dissolution of fossil tests occurs during their fall towards the sea floor and prior to their burial. (This effect on the preservation of the isotopic record is presently under investigation in various laboratories.) Subsequently, authigenic calcite forms and its importance grows with the depth of the burial. It is generally different in its characteristics from biogenic calcite, especially for isotope contents, because the temperature and isotope composition of interstitial waters may be completely different from those where the carbonate originally formed. Before the isotope analyses were made, samples of the oldest sediments were studied petrographically and appeared to be highly recrystallized. Higher in the stratigraphic series (Danian and Eocene chalks), the recrystallized carbonate component appeared to be less important, and it is only in the uppermost samples (Core 2) that recrystallized material is absent. The sequence of recrystallization seems to be the same as that observed in other sediment series, for instance in Legs 2 and 17 (Lancelot et al., 1972; Schlanger et al., 1973). Although the original O record is sensitive to such diagenesis, the same is not true for C which retains its original value more easily. In analyzing the data, several sets of points appear on the scatter diagram C/O (Figure 1), but the most discernible gap appears during the Eocene; the transition being approximately in Core 19 where a shift occurs in O values (Figure 2). Such an event already has been observed in most isotope studies of foraminifers at the Eocene-Oligocene transition, with some possible variations in its precise stratigraphic position (Savin et al., 1975; Douglas and Savin, 1971; Kennett and Shackleton, 1976) in both the Pacific and the southern Atlantic oceans. In every case, the shift is about l ‰ delta unit. One cannot ascertain if the transition here is later or earlier than at other locations, but it may be compared to the one observed by Shackleton and Kennett (1975a, b) at subantarctic sites (Campbell Plateau). It appears highly improbable that such a synchronism could be due to diagenesis for such widely spaced locations. More likely, its origin is a significant, geodynamic event. This could be either the intrusion of antarctic waters (Shackleton and Kennett, 1975a) to these northern latitudes, or the easier communication of arctic waters with the Atlantic Ocean. Regional tectonic events, such as the rotation of Spain, also seem to be synchronous with the isotopic change in carbonates. Other mineralogical indicators (Chamley et al., this volume) indicate a drastic change in water circulation as well as in continental climatic conditions at the same time. The following remarks can be made relative to the different episodes of carbonate sedimentation: 1) Pre-Cenomanian carbonates: Sedimentological and micropaleontological studies (this volume) show the multiple origins of carbonate and that recrystallization is important. However, oxygen isotope values differ from those observed in contemporary beds from Legs 11 and 44 in the western Atlantic (Brenneke, 1977; Létolle et al., 1978), where much lower O values are interpreted as being due to recrystallization in a high geothermal gradient setting and/or hydrothermal influences not evident at Hole 398D. 2) Campanian to late Eocene: Carbonate material is much less recrystallized in this interval. From Cores 48 to 22 a slow

Oxygen 18 and Carbon 13 Isotopes from Bulk Carbonate Samples, Leg 47B