Stable isotopic and carbonate cyclicity in deep sea sediments from different DSDP Holes

Oxygen and carbon isotopic variability of the dominant (<38 µm) carbonate fraction within bedded, organic-carbon rich Lower Cretaceous sediment intervals from various DSDP sites are closely correlated with preservational changes in the carbonates. Isotopic fluctuations are absent where carbonate contents vary little and where the carbonate fraction is dominated by biogenic phytoplankton remains. Within each of the studied intervals oxygen and carbon isotopic ratios become increasingly more negative in samples with carbonate contents higher than about 60% in which the proportion of diagenetic microcarbonate increases rapidly. Carbon isotopic ratios show a trend towards positive values in samples with carbonate contents of less than 40% and strong signs of dissolution. The taxonomic composition of nannofossil assemblages varies little within single intervals, despite significant differential diagenesis among individual beds; this points towards ecological stability of oceanic surface waters during the deposition of alternating beds. Bedding is, however, closely related to changing bioturbation intensity, indicating repeated fluctuations of the deep-water renewal rates and oxygen supply. Various microbial decomposition processes of organic matter leading to bed-specific differential carbonate diagenesis resulted in an amplification of primary bedding features and are considered responsible for most of the observed fluctuations in the stable isotopic ratios and carbonate contents.

Determination of absolute coccolith abundances in deep-sea sediments by spiking with microbeads and spraying - SMS-method (Table 1)

A quick new method is described for the quantification of absolute nannofossil proportions in deep-sea sediments. This method (SMS) is the combination of Spiking a sample with Microbeads and Spraying it on a cover slide. It is suitable for scanning electron microscope (SEM) analyses and for light microscope (LM) analyses. Repeated preparation and counting of the same sample (30 times) revealed a standard deviation of 10.5%. The application of tracer microbeads with different diameters and densities revealed no statistically significant differences between counts. The SMS-method yielded coccolith numbers that are statistically not significantly different from values obtained from the filtration-method. However, coccolith counts obtained by the random settling method are three times higher than the values obtained by the SMS- and the filtration-method.