Abstract
The present work represents an attempt to extrapolate new information about sea surface temperature, early diagenesis and sea floor oxic condition, from the finest fraction of Chalk. The work is based on the analysis of samples from two shallow cores, Stevns-1 and Dalbyover-1, both from the Danish Basin. The analyses are based on the combination and integration of conventional isotope paleothermometry (δ18O), clumped isotope paleothermometry (Δ47), trace elements distribution, pyrite framboids size distribution and Sulphur isotopes. Chalk has been disaggregated using a conservative method based on a combination between freeze and thaw cycles and sonication. The ooze was then separated using wet sieving down to 10μm and centrifuged to isolate the fraction 5-10μm (coccolith-enriched) from the 1-5μm (micarb-enriched). The results from the coccolith material indicate a sea surface temperature for the Late Campanian – Maastrichtian of 24-30ºC with an average of 25.9°C ±2. This is 8-10ºC higher than reported temperatures based on bulk δ18O data from the same core but comparable, in term of both trends and absolute values, with estimates of Late Cretaceous tropical sea surface temperatures. The Δ47 of the smaller fraction (1-5μm) show that this material precipitated at temperatures between 14°C and 21°C (average 16±1°C). These temperatures are colder than the coccolith counterpart but potentially in line with sea floor temperature at 400-600m of depth. Considering this and the general agreement between the Δ47 and δ18O temperature, these particles are interpreted as abiotic calcite precipitated close to the seafloor. Small differences in trace element between the two sets of samples seem to suggest calcite precipitation connected with early aragonite dissolution as final product of patchy redox mosaic of organic matter degradation. The last chapter of this thesis focuses on the marl layers of the Rørdal Mb.
In the marly units, framboids are always present, but size and size distribution are not constant indicating different degrees of dysoxia during the deposition. Since no evidence of lamination is found in connection with the occurrence of framboid, we suggest these events to be quite short-lived and therefore overprinted by reworking by successive generations of burrows.
In the marly units, framboids are always present, but size and size distribution are not constant indicating different degrees of dysoxia during the deposition. Since no evidence of lamination is found in connection with the occurrence of framboid, we suggest these events to be quite short-lived and therefore overprinted by reworking by successive generations of burrows.
| Original language | English |
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| Number of pages | 149 |
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| Publication status | Published - Jun 2019 |