Quantitative assessment and palaeoecologic significance of Turonian c-dinocysts assemblages in a high-resolution stratigraphic basin to basin margin transect
Zusammenfassung der Projektergebnisse
We investigated an Upper Turonian high frequency 3rd order sequence (Hyphantoceras Sequence, ca. 400 ky) for temporal and geographical changes in its calcareous dinoflagellate cyst (c-dinocyst) flora from the basin margin (Bohemia) into the fully pelagic setting (Chalk Facies) over a distance of ca. 1000 km. Systematics of Turonian c-dinocysts needed to be clarified and proximal-distal floral trends versus relative sea-level were mapped. Distinct character traits such as P. sphaerica/P. ovalis ratio, coarse/fine crystalline walls in Obliquipithonelloidea, ratio sphaerical/elongated forms, vertical size distribution and wall thickness were expected to provide additional information. Unfortunately, the proximal section (Bohemia) did not provide a single c-dinocysts, why we concentrated on the Salzgitter-Salder (Lower Saxony, Germany) and Bridgewick section (Sussex, England). The ratio coarse/fine crystalline walls in Obliquipithonelloidea was not measured due to the lack of Obliquipithonelloidea. The bulk of the material occurred in the 32µm fraction, used for our analyses. Ca. 16.000 cdinocysts were analysed (SEM) and documented on 2.646 photos. From 55 thin sections, 84 sets of measurements (wall thickness) were made from Salder (100 round, 100 oval), resulting in 16.800 measurements. The results are summarized below. - a literature review of c-dinocyst species number in the Cretaceous of the Lower Saxony Basin shows that diversity decreases with the progressive flooding of the European shelf due to the Mid-Cretaceous transgression. Progressive nutrient depletion caused a diversity breakdown. - Highest diversity (Hauterivian 32 taxa, Albian 38 taxa) face a Lower/Upper Turonian diversity loss (7 taxa), why the interval reflects a nutrient crisis in the Central European Cretaceous sea. - both in Salder and Bridgewick, the flora is dominated over 95 % by Pithonella sphaerica, P. ovalis and P. spiralis (new form). P. discoidea, P. cardiiformis, Normanida circumperforata, Pirumella sp.) are accessory. - the ratio P. sphaerica/P. ovalis enables the recognition of low relative sea-level (P. sphaerica peak). Maximum flooding zones (mfz) are characterized almost exclusively by P. ovalis. In accordance if the higher abundance of P. ovalis in more distal settings (Bridgewick). - the cumulative c-dinocyst composition, CCC, shows distinct and redundant cyclic floral changes during sea-level change. The TST is always reduced to a thin sheet of sediments and characterized by a shift towards species decrease and a positive P. ovalis shift. The mfz is depauperate of taxa and P. ovalis-dominated. The HST shows an increase of species, and the abundance of P. sphaerica and P. ovalis decrease in favour of P. spiralis and others. The stacking pattern of floral cycles suggest that it is possible to recognize an early HST (“stable cycling” geometry) and a late HST (“shifting cycles” geometry) based on the CCC. - the floral response is seen to reflect increasing nutrient supply during regression instead of increased internal cycling due to stronger water agitation during progressive sea-level fall. - the absolute lack of c-dinocysts in Ùpohlavy and the ostracod-foraminifera dominance suggest marine mesotrophic to eutrophic conditions, unfavourable for c-dinocyst mineralizing dinoflagellates. - the size curves of P. phaerica/spiralis/ovalis is indistinct without relation to sea-level changes. - the wall thickness of oval and spherical c-dinocysts is not related to cyst size. Instead, an increase of wall thickness during the lower Upper Turonian occurs. It is possible that this is a response to a cooling period in this interval described in the literature.
Projektbezogene Publikationen (Auswahl)
- 2012. Taxonomy and palaeoecologic significance of Turonian calcareous dinoflagellate cysts from the Upper Cretaceous of NW Germany (Salder Formation, Upper Turonian). Centenary Meeting of the Paläontologische Gesellschaft, Programme, Abstracts, and Field Guides, 24.09. – 29.09.2012, Museum für Naturkunde Berlin, p. 204
Zobel, K. & Wiese, F.
- 2014. Palaeoenvironmental significance of Turonian boreal calcareous dinoflagellate cyst assemblages (NW Germany Lower Saxony). 9th International Symposium on the Cretaceous System, Ankara, 01. – 05. September 2013, Abstract Volume, p. 12
Zobel, K., Keupp, H. & Wiese, F.
- 2015. Calcareous dinoflagellate cysts and the Turonian nutrient crisis - Data from the upper Turonian of the Lower Saxony Basin (northern Germany). Cretaceous Research 56, 673-688
Wiese, F., Zobel, K. & Keupp, H.
(Siehe online unter https://doi.org/10.1016/j.cretres.2015.06.007)
