PhD defence: Mads Engholm Jelby (virtual/hybrid)

Mads Engholm Jelby defends his thesis,

Depositional dynamics and holostratigraphy of the Lower Cretaceous Rurikfjellet Formation in Arctic Svalbard: From local storm sedimentation to global carbon cycling

Video link

Associate Professor Kresten Anderskouv, IGN
Professor Lars Stemmerik, GEUS
Professor Snorre Olaussen, UNIS, Svalbard

Assessment Committee:
Associate Professor Christian Haug Eide, University of Bergen
Senior Research Scientist Erik Skovbjerg Rasmussen, GEUS
Professor MSO Christian J. Bjerrum (chair), IGN

The Early Cretaceous epoch (145.7100.5 million years ago) was characterized by major changes related to plate tectonic reconfigurations, rapid and high-amplitude sea-level oscillations, and in particular dynamic oceanographic and climate fluctuations. Long-lived, humid greenhouse conditions prevailed, with dinosaurs, pterosaurs and alike roaming the Earth. The climate caused giant storms of today-unseen size and magnitude to occur relatively frequently, resulting in the common deposition of anomalously large quantities of sand in shallow-marine environments (expressed as sandy storm beds called 'tempestites'). In addition, the global carbon cycle was characterized by significant perturbations affecting the entire ocean–atmosphere system. However, neither the depositional mechanisms during storms, nor the carbon-cycle dynamics, in the Early Cretaceous are fully understood.
Environmental fluctuations and geochemical circulation are amplified at the poles, making the present-day Arctic areas ideal for studying the Early Cretaceous in time and space. Consequently, this PhD study set out to conduct a detailed geological study of the lowermost Cretaceous Rurikfjellet Formation in the High-Arctic archipelago of Svalbard, by integration of detailed sedimentological and various stratigraphic disciplines.
Investigated sandstone tempestites show that ancient marine storm deposition was commonly controlled by a combination of relatively steady and highly unsteady waves, downwelling storm flows, and flows (hyperpycnal) generated directly from plunging rivers during coupled storm-floods. Mapping out these process relationships in the Rurikfjellet Formation across Spitsbergen further indicates that general storm-depositional models require reevaluation for low-gradient marine settings. In addition, the carbon cycle of the northern high-latitude areas, including Svalbard, experienced long-term (ca. 30 million years) decoupling from and recoupling to the prevailing global signal. These changes were mainly caused by oceanographic reconfigurations of the northern high-latitude basins due to global eustatic sea-level changes. Notably, northern basinal isolation was followed by basin reconnection to lower latitudes, which resulted in a global carbon-cycle re-equilibration and resulting expression of the so-called Weissert Event.

A digital version of the PhD thesis can be obtained from the PhD secretary Anne Marie Faldt