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Paleocene-eocene dinoflagellate cysts from Africa, India and Belgium

Book - Dissertation

The early Paleogene was an epoch with generally much higher mean global temperatures and concentrations of greenhouse gasses than today. Superimposed on the general warm early Paleogene climate are several sudden transient (10-100 kyr scale) and extreme climatic events known as hyperthermals, which are characterized by a.o. rapid changes in surface and deep-sea temperatures, ocean circulation patterns, surface ocean acidification, precipitation and productivity. This makes the early Paleogene an extremely interesting period, as it likely represents the most recent and best analogue for the high CO2 greenhouse the world is trending toward. Additionally, most of the modern vertebrates have their origin and begin to diversify during the early Paleogene, including the iconic APP mammal taxa: Artiodactyls (even-toed ungulates, e.g. sheep, goats), Perissodactyls (odd- toed ungulates, e.g. horses, zebras) and Primates that appear almost simultaneously in all three Northern hemisphere continents around the Paleocene-Eocene boundary. The initial objective of this dissertation was to join and support the PalEurAfrica project, funded by Belgian Science Policy Office, which aimed to further evaluate the hypothesis that modern vertebrates and especially the APP taxa likely originated during the late Paleocene in tropical habitats farther south. These hypotheses unfortunately remain largely untested due to the lack of early Paleogene fossil data from Africa, especially from Sub-Saharan Africa. Therefore, the Paleogene fossiliferous localities of western central Africa (i.e. Angola and Congo) are a critical link in understanding the wider Cenozoic African faunal dynamics. The idea was to focus on the dinoflagellate cyst assemblages of selected localities with the aim of carrying out detailed biostratigraphic and paleoecological investigations in order to construct a robust stratigraphic and paleoecological framework to evaluate the vertebrate and invertebrate occurrences at each of these sites. The Landana locality in the Cabinda exclave in Angola was the first African site that was selected, with the specific objective to carry out a detailed, and if possible high-resolution, dinoflagellate cysts analysis of the available samples. Despite the relatively limited stratigraphic resolution and the excessive amounts of amorphous organic matter in the samples we were able to perform a successful and extensive analysis of the Landana locality. Analysis of the Landana samples revealed a diverse dinoflagellate cyst assemblage comprising more than 90 dinoflagellate cyst taxa from the ?Danian/early Selandian to Eocene/early Oligocene. Additionally, dinoflagellate cyst assemblages proved highly variable, with clearly defined periods of heterotrophic and autotrophic taxa dominance. These periods are likely connected to changes in terrestrial input, upwelling intensity, etc. Our work at Landana represents the first extensive sub-equatorial African Paleogene dinoflagellate cyst record ever. This provides a much needed first real glimpse into a region for which little to no information is available and allowed us to further refine the initial age constraints. This ultimately resulted in the establishment of a relatively robust biostratigraphic scheme that constrains the numerous vertebrate and invertebrate occurrences throughout the Landana section. Our record represents a major first step towards a reference Paleocene U+2013 Eocene dinoflagellate cyst record for sub-equatorial regions. After the completion of the Landana analysis our focus shifted towards time-equivalent sites from other parts of the world. This on one hand forced a shift away from the PalEurAfrica project, but on the other hand provided us with the opportunity to participate in a project in India and take a deep dive into the historic Belgian Ypresian Clays. The Indian subcontinent similarly long seemed to hold great promise in the pursuit of the birthplace of modern vertebrates since the subcontinent collided with Asia roughly at the same time these taxa appeared. With this in mind our Indian-American-Belgian team extensively explored several open pit mines in the Gujarat Province of India during the last decade. This in hopes of discovering vertebrate fossils that can increase our knowledge of the early Paleogene faunas of the Indian subcontinent. The most recent vertebrate bearing deposits were discovered at the Tadkeshwar mine. These discoveries rekindled the need for a conclusive age for the vertebrate and mammal bearing deposits of the Cambay Shale Formation, which has been a longstanding point of debate. In this regard forty-five samples collected from the Cambay Shale Formation at the Tadkeshwar were analysed with the idea of performing a detailed dinoflagellate cyst analysis in the hopes of further fine-tune the (bio)stratigraphic framework. This unfortunately proved to be an extremely difficult task due to the enormous amounts of amorphous organic matter. The few taxa that were recorded show a general semblance with several records from IX India and Pakistan that straddle the Paleocene U+2013 Eocene boundary which very tentatively suggests that the deposits might have been deposited around or shortly after the Paleocene-Eocene transition. No important lowest occurrences were recorded which would be in line with our suggestion that the deposits between the two major lignite seams at both mines represent a single land mammal age, and not an interval of up to five million years as was suggested by other authors. The recorded low diversity assemblage as well as prominence of Polysphaeridium cysts generally fits perfectly with the inferred at times near-shore very restricted marine depositional environment of the deposits in the Vastan and Tadkeshwar mines. Most of the early Eocene hyperthermal research has focused on the Arctic region and deep-sea settings and has resulted in a general lack of detailed biotic data from shelf settings. The Kallo site (Antwerp, Belgium), the reference site for the Belgian edge of the southern North Sea Basin, is one of the key sites to help mitigate this gap in our knowledge. A recent update of the regional stratigraphic framework, complemented with stable carbon isotope (U+03B413C), foraminiferal biostratigraphy and XRF revealed the existence of several of the post-Paleocene Eocene Thermal Maximum (PETM) hyperthermal events, including Eocene Thermal Maximum 2 (ETM-2) and ETM-3 at Kallo. This, combined with the wealth of newly published research on the early Eocene global climate and paleoecological preferences and taxonomy of dinoflagellate cysts, provided an ideal opportunity for a major re-evaluation of the dinoflagellate cyst record in the former Ypresian type area. With this in mind two sites were selected, the Kallo borehole (Antwerp, Belgium; early Ypresian) and the Mullier Quarry (Mouscron, Belgium; middle Ypresian), which were located on the midshelf of the southern edge of the intracratonic North Sea Basin during the early Eocene. The aim was to significantly enhance any existing resolution, re-evaluate and update the existing taxonomy as well as perform a detailed paleoecological analysis on the recorded dinoflagellate cysts assemblages. This will help to evaluate the changes affecting the basin during that period and evaluate and most likely significantly enhance our knowledge on the effects of the post-PETM hyperthermals and general climate perturbations on the dinoflagellate cyst assemblages. At Kallo more than 90 dinoflagellate cyst taxa were recorded, comprising a very diverse typical early Ypresian assemblages. The significantly enhanced resolution highlighted that dinoflagellate cyst lowest occurrences are far more staggered and gradual than was suggested by previous authors, with new taxa primarily first appearing during the 3rd order transgressive system tract. An updated dinoflagellate cyst zonation was proposed, including some regional subzones with potential stratigraphic value in the southern North Sea Basin as well as further afield. The general dinoflagellate cysts associations suggest relatively (open) marine conditions for the entire studied interval. The effects of H1 (ETM-2) on the dinoflagellate cysts seem relatively small and are likely trumped by the mfs of the 3rd order transgression affecting the North Sea Basin at the time. The origin of the major Phthanoperidinium influx that is observed during H2 remains unclear and could reflect climatic and or hydrological changes associated with H2 (ETM-3) or alternatively enhanced basinward transport of near-shore taxa. All in all, we observed that the effects of the hyperthermals in the studied interval at Kallo are relatively small and largely dictated by the roughly concurrent sea-level fluctuations. Analysis of the enigmatic secondarily decalcified Aalbeke Member at Mullier Quarry revealed relatively diverse dinoflagellate cysts assemblages, with a lot of different groups and complexes comprising a significant portion of the assemblage. The age of the Aalbeke Member, which equates to middle of NP12 and more specifically the Ochetodinium romanum zone of De Coninck (1991), was herein confirmed. Dinoflagellate cyst assemblages seem to point to (seasonally) stratified conditions and periods of enhanced nutrient availability and low salinity conditions. This could perhaps reflect the effects of the Early Eocene Climatic Optimum (EECO) in the deeper parts of the southern edge of the North Sea Basin or the limited stratigraphic resolution and the lack of high- resolution assemblage data for the transition into and out of the Aalbeke Member interval.
Number of pages: 1
Publication year:2020
Accessibility:Open