Biostratigraphy and Paleoenvironment in the Neogene of the High Northern Latitudes - Insights from the palynomorph record of ODP Hole 907A in the Iceland Sea

Abstract

The Neogene is of crucial importance for our understanding of Earth s climate evolution as it was finally pushed into the cold Quaternary climate with its major glaciations in the high northern latitudes. Notwithstanding, the response of the high northern latitudes to this Cenozoic transition from the Greenhouse into the Icehouse world yet remains poorly constrained due to the virtual absence of calcareous microfossils in high latitude Neogene sediments. This thesis examines the influence of the long-term climate deterioration on Miocene to Pliocene marine palynomorph assemblages (dinoflagellate cysts, prasinophyte algae, acritarchs) of ODP Hole 907A in the Iceland Sea, a sensitive area physically close to the growing Greenland ice-sheet, which experienced the effects of migrating ocean currents and sea-ice cover. Based on the pristine paleomagnetic record of Site 907, the principal aim of this thesis is to enhance the biostratigraphic application of organic-walled microfossils for (supra)regional correlations in these largely biogenic carbonate free sediments, and to utilize them for paleoenvironmental reconstruction in order to provide new insights into the Neogene history of the Iceland Sea. This thesis provides absolute age control on a suite of twenty-four dinoflagellate cyst and two acritarch bioevents for the first time in the Neogene of the high northern latitudes. These magnetostratigraphically-calibrated events have been compared across the North Atlantic and adjacent seas to critically evaluate their potential for biostratigraphic correlations within the high latitudes, but also with the mid- and low latitudes. Results show that seven species are useful for supraregional biostratigraphic correlations within the Nordic Seas, and four allow integrating the latter with the mid-latitude North Atlantic. Furthermore, this study highlights four species potentially useful for correlating mid-latitude sequences to the Central Arctic Ocean. Although an additional number of species appears to be valuable on a broader (million year) timescale, most events are clearly asynchronous indicating a strong latitudinal control even in the warmer Miocene. In a second study, the global paleobiogeographic distribution of the extinct Batiacasphaera micropapillata complex has been analysed in order to utilize it for future paleoenvironmental reconstructions. This major component of Neogene palynomorph assemblages in the high northern latitudes is a warm to cool-temperate species complex that may sustain seasonal sea-ice cover. It has an outer-neritic to oceanic affinity, a tolerance for somewhat higher salinities, and is apparently well adapted to enhanced nutrient availability and the prevailing light regime in the high latitudes. Its Late Miocene decline and its stratigraphically important highest occurrence in the Early Pliocene are closely related to climate deterioration towards the intensification of Northern Hemisphere glaciation. Based on distinctive changes in palynomorph assemblage composition, which are supplemented by alkenone sea surface temperature estimates, the paleoenvironmental evolution of the Iceland Sea has been reconstructed for the c. 15 2.5 Ma interval. Results indicate a close coupling of marine palynomorphs and sea surface temperatures not only to the long-term Neogene climate cooling that occurred after the Middle Miocene Climate Optimum, but also to local changes in ocean circulation due to the Fram Strait opening, sill depth variations of the Greenland-Scotland Ridge, and the shoaling and subsequent closure of the Panama Isthmus. Moreover, a distinct variability in the palynomorph assemblages reflects short-term climate variations that partly coincide with the short-lived Miocene glaciation events (Mi-3 to Mi-6), and the progressive strengthening and freshening of a proto-East Greenland Current towards modern conditions in the Late Miocene and Early Pliocene. The impoverished Pliocene assemblages suggest that major adjustments, with conditions seriously effecting marine productivity have already been established in the Iceland Sea well before the marked expansion of the Greenland ice-sheet at 3.3 Ma and the subsequent intensification of Northern Hemisphere glaciations. In conclusion, this thesis provides an improved, magnetostratigraphically-calibrated temporal framework for future analyses of high and mid-latitude Miocene to Pliocene sequences, first estimates on the paleoecology of an important Neogene species complex, and ultimately new insights into the paleoenvironmental evolution of an area sensitive for global climate change.