Organic matter in Late Devonian sediments as an indicator for environmental changes
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Beschreibung
In order to reveal the peculiarities distinguishing the geochemical differentiation of the
organic matter during the Late Devonian, sediment cores from the Rhenish, Michigan and
Appalachian Basins have been investigated. In the late Frasnian two major extinction
events took place: the Lower and the Upper Kellwasser Event. The latter was one of the
most striking mass extinction events in the Phanerozoic; (Sepkoski, 1996; Walliser,
1996). Although the effects of these events on the invertebrate fauna are well known
(Schindler, 1990), information related to the evolution of the phytoplankton is limited.
Sedimentary rocks from the above cited profiles were inspected with optical microscopy
and investigated for organic geochemical bulk parameters, selected trace metals,
carbon and nitrogen isotopy as well as extractable biomarkers.
To obtain reliable results from the geochemical analyses, the processing of nonweathered
samples with low maturity is required. The results of the maturity investigations
classified the material as - at the most - only marginal mature and a redistribution
of soluble organic matter has been considered very unlikely.
The Devonian time was outstanding due to the evolution of the land plants which
have influenced nearly all environmental factors such as climate, sea level and redox
conditions. The covering of terrestrial areas by plants caused drastic transformations
in respect to geosphere, hydrosphere and atmosphere (Walliser, 1996). Depending on
the input of nutrients into the oceans and on the subsequent phytoplankton bloom, the
palaeoredox conditions and the amount of burial of organic carbon have been affected.
The perturbations of the carbon dioxide level in the atmosphere and in the water modified
the climate and therefore the sea level which influenced the water circulation and oxygen
supply and thus the living conditions within the oceans.
Information regarding the sources of organic material specified whether it originated
from terrestrial or marine components and consisted of bacteria or algae. Pristane/
phytane ratio and selected trace element ratios unveiled the redox conditions within
the sediments. Aryl isoprenoids gave the opportunity to understand the palaeoredox conditions
within the water column. Moreover, the nitrogen isotopy delivered information
regarding the nutrient supply. Another isotopic analysis (δ13Corg) was focused to test
the global character of the positive carbon isotope shifts within numerous other Upper
Devonian profiles. Exploiting these results, the productivity of the oceans and the environmental
conditions during the Late Devonian have been successfully reconstructed.
Furthermore, possible causes for the mass extinction have been discussed in more detail.
Autor*in
Marina Kloppisch
Themen in »Organic matter in Late Devonian sediments as an indicator for environmental changes«
Environmental changes Late Devonian Organic matter