Alteration of organic matter in sinking particles
As part of MARUM project GB1, we study alteration processes affecting orgnic matter during sinking through the water column. We will focus on changes of biomarker abundances and assemblages during sinking as well as possible alterations of their isotopic signatures. In addition, we investigate the relative contribution of pre-aged resuspended material to particles entrained in nepheloid layers. The work will be performed as part as a highly interdisciplinary project.
Diagenetic alteration, preservation and isotopic changes of biomarkers
The focus of this research project is the investigation of diagenetic alteration processes, preservation potentials and changes in the isotopic signatures of organic compounds related to selective degradation/preservation.
Exposure to oxygen and residence time within oxygen replete environments are thought to be one of the major controlling factors of organic matter preservation. Previous studies have furthermore indicated that the preservation potential of different biomarkers varies with their structure and intrinsic resistivity to oxidization. Another potential influence on preserved radiocarbon ages of biomarkers is the preferential degradation of fresh autochthonous organic matter, while pre-aged material may be better preserved due to higher chemical stability or physical protection. We examine these processes by comparing surface sediments from oxygen-replete and dysoxic to anoxic locations. Various marine and terrigenous biomarkers are extracted and their compound-specific 14C and stable isotopic signatures are determined.
Conceptual diagram illustrating the effect of selective degradation of reactive versus labile and protected versus non-protected (fresh) organic compounds on their abundance and radiocarbon age in down-core sediment profiles (Mollenhauer and Eglinton, submitted 2006).
Triple-isotope measurements on chloropigments
Chlorophyll a is the major antenna pigment in virtually all aerobic photoautotrophic organisms on earth, produced by terrestrial higher plants as well as marine algae. First order derivative products of chlorophyll a include pheophytin a (demetallation), pyrochlorophyll a (decarbomethoxylation), chlorophyllide a (dephytolation), pyropheophytin a (demetallation, decarbomethoxylation), pheophorbide a (demetallation, dephytolation) and pyropheophorbide a (demetallation, decarbomethoxylation, dephytolation)
Transformation of chlorophyll a to its derivative products is likely caused by (autolytic) cell senescence, photo-oxidation, grazing, microbial or viral lysis and enzymatic or hydrolysis reactions. As a result of these multiple degradative processes, preservation of chlorophyll a is low. In marine environments only a minor fraction of the original chlorophyll biomass reaches the sediments.
However, the precise timescales of the single degradative transformation processes are yet unknown. Combined compound-specific δ15N, δ13C and Δ14C analysis of chloro- and pheopigments from marine sediments may provide a tool to determine these alteration timescales, and the stable isotopic composition provides information on the present and past biogeochemistry, i.e. physiology and ecology, of the photoautotrophic community during deposition. Insights from a core-top study will furthermore help establishing chlorophyll- and pheopigment-data as paleoproxies.