Soils as Sinks for Biomarkers and Eventual Sources of Paleoecological Information: Exploring n-Alkane Distributions and Carbon Isotope Ratios in Modern Ecosystems
Abstract
Plant wax biomarkers and their isotope ratios in terrestrial sediments are frequently used to reconstruct vegetation and hydroclimate in past ecosystems. Typically, this includes analyses of biomarkers from paleosols, alluvium, or lacustrine sediments. The first and often last stop for plant waxes in the terrestrial sedimentary record is the underlying soil in which the plant is growing. Although the transit distance from plant to soil is short, the residence time can be quite long. Furthermore, soils are biologically active, especially in the O and A horizons, leaving biomarkers prone to potential degradation or removal through advective processes of bioturbation and erosion. In order to interpret biomarker data from terrestrial sediments, it is essential to investigate biomarker systematics in modern soils.
We present n-alkane concentrations, molecular distributions, and compound specific carbon isotope data from modern soils to evaluate their ability to record vegetation composition (e.g., C3 vs. C4) and structure (woody cover). We find that n-alkyl lipid concentrations decline rapidly with depth in the soil, similar to TOC. We develop a method for reconstructing woody cover using n-alkane molecular distributions that are based on modern calibrations and include all available C3 andC4 grass data. This method has a distinct advantage over the carbon isotope-based method for reconstructing woody cover in that it can be applied to systems that predate the late Miocene expansion of C4 grasses, with the potential to reveal ecosystem structure over much of the Cenozoic in Africa. Finally, we evaluate carbon isotope enrichment factors between bulk soil and n-alkanes to improve isotope-based vegetation reconstructions. The enrichment factor ranges from about 5 to 15 ‰, with an average of 8 to 9 ‰ for the n-C31 alkane. The molecular distribution and carbon isotope data together show that biomarkers in modern soils reflect current vegetation composition and structure. These data provide a quantitative framework for reconstructing the proportion of C3 to C4 vegetation and woody cover in past environments using biomarkers in terrestrial sediments.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP31D1696U
- Keywords:
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- 0424 Biosignatures and proxies;
- BIOGEOSCIENCESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHYDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHY