Abstract. Plant wax n-alkane chain length distribution and isotopeshave been studied in modern ecosystems as proxies to reconstruct vegetationand climate of the past. However, most paleo-proxies focus on eitherconcentrations or isotopes, whereas both carry complementary information onthe mixing sources. We propose a multi-source mixing model in a Bayesianframework that evaluates both chain length distributions and isotopessimultaneously. The model consists of priors that include user-definedsource groups and their associated parametric distributions of n-alkaneconcentration and δ13C. The mixing process involves newlydefined mixing fractions such as fractional leaf mass contribution (FLMC)that can be used in vegetation reconstruction. Markov Chain Monte Carlo isused to generate samples from the posterior distribution of these parametersconditioned on both data types. We present three case studies from distinctsettings. The first involves n-C27, n-C29, and n-C31 alkanes in lake surface sediments of Lake Qinghai, China. The model provides more specific interpretations on the n-alkane input from aquatic sources than the conventional Paq proxy. The second involves n-C29, n-C31, and n-C33 alkanes in lake surface sediments in Cameroon, western Africa. Themodel produces mixing fractions of forest C3, savanna C3, andC4 plants, offering additional information on the dominant biomescompared to the traditional two-end-member mixing regime. The third couplesthe vegetation source model to a hydrogen isotope model component, usingbiome-specific apparent fractionation factors (εa) toestimate the δ2H of mean annual precipitation. By leveraging chain length distribution, δ13C, and δ2H data offour n-alkane chains, the model produces estimated precipitation δ2H with relatively small uncertainty limits. The new framework shows promise for interpretation of paleo-data but could be further improved by including processes associated with n-alkane turnover in plants, transport,and integration into sedimentary archives. Future studies on modern plantsand catchment systems will be critical to develop calibration datasets thatadvance the strength and utility of the framework.
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What plant is that? Chemotaxonomy from n-alkane molecular distributions of East African plants with implications for paleoecology
Plant wax n-alkanes serve as reliable biomarkers given their abundance, stability, and distribution in the sedimentary record. As a result, their utility as isotopic indicators of vegetation and hydroclimate is well-established. A less well studied aspect of plant n-alkanes is the use of their molecular distributions, or differences in the relative abundances of homologues, for chemotaxonomy. Limited plant n-alkane datasets from southern and western Africa suggest molecular distributions can differentiate C4 grasses from C3 woody vegetation. Here we examine a suite of plants from East Africa, where almost no plant biomarkers data exists from modern plants. In this study, over 100 samples of 19 species of plants were collected monthly from the Samburu National Reserve in Kenya from October 2001 to March 2003, across multiple growing seasons; n-alkane distributions and concentrations from both individual species and designated plant functional types (PFTs) - based on both photosynthetic pathway and growth form - were investigated. Previously published n-alkane data from western and southern Africa, or the "All Africa" dataset, were examined to further understand potential spatial differences in biomarker distributions. n-alkane distributions in both datasets vary in both individual species and within PFTs. Principal Components Analysis (PCA) was used to analyze distributions of n-alkanes in individual species and in PFTs, to determine the primary sources of variability. Results indicate that n-alkane distributions can be used to separate some individual species - namely, C4 grasses - and can be used to separate PFTs. C4 grasses and C3 woody vegetation were successfully separated in both datasets. Additionally, we found that n-alkane concentrations vary by four orders of magnitude across homologues and PFTs. A compiled African plant data set shows that C31 concentration is the most representative of the plant community for C4 grasses, C3 shrubs, and C3 trees and thus, is most ideal for stable isotope vegetation reconstructions. These data suggest that an organic geochemical approach to plant taxonomy is crucial to future biomarker applications for reconstructing vegetation distribution and structure in past ecosystems.
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- Award ID(s):
- 1757602
- NSF-PAR ID:
- 10091132
- Date Published:
- Journal Name:
- American Geophysical Union, Fall Meeting 2018, abstract #PP31D-1701
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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