ENVIRONMENTAL AND ECOLOGICAL CONSTRAINTS ON MOLECULAR AND ISOTOPIC SIGNATURES IN TERRESTRIAL ORGANIC CARBON

Open Access
Author:
Diefendorf, Aaron F.
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 08, 2010
Committee Members:
  • Katherine Haines Freeman, Dissertation Advisor
  • Katherine Haines Freeman, Committee Chair
  • Jennifer Macalady, Committee Chair
  • Scott W Wing, Committee Member
  • Lee Kump, Committee Member
  • Jason Philip Kaye, Committee Member
Keywords:
  • lipids
  • biomarker
  • fractionation
  • Carbon isotope
  • n-alkanes
  • Bighorn Basin
  • organic geochemistry
Abstract:
<p>Carbon isotope (&delta;<sup>13</sup>C) fractionation by plants during photosynthesis (&Delta;<sub>leaf</sub>) varies with environmental conditions and plant community. This suggests &delta;<sup>13</sup>C preserved in ancient terrestrial organic carbon encodes both climatic and ecological signals. Plant biomarkers—molecular fossils specific for plants—potentially provide community specific &delta;<sup>13</sup>C values and floral composition. Constructive interpretations of molecular and isotopic signatures of ancient plants require a more robust knowledge of quantitative patterns of &delta;<sub>leaf</sub> across environmental gradients and plant communities than have been available from the ecologic literature. Further, this approach requires isotope fractionation during lipid synthesis (&epsilon;<sub>lipid</sub>) and abundances patterns for <i>n</i>-alkyl lipids and diterpenoids (gymnosperm biomarker) and triterpenoids (angiosperm biomarker) be constrained across plant communities and growth conditions. </p> </p> <p> To addresses the role of climate and plant community in controlling carbon isotope fractionation during photosynthesis, my coauthors and I compiled 3,330 previously published leaf &delta;<sup>13</sup>C values (converted to &Delta;<sub>leaf</sub>) representing a wide range of environmental conditions and plant communities. Consistent with earlier, but more restricted studies, mean annual precipitation and plant community emerge as the strongest predictors of &Delta;<sub>leaf</sub>. To constrain lipid abundance patterns and &epsilon;<sub>lipid</sub>, leaves from different plant species were sampled. <i>n</i>-Alkane abundances were 200 times higher in angiosperms compared to gymnosperms and di- and triterpenoid abundances were positively correlated to leaf life-span. &epsilon;<sub>lipid</sub> values, as measured relative to leaf tissue, are consistent with previous reports of acetogenic lipids for <i>n</i>-alkanes (up to -5‰). However, &epsilon;<sub>lipid</sub> values were remarkably small for triterpenoids (-0.4‰, MVA pathway) and diterpenoids (-0.6‰, MEP pathway). </p> </p> <p>To evaluate how our new data for modern biomarker abundance, &Delta;<sub>leaf</sub>, and &epsilon; patterns can be applied to studies of ancient plants and climate, plant biomarkers and floral estimates of precipitation, temperature, and plant community were collected from Paleocene and Eocene sediments in the Bighorn Basin (WY, USA). Elevated diterpenoid and <i>n</i>-alkane abundances may indicate longer leaf life-spans and intensified insect herbivory; however, low triterpenoid abundances suggest a preservational bias. Importantly, as with modern plants, precipitation appears to be the primary control on &Delta;<sub>leaf</sub> when determined separately for angiosperms and gymnosperms. This finding suggests temperature and/or <i>p</i>CO<sub>2</sub> may not be important factors controlling &Delta;<sub>leaf</sub>.