STABLE ISOTOPE AND LIPID SIGNATURES OF PLANTS ACROSS A CLIMATE GRADIENT: IMPLICATIONS FOR BIOMARKER-BASED PALEOCLIMATE RECONSTRUCTIONS

Open Access
Author:
Doman, Christine Elizabeth
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 19, 2014
Committee Members:
  • Katherine Haines Freeman, Thesis Advisor
Keywords:
  • Isotpe fractionation
  • n-alkanes
  • C4
  • Kohala
  • Kikuyu grass
Abstract:
Plant wax n-alkanes are long, saturated hydrocarbons that form part of the protective, waxy cuticle on leaves. These lipids are pervasive and persistent in soils and sediments and are ideal biomarkers of ancient terrestrial organic matter. In ecosystems dominated by C3 plants, fractionation of carbon and hydrogen isotopes during production of whole leaves and hydrocarbon lipids are well documented, but the sensitivity of isotopic fractionation between leaves and lipids to climate has not been fully investigated. Further, there are few studies of stable isotopes in C4 plant lipids. In both cases, it is unclear if carbon and hydrogen isotopic fractionations during lipid production are sensitive to environmental conditions, such as moisture, or if they reflect inherited characteristics tied to taxonomic or phylogenetic affiliation. This study used a natural climate gradient on the Kohala peninsula of Hawaii to investigate relationships between climate and the δ13C and δ2H values of n-alkanes in three taxa of C3 and two taxa of C4 plants. At Kohala, δ13C values of C3 plant leaves and lipids decreased 5‰ from the driest to the wettest sites, consistent with published data. As expected, the carbon isotopic composition of C4 leaves varied less than 2.2‰ for sites that received less than 1060 mm mean annual precipitation (MAP). At sites above 1060 mm MAP, C4 leaf δ13C values were depleted by up to 15‰ compared to drier sites. This is likely due to shade-related carbon leakage from photosynthetic cells (which can result in a 5-10‰ depletion in δ13C), possibly enhanced by the influence of a forest canopy at the very wettest site. Biomass δ13C values are lower under a closed canopy where understory plants fix carbon from 13C depleted, previously respired CO2. The combination of these two factors could account for the unusually depleted leaf and lipid carbon isotope values observed at the wettest Kohala sites. In all C3 plants and in buffel grass, carbon isotopic fractionation between leaves and lipids (13εlipid) ranged between 5‰ and 8‰, and did not vary with MAP. In the C4 Kikuyu grass, 13εlipid was consistently lower, between 8 and 10‰, although it was more variable for samples from exceptionally wet sites. Estimates of leaf δ13C for C3 and for C4 plants below 1060 mm MAP derived from analyses of alkanes preserved in ancient sediments are not sensitive to climate. The apparent fractionation of carbon isotopes in leaves and lipids is sensitive to differences among plant functional type and species. Hydrogen isotopic values of n-alkanes from all plant types did not vary with MAP, but differed consistently between plant functional types and sampled taxa. The absence of a strong trend between fractionation and aridity supports the use of δ2Hlipid as a record of environmental waters at the time of lipid synthesis, provided that the relative contributions of each plant functional type can be identified with other proxies.