Matthew J. Lockheart

Variations in the stable carbon isotope compositions of individual lipids from the leaves of modern angiosperms: implications for the study of higher land plant-derived sedimentary organic matter

Seasonal changes in δ13C values for individual lipids from the leaves of several species of tree have been studied in order to provide essential background information for use in future investigations of the isotopic signatures of terrigenous sedimentary organic matter. The n-alkanes of Betula ermanii, Quercus castaneifolia and Fagus japonica revealed increased δ13C in autumn leaves compared with leaves sampled at the start of the growing season. Samples taken from Q. castaneifolia and F. sylvatica at monthly intervals showed gradual depletion of 13C in bulk tissues and n-alkanes through the growing season. This may be a consequence of either recycling of depleted internal carbon in order to replace weathered waxes, or increased fractionation against 13C by the enzyme ribulose 1,5-bisphosphate carboxylase in response to increasing summer temperatures. Sitosterol exhibited similar isotopic trends as the n-alkanes in F. sylvatica, but showed the opposite behaviour in Q. castaneifolia. The effect of sunlight intensity on δ13C was investigated in foliage sampled at different compass positions around two trees, Q. robur and F. sylvatica. Bulk tissue and lipids from inner shade leaves were consistently more depleted in 13C than those from the corresponding sun leaf. The leaves receiving the highest sunlight irradiance on average, i.e. southern foliage, exhibited the lowest δ13C in lipids and bulk tissues. The variability of δ13C values with irradiance level may be due to changes in photosynthetic assimilation rates and the adaptation of the leaf epidermis and stomata in response to its light environment. Lipids and bulk tissues from leaves of Quercus species were found to possess slightly more depleted δ13C values than those in Fagus species, although interspecies variability was quite large. This study has important implications for the study of terrestrially derived organic matter preserved in ancient sediments. The results demonstrate the importance of elucidating the environmental factors that influence δ13C values of individual lipids in modern leaves prior to using isotopic shifts in sedimentary and fossil lipids as indicators of palaeoenvironmental change.

The origin of faeces by means of biomarker detection

One of the defining characteristics of a biomarker is the retention, during diagenesis, of structural characteristics indicative of its original biogenic source (Peters and Moldowan, 1993). Since the 1960s an increasing interest has developed in the use of environmental biomarker compounds to monitor levels of human and animal waste introduced to marine and terrestrial environments; the relative stability of these compounds enabling both spatial and long-term temporal studies to be realized (e.g., Walker et al., 1982; Vivian, 1986; Evershed and Bethell, 1996 and references therein). The specificity of particular biomarkers for different faecal sources makes their use particularly attractive especially in circumstances where deducing the origin of sewage constituting a pollution event is crucial. Research into organicmatter in the marine and terrestrial environments over the last 20 years has led to the development of two main groups of organic compounds found in faecal matter that can be used to monitor sewage, namely: 5b-stanols and bile acids. This communication provides an overview for researchers not necessarily familiar with molecular methods of sewage monitoring. The two compound classes will be considered separately, and then their integration and future development discussed.

Molecular and isotopic biogeochemistry of the Miocene Clarkia Formation: hydrocarbons and alcohols

The Miocene lacustrine deposit at Clarkia, northern Idaho, U.S.A. is well known for its rich and exceptionally well preserved higher-plant leaf fossils. In the present study, the distributions, abundance and isotopic compositions of aliphatic hydrocarbons and alcohols isolated from two sediment samples and from two associated fossils (Magnolia latahensis and Platanus dissecta) are investigated. The general similarity of these characteristics in the sediments suggests a relatively uniform depositional environment and consistent terrestrial input. The sediment hydrocarbons are comprised mainly of compounds derived from higher plants and bacteria, whereas the alcohols reflect contributions from both higher plants and algae. A notable feature of the higher-plant derived terpenoid hydrocarbons is the prevalence of des-A-triterpenes, largely in the absence of their pentacyclic precursors, indicating the dominance of A-ring degradation for these pentacyclic triterpenoids. The des-A-triterpenes include several novel compounds, namely 19α(H)- and 19β(H)-28-nor-des-A-lup-17-ene and des-A-oleana-9,13(18)-diene. The sediment samples contain high abundances of C24C32 long-chain alkanediols, consisting of a series of α,ω-diols, α,(α − 1)-diols, 1,15-diols and an unusual series of 1,3-diols. Both fossil leaves show clear differences in their compound distributions and concentrations from the associated sediments, which suggests that they retain a composition distinct from the sediment groundmass. The δ 13C values of C25C33 n-alkanes and C24C32 n-alcohols in sediments and fossils are consistent with an origin from terrestrial C3 higher plants. Algal lipids in the two sediments are represented by dinostanol (−24.6 and −25.9‰) inherited from dinoflagellates, and long-chain 1,15-alkanediols (−26.4 to −29.7‰) derived from microalgae. The long chain α,ω-, α,(ω − 1)-, and 1,3-alkanediols are more enriched in 13C (−21.8 to −26.4‰), suggesting an origin from sources other than higher plants, probably aquatic photoautotrophic organisms. The 13C depletion of some hopanoids (−35.9 to −61.0‰) attests to contributions from methanotrophic bacteria and suggests an active methane cycle at the time of deposition.

The biomolecular paleontology of continental fossils

Abstract The preservation of compounds of biological origin (nucleic acids, proteins, carbohydrates, lipids, and resistant biopolymers) in terrigenous fossils and the chemical and structural changes that they undergo during fossilization are discussed over three critical stratigraphic levels or “time slices.” The youngest of these is the archeological record (e.g., <10 k.y. b.p.), when organic matter from living organisms undergoes the preliminary stages of fossilization (certain classes of biomolecule are selectively preserved while others undergo rapid degradation). The second time slice is the Tertiary. Well-preserved fossils of this age retain diagenetically modified biomarkers and biopolymers for which a product–precursor relationship with the original biological materials can still be identified. The final time slice is the Carboniferous. Organic material of this age has generally undergone such extensive diagenetic degradation that only the most resistant biopolymers remain and these have undergone substantial modification. Trends through time in the taphonomy and utility of ancient biomolecules in terrigenous fossils affect their potential for studies that involve chemosystematic and environmental data.

Leaf carbon isotope compositions and stomatal characters: important considerations for palaeoclimate reconstructions

Variations in stable carbon isotope composition and stomatal characters of fossil leaves are being used increasingly to provide information about palaeoatmospheric CO2 concentrations. However, differences in these parameters are not solely dependant on CO2 concentrations and factors such as aspect (i.e. sun and shade) must be considered in palaeoenvironmental reconstructions. Stable carbon isotope analyses in combination with stomatal analyses of canopy sun and shade leaves of modern Fagus sylvatica reveal that sun leaves contain more 13C and have higher stomatal indices when compared with shade leaves. These observations, probably related to stomatal conductance, have significant implications for studies using fossil leaves to reveal palaeoclimate signals.

Isotope and molecular evidence for the diverse origins of carboxylic acids in leaf fossils and sediments from the Miocene Lake Clarkia deposit, Idaho, U.S.A.

Abstract The normal fatty acids (C 14 C 32 ), ω-hydroxy acids (C 12 C 32 ), (ω-1)-hydroxy acids (C 28 to C 32 ), terpenoid acids and hopanoid acids in higher plant leaf fossils and enclosing sediments from the Miocene lacustrine Clarkia deposit (17–20 Ma) show a wide range (−18.5 to −55.2‰ vs PDB) of carbon isotopic compositions. Both molecular distributions and isotopic compositions display highly informative similarities (e.g., C 22 C 32 n -acids) and differences (e.g., C 16 and C 18 n -acids, C 22 ω- and C 28 and C 30 (ω-1) hydroxy acids) between the individual fossils and sediment matrix. These δ 13 C values are distinctive among compound classes. The molecular distributions and the ranges of δ 13 C values (−27.2 to −32.2‰) for long chain n -fatty acids (C 22 C 32 ), terpenoid acids and ω-hydroxy acids (except for C 22 ω-hydroxy acids) are in accord with derivation from mainly C 3 terrestrial higher plants. The (ω-1) hydroxy acids (C 28 and C 30 homologues), however, show a significant 13 C enrichment (−18.5 to −22.8‰) in the sediments, consistent with an origin in aquatic photosynthetic organisms whose habitats are severely CO 2 limited. The C 22 ω-hydroxy acid shows high abundance in sediments but is not detected in association with leaf fossils and has distinctive δ 13 C values (ca. −33‰), depleted in 13 C by about 4–5‰ compared with the other ω-hydroxy acid homologues (C 24 C 28 ), and is probably derived from micro-organisms living below the oxycline. Bacterial hopanoids are dominated by isomers with 22 R -17β(H),21β(H) configuration, consistent with the low thermal maturity of the sediment, and also display rather negative δ 13 C values (−38 to −55‰), suggesting their derivation from methanotrophic bacteria. From the isotopic data we infer the operation of a methane cycle, which is also supported by the palaeontological evidence for anoxia in the water column. A novel compound, des-A-olean-28-oic acid was also tentatively identified.

Chemotaxonomic classification of fossil leaves from the Miocene Clarkia lake deposit, Idaho, USA based on n-alkyl lipid distributions and principal component analyses

Fossil leaves which show excellent preservation of lipids and other organic compounds are preserved in abundance at the Miocene Clarkia lake deposit in northwestern Idaho, USA. Eighty three fossils from nine genera which are commonly found at the site, namely Quercus, Platanus, Magnolia, Pseudofagus, Fagus, Cocculus, Taxodium, Metasequoia and Sequoia were investigated to assess whether their chemical compositions can be used in chemotaxonomic comparisons. Chromatographically separated n-alkane and n-alkanol sub-fractions were examined by GC and GC/MS to establish whether characteristic distributions were evident between fossils and distinct from the enclosing sediment. Chemotaxonomic comparisons were performed between the individual fossil specimens and six sediments using the n-alkane and n-alkanol profiles and principal component analysis (PCA). Characteristic distributions were observed for certain genera, e.g. Platanus and Quercus, which were reproducible between multiple specimens of the same genus. Concentrations of the lipids in fossils were markedly greater than in the six sediments in all of the fossils except the conifers examined. These findings demonstrate that n-alkyl lipid distributions from fossil leaves have chemotaxonomic utility, with potential for comparisons with modern taxa.

Modeling Traditional Manuring Practice: Soil Organic Matter Sustainability of an Early Shetland Community?

Modeling of soil systems is an essential approach to discussions of the historical dimensions of soil sustainability, but as yet there has been no formal testing and application of such models. In this paper we first test the ability of the CENTURY agroecosystem model to predict soil organic carbon levels in anthropogenic plaggen soils from ethnographic and historical land management information of manuring practices on the Shetland island of Papa Stour. Observations suggest that the model makes accurate predictions and can be used to develop and test hypothetical land management scenarios. Results suggest that within historic time the arable areas of Papa Stour were manured at a level above that required to maintain soil organic carbon levels, and consequently the hill-land source of organic material was overexploited with no real apparent gain. Modeled evidence suggests that short-term observations of soil organic carbon levels would indicate a greater degree of manure application than was actually required over the longer term. Successful use of the CENTURY model in this historic context suggests that it may be applicable to questions of soil sustainability in other areas of the North Atlantic region.

C26 to C32 1,3-alkanediols, a Novel Series of Biological Markers Identified in Miocene (17-20 MA) Leaf Fossils and Sediment

Abstract A novel homologous series of long chain 1,3-alkanediols, ranging from C26 to C32 with strong even/odd predominance has been identified in a Miocene (17-20 million years old) deposit of fossil leaves and enclosing sediments by GC-MS. Their structures are confirmed by synthesis of their C28 homologue.

Carbon-Isotopic Compositions of Individual Alkanes/Alkenes in Leaf Fossils and Sediments from the P-33 Site of the Miocene Clarkia Deposit

The alkanes/alkenes from the Clarkia sediments and fossils were analysed by GC, GC/MS and irm-GC/MS. The results indicate that the sediment has an extremely low thermal maturity and the Sycamore fossil analysed still preserves a distinctive n-alkane distribution comparing with the surrounding sediments. The presence of isotopically depleted hopenes suggests input from methanotrophic bacteria and hence the presence of an active methane cycle.