Pf van Bergen

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.

Organic geochemical studies of soils from the Rothamsted classical experiments-IV. Preliminary results from a study of the effect of soil pH on organic matter decay

Total lipid extracts and solvent insoluble organic matter in soils from the Park Grass Exper- iment at Rothamsted Experimental Station, Harpenden, U.K. were studied to determine the eAect of pH on the preservation/degradation of plant derived biomolecules. Analyses involved high temperature- gas chromatography (HT-GC), HT-GC-mass spectrometry (HT-GC-MS), GC combustion-isotope ratio MS (GCC-IRMS) and flash pyrolysis-GC (Py-GC) and Py-GC-MS. The plots selected for study have pH values ranging from 3.7 to 7.3, with acidic soils exhibiting two distinct horizons (i.e. humic rich top layer and mineral soil). The total lipid extracts of the soil samples with low pH exhib- ited higher relative abundances of long-chain (>C20) organic acids believed to be derived largely from oxidation of plant lipids. The vegetation signature in the low molecular weight fraction is only retained in the humic rich top layer. The signal in the mineral layer is believed to derive primarily from previous vegetation. Compound specific stable carbon isotope (d 13 C) measurements of long-chain n-alkanols are considered to reflect diAerences in the rate of incorporation of plant lipids into the humic top layer re- lated to the grass species dominating the standing vegetation. In the soil samples of low pH, lignin con- tributes to the high molecular weight fraction of the humic layer. In contrast, the mineral layer of the same soil shows little evidence of intact lignin, but is instead dominated by amino acid pyrolysis pro- ducts, probably deriving from (degraded) polypeptides. The pyrolysates of the mineral soils of high pH yield a distribution of products similar to that found in the deeper layer of the low pH samples but with evidence of lignin derived moieties. Overall, soil pH was found to have a significant eAect on the preservation of higher plant derived biomolecules including ligno-cellulose. # 1998 Elsevier Science Ltd. All rights reserved

Formation of long-chain ketones in archaeological pottery vessels by pyrolysis of acyl lipids

Abstract Studies of organic residues preserved in unglazed archaeological pottery have revealed the presence of homologous series of long-chain ketones containing 29–35 carbon atoms. The C31, C33 and C35 ketones are particularly abundant and exhibit a distinct monomidal distribution. The presence of long-chain ketones in potsherds is usually ascribed to the absorption of epicuticular waxes into the pottery fabric during the cooking of leafy vegetables. However, compound specific stable carbon isotope (δ13C) analyses of the individual lipids present in the potsherd extracts, in combination with detailed structural information, indicates that these ketones do not derive from plant waxes. Isotopic and structural analysis of the fatty acids, which always co-occur with the ketones, suggest that a precursor-product relationship exists. Micro-scale pyrolysis of a range of free fatty acids and triacylglycerols in the presence of various inorganic matrices was undertaken in exploring the possibility of an abiological route to the formation of the ketones. Depending on the pyrolysis conditions employed, substantial yields of long mid-chain ketones were formed which were structurally and isotopically congruent to those observed in the ancient potsherds. The ketones are formed by ketonic decarboxylation (a type of head to head condensation reaction), probably involving fatty acid metal salts as intermediates, the metal being provided by the inorganic matrix. Apart from the abundant long mid-chain ketones various other products such as methylketones, methyl esters, alkanes, alk-1-enes and homologous series of minor ketones are formed as secondary pyrolysis products. These latter products are not found in the pottery probably due to less vigorous thermal conditions achieved during the original use of the vessel compared with those attained in the laboratory pyrolysis experiments. Evidence for this comes from the formation of the fatty acid methyl esters which are only produced under the most forcing of pyrolysis conditions.

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.

Chemical and morphological changes in archaeological seeds and fruits during preservation by desiccation

Abstract Soluble and insoluble constituents of modern and ancient (ca. 600 AD) desiccated barley kernels and radish seeds have been studied using high-temperature gas chromatography-mass spectrometry, scanning electron microscopy, and Curie-point pyrolysis-gas chromatography-mass spectrometry. Although some changes in colour are seen, the morphology and anatomy of the desiccated specimens are largely unchanged compared with their extant counterparts. The main chemical alterations arising through long-term desiccation are chemically rather than microbially mediated. Comparison between the lipid data from the modern and ancient barley and radish reveals that extensive ester hydrolysis has occurred over time, while oxidation has been retarded. The insoluble material of the modern barley kernel walls, which is composed of a lignin-cellulose complex characteristic of monocotyledons, undergoes upon desiccation chemical alterations resulting in a significant decrease in the abundance of polysaccharides and cinnamic acids moieties. In marked contrast, the insoluble constituents of the modern radish seed coat yields primarily amino acid moieties upon pyrolysis, most likely deriving from proteins. The seed coal also contains a polyphenolic macromolecule and a small contribution from a dicotyledon lignincellulose complex. This is the first time such a distinct chemical composition has been reported for modern sclerotic plant tissues. The chemical composition of the tissues of the ancient radish specimens appears little altered compared with their modern counterparts; the only obvious difference is the decrease in abundance of 2,6-dimethoxyphenol moieties in the lignin-cellulose pyrolysis products. Comparison of the microscopic and chemical data with that of walls of propagules, i.e., fruits and seeds, deposited in aquatic environments reveals no differences between the material deposited under desiccating and aquatic conditions.

Comparison of the analytical performance of filament and Curie-point pyrolysis devices

Abstract A wide range (16) of synthetic polymers, biological and organic geochemical samples was chosen to compare the performance of filament and Curie-point pyrolysis devices in combination with gas chromatography/mass spectrometry (py-GC/MS). The pyrolysis results were compared qualitatively, quantitatively and using statistical data visualization methods. Multivariate visualization methods showed a good reproducibility between consecutive runs of the same sample. Statistical analyses of processed py-GC/MS data and qualitative comparison of total ion and mass chromatograms revealed a high degree of comparability between the data obtained from the two pyrolysis devices. This investigation constitutes the first systematic comparison of the two most widely used pyrolysis devices and demonstrates that their results can be confidently cross-referenced providing that all other analytical variables, e.g. sample size, GC column stationary phase, carrier gas, etc. are strictly controlled.