Equal abundance of odd and even n-alkanes from cycad leaves: can the carbon preference index (CPI) faithfully record terrestrial organic matter input at low latitudes? (original) (raw)
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Organic Geochemistry, 2016
The carbon isotope composition of terrestrial plants records valuable ecophysiological and palaeoecological information. However, interspecies variability in 13 C/ 12 C, at both the bulk and compound-specific (CS) level, requires further exploration across a range of ecosystem types. Here, we present bulk and n-alkane δ 13 C values, and n-alkane concentrations, from seven plants (C 3 and C 4) growing in a temperate UK saltmarsh. Inter-and intra-species variation in n-alkane δ 13 C values among C 3 plants ranged from 8‰ (n-C 31) to 10‰ (n-C 27) across the 2011 and 2012 growing seasons, exceeding variability in bulk tissue (7‰). In contrast, the C 4 monocot showed < 2‰ seasonal shifts in bulk and CS values. As a result of the variability in our CS data, we calculate that n-alkane based C 3 /C 4 reconstructions in temperate saltmarshes have a maximum uncertainty of ~11%. For dicots and succulents, seasonal bulk and CS δ 13 C trends diverged, while for C 3 and C 4 monocots, bulk and CS values followed similar temporal patterns. Apparent biosynthetic fractionation varied from-4 to-10‰ for C 3 plants, and reached-13‰ for the C 4 monocot. We explain discrepancies between bulk and n-alkane δ 13 C values by referring to possible interspecies variation in salinity adaptation, which may influence the partitioning of pyruvate, shifting the isotopic composition of lipid biomarkers. These findings open new avenues for empirical studies to further understand the metabolic processes fractionating carbon during the synthesis of n-alkanes, enhancing interpretation of the biomarker signal from the geological record.
Organic Geochemistry, 2011
The molecular hydrogen isotope composition (dD) of leaf waxes from terrestrial plants is increasingly used to infer hydrological characteristics of ancient high latitude climates. Analysis of the hydrogen isotope composition of n-alkanes (dD n-alkane) from a global dataset of individual plants growing at low and middle latitudes indicates that plant ecological life form is an important factor in determining the hydrogen isotope fractionation. However, environmental and biological controls of high latitudinal leaf wax dD values are poorly understood because of a lack of dD records from modern flora in these regions. We previously noticed smaller apparent hydrogen isotope fractionations between n-alkanes and environmental water (e alk-water) in deciduous trees growing at high latitudes (>59°N; Liu, W.-G., Yang, H., 2008. Multiple controls for the variability of hydrogen isotopic compositions in higher plant n-alkanes from modern ecosystems. Global Change Biology 14, 2166-2177.) To further examine these issues, we measured dD n-alkane from a variety of plants that inhabit high latitude environments and added critically needed leaf wax dD data from grass and herbs to the existing global dD n-alkane database. Inclusion of these new data with the existing global dataset (n = 408) confirms plant ecological life form as an important control for leaf wax dD variation for terrestrial plants living at high latitudes. Our results suggest that, while precipitation dD is captured in these high latitude plants, physiological characters such as leaf area, venation pattern and hydraulic system, that enhance transpiration rate during summer growth, may impart dD n-alkane differences among plants with different ecological life forms.
Diagenesis effects on specific carbon isotope composition of plant n-alkanes
Organic Geochemistry, 2004
The stable carbon isotope compositions of individual n-alkanes extracted from leaves of (1) Ginkgo biloba degrading in a well-drained soil and (2) Eretmophyllum, a fossil Ginkgoale, were investigated in a first approach to refining the estimate of potential diagenetic effects on the isotopic composition of sedimentary alkanes derived from higher plants; n-alkanes extracted from fresh leaves of G. biloba appeared 13 C-depleted when compared with bulk leaves. Within a given extract they exhibited characteristic variability in specific isotope composition with respect to chain length. Although the isotope composition of bulk leaves remained unchanged through senescence and litter formation, the specific n-alkane isotopic composition varied in two respects: (i) general 13 C-enrichment in all n-alkanes and (ii) homogenisation of specific isotopic composition of all the n-alkanes of a given extract. As for fresh and degrading leaves of G. biloba, n-alkanes extracted from the fossil Eretmophyllum appeared 13 C-depleted when compared with bulk leaves. Their isotope distribution patterns suggested that the isotope composition of fossil alkanes may have been slightly affected by diagenesis. Results for degrading leaves and fossil leaves (1) indicated that the isotope composition of individual alkanes can be more affected by degradation than that of bulk leaves and (2) confirmed that the isotope signal in fossils, and sedimentary organic matter in general, may be less altered than in the organic matter of welldrained soil. #
Frontiers of Earth Science in China, 2009
Carbon Preference Index (CPI values) of higher plant-derived long-chain n-alkanes extracted from 62 surface soil samples in eastern China exhibited a specific pattern of variations, namely gradual increase with the increasing latitudes. Such regular variations existed in both forest soil and grassland soil. Our data implied that CPI values of higher plant-derived long-chain n-alkanes had a certain connection with climatic conditions, and such a connection was not influenced by vegetation types. Together with previous data from marine sediments, loess/paleosol sequences, tertiary red clay and modern plants, our observation made us conclude that CPI values of higher plant-derived long-chain n-alkanes may be used as an excellent proxy for paleoclimatic studies.
What does leaf wax δD from a mixed C3/C4 vegetation region tell us?
2013
Hydrogen isotope values (dD) of sedimentary terrestrial leaf wax such as n-alkanes or n-acids have been used to map and understand past changes in rainfall amount in the tropics because dD of precipitation is commonly assumed as the first order controlling factor of leaf wax dD. Plant functional types and their photosynthetic pathways can also affect leaf wax dD but these biological effects are rarely taken into account in paleo studies relying on this rainfall proxy. To investigate how biological effects may influence dD values we here present a 37,000-year old record of dD and stable carbon isotopes (d 13 C) measured on four n-alkanes (n-C 27 , n-C 29 , n-C 31 , n-C 33 ) from a marine sediment core collected off the Zambezi River mouth. Our paleo d 13 C records suggest that each individual n-alkanes had different C 3 /C 4 proportional contributions. n-C 29 was mostly derived from a C 3 dicots (trees, shrubs and forbs) dominant vegetation throughout the entire record. In contrast, the longer chain n-C 33 and n-C 31 were mostly contributed by C 4 grasses during the Glacial period but shifted to a mixture of C 4 grasses and C 3 dicots during the Holocene. Strong correlations between dD and d 13 C values of n-C 33 (correlation coefficient R 2 = 0.75, n = 58) and n-C 31 (R 2 = 0.48, n = 58) suggest that their dD values were strongly influenced by changes in the relative contributions of C 3 /C 4 plant types in contrast to n-C 29 (R 2 = 0.07, n = 58). Within regions with variable C 3 /C 4 input, we conclude that dD values of n-C 29 are the most reliable and unbiased indicator for past changes in rainfall, and that dD and d 13 C values of n-C 31 and n-C 33 are sensitive to C 3 /C 4 vegetation changes. Our results demonstrate that a robust interpretation of palaeohydrological data using n-alkane dD requires additional knowledge of regional vegetation changes from which nalkanes are synthesized, and that the combination of dD and d 13 C values of multiple n-alkanes can help to differentiate biological effects from those related to the hydrological cycle.
Organic Geochemistry, 2008
This study investigates stable carbon and hydrogen isotope compositions of leaf wax n-alkanes from 3 angiosperm and 2 conifer species from a semi-desert natural environment. The δ13C and δD n-C27 alkane data show a clear distinction between Populus tremuloides/Syringa vulgaris (−31.9 to −32.7‰ and −168 to −186‰, respectively) and Pinus sylvestris/Picea pungens (−28.8 to −30.6‰ and −190 to −212‰) throughout the 2005 growing season. Like the other angiosperm species, Betula pendula was D-enriched (−162 to −178‰) relative to the conifer species. However, its δ13C values were more similar to those of Pinus sylvestris and Picea pungens in May, but had a ∼4‰ positive shift later in the season. We suggest that the observed isotopic variations derive mainly from lower stomatal conductance for CO2 and H2O vapor in conifers relative to angiosperms. Betula pendula experienced severe environmental stress that affected its carbon metabolism for most of the season. Our results suggest that shifts in δD and δ13C values of sedimentary leaf waxes may result not only from shifts in the paleoclimatic variables but also from temporal shifts in the distribution of angiosperms and conifers as well as from changes in the isotopic signatures of higher plants undergoing metabolic changes.
2008
This study investigates stable carbon and hydrogen isotope compositions of leaf wax n-alkanes from 3 angiosperm and 2 conifer species from a semi-desert natural environment. The dC and dD n-C27 alkane data show a clear distinction between Populus tremuloides/Syringa vulgaris ( 31.9 to 32.7‰ and 168 to 186‰, respectively) and Pinus sylvestris/ Picea pungens ( 28.8 to 30.6‰ and 190 to 212‰) throughout the 2005 growing season. Like the other angiosperm species, Betula pendula was D-enriched ( 162 to 178‰) relative to the conifer species. However, its dC values were more similar to those of Pinus sylvestris and Picea pungens in May, but had a 4‰ positive shift later in the season. We suggest that the observed isotopic variations derive mainly from lower stomatal conductance for CO2 and H2O vapor in conifers relative to angiosperms. Betula pendula experienced severe environmental stress that affected its carbon metabolism for most of the season. Our results suggest that shifts in dD and ...
Geochimica Et Cosmochimica Acta, 1976
Even-carbon number C 22-C 28n-alkanes, are present in higher concentrations than their C 21-C 29 odd-carbon number homologs in an organic-rich rock of Mississippian ( ca. 310 × 10 6-yr-old) age, which contains algae-like remnants. Environmental indicators associated with this organic-rich rock permitted environmental comparison with ancient and modern environments that contain n-alkanes which exhibit similar distributions. The results of these comparisons suggest that even-carbon number n-alkanes are preferentially produced in highly saline, carbonate environments where aerobic and anaerobic bacteria have subsisted on the remains of blue-green algae.
Temporal variations in the δD of leaf n -alkanes from four riparian plant species
Organic Geochemistry
The hydrogen isotopic composition of terrestrial plant leaf waxes is widely used as a proxy for the isotopic composition of ambient water at the time of plant growth, yet there is considerable uncertainty about how environmental or plant-specific factors impact apparent hydrogen isotope fractionation. We sampled leaves from four riparian plant species (Pinus strobus, Tsuga canadensis, Phalaris arundinacea and Corylus americana) during the 2013 growing season (April to October) to evaluate the controls of hydrogen isotope fractionation in plant-derived nalkanes. Our data show that plants from different taxonomic classes produce distinct seasonal alkane δD that yields a more consistent isotopic record of changes in ambient ecosystem water than individual plants. Hydrogen (δD n-alkane) stable isotopes of terrestrial plant n-alkanes reflect the isotopic composition of ambient water modified by environmental parameters such as temperature, precipitation, stomatal regulation, water use efficiency and plant functional types (e.g., Lockheart
Carbon isotopic signatures of soil organic matter correlate with leaf area index across woody biomes
Journal of Ecology, 2014
Leaf area index (LAI), a measure of canopy density, is a key variable for modelling and understanding primary productivity, and also water use and energy exchange in forest ecosystems. However, LAI varies considerably with phenology and disturbance patterns so alternative approaches to quantifying stand level processes should be considered. The carbon isotope composition of soil organic matter (δ 13 C SOM ) provides a time-integrated, productivityweighted measure of physiological and stand-level processes, reflecting biomass deposition from seasonal to decadal time scales.