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What does leaf wax δD from a mixed C3/C4 vegetation region tell us?

AuthorsWang, Yiming V.; Larsen, T. ; Leduc, Guillaume; Andersen, Nils; Blanz, Thomas; Schneider, Ralph
Issue Date2013
CitationGeochimica et Cosmohimica Acta 111 : 128-139 (2013)
AbstractHydrogen 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 firstorder controlling factor of leaf wax dD. Plant functional types and their photosynthetic pathways can also affectleaf wax dD but these biological effects are rarely taken into account in paleo studies relying on this rainfall proxy.To investigate how bio- logical effects may influence dD values we here present a37,000-year old record of dD and stable carbon isotopes (d13C) mea- sured on four n-alkanes (n-C27, n-C29, n-C31, n-C33) from amarine sediment core collected offthe Zambezi River mouth.Our paleo d13C records suggest that each individual n-alkanes had differentC3/C4 proportional contributions. n-C29 was mostly derived from a C3 dicots (trees, shrubs and forbs)dominant vegetation throughout the entire record.In contrast,the longer chain n-C33 and n-C31 were mostly contributed by C4 grasses during the Glacial period but shifted to a mixture of C4 grasses and C3 dicots during the Holocene.Strong correlations between dD and d13C values of n-C33 (correlation coefficient R2 = 0.75, n = 58)and n-C31 (R2 = 0.48, n = 58)suggest that their dD values were strongly influencedby changes in the rel- ative contributions of C3/C4 plant types in contrast to n-C29 (R2 = 0.07, n = 58).Within regions with variable C3/C4 input, we conclude that dD values of n-C29 are the most reliable and unbiased indicator for past changes in rainfall,and that dD and d13C values of n-C31 and n-C33 are sensitive to C3/C4 vegetation changes. Our results demonstrate that arobust 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 d13C values of multiple n-alkanes can help to differentiate bio- logical effects from those related to the hydrological cycle.
Description12 páginas, 6 figuras, 2 tablas.
Publisher version (URL)http://dx.doi.org/10.1016/j.gca.2012.10.016
Appears in Collections:(CEAB) Artículos
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