| Summary: | The compound-specific δD values and molecular distributions of n-alkanes preserved in sediment archives are used as paleoclimate proxies. This use is based on two assumptions, 1) n-alkane δD values are highly correlated with source water δD values (Sachse 2012), and 2) the δD values of n-alkanes within a single site show little variation (Hou 2007). n-alkane molecular distributions are also used to infer the dominant source of n-alkanes input into sediment archives (Seki 2010). Despite this, paleoclimate reconstructions are being made without full understanding of how the δD values of nalkanes relate to the D isotopic compositions of meteoric waters in the region of synthesis, or how variable n-alkanes δD values could be within a single species. Using an in-depth investigation of Scots pine (Pinus sylvestris), significant variations in n-alkane molecular distributions and δD values were detected. These results suggest height of sample collection within a natural forest exerts a strong control on both n-alkane compositions, and δD values. Furthermore, this investigation indicates evergreen coniferous species synthesize both odd and even-chain-length n-alkanes, with high abundances of mid-chain n-alkanes, previously a diagnostic of aquatic plants. Both of these conclusions could significantly influence paleoclimate interpretations. Moreover, n-alkane δD values from Scots pine and Salix indicate little control of xylem water δD values on n-alkane δD values, suggesting that n-alkanes synthesized during spring are more representative of D-enriched stored assimilates. In addition, this study suggests too favourable conditions for photosynthesis could lead to reduced D-enrichment of leaf water, which is subsequently imprinted on the δD values of n-alkanes. These data suggest that under-estimations of climatic conditions present during the synthesis of n-alkane in sediments could occur if n-alkanes from existing plants are synthesized during periods of climatic stress.
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