Abstract
Palaeohydrological studies have increasingly utilised the 2H/1H composition of leaf wax n-alkyl lipids to extract information from the geological record. Interpretation of the sedimentary biomarker δ2H signal, however, requires detailed understanding of the mechanisms controlling hydrogen isotope fractionation between source water and n-alkly lipids (εl/w). The existence of large ranges in published n-alkyl δ2H and εl/w among modern plant species growing at a single location suggests that the lipid signal incorporated into the sedimentary record could be sensitive to relatively small-scale changes in vegetation assemblages. The mechanisms responsible for these interspecies differences are currently poorly constrained. Previous research has had limited success explaining n-alkyl δ2H by reference to physical processes controlling the movement of water inside/outside and within the leaf, while the relative importance of biochemical processes remains largely unexplored.
This project aims to identify the mechanisms controlling interspecies variation in n-alkane 2H/1H among a range of C3 and C4 plants from a Norfolk saltmarsh in the UK. To distinguish between environmental, physical and biochemical controls, we conducted 2H/1H analysis of soil, xylem, and leaf waters and n-alkanes (i) across multiple sampling sites within the marsh, (ii) throughout the 2012 growth season, and (iii) at different times of the day. We also measured the 2H/1H of chloroplast phytol in 7 samples collected at the end of 2012.
Leaf wax n-alkane δ2H varied among the sampled species by over 100‰ throughout the 2012 growth season. Environmental processes that could influence control source water 2H/1H did not fully account for this interspecies variation - soil water 2H/1H varied by only 35‰ with marsh sub-environment and exhibited site-specific seasonal shifts by no more than 31‰. Maximum interspecies variation in xylem water was 38‰, while leaf waters differed by only 29‰. We therefore concluded that mechanisms driving variation in the 2H/1H composition of leaf water are not sufficient to explain the range of n-alkane δ2H values we observed in north Norfolk salt marsh.
Comparison of the relationship between the δ2H of chloroplast phytol and leaf water reveals a strong relationship (R2 = 0.7), while that between the δ2H of n-alkanes vs. leaf water (R2 = 0.4) and the δ2H of n-alkanes vs. phytol (R2 = 0.1) does not. This implies that the δ2H values of leaf wax n-alkanes in some of the plants we have investigated are largely decoupled from the 2H/1H composition of leaf water and biosynthates in the chloroplast and, instead, are strongly influenced by biochemical process that control NADPH and/or other secondary compounds synthesised in the cytosol. Our data highlight the fact that further research is needed (i) to identify the biochemical mechanisms influencing the δ2H signal of leaf wax n-alkyl lipids across a broad range of modern plants, and (ii) to constrain the relationship between these mechanisms and source water 2H/1H. Only then can the hydrogen isotope information recorded in ancient sediments be fully understood.
This project aims to identify the mechanisms controlling interspecies variation in n-alkane 2H/1H among a range of C3 and C4 plants from a Norfolk saltmarsh in the UK. To distinguish between environmental, physical and biochemical controls, we conducted 2H/1H analysis of soil, xylem, and leaf waters and n-alkanes (i) across multiple sampling sites within the marsh, (ii) throughout the 2012 growth season, and (iii) at different times of the day. We also measured the 2H/1H of chloroplast phytol in 7 samples collected at the end of 2012.
Leaf wax n-alkane δ2H varied among the sampled species by over 100‰ throughout the 2012 growth season. Environmental processes that could influence control source water 2H/1H did not fully account for this interspecies variation - soil water 2H/1H varied by only 35‰ with marsh sub-environment and exhibited site-specific seasonal shifts by no more than 31‰. Maximum interspecies variation in xylem water was 38‰, while leaf waters differed by only 29‰. We therefore concluded that mechanisms driving variation in the 2H/1H composition of leaf water are not sufficient to explain the range of n-alkane δ2H values we observed in north Norfolk salt marsh.
Comparison of the relationship between the δ2H of chloroplast phytol and leaf water reveals a strong relationship (R2 = 0.7), while that between the δ2H of n-alkanes vs. leaf water (R2 = 0.4) and the δ2H of n-alkanes vs. phytol (R2 = 0.1) does not. This implies that the δ2H values of leaf wax n-alkanes in some of the plants we have investigated are largely decoupled from the 2H/1H composition of leaf water and biosynthates in the chloroplast and, instead, are strongly influenced by biochemical process that control NADPH and/or other secondary compounds synthesised in the cytosol. Our data highlight the fact that further research is needed (i) to identify the biochemical mechanisms influencing the δ2H signal of leaf wax n-alkyl lipids across a broad range of modern plants, and (ii) to constrain the relationship between these mechanisms and source water 2H/1H. Only then can the hydrogen isotope information recorded in ancient sediments be fully understood.
| Original language | English |
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| Publication status | Published - 2013 |
| Event | American Geophysical Union (AGU) Fall Meeting 2013 - USA, San Francisco, United States Duration: 9 Dec 2013 → 13 Dec 2013 |
Conference
| Conference | American Geophysical Union (AGU) Fall Meeting 2013 |
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| Country/Territory | United States |
| City | San Francisco |
| Period | 9/12/13 → 13/12/13 |