Inter-molecular variations of fatty acid δD in algae and submerged plants from the north-eastern Tibetan Plateau
American Geophysical Union Meeting
Hydrogen isotopic ratios (δD) of fatty acids (FAs) in lake sediments have been widely used in palaeoclimatic reconstruction, however, there are limited investigations on FA δD values in algae and submerged plants from natural lakes, greatly hampering the application and interpretation of their δD values in lacustrine environments. In this study, we systematically investigated δD values of short-chain (C14-C18), mid-chain (C20-C24), and long-chain (C26-C30) FAs in four algae and submerged macrophytes, including two algal genera Chara and Cladophora and two angiospermous genera Potamogeton and Ruppia, which commonly occur in six lakes on the north-eastern Tibetan Plateau. Our results are as below: (i) In C4-like taxa (Potamogeton, Ruppia, and Chara), the C16 and C26 FAs have minor differences in their δD values, while in the C3-like Cladophora, δD values of C16 FAs are significantly more negative (avg. -48 ± 7‰) than those of the C26 FAs. We propose that different photosynthetic mechanisms in these algae and submerged plants account for the different inter-molecular δD variations between C16 and C26 FAs. (ii) δD values of C26 FAs in the carbonate coated alga Chara are significantly correlated with the δ18O values of carbonate encrustations; such a co-variance suggests that in addition to lake water, other factors may also affect both lipid synthesis and carbonate encrustation building up in Chara, leading to a significantly positive apparent hydrogen isotope fractionation factors (εFA-W) between C26 FAs of Chara and lake water. (iii) δD values of C26 FAs from Potamogeton, Ruppia, and Cladophora are correlated well with lake water δD values (R2 = 0.84, p < 0.001, n = 19), suggesting that their δD values of long-chain FAs potentially record δD variations of lake water with the apparent hydrogen isotope fractionation factors (εFA-W) of C26 FAs ranging from -153 to -134‰. Our results highlight the impact of different photosynthetic pathways on the inter-molecular difference of FA δD values in algae and submerged plants and illustrate both the potential and the challenge of applying their FA δD values as a palaeo-hydrology proxy.