Water depth control on n-alkane distribution and organic carbon isotope in mid-latitude Asian lakes

The relative proportion of mid-chain to long chain n-alkane homologues and isotopic compositions of total organic carbon (P-aq and delta C-13(org)) have been proposed as potential lake-level proxies since such features mainly come from submerged aquatic plants living within an optimum range of lake water depth. With limited dataset available, the applicability of both indices and their sensitivity to lake water depth changes across a broad geographic extent need further investigation. Here we report P-aq and delta C-13(org) values in surface sediments collected from 55 lakes in mid-latitude Asia. We have found an arched relationship between P-aq index and water depth, with high Paq values occurring at the depth of similar to 1-10 m, while sediments from shallower and deeper water lakes have relatively low P-aq values. delta C-13(org) variation resembles an arched pattern with depth in freshwater and brackish lakes, with relatively high delta C-13(org) values also corresponding to the depth of similar to 1-10 m, but substantially high delta C-13(org) values in hypersaline environments (salinity > similar to 100,000 mg/L) obscure the arched delta C-13(org)-depth relationship. We suggest that both indices respond to the structure of aquatic plant community, especially the biomass of submerged plants. Our results confirm that P-aq and delta C-13(org) can be used to infer lake-level changes in mid-latitude Asia, but other influencing factors have to be considered before applying the two proxies to downcore reconstructions, and combined utilization of the P-aq and delta C-13(org) indicators could improve the reliability of paleohydrological reconstructions.

Automated summary

The relative proportion of mid-chain to long chain n-alkane homologues and isotopic compositions of total organic carbon have been proposed as lake-level proxies, based on their relationship with submerged aquatic plants. The study in mid-latitude Asia suggests that these proxies can be used to infer lake-level changes, but other influencing factors need to be considered before downcore reconstructions. Combining the P-aq and δC-13(org) indicators could improve the reliability of paleohydrological reconstructions.