An isotope, elemental, and n-alkane baseline for organic matter sources in sediments of high-altitude lakes in the Uinta Mountains, Utah, USA

This research provides a critical baseline for chemical and macromolecular characteristics of organic matter (OM) sources in high-elevation lake sediments. Such studies are essential for accurate paleoenvironmental interpretation of organic matter characteristics, particularly in mountain regions where steep environmental gradients lead to spatially variable OM inputs among lakes. We analyzed the carbon- and nitrogen-isotope compositions (δ¹³C_TOC, δ¹⁵N_TN), and total organic carbon to total nitrogen atomic ratios (TOC:TN), of whole tissues of modern vegetation, lichen and algae from Uinta Mountain lakes, along with the abundances and carbon- and hydrogen-isotope compositions of n-alkanes (δ¹³C_n-alkane, δ²H_n-alkane). TOC:TN can be used to distinguish between aquatic and terrestrial sources, but the differentiating values are higher in the Uinta Mountains than in low-elevation regions. Chain lengths of n-alkanes are more useful than n-alkane isotope compositions for differentiating between terrestrial and semi-aquatic sources in the Uinta Mountains. The ratio of n-alkanes C₂₃ to C₂₇ in mountain lake sediment OM can be used to detect inputs from coniferous krummholz trees and may serve as a proxy for treeline position. We used the isotope data from n-alkanes to calculate the carbon-isotope fractionation between atmospheric carbon dioxide and the n-alkane C₂₃ (ɛ_bulk), and the hydrogen-isotope fractionation between source water and the n-alkane C₂₅ (ɛ_water). These fractionation factors, and the isotope compositions of n-alkanes extracted from lake sediments, suggest that the δ¹³C of atmospheric CO₂ was − 7.0 ± 2‰ and the δ²H of precipitation was − 133 ± 7‰ at ~ 1400 CE in this region. These calculations demonstrate potential applications of our results for obtaining additional paleoclimatic information from paleolimnological organic matter archives.