Mid- and long-chain leaf wax δ2H values in modern plants and lake sediments from mid-latitude North America

Weak precipitation δ2H response to large Holocene hydroclimate changes in eastern North America

“…Pollen-inferred vegetation changes at and near our sites (Figure 4) show a shift from a gymnosperm-to an angiosperm-dominated vegetation during the Holocene. Average fractionation factors between n-C29-alkane and precipitation δ 2 H values are comparable for gymnosperms and angiosperms (within 6‰; Stefanescu et al, 2023). Moreover, the vegetation shift is not unique to Twin Ponds (Figure 4).…”

“…n-Alkanes were extracted from 2-8 grams freeze-dried sediment and analyzed for δ 2 H values on a Thermo Scientific Trace GC coupled to a Thermo Delta V IRMS following the methods of Stefanescu et al (2023). All samples were run in duplicates.…”

“…We use n-C29-alkane δ 2 H values to infer δ 2 HMAP values by applying the North American average apparent fractionation factor (εapp) of -131‰ (Stefanescu et al, 2023). The inferred Holocene δ 2 HMAP_C29 values at each lake were calculated as follows:…”

“…Even though n-C29-alkane is predominantly synthesized by terrestrial plants, aquatic plants can also synthesize n-C29-alkane in small amounts and could be an additional source of unusual isotopic values. An aquatic source may be particularly important at Twin Ponds where our primary core was collected from a productive littoral bench distal from the inlet, at a shallow depth of only 0.79 m. The fractionation factor between the aquatically produced n-C29-alkane and δ 2 HMAP (εapp) is ~30‰ larger than that of terrestrial plants (Stefanescu et al, 2023), therefore, a shift to aquatically produced n-alkanes would generate a negative excursion in reconstructed δ 2 HMAP_C29 values at Twin Ponds. Littoral terrestrial vegetation such as grasses may also be an important additional source.…”

“…Therefore, δ 2 Hwax reflects the δ 2 H composition of precipitation. Strong relationships at both regional and global scales between the δ 2 H values of mean annual precipitation (δ 2 HMAP) and those of terrestrial plants leaf-waxes (i.e., n-C29-alkane) make these compounds a suitable proxy for inferring the δ 2 H values of past precipitation (Sachse et al, 2004;Sachse et al, 2006;Garcin et al, 2012;Sachse et al, 2012;Tipple et al, 2013;Freimuth et al, 2017;McFarlin et al, 2019;Stefanescu et al, 2023). As the δ 2 H values of precipitation at a given location are strongly controlled by atmospheric circulation and rainout processes Gat 1996;Dansgaard 1964), the δ 2 H values of terrestrial leaf-waxes provide an opportunity to reconstruct the drivers behind past climate changes such as shifts in atmospheric circulation and moisture sources.…”