delta H-2(n-alkane) and delta O-18(sugar) biomarker proxies from leaves and topsoils of the Bale Mountains, Ethiopia, and implications for paleoclimate reconstructions

The hydrogen isotopic composition of leaf wax-derived n-alkane (delta H-2(n-alkane)) and oxygen isotopic composition of hemicellulose-derived sugar (delta O-18(sugar)) biomarkers are valuable proxies for paleoclimate reconstructions. Here, we present a calibration study along the Bale Mountains in Ethiopia to evaluate how accurately and precisely the isotopic composition of precipitation is imprinted in these biomarkers. n-Alkanes and sugars were extracted from the leaf and topsoil samples and compound-specific delta H-2(n-alkane) and delta O-18(sugar) values were measured using a gas chromatograph-thermal conversion-isotope ratio mass spectrometer (GC-TC-IRMS). The weighted mean delta H-2(n-alkane) and delta O-18(sugar) values range from - 186 to - 89 parts per thousand and from + 27 to + 46 parts per thousand, respectively. Degradation and root inputs did not appear to alter the isotopic composition of the biomarkers in the soil samples analyzed. Yet, the delta H-2(n-alkane) values show a statistically significant species dependence and delta O-18(sugar) yielded the same species-dependent trends. The reconstructed leaf water of Erica arborea and Erica trimera is H-2- and O-18-enriched by + 55 +/- 5 and + 9 +/- 1 parts per thousand, respectively, compared to precipitation. By contrast, Festuca abyssinica reveals the most negative delta H-2(n-alkane) and least positive delta O-18(sugar) values. This can be attributed to signal-dampening caused by basal grass leaf growth. The intermediate values for Alchemilla haumannii and Helichrysum splendidum can be likely explained with plant physiological differences or microclimatic conditions affecting relative humidity (RH) and thus RH-dependent leaf water isotope enrichment. While the actual RH values range from 69 to 82% (x = 80 +/- 3.4%), the reconstructed RH values based on a recently suggested coupled delta H-2(n-alkane) -delta O-18(sugar) (paleo-) hygrometer approach yielded a mean of 78 +/- 21%. Our findings corroborate (i) that vegetation changes, particularly in terms of grass versus non-grassy vegetation, need to be considered in paleoclimate studies based on delta H-2(n-alkane) and delta O-18(sugar) records and (ii) that the coupled delta H-2(n-alkane) -delta O-18(sugar) (paleo-) hygrometer approach holds great potential for deriving additional paleoclimatic information compared to single isotope approaches.

Automated summary

The study focused on the isotopic composition of leaf wax-derived n-alkane and hemicellulose-derived sugar biomarkers as valuable proxies for paleoclimate reconstructions. Isotopic values were measured from samples along the Bale Mountains in Ethiopia, showing species-dependence and highlighting the importance of considering vegetation changes in paleoclimate studies.