Description: The waxy coating that protects the leaves and other soft tissues of plants includes n-alkane and n-alkanoic acid compounds that are commonly used as biomarkers to reconstruct past environment. Plant waxes have geological relevance given their persistence in soils and paleosols, as well as in lake and marine sediments, yet diagenesis may alter their molecular and isotopic signatures from synthesis to deposition. This study seeks to understand the fate of plant wax biomarkers in soils after leaf-fall as characterized by a series of tropical soil profiles. We investigate the changes in abundance, molecular distributions, and hydrogen (δD) and carbon isotopic compositions (δ13C) of plant waxes (n-alkanes and n-alkanoic acids) in six litter-to-soil profiles along a 2740 m elevation transect from the eastern flank of the Andes mountains down to the lowland Amazon floodplain in Peru. From litter to soil, we find acid/alkane ratios increase, while absolute abundances decrease. In contrast, within each soil, acid/alkane ratios are roughly constant and we find an equivalent exponential decline in concentration in both compound classes with depth; with molecular distributions indicating some new production. We observe a 4 - 6‰ 13C-enrichment from litter to deeper soils for both C29 n-alkanes and C30 n-alkanoic acids; of which the Suess effect accounts for ≤ 2‰. We infer that microbial degradation and production (or 'turnover') processes influence the δ13C of plant waxes that survive in soils; in contrast, no systematic change in δD values is observed. The plant wax signal in soils includes averaging of inputs and diagenetic effects, so this signature is particularly relevant for the interpretation of plant waxes archives in paleosols and the plant waxes eroded from soils and exported to downstream sedimentary archives. We show that soils represent the major stock of plant wax under living ecosystems, suggesting that soils may be a quantitatively-important source of plant waxes available for fluvial erosion, with implications for studies of carbon cycling and paleoenvironmental reconstructions from downstream geological archives.

Description: Geochemical data, brGDGT fractions and calculated indices for soils and river samples during the wet and dry season in the Madre de Dios catchment, Peru.

Description: Soil profiles were collected along a transect across the Serengeti ecosystem, in Tanzania from 2-3S and 34-35.5E, 1153 to 1677 m above sea level and 0 to 1.6 m soil depth. The samples are modern soils and the temporal span of the soil depth profiles is unconstrained, likely centuries to millennia. The survey is intended to observe soil microbial lipid biomarkers that are commonly used as proxies for temperature and pH and to assess their robustness in alkaline carbonate-precipitating soil profiles, where soil carbonate proxies can also be applied. These modern calibrations can inform reconstructions of Eastern African paleoenvironments using the same proxies in geological archives. Lipid extractions and purifications were performed at USC in 2018-2019 and abundances of branched and isoprenoidal glycerol dialkyl glycerol tetraethers were obtained by high pressure liquid chromatography mass spectrometry, performed in 2020-2021 at the University of Arizona. Contextual data include total dissolved solids and pH measurements at the University of Houston in 2020-2021. For more information, please consult associated manuscript on the GDGTs within these soil profiles: Peaple et al., (2022) Identifying the drivers of GDGT distributions in alkaline soil profiles within the Serengeti ecosystem, Organic Geochemistry, in review. A publication on the bulk organics and compound specific carbon isotopic composition of plant waxes in the same soils: Zhang, et al. (2021) Carbon isotopic composition of plant waxes, bulk organics and carbonates from soils of the Serengeti grasslands, Geochimica et Cosmochimica Acta, 311, 316-331, doi:10.1016/j.gca.2021.07.005. That study includes stable hydrogen isotopic data on plant wax, available from doi:10.1594/PANGAEA.921002. A publication on multiple oxygen isotopes within carbonates in the same soil profiles can be found at: Beverly, E.J., Levin, N.E., Passey, B.H., Aron, P.G., Yarian, D.A., Page, M. and Pelletier, E.M. (2021) Triple oxygen and clumped isotopes in modern soil carbonate along an aridity gradient in the Serengeti, Tanzania. Earth and Planetary Science Letters, 567, 116952, doi:10.1016/j.epsl.2021.116952. That study includes stable carbon isotopic data on soil carbonates, available from doi:10.5281/zenodo.4919027.

Description: ntegrated terrestrial and marine records of northeast African vegetation are needed to provide long high resolution records of environmental variability with established links to specific terrestrial environments. In this study, we compare records of terrestrial vegetation preserved in marine sediments in the Gulf of Aden [Deep Sea Drilling Project (DSDP) Site 231] and an outcrop of lacustrine sediments in the Turkana Basin, Kenya, part of the East African Rift System. We analyzed higher plant biomarkers in sediments from both deposits of known equivalent age, corresponding to a ca. 50-100 ka humid interval prior to the b-Tulu Bor eruption ca. 3.40 Ma, when the Lokochot Lake occupied part of the Turkana Basin. Molecular abundance distributions indicate that long chain n-alkanoic acids in marine sediments are the most reliable proxy for terrestrial vegetation (Carbon Preference Index, CPI = 4.5), with more cautious interpretation needed for n-alkanes and lacustrine archives. Marine sediments record carbon isotopic variability in terrestrial biomarkers of 2-3 per mil, roughly equivalent to 20% variability in the C3/C4 vegetation contribution. The proportion of C4 vegetation apparently increased at times of low terrigenous dust input. Terrestrial sediments reveal much larger (2-10 per mil) shifts in n-alkanoic acid delta13C values. However, molecular abundance and isotopic composition suggest that microbial sources may also contribute fatty acids, contaminating the lacustrine sedimentary record of terrestrial vegetation.