Browsing by Author "Lowell, Thomas V."

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    Contrasting sensitivity of lake sediment n-alkanoic acids and n-alkanes to basin-scale vegetation and regional-scale precipitation δ2H in the Adirondack Mountains, NY (USA)
    (Elsevier, 2020-01) Freimuth, Erika J.; Diefendorf, Aaron F.; Lowell, Thomas V.; Bates, Benjamin R.; Schartman, Anna; Bird, Broxton W.; Landis, Joshua D.; Stewart, Alexander K.; Earth Sciences, School of Science
    The hydrogen isotope values of plant waxes (δ2Hwax) primarily reflect plant source water. δ2Hwax preserved in lake sediments has therefore been widely used to investigate past hydroclimate. The processes by which plant waxes are integrated at regional and catchment scales are poorly understood and may affect the δ2Hwax values recorded in sediments. Here, we assess the variability of sedimentary δ2Hwax for two plant wax compound classes (n-alkanes and n-alkanoic acids) across 12 lakes in the Adirondack Mountains that receive similar regional precipitation δ2H but vary at the catchment-scale in terms of vegetation structure and basin morphology. Total long-chain (n-C27 to n-C35) alkane concentrations were similar across all sites (191 ± 53 µg/g TOC) while total long-chain (n-C28 and n-C30) alkanoic acid concentrations were more variable (117 ± 116 µg/g TOC) and may reflect shoreline vegetation composition. Lakes with shorelines dominated by evergreen gymnosperm plants had significantly higher concentrations of long-chain n-alkanoic acids relative to n-alkanes, consistent with our observations that deciduous angiosperms produced more long-chain n-alkanes than evergreen gymnosperms (471 and 33 µg/g TOC, respectively). In sediments, the most abundant chain lengths in each compound class were n-C29 alkane and n-C28 alkanoic acid, which had mean δ2H values of −188 ± 6‰ and −164 ± 9‰, respectively. Across sites, the range in sedimentary n-C29 alkane (22‰) and n-C28 alkanoic acid δ2H (35‰) was larger than expected based on the total range in modeled mean annual precipitation δ2H (4‰). We observed larger mean εapp (based on absolute values) for n-alkanes (−123‰) than for n-alkanoic acids (−97‰). Across sites, the δ2H offset between n-C29 alkane and the biosynthetic precursor n-C30 alkanoic acid (εC29-C30) ranged from −8 to −58‰, which was more variable than expected based on observations in temperate trees (−20 to −30‰). Sediments with greater aquatic organic matter contributions (lower C/N ratios) had significantly larger (absolute) εC29-C30 values, which may reflect long-chain n-alkanoic acids from aquatic sources. Concentration and δ2Hwax data in Adirondack lakes suggest that long-chain n-alkanes are more sensitive to regional-scale precipitation signals, while n-alkanoic acids are more sensitive to basin-scale differences in catchment vegetation and wax sourcing.
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    Hydrogen isotopic composition (δ2H) of diatom-derived C20 highly branched isoprenoids from lake sediments tracks lake water δ2H
    (Elsevier, 2020-12) Corcoran, Megan C.; Diefendorf, Aaron F.; Lowell, Thomas V.; Freimuth, Erika J.; Schartman, Anna K.; Bates, Benjamin R.; Stewart, Alexander K.; Bird, Broxton W.; Earth Sciences, School of Science
    The hydrogen isotopic composition of lake water (δ2Hlw) reflects hydrological processes, which can yield information about evaporation and precipitation changes through time when preserved in lake sediment archives. Unfortunately, few proxies exist that record only δ2Hlw. Instead, most δ2Hlw records represent a mix of aquatic and terrestrial material. Highly branched isoprenoids (HBIs), known to be produced by diatoms in marine and lacustrine settings, may be used as a lake water proxy to directly reconstruct hydroclimate, if the hydrogen isotopic composition of HBIs (δ2HHBI) reflects the δ2Hlw. We test this hypothesis by analyzing 78 sediment samples from 12 lakes in the Adirondack Mountains in New York, for HBI concentrations and δ2H. δ2HHBI was compared to δ2Hlw, which showed an average fractionation (εHBI/lw) of −127.3 ± 15.0‰ (1σ) for all samples in all lakes. Consistency in εHBI/lw between samples implies that δ2HHBI may be used to reconstruct δ2Hlw through time, to help assess how lake systems have changed in the past. Sediment samples collected from deeper (>4 m) zones within the lake had smaller variability in εHBI/lw (±11.9‰, 1σ) than samples from shallower zones, suggesting that εHBI/lw may be sensitive to other factors, such as light availability, which may be related to differences in diatom growth habit (e.g., benthic, planktonic). Similarly, the carbon isotopes of HBIs (δ13CHBI) were higher for sediment samples collected in deeper zones in the lake, suggesting that δ13CHBI can be used to further understand differences in HBI synthesis in diatom communities living in different growth habitats.
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    Late Quaternary hydroclimate variability in the Colombian Andes and the potential for deep sediment core recovery at Lago de Tota revealed through CHIRP and airgun-sourced seismic reflection data
    (AGU, 2018-12) Gibson, Derek; Bird, Broxton W.; Watruss, Nigel; Escobar, Jaime; Lowell, Thomas V.; Ahmed, Maliha; Fonseca, Hector; Velasco, Felipe; Fernandez, Alejandro; Polissar, Pratigya J.
    A geophysical investigation of lacustrine stratigraphy at Lago de Tota (Tota), Boyaca, Colombia provides evidence for significant lake level fluctuations during the late Quaternary from ~60 ka to the present. CHIRP data collected in 2015 from this large, high-elevation lake in the Eastern Cordillera of the Northern Hemisphere (NH) Colombian Andes reveal a series of off-lap and on-lap sequences in the upper ~20 m of the lake’s sediment column that indicate large amplitude changes in lake level. These data, temporally constrained by 14C-dated sediment cores and extrapolated Holocene sedimentation rates, suggest that lake levels at Tota were lower than present during Marine Isotope Stage (MIS) 4 between 60 and 57 ka, relatively high during MIS 3 between 57 and 29 ka, fell to their lowest levels during MIS 2 between 29 and 14 ka, and gradually rose to the modern high-stand through a series of transgressions during MIS 1 and the Holocene from ~14 ka to the present. These fluctuations are broadly consistent with trends observed in other lake level reconstructions from the Northern (in phase) and Southern (out of phase) Hemisphere Andes, possibly supporting the idea that millennial-to-orbital-scale South American hydroclimate variability is linked to shifts in the mean latitude of the Intertropical Convergence Zone (ITCZ) due to the influence of insolation- and ocean circulation-driven hemispheric temperature gradients during glacial/stadial and interglacial/interstadial events. These results, as well as the presence of a thick (>300 m) sedimentary archive, indicate that Tota has significant potential to produce high-resolution, quantitative, paleo- hydroclimate data spanning much of the last 1 million years. Because geophysical surveys and long paleoclimate records from NH South America are exceedingly rare, these data provide critical insight into regional hydroclimate trends through the Late Quaternary.