Calibrating and Applying Sedimentary Leaf Wax Hydrogen Isotopes to Reconstruct Hydroclimate of Greenland during the Holocene and Last Interglacial

McFarlin, Jamie Marie

doi:https://doi.org/10.21985/n2-sqgj-4x27

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Calibrating and Applying Sedimentary Leaf Wax Hydrogen Isotopes to Reconstruct Hydroclimate of Greenland during the Holocene and Last Interglacial

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Records of temperature and precipitation during Quaternary interglacial periods on Greenland are notably scarce, limiting assessment of the magnitude and rate of local environmental response to orbitally-driven Arctic warming. It is largely unknown if warming across Greenland was spatially uniform through past interglacials, and if warming was accompanied by major changes to the hydrologic cycle, both of which have implications for the size of the Greenland Ice Sheet (GrIS). This dissertation research calibrates the relationship between the hydrogen (H) isotopic composition of modern sedimentary plant waxes and that of precipitation in Greenland for use as a proxy for paleo-precipitation isotopes. The data presented here demonstrate a robust relationship between the H isotopic composition of long-chain sedimentary plant waxes, commonly ascribed to terrestrial plants, and local precipitation on Greenland. These calibration data are used to reconstruct the H isotopic composition of terrestrial leaf water from lacustrine sedimentary waxes, and then combined with independent proxy evidence for the O isotopic composition of lake water (O isotopes of chironomid larvae) and summer air temperature (chironomid species assemblages) from the same lake records. This paired-proxy approach is employed to reconstruct atmospheric moisture during the Last Interglacial (~129 to 116 ka) and the Holocene (11.7 to 0 ka) in northwest Greenland, and the Holocene in south Greenland. In northwest Greenland at Wax Lips Lake, maximum summer air temperatures during the Last Interglacial and Holocene, inferred from chironomid species assemblages, indicate pronounced regional warming. This warming was accompanied by relatively wet conditions, as evidenced by leaf water and lake water that are statistically indistinguishable. In contrast, during the cool late Holocene leaf water is significantly 2H-enriched relative to lake water, indicating arid conditions that promoted evapotranspiration during the summer growth season. Relating these data to published records of the extent of the GrIS through the Holocene indicates the local margin responded to summer air temperature more strongly than to atmospheric moisture. The maximum local retreat of the GrIS occurred during the early Holocene, a period characterized by high relative humidity and relatively warm summers. In contrast, re-advance of the local GrIS occurred during the cool late Holocene, when relative humidity, and by inference precipitation amounts, in northwest Greenland were relatively low. In south Greenland, a comparison of the H isotopes of leaf wax to the O isotopes of chironomids at Pincushion Lake indicates no significant changes in relative humidity over the past 10,000 years (i.e. no periods when leaf water was enriched relative to lake water, indicative of arid conditions), likely reflecting a consistently wet maritime climate for much of the Holocene. Although other records from southwest, west, and southeast Greenland indicate periods of notable aridity during the Holocene, these records disagree on the timing of relatively wet vs. dry conditions. O isotopes in ice cores and dating of glacial geologic features indicate changes in the elevation and extent of the south GrIS through the Holocene, with major retraction in the early Holocene. Although there is no evidence from this data for arid conditions during the Holocene, the H isotopes of leaf wax express a similar magnitude of change in precipitation isotopes over the Holocene as isotopes of ice at NGRIP and GRIP. This suggests other climate drivers, in particular higher summer temperature in the early Holocene, were likely responsible for changes in the extent of the south GrIS. This dissertation adds to the literature new plant biomarker data from Greenland across a broad climate gradient, and further advances biomarker methods by calculating water isotopes from proxy isotope data using new Greenland-specific calibrations, facilitating interpretation of isotopic changes in these proxies through geologic time.

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