@article {thompson_use_2022,
	title = {Use of δ18Oatm in dating a Tibetan ice core record of Holocene/Late Glacial climate},
	journal = {Proceedings of the National Academy of Sciences},
	volume = {119},
	number = {45},
	year = {2022},
	note = {Publisher: Proceedings of the National Academy of Sciences},
	month = {11/2022},
	pages = {e2205545119},
	abstract = {Ice cores from the northwestern Tibetan Plateau (NWTP) contain long records of regional climate variability, but refrozen meltwater and dust in these cores has hampered development of robust timescales. Here, we introduce an approach to dating the ice via the isotopic composition of atmospheric O2 in air bubbles (δ18Oatm), along with annual layer counting and radiocarbon dating. We provide a robust chronology for water isotope records (δ18Oice and d-excess) from three ice cores from the Guliya ice cap in the NWTP. The measurement of δ18Oatm, although common in polar ice core timescales, has rarely been used on ice cores from low-latitude, high-altitude glaciers due to (1) low air pressure, (2) the common presence of refrozen melt that adds dissolved gases and reduces the amount of air available for analysis, and (3) the respiratory consumption of molecular oxygen (O2) by micro-organisms in the ice, which fractionates the δ18O of O2 from the atmospheric value. Here, we make corrections for melt and respiration to address these complications. The resulting records of water isotopes from the Guliya ice cores reveal climatic variations over the last 15,000 y, the timings of which correspond to those observed in independently dated lake and speleothem records and confirm that the Guliya ice cap existed before the Holocene. The millennial-scale drivers of δ18Oice are complex and temporally variable; however, Guliya δ18Oice values since the mid-20th century are the highest since the beginning of the Holocene and have increased with regional air temperature.},
	doi = {10.1073/pnas.2205545119},
	url = {https://www.pnas.org/doi/10.1073/pnas.2205545119},
	author = {Thompson, Lonnie G. and Severinghaus, Jeffrey P. and Yao, Tandong and Davis, Mary E. and Mosley-Thompson, Ellen and Beaudon, Emilie and Sierra-Hern{\'a}ndez, M. Roxana and Porter, Stacy E.}
}
@article {2989,
	title = {The triple argon isotope composition of groundwater on ten-thousand-year timescales},
	journal = {Chemical Geology},
	volume = {583},
	year = {2021},
	month = {11/2021},
	pages = {120458},
	abstract = {Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas 14C of dissolved inorganic carbon and dissolved 4He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic 40Ar in groundwater acquired from decay of 40K in aquifer minerals should be insensitive to some processes that impact 14C and 4He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic 40Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100~ka). Here we present the first high-precision (<1{\textperthousand}) measurements of triple Ar isotopes (40Ar, 38Ar, 36Ar) in groundwater. We introduce a model that distinguishes radiogenic 40Ar from atmospheric 40Ar by using the non-radiogenic Ar isotopes (36Ar, 38Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic 40Ar excess (Δ40Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ40Ar ranges from ~0{\textperthousand} (the expected minimum value) to +4.2{\textperthousand} across three study areas near Fresno, San Diego, and the western Mojave Desert. Based on measurements from a network of 23 scientific monitoring wells in San Diego, we find evidence for a strong dependence of Δ40Ar on aquifer lithology. We suggest that Δ40Ar is fundamentally controlled by the weathering of old K-bearing minerals and thus reflects both the degree of groundwater-rock interaction, which is related to groundwater age, and the integrated flow through different geological formations. Future studies of Late Pleistocene groundwater may benefit from high-precision triple Ar isotope measurements as a new tool to better interpret 14C- and 4He-based constraints on groundwater age and flow.},
	keywords = {groundwater, Groundwater dating, Isotope geochemistry, Noble gas},
	isbn = {0009-2541},
	doi = {10.1016/j.chemgeo.2021.120458},
	url = {https://www.sciencedirect.com/science/article/pii/S0009254121004010},
	author = {Seltzer, Alan M. and Krantz, John A. and Ng, Jessica and Danskin, Wesley R. and Bekaert, David V. and Barry, Peter H. and Kimbrough, David L. and Kulongoski, Justin T. and Severinghaus, Jeffrey P.}
}
