Recent summer warming in northwestern Canada exceeds the Holocene thermal maximum.

Eastern Beringia is one of the few Western Arctic regions where full Holocene climate reconstructions are possible. However, most full Holocene reconstructions in Eastern Beringia are based either on pollen or midges, which show conflicting early Holocene summer temperature histories. This discrepancy precludes understanding the factors that drove past (and potentially future) climate change and calls for independent proxies to advance the debate. We present a ~13.6 ka summer temperature reconstruction in central Yukon, part of Eastern Beringia, using precipitation isotopes in syngenetic permafrost. The reconstruction shows that early Holocene summers were consistently warmer than the Holocene mean, as supported by midges, and a thermal maximum at ~7.6-6.6 ka BP. This maximum was followed by a ~6 ka cooling, and later abruptly reversed by industrial-era warming leading to a modern climate that is unprecedented in the Holocene context and exceeds the Holocene thermal maximum by +1.7 ± 0.7 °C.

Routine high-precision analysis of triple water-isotope ratios using cavity ring-down spectroscopy.

RATIONALE: Water isotope analysis for δ(2) H and δ(18) O values via laser spectroscopy is routine for many laboratories. While recent work has added the δ(17) O value to the high-precision suite, it does not follow that researchers will routinely obtain high precision (17) O excess (Δ(17) O). We demonstrate the routine acquisition of high-precision δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values using a commercially available laser spectroscopy instrument. METHODS: We use a Picarro L2140-i cavity ring-down spectroscopy analyzer with discrete liquid injections into an A0211 vaporization module by a Leap Technologies LC PAL autosampler. The instrument is run in two modes: (1) as recommended by the manufacturer (default mode) and (2) after modifying select default settings and using alternative data types (advanced mode). Reference waters analyzed over the course of 15 months while running unknown samples are used to assess system performance. RESULTS: The default mode provides precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values that may be sufficient for many applications. When using the advanced mode, we reach a higher level of precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values (0.4 mUr, 0.04 mUr, 0.07 mUr, 0.5 mUr, and 8 µUr, respectively, where mUr = 0.001 = ‰, and µUr = 10(-6) ) in a shorter amount of time and with fewer syringe actuations than in the default mode. The improved performance results from an increase in the total integration time for each injected water pulse. CONCLUSIONS: Our recommended approach for routine δ(2) H, δ(17) O, δ(18) O, d and Δ(17) O measurements with the Picarro L2140-i is to make use of conditioning vials, use fewer injections (5 per vial) with greater pulse duration (520 seconds (s) per injection) and use only the first 120 s for δ(2) H measurements and all 520 s for δ(17) O and δ(18) O measurements. Although the sample throughput is 10 unknowns per day, our optimal approach reduces the number of syringe actuations, the effect of memory, and the total analysis time, while improving precision relative to the default approach. Copyright © 2016 John Wiley & Sons, Ltd.

Measurement of SLAP2 and GISP δ17O and proposed VSMOW-SLAP normalization for δ17O and 17O(excess).

RATIONALE: The absence of an agreed-upon δ(17)O value for the primary reference water SLAP leads to significant discrepancies in the reported values of δ(17)O and the parameter (17)O(excess). The accuracy of δ(17)O and (17)O(excess) values is significantly improved if the measurements are normalized using a two-point calibration, following the convention for δ(2)H and δ(18)O values. METHODS: New measurements of the δ(17)O values of SLAP2 and GISP are presented and compared with published data. Water samples were fluorinated with CoF(3). Helium carried the O(2) product to a 5A (4.2 to 4.4 Å) molecular sieve trap submerged in liquid nitrogen. The O(2) sample was introduced into a dual-inlet ThermoFinnigan MAT 253 isotope ratio mass spectrometer for measurement of m/z 32, 33, and 34. The δ(18)O and δ(17) values were calculated after 90 comparisons with an O(2) reference gas. RESULTS: We propose that the accepted δ(17)O value of SLAP be defined in terms of δ(18) O = -55.5 ‰ and (17)O(excess) = 0, yielding a δ(17)O value of approximately -29.6986 ‰ [corrected]. Using this definition for SLAP and the recommended normalization procedure, the δ(17)O value of GISP is -13.16 ± 0.05 ‰ and the (17)O(excess) value of GISP is 22 ± 11 per meg. Correcting previous published values of GISP δ(17)O to both VSMOW and SLAP improves the inter-laboratory precision by about 10 per meg. CONCLUSIONS: The data generated here and compiled from previous studies provide a substantial volume of evidence to evaluate the various normalization techniques currently used for triple oxygen isotope measurements. We recommend that reported δ(17) O and (17)O(excess) values be normalized to the VSMOW-SLAP scale, using a definition of SLAP such that its (17)O(excess) is exactly zero.