We present the WD2014 chronology for the upper part (0–2850 m; 31.2 ka BP) of the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core. The chronology is based on counting of annual layers observed in the chemical, dust and electrical conductivity records. These layers are caused by seasonal changes in the source, transport, and deposition of aerosols. The measurements were interpreted manually and with the aid of two automated methods. We validated the chronology by comparing to two high-accuracy, absolutely dated chronologies. For the Holocene, the cosmogenic isotope records of ¹⁰Be from WAIS Divide and ¹⁴C for IntCal13 demonstrated that WD2014 was consistently accurate to better than 0.5 % of the age. For the glacial period, comparisons to the Hulu Cave chronology demonstrated that WD2014 had an accuracy of better than 1 % of the age at three abrupt climate change events between 27 and 31 ka. WD2014 has consistently younger ages than Greenland ice core chronologies during most of the Holocene. For the Younger Dryas–Preboreal transition (11.595 ka; 24 years younger) and the Bølling–Allerød Warming (14.621 ka; 7 years younger), WD2014 ages are within the combined uncertainties of the timescales. Given its high accuracy, WD2014 can become a reference chronology for the Southern Hemisphere, with synchronization to other chronologies feasible using high-quality proxies of volcanism, solar activity, atmospheric mineral dust, and atmospheric methane concentrations.

Future precipitation changes in a warming climate depend regionally upon the response of natural climate modes to anthropogenic forcing. North Pacific hydroclimate is dominated by the Aleutian Low, a semi-permanent wintertime feature characterized by frequent low-pressure conditions that is influenced by tropical Pacific Ocean temperatures through the Pacific-North American (PNA) teleconnection pattern. Instrumental records show a recent increase in coastal Alaskan precipitation and Aleutian Low intensification, but are of insufficient length to accurately assess low frequency trends and forcing mechanisms. Here we present a 1200-year seasonally- to annually-resolved ice core record of snow accumulation from Mt. Hunter in the Alaska Range developed using annual layer counting and four ice-flow thinning models. Under a wide range of glacier flow conditions and layer counting uncertainty, our record shows a doubling of precipitation since ~1840 CE, with recent values exceeding the variability observed over the past millennium. The precipitation increase is nearly synchronous with the warming of western tropical Pacific and Indian Ocean sea surface temperatures. While regional 20^th Century warming may account for a portion of the observed precipitation increase on Mt. Hunter, the magnitude and seasonality of the precipitation change indicate a long-term strengthening of the Aleutian Low.

Remote sensing observations and climate models indicate that the Greenland Ice Sheet (GrIS) has been losing mass since the late 1990s, mostly due to enhanced surface melting from rising summer temperatures. However, in situ observational records of GrIS melt rates over recent decades are rare. Here, we develop a record of frozen meltwater in the West GrIS percolation zone preserved in seven firn cores. Quantifying ice layer distribution as a melt feature percentage (MFP), we find significant increases in MFP in the southernmost five cores over the past 50 years to unprecedented modern levels (since 1550 CE). Annual to decadal changes in summer temperatures and MFP are closely tied to changes in Greenland summer blocking activity and North Atlantic SSTs since 1870. However, summer warming of ~1.2°C since 1870‐1900, in addition to warming attributable to recent SST and blocking variability, is a critical driver of high modern MFP levels.

[1] A record of explosive eruptions over the last 1830 years reconstructed from a South Pole, Antarctica, ice core extends the coverage of volcanic history to the start of the first millennium A.D. The ice core dating by annual layer counting carries an uncertainty of ±2% of the number of years from time markers, with the largest dating error of ±20 years at the bottom of the 182 m core. Several aspects of the methodology of detecting and quantifying volcanic sulfate signals in ice cores are examined in developing this record. The new record is remarkably consistent with previous South Pole records. A comparison with records from several Antarctica locations suggests that the South Pole location is among the best for ice core reconstruction of volcanic records, owing to the excellent preservation of volcanic signals at the South Pole, the relatively low and uniform sulfate background, and the moderately high snow accumulation rates which allow for dating by annual layer counting. A prominent volcanic event dated at 531(±15) A.D., along with evidence from other records, indicates that an unusually large eruption took place in the tropics and was probably responsible for the “mystery cloud” climate episode of 536–537 A.D. The date of 536 is suggested for a prominent volcanic signal that appears in the first half of the sixth century A.D. in ice cores, which can in turn be used as a time stratigraphic marker in dating ice cores by annual layer counting or by computing average accumulation rates or layer thicknesses with such markers.