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Abstract. 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.

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

International audienceContinental intraplate volcanoes, such as Erebus volcano, Antarctica, are associated with extensional tectonics, mantle upwelling and high heat flow. Typically, erupted magmas are alkaline and rich in volatiles (especially CO2), inherited from low degrees of partial melting of mantle sources. We examine the degassing of the magmatic system at Erebus volcano using melt inclusion data and high temporal resolution open-path Fourier transform infrared (FTIR) spectroscopic measurements of gas emissions from the active lava lake. Remarkably different gas signatures are associated with passive and explosive gas emissions, representative of volatile contents and redox conditions that reveal contrasting shallow and deep degassing sources. We show that this unexpected degassing signature provides a unique probe for magma differentiation and transfer of CO2-rich oxidised fluids from the mantle to the surface, and evaluate how these processes operate in time and space. Extensive crystallisation driven by CO2 fluxing is responsible for isobaric fractionation of parental basanite magmas close to their source depth. Magma deeper than 4 kbar equilibrates under vapour-buffered conditions. At shallower depths, CO2-rich fluids accumulate and are then released either via convection-driven, open-system gas loss or as closed-system slugs that ascend and result in Strombolian eruptions in the lava lake. The open-system gases have a reduced state (below the QFM buffer) whereas the closed-system gases preserve their deep oxidised signatures (close to the NNO buffer)

Eruptive activity at Turrialba Volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San José. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here we use high‐frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions. Pulses of deeply derived CO 2 ‐rich gas (CO 2 /S total > 4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to 2 weeks before eruptions, which are accompanied by shallowly derived sulfur‐rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is ~8–10 km deep, whereas the shallow magmatic gas source is at ~3–5 km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H 2 S/SO 2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases, whereas the second did not. Massive degassing (>3000 T/d SO 2 and H 2 S/SO 2 > 1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H 2 S/SO 2 < 0.05) during the second eruptive period, reflecting the depletion of the hydrothermal system or the establishment of high‐temperature conduits bypassing remnant hydrothermal reservoirs, and the transition from phreatic to phreatomagmatic eruptive activity.

No abstract

Ar/ 39 Ar dates on basalts of Grand Canyon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40 K/ 40 Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40 Ar/ 39 Ar data cut this time frame approximately in half; new ages are all <723 ka, with age probability peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semicontinuous record for deciphering late Quaternary incision and faultslip rates and indicates that basalts fl owed into and preserved a record of a progressively deepening bedrock canyon. The Eastern Grand Canyon block (east of Toroweap fault) has bedrock incision rates of 150-175 m/Ma over approximately the last 500 ka; western Grand Canyon block (west of Hurricane fault) has bedrock incision rates of 50-75 m/Ma over approximately the last 720 ka. Fault displacement rates are 97-106 m/Ma on the Toroweap fault (last 500-600 ka) and 70-100 m/Ma on the Hurricane fault (last 200-300 ka). As the river crosses each fault, the apparent incision rate is lowest in the immediate hanging wall, and this rate, plus the displacement rate, is subequal to the incision rate in the footwall. At the reach scale, variation in apparent incision rates delineates ~100 m/Ma of cumulative relative vertical lowering of the western Grand Canyon block relative to the eastern block and 70-100 m of slip accommodated by formation of a hanging-wall anticline. Data from the Lake Mead region indicate that our refi ned fault-dampened incision model has operated over the last 6 Ma. Bedrock incision rate has been 20-30 m/Ma in the lower Colorado River block in the last 5.5 Ma, and displacement on the Wheeler fault has resulted in both lowering of the Lower Colorado River block and formation of a hanging-wall anticline of the 6-Ma Hualapai Limestone. In modeling long-term incision history, extrapolation of Quaternary fault displacement and incision rates linearly back 6 Ma only accounts for approximately two-thirds of eastern and approximately onethird of western Grand Canyon incision. This "incision discrepancy" for carving Grand Canyon is best explained by higher rates during early (5-to 6-Ma) incision in eastern Grand Canyon and the existence of Miocene paleocanyons in western Grand Canyon. Differential incision data provide evidence for relative vertical displacement across Neogene faults of the Colorado Plateau-Basin and Range transition, a key data set for evaluating uplift and incision models. Our data indicate that the Lower Colorado River block has lowered 25-50 m/Ma (150-300 m) relative to the western Grand Canyon block and 125-150 m/Ma (750-900 m) relative to the eastern Grand Canyon block in 6 Ma. The best model explaining the constrained reconstruction of the 5-to 6-Ma Colorado River paleoprofi le, and other geologic data, is that most of the 750-900 m of relative vertical block motion that accompanied canyon 40

[1] The exposed top of the conduit system at Mount Erebus Volcano, Ross Island, Antarctica, is a convecting lava (magma) lake hosting Strombolian eruptions caused by the explosive decompression of large (up to 5 m radius) gas slugs. Short-period (SP; f !1 Hz) seismoacoustic eruption seismograms are accompanied by oscillatory very long period (VLP) signals observed in the near field by broadband seismometers 0.7 to 2.5 km from the lava lake. A variable VLP onset, preceding eruptions by several seconds, is followed by a repeatable VLP coda that persists for several minutes until the lava lake recovers to its preeruptive level. VLP signals are dominated by distinct decaying nonharmonic modes, the largest at periods of 20.7, 11.3, and 7.8 s, with respective source Q values of approximately 11, 18, and 4. Particle motions indicate a temporally evolving source producing increasingly vertical posteruptive displacements as the signal decays. VLP scalar moments, up to $5Â10 11 N m, exceed SP moments by an order of magnitude or more, suggesting distinct, though genetically related, SP and VLP source mechanisms. We conclude that VLP signals arise from excitation of a quasi-linear resonator that is intimately associated with the conduit system and is excited by gravity and inertial forces associated with gas slug ascent, eruption, and magma recharge. VLP signal stability across hundreds of eruptions spanning 5 years, the persistence of the lava lake, and the rapid posteruptive lava lake recovery indicate a stable near-summit magma reservoir and VLP source process. INDEX TERMS: 4544