Nelia W. Dunbar

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Distinguishing subglacial till and glacial marine diamictons in the western Ross Sea, Antarctica: Implications for a last glacial maximum grounding line

Analyses of lithology, stratigraphy, and tephra from marine sediment cores collected from the western Ross Sea during cruises Eltanin 32 and 52 and Deep Freeze 80 and 87 indicate that subglacial till does not extend to the continental shelf edge. Subglacial till occurs as the lowest unit in most cores landward (south) of approximately 74°S, while seaward of approximately 74°S, the lowest diamicton units are glacial marine diamictons. Glacial marine diamictons are distinguished from subglacial tills by the presence of higher and more variable total organic carbon content downcore, distinct tephra layers, stratification, higher diatom and foraminifera abundances, higher sand content, and radiocarbon dates in chronological order downcore. Sand-sized tephra layers from two cores on the outer continental shelf are interpreted as single eruptive events, one likely to have been derived from the Mount Melbourne volcano and the other from the Pleiades volcano. Radiocarbon dates from sediment above and below the tephra layer in one of these cores (Df87-32) show that deposition indicative of open-water conditions occurred between 22 and 26 ka in the western Ross Sea.

Precise interpolar phasing of abrupt climate change during the last ice age

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.

Very long period oscillations of Mount Erebus Volcano

as the signal decays. VLP scalar moments, up to � 5� 10 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 Oceanography: Physical: Internal and inertial waves; 7280 Seismology: Volcano seismology (8419); 8414 Volcanology: Eruption mechanisms; 8419 Volcanology: Eruption monitoring (7280); KEYWORDS: Strombolian, very long period, volcano seismology

40Ar/39Ar geochronology of magmatic activity, magma flux and hazards at Ruapehu volcano, Taupo Volcanic Zone, New Zealand

We have determined precise eruption ages for andesites from Ruapehu volcano in the Tongariro Volcanic Centre of the Taupo Volcanic Zone (TVZ) using 40Ar/39Ar furnace step-heating of separated groundmass concentrates. The plateau ages indicate several eruptive pulses near 200, 134, 45, 22 and 300-m section of lavas in Whangaehu gorge as well as some lavas in Ohinepango and Waihianoa catchments on eastern Ruapehu, and this suite of lavas belongs to the Waihianoa Formation. This pulse of activity is not represented on nearby Tongariro volcano, indicating that the two volcanoes have independent magmatic systems. A younger group of lavas yields dates between 50 and 20 ka and includes lava flows from the Turoa skifield and in the Ohinepango and Mangatoetoenui catchments and is consistent with two pulses of magmatism around the time of the last glacial maximum, relating it broadly to the Mangawhero Formation. Syn- and post-last glacial activity lavas, with ages <15 ka are assigned to the Whakapapa Formation, and include the voluminous flows of the Rangataua Member on southern Ruapehu. Magma flux, integrated over 1000-yr periods, averages 0.6 km3 ka−1 assuming a volcano lifespan of 250 ka. Fluxes for the Te Herenga, Waihianoa and Mangawhero Formations are consistent at 0.93, 0.9 and 0.88 km3 ka−1, respectively. These fluxes are broadly comparable with those measured at other modern andesite arc volcanoes (e.g. Ngauruhoe, 0.88; Merapi, 1.2 and Karymsky 1.2 km3 ka−1). The relatively low flux (0.17 km3 ka−1) calculated for the Whakapapa Formation may derive from underestimates of erupted volume arising from an increase in phreatomagmatic explosive eruptions in postglacial times. However, using volume estimates for the 1995–1996 eruptions and a recurrence interval of 25 yr has yielded an integrated 1000-yr flux of 0.8 km3 ka−1 in remarkable agreement to estimates for the prehistoric eruptions. Overall, Ruapehu shows consistency in magma flux, but at time scales of the order of one hundred to some thousands of years, field evidence suggests that short bursts of activity may produce fluxes up to twenty times greater. This is significant from the perspective of future activity and hazard prediction.

Petrogenesis and volatile stratigraphy of the Bishop Tuff - evidence from melt inclusion analysis

The Bishop Tuff (BT), erupted from the Long Valley Caldera at 0.74 Ma, is composed of a Plinian tephra and ignimbrite (Hildreth, 1979). Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz and sanidine phenocrysts, a strong H2O gradient was present in the upper portion of the magma chamber, mainly in the first 120 km3 of erupted material. The H2O content of the magma which formed the Plinian tephra drops from 6 wt % to 3.5 wt %. In contrast, the magma which formed the ignimbrite contained a relatively constant amount of H2O, between 2 and 4 wt %. The strong drop in H2O content of the magma which formed the Plinian tephra suggests that only in its extreme upper part, if any, was the PH20 in the magma close to Ptotal. The halogen content of the BT magma was low and relatively constant at ∼700 ppm Cl and ∼ 500 ppm F. The trace and major element composition of MI from the Plinian and first-erupted ash flow lobes of the BT are similar to that of bulk tephra. The range in the trace element concentrations of inclusions suggests that fractional crystallization may have affected magmatic composition. However, in addition to fractional crystallization, the composition of MI, phenocrysts and bulk pumice lumps suggests that magma mixing may have been an important process in establishing the final trace element zonation within the Bishop magma chamber and may be responsible for some of the most dramatic observed trace element variations. The BT eruption appears to have removed sequential stratigraphic compositional layers from the magma chamber, whereas fhe Lower Bandelier Tuff (LBT) appears to have erupted chaotically, although both magma chambers are characterized by essentially the same volatile and therefore density zonation. However, the H2O content of the Plinian:ignimbrite transition is different for the two (∼3 wt % for the BT, 4–5 wt % for the LBT), suggesting that the LBT Plinian eruption may have ended prematurely, possibly due to caldera collapse. Therefore, the magma withdrawal dynamics of these eruptions may be more strongly controlled by external factors, such as vent configuration, rather that the volatile gradient of the melt.

Cause of chemical zoning in the Bishop (California) and Bandelier (New Mexico) magma chambers

Rhyolitic magma chambers often erupt to form deposits with a wide range of trace element chemistry, inferred to reflect zoning in the magma chamber prior to eruption. Ion probe microanalyses of trapped melt inclusions and matrix glass from the large Lower Bandelier Tuff and Bishop Tuff eruptions shows that much of this compositional variation can be blamed on the intrusion of a second rhyolitic magma into the base of the chambers. The second rhyolite was composed of similar major elements but contained significantly higher Ti, Sr, and Ba in both examples. Microanalyses of sanidine phenocrysts show pronounced trace element zoning profiles in accord with the glass chemistry. Applying the available diffusion coefficients for Sr in sanidine to the zoning suggests residence times on the order of 104 yrs after the mixing event. The source of the second magma is not known, but similarities in chemical zoning patterns in silicic magmas throughout the world point toward a common process. Mixing of less fractionated magma derived from similar source rocks is the simplest mechanism. Detailed isotopic studies may help distinguish different sources. Independent of the second magma, large variations in trace elements are observed in the melt inclusions from the Lower Bandelier and Bishop Tuffs which can be modeled by ∼ 40% fractional crystallization.

Pre-eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts

Abstract The ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for H2O, F and incompatible trace elements. Eleven melt inclusions from phenocrysts in air-fall tephra from Obsidian Dome near Long Valley, USA gave an average of 4.1±1.2 wt .% H2O and showed large variations in F and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types. Eight inclusions in phenocrysts from the Taupo “ultraplinian” event, New Zealand gave 4.5 ± 0.8 wt .% H2O with small variations in the other elements analyzed. Measured water contents are similar to earlier estimates of the pre-eruptive water contents of these and similar rhyolites and major- and trace-element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment.

Dating the Siple Dome (Antarctica) ice core by manual and computer interpretation of annual layering

The Holocene portion of the Siple Dome (Antarctica) ice core was dated by interpreting the electrical, visual and chemical properties of the core.The data were interpreted manuallyandwith acomputeralgorithm.The algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the GISP2 (Greenland Ice Sheet Project 2) ice core, 10 Be stratigraphic ties to the dendrochronology 14 C recordandthedatedvolcanic stratigraphy.Thealgorithm interpretation ismorecon- sistent andbetter quantifiedthanthe tedious and subjective manual interpretation.

Determination of pre-eruptive H2O, F and Cl contents of silicic magmas using melt inclusions: Examples from Taupo volcanic center, New Zealand

Water, F, and Cl contents of melt inclusions in phenocrysts from the 2-ka-old Taupo and Hatepe plinian tephras, and the ∼22-ka-old Okaia tephra from the Taupo volcanic center, New Zealand, were measured by electron and ion microprobe. Major and trace element chemistry of the inclusions is similar to that of bulk rock, supporting our assumption that volatile contents of inclusions are representative of the magma in which the crystals grew. Inclusions in the 2-ka Taupo plinian tephra contain a mean of 4.3 wt% H2O, 450 ppm F, and 1700 ppm Cl; from the Hatepe plinian tephra 4.3 wt% H2O, 430 ppm F, and 1700 ppm Cl; and from the Okaia tephra 5.9 wt% H2O, 470 ppm F, and 2100 ppm Cl. Sulfur was below the detection limit of 200 ppm. The constant H2O, F and Cl from a number of stratigraphic horizons in the tephra deposits suggest that the Taupo and Hatepe plinian tephras (>8.2 km3 magma volume) were derived from a magma body that did not contain a strong volatile gradient. By inference, there is no pre-eruptive volatile difference between these plinian eruptions and a phrea-toplinian eruption which occurred between the two. Virtually no major element zonation is seen in this eruptive sequence. Although the Okaia tephra was also erupted from the Taupo volcanic center, probably from a similar vent area, its higher volatile contents and distinct composition as compared to the Taupo tephras show that it was derived from a different, and possibly deeper, magma body.