Showing papers by "Nelia W. Dunbar published in 2009"

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Abstract: 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 ( approximately 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, approximately 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 approximately 3 degrees C warmer than today and atmospheric CO(2) concentration was as high as approximately 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 CO(2).

Climate forcing by iron fertilization from repeated ignimbrite eruptions: the icehouse-silicic large igneous province (SLIP) hypothesis.

Abstract: During middle Eocene to middle Miocene time, development of the Cenozoic icehouse was coincident with a prolonged episode of explosive silicic volcanism, the ignimbrite flare-up of southwestern North America. We present geochronologic and biogeochemical data suggesting that, prior to the establishment of full glacial conditions with attendant increased eolian dust emission and oceanic upwelling, iron fertilization by great volumes of silicic volcanic ash was an effective climatic forcing mechanism that helped to establish the Cenozoic icehouse. Most Phanerozoic cool-climate episodes were coeval with major explosive volcanism in silicic large igneous provinces, suggesting a common link between these phenomena.

Geologic and taphonomic context of El Bosque Petrificado Piedra Chamana (Cajamarca, Peru)

Abstract: Volcanic and volcaniclastic rocks in the northern Peruvian Andes (central Caja-marca, 79°10 ′ W, 6°35 ′ S) contain a diverse assemblage of permineralized woods known as El Bosque Petrifi cado Piedra Chamana. The fossil forest and associated paleosol are preserved in ash-fall and lahar deposits of the Huambos Formation. Dating of plagio-clase from the ash-fall deposit using 40 Ar/ 39 Ar methods yields a middle Eocene age of 39.35 ± 0.21 Ma. Accuracy of this age determination is supported by a more robust sanidine age of 39.52 ± 0.11 Ma from an underlying welded ignimbrite. Fossil wood and leaves associated with the ash-fall deposit include vertical trees rooted in the paleosol and buried in situ by the ash. Fossil wood is also present in high abun-dance and diversity in the overlying lahar. The fossils are signifi cant as a low-latitude assem-blage including a diversity of both monocots and dicots and in having fossil leaves occur-ring in close proximity to fossil woods. Pre-liminary analyses of wood and leaf characters suggest a megathermal climate with some limitations on plant growth associated with limited (seasonal) moisture availability. The assemblage represents lowland tropical forest that was probably growing near sea level and subsequently uplifted to the current elevation at the site (~2400–2600 m).INTRODUCTION

New sites of 3.1-Ma pumice beds in axial-fluvial strata of the Camp Rice and Palomas Formations, southern Rio Grande rift

Abstract: Newly discovered pumice beds in axial-fluvial strata of the Pliocene–lower Pleistocene Camp Rice and Palomas Formations in the southern Rio Grande rift are geochemically correlated to a previously dated 3.1-Ma pumice bed at Hatch Siphon. The Lucero pumice in the Dona Ana Mountains is 1–1.5 m thick and consists of granule- and pebble-sized pumice intercalated with fluvial sand, whereas the Mud Springs pumice along the southeastern flank of the Mud Springs Mountains is 10 cm thick and is composed of sand-sized pumice. Samples from all three sites consist of vesicular, rhyolitic glass fragments and are compositionally identical, particularly with respect to Fe, Ca, and Mn, suggesting derivation from the same 3.1-Ma volcanic eruption. The composition and age of this erupted material is consistent with derivation from the Grants Ridge area, south of Mt. Taylor, implying transport to the ancestral Rio Grande via the Rio Puerco drainage system. If the correlation is correct, the Lucero pumice, along with a bed of 1.6-Ma pumice and the constructional top of the Camp Rice Formation (~0.8 Ma), provide chronologic constraints on the rate of onlap of the northwestern Dona Ana Mountains by axial-fluvial sediment of the Camp Rice Formation. From 3.1 to 1.6 Ma, the sediment accumulation rate was 46.7 m/ Ma, and the lateral rate of eastward onlap was 2 km/Ma. The corresponding values diminished to 18.8 m/Ma and 0.89 km/Ma, respectively, from 1.6 to 0.8 Ma, perhaps due to activity on the Jornada fault, which borders the northern Dona Ana Mountains. If the correlation between the Mud Springs pumice and 3.1-Ma Hatch Siphon pumice is correct, then the Mud Springs pumice provides a reliable chronologic marker within the Palomas Formation that can be compared to existing biostratigraphic data from the same region. The 3.1-Ma Mud Springs pumice is located within the stratigraphic range of the vertebrate collection of Lucas and Oakes (1986) and is consistent with their interpretation of the fauna as medial Blancan in age (~3 Ma). In contrast, the vertebrate collection of Repenning and May (1986), which has been interpreted as very early Blancan II in age (~4.5 Ma), seems anomalously old, given the fact that it is only ~20 m beneath the Mud Springs pumice.

Clay mineralogy of the goathill north rock pile, questa mine, taos county, nm: origins and indications of in-situ weathering

Abstract: Clay minerals play an important role in the gravitational stability of man-made rock piles by affecting geotechnical and hydrological characteristics of the material. The Goathill North rock pile at the Questa Mine in northern New Mexico has been the focus of a multidisciplinary study to determine the effect of weathering on rock pile stability and in particular to determine if clay minerals are forming in the rock pile due to surface weathering. The results of x-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses indicate the clay minerals found within the fine-grained soil matrix of the rock pile are of hydrothermal origins and did not detect any neoformation of clay minerals. The foremost mechanism for an increase in clay minerals within the rock pile is the physical break down of hydrothermally altered rock fragments to release hydrothermal clay minerals. The hydrothermal clay minerals within the rock pile show evidence of weathering by the dehydration of smectite from 2-water to 1-water interlayers within the clay mineral structure. This dehydration is most likely due to higher temperatures and low relative humidities in the Goathill north rock pile during surface exposure.