Middle Wisconsinan Climate Fluctuations Recorded in Central Alaskan Loess

Fluctuations in Middle Wisconsinan environments are recorded in high resolution proxy climatic curves derived from magnetic susceptibility profiling of central Alaskan loess. Two intervals of low wind intensity and climatic amelioration, separated by a period of somewhat higher wind intensity, can be recognized in several loess records of the Middle Wisconsinan. Radiocarbon dates from loess in the Fox Permafrost Tunnel indicate that the culmination of the later period of low wind intensity occurred ca. 30-32.000 yr BP, and was associated with thermal degradation of permafrost. An older period of low wind intensity early in the Middle Wisconsinan, ca. 50-60,000 yr BP, is correlative with a fossil wood horizon in the permafrost tunnel and a widespread paleosol in loess sections. Warm intervals of similar age are recorded in the Grande Pile pollen record, in ice cores and in marines cores. The 30-32,000 yr BP and the 50-60,000 yr BP warm events recorded in Alaskan loess sequences may have been caused by "greenhouse" warming produced by transient increases in atmospheric CO2 as recorded in the Vostok ice core. A Middle Wisconsinan interval of higher wind intensity, associated with the development of ice wedges, may reflect climatic cooling due to low atmospheric CO2 values ca. 42,000 yr BP.Les courbes climatiques très précises tracées à partir des profils de susceptibilité magnétique faites sur le loess du centre de l'Alaska ont montré des fluctuations au Wisconsinien moyen. Deux intervalles de vents de faible intensité et de réchauffement climatique, séparés par une période de vents plus forts, ont pu être reconstitués à partir de plusieurs inventaires loessiques du Wisconsinien moyen. Les dates au radiocarbone recueillies sur le loess du Fox Permafrost Tunnel montre que l'optimum de la dernière période de vents faibles a eu lieu vers 30-32 ka BP, et qu'il était associé à une régression du pergélisol. Une période plus ancienne de vents faibles au début du Wisconsinien moyen (vers 50-60 ka BP) correspond à un horizon de bois fossile du Permafrost Tunnel et à un paléosol d'une grande superficie dans les régions de loess. Des intervalles datant de la même époque ont été observés à partir des données polliniques de Grande Pile, provenant de carottes marines et de glace. Les périodes chaudes de 30-32 ka et de 50-60 ka BP relevées dans les séquences de loess de l'Alaska ont peut-être été provoquées par l'effet de serre en raison de l'augmentation du CO2 atmosphérique comme l'a enregistrée la carotte de glace de Vostok. L'intervalle de vents plus intenses associé au développement de coins de glace reflète peut-être un refroidissement climatique lié aux faibles valeurs de CO2 atmosphérique, vers 42 ka BP.Die Fluktuationen in der Umwelt des mittleren Wiskonsiniums sind in sehr pràzisen KIimakurven abgebildet, die mittels Magnet-Empfindlichkeitsprofilen auf dem Loss von Zentral-Alaska aufgezeichnet wurden. Zwei Intervalle niedriger Windintensitât und klimatischer Verbesserung, die durch eine Période relativ hôherer Windintensitât unterbrochen wurden, kônnen in einigen LÔssaufzeichnungen aus dem mittleren Wiskonsinium erkannt werden. Radiokarbondatierungen von Loss in dem Fox-Permafrost-Tunnel zeigen, dass der Hôhepunkt der spàteren Période mit geringer Windintensitât um ca. 30-32 000 Jahre v.u.Z. eintrat und mit der thermischen Abtragung des Permafrostbodens verbunden war. Eine altère Période geringer Windintensitât zu Beginn des mittleren Wiskonsiniums, ca. 50-60 000 Jahre v.u.Z. korreliert mit einem fossilen Holz-Horizont im Permafrost-Tunnel und einem ausgedehnten Palâoboden in den Lôssabschnitten. Warme Intervalle ahnlichen Alters sind in dem Pollen-Zeugnis von Grande-PiIe aufgezeichnet, in den Eis und in den marinen Kernen. Die warmen Perioden um 30-32 000 Jahre v.u.Z. und 50-50 000 Jahre v.u.Z., die in Lôsssequenzen von Alaska aufgezeichnet sind, kônnten durch "Treibhaus-Effekt" wegen der Erhôhung des CO2 in der Atmosphère hervorgerufen worden sein, wie im Eisbohrkern von Vostok nachweisbar. Ein Intervall hôherer Windintensitât im mittleren Wiskonsinium verbunden mit der Entwicklung von Eiskeilen mag eine klimatische Abkùhlung als Folge von niedrigen C02-Werten in der Atmosphère um ca, 42 000 Jahre v.u.Z. spiegeln

The Largest Known Maars on Earth, Seward Peninsula, Northwest Alaska

The Espenberg Maars on the northern Seward Peninsula of Alaska were formed by a series of Pleistocene basaltic eruptions through thick permafrost. The maars were excavated as much as 300 m into older lithologies; ranging from 4 to 8 km in diameter, they are the four largest known maars on earth. Hydromagmatic eruptions which derive water from ground ice are evidently extremely explosive. The high heat capacity of ice in permafrost modulates the supply of water interacting with magma during the eruption, producing consistently low coolant-to-fuel ratios in an environment with a sustained, abundant water supply. The Espenberg Maars demonstrate that, under certain conditions, eruptions which involve the interaction of lava and permafrost are powerful enough to produce craters as large as small calderas.Les maars de l'Espenberg situés dans la partie septentrionale de la péninsule Seward en Alaska ont été formés par une série d'éruptions basaltiques datant du pléistocène, à travers une forte épaisseur de pergélisol. Les maars ont été creusés à une profondeur allant jusqu'à 300 m dans d'anciennes roches; d'un diamètre variant entre 4 et 8 km, ils sont les quatre plus grands maars connus sur Terre. Les éruptions hydromagmatiques qui tirent l'eau de la glace de sol sont, comme on l'a déjà constaté, extrêmement explosives. La grande capacité thermique de la glace dans le pergélisol détermine l'approvisionnement en eau qui interagit avec le magma au cours de l'éruption, donnant régulièrement lieu à un faible rapport refroidissant / combustible dans un environnement où l'eau est constamment abondante. Les maars de l'Espenberg démontrent que, dans certaines conditions, les éruptions qui déclenchent une interaction lave-pergélisol sont suffisamment puissantes pour donner naissance à des cratères de la grandeur de petites calderas

Magnetic properties of the Old Crow tephra: Identification of a complex iron titanium oxide mineralogy

International audience[1] The mineralogy and grain-size distribution of the Fe-Ti oxide population of the Old Crow tephra bed, outcropping at the Halfway House loess deposit in central Alaska, are characterized through multiple low-and high-temperature magnetization experiments. The characterization is facilitated by heavy liquid separation of the bulk sample into a low-density ( 0.8 and may play an equally important role as magnetic indicator of titanomagnetite. Furthermore, we demonstrate the ability of low-temperature magnetism to locate a 1 mm thick tephra bed dispersed in loess over 10 cm depth, through the identification of very low concentrations of a titanohematite phase with y = 0.9. The potential for advancing regional correlation of sedimentary deposits through the identification of Fe-Ti oxides common to tephra beds by low-temperature magnetism is illustrated in this study. INDEX TERMS: 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1512 Geomagnetism and Paleomagnetism: Environmental magnetism; 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 8404 Volcanology: Ash deposits; 5109 Physical Properties of Rocks: Magnetic and electrical properties; KEYWORDS: low-temperature magnetism, frequency and amplitude dependence of AC susceptibility, ilmenite-hematite and magnetite-ulvospinel solid solution series, tephra, stratigraphic correlatio

The presence of Holocene cryptotephra in Wales and southern England

There have been few detailed studies into the tephrostratigraphy of southern Britain. We report the tephrostratigraphy of two sites, one in southern England (Rough Tor, Cornwall) and one in Wales (Cors Fochno, west Wales). Our study extends the known southernmost reach of Icelandic cryptotephra in northern Europe. Given the large distance between sites in southern England and eruptive sources (e.g. Iceland 1500–1700 km distant), most of the cryptotephra layers consist of sparse numbers of shards, even by the standards of distal tephrostratigraphy (as low as 3 shards cm−1), each layer spanning only 1 or 2 cm in depth. We identify multiple cryptotephra layers in both sites, extending the known distribution of several tephra layers including the MOR-T4 tephra (∼AD 1000) most probably of Icelandic origin, and the AD 860 B tephra correlated to an eruption of Mount Churchill, Alaska. The two sites record contrasting tephrostratigraphies, illustrating the need for the inclusion of multiple sites in the construction of a regional tephrostratigraphic framework. The tephra layers we describe may provide important isochrons for the dating and correlation of palaeoenvironmental sequences in the south of Britain

Volcanic impacts on peatland microbial communities: A tephropalaeoecological hypothesis-test

Volcanic eruptions affect peatlands around the world, depositing volcanic ash (tephra) and a variety of chemicals including compounds of sulphur. These volcanic impacts may be important for many reasons, in particular sulphur deposition has been shown to suppress peatland methane flux, potentially reinforcing climatic cooling. Experiments have shown that sulphur deposition also forces changes in testate amoeba communities, potentially relating to the reduced methane flux. Large volcanic eruptions in regions with extensive peatlands are relatively rare so it is difficult to assess the extent to which volcanic eruptions affect peatland microbial communities; palaeoecological analyses across tephra layers provide a means to resolve this uncertainty. In this study, testate amoebae were analysed across multiple monoliths from a peatland in southern Alaska containing two tephras, probably representing the 1883 eruption of Augustine Volcano and a 20th Century eruption of Redoubt Volcano. Results showed relatively distinct and often statistically significant changes in testate amoeba community coincident with tephra layers which largely matched the response found in experimental studies of sulphur deposition. The results suggest volcanic impacts on peatland microbial communities which might relate to changes in methane flux

Distal volcanic impacts on peatlands: palaeoecological evidence from Alaska

Despite the fact that volcanic ash (tephra) layers are found preserved in peat deposits around the world, comparatively little research has investigated the impacts of distal volcanic emissions on peatlands. This study investigates the impacts of several late-Holocene volcanic eruptions on five peatlands in southern Alaska using a palaeoecological approach. Testate amoebae analysis, peat humification analysis and a basic analysis of plant macrofossil components were applied across 11 tephra layers. Changes in macrofossil and testate amoebae assemblages occur across several of the tephra layers. The humification results were considered unreliable because of a methodological problem, a finding which may have implications for other studies using this technique. Redundancy analyses on testate amoebae data show statistically significant changes associated with two tephras. The most likely causes of the impacts are volcanic gases, acidic precipitation or tephra-derived leachates. The finding that some tephras are associated with impacts whereas others are not may relate to the season of the eruption or meteorological conditions at the time of ash fall. These results suggest the sensitivity of peatlands and peatland microbial communities to distal volcanic products and imply that changes in key palaeoclimatic proxies may be caused by a mechanism independent of climate change. Implications of the results for peat-based palaeoclimatic studies are discussed, as are possible directions for future research

Tephrochronology and its application: A review

Tephrochronology (from tephra, Gk ‘ashes’) is a unique stratigraphic method for linking, dating, and synchronizing geological, palaeoenvironmental, or archaeological sequences or events. As well as utilising the Law of Superposition, tephrochronology in practise requires tephra deposits to be characterized (or ‘fingerprinted’) using physical properties evident in the field together with those obtained from laboratory analyses. Such analyses include mineralogical examination (petrography) or geochemical analysis of glass shards or crystals using an electron microprobe or other analytical tools including laser-ablation-based mass spectrometry or the ion microprobe. The palaeoenvironmental or archaeological context in which a tephra occurs may also be useful for correlational purposes. Tephrochronology provides greatest utility when a numerical age obtained for a tephra or cryptotephra is transferrable from one site to another using stratigraphy and by comparing and matching inherent compositional features of the deposits with a high degree of likelihood. Used this way, tephrochronology is an age-equivalent dating method that provides an exceptionally precise volcanic-event stratigraphy. Such age transfers are valid because the primary tephra deposits from an eruption essentially have the same short-lived age everywhere they occur, forming isochrons very soon after the eruption (normally within a year). As well as providing isochrons for palaeoenvironmental and archaeological reconstructions, tephras through their geochemical analysis allow insight into volcanic and magmatic processes, and provide a comprehensive record of explosive volcanism and recurrence rates in the Quaternary (or earlier) that can be used to establish time-space relationships of relevance to volcanic hazard analysis. The basis and application of tephrochronology as a central stratigraphic and geochronological tool for Quaternary studies are presented and discussed in this review. Topics covered include principles of tephrochronology, defining isochrons, tephra nomenclature, mapping and correlating tephras from proximal to distal locations at metre- through to sub-millimetre-scale, cryptotephras, mineralogical and geochemical fingerprinting methods, numerical and statistical correlation techniques, and developments and applications in dating including the use of flexible depositional age-modelling techniques based on Bayesian statistics. Along with reference to wide-ranging examples and the identification of important recent advances in tephrochronology, such as the development of new geoanalytical approaches that enable individual small glass shards to be analysed near-routinely for major, trace, and rare-earth elements, potential problems such as miscorrelation, erroneous-age transfer, and tephra reworking and taphonomy (especially relating to cryptotephras) are also examined. Some of the challenges for future tephrochronological studies include refining geochemical analytical methods further, improving understanding of cryptotephra distribution and preservation patterns, improving age modelling including via new or enhanced radiometric or incremental techniques and Bayesian-derived models, evaluating and quantifying uncertainty in tephrochronology to a greater degree than at present, constructing comprehensive regional databases, and integrating tephrochronology with spatially referenced environmental and archaeometric data into 3-D reconstructions using GIS and geostatistics