The hydrochemistry of Lake Vostok and the potential for life in Antarctic subglacial lakes     

Martin J. Siegert  Martyn Tranter  J. Cynan Ellis‐Evans  John C. Priscu  W. Berry Lyons 《水文研究》2003,17(4):795-814

Our understanding of Lake Vostok, the huge subglacial lake beneath the East Antarctic Ice Sheet, has improved recently through the identification of key physical and chemical interactions between the ice sheet and the lake. The north of the lake, where the overlying ice sheet is thickest, is characterized by subglacial melting, whereas freezing of lake water occurs in the south, resulting in ～210 m of ice accretion to the underside of the ice sheet. The accreted ice contains lower concentrations of the impurities normally found in glacier ice, suggesting a net transfer of material from meltwater into the lake. The small numbers of microbes found so far within the accreted ice have DNA profiles similar to those of contemporary surface microbes. Microbiologists expect, however, that Lake Vostok, and other subglacial lakes, will harbour unique species, particularly within the deeper waters and associated sediments. The extreme environments of subglacial lakes are characterized by high pressures, low temperatures, permanent darkness, limited nutrient availability, and oxygen concentrations that are derived from the ice that provides the meltwater. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Strontium Isotopic Signatures of Streams from Taylor Valley, Antarctica, Revisited: The Role of Carbonate Mineral Dissolution     

Carolyn B. Dowling  W. B. Lyons  Kathleen A. Welch 《Aquatic Geochemistry》2013,19(3):231-240

We have collected and analyzed a larger set of stream waters for major ions and strontium isotopes in Taylor Valley, McMurdo Dry Valleys, Antarctica. These new data substantiate the concept that the dissolution of carbonate minerals is a significant source of strontium, even in polar desert environments where liquid water is primarily limited to stream channels. In Taylor Valley, most of the carbonate minerals present are the result of secondary processes, such as mineral precipitation and/or eolian deposition, and not through primary geologic sources, such as bedrock or till sources. In Von Guerard Stream (Lake Fryxell basin) and Andersen Creek (Lake Hoare basin), water samples were collected during the austral summer to determine short-term ⁸⁷Sr/⁸⁶Sr patterns. The observed variability in both time and space may be dependent on the relative amounts of primary and secondary carbonates present in the streambeds and hyporheic zone as well as contingent on the relative proportions of carbonate and aluminosilicate weathering.  相似文献   

Constraining pathways of microbial mediation for carbonate concretions of the Miocene Monterey Formation using carbonate-associated sulfate     

Sean J. Loyd  William M. Berelson  Timothy W. Lyons  Douglas E. Hammond  Frank A. Corsetti 《Geochimica et cosmochimica acta》2012

Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and timing relationships to particular concretions has remained elusive. Previously employed proxies (e.g., carbon and oxygen isotopes) cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen, nitrate, metal oxides, and sulfate as electron acceptors, in addition to degradation by thermal decarboxylation. Here, we employ concentrations of carbonate-associated sulfate (CAS) and δ³⁴S_CAS (along with more traditional approaches) to determine the specific nature of concretion authigenesis within the Miocene Monterey Formation.Integrated geochemical analyses reveal that at least three specific organo-diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample-site specific. One calcitic concretion from the Phosphatic Shale Member at Naples Beach yields δ³⁴S_CAS values near Miocene seawater sulfate (～+22‰ VCDT), abundant CAS (ca. 1000 ppm), depleted δ¹³C_carb (～?11‰ VPDB), and very low concentrations of Fe (ca. 700 ppm) and Mn (ca. 15 ppm)—characteristics most consistent with shallow formation in association with organic matter degradation by nitrate, iron-oxides and/or minor sulfate reduction. Cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated δ³⁴S_CAS (up to ～+37‰), CAS concentrations of ～600 ppm, mildly depleted δ¹³C_carb (～?6‰), moderate amounts of Mn (ca. 250 ppm), and relatively low Fe (ca. 1700 ppm), indicative of formation in sediments dominated by sulfate reduction. Finally, concretions within a siliceous host at Montaña de Oro and Naples Beach show minimal CAS concentrations, positive δ¹³C values, and the highest concentrations of Fe (ca. 11,300 ppm) and Mn (ca. 440 ppm), consistent with formation in sediments experiencing methanogenesis in a highly reducing environment. This study highlights the promise in combining CAS analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity.  相似文献   

The possible role of magnetic measurements in the discrimination of Sahara/Sahel dust sources     

Richard Lyons  Frank Oldfield  Earle Williams 《地球表面变化过程与地形》2012,37(6):594-606

The mineral magnetic properties of deposited dusts collected along a broadly north‐to‐south transect across Niger have been investigated on both a bulk and particle size‐specific basis. Dusts display a general north‐to‐south gradient in fine‐grained ferrimagnetic mineral (magnetite/maghemite) concentrations, with samples south of the Sahara/Sahel transition (south of ~15·5^o N) generally containing greater concentrations than dusts from further north where the climate is much drier. This distinction is seen especially clearly in the clay (< 2 µm) fraction, which harbours the products of weathering and pedogenesis. This gradient in ferrimagnetic mineral concentrations broadly parallels that previously reported for surface soils/sands across a similar latitudinal range. We suggest that the regional distinction in both dust and surface soil/sand magnetic properties can be related to differences in weathering regime between the arid Saharan parts of the transect and the Sahel sites where higher rainfall has permitted stronger weathering and pedogenesis. Given that the weathering‐related magnetic signatures in the clays dominate the magnetic properties of the < 16 µm fractions in these samples, and that this particle size component is most likely to be involved in long‐range transport, magnetic measurements hold out the potential of discriminating Sahara and Sahel dusts deposited in remote areas. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Facilitating adaptation of biodiversity to climate change: a conceptual framework applied to the world’s largest Mediterranean-climate woodland     

Suzanne M. Prober  Kevin R. Thiele  Philip W. Rundel  Colin J. Yates  Sandra L. Berry  Margaret Byrne  Les Christidis  Carl R. Gosper  Pauline F. Grierson  Kristina Lemson  Tom Lyons  Craig Macfarlane  Michael H. O’Connor  John K. Scott  Rachel J. Standish  William D. Stock  Eddie J. B. van Etten  Grant W. Wardell-Johnson  Alexander Watson 《Climatic change》2012,110(1-2):227-248

The importance of ecological management for reducing the vulnerability of biodiversity to climate change is increasingly recognized, yet frameworks to facilitate a structured approach to climate adaptation management are lacking. We developed a conceptual framework that can guide identification of climate change impacts and adaptive management options in a given region or biome. The framework focuses on potential points of early climate change impact, and organizes these along two main axes. First, it recognizes that climate change can act at a range of ecological scales. Secondly, it emphasizes that outcomes are dependent on two potentially interacting and countervailing forces: (1) changes to environmental parameters and ecological processes brought about by climate change, and (2) responses of component systems as determined by attributes of resistance and resilience. Through this structure, the framework draws together a broad range of ecological concepts, with a novel emphasis on attributes of resistance and resilience that can temper the response of species, ecosystems and landscapes to climate change. We applied the framework to the world’s largest remaining Mediterranean-climate woodland, the ‘Great Western Woodlands’ of south-western Australia. In this relatively intact region, maintaining inherent resistance and resilience by preventing anthropogenic degradation is of highest priority and lowest risk. Limited, higher risk options such as fire management, protection of refugia and translocation of adaptive genes may be justifiable under more extreme change, hence our capacity to predict the extent of change strongly impinges on such management decisions. These conclusions may contrast with similar analyses in degraded landscapes, where natural integrity is already compromised, and existing investment in restoration may facilitate experimentation with higher risk?options.  相似文献   

Timescales for the evolution of oxygen isotope compositions in the solar nebula     

J.R. Lyons  E.A. Bergin  A.M. Davis  K. Hashizume 《Geochimica et cosmochimica acta》2009,73(17):4998-6266

We review two models for the origin of the calcium-, aluminum-rich inclusion (CAI) oxygen isotope mixing line in the solar nebula: (1) CO self-shielding, and (2) chemical mass-independent fractionation (MIF). We consider the timescales associated with formation of an isotopically anomalous water reservoir derived from CO self-shielding, and also the vertical and radial transport timescales of gas and solids in the nebula. The timescales for chemical MIF are very rapid. CO self-shielding models predict that the Sun has Δ¹⁷O_SMOW ∼ −20‰ (Clayton, 2002), and chemical mass-independent fractionation models predict Δ¹⁷O_SMOW ∼0‰. Preliminary Genesis results have been reported by McKeegan et al. (McKeegan K. D., Coath C. D., Heber, V., Jarzebinski G., Kallio A. P., Kunihiro T., Mao P. H. and Burnett D. S. (2008b) The oxygen isotopic composition of captured solar wind: first results from the Genesis. EOS Trans. AGU 89(53), Fall Meet. Suppl., P42A-07 (abstr)) and yield a Δ¹⁷O_SMOW of ∼ −25‰, consistent with a CO self-shielding scenario. Assuming that subsequent Genesis analyses support the preliminary results, it then remains to determine the relative contributions of CO self-shielding from the X-point, the surface of the solar nebula and the parent molecular cloud.The relative formation ages of chondritic components can be related to several timescales in the self-shielding theories. Most importantly the age difference of ∼1-3 My between CAIs and chondrules is consistent with radial transport from the outer solar nebula (>10 AU) to the meteorite-forming region, which supports both the nebular surface and parent cloud self-shielding scenarios. An elevated radiation field intensity is predicted by the surface shielding model, and yields substantial CO photolysis (∼50%) on timescales of 0.1-1 My. An elevated radiation field is also consistent with the parent cloud model. The elevated radiation intensities may indicate solar nebula birth in a medium to large cluster, and may be consistent with the injection of ⁶⁰Fe from a nearby supernova and with the photoevaporative truncation of the solar nebula at KBO orbital distances (∼47 AU). CO self-shielding is operative at the X-point even when H₂ absorption is included, but it is not yet clear whether the self-shielding signature can be imparted to silicates. A simple analysis of diffusion times shows that oxygen isotope exchange between ¹⁶O-depleted nebular H₂O and chondrules during chondrule formation events is rapid (∼minutes), but is also expected to be rapid for most components of CAIs, with the exception of spinel. This is consistent with the observation that spinel grains are often the most ¹⁶O-rich component of CAIs, but is only broadly consistent with the greater degree of exchange in other CAI components. Preliminary disk model calculations of self-shielding by N₂ demonstrate that large δ¹⁵N enrichments (∼ +800‰) are possible in HCN formed by reaction of N atoms with organic radicals (e.g., CH₂), which may account for ¹⁵N-rich hotspots observed in lithic clasts in some carbonaceous chondrites and which lends support to the CO self-shielding model for oxygen isotopes.  相似文献   

Bank erosion of legacy sediment at the transition from vertical to lateral stream incision          下载免费PDF全文

Nathan J. Lyons  Michael J. Starek  Karl W. Wegmann  Helena Mitasova 《地球表面变化过程与地形》2015,40(13):1764-1778

Streambank erosion is a primary source of suspended sediments in many waterways of the US Atlantic Piedmont. This problem is exacerbated where banks are comprised of fine sediment produced by the intensive land use practices of early European settlers. A stream in this region, Richland Creek incises into banks comprised of three stratigraphic layers associated with historic land use: pre‐European settlement, early European agriculture and development, and water‐powered milldam operation. This study aims to identify the bank processes along a reach of Richland Creek that is eroding towards its pre‐disturbance elevation. The volume of material that has eroded along this stream since the milldam breached was calculated by differencing a reconstructed surface of the pond bed and an aerial lidar digital terrain model (DTM). Immediately downstream from the study reach, the channel is floored by bedrock and immediately upstream the rate of channel erosion approximately doubled along the longitudinal profile of Richland Creek, which indicate that the study reach spans the transition from a channel dominated by vertical incision in the upstream direction to horizontal widening in the downstream direction. The combined hydrometeorological conditions and dominant processes causing reach‐scale cut bank erosion were investigated with analyses of stream stage, precipitation, and streambank volumetric and surfaces change that was measured during nine terrestrial lidar surveys in 2010–2012. The spatial variability of erosion during a simulated precipitation event was examined in a field‐based experiment. Erosion was greatest where mill pond sediment columns detached along vertical desiccation and horizontal seepage cracks. This sediment accumulated on the bank toe throughout the study and was a source of readily‐entrained fine sediment contrary to the upper reaches where depositional accommodation space is more limited. Findings suggest that hotspots of sediment excavation progress upstream, indicating that restoration efforts should focus upon stabilizing banks at these locations. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

The Saline Lakes of the McMurdo Dry Valleys, Antarctica     

William J. Green  W. Berry Lyons 《Aquatic Geochemistry》2009,15(1-2):321-348

The McMurdo Dry Valleys of Antarctica are among a rare group of ice-free regions lying along the coast of an otherwise ice-burdened continent. For Antarctica, these are highly atypical regions of exposed rock and barren soils. Within their 4,000 km² expanse, the valleys contain a number of permanently ice-covered, closed-basin lakes, which range from freshwater to highly saline environments. This paper examines the physical structure, geochemistry, nutrient and trace metal dynamics, biology, and hydrologic history of saline Lake Bonney (both east and west lobes), Fryxell, Vanda, and Joyce and provides an update to recently published volumes on these pristine systems.  相似文献   

Acoustic Propagation Through Clustered Bubble Clouds     

Weber  T.C. Lyons  A.P. Bradley  D.L. 《Oceanic Engineering, IEEE Journal of》2007,32(2):513-523

One of the underlying assumptions in the effective medium theory describing the propagation of acoustic waves through bubble clouds is that the probability of an individual bubble being located at some position in space is independent of the locations of other bubbles. However, bubbles within naturally occurring clouds may be influenced by the dynamics of the fluids in which they are entrained so that they become preferentially concentrated, or clustered, leading to statistical dependence in their positions. For bubble clouds in which the important scattering terms include those with interactions between at least two bubbles, statistical dependence between bubble positions leads to a reduction in the attenuation of the coherent acoustic pressure field from that which would be predicted for a nonclustered bubble cloud. Bubble clustering can be accommodated in effective medium theories using correlation functions describing the relationship between the positions of the bubbles. For double scattering, the two-bubble correlation (i.e., the pair correlation function) must be used, for triple scattering, the three bubble correlation must be used, and so on. In contrast to the three attenuation of the coherent field, making the assumption of independent bubble positions leads to an underestimate of the incoherent field. Both the coherent and incoherent acoustic fields for bubble clouds exhibiting correlated bubble positions are explored in this paper with the use of numerical simulations.  相似文献   

A palaeomagnetic secular variation record from c. 120 000 yr-old New Zealand cave sediments     

Gillian M. Turner  Ruth G. Lyons 《Geophysical Journal International》1986,87(3):1181-1192

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