Detrital input and early diagenesis in sediments from Lake Baikal revealed by rock magnetism

A rock magnetic study was performed on sediment cores from six locations in Lake Baikal. For a comprehensive approach of the processes influencing the rock magnetic signal, additional data are presented such as total organic carbon (TOC), total sulphur (TS), opal, water content and relative variations in iron and titanium measured on selected intervals. In glacial sediments, the magnetic signal is dominated by magnetite, which is considered to be of detrital origin. This predominance of magnetite is interrupted by distinct horizons of authigenic greigite, probably confined to reductive microenvironments. In interglacial stages, besides dilution by biogenic silica and a decreasing detrital input, the weakness of the rock magnetic signal is also due to a reductive dissolution of magnetic particles. The magnetic assemblage is strongly linked to the redox history of interglacial sediment. In the oxidised bottom sediments of Lake Baikal, a biogenic magnetite is observed [Peck, J.A., King, J.W., 1996. Magnetofossils in the sediments of lake Baikal, Siberia. Earth Planet. Sci. Lett. 140 (1–4), 159–172]. After burial under the redox front, the magnetite is preferentially dissolved, and detrital hematite remains dominant when the sedimentation rate is low and when the residence time of the magnetite close to the redox boundary is long. During these low sedimentation rate conditions, the redox front is preserved [Granina, L., Müller, B. and Wehrli, B., 2004. Origin and dynamics of Fe and Mn sedimentary layers in Lake Baikal. Chem. Geol. 205 (1-2), 55-72]. At constant sedimentation rate and fast burial, the magnetite is preserved or transformed into greigite when sulphate-reducing conditions are reached in the sediment. In interglacial sediments, the magnetic assemblages depict changes in the sedimentation rate, which are traced using the ratio of magnetite over hematite (S-ratio). At the beginning of interglacials, the sedimentation rate is constant with an assemblage magnetite+greigite (high S-ratio), and at the end of some interglacials, the sedimentation rate decreases with a predominance of hematite (low S-ratio).     

Variation in Lake Baikal's phytoplankton distribution and fluvial input assessed by SeaWiFS satellite data

The current behaviour of selected climate proxies in Lake Baikal was assessed by remote sensing analyses of ‘Sea viewing Wide Field of view Sensor’ (SeaWiFS) satellite data. Suitable proxies include optically visible water constituents such as phytoplankton, suspended terrigenous matter and yellow substance. These limnological parameters reflect the present-day climate bioproductivity and the river discharge in the catchment area.A biological and geochemical ground truth data set for Lake Baikal was established with the help of members of the paleoclimate project ‘high-resolution CONTINENTal paleoclimate record in Lake Baikal’ (CONTINENT). For processing the SeaWiFS satellite data, the atmospheric correction was adapted to the case of Lake Baikal. Chlorophyll as a proxy for phytoplankton was quantified using global NASA ocean colour algorithms OC2 and OC4. In cases of no optical interferences by terrigenous input, the calculated chlorophyll concentrations in clear pelagic waters were within ±30% accuracy with the CONTINENT cruise data during the summers of 2001 and 2002. Within this range of accuracy, the SeaWiFS time series will be able to show the seasonal variations of chlorophyll of specified bio-optical provinces of Lake Baikal and of CONTINENT sites. In this study, the suspended matter as a proxy for the terrigenous input was calculated according to an empirical algorithm using ground truth data in the time frame of flooding events in summer 2001. The approach chosen correlates the suspended matter concentration with the remotely sensed parameter of ‘attenuation coefficient’ to account for the organic-rich terrigenous input that originates from the swampy watersheds.Seasonal and spatial information that is provided by the analyses of the SeaWiFS satellite data will assist paleoclimate researchers to interpret the autochthonous and allochthonous influences at the CONTINENT coring sites.     

Magnetic characterization of Cretaceous‐Tertiary boundary sediments

Abstract— Rock magnetic properties across several K‐T boundary sections have been investigated to reveal any possible magnetic signature associated with the remains of the impact event at the end of the Cretaceous. Studied sections' locations vary in distance to the Chicxulub structure from distal (Agost and Caravaca, Spain), through closer (ODP Hole 1049A, Blake Nose, North Atlantic), to proximal (El Mimbral and La Lajilla, Mexico). A clear magnetic signature is associated with the fireball layer in the most distal sections, consisting of a sharp increase in susceptibility and saturation isothermal remanent magnetization (SIRM), and a decrease in remanence coercivity. Magnetic properties in these sections point to a distinctive ferrimagnetic phase, probably corresponding to the reported Mg‐ and Ni‐rich, highly oxidized spinels of meteoritic origin. At closer and proximal sections magnetic properties are different. Although there is an increase in susceptibility and SIRM associated with a rusty layer placed on top of the siliciclastic deposit in proximal sections, and with a similar limonitic layer on top of the spherule bed that defines the boundary at Blake Nose, the magnetic properties indicate a mixture of iron oxyhydroxides dominated by fine‐grained goethite. Based on previous geochemical studies at Blake Nose and new geochemical and PGE abundance measurements performed in this work at El Mimbral, this goethite‐rich layer can be interpreted as an effect of diagenetic remobilization and precipitation of Fe. There is not enough evidence to assert that this Fe concentration layer at proximal sections is directly related to deposition of fine meteoritic material. Magnetic, geochemical, and iridium data reject it as a primary meteoritic phase.     

Geophysical survey of the proposed Tsenkher impact structure,Gobi Altai,Mongolia

Abstract– We have performed forward magnetic and gravity modeling of data obtained during the 2007 expedition to the 3.7 km in diameter, circular, Tsenkher structure, Mongolia, in order to evaluate the cause of its formation. Extensive occurrences of brecciated rocks, mainly in the form of an ejecta blanket outside the elevated rim of the structure, support an explosive origin (e.g., cosmic impact, explosive volcanism). The host rocks in the area are mainly weakly magnetic, silica‐rich sandstones, and siltstones. A near absence of surface exposures of volcanic rocks makes any major volcanic structures (e.g., caldera) unlikely. Likewise, the magnetic models exclude any large, subsurface, intrusive body. This is supported by an 8 mGal gravity low over the structure indicating a subsurface low density body. Instead, the best fit is achieved for a bowl‐shaped structure with a slight central rise as expected for an impact crater of this size in mainly sedimentary target. The structure can be either root‐less (i.e., impact crater) or rooted with a narrow feeder dyke with relatively higher magnetic susceptibility and density (i.e., volcanic maar crater). The geophysical signature, the solitary appearance, the predominantly sedimentary setting, and the comparably large size of the Tsenkher structure favor the impact crater alternative. However, until mineralogical/geochemical evidence for an impact is presented, the maar alternative remains plausible although exceptional as it would make the Tsenkher structure one of the largest in the world in an unusual setting for maar craters.     

Geological features indicative of processes related to the hematite formation in Meridiani Planum and Aram Chaos, Mars: a comparison with diagenetic hematite deposits in southern Utah, USA

In order to understand the formation of the few but large, hematite deposits on Mars, comparisons are often made with terrestrial hematite occurrences. In southern Utah, hematite concretions have formed within continental sandstones and are exposed as extensive weathered-out beds. The hematite deposits are linked to geological and geomorphological features such as knobs, buttes, bleached beds, fractures and rings. These terrestrial features are visible in aerial and satellite images, which enables a comparison with similar features occurring extensively in the martian hematite-rich areas. The combination of processes involved in the movement and precipitation of iron in southern Utah can provide new insights in the context of the hematite formation on Mars. Here we present a mapping of the analogue geological and geomorphological features in parts of Meridiani Planum and Aram Chaos. Based on mapping comparisons with the Utah occurrences, we present models for the formation of the martian analogues, as well as a model for iron transport and precipitation on Mars. Following the Utah model, high albedo layers and rings in the mapped area on Mars are due to removal or lack of iron, and precipitation of secondary diagenetic minerals as fluids moved up along fractures and permeable materials. Hematite was precipitated intraformationally where the fluid transporting the reduced iron met oxidizing conditions. Our study shows that certain geological/geomorphological features can be linked to the hematite formation on Mars and that pH differences could suffice for the transport of the iron from an orthopyroxene volcanoclastic source rock. The presence of organic compounds can enhance the iron mobilization and precipitation processes. Continued studies will focus on possible influence of biological activity and/or methane in the formation of the hematite concretions in Utah and on Mars.     

Climatic change in Central Asia during MIS 3/2: a case study using biological responses from Lake Baikal

A Marine Isotope Stage (MIS) 3/early MIS 2 section from a structural high along the east coast of the North Basin of Lake Baikal was analysed for diatoms, C/N ratios, and organic carbon isotope ratios. Diatoms were present throughout MIS 3 and early MIS 2, with high concentrations of the planktonic taxa Cyclotella sp. c.f. gracilis between 54 and 51.5 kyr BP indicating relatively warm, interstadial, conditions. Following a %TOC inferred climatic cooling between 43.2 and 39.1 kyr BP, evidence of a more muted δ¹³C_(organic) and %TOC inferred climatic warming from c. 39.1–34.7 kyr BP coincides with a period of very high diatom concentrations, indicating high aquatic productivity, at the Buguldeika Saddle in the South Basin of Lake Baikal. No evidence exists for a ‘Kuzmin’ catchment erosional event in the North Basin during MIS 3. This, however, may reflect the location of the coring site away from major riverine inputs. Abrupt climatic cooling at the culmination of both warm phases in the North Basin are associated, on the basis of the palaeomagnetic age-model and correlations to existing sites in Lake Baikal, with the initiation of Heinrich events 5 (c. 50 kyr BP) and 4 (c. 35 kyr BP), respectively, in the North Atlantic. The amount of organic material declines across the MIS 3/MIS 2 transition while constant C/N ratios suggest organic material to be predominantly derived from phytoplankton. An increase in δ¹³C_(organic) at the MIS 3/MIS 2 transition may therefore indicate changes in aquatic productivity, pCO₂ or the inorganic carbon pool.     

Metal-catalyzed degradation and racemization of amino acids in iron sulfate brines under simulated martian surface conditions

Recent lines of evidence indicate that Mars may have been substantially warmer and wetter, raising questions about whether the planet’s surface was once more conducive to the possibility of life. To better constrain the extent of preservation of ancient amino acid biosignatures, the effects of metal ions and UV radiation were investigated with respect to the rates of diagenesis of amino acids in metal-rich sulfate brines. Individual experimental results indicate that the oxidation rates of amino acids loosely followed trends characteristic of a radiolytic oxidation mechanism that is dependent on variations in side-chain constituents. Relative rates of racemization only show a limited dependence on the presence of metals; overall, rates of racemization in the presence of metals in solution are an order of magnitude slower than oxidation and both are several orders of magnitude faster than previously reported. This increase in rate constant is indicative of an extremely efficient catalytic enhancement of this destructive diagenetic pathway. These findings imply that chiral life-detection strategies should focus on specific geochemical environments that exhibit relatively slow rates of both degradation and racemization. Episodic liquid water during periods of increased water activity on the martian surface may have significant implications for the oxidation of organic matter through secondary diagenetic reactions and may act as a leveling mechanism for organic compounds.     

Vegetation and climate variability during the Last Interglacial evidenced in the pollen record from Lake Baikal

A pollen record from the core sediments collected in the northern part of Lake Baikal represents the latest stage of the Taz (Saale) Glaciation, Kazantsevo (Eemian) Interglacial (namely the Last Interglacial), and the earliest stage of the Zyryanka (Weichselian) Glaciation. According to the palaeomagnetic-based age model applied to the core, the Last Interglacial in the Lake Baikal record lasted about 10.6 ky from 128 to 117.4 ky BP, being more or less synchronous with the Marine Isotope Stage 5e. The reconstructed changes in the south Siberian vegetation and climate are summarised as follows: a major spread of shrub alder (Alnus fruticosa) and shrub birches (Betula sect. Nanae/Fruticosae) in the study area was a characteristic feature during the late glacial phase of the Taz Glaciation. Boreal trees e.g. spruce (Picea obovata) and birch (Betula sect. Albae) started to play an important role in the regional vegetation with the onset of the interglacial conditions. Optimal conditions for Abies sibirica–P. obovata taiga development occurred ca. 126.3 ky BP. The maximum spread of birch forest-steppe communities took place at the low altitudes ca. 126.5–125.5 ky BP and Pinus sylvestris started to form forests in the northern Baikal area after ca. 124.4 ky BP. Re-expansion of the steppe communities, as well as shrubby alder and willow communities and the disappearance of forest vegetation occurred at about 117.4 ky BP, suggesting the end of the interglacial succession. The changes in the pollen assemblages recorded in the sediments from northern Baikal point to a certain instability of the interglacial climate. Three phases of climate deterioration have been distinguished: 126–125.5, 121.5–120, and 119.5–119 ky BP. The penultimate cooling signal may be correlated with the cool oscillation recorded in European pollen records. However, such far distant correlation requires more careful investigation.     

Similarities between the shergottites and terrestrial ferropicrites

Element abundance ratios have been used to both distinguish terrestrial and martian basalts and make estimates on the bulk planetary chemistry. However, these estimates are based upon ratios that are assumed to have been unaffected by igneous processes. Since the extent to which this is valid is unknown, comparisons of terrestrial and martian rocks are best conducted on rocks with similar mineralogy and petrology, and therefore a good likelihood of similar crystallization histories. When the geochemistry of terrestrial ferropicrites is compared with the olivine-phyric and basaltic shergottites, previously observed differences in chemistry are no longer definitive (i.e. Mg/Si, Al/Si, Ca/Si, Fe/Si, FeO/MnO, Al/Ti, Na/Ti, Na/Al, K/La, K/Rb, K/Th, K/U, Th/U ratios). Since ferropicrites are geochemical terrestrial analogs for the shergottites, their formation history can provide useful information about the formation of the shergottites. This suggests that both ferropicrites and shergottites formed from a heavily processed mantle source region.     

Late glacial and Holocene environmental change in the Lake Baikal region documented by oxygen isotopes from diatom silica

We investigate late glacial and Holocene climate change recorded in Lake Baikal using the oxygen isotope composition of diatom silica (δ¹⁸O_DIAT). Evaporation from the lake is minor, and the temperature fractionations of δ¹⁸O are unable to explain variations in the δ¹⁸O_DIAT record alone. Isotopically, low meltwater input from glaciers may have some influence on δ¹⁸O_DIAT, but the assumed periods of climatic warming and wastage do not coincide with large shifts in δ¹⁸O_DIAT. There is a gradual oxygen isotope lowering from 27.0‰ to 20.6‰ over the late glacial, while, during the Holocene, δ¹⁸O_DIAT values return to relatively high values. Previous studies of the modern oxygen and hydrogen isotope composition of Lake Baikal's inputs reveal that fluvial input to the lake's North Basin are isotopically lower than fluvial input from South Basin rivers. This north–south gradient of river δ¹⁸O and δD is mainly due to the greater input from isotopically low winter precipitation in the north and isotopically higher summer precipitation in the south. As a result, the δ¹⁸O_DIAT record from Lake Baikal can at least in part be explained by varying input from these sources related to seasonal changes in precipitation. Changes in atmospheric conditions may have a role in altering seasonality and the distribution of precipitation over Lake Baikal's catchment. A feedback mechanism is well known linking higher Eurasian spring snow cover extent (ESSC) to the development of anticyclonic conditions and low precipitation the following summer in the areas south of Lake Baikal. A simultaneous increase in the importance of depleted water (snowmelt) input from the north and decreased enriched summer precipitation in the south is needed to explain depletions in δ¹⁸O of lake water and subsequently δ¹⁸O_DIAT during colder periods. The opposite of this situation is required to enrich lake water during warmer periods. The analysis of δ¹⁸O from diatom silica is a useful proxy for environmental change, especially in lakes, like Lake Baikal, where carbonates are absent or diluted. However, analysis must be based on near pure diatom samples as even trace amounts of silt can have a dominating effect on δ¹⁸O_DIAT values.     

Mired in the past — looking to the future: Geochemistry of peat and the analysis of past environmental changes

Peat cores from ombrotrophic bogs have been used as a valuable archive to study environmental change for over a century. Much of this focus on the peat record has been on biological proxies of environmental change, such as pollen and peat-forming macrofossils, but there is growing interest in the geochemical record to study environmental changes. Several studies of long-term peat records in Europe have reconstructed past changes in atmospheric lead pollution, for example, and the general cohesiveness of the results and their agreement with known historical trends in metal production exemplify the best potential of peat geochemistry as an environmental archive. Based on the success with lead, a current emphasis in peat reconstructions is to assess the record of past mercury deposition and results thus far show generally consistent trends, e.g., a pre-anthropogenic mercury accumulation rate of about 0.5–1.5 μg Hg m^− 2 year^− 1. Despite this general consistency there is increasing concern that there may be diagenetic effects on the quantitative record of some metals, which can be inferred based on a strong relationship between mercury and other organically bound elements and proxies for peat decomposition (C/N ratio). However, it is possible that changes in decomposition and the alteration of some metal records could provide climatic information. A few recent studies show that closer examination of the geochemical matrix, in some cases along with biological proxies, can provide valuable information on landscape changes and climate; for example, partitioning metals into different weight fractions and source regions can be applied to climate studies. The best interpretations of the peat geochemical record in the context of environmental and climate change will likely come when geochemical and biological records are considered simultaneously.     

A qualitative assessment of the influence of bioturbation in Lake Baikal sediments

The impact of bioturbation in Lake Baikal sediments, particularly on rhythmic layering and mixing, was assessed by studying the actual vertical distribution of benthic animals in continuous accumulation zones selected by seismic survey (Vydrino Shoulder, Posolskoe Bank, Continent Ridge). To assess the influence of the bioturbation, animals were extracted from short cores and identified at the relevant taxonomic level. The faunal distribution is examined in parallel with the bioturbation tracks observed in thin section. Oligochaeta, Nematoda, Ostracoda, Copepoda, Gammaridae, Chironomidae and Hydrachnidia were found inhabiting the sediment. Among them, only oligochaete worms were assumed to have a significant impact on sediment mixing because of their “conveyor belt” feeding. The other two most abundantly sampled groups, nematods and copepods, belong to the interstitial fauna that has no significant impact on the vertical displacement of sediment particles and do not ingest the sediment. The presence of a benthic fauna as deep as 15 cm in the sediment indicates that the possibility of sediment disturbance by invertebrate activity cannot be dismissed in Lake Baikal. The effect of biological mixing is more limited in the deepest stations because the number of potential bioturbators is reduced, qualitatively as well as quantitatively. Located in the abyssal zone, Continent and Vydrino (but outside turbidites) deep stations appear to be most promising sediment records for tracking climate signal at high resolution.     

High Lake gossan deposit: An Arctic analogue for ancient Martian surficial processes?

Gossan samples collected during a reconnaissance expedition to High Lake in Nunavut, Canada, were analyzed to determine their mineral components and to define parameters for the geochemical environment in which they formed. The gossan represents a natural acid drainage site in an arctic environment that serves as an analogue to the conditions under which sulfate and Fe-oxide possibly formed on Mars. Rock and soil samples were taken from three different outcrops and analyzed using XRD, SEM/EDS and Mössbauer. Two main mineral assemblages were observed. The first assemblage, which was found primarily in samples from the first outcrop, contained chlorite, Fe-phosphates, Fe-oxide and quartz. The second assemblage, which was found at the second and third outcrops, was primarily quartz, mica and jarosite. One sample (G41), containing Fe-oxide, jarosite and gypsum, appears to be transitional between a Fe-oxide dominant assemblage to a jarosite dominant assemblage. Thermodynamic equilibria predicts that the gossan pore water should range from mildly acidic, relatively sulfate-poor (pH 3-6; SO₄ <1000 mg l⁻¹) to highly acidic and relatively sulfate-rich (pH 0.5-3; SO₄ >3000 mg l⁻¹) for the first and second mineral assemblages, respectively. Kinetic reaction models indicate that the second assemblage replaces the first during evaporation or freezing of water. Compared to acid mine drainage (AMD) sites located in temperate regions, the arctic High Lake gossan lacks diversity in sulfate species and has smaller diagenetic crystal sizes. The smaller crystal size may reflect the slower reaction rates at colder temperatures and the seasonal water saturation. These initial results indicate that the High Lake gossan deposit does record mechanisms for which minerals like hematite, goethite, gypsum and jarosite, which are found on Mars, can form in an environment that involves seasonal water occurrence in a cold climate.     

Associations of organic matter with minerals in Tagish Lake meteorite via high spatial resolution synchrotron‐based FTIR microspectroscopy

We have investigated spatial and spectral associations between mineral species and organic matter in the Tagish Lake meteorite. Synchrotron‐based infrared microspectroscopy allowed us to spatially locate specific organic and inorganic compounds within multiple Tagish Lake grains with high spatial resolution. Generated two‐dimensional infrared maps present strong spatial association between aliphatic C‐H and OH in phyllosilicates in Tagish Lake grains. These observations indicate possible roles of phyllosilicates for the formation, evolution, and preservation of organic matter. Infared spectra of all studied Tagish Lake grains show a strong carbonate band, which also shows a weak but positive correlation with organic matter in some grains. However, intergrain correlation was not observed between carbonates and organics, which is likely due to the difference of carbonate occurrence, e.g., presence of larger grains or intergrowth of carbonates on phyllosilicates. Possible scenarios further explaining the observed associations of organics with phyllosilicates and carbonates are presented.     

Acid drainage generation and associated Ca-Fe-SO4 minerals in a periglacial environment, Eagle Plains, Northern Yukon, Canada: A potential analogue for low-temperature sulfate formation on Mars

Near Eagle Plains, northern Yukon, Canada, acidic Ca-Fe-Mg sulfate waters are discharging year-long from disturbed permafrosted sandstone bedrock overlying pyritiferous black shales. These acidic waters are precipitating gypsum with minor amounts of jarosite-K (Na), schwertmannite and hematite. This mineral assemblage is similar to that observed at Meridiani Planum (and other location on Mars), making this site a valuable analogue for low-temperature sulfate geochemistry and mineral formation on Mars. Stable O-S isotope analysis of the acidic waters near Eagle Plains revealed that the oxygen in the dissolved sulfate is mostly derived from water (ca. 70%), suggesting that the sulfide oxidation process could be in part biomediated (i.e., accelerated by acidophilic Fe-oxidizing bacteria). However, unlike the dissolved sulfate in the waters, the formation of the Ca-Fe-SO₄ minerals appears to be purely abiotic. The stable O-S isotope composition of the sulfate minerals is well within the predicted equilibrium range at low temperature, suggesting that they formed through physico-chemical processes (i.e., evaporation or freezing). Low-temperature geochemical modeling with FREZCHEM and PHREEQC suggests that the mineral assemblage at Eagle Plains precipitated mainly through the freezing of Ca-Fe-Mg-SO₄ acidic waters, rather than through evaporation during the dry summer season, although the latter is still possible. This suggests that the sulfate mineral assemblage observed on Mars could have also formed under a periglacial-type climate. Considering that the active layer in the zone affected by acid drainage does not freeze-over during winter, the residual talik offers a localized niche environment to support acidophilic microorganisms. Overall, the fact that acid drainage is presently active near Eagle Plains allows the direct observation of the low-temperature geochemical processes responsible for generating acid drainage conditions and precipitation of gypsum, schwertmannite, jarosite-K, jarosite-Na, goethite and hematite.     

An analysis of Apollo lunar soil samples 12070,889, 12030,187, and 12070,891: Basaltic diversity at the Apollo 12 landing site and implications for classification of small‐sized lunar samples

Lunar mare basalts provide insights into the compositional diversity of the Moon's interior. Basalt fragments from the lunar regolith can potentially sample lava flows from regions of the Moon not previously visited, thus, increasing our understanding of lunar geological evolution. As part of a study of basaltic diversity at the Apollo 12 landing site, detailed petrological and geochemical data are provided here for 13 basaltic chips. In addition to bulk chemistry, we have analyzed the major, minor, and trace element chemistry of mineral phases which highlight differences between basalt groups. Where samples contain olivine, the equilibrium parent melt magnesium number (Mg#; atomic Mg/[Mg + Fe]) can be calculated to estimate parent melt composition. Ilmenite and plagioclase chemistry can also determine differences between basalt groups. We conclude that samples of approximately 1–2 mm in size can be categorized provided that appropriate mineral phases (olivine, plagioclase, and ilmenite) are present. Where samples are fine‐grained (grain size <0.3 mm), a “paired samples t‐test” can provide a statistical comparison between a particular sample and known lunar basalts. Of the fragments analyzed here, three are found to belong to each of the previously identified olivine and ilmenite basalt suites, four to the pigeonite basalt suite, one is an olivine cumulate, and two could not be categorized because of their coarse grain sizes and lack of appropriate mineral phases. Our approach introduces methods that can be used to investigate small sample sizes (i.e., fines) from future sample return missions to investigate lava flow diversity and petrological significance.     

Observational Indicators of Formation Excitation of H2

Theoretical predictions by Farebrother et al. and Meijer et al. of rovibrational excitation probabilities in H₂ arising from formation by Eley-Rideal processes on a graphite surface are incorporated into a model of the chemistry and excitation of interstellar H₂. The model includes the usual radiative and collisional pumping of H₂ rotational and vibrational states, in addition to the formation processes. Predictions are made for HH₂ rovibrational emission line intensities for representative points in diffuse and in dark interstellar clouds. We find that – if all the interstellar HH₂ is formed by this Eley-Rideal process – then the consequences of formation pumping, as distinct from collisional and radiative pumping, should be clearly evident in both cases. In particular, we predict a clear spectral signature of this direct HH₂ formation process on graphite, distinct from radiative and collisional pumping; this signature should be evident in both diffuse and dark clouds; but the emissivity for dark clouds is predicted to be some 500 times greater than that in diffuse clouds in which the dense material may be embedded. An observational search for this signature in two dark cloud sources was made, but a preliminary analysis of the data did not yield a detection. The implications of and possible reasons for this preliminary conclusion are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Prototype detector for ultrahigh energy neutrino detection

Necessary technical experience is being gained from successful construction and deployment of current prototype detectors to search for UHE neutrinos in Antarctica, Lake Baikal in Russia, and the Mediterranean. The prototype detectors have also the important central purpose of determining whether or not UHE neutrinos do in fact exist in nature by observation of at least a few UHE neutrino-induced leptons with properties that are not consistent with expected backgrounds. We discuss here the criteria for a prototype detector to accomplish that purpose in a convincing way even if the UHE neutrino flux is substantially lower than predicted at present.     

The oxygen isotopic composition of water from Tagish Lake: Its relationship to low‐temperature phases and to other carbonaceous chondrites

Abstract— The fall and recovery of the Tagish Lake meteorite in British Columbia in January 2000 provided a unique opportunity to study relatively pristine samples of carbonaceous chondrite material. Measurements of the oxygen isotopic composition of water extracted under stepped pyrolysis from a bulk sample of this meteorite have allowed us to make comparisons with similar data obtained from CI and CM chondrites and so further investigate any relationships that may exist between these meteorites. The much lower yield of water bearing a terrestrial signature in Tagish Lake is indicative of the pristine nature of the meteorite. The relationship between the isotopic composition of this water and reported isotopic values for carbonates, bulk matrix and whole rock have been used to infer the extent and conditions under which parent‐body aqueous alteration occurred. In Tagish Lake the difference in Δ17O isotopic composition between the water and other phases is greater than that found in either CM or CI chondrites suggesting that reaction and isotopic exchange between components was more limited. This in turn suggests that in the case of Tagish Lake conditions during the processes of aqueous alteration on the parent body, which ultimately controlled the formation of new minerals, were distinct from those on both CI and CM parent bodies.     

Preliminary study of fluxes of major lipid biomarker classes in the water column and sediments of Lake Baikal, Russia

The application of biomarker climate proxies to lacustrine settings requires a thorough understanding of those processes that give rise to and mediate in the burial and preservation of organic matter in the sediments. This information is to date missing for Lake Baikal in Central Asia. The biogeochemistry of the lake cannot be considered analogous to that in other lacustrine environments given its enormous size, depth, remote location and unique biology. The aim of this paper is to report on the main compound classes in the water column and sediments. As part of an ongoing evaluation of the inputs and fluxes of organic matter in Lake Baikal, we have found that there are significant differences in the inputs and preservation of organic matter between the North and South Basins. Both basins have dominant algal input and a contribution from the terrestrial plants of the lake edges and surrounding environments. However, the concentration of organic material in the sediment traps and sediments of the South Basin is much higher than that of the North Basin. In addition, the South Basin contains much higher proportions of the more labile material than does the North Basin. This is likely due to the South Basin being free of surface ice for a much longer time than the North, and in consequence, primary producers having an overall longer productive season. There is some evidence of microbial activity in the sediment traps and sediments from both the North and South Basins, but it does not appear to be more predominant in one basin than the other. It is probable that the differences in the composition and concentration of biomarkers in sediments between basins is due to the length of and the intensity of the productive season, which is in turn influenced by the climate.