1000 years of climate variability in central Asia: assessing the evidence using Lake Baikal (Russia) diatom assemblages and the application of a diatom-inferred model of snow cover on the lake

The mainly endemic phytoplankton record of Lake Baikal has been used in this study to help interpret climate variability during the last 1000 years in central Asia. The diatom record was derived from a short core taken from the south basin and has been shown to be free from any sedimentary heterogeneities. We employ here a diatom-based inference model of snow accumulation on the frozen lake for the first time (r²_boot=0.709; RMSEP=0.120 log cm). However, palaeoenvironmental reconstructions have been improved by the use of correction factors, specifically developed for the dominant phytoplankton (Aulacoseira baicalensis, Aulacoseira skvortzowii, Cyclotella minuta, Stephanodiscus meyerii and Synedra acus) in the south basin of Lake Baikal. Cluster analysis identifies three significant zones in the core, zone 1 (c. 880 AD–c. 1180 AD), zone 2 (c. 1180–1840 AD) and zone 3 (c. 1840–1994 AD), coincident with the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the period of recent warming, respectively. Our results indicate that S. acus dominated the diatom phytoplankton within zone 1 coincident with the MWP. S. acus is an opportunistic species that is able to increase its net growth when A. baicalensis does not. During this period, conditions are likely to have been unfavourable for the net increases in A. baicalensis growth due to the persistence of warm water in the lake, together with an increased length of summer stratification and delay in timing of the autumnal overturn. In zone 2, spring diatom crops blooming under the ice declined in abundances due in part to increased winter severity and snow cover on the lake. Accumulating snow on the lake is likely to have arisen from increased anticyclonic activity, resulting in prolonged winters expressed during the LIA. Thick, accumulating snow cover inhibits light penetration through the ice, thereby having negative effects on cell division rate and extent of turbulence underneath the ice. Consequently, only taxa whose net growth occurs during autumn overturn (C. minuta) predominate in the lake at this time. Diatom census data and reconstructions of snow accumulation suggest that warming in the Lake Baikal region started as early as c. 1750 AD, with a shift from taxa that bloom during autumn overturn to assemblages that begin to grow underneath the frozen lake in spring. Very recent increases and subsequent decline of S. acus in the surface sediments of the lake mirror monitoring records of this species over the last 50 years. Our study confirms that, over the last 1000 years, physical processes are important in determining planktonic diatom populations in the lake and highlights the value of integrated plankton, trap, and sediment studies for improving quantitative palaeoenvironmental reconstructions from fossil material.     

A late Quaternary lake record from the Qilian Mountains (NW China): evolution of the primary production and the water depth reconstructed from macrofossil, pollen, biomarker, and isotope data

The history (45–0 ka BP) of the aquatic vegetation composition of the shallow alpine Lake Luanhaizi from the NE Tibetan Plateau is inferred from aquatic plant macrofossil frequencies and aquatic pollen and algae concentrations in the sediments. C/N (range: 0.3–100), δ¹³C (range: −28 to −15‰), and n-alkane measurements yielded further information on the quantitative composition of sedimentary organic matter. The inferred primary production of the former lake ecosystem has been examined in respect of the alternative stable state theory of shallow lakes [Scheffer, M., 1989. Alternative stable states in eutrophic, shallow freshwater systems: a minimal model. Hydrobiological Bulletin 23, 73–83]. Switches between clear and turbid water conditions are explained by a colder climate and forest decline in the catchment area of Lake Luanhaizi. The macrofossil-based reconstruction of past water depth and salinity ranges, as well as other organic matter (OM) proxies allowed climatic inferences of the summer monsoon intensity during the late Quaternary. Around 45 ka BP, conditions similar to or even moister than present-day climate occurred. The Lake Luanhaizi record is further evidence against an extensive glaciation of the Tibetan Plateau and its bordering mountain ranges during the Last Glacial Maximum. Highest lake levels and consequently a strong summer monsoon are recorded for the early Holocene period, while gradually decreasing lake levels are reconstructed for the middle and late Holocene.     

Climate changes detected through the world's longest sea level series

The sea level series of Stockholm in the Baltic Sea, commencing already in 1774, is analysed in various ways together with contemporary climate data, in order to investigate long-term sea level changes and their relations to climate changes.First, a study of the eustatic rise of sea level, based on annual mean sea levels, is peformed, and compared with other sea level and climate studies. It is concluded that the general climatic rise of sea level has increased significantly (99.9%) from about 0.0 mm/year during the end of the Little Ice Age, to about 1.0 mm/year during the past century, characterized by melting of glaciers. Such sea level changes due to northern hemisphere climate variations since 800 A.D. have (hitherto) probably always kept within −1.5 and +1.5 mm/year, with an average fairly close to zero.Second, an investigation of the sea level variability, also based on annual mean sea levels, is performed together with temperature and wind variabilities. It is found that the interannual sea level variability of the Baltic Sea has decreased significantly (98%) from the end of the 1700s to the beginning of the 1900s; after that it has increased significantly (95%) again. Precisely the same is found to apply to winter climate or, more specifically, to the interannual winter temperature variability and the interannual winter wind variability. The common origin of all these long-term changes turn out to be two consecutive winter wind processes over the North and Baltic Seas, especially the Baltic entrance. From the end of the 1700s to the beginning of the 1900s, there has been a rapidly decreasing number of dominating winter winds from northeast, and after that there has been an increasing number of dominating winter winds from southwest. This may indicate corresponding long-term changes in the North Atlantic Oscillation.Third, using monthly mean sea levels together with corresponding wind data, seasonal variations are investigated. The seasonal sea level variation in the Baltic Sea has increased significantly (99%) since the early 1800s, together with a shift of the maximum from late summer to early winter. It is found that the main origin is a secular change of the winter wind conditions over the Baltic entrance, with increasing southwesterly winds in early winter. This might also be related to a long-term change in the North Atlantic Oscillation.     

Effects of a 2 × CO2 climate on two large lake systems: Pyramid Lake, Nevada, and Yellowstone Lake, Wyoming

The possible effects of trace-gas induced climatic changes on Pyramid and Yellowstone Lakes are assessed using a model of lake temperature. The model is driven by years of hourly meteorological data obtained directly from the output of double-CO₂ experiments (2 × CO₂) conducted with a regional climate model nested in a general circulation model. The regional atmospheric model is the climate version of the National Center for Atmospheric Research/Pennsylvania State University mesoscale model, MM4.Average annual surface temperature of Pyramid Lake for the 2 × CO₂ climate is 15.5 ± 5.4°C (±1 σ), 2.8°C higher than the control. Annual overturn of the lake ceases as a result of these higher temperatures for the 2 × CO₂ climate. Evaporation increases from 1400 mm yr⁻¹ in the control to 1595 mm yr⁻¹ in the 2 × CO₂ simulation, but net water supplied to the Pyramid Lake basin increases from −6 mm yr⁻¹ in the control to +27 mm yr⁻¹ in the 2 × CO₂ simulation due to increased precipitation.For the open water periods, the average annual surface temperature of Yellowstone Lake is 13.2 ± 5.1°C for the 2 × CO₂ climate, a temperature 1.6°C higher than the control. The annual duration of ice cover on the lake is 152 days in the 2 × CO₂ simulation, a reduction of 44 days relative to the control. Warming of the lake for the 2 × CO₂ climate is mostly confined to the near-surface. Simulated spring overturn for the 2 × CO₂ climate occurs earlier in the year and fall overturn later than in the control. Evaporation increases from 544 mm yr⁻¹ to 600 mm yr⁻¹ in the 2 × CO₂ simulation, but net water supplied to the Yellowstone Lake basin increases from +373 mm yr⁻¹ in the control to +619 mm yr⁻¹ due to increased precipitation. The effects of these climatic changes suggest possible deterioration of water quality and productivity in Pyramid Lake and possible enhancement of productivity in Yellowstone Lake.     

Twentieth century thinning of Mendenhall Glacier,Alaska, and its relationship to climate,lake calving,and glacier run-off

Mendenhall Glacier is a dynamic maritime glacier in southeast Alaska that is undergoing substantial recession and thinning. The terminus has retreated 3 km during the 20th century and the lower part of the glacier has thinned 200 m or more since 1909. Glacier-wide volume loss between 1948 and 2000 is estimated at 5.5 km³. Wastage has been the strongest in the glacier's lower reaches, but the glacier has also thinned at higher elevations. The shrinkage of Mendenhall Glacier appears to be due primarily to surface melting and secondarily to lake calving. The change in the average rate of thinning on the lower glacier, <1 m a⁻¹ between 1948 and 1982 and >2 m a⁻¹ since 1982, agrees qualitatively with observed warming trends in the region. Mean annual temperatures in Juneau decreased slightly from 1947 to 1976; they then began to increase, leading to an overall warming of ∼1.6 °C since 1943. Lake calving losses have periodically been a small but significant fraction of glacier ablation. The portion of the terminus that ends in the lake is becoming increasingly vulnerable to calving because of a deep pro-glacial lake basin. If current climatic trends persist, the glacier will continue to shrink and the terminus will recede onto land at a position about 500 m inland within one to two decades. The glacier and the meltwaters that flow from it are integral components of the Mendenhall Valley hydrologic system. Approximately 13% of the recent average annual discharge of the Mendenhall River is attributable to glacier shrinkage. Glacier melt contributes 50% of the total river discharge in summer.     

El'gygytgyn impact crater,Russia: Structure,tectonics, and morphology

Abstract— The 3.6 Myr old El'gygytgyn impact crater is located in central Chukotka, northeastern Russia. The crater is a well‐preserved impact structure with an inner basin about 15 km in diameter, surrounded by an uplifted rim about 18 km in diameter. The flat floor of the crater is in part occupied by Lake El'gygytgyn, 12 km in diameter, and surrounding terraces. The average profile of the rim is asymmetric, with a steep inner wall and a gentle outer flank. The rim height is about 180 m above the lake level and 140 m above the surrounding area. An outer ring feature, on average 14 m high, occurs at about 1.75 crater radii from the center of the structure. El'gygytgyn crater is surrounded by a complex network of faults. The density of the faults decreases from the bottom of the rim to the rim crest and outside the crater to a distance of about 2.7 crater radii. Lake El'gygytgyn is surrounded by a number of lacustrine terraces. Only minor remnants are preserved of the highest terraces, 80 and 60 m above the present‐day lake level. The widest of the terraces is 40 m above the current lake level and surrounds the lake on the west and northwest sides. The only outlet of the lake is the Enmivaam River, which cuts through the crater rim in the southeast. In terms of structure, El'gygytgyn is well preserved and displays some interesting, but not well understood, features (e.g., an outer ring), similar to those observed at a few other impact structures.     

Climate change and recent water level variability in Patagonian proglacial lakes, Argentina

A long series of lakes (~ 150) borders the Patagonian Andes (south of ~ 38°S), most of which are a geomorphologic relict of Pleistocene glaciations. Employing instrumental records, we inspected lake water level departures from seasonal variations in seven proglacial lakes: Lacar, Mascardi, Steffen, Escondido, Puelo, Vinter, and Argentino. Lakes north of ~ 42°S show maximum gage (water) level during austral winter months; lakes between ~ 42° and ~ 45°S appear transitional; the one lake south of ~ 50°S (Argentino) shows maximum water level in early autumn. Most lakes show moderate level fluctuation throughout yearly records and, in general, show heteroscedacity. Furthermore, Hurst exponents reveal persistent behavior (i.e., long-term memory effect) in all water level series. In most lakes there are no trends in deseasonalized mean and maximum water levels (Seasonal Kendall test). Lake Mascardi–Manso River system (mostly fed by melt water from the retreating Manso Glacier) is a contrasting example that shows a decreasing trend during summer months that we ascribe to the also declining ice volume. Harmonic analysis (Fourier and wavelet transform) of deseasonalized mean and maximum water level time series shows interannual and decadal periodicities that we link to the occurrence of El Niño and/or the Antarctic Oscillation. The associated phase spectrum indicates that there is a ~ 13-month lag between ENSO occurrences and its effect on anomalous lake water levels. Increased snow accumulation during austral winters usually follows summertime El Niño events, which normally result in increased melt water volume that occurs with about one-year delay during the following (austral) spring/summer.     

The chronology of a large ice-dammed lake and the Barents–Kara Ice Sheet advances, Northern Russia

Beach and shoreface sediments deposited in the more than 800-km long ice-dammed Lake Komi in northern European Russia have been investigated and dated. The lake flooded the lowland areas between the Barents–Kara Ice Sheet in the north and the continental drainage divide in the south. Shoreline facies have been dated by 18 optical stimulated luminescence (OSL) dates, most of which are closely grouped in the range 80–100 ka, with a mean of 88±3 ka. This implies that that the Barents–Kara Ice Sheet had its Late Pleistocene maximum extension during the Early Weichselian, probably in the cold interval (Rederstall) between the Brørup and Odderade interstadials of western Europe, correlated with marine isotope stage 5b. This is in strong contrast to the Scandinavian and North American ice sheets, which had their maxima in isotope stage 2, about 20 ka. Field and air photo interpretations suggest that Lake Komi was dammed by the ice advance, which formed the Harbei–Harmon–Sopkay Moraines. These has earlier been correlated with the Markhida moraine across the Pechora River Valley and its western extension. However, OSL dates on fluvial sediments below the Markhida moraine have yielded ages as young as 60 ka. This suggests that the Russian mainland was inundated by two major ice sheet advances from the Barents–Kara seas after the last interglacial: one during the Early Weichselian (about 90 ka) that dammed Lake Komi and one during the Middle Weichselian (about 60 ka). Normal fluvial drainage prevailed during the Late Weichselian, when the ice front was located offshore.     

Sedimentary effects on the expansion of a Himalayan supraglacial lake

Supraglacial Tsho Rolpa Lake in the Nepal Himalaya has been increasing rapidly in size since the 1950s, corresponding to the mountain-glacier shrinkage after the Little Ice Age. The lake basin expansion results from the subsidence by dead-ice melt below the bottom of the lake, and the retreat of the glacier terminus. Field observations of Tsho Rolpa in 1996 revealed that the retreat of glacier terminus is connected to a wind-induced vertical circulation of surface water heated by solar radiation. In order to clarify the mechanism of the lake expansion associated with sedimentary processes, we measured bottom sedimentation rate with some sediment traps, and vertical suspended sediment concentration (SSC) and water temperature, and analyzed the grain size of suspended and trapped sediments. The sediments, mostly composed of clay-sized grains, are dominantly supplied by glacier-melt water inflow at the glacier terminus. Sedimentary processes of such fine sediment comprise: (1) suspended-sediment fallout from intrusion of horizontal currents; (2) sediment sorting by sediment-laden underflows; and (3) the debris supply from the ice collapse at the glacier terminus. The (1) and (2) processes produce the density stratification of the lake, accompanied by a pycnocline at a depth of about 27 m. The existence of the pycnocline builds up the vertical water circulation in the surface layer to enhance the glacier-melt at the terminus. With respect to the subsidence of the lake bottom, nearly molecular thermal diffusion is probably dominant near the bottom of the deepest point, which results from the kinetic-energy dissipation of sediment-laden underflows. The stable existence of the bottom turbid water throughout the year could cause continuous dead-ice melt below the lake bottom.     

Pollution and eutrophication history AD 1800–2005 as recorded in sediments from five lakes in Central Chile

One of the fundamental problems to quantifying past impact of anthropogenic activities is that long series of observational data for pollutant deposition and changes in the nutrient cycling of ecosystems (eutrophication) are often not available. Lake sediments may provide suitable archives to decipher the history of local and regional pollution and eutrophication.Here we provide quantitative high-resolution data for the history of airborne pollutants and eutrophication from sediments of five lakes in Central Chile between ca. AD 1800–2005. We use spheroidal carbonaceous particles (SCPs) from fossil fuel combustion and excess atmospheric Cu deposition from mining activities as a proxy for atmospheric deposition. Organic carbon and nitrogen flux rates to the sediments and C/N ratios are used as a proxy for aquatic primary production and eutrophication.We show that the lake sediment SCP and Cu records are highly consistent and depict in great detail the local and regional history of urban, industrial and transportation history as reported in independent documentary sources and statistics. The pre-industrial and pre-1950 background concentrations (and flux rates) of the substances can be quantified. We can also show that technical measures taken in the early 1980s to trim down Cu emissions from the copper mines reduced the excess atmospheric Cu fallout to the lakes by about 50%.Eutrophication of the lakes did not start before ca. 1980. Prior to that time, warm season temperatures explain most of the variance in TOC and N flux to the sediments. The three dimictic lakes show only moderate eutrophication responses to enhanced N supply (as atmospheric fallout; enrichment factors for TOC and N 1.1–2.6), suggesting that mainly phosphorus controls aquatic primary production. The meromictic lake, where phosphorus recycling is likely, shows the largest response (enrichment factors for TOC and N between 9–20).While all five lakes show overall consistent and similar trends for the pollution history during the 19th and 20th century, there are significant differences in the details of the individual profiles. This suggests that local sources are highly important and the common regional (background) signal is relatively marginal. This is very different from Europe.     

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.     

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).     

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.     

Site Characterization Using Solar Hα Images

A Multi-Application Solar Telescope (MAST) is proposed to be installed at the lake site (Lake Fatehsagar) of Udaipur Solar Observatory (USO) in India. The lake site Observatory of USO is located on a small island in the middle of the lake. To determine the optimum size of the MAST (for use with an adaptive optics system), it was decided to quantify the seeing conditions prevailing at the lake site during the different months of the year. For this purpose, we have used short-exposure (3 ms) high-resolution Hα (6563 Å) images (spatial scale of ～0.55 arc sec per pixel) of the Sun taken in burst mode with the 15-cm refractor Spar telescope located at the lake site of USO. Spectral ratio technique as reported by von der Lühe (1984, J. Opt. Soc. Am. A1, 510) has been used to estimate the Fried’s parameter (r ₀) at this site, which gives the quantitative measure of astronomical seeing. This study has been carried out daily on an hourly basis during 4:30?–?10:30 UT over the months January?–?June of the years 2005 and 2006 to understand the diurnal and seasonal variations in r ₀ at this site. It is noteworthy that the lake was almost dry during the observing period in 2005, while it overflowed during our observations in 2006 because of abundant monsoon rains. The seeing in the presence of water shows improvement in r ₀ by about 1.0 cm with respect to the previous year’s dry condition and mean r ₀ varies between 4.0 and 4.5 cm as evident from the data obtained between January and June, 2006.     

A Holocene and latest Pleistocene pollen record from Lake Poukawa, Hawke's Bay, New Zealand

Lake Poukawa is a small, shallow lake lying in the middle of extensive peatland in the Poukawa depression, central Hawke's Bay. Holocene peats (10 m at deepest point) overlie more than 200 m of sand, silt, clastic debris and infrequent thin peats and lacustrine sediments deposited during the late Pleistocene. Pollen analyses are presented for: a peat possibly dating to a late stage of the last interglacial or a warm interstadial of the last glacial; cool climate last glacial sediments; and a Holocene peat. The last interglacial or interstadial peat records a cool climate Nothofagus podocarp forest. During the last glacial, sparse shrubland and grassland grew within the depression under much drier and colder conditions than now. There is no pollen record for the Late Glacial and early Holocene period as conditions remained too dry for peat formation. Avian fossils indicate scrub and grassland persisted through until at least 10,600 years BP, and scrub or open forest may have prevailed until c. 6500 years BP. Closed podocarp broadleaved forest (Prumnopitys taxifolia dominant) occupied the depression from at least 6500 years BP until its destruction by Polynesian settlers after 800 years BP. Water levels rose from 6500 to 4500 years BP, culminating in the establishment of the present fluctuating lake-peatland system. Dry conditions in the Late Glacial and early Holocene may reflect a predominant northwesterly air flow, and a change to more easterly and southerly air flow in the mid- to late Holocene resulted in increased rainfall.     

Elemental,isotopic, and structural changes in Tagish Lake insoluble organic matter produced by parent body processes

Here, we present the results of a multitechnique study of the bulk properties of insoluble organic material (IOM) from the Tagish Lake meteorite, including four lithologies that have undergone different degrees of aqueous alteration. The IOM C contents of all four lithologies are very uniform and comprise about half the bulk C and N contents of the lithologies. However, the bulk IOM elemental and isotopic compositions vary significantly. In particular, there is a correlated decrease in bulk IOM H/C ratios and δD values with increasing degree of alteration—the IOM in the least altered lithology is intermediate between CM and CR IOM, while that in the more altered lithologies resembles the very aromatic IOM in mildly metamorphosed CV and CO chondrites, and heated CMs. Nuclear magnetic resonance (NMR) spectroscopy, C X‐ray absorption near‐edge (XANES), and Fourier transform infrared (FTIR) spectroscopy confirm and quantitate this transformation from CR‐like, relatively aliphatic IOM functional group chemistry to a highly aromatic one. The transformation is almost certainly thermally driven, and probably occurred under hydrothermal conditions. The lack of a paramagnetic shift in ¹³C NMR spectra and 1s‐σ* exciton in the C‐XANES spectra, both typically seen in metamorphosed chondrites, shows that the temperatures were lower and/or the timescales were shorter than experienced by even the least metamorphosed type 3 chondrites. Two endmember models were considered to quantitatively account for the changes in IOM functional group chemistry, but the one in which the transformations involved quantitative conversion of aliphatic material to aromatic material was the more successful. It seems likely that similar processes were involved in producing the diversity of IOM compositions and functional group chemistries among CR, CM, and CI chondrites. If correct, CRs experienced the lowest temperatures, while CM and CI chondrites experienced similar more elevated temperatures. This ordering is inconsistent with alteration temperatures based on mineralogy and O isotopes.     

Valley network-fed, open-basin lakes on Mars: Distribution and implications for Noachian surface and subsurface hydrology

A new catalog of 210 open-basin lakes (lakes with outlet valleys) fed by valley networks shows that they are widely distributed in the Noachian uplands of Mars. In order for an outlet valley to form, water must have ponded in the basin to at least the level of the outlet. We use this relationship and the present topography to directly estimate the minimum amount of water necessary to flood these basins in the past. The volumes derived for the largest lakes (∼3×¹⁰⁴ to ∼2×¹⁰⁵ km³) are comparable to the largest lakes and small seas on modern Earth, such as the Caspian Sea, Black Sea, and Lake Baikal. We determine a variety of other morphometric properties of these lakes and their catchments (lake area, mean depth, volume, shoreline development, outlet elevation, and watershed area). Most candidate lakes have volumes proportional to and commensurate with their watershed area, consistent with precipitation as their primary source. However, other lakes have volumes that are anomalously large relative to their watershed areas, implying that groundwater may have been important in their filling. Candidate groundwater-sourced lakes are generally concentrated in the Arabia Terra region but also include the Eridania basin [Irwin, R.P., Howard, A.D., Maxwell, T.A., 2004a. J. Geophys. Res. 109, doi: 10.1029/2004JE002287. E12009; Irwin, R.P., Watters, T.R., Howard, A.D. Zimbelman, J.R., 2004b. J. Geophys. Res. 109, doi: 10.1029/2004JE002248. E09011] and several lakes near the dichotomy boundary. This areal distribution is broadly consistent with where groundwater should have reached the surface as predicted by current models. Both surface runoff and groundwater flow appear to have been important sources for lakes and lake chains, suggesting a vertically integrated hydrological system, the absence of a global cryosphere, and direct communication between the surface and subsurface hydrosphere of early Mars.     

Transient surface liquid in Titan’s polar regions from Cassini

Cassini RADAR images of Titan’s south polar region acquired during southern summer contain lake features which disappear between observations. These features show a tenfold increases in backscatter cross-section between images acquired one year apart, which is inconsistent with common scattering models without invoking temporal variability. The morphologic boundaries are transient, further supporting changes in lake level. These observations are consistent with the exposure of diffusely scattering lakebeds that were previously hidden by an attenuating liquid medium. We use a two-layer model to explain backscatter variations and estimate a drop in liquid depth of approximately 1-m-per-year. On larger scales, we observe shoreline recession between ISS and RADAR images of Ontario Lacus, the largest lake in Titan’s south polar region. The recession, occurring between June 2005 and July 2009, is inversely proportional to slopes estimated from altimetric profiles and the exponential decay of near-shore backscatter, consistent with a uniform reduction of 4 ± 1.3 m in lake depth.Of the potential explanations for observed surface changes, we favor evaporation and infiltration. The disappearance of dark features and the recession of Ontario’s shoreline represents volatile transport in an active methane-based hydrologic cycle. Observed loss rates are compared and shown to be consistent with available global circulation models. To date, no unambiguous changes in lake level have been observed between repeat images in the north polar region, although further investigation is warranted. These observations constrain volatile flux rates in Titan’s hydrologic system and demonstrate that the surface plays an active role in its evolution. Constraining these seasonal changes represents the first step toward our understanding of longer climate cycles that may determine liquid distribution on Titan over orbital time periods.     

Evidence for episodic alluvial fan formation in far western Terra Tyrrhena, Mars

A Late Noachian-aged alluvial fan complex within Harris Crater in far western Terra Tyrrhena, Mars, is comprised of two well-defined source regions and associated discrete depositional lobes. Three fan units were recognized based on common morphological characteristics, thermal properties and spectral signatures. Although the entire fan complex has been subjected to extensive erosional degradation, the preserved morphologies record episodic fan formation and indicate the type of flow processes that occurred; the bulk of the fan surface has morphology consistent with fluvial emplacement while one fan unit exhibits a rugged surface texture with boulders consistent with a debris flow. This transition from fluvial to late-stage debris flow(s) suggests a decline in available water and/or change in sediment supply. The thermal inertia values obtained for all three fan surface units (mean values ranged from 318 to 344 J m⁻² K⁻¹ s^−1/2) are typical for coarse-grained and/or well-indurated materials on Mars, but subtle variations point to important distinctions. Variations in aeolian bedform coverage as well as the density of ridges (inferred inverted channels) and boulders contribute to these subtle fan thermophysical differences and likely reflect changes in the fan depositional mechanisms and variations in post-depositional modification histories. The majority of the alluvial fan surface has a spectral signature that is broadly similar to TES “Surface Type 2” (ST2), with some important exceptions at long wavelengths. However, a unique spectral component was identified in one of the fan units (unit 3), that likely reflects lithological differences from other fan materials. This spectral attribute of unit 3 matched locations within the western catchment providing confirmation of provenance and supporting the contention that sediment supply changed over time as the fan developed. Finally, we applied simple modeling to a well preserved subsection of the fan complex to quantify the developmental history. Using the computed eastern fan volume (32 km³), significant water, likely from precipitation, was involved in fan construction (>50 km³) and an extensive period of fan formation occurred over millennia or longer.     

High‐albedo asteroid 434 Hungaria: Spectrum,composition and genetic connections

Abstract— New data in the wavelength region of approximately 0.4–2.5 μm have been obtained for asteroid 434 Hungaria. This is the most complete visible to near‐infrared spectrum to date for this object. The near‐infrared portion of the spectrum (about 0.8–2.5 μm) is smooth, featureless, and agrees well in the overlap region with new visible region data. However, visible region (about 0.45–0.9 μm) data appear to exhibit weak, broad spectral absorption features near 0.5, 0.6–0.7, and 1 μm. If real, the presence of such features would strongly constrain the compositional determination of Hungaria since it has a relatively high albedo of 46%. Most minerals that exhibit similar absorption features, and are commonly found in meteorites, have a much lower albedo. Asteroid 434 Hungaria has been observed more than six times in these overlapping spectral regions, and it is now possible to assess its mineral composition with some confidence. The dominant phase on this asteroid is an iron‐free mineral, probably enstatite. Hungaria may contain secondary phases causing subtle, visible‐region absorption features. Alternatively, the surface layer(s) of the asteroid may be contaminated by an absorbing species from an external source.