Lithological Evidence Research Articles

The origin of life on Earth

Origin of life is one of the greatest mysteries for mankind. Life is the most complex outcome of the 13.7 billion years evolution of our Universe. Human has just recently found that life has experienced a long coevolution with its earth environments through the deep geological time. At the beginning of its planetary evolution, Earth lost its highly reduced atmosphere that contained relatively high of hydrogen, ammornia and methane and turned to a secondary atmosphere contained high of carbon dioxide, dinitrogen and water by differentiation. In the first 700 million years with a hot, global climatic temperature, the strong reactions between the water-carbon dioxide atmosphere and the ultramafic crust produced abundant secondary earth materials including clay minerals and carbonates. Those clay minerals migh have catalyzed the indigenous synthesis of organic matter for the prebiotic evolution toward life; while the deposition of massive carbonates sequestrated the atmospheric carbon dioxide that significantly weakened the greenhouse effect and led to the formation of the earliest oceans only ca. 150 million years after Earth’s formation. Though the Earth environments before the Later Heavy Bombardment at ca. 4.1–3.8 billion years was already suitable for life to start, the oldest lithological evidence of life is around 3.71 billion years old. The current controversies on the origin of life include two basic questions: hot or cold, gene first or metabolism first. The Hot Origin theory is supported by the observed interactions between hot water and mafic rocks in producing small organic molecules; the Cold Origin theory insists that the cold temperature is essential for the inorganic synthesis of amino acids and the stability of primary genetic materials. On the other side, astronomists prefer that the exogenous input of organic carbon, water and transition metals by comets and meteorites might be a significant contribution to the prebiotic evolution of life in the early time. Life is a star material. All the cosmic evolutionary stages contributed to the emergence and evolution of life on Earth. The Big Bang produced hydrogen, the main sequence evolution of stars cooked carbon, oxygen, nitrogen, sulfur, phosphorus, silicon and iron, and even supernova, the dramatic ending of massive stars, produced various heavy radioactive isotopes that powers the highly active geodynamic processes of Earth’s interior. Hydrogen, nitrogen, sulfur, phosphorus, silicon and some metals find their usefulness in biochemistry by having chemical reactions with water-carbon based chemical systems. The various extreme conditions on Earth, such as high or low temperatures, strong photo or ionic radiations, high or low pH values, lacking of nutrients, etc., provide constraints for life on the other planets/moons of our solar system, or on the increasing numbers of Earth-like exoplents. In the observable universe, water is the best chemical solvent for life, and only carbon can make the backbone for life. The unique physical properties of water and incomparable biochemical properties of carbon, combined with their high cosmic abundances, strongly suggest life as we can understand should be common in the Universe; and our understanding of extremophiles and the evolutionary pathway of life on Earth suggest that at least microbial life should be common in the Universe.

Devonian/Carboniferous boundary glacioeustatic fluctuations in a platform-to-basin direction: A geochemical approach of sequence stratigraphy in pelagic settings

We investigated high-resolution stratigraphic distribution of selected major and trace elements and gamma-ray spectra of fourteen Devonian/Carboniferous (D/C) boundary sections of Europe located in the late Palaeozoic Laurussia and Gondwana. The aim was to trace the geochemical signature of a marked forced and normal regressive interval which was associated with rapid progradation of siliciclastics into the marine carbonate systems (Rhenish Massif) and a prominent hiatus in shallow-water ramp settings (Namur–Dinant Basin). This interval represents the late Devonian Hangenberg event (HBE) sensu lato (middle praesulcata conodont zone) as defined by previous authors. This regressive interval (FSST to LST) correlates with thin shale layers (HBE shale) sandwiched between monotonous nodular calcilutite/calcisiltite successions at five pelagic sections of Moravia, Carnic Alps, Montagne Noire, and Pyrenees. In all sections with continuous D/C sedimentation (i.e., except those of the Namur–Dinant Basin), the HBE s.l. interval is accompanied by elevated percentages of detrital proxies (Al, K, Rb, Zr) and changes in their ratios (Zr/Rb, K/Al, Rb/K) which are normally interpreted as indicators of increased siliciclastic input, provenance, and grain size. Zr/Rb and other proxies are traceable even without apparent lithological evidence and can, therefore, facilitate stratigraphic correlation. Paleoredox and productivity proxies (U/Th and Ni/Rb enrichment factors) only rarely show elevated values in the Hangenberg black shale interval, indicating that the associated water dysoxia/anoxia was a local rather than global phenomenon. Global correlations based on the HBE black shales should therefore be dropped in favor of the HBE s.l. interval. Moreover, analysis of sedimentation rates in the upper expansa to kockeli zone interval using the published radiometric ages suggests that the HBE s.l. was a time of significant increase in the rate of siliciclastic supply into the ocean, even in the most distal pelagic sections. Consequently, the previous interpretation of the HBE black shale as a condensed succession deposited during rapid sea-level rise seems unlikely. We interpret the HBE s.l. (i.e., including the HBE black shale) as a marine record of glacioeustatic sea-level drop and increased aeolian transport in connection with late Devonian climatic cooling and glaciation. The set of geochemical markers related to the late Devonian sea-level fluctuation can be used for super-regional to global correlations from platform to basin settings. Moreover, they can facilitate current efforts to determine a new D/C boundary definition.

Integrated Upper Ordovician–lower Silurian biostratigraphy of the Grötlingbo-1 core section, Sweden

The biostratigraphical data obtained from the Grötlingbo-1 core section (southern Gotland, Sweden) enable recognition of most of the regional stages known from the Sandbian (Upper Ordovician) to lower Sheinwoodian (Wenlock, Silurian) interval in the East Baltic. The strata corresponding to the Upper Ordovician Keila–Nabala stages have markedly reduced thicknesses in the Grötlingbo-1 core section. Gaps of considerable duration were recognised in some parts of the section: lithological evidence suggest that only lower part of the Pirgu Stage (corresponds to all or part of the Jonstorp Formation) is represented in the section; in the Llandovery, strata corresponding to the Aspelundia? fluegeli and Distomodus staurognathoides Conodont Zones (CZs), and to the Pterospathodus eopennatus CZ (equivalent to the Conochitina alargada and Eisenackitina dolioliformisChitinozoan Zones (CtZs)) are missing. Graptolites from the core indicate that the sedimentary record above the Aeronian to middle Telychian gap recommences in the lower part of the Oktavites spiralis Graptolite Zone (GZ). Co-occurrences of conodonts, graptolites and chitinozoans agree with earlier data suggesting correlation between the lower boundaries of the Cyrtograptus lapworthiGZ and Pterospathodus amorphognathoides amorphognathoides CZ, and between the lower and middle parts of the Ireviken Event and the upper Cyrtograptus murchisoni GZ.

Caracterização palinoflorística e paleoambiental da Formação Santana (Cretáceo Inferior), Poço 4-BO-1-PE, bacia do Araripe, Nordeste do Brasil

A paleoenvironmental characterization of the Santana Formation sedimentary rocks (4-BO-1-PE well), Araripe Basin, Late Aptian-Early Albian, was accomplished through qualitative and quantitative analyses of the palynofl ora. Palynological and lithological evidence indicate a predominantly continental depositional environment for this section, composed by a fl uvial, lacustrine deltaic system in a semi-arid climate. The detection of a single dinofl agellate cyst suggests that small sea ingressions took place during Aptian-Albian times in the region, as also verifi ed in other coeval strata of marginal Brazilian Cretaceous basins. Comparisons with palynofl oras from other continents support the hypothesis that this assemblage belongs to the North-Gondwana microfl oristic province.

Late Cretaceous paleoenvironmental evolution of the Tarfaya Atlantic coastal Basin, SW Morocco

Lithological evidence, benthic foraminiferal census counts, and X-ray fluorescence (XRF) scanner-derived elemental data were integrated with planktonic foraminiferal biostratigraphy and bulk carbonate stable isotopes to retrace the Turonian to early Campanian paleoenvironmental evolution and sea-level history of the Tarfaya Atlantic coastal basin (SW Morocco). The lower Turonian is characterized by laminated organic-rich deposits, which contain impoverished benthic foraminiferal assemblages, reflecting impingement of the oxygen minimum zone on the shelf during a sea-level highstand. This highstand level is correlated to the global transgressive pulse above the sequence boundary Tu1. The appearance of low-oxygen tolerant benthic foraminiferal assemblages dominated by Gavelinella sp. in the middle to upper Turonian indicates an improvement in bottom water oxygenation, probably linked to offshore retraction of the oxygen minimum zone during a regressive phase. This interval is marked by major regressive events expressed by a series of erosional truncations associated with the prominent sequence boundaries Tu3 and/or Tu4. Dysoxic–anoxic conditions recorded in the upper Santonian of the Tarfaya Basin coincide with the eustatic sea-level rise prior to Sa3 sequence boundary. The lower Campanian transgression, only recorded in the southern part of the Tarfaya Basin, coincided with substantial deepening, enhanced accumulation of fine-grained clay-rich hemipelagic sediments and improved oxygenation at the seafloor (highest diversity and abundance of benthic foraminiferal assemblages). Stable isotope data from bulk carbonates are tentatively correlated to the English Chalk carbon isotope reference curve, in particular the Hitch Wood Event in the upper Turonian, the Navigation Event in the lower Coniacian, the Horseshoe Bay Event in the Santonian and the Santonian/Campanian Boundary Event.

Methanotrophy in a Paleoproterozoic oil field ecosystem, Zaonega Formation, Karelia, Russia

Organic carbon rich rocks in the c. 2.0 Ga Zaonega Formation (ZF), Karelia, Russia, preserve isotopic characteristics of a Paleoproterozoic ecosystem and record some of the oldest known oil generation and migration. Isotopic data derived from drill core material from the ZF show a shift in δ(13) C(org) from c. -25‰ in the lower part of the succession to c. -40‰ in the upper part. This stratigraphic shift is a primary feature and cannot be explained by oil migration, maturation effects, or metamorphic overprints. The shift toward (13) C-depleted organic matter (δ(13) C(org) < -25‰) broadly coincides with lithological evidence for the generation of oil and gas in the underlying sediments and seepage onto the sea floor. We propose that the availability of thermogenic CH(4) triggered the activity of methanotrophic organisms, resulting in the production of anomalously (13) C-depleted biomass. The stratigraphic shift in δ(13) C(org) records the change from CO(2) -fixing autotrophic biomass to biomass containing a significant contribution from methanotrophy. It has been suggested recently that this shift in δ(13) C(org) reflects global forcing and progressive oxidation of the Earth. However, the lithologic indication for local thermogenic CH(4) , sourced within the oil field, is consistent with basinal methanotrophy. This indicates that regional/basinal processes can also explain the δ(13) C(org) negative isotopic shift observed in the ZF.

Carbon isotope stratigraphy and depositional oxia through Cenomanian/Turonian boundary sequences (Upper Cretaceous) in New Zealand

Stratigraphic sections across the Cenomanian/Turonian boundary (C/T boundary) are identified in New Zealand and were deposited in southern high latitudes of the palaeo-Pacific. Lithological evidence for Cretaceous Oceanic Anoxic Event 2 (OAE2), which preceded and spanned the C/T boundary, is lacking in these sections. The correlative interval is identified, however, from a positive 2‰ carbon isotope excursion (CIE) and from clustered highest occurrences of Cenomanian-restricted dinoflagellate taxa together with the lowest occurrence of Turonian Heterosphaeridium difficile. A zone lacking benthic macrofossils encompasses the CIE. In some sections, this interval is also characterized by distinctive red mudstone beds; the thickest such red bed (6–18m thick) may overlap or just overlie the main part of the CIE interval. Shelly macrobenthos, notably inoceramid bivalves, disappeared >500kyr prior to the CIE. This suggests that environmental deterioration associated with OAE2 may have preceded the inferred volcanic trigger that has been identified from other regions. Strong intermediate water depth oxia during OAE2, which contrasts with oceanic anoxic conditions that occurred elsewhere on the globe, apparently prevailed during the later phase of OAE2 in the southernmost Pacific. New data from New Zealand indicate that causal mechanism(s) of OAE2 may be complex.

Conodont apatite δ 18O values from a platform margin setting, Oklahoma, USA: Implications for initiation of Late Ordovician icehouse conditions

The uppermost Sandbian–lower Katian stratigraphic record documents a critical time period in Earth history. Lithological evidence and geochemical proxy records suggest that the uppermost Sandbian–lower Katian interval could represent a transitional phase from a long Cambrian–Ordovician greenhouse world to the Late Ordovician icehouse world. To examine this interval of Earth's history, oxygen isotopes of conodont apatite from the Bromide and Viola Springs Formations in southern Oklahoma were analyzed in order to provide sea-surface temperature estimates at a location that had little influence by terrigenous sedimentation. Our results are inconsistent with 1) previous suggestions that global cooling occurred across a prominent sequence boundary close to the Sandbian–Katian boundary across North America and 2) the hypothesis that there is a direct connection between volcanic mega eruption(s) and a significant long term impact on climate during the Sandbian–Katian of Laurentia. Furthermore, δ 18O values spanning the Guttenberg Carbon Isotope Excursion (GICE), an interval that has been interpreted as evidence for an early onset of the Late Ordovician glaciation, are variable. A + 1.5‰ δ 18O shift is observed in the lower GICE that is followed by a return to δ 18O values that fluctuate around a mean of 19.0‰ in the upper GICE. Together, these observations suggest a short-lived cooling event within the lower GICE. This cooling event may represent enhanced carbon burial and drawdown of atmospheric pCO 2 followed by a return to more stable low-latitude temperatures in the upper GICE.

Molecular carbon isotope variations in core samples taken at the Permian–Triassic boundary layers in southern China

Stable carbon isotope composition (δ13C) of carbonate sediments and the molecular (biomarker) characteristics of a continuous Permian–Triassic (PT) layer in southern China were studied to obtain geochemical signals of global change at the Permian–Triassic boundary (PTB). Carbonate carbon isotope values shifted toward positive before the end of the Permian period and then shifted negative above the PTB into the Triassic period. Molecular carbon isotope values of biomarkers followed the same trend at and below the PTB and remained negative in the Triassic layer. These biomarkers were acyclic isoprenoids, ranging from C15 to C40, steranes (C27 dominates) and terpenoids that were all significantly more abundant in samples from the Permian layer than those from the Triassic layer. The Triassic layer was distinguished by the dominance of higher molecular weight (waxy) n-alkanes. Stable carbon isotope values of individual components, including n-alkanes and acyclic isoprenoids such as phytane, isop-C25, and squalane, are depleted in δ13C by up to 8–10‰ in the Triassic samples as compared to the Permian. Measured molecular and isotopic variations of organic matter in the PT layers support the generally accepted view of Permian oceanic stagnation followed by a massive upwelling of toxic deep waters at the PTB. A series of large-scale (global) outgassing events may be associated with the carbon isotope shift we measured. This is also consistent with the lithological evidence we observed of white thin-clay layers in this region. Our findings, in context with a generally accepted stagnant Permian ocean, followed by massive upwelling of toxic deep waters might be the major causes of the largest global mass extinction event that occurred at the Permian–Triassic boundary.

Incised palaeo-channels of the late Middle Pleistocene Thames: age, origins and implications for fluvial palaeogeography and sea-level reconstruction in the southern North Sea basin

Multidisciplinary investigations of the infills of steeply-incised buried channels on the coast of Essex, England, provide important insights into late Middle Pleistocene climate and sea-level change and have a direct bearing on the differentiation of MIS 11 and MIS 9 in terrestrial records. New data are presented from Rochford and Burnham-on-Crouch where remnants of two substantial palaeo-channels filled with interglacial sediment can be directly related to the terrace stratigraphy of the Thames. The sediments in both channels accumulated in an estuarine environment early in an interglacial when mixed oak forest was becoming established. Lithological evidence suggests that the interglacial beds post-date the brackish-water infill of an older palaeo-channel ascribed to the Hoxnian and correlated with part of MIS 11, and pre-date terrace gravels (Barling Gravel) ascribed to MIS 8. An MIS 9 attribution is supported by molluscan biostratigraphy, palaeo-salinity and amino-acid racemization data. The relative sea-level record in this area thus includes evidence for two major marine transgressions during MIS 11 and MIS 9, with local maxima of >10 m O.D. Both are associated with sediments that show ‘Hoxnian’ palynological affinities. The wider significance of these findings, and of an intermediate phase of pronounced fluvial incision during MIS 10, is discussed.

New structural model to explain geophysical features in northwestern New South Wales: implications for the tectonic framework of the Tasmanides

Paleozoic and older basement rocks in northwestern New South Wales and southern and central Queensland are largely obscured by sedimentary rocks of the Mesozoic Eromanga Basin and younger Cenozoic sediments. Interpretations of the basement geology for that area have been heavily reliant upon interpretations of the regional aeromagnetic and gravity data. Geophysically, northwestern New South Wales is characterised by an east–west-trending belt of magnetic features which trend northwest in the western part and northeast to east-northeast in the eastern part; and by an east–west-trending gravity ridge. These east–west-trending features have previously been interpreted to represent the southern part of the Thomson Orogen, a separate tectonic domain to the Lachlan Orogen to the south. However, apart from these east–west geophysical trends there is sufficient geophysical and lithological evidence to suggest continuity of the Lachlan Orogen into southern and central Queensland. A new model is herein proposed whereby the geophysical features are attributed to structures resulting from crustal thickening of a Cambro-Ordovician backarc package, west of a volcanic arc, which occurred during the Late Ordovician to Early Silurian Benambran Orogeny. While the principal stress direction was oriented east-northeast–west-southwest, the structures formed via a process of tectonic escape or extrusion due to differential strain caused by the shape of the Precambrian margin in western New South Wales. The model is consistent with field observations, geophysical imagery, borehole data and seismic data. Analogue models using sand and gelatine were constructed to illustrate the concept and demonstrate that it is feasible. The model permits the extension of the Delamerian and Lachlan Orogens into southern and central Queensland to join with similar rock packages in northern Queensland, consistent with some previous tectonic models for the Tasmanides. This brings into question the need to delineate a separate Thomson Orogen.

The late Oligocene molluscan fauna from Otranto (Apulia, Southern Italy): an example of alternating freshwater, lagoonal and emerged environments

Abstract: Eighteen gastropod and five bivalve species are taxonomically described from an Upper Oligocene succession cropping out near Otranto (southern Salento, Apulia, Italy). Among these, Tectarius (Echininus) japigiae, Hydrobia dubuissoni hydruntina, Pseudamnicola messapica, Pseudamnicola palmariggii and Stenothyrella salentina are new taxa. Nonmarine and lagoonal environments characterize almost the whole succession, as testified by species-poor assemblages rich in specimens of freshwater and oligo- to mesohaline prosobranchs, such as Neritinidae, Hydrobiidae, Stenothyridae, Thiaridae, Potamididae, Batillariidae, pulmonates, including Planorbidae and bivalves comprising Dreissenidae and Cyrenidae. The genera Theodoxus, Hydrobia, Melanoides, Potamides, Terebralia, Batillaria, Granulolabium, Mytilopsis and Polymesoda alternately dominate. Littoral marine elements, such as Tectarius, Turritella, Barbatia, Anadara and Chama, are scattered present in the succession, but some prevail towards the top. The fauna is palaeobiogeographically significant having strong affinities with those of the Oligo–Miocene basins of Aquitaine, Mainz, Bavaria, North Alpine Foreland, Vienna, Greece and Turkey. The analysed Otranto succession is assigned to the Chattian Galatone Formation of southernmost Apulia (Salento) based on lithological evidence and supported by the palaeontological data.

British Ice Sheet dynamics inferred from North Atlantic ice‐rafted debris records spanning the last 175 000 years

AbstractSediment core MD04‐2822 from the Rockall Trough, northeast Atlantic, provides lithological evidence for the expansion of the British Ice Sheet (BIS) onto the Hebridean margin during both the penultimate (Marine Isotope Stage (MIS) 6/Wolstonian/Saalian) and last glaciation (Devensian/Weichselian). A composite chronology is constructed using: (i) the benthic foraminiferal oxygen isotope record, matched to an orbitally tuned stacked record of global ice volume (SPECMAP); (ii) the synchronisation of the planktonic foraminiferal (Neogloboquadrina pachyderma (sinistral)) and ITRAX XRF Ca records to the Greenland ice core (NGRIP) back to ca. 123 ka; and (iii) the synchronisation of the same N. pachyderma (sinistral)) and ITRAX XRF Ca records to the atmospheric methane record of the Antarctic ice core (EPICA Dome C) beyond 123 ka. The first offshore evidence for the expansion of the last BIS within MIS 4 is presented, suggesting extensive glaciation of northwestern Britain during the Devensian/Weichselian as early as 70 ka. The proximity of this long sediment core to the glaciated margin of northwestern Scotland makes it ideal to capture the ice‐rafted debris (IRD) signal of the BIS for the last 175 ka, via a continuous flux record of IRD. British Geological Survey © NERC 2009. All rights reserved.

Climate signals in sediment mineralogy of Lake Baikal and Lake Hovsgol during the LGM-Holocene transition and the 1-Ma carbonate record from the HDP-04 drill core

Modeling the bulk sediment XRD patterns allows insight into the environmental and depositional histories of two neighboring rift lake basins within the Baikal watershed. Parallel 14C-dated LGM-Holocene records in Lakes Baikal and Hovsgol are used to discuss the mineralogical signatures of regional climate change. In both basins, it is possible to distinguish ‘glacial’ and ‘interglacial’ mineral associations. Clay minerals comprise in excess of 50% of layered silicates in bulk sediment. The abundance of smectite (expandable) layers in mixed-layer illite–smectites and the total illite abundance are the main paleoclimatic indices in the clay mineral assemblage. Both indices exhibit coherent responses to the Bølling–Allerød and the Younger Dryas. The smectite layer index is not equivalent to the abundance of illite–smectite, because illite–smectite tends to transform into illite. Repeated wetting–drying cycles in soils and high abundance of expandable layers in illite–smectites (>42%) favor the process of illitization. This relationship is clearly shown in both Baikal and Hovsgol records for the first time. The opposite late Holocene trends in illite abundance in Lake Baikal and Lake Hovsgol records suggest that a sensitive optimal regime may exist for illite formation in the Baikal watershed with regard to warmth and effective moisture. The Lake Hovsgol sediments of the last glacial contain carbonates, suggesting a positive trend in the lake's water budget. A progressive change towards lower Mg content in carbonates indicates lowering mineralization of lake waters. This trend is consistent with the lithologic evidence for lake-level rise in the Hovsgol basin. The pattern of mineralogical changes during the past 20 ka is used to interpret bulk sediment and carbonate mineralogy of the long 81-m Lake Hovsgol drill core (HDP-04) with a basal age of 1 Ma. The interglacial-type silicate mineral associations are confined to several thin intervals; most of the sediment record is calcareous. Carbonates are represented by six main mineral phases: calcite, low-Mg calcite, intermediate/high-Mg calcite, dolomite, excess-Ca dolomite and metastable monohydrocalcite. These mineral phases tend to form stratigraphic successions indicative of progressive changes in lake water chemistry. Five sediment layers with abundant Mg-calcites in the HDP-04 section suggest deposition in a low standing lake with high mineralization (salinity) and high Mg/Ca ratios of lake waters. Lake Hovsgol sediments contain the oldest known monohydrocalcite, found tens of meters below lake bottom in sediments as old as 800 ka. This unusual find is likely due to the conditions favorable to preservation of this metastable carbonate.

Sedimentary record from Lake Hovsgol, NW Mongolia: Results from the HDP-04 and HDP-06 drill cores

The Hovsgol Drilling Project retrieved Pleistocene sediment section with the basal age of ca. 1 Ma from the Hovsgol rift basin, NW Mongolia. Detailed lithologic data on drill cores is presented and compared with analogous sediment facies in the radiocarbon-dated records of the last glacial–interglacial transition. Drill cores from two sites, presently in 239 m and 235 m water depth, represent somewhat different depositional settings. The shorter HDP-06 drill core (26 m) at the base of the gentler SE slope of the rift basin contains lithologic evidence for several recent lake lowstands on the order of −200 m. The longer HDP-04 drill core (81 m) some 8 km away at the base of the steep NW underwater slope is composed of finer sediments and contains at least 10 characteristic transitions from calcareous to carbonate-free (diatomaceous) layers. These lithologic transitions are interpreted here as signals of repeated Pleistocene lake transgressions in the Hovsgol basin. Transgressions appear to have been associated with lower sedimentation rates and with the deposition of thin turbidite beds at the drill site. Comparison of drill core lithology with the available seismic data shows reasonable agreement in terms of the number of lowstand events and the general trends of changing lake level. HDP-04 drill core retrieved shallow-water facies containing sand and carbonate oolites deposited at the time of the most dramatic mid-Pleistocene regression of the lake. At ca. 24 m core depth, this interval corresponds to a major basinwide angular unconformity apparent in the seismic pattern. Lake Hovsgol, a smaller sister rift lake of the grand Lake Baikal, has a confined local catchment, which makes it very sensitive to regional variations in the effective moisture. Consisting primarily of calcareous mud, the sedimentary record of Lake Hovsgol provides a unique regional sedimentary archive. Future multi-proxy studies of the Hovsgol sedimentary records will allow constraint of the mid-late Pleistocene history of the hydrologic budget in the Baikal region of continental interior Asia.

Regional and local controls on postglacial vegetation and fire in the Siskiyou Mountains, northern California, USA

The Siskiyou Mountains of northwestern California and southwestern Oregon are a floristic hotspot, and the high diversity of conifers there likely results from a combination of geological, ecological, climatological and historical factors. To evaluate how past climate variability has influenced the composition, structure and fire regime of the Siskiyou forests, pollen, charcoal, and lithological evidence was examined from two lakes along a moisture gradient to reconstruct the vegetation, fire and climate history. The late-glacial period was characterized by subalpine parkland and infrequent fire at both sites. During the late-glacial/Early Holocene transition period, subalpine parkland was replaced by a closed forest of Pinus, Cupressaceae, Abies and Pseudotsuga and more frequent fires a 1000 years earlier at the wetter site, and it is likely that reduced Pacific Ocean upwelling created warmer drier conditions at the coast. In the Early Holocene, Pinus, Cupressaceae were less abundant and fire less frequent at the coastal site during a period of increased coastal upwelling and fog production. In the Late Holocene, Abies, Pseudotsuga, Pinus, and Quercus vaccinifolia increased in the forest at both sites suggesting a widespread response to cooling. Fewer fires at the wetter site may account for the abundance of Picea breweriana within the last 1000 years. The comparison of the two records implies that large-scale controls in climate during the last 14,000 cal yr BP have resulted in major changes in vegetation and fire regime. Asynchrony in the ecosystem response of wetter and drier sites arises from small-scale spatial variations in effective moisture and temperature resulting from topographically-influenced microclimates and coastal-to-inland climate gradients.

Biostratigraphic and paleogeographic significance of a palynological assemblage from the Middle Devonian Ulusubasite Formation, eastern Junggar Basin, Xinjiang, China

Abundant palynomorphs are here reported for the first time from tuffaceous siltstones of the Middle Devonian Ulusubasite Formation at the Zhifang Section, eastern Junggar Basin, Xinjiang, Northwest China. Thirty-four miospore species assigned to 23 genera (four established species, two cf., 19 sp., and nine spp.), 19 acritarch species assigned to 19 genera (10 established species, three cf., five sp., and one spp.), and scolecodonts are identified. The biostratigraphic ranges of the acritarchs and miospores indicate a Mid Devonian age, corroborating the age assignment previously based on associated corals, brachiopods, and plant fossils. Paleogeographic reconstruction for the Middle Devonian indicates that the eastern Junggar Basin was part of the Kazakhstan Plate and situated in the low-mid latitudes of the Northern hemisphere between the Euramerican landmass to the west, and the North and South China, and Gondwana landmasses to the east and south. Although there is some miospore compositional similarity between the eastern Junggar Basin and the landmasses of Euramerica, South and North China, and Gondwana, it is not particularly high, and is mainly at the generic level. The acritarch assemblage, however, consists mostly of cosmopolitan species and provides new evidence for extending the marine linkage between North America, Baltica, the Junggar Basin of Kazakhstan, and Gondwana. Paleontologic and lithologic evidence indicates that the Ulusubasite Formation was deposited in a near-shore marine environment.

Palaeolimnological evidence for widespread late 18th century drought across equatorial East Africa

The sediment records of three shallow climate-sensitive lakes in equatorial East Africa (Chibwera and Kanyamukali in western Uganda, and Baringo in central Kenya) contain clear lithological evidence of an episode of complete desiccation in the relatively recent past. Quantitative stratigraphic analyses of bulk sediment composition, texture (% sand and coarse organic detritus), the C/N ratio of bulk organic matter, and magnetic susceptibility allowed us to define a common succession of sedimentary units in all three lakes. This common stratigraphy includes a basal unit of dry and stiff low-organic clays capped by a horizon of peaty mud or peat, overlain by organic lake sediment. We interpret this stratigraphy to represent an episode of (nearly) complete desiccation of these lake basins. This was followed first by an early phase of modest lake refilling, during which time extensive littoral and submerged vegetation developed, producing peaty lake deposits. Mostly higher lake levels later on resulted in deposition of fine-grained lake muds. Using 210Pb and 137Cs activity profiles combined with four AMS 14C dates on single terrestrial plant macrofossils or multiple charred grass particles indicate that in all three lakes the episode of most recent complete desiccation occurred in the late 18th and early 19th century. These results add to an accumulating body of palaeoenvironmental data revealing a prolonged period of extreme drought in equatorial East Africa about 200 years ago, also documented in the cultural history of the African Great Lakes region. Given the geographical spread of the study lakes across the East African plateau, these results indicate that the climatic anomaly of reduced rainfall responsible for this drought was at least sub-continental in scale.

Deformation of the Chukchi microcontinent: Structural, lithologic, and geochronological evidence

This paper presents the results of structural, lithologic, and geochronological (K-Ar, Rb-Sr) studies of the Carnian terrigenous rocks in the sedimentary cover of the Chukchi microcontinent and U-Pb dating of detrital zircons. From the lithological features, three types of sections are recognized. Terrigenous sequences of the first type were deposited on the outer shelf in the distal zone of the prograding delta, sequences of the second type accumulated at the rise of continental slope, and sediments of the third type are characteristic of the pelagic zone. In mineralogy and geochemistry, sandstones are rather uniform and inherit the sialic composition of provenance. The detrital zircons comprise several populations with predominance of the varieties derived from igneous rocks. The U-Pb age of the youngest population is 236–255 Ma. The conditions of postsedimentation alteration reached those of greenschist metamorphic facies and anchimetamorphism. Several cleavage systems have been established. Sericite related to the oldest system is distinguished by elevated Ti, Mn, and Fe components. The first stage of deformation of the Carnian sedimentary rocks about 200 Ma ago resulted in the rearrangement of K-Ar and Rb-Sr isotopic systems in whole-rock samples and minerals and is clearly recorded in isotopic data. It is suggested that the deformation related to the normal faulting in Triassic rocks and the emergence of the Lesser Anyui Block were plausible causes of the first structural rearrangement.

Authigenic carbon entombed in methane-soaked sediments from the northeastern transform margin of the Guaymas Basin, Gulf of California

Extensive ROV-based sampling and exploration of the seafloor was conducted along an eroded transform-parallel fault scarp on the northeastern side of the Guaymas Basin in the Gulf of California to observe the nature of fluids venting from the seafloor, measure the record left by methane-venting on the carbonates from this area, and determine the association with gas hydrate. One gas vent vigorous enough to generate a water-column gas plume traceable for over 800 m above the seafloor was found to emanate from a ∼10-cm-wide orifice on the eroded scarp face. Sediment temperature measurements and topography on a sub-bottom reflector recorded in a transform-parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent. Active chemosynthetic biological communities (CBCs) and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform-parallel scarp. The carbon isotopic composition of methane bubbles flowing vigorously from the gas vent (−53.6±0.8‰ PDB) is comparable to methane found in sediment cores taken within the CBCs distributed along the scarp (−51.9±8.1‰ PDB). However, the δ13C value of the CO2 in the vent gas (+12.4±1.1‰ PDB) is very distinct from those for dissolved inorganic carbon (DIC) (−35.8‰ to −2.9‰ PDB) found elsewhere along the scarp, including underneath CBCs. The δ13C values of the carbonate-rich sediments and rocks exposed on the seafloor today also span an unusually large range (−40.9‰ to +12.9‰ PDB) and suggest two distinct populations of authigenic carbonate materials were sampled. Unconsolidated sediments and some carbonate rocks, which have lithologic evidence for near-seafloor formation, have negative δ13C values, while carbonate rocks that clearly formed in the subsurface have positive δ13C values (up to +23.0‰) close to that measured for CO2 in the vent gas. There appears to be two carbon sources for the authigenic carbonates: (1) deeply-sourced, isotopically heavy CO2 (∼+12‰); and (2) isotopically light DIC derived from local anaerobic oxidation of methane at the sulfate–methane interface in the shallow subsurface. Addition of isotopically light methane-derived carbon at the seafloor may completely mask the isotopically heavy CO2 signature (+12.4‰) in the underlying sediments. Thus, the authigenic carbonates may have formed from the same methane- and carbon dioxide-bearing fluid, but under different migration and alteration conditions, depending on how it migrated through the sediment column.