“Mind the gap”: Caves,radiocarbon sequences,and the Mesolithic–Neolithic transition in Europe—lessons from the Mala Triglavca rockshelter site

Radiocarbon sequences from some northern Mediterranean cave sites show a temporal gap between Mesolithic and Neolithic occupations. Some authors regard this as a regional phenomenon and have sought to explain it in terms of a general population decline in the late Mesolithic, which facilitated the replacement of indigenous foragers by immigrant farmers. New evidence from the rockshelter site of Mala Triglavca, in Slovenia, leads us to question this view. We describe the deposits in the rockshelter and discuss the results of AMS radiocarbon dating of bone samples recovered in excavations in the 1980s. New archaeological investigations and associated soil/sediment analyses show that in the central part of the rockshelter a well‐defined stratigraphic sequence can be established, despite post‐depositional modification by soil forming processes. There is also evidence of substantial post‐depositional disturbance of the cave sediments by human agency and geomorphological processes, which have created “temporal gaps” and “inversions” in the radiocarbon sequence. The relatively large series of radiocarbon dates obtained enables some of the post‐depositional processes to be identified. © 2008 Wiley Periodicals, Inc.     

A multiple cave deposit assessment of suitability of speleothem isotopes for reconstructing palaeo‐vegetation and palaeo‐temperature

The suitability of speleothems for interpreting palaeoclimate is typically determined by using either the Hendy Test, overlapping analysis or long‐term cave environment monitoring. However, in many cases, these methods are not applicable, because a speleothem lacks clearly traceable layers for the Hendy Test, it is difficult to obtain an overlapping speleothem nearby, or long‐term cave monitoring is impractical. The authors propose a multiple cave deposit approach to assess the suitability of speleothems for palaeoclimate study. Speleothems collected from two sites within Raccoon Mountain Cave, Tennessee (USA) exhibit remarkable spatial variation (δ¹³C: ?10·3‰ to ?2·2‰) over a relatively short distance (ca 260 m). Drip water δ¹⁸O values exhibit a seasonal precipitation signal at Site 1 and an annual signal at Site 2. Combining field observations, water isotope analysis and trace‐element data, the authors propose that the speleothem formation at Site 1 and Site 2 tapped distinct sources of CO₂: (i) CO₂ derived from overlying soils for Site 1; and (ii) limestone dissolved inorganic carbon induced by ground water dissolution for Site 2. Using fresh cave deposits (modern speleothem) δ¹³C (100% C3 vegetation) as an analogue, a simple model was developed to estimate land surface vegetation for speleothems. The speleothem formation temperature estimated using fresh cave deposit δ¹⁸O values generally reflects the mean annual temperature in this region. This study indicates that spatial variations in carbon isotopes could be caused by different carbon sources dominating in different parts of the cave, which should be taken into consideration by researchers when using speleothem δ¹³C values to reconstruct temporal palaeo‐vegetation changes. This study demonstrates a practical sampling strategy for verifying suitability of speleothems for palaeo‐vegetation and palaeo‐temperature reconstructions by analysing multiple cave deposits, especially for cases in which the Hendy Test, parallel sampling and long‐term monitoring of cave environment are not feasible.     

Additive effects of climate change and human hunting explain population decline and extinction in cave bears

Cave bears (Ursus spelaeus) are an iconic component of the European late Quaternary Ice Age megafauna. Recent demographic analyses based on cave bear mtDNA sequences and refined radiocarbon dating indicate that cave bear population size and genetic diversity started to decline some 50 kilo years ago (kya). Hence, neither the coldest phase of the last glaciation (started some 24 kya), nor the colonization of Europe by Palaeolithic hunters (started some 45 kya) coincides with the beginning of population decline. Here, we reconstructed cave bear climatic niche evolution through time. Then, we performed spatially explicit population viability analyses to assess cave bear demographics through time in response to climatic changes, human effects on bear survival and their combination. We found that climate change was responsible for a 10‐fold decrease in cave bear population size after 40 kya. However, climate change on its own could not explain U. spelaeus extinction at 24 kya. Additional negative effects consistent with human population expansion are required to explain both U. spelaeus' retreat from eastern Europe since 40 kya and its final extinction.     

A micromorphological and mineralogical study of site formation processes at the late Pleistocene site of Obi‐Rakhmat,Uzbekistan

Site formation processes at the Late Pleistocene rockshelter deposit of Obi‐Rakhmat were reconstructed through soil micromorphology and Fourier transform infrared spectroscopy (FTIR). The entire sequence has undergone limited diagenesis and is well preserved. The base of the stratified sequence represents a karstic setting with intermittent, low‐energy deposition of autochthonous gravitational debris and anthropogenic material in a wet, muddy environment. These sediments were post‐depositionally affected by episodic waterlogging. The bulk of the sequence overlying the karstic layers comprises a continuous series of primary freshwater spring deposits containing reworked anthropogenic material that was buried penecontemporaneously with calcium carbonate deposition. The top of the sequence is weakly cryoturbated, indicating a periodically cold, wet environment. No alluvial elements that could suggest sediment inputs from the nearby river terraces were documented. A single exogenous layer was identified, representing an episode of colluviation from directly above the rockshelter preceding a major roofspall event. The basal part of the sequence contains slightly reworked anthropogenic remnants of intense activities, including combustion. The anthropogenic elements present in the spring deposits show higher degrees of reworking, suggesting within‐layer translocation. The development of spring activity at the site did not cause humans to abandon the rockshelter; they continued to carry out their activities throughout a changing local environment. © 2009 Wiley Periodicals, Inc.     

An example of a highly bioturbated,storm‐influenced shoreface deposit: Upper Jurassic Ula Formation,Norwegian North Sea

Integrated ichnological and sedimentological analyses of core samples from the Upper Jurassic Ula Formation in the Norwegian Central Graben were undertaken to quantify the influence of storm waves on sedimentation. Two main facies associations (offshore and shoreface) that form a progradational coarsening upward succession are recognizable within the cores. The offshore deposits are characterized by massive to finely laminated mudstones and fine‐grained sandstones, within a moderately to highly bioturbated complex. The trace fossil assemblage is dominated by deposit‐feeding structures (for example, Planolites, Phycosiphon and Rosselia) and constitutes an expression of the proximal Zoophycos to distal Cruziana ichnofacies. The absence of grazing behaviours and dominance of deposit‐feeding ichnofossils is a reflection of the increased wave energies present (i.e. storm‐generated currents) within an offshore setting. The shoreface succession is represented by highly bioturbated fine‐grained to medium‐grained sandstones, with intervals of planar and trough cross‐bedding, thin pebble lags and bivalve‐rich shell layers. The ichnofossil assemblage, forming part of the Skolithos ichnofacies, is dominated by higher energy Ophiomorpha nodosa ichnofossils and lower energy Ophiomorpha irregulaire and Siphonichnus ichnofossils. The presence of sporadic wave‐generated sedimentary structures and variability in ichnofossil diversity and abundance attests to the influence of storm‐generated currents during deposition. As a whole, the Ula Formation strongly reflects the influence of storm deposits on sediment deposition; consequently, storm‐influenced shoreface most accurately describes these depositional environments.     

Variations in the architecture of hydraulic‐jump bar complexes on non‐eroding beds

Sediment accumulation downstream of hydraulic jumps can occur in many settings but the architectures of such deposits are poorly documented. Here, three flume runs were used to examine the influence of sediment grain size and transport rate on the characteristics of hydraulic‐jump unit bars. In one of these runs six hydraulic‐jump unit bars formed a hydraulic‐jump bar complex. In another, the same sediment was supplied more quickly and only two unit bars formed. In the third run with the same sediment supply rate, but different grain size, only one large unit bar formed. All unit bars developed in a similar way but their size and internal architecture differed; they all resulted from a reduction in sediment transport capacity at the transition from supercritical flow to subcritical flow in the hydraulic jump. After initial onset of sedimentation and unit bar formation, generation of subsequent unit bars may be: (i) related to small changes in sediment flux; and (ii) independent of changes in the hydraulic jump. Continued sedimentation caused changes from oscillating to weak hydraulic jumps and hydraulic‐jump unit bars formed in both circumstances. The flow of water and suspended sediment becomes shallower over the lee of the bar complex. This leads to flow acceleration and a return to supercritical flow conditions. In turn, a chain of such features can form and generate a chute and pool bed morphology. There is an inherent upper size limit to a hydraulic‐jump bar complex due to the changing flow conditions over the growing deposit as the water above it becomes shallower. There is also an amplitude minimum for the development of foresets and subsequent unit bar growth. Hydraulic‐jump unit bars have architectures that should be recognizable in the rock record and because their size is constrained by the flow conditions, their identification should be useful for interpreting palaeoenvironment.     

Cave bear occupation in Schwabenreith Cave,Austria, during the early last glacial: constraints from 230Th/U‐dated speleothems

The cave bear was a prominent member of the Upper Pleistocene fauna in Eurasia. While breakthroughs were recently achieved with respect to its phylogeny using ancient DNA techniques, it is still challenging to date cave bear fossils beyond the radiocarbon age range. Without an accurate and precise chronological framework, however, key questions regarding the palaeoecology cannot be addressed, such as the extent to which large climate swings during the last glacial affected the habitat and possibly even conditioned the final extinction of this mammal. Key to constraining the age of cave bear fossils older than the lower limit of radiocarbon dating is to date interlayered speleothems using ²³⁰Th/U. Here we report new results from one such site in the Eastern European Alps (Schwabenreith Cave), which yielded the highest density of bones of cave bear (Ursus spelaeus eremus). Although dating of the flowstones overlying this fossiliferous succession was partly compromised by diagenetic alteration, the ²³⁰Th/U dates indicate that the bear hibernated in this cave after about 113 ka and before about 109 ka. This time interval coincides with the equivalent of Greenland Stadial 25, suggesting possible climate control on the cave bear's habitat and behaviour. © 2019 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd     

Depositional processes on oceanic island shelves – Evidence from storm‐generated Neogene deposits from the mid‐North Atlantic

Oceanic islands – such as the Azores in the mid‐North Atlantic – are periodically exposed to large storms that often remobilize and transport marine sediments along coastlines, and into deeper environments. Such disruptive events create deposits – denominated tempestites – whose characteristics reflect the highly dynamic environment in which they were formed. Tempestites from oceanic islands, however, are seldom described in the literature and little is known about storm‐related sediment dynamics affecting oceanic island shelves. Therefore, the geological record of tempestite deposits at oceanic islands can provide invaluable information on the processes of sediment remobilization, transport and deposition taking place on insular shelves during and after major storms. In Santa Maria Island (Azores), a sequence of Neogene tempestite deposits was incorporated in the island edifice by the ongoing volcanic activity (thus preserved) and later exposed through uplift and erosion. Because it was overlain by a contemporary coastal lava delta, the water depth at the time of deposition could be inferred, constituting an excellent case‐study to gain insight on the still enigmatic processes of insular shelf deposition. Sedimentological, palaeontological, petrographic and palaeo‐water depth information allowed the reconstruction of the depositional environment of these sediments. The sequence typifies the characteristics of a tempestite (or successive tempestites) formed at ca 50 m depth, in a steep, energetic open insular shelf, and with evidence for massive sediment remobilization from the nearshore to the middle or outer shelf. The authors claim that cross‐shelf transport induced by storm events is the main process of sediment deposition acting on steep and narrow shelves subjected to high‐energetic environments, such as the insular shelves of open‐sea volcanic islands.     

A facies‐based depositional model for ancient and modern,tectonically–confined tidal straits

Modern and ancient tidal straits are the least well understood of all tide‐dominated depositional systems. To provide an increased understanding of these systems, a facies‐based depositional model is assessed by comparing multibeam surveys of three present‐day tidally dominated seaways with a number of superbly exposed Neogene‐to‐Quaternary strait‐fill successions of Calabria (south Italy). The model points out the existence of four depositional zones, laterally adjacent from the narrowest strait centre to its terminations, distributed along symmetrical or asymmetrical seaways. These zones, whose signature is recorded in four facies associations in the Calabrian tidal straits, are as follows: (i) the strait‐centre zone, associated with the tidal current maxima and where sediments are scarce or absent; (ii) the dune‐bedded zone, where sediments form dune complexes due to tidal flow expansion; (iii) the strait‐end zone, where currents decelerate accumulating thinly bedded, fine‐grained deposits; and (iv) the strait‐margin zone, where sediment massflows descend tectonically active, steep margins towards the strait axis. In ancient, tectonically confined, narrow seaways, these facies generate a distinctive deepening‐upward vertical succession, where tidal currents are the dominant process in the sediment distribution.     

The lowermost Mississippi River: a mixed bedrock‐alluvial channel

In this study, the distribution of channel‐bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high‐intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio‐deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach‐scale distribution of channel‐bed facies is related to river planform. From a broader perspective, the distribution of channel‐bed facies is related to channel sinuosity — higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1·5:1 for a 45 km segment of the river that has a sinuosity of 1·76 and this ratio increases to ca 3:1 for a 120 km segment of the river that has a sinuosity of 1·21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock‐alluvial channel. The analyses demonstrate that a mixed bedrock‐alluvial channel boundary can be associated with low‐gradient and sand‐bed rivers near their marine outlet.     

Morphometry of upper cheek teeth of cave bears (Carnivora,Ursidae)

Univariate and multivariate statistics were applied to analyse the morphometrical variability of 4920 upper cheek teeth (P4, M1 and M2) of cave bears from 123 geographical sites (180 samples) of different Pliocene – Pleistocene ages. The analysed specimens included those belonging to the big cave bears Ursus kudarensis, U. deningeri, U. spelaeus (three subspecies) and U. kanivetz (including U. ingressus), as well as the small cave bear U. rossicus. The information‐theoretical parameters (Shannon entropy and orderliness (Von Foerster, 1960: On self‐organizing systems and their environments. In Self‐Organizing Systems, 31–50. Pergamon Press, London) were used to estimate tooth diversity in different teeth, different taxa and in selected local chrono‐populations. Multivariate allometry coefficients (Klingenberg, 1996: Multivariate allometry. In Advances in Morphometrics, 23‐49. Plenum Press, New York) were used to describe the relationships of different ‘parts’ of a tooth and to compare allometric patterns amongst species or selected local samples. A multivariate analysis showed a significant overlap of the size/shape parameter ranges in deningeroid and spelaeoid bears within morphological spaces. Within the cave bear lineage, the Deninger's bear has the greatest morphological diversity index (entropy) of all the teeth overall, and the lowest diversity is observed in the final taxon of this lineage – U. kanivetz (=ingressus). The P4 and M2 diversity showed multidirectional correlations with elevation above sea level amongst several ‘local’ populations of Late Pleistocene cave bears. The morphological disparities between the studied taxa are in close agreement with the distances in the available schemes of genetic differentiation based on ancient mitochondrial DNA. The split of U. kudarensis and U. deningeri has a good bootstrap support, which corresponds to the hypothesis about their parallel evolution. The small cave bear U. rossicus is placed between U. arctos and U. deningeri. The phylogenetic signal is more pronounced in the variability of teeth in comparison with other skeletal remains of cave bears (cranium, mandible, or metapodial bones).     

Sedimentological–ichnological model for tide‐dominated shelf sandbodies: Lower Cambrian Gog Group of western Canada

Dunes and bars are common elements in tide‐dominated shelf settings. However, there is no consensus on a unifying terminology or a systematic classification for thick sets of cross‐stratified sandstones. In addition, their ichnological attributes have hardly been explored. To address these issues, the properties, architecture and ichnology of compound cross‐stratified sandstone bodies contained in the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains are described here. In these transgressive sandstones, five types of compound cross‐stratified sandstone are distinguished based on foreset geometry, sedimentary structures and internal heterogeneity. These represent four broad categories of subtidal sandbodies: (i) compound‐dune fields; (ii) sand sheets; (iii) sand ridges; and (iv) isolated dune patches; tidal bars comprise a fifth category but are not present in the Gog Group. Compound‐dune fields are characterized by sigmoidal and planar cross‐stratified sandstone in coarsening‐upward and thickening‐upward packages (Type 1); these are mostly unburrowed, or locally contain representatives of the Skolithos ichnofacies, but are intercalated with intensely bioturbated sandstone containing the archetypal Cruziana ichnofacies. Sand‐sheet complexes, also composed of compound dunes, cover more extensive subtidal areas, and comprise three adjacent subenvironments: core, front and margin. The core is characterized by thick‐bedded sets of cross‐stratified sandstone (Type 2). A decrease of bedform size at the front is recorded by wedges of thinner‐bedded, low‐angle and planar cross‐stratified sandstone (Type 3) exhibiting dense Skolithos pipe‐rock ichnofabric. The margin is characterized by interbedded sandstone and mudstone, and hummocky cross‐stratified sandstone. Sand‐sheet deposits exhibit clear trends in trace‐fossil distribution along the sediment transport path, from non‐bioturbated beds in the core to Skolithos ichnofacies at the front, and a depauperate Cruziana ichnofacies at the margin. Tidal sand ridges are large elongate sandbodies characterized by large sigmoid‐shaped reactivation surfaces (Type 4). Sand ridges display clear ichnological trends perpendicular to the axis of the ridge, with no bioturbation or a poorly developed Skolithos ichnofacies in the core, a depauperate Cruziana ichnofacies in lee‐side deposits, and Cruziana ichnofacies at the margin. While both tidal ridges and tidal bars migrate by means of lateral accretion, the latter occur in association with channels while the former do not. Because tidal bars tend to occur in brackish‐water marginal‐marine settings, their ichnofauna are typically of low diversity, representing a depauperate Cruziana ichnofacies. Isolated dune patches developed on sand‐starved areas of the shelf, and are represented by lenticular sandbodies with sigmoidal reactivation surfaces (Type 5); they typically lack trace fossils, but the interfingering muddy deposits are intensely bioturbated by a high‐diversity fauna recording the Cruziana ichnofacies. The variety of sandbody types in the Gog Group reflects varying sediment supply and location on the inner continental shelf. These, in turn, governed substrate mobility, grain size, turbidity, water‐column productivity and sediment organic matter which controlled trace fossil distribution.     

Evolution and architecture of a West Mediterranean Upper Pleistocene to Holocene coastal apron‐fan system

The Quaternary deposits of tectonically stable areas are a powerful tool to investigate high‐frequency climate variations (<10 ka) and to distinguish allogenic and autogenic factors controlling deposition. Therefore, an Upper Pleistocene–Holocene coastal apron‐fan system in north–western Sardinia (Porto Palmas, Italy) was studied to investigate the relations between climate changes, sea‐level fluctuations and sediment source‐supply that controlled its development. The sedimentary sequence records the strong influence of local (wet/dry) and worldwide (sea‐level) environmental variations in the sedimentation and preservation of the deposits. A multi‐disciplinary approach allowed subdivision of the succession into four major, unconformity‐bounded stratigraphic units: U1 U2, U3 and U4. Unit U1, tentatively dated to the warm and humid Marine Isotopic Stage (MIS) 5, consists of sandy, gravelly coastal/beach deposits developed during high sea‐level in low‐lying areas. Unit U2 consists of debris‐flow dominated fan‐deposits (ca 74 ka; MIS 4), preserved as partial fills of small valleys and coves. Unit U2 is mainly composed of reddish silty conglomerate to pebbly siltstones sourced from the Palaeozoic metamorphic inland hills (bedrock), superficially disintegrated during the preceding warm, vegetation‐rich MIS 5. The cold and semi‐arid climate strongly reduced vegetation cover along the valley flanks. Therefore, sediment gravity‐flow processes, possibly activated by rainstorms, led to deposition of debris‐flow dominated fans. Unit U3 consists of water‐flow dominated alluvial‐fan deposits (ca 47 to 23 ka; MIS 3), developed on a slightly inclined coastal plain. Unit U3 is composed of sandstone and sandy conglomerate fed from two main sediment sources: metamorphic inland bedrock and Quaternary bioclastic‐rich shelf‐derived sands. During this cold phase, sea‐level dropped sufficiently to expose bioclastic sands accumulated on the shelf. Frequent climate fluctuations favoured inland aeolian transport of sand during dry phases, followed by reworking of the aeolian bodies by flash floods during wet phases. Bedrock‐derived fragments mixed with water‐reworked, wind‐blown sands led to the development of water‐flow dominated fans. The Dansgaard–Oeschger events possibly associated with sand landward deflation and main fan formations are Dansgaard–Oeschger 13 (ca 47 ka), Dansgaard–Oeschger 8 (ca 39 ka) and Dansgaard–Oeschger 2 (ca 23 ka). No record of sedimentation during MIS 2 was observed. Finally, bioclastic‐rich aeolianites (Unit U4, ca 10 to 5 ka; MIS 1), preserved on a coastal slope, were developed during the Holocene transgression (ca 10 to 5 ka; MIS 1). The studied sequence shows strong similarities with those of other Mediterranean sites; it is, however, one of the few where the main MIS 4 and MIS 3 climatic fluctuations are registered in the sedimentary record.     

Geoarchaeology and Paleoenvironmental Context of the Burntwood Creek Rockshelter,High Plains of Northwestern Kansas,U.S.A

The first systematic geoarchaeological study of a rockshelter in Kansas yielded information about rockshelter formation, Holocene paleoenvironments, archaeological site formation processes, and prehistoric human habitation and site use. The rockshelter was carved by groundwater sapping; colluvium, alluvium, and roof‐fall debris subsequently accumulated on its floor. An assemblage of Late Archaic (∼2000–4000 ¹⁴C years before present [yr B.P.]) cultural deposits (lithic material, bone, and charcoal) is preserved in a 2‐m thick package of colluvium and alluvium in front of the shelter. Chipped stone analysis indicates that Late Archaic occupants of the shelter were using mostly local materials and resharpening their stone tools at the shelter. Stable carbon isotope and phytolith analyses indicate that a mixed C₃ and C₄ plant community existed in the study area and that it changed in composition through the late Holocene. Based on the phytolith assemblage from an unlined fire pit at the front of the shelter, Late Archaic people used local hackberry trees for fuel and/or food.     

A 2000‐year lipid biomarker record preserved in a stalagmite from north‐west Scotland

Previous studies on lipid biomarkers preserved in Chinese stalagmites have indicated that ratios of low‐molecular‐weight (LMW) to high‐molecular‐weight (HMW) n‐alkanes, n‐alkan‐2‐ones, n‐alkanols and n‐alkanoic acids can be used as an index of vegetation versus microbial organic matter input to the system and, by extension, a marker of climatic changes, with increases in the proportion of LMW compounds coinciding with colder periods. Here we test whether this hypothesis is equally applicable to a different geographical region (north‐west Scotland), by examining a stalagmite record of the past 200 years, and a wider range of lipid markers. We also test the applicability of other lipid proxies in this context, including the use of n‐alkane ratios, to interpret vegetation changes, and unsaturated alkanoic acid ratios as climatic indicators. The results show that lipid proxies preserved in stalagmites, and especially those related to vegetation, are potentially extremely useful in palaeoenvironmental research. Of particular value is the use of C₂₇/C₃₁ n‐alkane ratios as a proxy for vegetation change, clearly indicating variations between herbaceous and arboreal cover. This proxy has now been successfully applied to samples from diverse environments, and can be considered sufficiently robust to be of use in analysing future stalagmite records. It will be of particular value in areas where reliable pollen records are not available, as is often the case with deeper cave deposits. However, the division between LMW and HMW aliphatic compounds is not a clear‐cut case of microbial versus plant activity, with the changes in LMW compounds relating more closely to those in their HMW analogues than in specific bacterial biomarkers. The use of unsaturated alkanoic acid ratios here gives conflicting results, with the observed variation through time depending on the isomer measured. The discrepancies between the findings of this study and previous work are likely to be due to the varying controls on the lipids (original organic matter input, and compound degradation), which in turn will be affected by whether the main climatic limiting factor on the soil is temperature or precipitation. This suggests that lipid proxies preserved in stalagmites must be interpreted with care, particularly in the case of bacterial compounds which may be derived from within the cave or from the soil. However, many of these issues can be resolved by the use of multi‐proxy studies. Copyright © 2011 John Wiley & Sons, Ltd.     

Coniferization of the mixed‐wood boreal forests under warm climate

Mixed‐wood boreal forests are characterized by a heterogeneous landscape dominated by coniferous or deciduous species depending on stand moisture and fire activity. Our study highlights the long‐term drivers of these differences between landscapes across mixed‐wood boreal forests to improve simulated vegetation dynamics under predicted climate changes. We investigate the effects of main climate trends and wildfire activities on the vegetation dynamics of two areas characterized by different stand moisture regimes during the last 9000 years. We performed paleofire and pollen analyses in the mixed‐wood boreal forest of north‐western Ontario, derived from lacustrine sediment deposits, to reconstruct historical vegetation dynamics, which encompassed both the Holocene climatic optimum (ca. 8000–4000 a bp ) and the Neoglacial period (ca. 4000 a bp ). The past warm and dry period (Holocene climatic optimum) promoted higher fire activity that resulted in an increase in coniferous species abundance in the xeric area. The predicted warmer climate and an increase in drought events should lead to a coniferization of the xeric areas affected by high fire activity while the mesic areas may retain a higher broadleaf abundance, as these areas are not prone to an increase in fire activity. Copyright © 2019 John Wiley & Sons, Ltd.     

Sedimentary evolution and environmental history of Lake Van (Turkey) over the past 600 000 years

The lithostratigraphic framework of Lake Van, eastern Turkey, has been systematically analysed to document the sedimentary evolution and the environmental history of the lake during the past ca 600 000 years. The lithostratigraphy and chemostratigraphy of a 219 m long drill core from Lake Van serve to separate global climate oscillations from local factors caused by tectonic and volcanic activity. An age model was established based on the climatostratigraphic alignment of chemical and lithological signatures, validated by ⁴⁰Ar/³⁹Ar ages. The drilled sequence consists of ca 76% lacustrine carbonaceous clayey silt, ca 2% fluvial deposits, ca 17% volcaniclastic deposits and 5% gaps. Six lacustrine lithotypes were separated from the fluvial and event deposits, such as volcaniclastics (ca 300 layers) and graded beds (ca 375 layers), and their depositional environments are documented. These lithotypes are: (i) graded beds frequently intercalated with varved clayey silts reflecting rising lake levels during the terminations; (ii) varved clayey silts reflecting strong seasonality and an intralake oxic–anoxic boundary, for example, lake‐level highstands during interglacials/interstadials; (iii) CaCO₃‐rich banded sediments which are representative of a lowering of the oxic–anoxic boundary, for example, lake level decreases during glacial inceptions; (iv) CaCO₃‐poor banded and mottled clayey silts reflecting an oxic–anoxic boundary close to the sediment–water interface, for example, lake‐level lowstands during glacials/stadials; (v) diatomaceous muds were deposited during the early beginning of the lake as a fresh water system; and (vi) fluvial sands and gravels indicating the initial flooding of the lake basin. The recurrence of lithologies (i) to (iv) follows the past five glacial/interglacial cycles. A 20 m thick disturbed unit reflects an interval of major tectonic activity in Lake Van at ca 414 ka bp . Although local environmental processes such as tectonic and volcanic activity influenced sedimentation, the lithostratigraphic pattern and organic matter content clearly reflect past global climate changes, making Lake Van an outstanding terrestrial archive of unprecedented sensitivity for the reconstruction of the regional climate over the last 600 000 years.     

Exceptional silica speleothems in a volcanic cave: A unique example of silicification and sub‐aquatic opaline stromatolite formation (Terceira,Azores)

Silica stromatolites occur in a number of modern hydrothermal environments, but their formation in caves is very rare. The silica stromatolitic speleothems of the Branca Opala cave (Terceira Island, Azores), however, provide an excellent opportunity for their study. These formations may be analogous to ancient silica stromatolites seen around the world. Petrographic, mineralogical and geochemical analyses were undertaken on the silica speleothems of the above cave, and on the silica‐tufa deposits outside it, with the aim of understanding their genesis. The possible hydrothermal origin of their silica is discussed. X‐ray diffraction analyses showed opal‐A to be the sole silica phase. Negligible ordering of this opal‐A showed ageing to be insignificant, as expected for recent silica deposits. Most of the silica speleothems examined were definable as sub‐aquatic opaline stromatolites that are not currently growing. Optical microscopy clearly revealed a lower microlaminated, an intermediate and an upper microlaminated zone within the stromatolites. Stromatolite types (I, II and III) were classified with respect to their internal structure and distribution throughout the cave. Scanning electron microscopy showed silicified bacterial filaments within the stromatolites, the silicified plant remains and the silica‐tufa deposits. Bacteria therefore played a major role in the precipitation of the opal‐A. Plasma emission/mass spectrometry showed major, minor and rare earth elements to be present in only small quantities. The rare earth elements were mainly hosted within volcanic grains. Rapid silica precipitation from highly super‐saturated water would explain the intense silicification of the plant remains found inside and outside the cave. The opaline stromatolites, the silica‐tufa deposits and the above‐mentioned intense general silicification suggest a local hydrothermal source for the silica. Indeed, these deposits strongly resemble plant‐rich silica sinter associated with low‐temperature hot spring deposits that include bacterial filaments. However, no geochemical signals that might indicate a hydrothermal origin could be found.     

Stratigraphy and noncultural site formation at the Shurmai Rockshelter (GnJm1) in the Mukogodo Hills of North-Central Kenya

The Shurmai rockshelter (GnJm1) is located in the semiarid Mukogodo Hills region of north central Kenya. The rockshelter cavity is formed in Precambrian granite gneiss. The site contains a stratified sequence of sediments and archaeological materials that date from the end of the African Middle Stone Age (at or before ca. 45,000 yr B.P.) to modern times. The stratigraphic sequence at the Shurmai rockshelter is described and the principal noncultural formation processes responsible for its creation are reconstructed. Extant rockshelter sediments consist predominately of coarse angular rubble produced by rock fall, and additional sands and gravels produced by granular disintegration, slopewash, debris flow activity, and, to a lesser extent, human activity. The more identifiable aspects of the geologic history of the shelter include a number of episodes of rock fall deposition, in situ rock fall weathering, and erosional truncation. The reconstruction of the formation processes active at the Shurmai Rockshelter is used to illustrate how such geoarchaeological analysis clarifies understanding of prehistoric human occupation in rockshelter contexts in general, and in granitic terrane in particular. © 1999 John Wiley & Sons, Inc.     

Sedimentology of tsunami inflow and backflow deposits: key differences revealed in a modern example

Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes can range from clay to boulders of several metres in diameter. Grain‐size distributions reflect the mode of deposition and may be used to explore the hydrodynamic conditions of transport. The absence of unique sedimentary features identifying tsunami deposits makes it difficult in some cases to distinguish inflow from backflow deposits. On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow and backflow deposits of highly variable character. Tsunami inflow entrained sands, gravels and boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to 12 t were transported up to 300 m inland and 13 m above sea‐level. Thin veneers of coarse sand were found up to the maximum runup at 600 m inland and 19 m above sea‐level. Backflow re‐mobilized most of the sands and gravels deposited during inflow. The orientation of erosional structures indicates that significant volumes of sediment were entrained also during backflow. A major feature of the backflow deposits are widespread prograding fans of coarse sediment developed downcurrent of terrace steps. Fan sediments are mostly structureless but include cross‐bedding, imbrication and ripples, indicating deposition from bedload traction currents. The sediments are poorly sorted, grain sizes range between medium to coarse sand to gravel and pebbles. An assessment of the backflow transport conditions of this mixed material suggests that bedload transport at Rouse numbers >2·5 was achieved by supercritical flows, whereas deposition occurred when currents had decelerated sufficiently on the low‐gradient lower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists of backflow deposits to more than 90%. Due to the lack of sedimentary structures, many previous studies of modern tsunami sediments found that most of the detritus was deposited during inflow. This study demonstrates that an uncritical use of this assumption may lead to erroneous interpretations of palaeotsunami magnitudes and sedimentary processes if unknowingly applied to backflow deposits.