Palaeoenvironmental Indicators Research Articles

Thermal behavior of borax, Na2B4O5(OH)4·8H2O

Abstract Globally, borax is one of the most important borate minerals, both industrially and from a technological viewpoint. Despite its importance, there have been only a few reports on the structural changes of borax upon heating. In this study, we have investigated the thermal behavior and structural characteristics of borax using thermal analysis, in situ high-temperature synchrotron X-ray diffraction, in situ variable-temperature single-crystal X-ray diffraction, and quantum chemical calculations. Differential scanning calorimetry (DSC) curve showed a large endothermic peak at 349 K corresponding to the dehydration reaction of borax. X-ray diffraction (XRD) pattern remained almost unchanged up to 353 K. Above 363 K, the XRD peaks gradually became less intense until they disappeared at 403 K. The unit-cell volume continuously increased with increasing temperature and became constant just before its phase transition to tincalconite. The volumetric thermal expansion coefficient between 113 and 323 K was 7.84 × 10–5 K–1. The a and c lattice parameters exhibited a slight increase trend with increasing temperature, whereas the b lattice parameter increased significantly. Therefore, the thermal expansibilities followed the order b/b0 >> c/c0 > a/a0. The increasing b lattice parameter was closely related to the elongation of the O4···H7B hydrogen-bond interaction along the b-axis. Na-O bond lengths were isotropically expanded with increasing temperature, whereas the B-O bond lengths and angles remained unchanged even after the phase transition to tincalconite. Molecular orbital calculations revealed that an electron cloud shared by two borate tetrahedra led to the formation of a large electron cloud distribution over the B4O5(OH)4 cluster. The intramolecular interactions with substantial covalent character made the cluster quite rigid. The existence of borate minerals containing the B4O5(OH)4 clusters in their structure evidenced the presence of moderately acidic or alkaline water, wherein the borate minerals grew via the incorporation of B4O5(OH)4 clusters. Our results indicate that the connection geometry of the fundamental building block consisting of Bφ3 triangles and Bφ4 tetrahedra (φ: O2–, OH–) can potentially be used as a palaeoenvironmental indicator.

Karst-bauxite formation during the Great Oxidation Event indicated by dating of authigenic rutile and its thorium content

Aluminium (Al)-rich palaeosols—i.e., palaeobauxite deposits—should have formed in karst depressions in carbonate sequences as a result of acidic solutions from oxidative weathering of sulfide minerals during the Great Oxidation Event (GOE), but no GOE-related karst-palaeobauxite deposits have so far been recorded. Here, we report results of in situ uranium–lead (U–Pb) dating of detrital zircon and spatially associated rutile from a metamorphosed Al-rich rock within a dolomite sequence in the Quadrilátero Ferrífero (QF) of Minas Gerais, Brazil, known as the Gandarela Formation. Rutile grains are highly enriched in thorium (Th = 3–46 ppm; Th/U ratio = 0.3–3.7) and yielded an isochron, lower-intercept age of ca. 2.12 Ga, which coincides with the final phase of the GOE—i.e., the Lomagundi event. The rutile age represents either authigenic growth of TiO2 enriched in Th, U and Pb during bauxite formation, or subsequent rutile crystallisation during metamorphic overprint. Both cases require an authigenic origin for the rutile. Its high Th contents can be used as a palaeoenvironmental indicator for decreased soil pH during the GOE. Our results also have implications for iron (Fe)-ore genesis in the QF. This study demonstrates that in situ U–Th–Pb-isotope analyses of rutile can place tight constraints on the age and nature of palaeosols.

Middle Devonian Foraminifera from the Holy Cross Mountains

The Middle Devonian was an exceptional time for foraminiferal evolution because of the emergence of the first true calcareous multilocular taxa. Despite being key forms for an understanding of the origin of foraminiferal multilocularity, which is one of the most intriguing events in the evolutionary history of the group, they are largely unstudied. These unique foraminifera, including representatives of the genera Cremsia , Moravammina , Pseudopalmula , Semitextularia, and Vasicekia, are presented herein as well as foraminifer-like microfossils of uncertain biological position. The studied material comes from the Middle Givetian strata of the Miłoszów section and the Eifelian part of the Grzegorzowice-Skały section (Holy Cross Mountains, Poland). Described isolated specimens are in an exceptionally good state of preservation on a worldwide scale, which enabled the detailed analysis of test morphologies, complementing previous papers based on less well-preserved material. The suggested systematics of the foraminifera collected attempts to revise scarce taxonomic data that is still under debate, especially the classification of Moravammina , Cremsia, and Vasicekia . The Devonian foraminifers presented were prominent endobenthic and epibenthic inhabitants of common organic coral-stromatoporoid buildups. Palaeobiogeographical records show that during the Middle Devonian, the studied forms extended their exclusive European distribution into further Laurussian shelves and shallow seas, located in the northern and southeastern parts of the Rheic basin. This assumes that such assemblages may be used as palaeoenvironmental indicators. However, at present, their correlative potential is unknown.

Ostracods had colonized estuaries by the late Silurian.

The fossil record of terrestrialization documents notable shifts in the environmental and physiological tolerances of many animal and plant groups. However, for certain significant components of modern freshwater and terrestrial environments, the transition out of marine settings remains largely unconstrained. Ostracod crustaceans occupy an exceptional range of modern aquatic environments and are invaluable palaeoenvironmental indicators in the fossil record. However, pre-Carboniferous records of supposed non-marine and marginal marine ostracods are sparse, and the timing of their marine to non-marine transition has proven elusive. Here, we reassess the early environmental history of ostracods in light of new assemblages from the late Silurian of Vietnam. Two, low diversity but distinct ostracod assemblages are associated with estuarine deposits. This occurrence is consistent with previous incidental reports of ostracods occupying marginal and brackish settings through the late Silurian and Devonian. Therefore, ostracods were pioneering the occupation of marginal marine and estuarine settings 60 Myr before the Carboniferous and they were a component of the early phase of transition from marine to non-marine environments.

Neanderthal palaeoecology in the late Middle Palaeolithic of western Iberia: a stable isotope analysis of ungulate teeth from Lapa do Picareiro (Portugal)

ABSTRACTAdaptation to Late Pleistocene climate change is an oft‐cited potential contributor to Neanderthal disappearance in Eurasia. Accordingly, research on Neanderthal behaviour – including subsistence strategies, mobility, lithic technology, raw material procurement and demography – often focuses on linking changes observable in the archaeological record to specific phases of climate and environmental change. However, these correspondences are often tenuous because palaeoclimatic and archaeological records are rarely available on the same scale. In Iberia, a critical location for understanding the demise of Neanderthals, some research indicates that Neanderthal populations were unable to recover from environmental degradations known as Heinrich Events, while other studies suggest that enclaves of Neanderthal populations survived for several millennia longer in refugial zones. Here, we present a palaeoenvironmental reconstruction study using analysis of δ13C and δ18O of herbivore tooth enamel recovered from two Mousterian deposits at Lapa do Picareiro, a site located in Portuguese Estremadura. We then use these data, combined with other site‐based palaeoenvironmental indicators, to assess whether central Portugal acted as a refugium during periods of unfavourable climate, and to test whether Neanderthals in Portuguese Estremadura reorganised their mobility strategies after severe climate episodes.

The functional performance of productidine brachiopods in relation to environmental variables

Fossils are useful as palaeoenvironmental indicators when evaluated using large occurrence and lithology association data sets and/or functional morphology. However, when functional morphology is invoked on an ad hoc basis there exists a risk of circular reasoning. Performance spaces/landscapes constructed using biomechanical experiments can be used to tie performance to morphology quantitatively. They can be used to estimate fitness and function in specific environments by allowing multiple specific measures of function to be evaluated simultaneously. Absolute performance can be used to constrain palaeoenvironmental interpretations by detecting functional limits. We constructed a performance space with four variables (transport resistance, settling time, settling orientation and respiration potential) relating to hydrodynamic and metabolic variables. We combine new performance data with existing palaeoenvironmental interpretations and associated distributions of brachiopods of the suborder Productidina during the late Carboniferous and early Permian in the North American Midcontinent. Productidines were chosen because they lacked pedicles and relied on body and spine morphology to interact with their environment. Offshore dysoxia-associated taxa had increased resistance to sinking and proportionally larger lophophores, potentially improving survivorship in low-oxygen, soft substrate environments. Taxa associated with nearshore and oxygenated offshore environments display a range in performance consistent with a range of conditions, but all had proportionally smaller lophophores. Models that are less stable in high-velocity flows consistently settle in the correct convex down orientation if entrained, while more stable models did not. Our study suggests that as productidines radiated during the Carboniferous, they also differentiated within the performance space.

New Viverridae specimens from the Malapa Hominin Site and their utility as palaeoenvironmental indicators

The use of small-bodied animals, especially small Carnivora, as indicators of palaeoenvironment, is becoming more pronounced. These smaller taxa are more spatiotemporally constrained, owing to high territoriality, ecosystem sensitivity and physical limitations on their ability to travel large distances within their lifespan. This article describes seven new Viverridae specimens, together with a re-evaluation of a previously published specimen, from the hominin-bearing site of Malapa, located in the Cradle of Humankind World Heritage Site, South Africa. When combined with other palaeoenvironmental indicators, the addition of these eight specimens, consisting of Genetta sp., Genetta genetta, and Genetta cf. tigrina, allows for a more precise reconstruction of the environment surrounding Malapa as well as providing additional insights into possible depositional factors that could have resulted in the accumulation of the Malapa deposit.

Pycnodont fishes: morphologic variation, ecomorphologic plasticity, and a new interpretation of their evolutionary history

Some examples of morphologic variation of body and fins morphology in Pycnodontiformes are shown; not all are butterfly fish-like, as the common place assumes. Pycnodonts are characterized by a heterodontous dentition; teeth on the vomer and the prearticulars are molariform, yet of diverse shapes, whereas teeth on the premaxilla and the dentary do exhibit even more diverse morphologies. This morphologic variation is also analyzed, and the inaccuracy of another common place, the crushing or durophagous dentition of the pycnodonts, is explained: these terms refer to function, not to form. The ecomorphologic evaluation of both sources of morphologic variation, body and dentition, indicate that pycnodonts may have been adapted to a large variety of potential diets and environments. The environment of pycnodont fishes is often believed to have been reefal, but this is not necessarily the case, since their adaptations are not exclusively functional in reefs or even in marine environments only. Examples of freshwater pycnodonts are mentioned, showing that these fishes are potentially misleading palaeoenvironmental indicators: their mere presence in any given locality is not an un ambiguous indication of its palaeoenvironment. The ecomorphologic plasticity of pycnodonts was a key factor for their success as a group. This, together with a re-evaluation of their fossil record, leads to a new interpretation of the evolutionary history of the Pycnodontiformes.

Sequence analysis, cyclostratigraphy and palynofacies of early Anisian carbonate ramp deposits, NW Bulgaria

A sequence- and cyclostratigraphic interpretation of early Anisian (Aegean) shallow-marine carbonate ramp deposits, exposed in outcrop sections west of Tserovo village, NW Bulgaria, is presented. The hierarchical pattern identified can be interpreted in terms of Milankovitch cyclicity with elementary sequences representing the precession (20-kyr) cycle, small-scale sequences the short eccentricity (100-kyr), and medium-scale sequences the long eccentricity (400-kyr) cycle. Palynology provides a robust stratigraphic framework. The study of sedimentary organic matter, revealing variations of terrestrial input, sorting and fragmentation of phytoclasts, and prominent acritarch peaks, allows the interpretation of environmental changes and contributes to the cyclostratigraphic and sequence-stratigraphic framework. The detailed documentation of syndepositional soft-sediment deformation structures confirms their laterally traceable distribution within the depositional sequences and makes them good palaeoenvironmental indicators. Anisian ramp systems of the western Tethyan realm thus were subjected to highly dynamic regimes, recording the interplay between sea-level changes in tune with orbital cycles and ramp morphology.

Origin and palaeoenvironmental indications of Eocene to Oligocene primary lacustrine dolomite, Northern Tianshan Mountains, NW China

During the Eocene to Oligocene, lacustrine dolostone developed in the Anjihaihe Formation on the northern margin of the Tianshan Mountains in the southern Junggar Basin. Dolostone formation is usually restricted to specific environmental and geochemical conditions; therefore, dolostone contains considerable information on palaeoenvironment and palaeoclimate changes. Furthermore, this dolostone provides more information on the formation process and formation mechanisms of dolostone. The studied dolostone is white to greyish white and consists of dolomite crystals that are 3–10 µm in size, homogeneous, subhedral to euhedral and nearly stoichiometric (with average Mg/Ca molar ratios of 0.97). The high Sr content (average 1996 ppm), relatively high δ18O values (average −1.8‰) and co-occurrence with halite and gypsum reflect evaporative and saline conditions. The δ26Mg values of the dolostone range from −0.81 to −2.44‰, with a mean of −1.78‰. Strong positive correlations exist between the δ26Mg values and depth (r = 0.76) and the Mg isotope Rayleigh distillation model results, which indicates that δ26Mg fractionation occurred in conjunction with evaporation and that most of the Anjihaihe lacustrine dolomite developed in a natural environment at 25–50 °C. Moreover, evidence of microbes can be found covering the surface of the dolostone in the presence of a few framboids of pyrite. Additionally, clay layers in the dolostone are primarily composed of bentonite (40–60%) but also contain small amounts of montmorillonite (0.8–3.5%) and zeolite (0.1–0.5%). Detrital zircon ages of 40–44 Ma are presented for the first time for the Anjihaihe Formation. The results show that understanding the primary lacustrine dolostone is significant and essential to reconstructing the evolution of palaeoenvironments and synsedimentary volcanic activity in the northern Tianshan Mountains during the Eocene to Oligocene. This work also provides a possible model that explains the formation of the studied dolostone in an evaporative saline lacustrine environment.

The dynamics of mountain erosion: Cirque growth slows as landscapes age

AbstractGlacial cirques are widely used palaeoenvironmental indicators, and are key to understanding the role of glaciers in shaping mountain topography. However, notable uncertainty persists regarding the rate and timing of cirque erosion. In order to address this uncertainty, we analyse the dimensions of 2208 cirques in Britain and Ireland and model ice accumulation to investigate the degree of coupling between glacier occupation times and cirque growth. Results indicate that during the last ~120 ka, cirques were glacier‐free for an average of 52.0 ± 21.2 ka (43 ± 18%); occupied by small (largely cirque‐confined) glaciers for 16.2 ± 9.9 ka (14 ± 8%); and occupied by large glaciers, including ice sheets, for 51.8 ± 18.6 ka (43 ± 16%). Over the entire Quaternary (i.e. 2.6 Ma), we estimate that cirques were glacier‐free for 1.1 ± 0.5 Ma; occupied by small glaciers for 0.3 ± 0.2 Ma; and occupied by large glaciers for 1.1 ± 0.4 Ma. Comparing occupation times to cirque depths, and calculating required erosion rates, reveals that continuous cirque growth during glacier occupation is unlikely. Instead, we propose that cirques attained much of their size during the first occupation of a non‐glacially sculpted landscape (perhaps during the timeframe of a single glacial cycle). During subsequent glacier occupations, cirque growth may have slowed considerably, with the highest rates of subglacial erosion focused during periods of marginal (small glacier) glaciation. We propose comparatively slow rates of growth following initial cirque development because a ‘least resistance’ shape is formed, and as cirques deepen, sediment becomes trapped subglacially, partly protecting the bedrock from subsequent erosion. In support of the idea of rapid cirque growth, we present evidence from northern British Columbia, where cirques of comparable size to those in Britain and Ireland developed in less than 140 ka. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

Tomographic reconstructions of crab burrows from deltaic tidal flat: Contribution to palaeoecology of decapod trace fossils in coastal settings

Trace fossils play an important role in reconstructing palaeoecology and depositional environment. Their palaeoecological implications can be better understood when the trace makers are correctly identified. Neoichnology studies the burrow morphologies, behavioral ecology of modern organisms and their interaction with the environment. Neoichnology is of great significance in linking burrowing behaviors of extant organisms with interpretation of trace fossil palaeoecology. This paper reports a study on burrows produced by the ocypodid crab (Macrophthalmus japonicus De Haan) in the tidal region of the Yellow River Delta using CT scanning techniques. Our results show that M. japonicus produces abundant I-, U-, and Y-shaped burrows in a mudflat of the delta. These domicile structures are similar in morphological characteristics to the trace fossil Psilonichnus, suggesting M. japonicus as its possible additional trace maker. Our study supports the usage of Psilonichnus as a significant palaeoenvironmental indicator for coastal and shelf environments. Furthermore, this study highlights the great potential of applying CT techniques in future ichnological-related studies.

Bivalves and oncoids as palaeoenvironmental indicators at Late Jurassic and Early Cretaceous dinosaur sites from Spain

Bivalves from the Late Jurassic and Early Cretaceous stratigraphic section at Las Zabacheras (Galve Sub-basin Teruel, northern Spain), are reviewed from both systematic and palaeoautoecological perspectives. For this study the Villar del Arzobispo Formation, well known for important dinosaur occurrences, was sampled from the boundary with the underlying Higueruelas Formation (Late Jurassic), to the first levels of the overlying El Castellar Formation (Early Cretaceous). The bivalve taxa have Late Jurassic affinity, pointing to a possible Jurassic-Cretaceous boundary towards the top of the Villar del Arzobispo Formation. We have sampled oncoids, whose nuclei are bivalves, through the section to study environment change in this lithostratigraphical formation. Geochemical trace elements and δ13C and δ18O stable isotope analysis of the oncoids enable us to determine the conditions in which the microbialites were formed and provide further palaeoenvironmental data from the deposits containing the bivalves. Bivalve taxa change from the lower part of the Villar del Arzobispo Formation, where Ceratomya excentrica and Unicardium cf. subregulare are characteristic of marine conditions, becoming more continental towards the top of the formation with the presence of Unionoidean bivalves, and in the “Wealden” facies of the El Castellar Formation, where Teruella gautieri, has been found. Bivalves and oncoids allow us to recognize continental conditions where the first dinosaur of Spain, the sauropod Aragosaurus ischiatus was found, in an open water system, where there was limited evaporation, and with enough energy to produce well oxygenated water.

Large rotaliid foraminifera as biostratigraphic and palaeoenvironmental indicators in northwest Borneo: An example from a late Miocene section in Brunei Darussalam

Palaeonvironmental interpretations and biostratigraphic zonations in northwest Borneo are still a matter of debate but are important for petroleum related applications. The outcrop of Ambug Hill, a late Miocene marine siliciclastic sequence in Brunei Darussalam, has been investigated for its foraminifera assemblage. From a 9.5 m clay-rich interval 50 taxa have been recognized and classified, whenever possible to species level. The studied fauna is characterized by mostly hyaline foraminifera, dominated by two rotaliid species: Cavarotalia annectens and Heterolepa dutemplei. Additionally, a number of miliolids, lageniids, bolivinids and also planktonic foraminifera are present but in much lower abundance. The dominance of the rotaliids points to a sea floor rich in organic carbon characterized by muddy substrates with very low light penetration and the possible growth of patchy sea grass or macroalgae in the system, as indicated by epiphytic foraminifera in the samples. The results obtained give additional insights on how the environmental conditions might have been during the deposition of the deltaic sequences very widespread in the region, and how the fauna coped with such settings. Throughout the investigated samples, the dominance of the assemblages clearly shows alternating stress conditions, typical of pro- deltaic environments.The large abundance of rotaliid foraminifera, points toward possible biostratigraphic applications as several taxa within this group have short biozones in the region even though have strong facies dependency. If their role as biostratigraphic indicators could be finally proven, they might be used to date the large depositional sequences occurring in the region, since planktonic index fossils are scarce due to the very proximal settings of the sediments.

Coupled microbial bloom and oxygenation decline recorded by magnetofossils during the Palaeocene\u2013Eocene Thermal Maximum

Understanding marine environmental change and associated biological turnover across the Palaeocene–Eocene Thermal Maximum (PETM; ~56 Ma)—the most pronounced Cenozoic short-term global warming event—is important because of the potential role of the ocean in atmospheric CO2 drawdown, yet proxies for tracing marine productivity and oxygenation across the PETM are limited and results remain controversial. Here we show that a high-resolution record of South Atlantic Ocean bottom water oxygenation can be extracted from exceptionally preserved magnetofossils—the bioinorganic magnetite nanocrystals produced by magnetotactic bacteria (MTB) using a new multiscale environmental magnetic approach. Our results suggest that a transient MTB bloom occurred due to increased nutrient supply. Bottom water oxygenation decreased gradually from the onset to the peak PETM. These observations provide a record of microbial response to the PETM and establish the value of magnetofossils as palaeoenvironmental indicators.

Fossils of the southern Baltic Sea as palaeoenvironmental indicators in multi-proxy studies

Many remains of organisms in Holocene sediments from the Baltic Sea enter the fossil record. These (sub)fossils bear a wealth of information about past environments. The present paper describes all relevant taxonomic groups larger than 63 μm and gives data on hard part identification, preservation, ecology and applications. Many important taxa are presented by light-microscopic photographs and ecological data. If possible, references are given for taxonomic keys and ecology referring to the area of the southern Baltic Sea. Typical associations and indicator taxa are listed as references for palaeoenvironmental reconstructions. This review is intended to serve students and scientists of geosciences, biology and archaeology as an introduction to the identification and use of microfossils from the Baltic Sea or other Holocene brackish-water sediments.

Environmental and taxonomic controls of carbon and oxygen stable isotope composition in <i>Sphagnum</i> across broad climatic and geographic ranges

Abstract. Rain-fed peatlands are dominated by peat mosses (Sphagnum sp.), which for their growth depend on nutrients, water and CO2 uptake from the atmosphere. As the isotopic composition of carbon (12,13C) and oxygen (16,18O) of these Sphagnum mosses are affected by environmental conditions, Sphagnum tissue accumulated in peat constitutes a potential long-term archive that can be used for climate reconstruction. However, there is inadequate understanding of how isotope values are influenced by environmental conditions, which restricts their current use as environmental and palaeoenvironmental indicators. Here we tested (i) to what extent C and O isotopic variation in living tissue of Sphagnum is species-specific and associated with local hydrological gradients, climatic gradients (evapotranspiration, temperature, precipitation) and elevation; (ii) whether the C isotopic signature can be a proxy for net primary productivity (NPP) of Sphagnum; and (iii) to what extent Sphagnum tissue δ18O tracks the δ18O isotope signature of precipitation. In total, we analysed 337 samples from 93 sites across North America and Eurasia using two important peat-forming Sphagnum species (S. magellanicum, S. fuscum) common to the Holarctic realm. There were differences in δ13C values between species. For S. magellanicum δ13C decreased with increasing height above the water table (HWT, R2=17 %) and was positively correlated to productivity (R2=7 %). Together these two variables explained 46 % of the between-site variation in δ13C values. For S. fuscum, productivity was the only significant predictor of δ13C but had low explanatory power (total R2=6 %). For δ18O values, approximately 90 % of the variation was found between sites. Globally modelled annual δ18O values in precipitation explained 69 % of the between-site variation in tissue δ18O. S. magellanicum showed lower δ18O enrichment than S. fuscum (−0.83 ‰ lower). Elevation and climatic variables were weak predictors of tissue δ18O values after controlling for δ18O values of the precipitation. To summarize, our study provides evidence for (a) good predictability of tissue δ18O values from modelled annual δ18O values in precipitation, and (b) the possibility of relating tissue δ13C values to HWT and NPP, but this appears to be species-dependent. These results suggest that isotope composition can be used on a large scale for climatic reconstructions but that such models should be species-specific.

Brackish to hypersaline facies in lacustrine carbonates: Purbeck Limestone Group, Upper Jurassic\u2013Lower Cretaceous, Wessex Basin, Dorset, UK

Sedimentary facies and stratigraphic architecture of non-marine carbonates are controlled by a range of environmental parameters, such as climate, hydrology and tectonic setting, but the few published facies models do not account for this variability. Outcrop and petrographic observations from the Mupe Member of the Purbeck Limestone Group (Upper Jurassic–Lower Cretaceous) in Dorset, southern England, are the basis for new depositional models of non-marine microbialites and associated carbonates in an extensional basin. Ten facies are defined, described and grouped into five facies associations. The Mupe Member is characterised by accumulation of in situ microbial mounds developed around tree remains preserved as moulds and silicified wood. Mounds occur within three stratigraphic units, separated by three palaeosoils, characterised by less-porous, bedded, inter-mound packstone–grainstone that commonly onlap mound margins. Mounds are developed mainly in the shallowest areas of the lake, as indicated by their shapes, facies relationships and association with palaeosoils. These microbial mounds are compared to modern (Laguna Bacalar, Mexico and Great Salt Lake, Utah, USA) and ancient (Eocene Green River Formation, Uinta Basin, Utah, USA) analogues to assess their value as palaeoenvironmental indicators. Facies transitions indicate an earlier, brackish-water lake and a later hypersaline lake for the Mupe Member, both within a semi-arid climate setting in an extensional basin. The fact that the microbialites are covered by evaporitic strata, together with sedimentological, palaeontological and stable isotope data, suggest that there was a sharp change from through-flowing brackish-water, to a closed hypersaline, lacustrine system.

Functional attributes of epilithic diatoms for palaeoenvironmental interpretations in South-West Greenland lakes

Benthic diatoms are commonly used for palaeoenvironmental reconstruction in Arctic regions, but interpretation of their ecology remains challenging. We studied epilithic diatom assemblages from the shallow margins of 19 lakes from three areas (coast-inland-ice sheet margin) along a climate gradient in Kangerlussuaq, West Greenland during two periods; shortly after ice-off (spring) and in the middle of the growth season (summer). We aimed to understand the distribution of Arctic epilithic diatoms in relation to water chemistry gradients during the two seasons, to investigate their incorporation into lake sediments and to assess their applicability as palaeoenvironmental indicators. Diatoms were correlated with nutrients in the spring and alkalinity/major ions in the summer, when nutrients were depleted; approximately half of the variance explained was independent of spatial factors. When categorised by functional attributes, diatom seasonal succession differed among regions with the most obvious changes in inland lakes where summer temperatures are warmer, organic nutrient processing is prevalent and silicate is limiting. These conditions led to small, motile and adnate diatoms being abundant in inland lakes during the summer (Nitzschia spp., Encyonopsis microcephala), as these functional attributes are suited to living within complex mats of non-siliceous microbial biofilms. Seasonal succession in silica-rich lakes at the coast was less pronounced and assemblages included Tabellaria flocculosa (indicating more acidic conditions) and Hannaea arcus (indicating input from inflowing rivers). The nitrogen-fixing diatom Epithemia sorex increased from the coast to the ice sheet, negatively correlating with a gradient of reactive nitrogen. The presence of this diatom in Holocene sediment records alongside cyanobacterial carotenoids during arid periods of low nitrogen delivery, suggests that it is a useful indicator of nitrogen limitation. Nitzschia species appear to be associated with high concentrations of organic carbon and heterotrophy, but their poor representation in West Greenland lake sediments due to taphonomic processes limits their palaeoenvironmental application in this region. Proportions of epilithic taxa in lake sediment records of coastal lakes increased during some wetter periods of the Holocene, suggesting that snowpack-derived nutrient delivery may offer diatom taxa living at lake margins a competitive advantage over planktonic diatoms during the “moating” ice melt period. Thus, further research investigating linkages between epilithic diatoms, snowpack and nutrient delivery in seasonally frozen lakes is recommended as these taxa live on the ‘front-line’ during the spring and may be especially sensitive to changes in snowmelt conditions.

Effect of Climate on Morphology and Development of Sorted Circles and Polygons

AbstractSorted circles and polygons are widespread features of periglacial landscapes, but the controls on their development remain poorly understood, impeding their use as palaeoenvironmental indicators. We investigate the relationship of sorted circles and polygons to altitude in the northern Billefjorden area, central Svalbard. The patterns occur in two distinct elevation zones, below 200–250 m asl and above 600 m asl. The higher‐elevated patterns have smaller diameters and shallower sorting depths due to a thinner active layer at higher elevations, suggesting that sorted patterns can indicate climate conditions and ground thermal state when the patterns initiated. Geology is believed to be of less importance for pattern morphology in the study area, causing only its fine‐scale variations. The pattern diameter‐to‐sorting depth ratios have a median value of 3.57, consistent with previous studies and theoretical models of patterned‐ground formation involving circulation mechanisms. Large‐scale sorted patterns may develop over centennial timescales in this high‐Arctic environment. They are probably not in equilibrium with present‐day climate conditions and have probably formed throughout the Holocene. Copyright © 2017 John Wiley & Sons, Ltd.