Arid Phase Research Articles

A long arid interlude in the Indian summer monsoon during ∼4,350 to 3,450 cal. yr BP contemporaneous to displacement of the Indus valley civilization

Climatic variability played a pivotal role in the development of human societies and stability of civilizations in the past. To constrain the causes that drove displacement and deurbanization of the Indus Valley civilization, we present a 4500 yr old proxy record from the Tso Moriri Lake, northwestern (NW) Himalaya. The record indicates an interval of warm and wet climatic conditions from 4500 to 4350 calibrated years before Present (cal. yr BP) with a trend toward deteriorating climate, culminating in a pronounced, long cold and arid phase from∼4350 to 3450 cal. yr BP in the region. The Indian summer monsoon (ISM) weakened during this long arid phase depleting the water resources in the area of the Indus Valley settlements that likely have triggered the gradual abandonment of the Harappan cities. This long interval of ISM weakening and thus aridity in South Asia was driven by enhanced El Niño intensity and frequency.

Formation of desert pavements and the interpretation of lithic-strewn landscapes of the central Sahara

This study focuses on two different but interlinked lines of evidence that critically examine land surface processes contributing to the formation of desert pavements in the central Sahara. (1) Soil pedostratigraphies from the Messak plateau (SW Libya) illustrate phases of land surface stability and instability that reflect humid and arid phases of Quaternary climate, respectively. (2) The density and morphologies of surface lithic scatters in the same region were re-examined, based on data previously published by Foley and Lahr (2015, PLoS ONE). This re-examination shows that many surface clasts previously interpreted as lithics are better interpreted as formed by in situ weathering and wind abrasion. Furthermore, weathering, abrasion and deflation are the major processes by which desert clasts are formed and concentrated on the land surface, not human agency. Erosional Quaternary periods allowed for the formation of condensed pedostratigraphies by which surface clasts and lithics were mixed and concentrated on the land surface. These two independent lines of evidence show that desert land surfaces do not reflect a single time period of formation, and that Quaternary desert pavements (including any lithics located thereon) evolved in response to subaerial weathering and erosion processes.

Fishes in the desert: mitochondrial variation and phylogeography of Danakilia (Actinopterygii: Cichlidae) and Aphanius (Actinopterygii: Cyprinodontidae) in the Danakil Depression of northeastern Africa

The Danakil Depression in northeastern Africa represents one of the harshest arid environments on Earth, yet two genera of fishes, Danakilia (Cichlidae) and Aphanius (Cyprinodontidae), share its sparse aquatic habitats. The evolutionary history of these fishes is investigated here in the context of genetic, geological and paleoenvironmental information. We collected samples from seven sites and assessed phylogeographic relationships using concatenated COI and cytb mtDNA genes. Danakilia morphospecies show low differentiation at mitochondrial markers, but variation is partitioned between a northern cluster containing D. dinicolai plus three undescribed riverine populations, and a southern cluster including two creek populations of D. franchettii separated by the hypersaline waters of Lake Afrera. Aphanius displayed four genetically distinct clades (A. stiassnyae in Lake Afrera; one distributed across the entire area; one in Lake Abaeded; and one in the Shukoray River), but without clear large-scale geographic structure. However, Danakil Aphanius are clearly differentiated from A. dispar sensu stricto from the Sinai Peninsula. Geological evidence suggests that after the Late Pleistocene closure of the Danakil-Red Sea connection, increased post-glacial groundwater availability caused the formation of a brackish paleo-lake flooding the entire region below the –50 m contour. Fish populations previously isolated in coastal oases during glaciation were able to mix in the paleo-lake. Subsequently, in a more arid phase starting ∼7300 BP, paleo-lake regression isolated fishes in separate drainages, triggering their still ongoing diversification.

Fabric of soil derived from parna and the riddle of transported pellets

It is generally accepted that large areas of southern New South Wales (NSW) and northern Victoria are mantled by soils derived from clayey loess deposits called parna. These parna deposits are believed to have formed during arid phases of the Pleistocene and the mineral constituents are assumed to have been transported as silt- and fine sand-sized pellets of calcareous clay, with some companion quartz grains of a similar size. A common property of parna-derived soils is subplasticity, where the apparent field texture grade becomes more clayey with increasing mechanical working of the bolus. This propensity for subplastic behaviour suggests that parna-derived soils contain stable silt- and fine sand-sized pellets of clay, yet there has been little direct micromorphological evidence of these pellets ever published. In the present study, thin section samples from several parna type-sites in southern NSW were examined micromorphologically to reveal the presence of very well size-sorted quartz grain populations (companion grains) and identifiable prolate clay aggregations of a similar silt to fine sand size. These prolate pellets, comprised of quartz, illite, kaolinite and chlorite, have a very distinctive mosaic-speckled b-fabric and are more widely distributed throughout the soil than the similarly sized and shaped faunal faecal pellets. Where these pelletal aggregations are not evident, such as in the deeper parna deposits, abundant illuviation features suggest that clay particles deposited within the parna, whether as pellets or coatings on grains, have subsequently undergone considerable weathering and a range of pedogenic processes.

An aeolian sediment reconstruction of regional wind intensity and links to larger scale climate variability since the last deglaciation from the east coast of southern Africa

Few long-term environmental records are available for southern Africa where shifts in atmospheric circulation and changes in sea surface temperatures interact to influence regional climate dynamics. We present downcore grain size and inorganic geochemistry data covering the last ~23,000years from a peatland on the east coast of South Africa and examine links between shifts in regional wind activity and palaeoclimatic variability. Our record documents substantial variations in aeolian flux associated with changes in regional climate and wind patterns that reflect larger scale atmospheric circulation patterns. Substantially higher fluxes observed during the Last Glacial Maximum (LGM) are linked to widespread aridification and an expansion in local source areas brought about by a clear shift to dry and cool conditions. Variations in grain size distribution reveal that the aeolian record from Mfabeni comprises two dominant end-members; locally-derived coarse-grained material and a more fine-grained dust component. Marked changes in composition and modal grain size suggest that hydrological shifts in the region during the LGM were accompanied by an increase in storm frequency and wind strength that we link to a northward displacement in the westerly wind belt and a strengthening in wind intensity. Coupling between a rapid increase in sea surface temperature (SST) and an approximate three-fold decrease in aeolian activity after 15kcalyrBP suggests that changes in SST and its effect on the position and intensity of the westerlies in the Southern Ocean was the dominant climatic driver in the region during deglaciation. Substantially lower aeolian activities through the early Holocene indicate a warming in regional climate and the establishment of more humid conditions under the influence of enhanced tropical easterly flow. Our record also documents more subtle changes in climate over the mid to late Holocene and provides support for an arid phase in southern African climate 6–4kcalyrBP, as well as an increase in climate variability associated with a strengthening in El Niño–Southern Oscillation (ENSO) activity ~2kcalyrBP. The study contributes to current knowledge of atmospheric circulation patterns in the Southern Hemisphere and provides new insight into links between aeolian activity, regional wind patterns and climatic variability over glacial-interglacial timescales for a region where existing palaeoclimate records are scarce.

Millennial-scale fluctuations in Saharan dust supply across the decline of the African Humid Period

The Sahara is the world's largest dust source with significant impacts on trans-Atlantic terrestrial and large-scale marine ecosystems. Contested views about a gradual or abrupt onset of Saharan aridity at the end of the African Humid Period dominate the current scientific debate about the Holocene Saharan desiccation. In this study, we present a 19.63 m sediment core sequence from Lake Sidi Ali (Middle Atlas, Morocco) at the North African desert margin. We reconstruct the interaction between Saharan dust supply and Western Mediterranean hydro-climatic variability during the last 12,000 yr based on analyses of lithogenic grain-sizes, XRF geochemistry and stable isotopes of ostracod shells. A robust chronological model based on AMS 14C dated pollen concentrates supports our multi-proxy study. At orbital-scale there is an overall increase in southern dust supply from the Early Holocene to the Late Holocene, but our Northern Saharan dust record indicates that a gradual Saharan desiccation was interrupted by multiple abrupt dust increases before the ‘southern dust mode‘ was finally established at 4.7 cal ka BP. The Sidi Ali record features millennial peaks in Saharan dust increase at about 11.1, 10.2, 9.4, 8.2, 7.3, 6.6, 6.0, and 5.0 cal ka BP. Early Holocene Saharan dust peaks coincide with Western Mediterranean winter rain minima and North Atlantic cooling events. In contrast, Late Holocene dust peaks correspond mostly with prevailing positive phases of the North Atlantic Oscillation. By comparing with other North African records, we suggest that increases in Northern Saharan dust supply do not solely indicate sub-regional to regional aridity in Mediterranean Northwest Africa but might reflect aridity at a trans-Saharan scale. In particular, our findings support major bimillennial phases of trans-Saharan aridity at 10.2, 8.2, 6.0 and 4.2 cal ka BP. These phases coincide with North Atlantic cooling and a weak African monsoon.

Carbon and nitrogen isotopic ecology of Holocene camelids in the Southern Puna (Antofagasta de la Sierra, Catamarca, Argentina): Archaeological and environmental implications

This article focuses on camelid isotope ecology from archaeological sites dating to the Holocene (c. 9800–420 BP) located in the Southern Argentine Puna. We present 92 carbon and nitrogen stable isotope values (δ13C and δ15N) extracted from wild (Vicugna vicugna and Lama guanicoe) and domesticated (L. glama) camelid bone collagen. The samples come from the following archaeological sites: Quebrada Seca 3, Cueva Salamanca 1, Peñas de la Cruz 1.1, Casa Chávez Montículos 1 and 4, Bajo del Coypar II, Cueva Cacao 1A, Real Grande 1, and Real Grande 6, all of them located over 3300masl in Antofagasta de la Sierra, Catamarca, Argentina. Mean carbon isotopic relationship (δ13C) was −17.2‰, ranging from −20.1‰ to −11.6‰, and mean nitrogen isotopic relationship (δ15N) was 6.8‰, ranging from 3.4‰ to 10.8‰. We interpret these isotope values against the background of environmental and cultural changes that occurred throughout this period. Variations in δ13C and δ15N values were generally not structured per the availability of different pasture areas across varying altitude. Instead, there was an appreciable increase of some δ13C values in domesticated camelids, suggesting the use of complementary forage in their diet. That aside, the isotope values do not differ significantly among the different camelid taxa. More generally, a noticeable increase in δ15N values, and to a lesser extent in δ13C values, was noted at around 8000 BP, coinciding with a palaeoclimatic phase of extreme regional aridity. Yet, no significant increase of δ13C and δ15N values was recorded during a later arid phase (1500 BP), and no significant variations were observed in isotope values during the arid and wet fluctuations (sub-phases) within each of the principal Holocene palaeoenvironmental phases recorded for the region. These results highlight the varying magnitude of past climatic changes and their differential impact, and emphasise the need to investigate further into the environmental aspects that produce variation in isotope data.

Paleoenvironmental conditions across the Jurassic–Cretaceous boundary in central-eastern Mexico

The Padni section of central-eastern Mexico is characterized by pelagic, organic-rich carbonates and shales dated in this study by calpionellid biostratigraphy to the late Tithonian-late Berriasian time interval. Microfacies, pyrite framboid size, spectrometric gamma-ray and mineralogical data are herein integrated in order to reconstruct the paleoenvironmental change during the Jurassic-Cretaceous boundary. Deposits of the late Tithonian–early Berriasian are characterized by laminated, organic-rich facies with abundant radiolarian, tiny pyrite framboids and low Th/U ratios. They are linked to upwelling in a semi-restricted basin, high marine productivity and anoxic bottom waters. The early incursions of Tethyan oceanic waters into the proto-Gulf of Mexico occurred during late Tithonian as attested the appearance of calpionellids. Short and intermittent accumulations of saccocomids during early Berriasian suggest episodes of sporadic connection between the Tethys, the proto-Atlantic and the Pacific ocean during sea-level rise events. A full and stable connection between the Tethys and proto-Gulf of Mexico was established until the late Berriasian. This event is supported by the presence of open marine and bioturbated facies with a framboid population typical of dysoxic conditions, higher Th/U ratios and a decreasing pattern of the total organic carbon content. In addition to highlighting the replenishment of the oxygen supply to the basin, this facies also points to a younger age for the finalization of the Yucatán Block rotation and the end of the Gulf of Mexico opening. Deposition of the studied section occurred mostly during a Tithonian–Berriasian arid phase reported in other Tethyan and Atlantic regions. The similarity between the discrete segments of the standard gamma-ray curve defined in the studied outcrop and those reported from subsurface implies their regional continuity allowing their use for correlation purposes.

Palaeoclimatic reconstruction by using the varvite sediments of Bharatpur, Upper Lahaul Valley, NW Himalaya, India

A 6 m thick varvite profile exposed at Bharatpur (77° 27′ E: 32° 48′ N, altitude 4692 m) in the NW Himalaya was investigated to understand the past climatic signatures during the Late Quaternary period. The radiocarbon chronology brackets the varvite sedimentation between ∼24.3 and 7.2 ka BP and the sedimentary sequence is bounded by river born gravel at the top and glacial outwash at the bottom. The proxies used were lithology, chronology (AMS/radiocarbon dating), geochemical analysis, loss on ignition (LOI), and mineral magnetism. Other parameters, e.g., Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Al2O3/SiO2, Fe2O3, CaO, MgO and Al/Na ratio were also used. Based on the combined data set, six phases of warm and cold climatic conditions are reconstructed. The mean CIA, CIW and Al/Na ratios reveal that the sediments were incorporated from sedimentary rocks by moderate silicate chemical weathering process. Based on the available chronology, the basal most glacial outwash event can be estimated as ca. 24.3 ka BP and the laminated section shows continuity of sedimentation from the Late Quaternary to Holocene. The period between ∼ ca. 24.3 and 22.6 ka can be regarded as an arid phase. From ∼ ca. 22.6–20.5 ka BP, a slight increase in monsoonal precipitation is observed, followed by decreased precipitation from ∼ca. 20.5 to 18 ka BP. The period from ca. ∼18 to 15.7 reveals warm and humid climatic conditions. We suggest that a cold event from ca. ∼15.7 to 11.5 may correspond to the Younger Dryas (YD) which in turn is followed by a phase of maximum moisture of the Holocene Climate Optimum (HCO) (ca. ∼11.5–7.2 ka BP) in the study area. The increased CIA during deglaciation and HCO suggest high chemical weathering, whereas, the LGM (Last Glacial Maximum) and YD are represented by the low CIA. The observed climatic phases/events are in agreement with the records from other parts of the Himalaya.

Early- to mid-Holocene paleohydrology in northeast Mediterranean: The detrital record of Aliakmon River in Loudias Lake, Greece

One of the prominent features of northeast Mediterranean (NEM) Holocene climate are recurrent phases of cold and aridity; their impacts on the hydrological cycle remain at large unknown, as few existing paleohydrological records are either restricted to lake-level fluctuations or focus near the ‘8.2 kyr BP’ event. Here, we present the detrital record of Aliakmon River in Lake Loudias between 9500 and 3000 cal. BP. Magnetic susceptibility (MS) exhibits high correlation with mean grain size ( r = 0.7) of silt-sized fractions and is used as a proxy of the distal clastic input of Aliakmon River, whereas organic matter (OM), carbonate content (CaCO3), water content (WC), and clay concentrations decipher sedimentological and biological processes in Lake Loudias. Periods of high hydrological activity were interrupted by short intervals of low river discharge at c. 9400, 8600, 8200, 7500, 7000, 6200, 5300, and 4500 cal. BP and during a multi-century event centered at 3500 cal. BP, in agreement with marine and terrestrial paleoclimatic reconstructions from NEM. With exception of the wet period between c. 8.6 and 7.9 cal. BP, periods of increased hydrological activity are synchronous to contraction of Pinus forests and increased sea surface temperatures and silt transport in the Aegean Sea. The long-term (~580 years) variability of MS is in-phase with southeast Europe pollen-inferred annual temperature variations and with North Atlantic Ice Rafted Debris (IRD) events.

Geomorphological Holocene events as deduced from lithological records of the fluvial basins of Dauria

A conjugate analysis of the processes of morphogenesis and sedimentation within small fluvial basins of Dauria revealed a rhythmic character of change of the Holocene processes which are combined into cycles of a different hierarchical level. It is shown that each cycle begins with an extreme phase of intense fluvial drift and delivery of matter from the system. The fluvial horizons comprise the lower part of deposits of the Holocene macrocycle in the Vesnyanka pad’ (a valley without a permanent watercourse). A disastrous fluvial drift was recorded repeatedly for the period 7–8.5 cal. ka and at the transition time from the Atlantic to Subboreal time. A normal zonal phase of integration of matter within the basins is exhibited by buried humus horizons of soils which give clear evidence of 1800-year cycles. A regional development in Dauria is recorded for the soils with the age of about 8, 6.4 and 4.6 as well as 1.2–1.4 cal. ka. The upper part of the deposits of each cycle is characterized by aeolian horizons of extreme arid phases. The profiles of proluvial trails are indicative of 130-year cycles. Based on results from studying buried soils, data of pollen analysis and radiocarbon dating of the deposits, we determined an enhancement in climate aridization for the last 10 ka and, in general, a decreased of the sedimentation and morphogenesis rate.

Palaeoenvironmental dynamics and Palaeolithic occupation at Katoati, Thar Desert, India

AbstractLate Pleistocene palaeoenvironments in the Thar Desert (India), located at the eastern extent of the Saharo-Arabian desert belt, have fluctuated considerably as a result of the varying range and intensity of the Indian summer monsoon. Phases of widespread Pleistocene aridity are well documented in the Thar Desert, but research focusing on humid proxies is critical to examine how the region may have facilitated population expansions across southern Asia. At Katoati, located on the northeast margin of the Thar Desert, the combination of field recording of sediment sections with detailed analyses (micromorphology, stable isotope, loss on ignition, magnetic susceptibility, and X-ray fluorescence) from an archaeological site identify a series of hominin occupations during phases of enhanced humidity between ~96 and 60 ka. A gradient of humidity on the eastern margin of the Thar Desert during the late Pleistocene is identified, with the periodic humidity evident at Katoati occurring more frequently and with longer duration towards the southern margin. This uneven distribution of humidity in the Thar Desert is likely to have strongly influenced when and where hominin populations could expand into and across the region.

Palaeohydrological changes during the mid and late Holocene in the Carpathian area, central-eastern Europe

Multi-proxy, high-resolution analyses (lithological, geochemical, environmental magnetism) anchored by 22 14C dates, of a 5.53m long sediment core from Lake Ighiel (Romanian Carpathians, central-eastern Europe) allowed the reconstruction of key local, catchment-lacustrine dynamics and an appraisal of palaeohydrological and palaeoclimatic gradients acting regionally over the last 6000years. The first sedimentological phase of the record from 6030 to 4200cal yrBP is characterised by low productivity and high detrital input indicating surface runoff processes due to enhanced rainfall. This interpretation is in agreement with other hydrological reconstructions reporting increased precipitation also in CE Europe, NE Mediterranean and also inferred summer and winter latitudinal temperature gradients (LTG) (as defined by Davis and Brewer, 2009), showing a strong connectivity between basin-lacustrine dynamics and the establishment of the dominant, Atlantic atmospheric circulation pattern in the area. The lacustrine system was more stable between 4200 and 2500cal yrBP when clastic inputs diminished and biological productivity increased. During this interval, the coherence of Lake Ighiel's multi-century detrital events, identified in a range of proxy-data (albeit different in frequency and magnitude), with flood activity in central Europe (the Alps) suggests a common moisture forcing (Atlantic and periodically Mediterranean influences). In contrast, different reconstructions from the NE Mediterranean indicate a distinct NW-SE hydro-climatic gradient. A more complex and variable trend is depicted in Lake Ighiel sedimentation during the last ~2500years showing a variable detrital trend likely reflecting an intriguing hydrological pattern which is in agreement with intervals of increased aridity phases during a generally moister period. Comparing our sedimentological results with published pollen records from the nearby area clearly indicates anthropogenic imprints during the Dacian-Roman Period and especially from the Middle Ages towards the present. The ~6000-year long sedimentary record of Lake Ighiel contributes to our understanding of mid and late Holocene palaeohydrological changes in the Carpathians and highlights the importance of latitudinal gradients in driving hydrological variability in continental Europe.

Microbialitos da Sequência Balbuena IV (Daniano), Bacia de Salta, Argentina: caracterização de intrabioarquiteturas e de microciclos

Microbialitos gerados em ambiente deposicional lacustre apresentam grande diversidade de estruturas e texturas internas. A organização e a caracterização do arranjo formado por esses atributos correspondem ao que se denomina intrabioarquitetura. Este trabalho tem o objetivo de identificar os principais tipos de intrabioarquiteturas nos microbialitos da Sequência Balbuena IV (Daniano, Bacia de Salta, Argentina) em escala mesoscópica (100 – 10-2 m) e a interpretação cíclica da recorrência dos componentes que formam estas intrabioarquiteturas, em escala microscópica. Foram reconhecidos sete padrões intrabioarquiteturais, assim denominados: Microbialito Aglutinante de grão grosso (MA-gg); Microbialito Bandeado Fasciculado/Aglutinante de grão fino (MBFA-gf); Microbialito Pseudo-microcolunar Aglutinante de micrita (MPMA-m); Microbialito Arbustiforme Microgrumoso Aglutinante de grão fino (MArbMA-gf); Microbialito Dendriforme Microgrumoso Aglutinante de grão fino (MDMA-gf); Microbialito Bandeado Fasciculado/Aglutinante de grão fino com esferulito (MBFA-gf,esf) e Estromatólito Microgrumoso/Aglutinante de grão grosso (EMA-gg). Essas intrabioarquiteturas foram agrupadas e relacionadas a três tipos de microciclos deposicionais. Os microciclos possibilitam organizar o registro microbial em escala meso- e microscópica. Essa organização cíclica está relacionada às variações climáticas, que afetam o nível de base, o aporte sedimentar e os processos biológicos e geoquímicos do sistema lacustre. Portanto, foi possível associar a formação dos microbialitos a períodos de maior ou de menor aridez no ambiente deposicional.

The Medieval Climate Anomaly as a factor in the history of Sijilmasa, southeastern Morocco

ABSTRACTThis paper explores how the Medieval Climate Anomaly (MCA) may have impacted the city of Sijilmasa, an important trading centre situated in the Tafilalt oasis of southeastern Morocco during the eleventh century CE, as well as some adaptation strategies that may have been employed by the people of the region in response to these changing environmental conditions. Palaeoclimate proxy evidence indicates that the MCA ushered in a period of warmer and drier conditions throughout Morocco, and there are some indications in the archaeological and historical records that this may have caused some substantial disruptions in the Tafilalt oasis during the first half of the eleventh century CE. However, despite the continuation of this arid phase, the region experienced a period of rapid growth and economic development after the Almoravids conquered Sijilmasa in 1055 CE. This florescence during a period of frequent drought events may have been enabled in part by Almoravid efforts to alter the natural course of the region’s principal river, the Oued Ziz, so that it would flow directly through the middle of the Tafilalt oasis and thus make the region more drought-resistant.

The Chester Formation (Early Triassic, southern Britain): sedimentary response to extreme greenhouse climate?

The Early Triassic of onshore southern Britain is dominated by the Chester Formation; a red-bed succession of braidplain origin, sourced mainly from the Armorican Massif over what is now north-west France. At outcrop in south-west England and the English Midlands the Chester Formation comprises an abrupt conglomeratic outbreak that fines further northwards into arenaceous strata. Previous interpretations have invoked tectonic or climatic causes for the Early Triassic development of the Budleighensis river system and emplacement of the Chester Formation. Climatic models variously invoke a shift to a wetter climate, high erosion rates associated with the end-Permian mass extinction (EPME), and an Early Triassic (Olenekian) arid phase. We support the concept of a fundamental climatic control, and suggest that the Chester Formation reflects an extreme Early Triassic greenhouse climate characterised by high temperatures, aridification, de-vegetation, heavy floods and high erosion rates.

Devonian lacustrine shore zone architecture: Giving perspective to cliff exposures with ground penetrating radar

AbstractLake margin sedimentary systems have been the subject of only limited study. The cyclic Middle Devonian lacustrine succession of Northern Scotland contains repeated developments of shore zone sandstones and thus provides an ideal location for the study of these units. The cycles comprise deep lake, shallow lake, playa and shore zone facies. Detailed field observations are presented alongside ground penetrating radar data which has aided large‐scale and three‐dimensional characterization of the shore zone sand bodies. Loading and discrete channel forms are recognized in thin‐bedded sandstones within the lower portion of the lake shore zone successions. Up‐section, the sandstone beds appear to become amalgamated, forming subtle low angle accretionary bar complexes. Where imaged on the radar profiles, the repeated development of shoreward migrating features succeeded by more shallow angled lakeward accreting surfaces is recognized; these are ascribed to washover and swash–backwash processes, respectively. The orientation of these features is similar to palaeocurrent measurements from oscillation ripples, suggesting an alignment of the shore zone bars perpendicular to the prevailing wind direction. Further loaded sandstone beds and sand‐filled shallow channel features overlie the bar forms. The context of the shore zone facies allows the controls on its formation to be examined. The shore zone sandstones overlie playa facies which contain abundant desiccation horizons, reflecting the most arid phase in the climatically controlled lacustrine cycle. As climatic conditions ameliorated, the rejuvenation of fluvial systems resulted in the transport of sand out into the basin. Initial deposition was limited to intermittent events where sediment was laid down on a water‐saturated substrate. High resolution fluctuations in lake level resulted in periodic short‐lived reworking events along the lake margins which produced amalgamated sands, forming low relief bars. Shore zone reworking is likely to have occurred over a wide area as the lake margin migrated back and forth, and gradually transgressed.

Disentangling adaptive evolutionary radiations and the role of diet in promoting diversification on islands.

Although the initial formulation of modern concepts of adaptive radiation arose from consideration of the fossil data, rigorous attempts to identify this phenomenon in the fossil record are largely uncommon. Here I focus on direct evidence of the diet (through tooth-wear patterns) and ecologically-relevant traits of one of the most renowned fossil vertebrates-the Miocene ruminant Hoplitomeryx from the island of Gargano-to deepen our understanding of the most likely causal forces under which adaptive radiations emerge on islands. Results show how accelerated accumulation of species and early-bursts of ecological diversification occur after invading an island, and provide insights on the interplay between diet and demographic (population-density), ecological (competition/food requirements) and abiotic (climate-instability) factors, identified as drivers of adaptive diversification. A pronounced event of overpopulation and a phase of aridity determined most of the rate and magnitude of radiation, and pushed species to expand diets from soft-leafy foods to tougher-harder items. Unexpectedly, results show that herbivorous mammals are restricted to browsing habits on small-islands, even if bursts of ecological diversification and dietary divergence occur. This study deepens our understanding of the mechanisms promoting adaptive radiations, and forces us to reevaluate the role of diet in the origins and evolution of islands mammals.

Timing of wet episodes in Atacama Desert over the last 15 ka. The Groundwater Discharge Deposits (GWD) from Domeyko Range at 25°S.

Abstract A chronologically robust reconstruction of timing and dynamics of millennial time scale wet episodes encompassing the entire Atacama Desert during the last 15 ka has been constructed. To accomplish this, a new composite paleoclimatic record from Groundwater Discharge Deposits (GWD) in the Sierra de Varas (Domeyko Range, southern Atacama in Chile at 25°S) has been compiled and compared with other published paleohydrologic records from the Atacama region. In Sierra de Varas (SV), three millennial timescale wet climate phases have been characterized: around 14.5 ka cal BP, 12.2–9.8 ka cal BP, and 4.7 ka cal BP to the present day. These wet phases are interpreted from intervals of GWD facies formed during periods when the springs were active. GWD facies include: (1) black organic peat, rooted mudstones and sandstones formed in local wetland environments, and (2) gypsum-carbonate rich layers formed by interstitial growth. GWD intervals alternate with gravelly alluvial material deposited during arid phases. A trend towards less humid conditions during the Late Holocene wet episode characterizes GWD sedimentary series in Sierra the Varas, suggesting the onset of a dry episode over the last few centuries. Around 0.7 ka BP a very short wet episode is recorded in the central part of the desert suggesting this was the time of maximum humidity for the entire late Holocene wet period. A brief arid phase occurred between 1.5 and 2.0 ka BP indicated by the absence of GWD in the Domeyko Range. The paleoclimatic reconstruction encompassing the entire Atacama region shows that both the intensity and occurrence of wetter conditions were governed mainly by the distance to the source of moisture, and secondarily by the elevation of the sites. In the northern Atacama (16–20°S), four wet phases fed by N-NE summer monsoon precipitations have been proposed: Tauca phase (18–14 ka cal BP) and Coipasa phase (13–10 ka cal BP) during the Late Glacial, followed by Early Holocene and Late Holocene phases. In contrast, southern Atacama records (23–28°S) display only three pluvial periods which result from SE summer monsoon precipitation and outbreaks from the Westerlies during wintertime. The Early Holocene in the southern Atacama was a period of aridity, generating important landscape differences to those in the Northern Atacama where conditions were wetter. The core of Atacama (20–23°S) is the overall driest part of the desert because it is located in the distal limits of both N-NE and SE sources of moisture, the Amazon Basin and Gran Chaco areas, respectively.

Low‐latitude Holocene hydroclimate derived from lake sediment flux and geochemistry

ABSTRACTThis study investigates hydrological responses to climatic shifts using sediment flux data derived from two dated palaeolake records in south‐east Arabia. Flux values are generally low during the early Holocene humid period (EHHP) (∼9.0–6.4k cal a BP) although several short‐lived pulses of increased detrital input are recorded, the most prominent of which is dated between ∼8.3 and 7.9k cal a BP. The EHHP is separated from the mid‐Holocene humid period (MHHP) (∼5.0–4.3k cal a BP) by a phase of increased sediment flux and aridity, which began between ∼6.4 and 5.9k cal a BP and peaked between ∼5.2 and 5.0k cal a BP. The termination of the MHHP is marked by a phase of high detrital sediment flux between ∼4.3 and 3.9k cal a BP. While long‐term shifts in climate are most likely linked to changes in the summer position of the Intertropical Convergence Zone and associated Indian and African monsoon systems, it is noted that the abrupt, short‐term phases of aridity observed in both records are coeval with intervals of rapid climate change globally, which triggered non‐linear, widespread landscape reconfigurations throughout south‐east Arabia.