Humid Holocene Research Articles

A Multi-Proxy Investigation of the Anthropogenic Landscape Near the Early Bronze Age Building VII at Al-Khashbah, Oman

The Early Bronze Age in southeastern Arabia is characterized by a significant shift in the archaeological record that coincides with the regional deterioration of climatic conditions associated with the end of the Holocene Humid Period. This study presents evidence for human modification of the local landscape around Building VII, a large circular structure at Al-Khashbah, Oman dating to the late third millennium BC. Through multiproxy analyses combining archaeological, geophysical, geomorphological, pedological and malacological, geochemical, palynological, and archaeobotanical datasets, we found evidence for the excavation of sediments at the foot of Building VII during the Umm an-Nar period. This excavation was subsequently filled by sediment presenting evidence for soil development, increased organic carbon, salt enrichment, microcharcoal, and snails. We suggest that both the excavation and subsequent aggradation may be related to gardening activities, though archaeobotanical and palynological analyses did not yield sufficient preserved material for local vegetation reconstruction. Nevertheless, our results contribute valuable insights into local human-environmental interactions and show that investigations beyond the visible archaeology of the monumental buildings of Early Bronze Age Oman can record a wide range of human activities and land modification near these important sites. These results have the potential to provide information on the resilience and adaptation to climate aridification by Umm an-Nar period peoples in central Oman and landscape modification during this period of economic transformation, increasing sedentarization, and growing social differentiation.

Tracing the range shifts of African tree ferns: Insights from the last glacial maximum and beyond

African tropical forests are experiencing rapid decline as a result of several factors, including increasing population pressure, recurrent wildfires, selective logging practices, land use changes, intensified agricultural activities, and other social and economic issues. Using MaxEnt, paleoclimatic data, and future climate scenarios, the present study seeks to explore the presence of tree ferns in tropical and Saharan Africa during the Last Glacial Maximum, African Holocene Humid Period (AHHP; ca. 14,500–5000 years ago) and to project the effects of climate change on the distribution of tree ferns in Africa under two future climatic scenarios, Representation Concentration Pathways (RCP) 4.5 and 8.5. Our study reveals that despite a significant increase in precipitation during the AHHP, precipitation distribution was variable and insufficient to support the five tree fern species examined in this study. While some tree fern species have experienced range shifts over time, we found that most of them have maintained their presence within refuge areas that probably endured the late Pleistocene extinction event. These refugia provided a haven for some tree ferns, allowing them to persist and survive amidst challenging and varying environmental conditions. This highlights tree ferns' remarkable adaptability in changing climate as well as the critical importance of these refugial areas in safeguarding their populations during climatic upheaval. Our study further demonstrates that different species respond to climate change differently, with some experiencing minimal range contractions of 2.0 %, up to more than 57.0 % range expansion in other species. Preserving refugia not only safeguards tree fern populations but also contributes to conserving overall forest biodiversity and ecosystem functioning. This knowledge is crucial for implementing targeted conservation actions that promote sustainable forest management and can mitigate the threats posed by climate change and anthropogenic activities in African closed wet forests.

Tracking orbital and suborbital climate variability in the westernmost Mediterranean over the past 13,000 years: New insights from paleoperspectives on marine productivity responses

This study presents a comprehensive analysis of a sediment record from the Western Alboran Basin (core GP04PC), utilizing palynological and geochemical tools to investigate marine productivity responses to orbital and suborbital climate variability over the past 13,000 years. High productivity during the Younger Dryas humid phase (∼12.4–11.7 ka) and the Holocene humidity optimum (∼10.5–8.5 ka) was driven by increased local river discharges resulting from rapid mountain glaciers melting and enhanced regional precipitation. During the late Holocene, frequent flood events linked to negative North Atlantic Oscillation (NAO) incursions potentially led to multicentennial-scale productivity increases. The findings indicate that periods characterized by wet regional conditions and increased river run-off, influenced by orbital (e.g., insolation cycles) and suborbital factors (e.g., NAO and Atlantic Meridional Overturning Circulation changes), consistently enhanced marine productivity in the Western Alboran Basin. The study also reveals that the current high productivity and carbon export in the Western Alboran Basin are maintained by active upwelling and downwelling systems driven by a persistent positive NAO phase following the southward migration of the Intertropical Convergence Zone (ITCZ) that occurred around 6.5 ka. Furthermore, geochemical proxies support a strong detrital influence on trace metal concentrations, including barium (Ba), in deep Western Alboran sediments during the Holocene. This limits the use of Ba/Al ratios for accurately reconstructing productivity changes and highlights the importance of dinocyst analysis as a complementary tool for robust marine productivity reconstructions in this region. These observations provide valuable paleoperspectives on marine ecosystem responses to climate variability, contributing to the development of robust long-term productivity models essential for adapting to ongoing environmental changes in the region, and demonstrating the strong influence of North Atlantic climate and ocean dynamics on centennial-scale productivity oscillations in this region.

Sedimentary record of primary productivity changes in a crater lake in Northeast China since the Last Glacial Maximum

High-resolution primary productivity records are valuable for identifying lacustrine ecosystem degradation in the context of long-term climatic variability. In this paper, we analyse primary productivity changes in a seasonally ice-covered crater lake (Lake Tuofengling) in northeast China since the Last Glacial Maximum using biogenic silica content and the carbon preference index of short-chain n-alkanes as proxies. Results show that primary productivity was lower during the cold and dry glacial period, whereas it was higher during the warm and humid Holocene. During the last deglaciation, primary productivity experienced a ∼ 1.5-kyr lagged response to Bølling-Allerød warming, and multiple oscillations during the Younger Dryas. Additionally, the peak productivity of the entire profile also occurred during this period. These fluctuations and the highest primary productivity are suggested to be the result of the combined influences of increased summer temperatures and regional permafrost melting. The current state of primary productivity in Lake Tuofengling is moderate but has been increasing since the abrupt increase in diatom productivity after ∼3 ka B.P. This stage is associated with increased dust deposition and longer ice-free periods under winter-warming conditions. We suggest that longer seasonal stratification under the current warmer winters and increased nutrient concentrations caused by decreased lake level will increase the risk of lake ecosystem degradation in the future.

The African Holocene Humid Period in the Tibesti mountains (central Sahara, Chad): Climate reconstruction inferred from fossil diatoms and their oxygen isotope composition

The climate of the African Holocene Humid Period (AHHP) is reconstructed in the Tibesti Volcanic Massif (TVM) in the central Sahara from well-preserved diatomaceous deposits in the two crater palaeolakes of Trou au Natron at Pic Toussidé and Era Kohor at Emi Koussi. The two records cover the period from ∼9500 to 4500 cal yr BP. Climate and palaeoenvironmental changes during this period were inferred from diatom assemblages, interpretation of variations in their oxygen isotope composition (δ18Odiatom), reconstruction of lake water conductivity from diatom-based transfer functions, and estimation of the lake water balance (Evaporation/Inflow ratio, E/I). Our findings provide evidence for two distinct lacustrine episodes. During the early to mid-Holocene transition, low δ18Odiatom values, high percentages of planktonic diatoms, low lake water conductivity and a positive water balance (E/I < 1) suggest wet conditions, which were likely related to the optimum of the AHHP. From the mid-to late Holocene transition, an aridification trend is revealed by increasing δ18Odiatom values, high percentages of benthic diatoms and a negative water budget (E/I > 1), occurring as early as 6500 cal yr BP and intensifying after 5300 cal yr BP. Moreover, our data show on average a decrease in precipitation amounts of ∼35% between the peak and the end of the AHHP in the Tibesti region. This timing of the AHHP in the mountainous Tibesti is consistent with the aridification of the central Sahara recorded at lowland sites, which has mainly been related to the southward retreat of the Intertropical Convergence Zone (ITCZ) and the associated African monsoonal rainfall belt, following the gradually declining summer insolation that led to the termination of the AHHP. Our results prove the existence of Holocene lakes in the TVM craters that developed contemporaneously with the lakes of the Chadian basin and the Libyan Sahara. On a broader scale, our data share similar hydroclimatic patterns with studies from the eastern and northern Sahara.

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The investigations presented in this paper focus on the reconstruction of Holocene environmental change in Mongolia based on published paleoclimate records, including lake sediments, lake levels, pollen records and eolian sediment records. These data indicate that the early Holocene of Mongolia is characterized by increasing temperature and humidity. The mid Holocene is characterized by enhanced aridity, and during late Holocene humidity increased, evaporation decreased.

Abrupt Contraction of the Indo‐East Asian Monsoons Ended the Holocene Humid Period

AbstractRain‐belt migration, and in particular monsoon migration, are a cardinal, yet under‐constrained aspect of climate change. Here, we present a new lake‐level record from a closed‐basin lake in northern China and a compilation of available lake level records from closed‐basin lakes located at the northern margins of East Asian and Indian monsoon regions. These data show that ca. 6 ka ago, changes in water availability, as recorded by the surface areas of the closed‐basin lakes, shrunk dramatically, within a few decades, and have not been restored to their pre‐6 ka sizes since. These data imply that the Indo‐East Asian monsoon systems underwent a large coeval southward contraction, which preceded the gradual decline in insolation forcing, and abruptly ended the Holocene Humid Period (HHP). This new compilation enables to constrain potential feedback mechanisms involved in the early termination of the HHP at the monsoon fringes.

Rare Holocene sediment deposits from Sodmein Playa (Eastern Desert, Egypt)—Stratigraphic assessment and environmental setting

AbstractSodmein Playa is one of the rare Pleistocene open‐air sites in the Eastern Desert of Egypt. Based on the associated stone artefact material, it could be assigned to the Middle Stone Age/Last Interglacial. However, it has not yet been possible to clarify whether the sediments at the basin originated during the Pleistocene or later during Holocene wet phases. Our integrative approach combining Optically Stimulated Luminescence chronology, and cryptotephra analysis, allows us to link the environmental archive of Sodmein Playa with the site of Sodmein Cave. Sodmein Playa indicates wetter climate conditions starting around 9 ka with a (relative) maximum around 7 ka, in line with the general framework of the Holocene Humid Period in Northeast Africa. Despite the climatic similarity, regional environmental differences can still be identified and the effective available water around Sodmein Playa is reduced. The results are well integrated into the current archaeological knowledge with the change from hunter‐gatherers to herders during the Holocene in the area. Analyses of cryptotephra reveal a wide range of source regions, including Eastern and Central Anatolian, the Azores, and the Aegean, as well as those which remain uncorrelated. A tentative correlation with the Holocene cryptotephra record from Sodmein Cave is established.

From Dilmun to Wādī al‐Fāw: A forgotten desert corridor, c. 2000 BC

AbstractThere is a lacuna of knowledge on the inland trade routes across Bronze Age central Arabia, which this article seeks to fill based on new evidence from Wādī al‐Fāw, Saudi Arabia. Contrary to a common belief that interior Southeast Arabia after the Holocene Humid Phase and until the domestication of the dromedary had turned desolate Badlands, this study offers documentation that during the early Bronze Age, a commercial corridor connected the Kingdom of Dilmun on the Arabian Gulf coast with the southern parts of Saudi Arabia, probably Yemen. Seals of Dilmun Type, Dilmun pottery and related burial praxis make up the gist of the evidence from Wādī al‐Fāw. A dry mummification mound burial custom is possibly identified at al‐Fāw and probably Taymāʾ, which contrasts the classic Dilmun mound burial custom. An attempt is made to reconstruct the most likely route that connected Dilmun and Wādī al‐Fāw. The emergence around 2000 BC of this trade network, likely based on donkey trains, closely coincides with the rise of the Kingdom of Dilmun, but surprisingly also with a time when Arabia witnessed unusually arid conditions. Identification of this unexpected ancient corridor should profoundly affect how upcoming models consider linguistic, ideological, genetic, cultural and technological transmission across Bronze Age Arabia.

A Chironomid Record of Early-Middle Holocene Environmental Evolution in the Darhad Basin, Northern Mongolia.

Simple SummaryThe Holocene humidity evolution presents spatial heterogeneity in the Mongolian Plateau where multiple circulation systems converged. Inconsistent records of regional environmental evolution affect accurate assessment of lake ecosystems. Chironomid larvae are one of the most abundant benthic invertebrates in lakes. The taxa composition of their functional groups is largely determined by aquatic habitats. Therefore, analyzing the changes of their functional traits can effectively indicate the hydrological dynamics in the past. In this study, we analyzed the early-middle Holocene (9.0–4.5 cal kyr BP) subfossil chironomid assemblages of a sedimentary sequence from the Darhad Basin in northern Mongolia. At 9.0 cal kyr BP, the community structure suddenly changed from littoral taxa to sublittoral/profundal taxa, reflecting an environmental transition from a river or shallow lake condition to a deep lake environment, which lasted until 4.5 cal kyr BP. Those hydrological patterns are consistent with the humidity evolution in the Westerlies dominated region, except that the onset of wetness occurred one thousand years earlier when comparing our results with previous ones, which may be related to the melting of regional glaciers and permafrost caused by increased summer solar insolation.Under the influence of various circulation systems, the Holocene humidity conditions on the Mongolian Plateau are spatially heterogeneous and the underlying mechanism is still ambiguous. The complexity of climate change may affect the accuracy of assessing lake ecosystem evolution. In this study, based on the precise chronology, a chironomid assemblage sequence from the Darhad Basin in northern Mongolia is analyzed to elucidate the hydroclimate variation during the early-middle Holocene. The results show that the chironomid communities changed suddenly from littoral taxa to sublittoral/profundal taxa at about 9 cal kyr BP, reflecting an environmental transition from a river or shallow lake condition to a deep lake environment. Thereafter, most parts of the paleolake remained at a relatively high level until 4.5 cal kyr BP. This hydrological pattern resembles the typical humidity variations in the Westerlies affected regions, except that the onset of wetter conditions occurred one thousand years earlier as reflected in our results. The melting of glaciers and permafrost in the basin resulting from the early increased summer solar insolation could be a feasible explanation for these time advances.

The unexpectedly short Holocene Humid Period in Northern Arabia

The early to middle Holocene Humid Period led to a greening of today’s arid Saharo-Arabian desert belt. While this phase is well defined in North Africa and the Southern Arabian Peninsula, robust evidence from Northern Arabia is lacking. Here we fill this gap with unprecedented annually to sub-decadally resolved proxy data from Tayma, the only known varved lake sediments in Northern Arabia. Based on stable isotopes, micro-facies analyses and varve and radiocarbon dating, we distinguish five phases of lake development and show that the wet phase in Northern Arabia from 8800–7900 years BP is considerably shorter than the commonly defined Holocene Humid Period (~11,000–5500 years BP). Moreover, we find a two century-long peak humidity at times when a centennial-scale dry anomaly around 8200 years BP interrupted the Holocene Humid Period in adjacent regions. The short humid phase possibly favoured Neolithic migrations into Northern Arabia representing a strong human response to environmental changes.

Human landscape modification and expansion of tropical woodland in southern Arabia during the mid‐Holocene from rock hyrax middens

AbstractAimDryland ecosystems, such as those in southern Arabia, are particularly vulnerable to climate and land‐use change. Aridification and human subsistence changes at the end of the Holocene Humid Period at 6–4 ka have been used as an iconic example for evaluating such impacts to resilience of arid systems. Although records of ancient environments can provide critical insight into the biotic and abiotic mechanisms that alter ecosystems, traditional archives, such as lake deposits, are not common in southern Arabia after 6 ka; thus, we must use alternative archives.LocationYemen.TaxonPlants; Rock Hyrax (Procavia capensis).MethodsWe use fossil pollen and stable isotopes (δ15N; δ13C) of rock hyrax (P. capensis) middens from Wadi Sana, Yemen, to look at changes in ecosystem structure and function across this key transition from 6 ka to the present. A total of 17 middens were radiocarbon‐dated and stable isotopes were measured. Of these, pollen was extracted from hyraceum of 14 middens and identified using a light microscope. Fossil pollen assemblages were then compared to existing modern pollen samples from throughout Arabia.ResultsDuring the mid‐Holocene from at least 6 to 4.7 ka, the pollen flora reflects a landscape with abundant tropical trees. These included foundational woody taxa, such as Terminalia and Boswellia sacra (frankincense), which had a strong positive influence on local hydrology and the economy, respectively. Increased charcoal abundances also suggest that wildfire occurred periodically.Main conclusionsConnections with archaeological evidence suggest a strong link between human management and the presence of Terminalia woodland during the mid‐Holocene. These may have promoted increased groundwater storage and ponding as regional rainfall was decreasing. The mid‐Holocene expansion of Boswellia sacra (frankincense) may have helped support resin trade that became a critical export from the region. Fires were more common than today, suggesting semi‐arid but continuous vegetation cover across what is now bare ground. Finally, after 1 ka, increased sedge abundances at the expense of grasses and trees suggest the development of desert conditions.

Holocene Fire Regime Changes in the Southern Lake Baikal Region Influenced by Climate-Vegetation-Anthropogenic Activity Interactions

Catastrophic fire years that have taken place during the last decade in Siberia, and more generally within the boreal forest, have been directly linked to global warming and had strong repercussions on boreal ecosystems and human populations. In this context the study of the past dynamics of these fires is essential for understanding their links with climate, vegetation and human activity changes on longer time scales than the last few decades. However, few studies on fire dynamics are available for Siberia, and none have been conducted for the entire Holocene period. This study presents the first fire history reconstruction of this area during the Holocene based on charcoals sequestered in sediments of two lakes located on the southern shore of Lake Baikal, in Siberia. The results show a similar trend in the two lakes, with high frequency and high peak magnitude during the Early Holocene and low magnitudes after 6500 cal. yr BP. This difference is interpreted as crown fires versus surface fires. According to pollen records (Dulikha, Vydrino, Ochkovoe) available near the studied lakes, a vegetation transition occurred at the same time. Picea obovata, which has a tree structure prone to crown fires, was dominant during the Early humid Holocene. After 6500 cal. yr BP, conditions were drier and Pinus sylvestris and Pinus sibirica became the dominant species; their tree structure favors surface fires. In addition to vegetation dynamics, the nearby pollen sequence from Dulikha has been used to provide quantitative estimates of past climate, indicating an Early to Middle Holocene climatic optimum between 8000 and 5000 cal. yr BP and an increase in temperatures at the end of the Holocene. These results have been compared to outputs from regional climate models for the Lake Baikal latitudes. Fire dynamics appear to have been more linked to the vegetation than climatic conditions. Over the past 1500 years, the greater presence of human populations has firstly resulted in an increase in the fire frequency, then in its maintenance and finally in its suppression, which may possibly have been due to very recent fire management, i.e., after ca 500 cal. BP.

Palaeoenvironmental and climatic changes during the Late Glacial and Holocene in the Mongolia and Baikal region: A review

Multilayer sections in lake bottom sediments and continental sections of loose sediments in Mongolia and the Baikal region provide rich palaeontological material and radiocarbon dating for palaeoenvironmental reconstructions of the Late Pleistocene (Late Glacial) to Holocene. Multidisciplinary data from lacustrine bottom sediments provide evidence of past lake level and vegetation dynamics in the watersheds. Palaeontological data from terrestrial deposits indicate a mosaic landscape structure and a temperate warm and humid climate during the Late Glacial and Holocene, with some regional variability.Global and regional climatic changes at the end of the Pleistocene resulted in the mass extinction of components of the Mammoth faunistic complex, such as the woolly mammoth, cave lion, cave hyena, and big-horned deer; the number of tundra species decreased and migrated to the north, and arid animal species migrated to the south. During the Holocene, four vegetation types (steppe, forest, taiga, and desert) did not show radical changes, but their ratio and spatial distribution changed. Steppe landscapes were replaced by desert steppes during the arid phases of the Holocene. In contrast, the area of taiga forests expanded during humid Holocene phases, but the steppe landscapes remained dominant in the studied territory.

Distribution of n‐alkan‐2‐ones in Qionghai Lake sediments, southwest China, and its potential for late Quaternary paleoclimate reconstruction

AbstractStudies on long‐chain n‐alkan‐2‐ones from lake sediments remain sparse. In this study, we present an n‐alkan‐2‐one record from Qionghai Lake, southwest China, to assess the paleoclimate significance of variations in their compositions. A homologous series of n‐alkan‐2‐ones ranging from C21 to C35 were identified, with maximum concentrations of the C29 or C31 chain lengths and a strong odd‐over‐even predominance. This type of n‐alkan‐2‐one is considered to derive mainly from microbial oxidation of the corresponding n‐alkanes, and partial inputs from plants. The n‐alkan‐2‐one‐derived average chain length (ACL) and carbon preference index (CPI) values changed significantly over the past 28k cal a bp, consistent with the sediment grain size and n‐alkane proxies from the same core. Generally, the high CPI27‐33‐ket and low ACL27‐33‐ket values indicated cold and dry climates such as for the Last Glacial Maximum (23.2–19.7k cal a bp), Heinrich 1 event (17.6–15.6k cal a bp) and Younger Dryas (12.8–11.6k cal a bp), but low CPI27‐33‐ket and high ACL27‐33‐ket values denoted a warm and humid Holocene Climatic Optimum (7.0–4.3k cal a bp). Therefore, n‐alkan‐2‐ones have great paleoclimatic potential and can be applied together with other biomarkers to reconstruct a reliable paleoclimate record in lake sediments.

The origin of the forest-grassland mosaic of central Cameroon: What we learn from the isotopic geochemistry of soil organic matter

The end of the Holocene Humid Period was characterized by the massive collapse of forest ecosystems in central Africa. Was this large-scale environmental crisis dated between 3300 and 2500 years BP at the origin of the present-day wooded-grassland/forest mosaic of central Cameroon? Here we present carbon isotope data on soil organic matter collected from 29 sites from this area, including the Grassfield plateaus, with the aim of characterizing this environmental crisis and discussing the origin of the present-day landscape. 14C data on total soil organic matter and charcoal fragments show that the organic matter in these soils originates from the Holocene. In our forest sites, the δ13C profiles do not show any marked alteration of the forest cover throughout the Holocene. Discrete traces of forest expansion or retreat are observed only punctually at the forest margins. In the wooded-grassland/forest mosaic zone, the environmental crisis was of reduced amplitude. In the present grassland and wooded grassland areas, there was no true forest during the lower to middle Holocene, but a markedly more tree-covered landscape. The timing of this crisis revealed by our 14C measurements, although only indicative at this stage, suggests that it occurred earlier at higher altitudes than in the lowlands due to marked topographic relief favoring drainage along the slopes. These results fit a climatic rather than anthropogenic origin of the vegetation opening in this region.

Sahel environmental variability during the last millennium: Insight from a pollen, charcoal and algae record from the Niayes area, Senegal

The pollen, algae and charcoal record from the “Baobab” depression near Mboro in the Niayes area of Senegal shows that the last millennium was characterized by a gradual aridification which started 1340 CE and culminated 1845 CE. The sub-Guinean gallery forests, which survived the previous environmental crisis dated from the end of the Holocene Humid Period, suddenly collapsed and the Niayes region suffered a dramatic loss of biodiversity. We show that the tipping point between a forested and non-forested state occurred ca 400 centuries after the onset of aridity recorded by the lowering of the water level and the progressive salinization of the aquatic environment.

Neolithic pastoralism in marginal environments during the Holocene Humid Period, northern Saudi Arabia

The origins of agriculture in South-west Asia is a topic of continued archaeological debate. Of particular interest is how agricultural populations and practices spread inter-regionally. Was the Arabian Neolithic, for example, spread through the movement of pastoral groups, or did ideas perhaps develop independently? Here, the authors report on recent excavations at Alshabah, one of the first Neolithic sites discovered in Northern Arabia. The site’s material culture, environmental context and chronology provide evidence suggesting that well-adapted, seasonally mobile, pastoralist groups played a key role in the Neolithisation of the Arabian Peninsula.

Geochemical, multi-isotopic studies and geothermal potential evaluation of the complex Djibouti volcanic aquifer (republic of Djibouti)

The complex Djibouti volcanic aquifer system was studied to improve understanding of the recharge conditions of the Awrlofoul low-enthalpy geothermal system located in the middle of the aquifer. Forty-four thermal and non-thermal groundwater samples were analyzed to determine their major chemical compositions, trace element compositions, and multi-isotopic compositions (δ2H(H2O), δ18O(H2O), δ18O(SO4), δ34S(SO4), δ13C(DIC), 14C, 87Sr/86Sr, δ11B, δ15N(NO3−), and δ18O(NO3−)). Statistical analysis (Hierarchical Cluster Analysis and Principal Component Analysis) of chemical composition identified three main water groups, two affected by salinization (C1 and C2) and one fresh water group useful for drinking (C3). The latter group includes thermal water from the Awrlofoul geothermal field. This separation into three different water groups is also clear on a Langelier-Ludwig plot and is confirmed by analysis of historical chemical data over the last 30 years. The main causes of salinization are contamination of the fresh groundwater either by recent seawater intrusions (C2) or mixing with Ca-Cl fossil saline water (C1). The C1 waters are also highly affected by Mg/Ca-Na clay exchange. As expected, the 11B/10B isotope ratio of the intruded salt water, both recent and fossil, was much higher than that of seawater (δ11B up to +55‰). Unexpectedly, groundwater of meteoric origin (i.e., unaffected by a seawater intrusion), also showed a δ11B higher than that of seawater (46.3‰ < δ11B < 51.3‰). That the unexpectedly high δ11B values are likely due to 10B sequestration resulting from interaction with clay and/or carbonate precipitation is demonstrated by activity diagrams and saturation indices. The C1 water group is also affected by nitrate contamination (56.8 ± 19.2 mg/l). That the nitrate contamination is likely due to manure contamination is indicated by comparing the dual isotopic composition of nitrate to the boron isotope ratios. The isotopic composition of sulfate highlighted the importance of SO2-disproportionation to the local sulfate minerals that interacted with the meteoric recharge, while the strontium isotope ratios showed the importance of the seawater-basalt interaction with the fossil saline water component. The results of the mixing analysis using chemical composition, δ13C(DIC), and 14C data by geochemical software (NetpathXL) confirmed the presence of ternary mixing with at least three sources (seawater, meteoric, and fossil) in the waters with the highest chloride concentrations. The estimation of groundwater age by 14C was complicated by overexploitation (as testified by the lumped parameters approach). However, the fossil saline water component was dated back to the Holocene Humid Period.To estimate the temperature of the Awrlofoul low-enthalpy geothermal system, a multi-method geothermometric approach was applied. Chemical (mainly SiO2) and isotope (sulfate-water oxygen fractionation) geothermometers were employed together with multiple mineral equilibria. These different geothermometric approaches estimated a temperature range of 102 °C–140 °C for the geothermal reservoir, with a mean temperature of about 110 °C.Finally, a conceptual model was proposed for the Awrlofoul low-enthalpy geothermal system on the basis of the geochemical and isotopic data of the thermal and non-thermal groundwaters combined with the geology and hydrogeology of the study area.

Millennial-scale cyclical environment and climate variability during the Holocene in the western Mediterranean region deduced from a new multi-proxy analysis from the Padul record (Sierra Nevada, Spain)

Abstract A high-resolution multi-proxy approach, integrating pollen, inorganic and organic geochemical and sedimentological analyses, has been carried out on the Holocene section of the Padul sedimentary record in the southern Iberian Peninsula reconstructing vegetation, environment and climate throughout the last ~ 11.6 cal kyr BP in the western Mediterranean. The study of the entire Holocene allows us to determine the significant climate shift that occurred during the middle-to-late Holocene transition. The highest occurrence of deciduous forest in the Padul area from ~ 9.5 to 7.6 cal kyr BP represents the Holocene humidity optimum probably due to enhanced winter precipitation during a phase of highest seasonal anomaly and maximum summer insolation. Locally, insolation maxima induced high evaporation, counterbalancing the effect of relatively high precipitation, and triggered very low water table in Padul and the deposition of peat sediments. A transitional environmental change towards more regional aridity occurred from ~ 7.6 to 4.7 cal kyr BP and then aridification enhanced in the late Holocene most likely related to decreasing summer insolation. This translated into higher water levels and a sedimentary change at ~ 4.7 cal kyr BP in the Padul wetland, probably related to reduced evaporation during summer in response to decreased in seasonality. Millennial-scale variability is superimposed on the Holocene long-term trends. The Mediterranean forest regional climate proxy studied here shows significant cold-arid events around ~ 9.6, 8.5, 7.5, 6.5 and 5.4 cal kyr BP with cyclical periodicities (~1100 and 2100 yr) during the early and middle Holocene. A change is observed in the periodicity of these cold-arid events towards ~1430 yr in the late Holocene, with forest declines around ~ 4.7–4, 2.7 and 1.3 cal kyr BP. The comparison between the Padul-15-05 data with published North Atlantic and Mediterranean paleoclimate records suggests common triggers for the observed climate variability, with the early and middle Holocene forest declines at least partially controlled by external forcing (i.e. solar activity) and the late Holocene variability associated with internal mechanisms (oceanic-atmospheric).