The presence of Mg-rich calcite in foraminifera shells has impeded traditional calcite-based palaeothermometry in the eastern Mediterranean, complicating our understanding of the region's past climate. We analysed the Ca isotopic composition (delta Ca-44/40) in the shells of the planktonic foraminifera species Globigerinoides ruber (white), collected from 12 core-top sediment samples spanning the Aegean and Levantine seas in the eastern Mediterranean. The shells exhibited varying degrees of early diagenetic Mg-rich authigenic calcite coatings, which complicate traditional calcite-based palaeothermometry in the region. Our findings demonstrate a significant correlation between the delta Ca-44/40 data and the Mg/Ca ratios of the shells and suggest that the inorganic calcite overgrowth originates from seawater. Moreover, the Ca isotopic composition increases with the amount of calcite overgrowth. This relationship allowed us to quantify the maximum potential inorganic end-member. The highest amount of post-depositional calcite precipitation was observed in the warm, shallow basins of the South Aegean Sea. The diagenetic alteration of the primary biogenic delta Ca-44/40 geochemical signal is linked to the quantity of secondary calcite present. By combining Ca isotope and Mg/Ca measurements on isolated foraminifera tests, we provide novel insights into the early diagenetic history of carbonate-bound geochemical proxies.

A lack of quantitative rainfall reconstruction has hindered understanding of the role of hydrological disturbances at ∼4.2 kyr BP (1000 years before present) in the collapse of the Shijiahe culture-an advanced Neolithic society in the Middle Yangtze Valley (MYV). We provide a quantitative paleohydrology reconstruction for the period 4.6-3.5 kyr BP by using calcium isotopes, trace elements and δ13C from an annually laminated stalagmite from the MYV. Our reconstructed rainfall shows three drier intervals with rainfall of <700 mm/yr (4.36-4.33 kyr BP, 4.23-4.10 kyr BP, 3.57-3.55 kyr BP) and two wetter intervals with rainfall of >1000 mm/yr (3.95-3.84 kyr BP, 3.70-3.59 kyr BP), with suggestions of tripole/dipole rainfall patterns. Combined with archaeological and paleoflood evidence, these data suggest that the Shijiahe culture underwent transformation during drier periods, but abandoned the region when the rainfall was >1000 mm/yr. This robust, multiproxy record demonstrates that water excess could be as problematic as water shortage, even for advanced civilizations, and contributes to understanding hydrological perturbations at ∼4.2 kyr BP.

The West African Monsoon (WAM), Atlantic north-westerlies and Mediterranean cyclones are significant sources of rainfall in north-west Africa, supplying moisture to the fringes of the Sahara. Rainfall patterns and the extent of the desert vary through time with strong evidence of a wetter Sahara during the early- to mid-Holocene (widely referred to as the African Humid Period). North of 28 degrees N there is a particular lack of palaeorainfall reconstructions, with higher spatial- and temporal-resolution required to constrain the mechanisms responsible for past sub-tropical climate change, and the impacts of environmental change on human developments. We provide palaeorainfall reconstructions from stalagmites from today's arid north-west Sahara, inland from the coast and south of the Atlas Mountains in Morocco (30-32 degrees N). The records show increased rainfall between 8.7-4.3 kyr BP. The timing, and oxygen isotopes of the speleothem growth, in comparison with other records, strongly suggest that South-of-Atlas rainfall continued after the decline of the West African Monsoon in the mid-Holocene. We propose that additional rainfall was supplied by increased tropical-plume rainfall in the South-of-Atlas region. We suggest that an increased North-South inter-hemispheric temperature anomaly, shifting the ITCZ northwards, increased the supply of tropical-moisture to tropical-plumes. For the first time this study provides evidence supporting tropical-plumes as an additional source of past-rainfall, helping to reconcile palaeo-archives and modelling studies. Increased South-of-Atlas rainfall improved habitability and increased recharge to rivers flowing south into the Sahara, which likely facilitated connectivity through the Sahara, during a key period in the development of land use and animal production.

The success of industrial scale carbon capture and storage in geologic reservoirs depends on the permanence of the stored carbon dioxide (CO2). Carbon dioxide capture and mineralisation (CCM) or mineral carbonation, which is the conversion of CO2 to carbonate minerals via fluid-rock reactions provides low risk and permanent CO2 removal. Here, we demonstrate rapid mineralisation of industrial CO2 emissions in mantle peridotites. Captured CO2 from an ammonia plant in the Sultanate of Oman has been injected into peridotite at a pilot test site in the Samail ophiolite. Chemical and isotopic results indicate rapid carbonate mineral precipitation. Mass balance calculations suggest that similar to 88% of the injected CO2 was mineralised as carbonate minerals within 45 days after injection. This successful approach of CCM unlocks peridotite as a promising new type of reservoir for the safe and permanent disposal of anthropogenic CO2 emissions.

AbstractThe timing and geographic extent of a potential “4.2 ky event” remain highly contested. Here we present records of ENSO variability at 3.8 kyBP and 4.1 kyBP derived from a Borneo stalagmite, which suggest a significant change in ENSO properties between these time intervals. The Borneo records show evidence of significantly reduced ENSO activity at 4.1 kyBP, relative to other measured windows within the Holocene. This reduced ENSO activity coincides with a period of drier conditions and enhanced dust events in the Middle East that took place ∼4.0–4.3 kyBP. The Borneo records show evidence of enhanced ENSO activity at 3.8 kyBP. Various hydroclimate changes attributed to the “4.2 ky event” in many regions may thus be reflecting a shift from reduced to enhanced El Niño activity that occurred between 3.8 kyBP to 4.0 kyBP.

Abstract Archaeological sites in Northwest Africa are rich in human fossils and artefacts providing proxies for behavioural and evolutionary studies. However, these records are difficult to underpin on a precise chronology, which can prevent robust assessments of the drivers of cultural/behavioural transitions. Past investigations have revealed that numerous volcanic ash (tephra) layers are interbedded within the Palaeolithic sequences and likely originate from large volcanic eruptions in the North Atlantic (e.g. the Azores, Canary Islands, Cape Verde). Critically, these ash layers offer a unique opportunity to provide new relative and absolute dating constraints (via tephrochronology) to synchronise key archaeological and palaeoenvironmental records in this region. Here, we provide an overview of the known eruptive histories of the potential source volcanoes capable of widespread ashfall in the region during the last ~300,000 years, and discuss the diagnostic glass compositions essential for robust tephra correlations. To investigate the eruption source parameters and weather patterns required for ash dispersal towards NW Africa, we simulate plausible ashfall distributions using the Ash3D model. This work constitutes the first step in developing a more robust tephrostratigraphic framework for distal ash layers in NW Africa and highlights how tephrochronology may be used to reliably synchronise and date key climatic and cultural transitions during the Palaeolithic.

Abstract Despite all the effort made towards an understanding of the sedimentary, tectonic, and diagenetic evolution of the presalt sequence and the Pre-Salt reservoirs of the North Campos Basin (East Brazil), two knowledge gaps have yet to be filled: 1) a detailed study of diagenesis in the crystalline basement and rift phases, and 2) the timing of diagenetic events that affected the presalt succession. In this study, samples from these geologic units were analysed for mineral composition and paragenetic evolution, fluid temperature and salinity, stable isotope compositions, and LA-ICP-MS derived U-Pb ages of carbonate phases. The U-Pb ages of replacive and vein-filling cements reveal three tectono-diagenetic events, named Barremian-Aptian (BADE, 125-117 Ma), Albo-Cenomanian (ACDE, 103-98 Ma), and Campanian-Maastrichtian (CMDE, 83-70 Ma). Each phase is characterised by distinct minerals, precipitation temperatures, and burial conditions. The hydrothermal qualifier, identified by the temperature contrast between fluid and host rock, was initially high during BADE, then diminished over time (through ACDE) until it achieved equilibrium with the host rocks during CMDE. Diagenetic events are not coeval with magmatism but do coincide with known regional tectonic events described in the literature and are interpreted to be the result of increasing intraplate stresses. Multidisciplinary studies that include diagenetic events constrained by geochronological data will certainly lead to more robust conceptual geologic models, and therefore, to a more reliable management of resources and strategies such as Enhance Oil Recovery (EOR), Carbon Capture, Utilisation and Storage (CCUS), and drinking water. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7103900

Agriculture is the most important intersection between farming communities and the natural world, with major implications for land exploitation and labour organisation. In Italy, at the heart of the Roman Empire, understanding of agriculture remains heavily dependent on ancient sources, which are unable to provide a regional or diachronic view of practices across the socio-economic spectrum. In order to gain insight into agricultural economies in Roman Italy and their social and environmental implications, this article reconstructs agro-pastoral strategies at an imperial estate in southern Italy through a multi-isotope investigation of livestock bone collagen and tooth enamel. Analysis of carbon, nitrogen, oxygen and strontium isotopes (delta 13C, delta 15N, delta 18O, 87Sr/86Sr) are combined to evaluate animal management and mobility at Vagnari vicus and the villa of San Felice in the Basentello Valley. Results reveal taxon-specific herding strategies with the potential for significant inputs from legume forage/fodder and/or natural environments. Caprine herding did not appear to include long-distance transhumance. This analysis moves past previous text-based generalisations to provide a new and nuanced perspective on animal production in rural southern Italy and its economic and environmental implications.

Speleothems, secondary cave carbonate precipitates, can serve as tools to reconstruct past terrestrial ecosystem processes, particularly related to soil and vegetation. Radiocarbon, often in conjunction with other geochemical proxies, has been increasingly used for this purpose, as the speleothem reservoir effect retains useful information on local ecosystem conditions. On the other hand, speleothems are also of interest to the radiocarbon community as they can be dated very precisely with the U-Th method, which may allow the reconstruction of atmospheric radiocarbon levels in time if the reservoir effect remained constant.Over the past decades, a growing number of speleothem radiocarbon records have been generated from vastly different climate zones and ecosystem types. While much progress has been made in the interpretation of these records, a unified and global view of the factors driving variability in speleothem radiocarbon is still lacking. We compiled a global dataset of averaged speleothem radiocarbon measurements, and provide a critical evaluation of the applicability of the radiocarbon reservoir effect as a proxy for past ecosystem conditions. We compare our dataset to geographic (latitude, elevation), climatic (temperature and precipitation), and ecosystem and geological parameters (soil and bedrock thickness, soil age, vegetation type, and land cover). Our preliminary results show that it is difficult to extract a strong globally relevant driving factor for the mean absolute value in the reservoir effect at the investigated cave sites, and highlights the importance of detailed reporting of local conditions.To provide insight into the amplitude range of processes affecting published speleothem radiocarbon records we perform a series of numerical forward modeling experiments. We test how the effects of changing soil age, soil pCO2, carbonate dissolution regime, and pyrite oxidation affect carbon isotopes in stalagmites.Together, the global synthesis and modeling experiments provide us with the first global overview of how cave site parameters and climate and ecosystem processes affect speleothem radiocarbon records, and allow us to assess the sensitivity of this proxy as a tool for past ecosystem conditions.

Calcium isotope measurements in cave carbonate samples are showing significant capability as a proxy for reconstructing past rainfall amount [1,2] -an important step in developing more robust, quantitative palaeo-environmental reconstructions.To date this new proxy has focussed on calcite cave samples only.There is significant value in expanding this capability to aragonite forms of freshwater carbonates, and in improving our understanding of isotopic fractionation in aragonite sample archives.We conducted cave-analogue experiments to derive calcium isotope fractionation factors in a controlled environment.The experimental setup closely mimics natural processes (e.g.precipitation driven by CO 2 -degassing, low ionic strength solution, thin solution film) but with a tight control on growth conditions (temperature, pCO 2 , drip rate, saturation index and solution chemistry), with full details in [3].CaCO 3 was dissolved in deionized water in a 20,000 ppmV pCO 2 environment, with trace-elements (Li, Na, Mg, Co, Sr, Cd, Ba, U) at appropriate concentrations to mimic natural solutions.Mg concentrations were used to control the growth mineralogy.We then test this new calibration of Ca isotope fractionation in aragonite speleothem carbonates on North African speleothem samples, to help constrain changes in Holocene rainfall amount in the South-of-Atlas region.This adds to the existing work of [4] that focussed on the timing and on the source of additional rainfall along the northwestern boundary of the Sahara.

Calcite veins are common in organic-rich mudrocks, but their genesis and ability to transmit fluids are debated. A combined microstructural and isotopic investigation of an array of calcite veins recovered in core from the Marcellus Formation reveals that the veins grew via a combination of continuous fibrous growth and punctuated fracture-opening increments. Continuous opening is the result of pressure-solution creep and involves no mechanical fracturing, but rather the growth of a pressure fringe around a pre-existing, sealed fracture. In contrast, incremental opening is accomplished by overpressured, mineral-saturated fluid, which repeatedly ruptures the rock at the cement / host-rock interface. Punctuated growth increments occurred repeatedly throughout an otherwise protracted, continuous growth history, indicating that the present structures preserve hybrid deformation conditions between brittle, fluid-assisted cracking and plastic strain. Stable isotopic signatures match those of a regional opening-mode fracture set that formed in response to catagenetic fluid overpressures amid a tectonically imposed (Alleghanian) stress field. It is concluded that calcite veins form as opening-mode hydraulic fractures and are susceptible to increments of brittle reactivation, even while inelastic growth processes widen and fill the veins with fibrous cement.

Recent paleoclimate reconstructions have suggested millennial‐scale variability in the Indo‐Pacific Warm Pool region coincident with events of the last deglaciation. Here, we present a new stalagmite oxygen isotope record from northern Borneo, which today is located near the center of the region's mean annual intertropical convergence zone. The record spans the full deglaciation, and reveals for the first time distinct oxygen isotope variations at this location connected with the Bølling‐Allerød onset and the Younger Dryas event. The full deglaciation in the Borneo stalagmite proxy reconstruction appears remarkably similar to a 20–11 ka transient simulation of rainfall over the area produced using the isotope‐enabled Community Earth System Model. In this model, periods of weakened Atlantic Ocean meridional overturning circulation are associated with an anomalous Western North Pacific anticyclone, which is produced in boreal autumn and shifts south over Borneo during boreal winter, causing dry conditions.

Summary The pre-salt succession in the East Brazilian Margin consists of Ediacaran to Cambrian metamorphic basement overlain by Berriasian to Aptian mixed volcanics, clastics and carbonates, and topped by Aptian evaporites. The so called “Pre-Salt” reservoirs represent the upper part of this succession thus comprising the rift transition to post-rift tectonic stages and have been subject of several studies focused on sedimentology, petrography, geochemistry and diagenesis in the North of the Campos Basin. Despite the products of hydrothermal diagenesis being well known in the Pre-Salt reservoirs, their effects in the lower successions, the role of the faults as conduits for hydrothermal fluids, and the effects of lithology in the lateral extent of hydrothermal diagenesis have been only assumed though not fully understood. This works aims to provide evidence obtained through petrography, geochemistry, stable isotopes, fluid inclusion thermometry and U-Pb geochronology in carbonates for an extensive fault-related hydrothermal diagenesis throughout the basement and sedimentary section. The results are integrated through 3D seismic interpretation and inter-well correlations using lithogeochemical logs with recently published works that present, for the first time, comprehensive studies on the tectono-stratigraphic evolution and new age constraints for the deposition of the pre-salt succession.

The hyper-arid Saharan desert belt stretching across North Africa is an important part of the global climate system, with dust export shown to influence climate systems such as ENSO and distant monsoon systems. Understanding climate dynamics and potential future changes in this region is however difficult due to a paucity in both instrumental and high-resolution paleoclimate data. There is strong evidence for periods of increased rainfall across large parts of North Africa during the late Quaternary, termed ‘Green Sahara’ periods, which contribute to regional aquifer recharge and improved human population connectivity across the Sahara. There is, however, currently limited evidence regarding: i) precisely where and when rainfall occurred and; ii) the sources of moisture contributing to increased rainfall at the northern-most reaches of the Sahara. In this study, we present new proxy reconstructions from the northern limits of the presently hyper-arid Sahara Desert, to identify moisture sources, timing and latitudinal extent of rainfall change during these so-called Green Sahara periods. We do this using several ancient fossil stalagmites collected from cave sites in the desert foothills of the central Saharan Atlas Mountains, Algeria. High-precision U-Th chronology and stable-isotope measurements on calcite samples from multiple cave sites contribute towards an east-west transect of records. Due to the locations of the caves, stalagmite growth periods and stable isotope records provide direct evidence of where and when there was significantly increased rainfall in this region, and help us to identify potential sources of moisture through time. We present these results, and their implications for a more detailed reconstruction of the occurrence of Green Sahara periods in northwest Africa.

The temperate region of Western Europe underwent dramatic climatic and environmental change during the last deglaciation. Much of what is known about the terrestrial ecosystem response to deglacial warming stems from pollen preserved in sediment sequences, providing information on vegetation composition. Other ecosystem processes, such as soil respiration, remain poorly constrained over past climatic transitions, but are critical for understanding the global carbon cycle and its response to ongoing anthropogenic warming. Here we show that speleothem carbon isotope (δ13Cspel) records may retain information on local soil respiration, and allow its reconstruction over time. While this notion has been proposed in the past, our study is the first to rigorously test it, using a combination of multi-proxy geochemical analysis (δ13C, Ca isotopes, and radiocarbon) on three speleothems from Northern Spain, and quantitative forward modelling of processes in soil, karst, and cave. Our study is the first to quantify and remove the effects of prior calcite precipitation (PCP, using Ca isotopes) and bedrock dissolution (using the radiocarbon reservoir effect) from the δ13Cspel signal to derive changes in respired δ13C. Coupling of soil gas pCO2 and δ13C via a mixing line describing diffusive gas transport between an atmospheric and a respired end member allows modelling of changes in soil respiration in response to temperature. Using this coupling and a range of other parameters describing carbonate dissolution and cave atmospheric conditions, we generate large simulation ensembles from which the results most closely matching the measured speleothem data are selected. Our results robustly show that an increase in soil pCO2 (and thus respiration) is needed to explain the observed deglacial trend in δ13Cspel. However, the Q10 (temperature sensitivity) derived from the model results is higher than current measurements, suggesting that part of the signal may be related to a change in the composition of the soil respired δ13C, likely from changing substrate through increasing contribution from vegetation biomass with the onset of the Holocene.

We report Li measurements from cave-analogue carbonate-precipitation experiments in order to: i) assess the expected isotope fractionation factors applicable to speleothem growth, and ii) contribute to the wider understanding of lithium incorporation in carbonates. The experimental setup closely mimics natural processes (e.g. precipitation driven by CO2 degassing, low ionic strength solution, thin solution film) but within a laboratory setting that allows for controlled growth conditions (temperature, pCO(2), drip rate, carbonate saturation index and the composition of the initial solution). For the main batch of calcite growth experiments our average 1000ln alpha(calcite-solution )is -8.5 +/- 2(2 sigma). This low sensitivity of 1000ln alpha to in-cave growth conditions is encouraging for weathering intensity reconstruction using speleothem samples. At each temperature, growth rate was varied independently with calcite saturation indices ranging between 0.1 and 0.6, with no significant impact on either D(Li) or 1000ln alpha. For the full range of growth conditions (considering all temperatures and calcite saturation indices) we observe a small but significant decrease in 1000ln alpha, which we do not attribute to temperature. Based on an inter-study comparison we define LogDLi = 0.09(+/- 0.04) x GR - 3.2(+/- 0.2), where GR is growth rate in units of 10(-8) mmol cm(-2) s(-1). This is similar to the previously defined relationship over a reduced range of growth rates. Over this study's full range of growth rates, 1000ln alpha = -0.13 (+/- 0.04) x GR - 7.1(+/- 0.3) is significant at the rho = 0.018 level, consistent with a surface entrapment control. Future experiments should test factors such as growth rate and pH, independently of other variables, to further assess their role in defining 1000ln alpha. A subset of 20 degrees C experiments, with increasing [Mg](solution), provides 1000ln alpha for calcite, high-magnesian calcite (HMC) and aragonite. Our values of 1000ln alpha for these three mineralogies range between -7.3 and -10.7. For HMC and aragonite in particular there is close agreement with other published values of 1000ln alpha for CaCO3 grown in laboratory and in natural settings. In agreement with measurements in bulk carbonates, benthic foraminifera and brachiopods, this suggests that differences in 1000ln alpha between carbonate mineralogies may not always be as large as suggested by some earlier studies. (C) 2021 Elsevier Ltd. All rights reserved.

Terrestrial ecosystems are intimately linked with the global climate system, but their response to ongoing and future anthropogenic climate change remains poorly understood. Reconstructing the response of terrestrial ecosystem processes over past periods of rapid and substantial climate change can serve as a tool to better constrain the sensitivity in the ecosystem-climate response.In this talk, we will present a new reconstruction of soil respiration in the temperate region of Western Europe based on speleothem carbon isotopes (δ13C). Soil respiration remains poorly constrained over past climatic transitions, but is critical for understanding the global carbon cycle and its response to ongoing anthropogenic warming. Our study builds upon two decades of speleothem research in Western Europe, which has shown clear correlation between δ13C and regional temperature reconstructions during the last glacial and the deglaciation, with exceptional regional coherency in timing, amplitude, and absolute δ13C variation. By combining innovative multi-proxy geochemical analysis (δ13C, Ca isotopes, and radiocarbon) on three speleothems from Northern Spain, and quantitative forward modelling of processes in soil, karst, and cave, we show how deglacial variability in speleothem δ13C is best explained by increasing soil respiration. Our study is the first to quantify and remove the effects of prior calcite precipitation (PCP, using Ca isotopes) and bedrock dissolution (open vs closed system, using the radiocarbon reservoir effect) from the speleothem δ13C signal to derive changes in respired δ13C over time. Our approach allows us to estimate the temperature sensitivity of soil respiration (Q10), which is higher than current measurements, suggesting that part of the speleothem signal may be related to a change in the composition of the soil respired δ13C. This is likely related to changing substrate through increasing contribution from vegetation biomass with the onset of the Holocene.These results highlight the exciting possibilities speleothems offer as a coupled archive for quantitative proxy-based reconstructions of climate and ecosystem conditions.

Speleothems (secondary calcium carbonate formations) offer significant potential for recording environmental processes above caves, an area increasingly referred to as the Critical Zone.Speleothems grow for hundreds to millions of years, with absolute chronology from U-Th and U-Pb chronometers.Rainwater infiltrating the soil and flowing down through the cave responds to environmental processes.These complex environmental signals can be preserved within speleothem carbonates.Recent efforts to calibrate, model and interpret this complex geochemistry has progressed along multiple paths.Here we bring together recent examples, including: i) calibrating and using Li isotopes for reconstructing weathering intensity [1,2]; ii) the use of Ca isotopes for reconstructing changes in rainfall amount [3]; iii) the combined use of d 13 C, 14 C and d 44 Ca to demonstrate changes in soil respiration [4]; iv) phasehomogenization micro-thermometry for absolute temperature [5].In future years, combining these proxies provides the potential of regional-scale input into climate, weathering and the chemical cycling of elements, on timescales from thousands to millions of years.[1] C.C. Day et al.In press.Lithium isotopes and partition coefficients in inorganic carbonates: proxy calibration for weathering reconstruction.

To inform cost-effective monitoring of offshore geological storage of carbon dioxide (CO2), a unique field experiment, designed to simulate leakage of CO2 from a sub-seafloor storage reservoir, was carried out in the central North Sea. A total of 675 kg of CO2 were released into the shallow sediments (similar to 3 m below seafloor) for 11 days at flow rates between 6 and 143 kg d(-1). A set of natural, inherent tracers (C-13, O-18) of injected CO2 and added, non-toxic tracer gases (octafluoropropane, sulfur hexafluoride, krypton, methane) were used to test their applicability for CO2 leakage attribution and quantification in the marine environment. All tracers except O-18 were capable of attributing the CO2 source. Tracer analyses indicate that CO2 dissolution in sediment pore waters ranged from 35 % at the lowest injection rate to 41% at the highest injection rate. Direct measurements of gas released from the sediment into the water column suggest that 22 % to 48 % of the injected CO2 exited the seafloor at, respectively, the lowest and the highest injection rate. The remainder of injected CO2 accumulated in gas pockets in the sediment. The methodologies can be used to rapidly confirm the source of leaking CO2 once seabed samples are retrieved.