Abstract In southern Belgium, 23% of abstracted groundwater volumes are from chalk aquifers, representing strategic resources for the region. Due to their specific nature, these chalk aquifers often exhibit singular behaviour and require specific analysis. The quantitative evolution of these groundwater resources is analysed for the Mons Basin and Hesbaye chalk aquifers as a function of past evolution, in the short and long term. Groundwater level time series exhibit decreases when analysed over different periods. This is particularly visible for the Hesbaye chalk aquifer when comparing the 1960–90 and 1990–2020 periods. Such decreases are associated with observed temperature increases and precipitation decreases, inducing a decrease of aquifer recharge, and a probable increase of groundwater abstraction in the adjacent catchment. Past evolution is also discussed considering recent winter and summer drought events. The aquifers exhibit long delays in response to recharge events, particularly where the thickness of the partially saturated zone plays a crucial role in observed delays. Regarding future evolution, simulations of the impact of climate changes using medium–high emission scenarios indicate a probable decrease of the groundwater levels over the Hesbaye chalk aquifer.

Abstract A series of vegetation changes take place in tropical ecosystems during the Pennsylvanian Subperiod. The most notable change, recognizable from palynology and plant macrofossils at the Middle and Late Pennsylvanian boundary in the Illinois Basin, is the extirpation, or local extinction, of certain lineages of arborescent lycopsids, followed by their replacement by stem group marattialean tree ferns. The leading hypothesis suggests a significant change in precipitation regime as the cause. To test this hypothesis, we examine the vascular anatomy and physiology of key lineages of Pennsylvanian plants: the sphenopsids, tree ferns, cordaitaleans, medullosans, lycophytes and extrabasinal stem group coniferophytes. Using scanning electron and light microscopy of fossilized anatomy, we provide new data on these plants’ vascular systems, quantifying their physiological capacity and drought resistance. We find that three Pennsylvanian plant lineages – the medullosans, arborescent lycopsids and Sphenophyllum – contain high hydraulic conductivity but are vulnerable to drought-induced damage, whereas others are resistant, including stem group tree ferns and coniferophytes. Relative abundance changes among these plants were likely driven by drought, and differences in water use efficiency would have amplified drought events as plant communities changed. The interaction of physiological selectivity and positive feedback between aridity and drought tolerance likely played a significant role in Late Paleozoic floral changes.

Abstract For the first time, we present a decadal-scale stable isotope record (δ 18 O, δ 13 C) of 67 speleothem calcite samples coming from an artificial tunnel network located in Graz, Austria. Stable isotope data are interpreted with the help of time series (TS) analysis of mean air temperatures (MAT) and mean annual precipitations (MAP) that have been monitored and recorded in a neighbouring meteorological station. Speleothem records have proved to be very useful in reconstructing changes of environmental conditions. For studied stalagmites, which grew between 1945 and 2018, the δ 18 O values average −18.64‰ and range from −23‰ to −17‰ (VPDB, Vienna Pee Dee Belemnite), suggesting variable climatic conditions. The δ 18 O values of calcite increase along the growth axis and are correlated with high temporal resolution MAT, MAP and weighted mean annual δ 18 O of precipitations. For the same time interval, while the temperature TS show an increasing trend, with a steeper gradient since the 1980s, the precipitation TS presents a weak decreasing tendency. Increase in the δ 13 C values of speleothems from −33‰ to −24‰ (VPDB) is correlated with increasing temperature and drought, associated CO 2 degassing and soil erosion over the tunnel system.

ABSTRACT We extend a published 9000 yr fire history record from Little Lake, in the Oregon Coast Range, to 35,000 yr and compare it with the established pollen record from the site. The fire history is based on a high-resolution analysis of charcoal preserved in lake sediments, providing a fire history record that spans the Last Glacial Maximum in North America. The data enabled us to address questions regarding the interactions between large-scale climate changes associated with the shift from glacial to interglacial conditions and the accompanying changes in forest vegetation and fire regimes. The vegetation history indicates a change from open subalpine forests to closed western hemlock and Douglas fir forests as climate moved from cold and dry full glacial to warm and wet Holocene conditions. The fire history indicates that although there was more biomass burned in the Holocene, the frequency of fires between glacial and interglacial conditions was not significantly different, and the fire frequency did not change in concert with regional shifts in vegetation. This suggests that fire is a product of seasonal or multiyear variations in climate that may not cause significant shifts in vegetation. Also, as this short-term climate variability becomes more common in the near future, conditions for fires in these mesic forests may become more common as well.

ABSTRACT Shorelines formed by terminal lakes record past changes in regional moisture budgets. In the western Great Basin of North America, winter precipitation accounts for nearly half of the annual total and is well correlated with northeast Pacific storm track activity and moisture transport. We evaluated these relationships and found that historical precipitation between 1910 and 2012 was better correlated to moisture transport (0.78, p < 0.01) than to storm track activity (0.54, p < 0.01) because moisture transport better captures dynamics associated with the Sierra Nevada rain shadow. We derived modern analogs of enhanced and reduced storm track activity and moisture transport from reanalysis products and used associated winter precipitation anomalies with these analogs as inputs to a coupled water balance and lake evaporation model of the Walker Lake basin. Simulated lake-level responses were compared with a radiocarbon-dated lakeshore chronology spanning the past 3700 yr. Wet analogs developed from winters in the 90th and 75th percentiles for storminess and moisture transport produced lake levels that exceeded estimated late Holocene highstands by 50 m. Dry analogs (10th and 25th percentiles) produced lake levels corresponding to Medieval megadrought lowstands. The twentieth century is shown to be as wet as any century in the past 3700 yr. Our results demonstrate the sensitivity of terminal lakes to winter season circulations and highlight the value of using moisture transport as a predictor of cool season precipitation and to evaluate how past or future changes in regional circulations will influence the water balance of dryland regions.

ABSTRACT While drought represents a serious threat to the Pacific southwestern United States, floods represent an equally formidable threat. This risk is so significant that the U.S. Geological Survey created the ARkStorm Project. This project aims to prepare California for a future storm(s) on the scale of the disastrous A.D. 1861–1862 events. Unfortunately, our knowledge of premeasurement floods in the Pacific southwestern United States is sparse. To date, the best paleoflood record consists of flood layers in the Santa Barbara Basin, spanning the past 9000 calendar yr B.P. (cal yr B.P.). As an alternative to marine archives, the lakes of the Pacific southwestern United States represent untapped resources for possible premeasurement flood reconstructions. Here, we present evidence for a flood between ca. 4860 and 4820 cal yr B.P. using sediment from Lake Elsinore core LEGC03-4. Core LEGC03-4 is predominantly clayey silt with occasional sandy silt units of variable centimeter-scale thickness. Here, we focus on a specific core section between 350 and 315 cm, where an ~11-cm-thick “unusual” sediment unit (330–319 cm) is well preserved and complete. The core section was analyzed for a variety of physical and chemical properties, including magnetic susceptibility, loss-on-ignition (LOI) at 550 °C and 950 °C, grain size, C org :N total ratios, and δ 13 C (bulk organic matter) . The unit is characterized by an erosional basal contact and microflame structures. It is normally graded, with laminae occurring in the upper section of the unit. It contains predominantly terrestrial organic matter, and the upper boundary is gradational. It is coeval with the fourth highest sand peak in a previously dated central basin core. Consequently, it is our conclusion that the unusual sediment unit represents a turbidite associated with a large flood-producing precipitation event with a maximum limiting age between 4860 and 4820 cal yr B.P.

ABSTRACT This paper summarizes the hydrological variability in eastern California (central Sierra Nevada) for the past 3000 yr based on three distinct paleoclimate proxies, δ 18 O, total inorganic carbon (TIC), and magnetic susceptibility (chi). These proxies, which are recorded in lake sediments of Pyramid Lake and Walker Lake, Nevada, and Mono Lake and Owens Lake, California, indicate lake-level changes that are mostly due to variations in Sierra Nevada snowpack and rainfall. We evaluated lake-level changes in the four Great Basin lake systems with regard to sediment-core locations and lake-basin morphologies, to the extent that these two factors influence the paleoclimate proxy records. We documented the strengths and weaknesses of each proxy and argue that a systematic study of all three proxies together significantly enhances our ability to characterize the regional pattern, chronology, and resolution of hydrological variability. We used paleomagnetic secular variation (PSV) to develop paleomagnetic chronostratigraphies for all four lakes. We previously published PSV records for three of the lakes (Mono, Owens, Pyramid) and developed a new PSV record herein for Walker Lake. We show that our PSV chronostratigraphies are almost identical to previously established radiocarbon-based chronologies, but that there are differences of 20–200 yr in individual age records. In addition, we used eight of the PSV inclination features to provide isochrons that permit exacting correlations between lake records. We also evaluated the temporal resolution of our proxies. Most can document decadal-scale variability over the past 1000 yr, multidecadal-scale variability for the past 2000 yr, and centennial-scale variability between 2000 and 3000 yr ago. Comparisons among our proxies show a strong coherence in the pattern of lake-level variability for all four lakes. Pyramid Lake and Walker Lake have the longest and highest-resolution records. The δ 18 O and TIC records yield the same pattern of lake-level variability; however, TIC may allow a somewhat higher-frequency resolution. It is not clear, however, which proxy best estimates the absolute amplitude of lake-level variability. Chi is the only available proxy that records lake-level variability in all four lakes prior to 2000 yr ago, and it shows consistent evidence of a large multicentennial period of drought. TIC, chi, and δ 18 O are integrative proxies in that they display the cumulative record of hydrologic variability in each lake basin. Tree-ring estimations of hydrological variability, by contrast, are incremental proxies that estimate annual variability. We compared our integrated proxies with tree-ring incremental proxies and found a strong correspondence among the two groups of proxies if the tree-ring proxies are smoothed to decadal or multidecadal averages. Together, these results indicate a common pattern of wet/dry variability in California (Sierra Nevada snowpack/rainfall) extending from a few years (notable only in the tree-ring data) to perhaps 1000 yr. Notable hydrologic variability has occurred at all time scales and should continue into the future.