Abstract
The controls that affect the structure and timing of terminations are still poorly understood. We studied a tufa deposit from the Iberian Peninsula that covers Termination II (T-II) and ...whose chronology was synchronized to speleothem records. We used the same chronology to synchronize ocean sediments from the North Atlantic to correlate major climate events in a common timescale. We identify two stages within T-II. The first stage started with the increase of boreal summer integrated solar insolation, and during this stage three millennial climate oscillations were recorded. These oscillations resulted from complex ocean–atmosphere interactions in the Nordic seas, caused by the progressive decay of Northern Hemisphere ice-sheets. The second stage commenced after a glacial outburst that caused the collapse of the Thermohaline Circulation, a massive Heinrich event, and the onset of the Bipolar Seesaw Mechanism (BSM) that eventually permitted the completion of T-II. The pace of the millennial oscillations during the first stage of T-II controlled the onset of the second stage, when the termination became a non-reversible and global phenomenon that accelerated the deglaciation. During the last the two terminations, the BSM was triggered by different detailed climate interactions, which suggests the occurrence of different modes of terminations.
Rock tablets of known weight were buried in the soil of a karst region in Central Spain to evaluate the carbonate weathering during a period of a year. The experiment was conducted at two different ...soil depths: 5–10 and 50–55cm from the surface. The parental rock used in the experiment is composed of dolomite and magnesite with variable proportion of accessory minerals and minor elements. Soil mineral and chemical composition as well as its texture was also characterized. Meteorological conditions at the site together with temperature and CO2 in both soil levels were monitored. Sets of tablets were retrieved after 6 and 12months of the start of the experiment to account for seasonal weathering. Different lithologies do not exhibit significant differences in weathering, although a large inter-sample variability is attributed to variable size and distribution of the porosity. Results show an enhanced weathering during the wet and cold season that accounts for 78±1% of the total annual weathering. Rock tablets examined under scanning electron microscopy prior and after exposure to natural environment show that most of the material lost occurred along cracks, edges or large pores. Although dissolution is a common process, most of the weathering is due to crystal detachment. Rock tablets at the depth of 5–10cm were weathered 68±1% more than those set at 50–55cm from the surface. Higher soil moisture and concentration of CO2 were found deeper in the soil, which likely enhanced the dissolution of carbonate. However, physical weathering dominated weight loss of rock tablets at both soil depths; especially at the 5–10cm level where soil thermal and moisture cycles were more frequent and greater. Denudation rate calculated from the 12months set provides values of 2.48±1.07μm/yr and 1.75±0.66μm/yr at the depths of 5–10 and 50–55cm, respectively. Since the conditions at the average contact between soil and bedrock are similar to those at the 50–55cm depth, we consider that this is a more reliable denudation rate for the studied location during the studied period. The calculated weathering rate suggests that denudation has a limited contribution to the thinning of bedrock over caves at this site. Therefore, we consider that the formation of unroofed caves in this region most likely results from the thinning of bedrock cover over caves due to collapse of blocks from their ceilings.
•We perform a rock tablets experiment to measure subsurface weathering of carbonates.•Physical and chemical weathering account for denudation of karst regions.•Thermal and moisture cycles cause enhanced physical weathering of carbonate rocks.•Higher subsurface weathering of carbonates occurs during wet and cold periods.•In the studied region unroofed caves are unlikely to result from denudation.
•Moisture sources of precipitation were calculated in central Europe during 2009–2013.•The impact of moisture sources on δ18O composition of precipitation was quantified.•Moisture sources do not ...contribute significantly to the δ18O values of precipitation.•The δ18O variability is dominated by temperature.
The stable isotope composition of precipitation records processes taking place within the hydrological cycle. Potentially, moisture sources are important controls on the stable isotope composition of precipitation, but studies focused on this topic are still scarce. We studied the moisture sources contributing to precipitation at Postojna (Slovenia) from 2009 to 2013. Back trajectory analyses were computed for the days with precipitation at Postojna. The moisture uptake locations were identified along these trajectories using standard hydrometeorological formulation. The moisture uptake locations were integrated in eight source regions to facilitate its comparison to the monthly oxygen isotope composition (δ18O values) of precipitation. Nearly half of the precipitation originated from continental sources (recycled moisture), and >40% was from central and western Mediterranean. Results show that moisture sources do not have a significant impact on the oxygen isotope composition at this site. We suggest that the large proportion of recycled moisture originated from transpiration rather than evaporation, which produced water vapour with less negative δ18O values. Thus the difference between the oceanic and local vapour source was reduced, which prevented the distinction of the moisture sources based on their oxygen isotope signature. Nevertheless, δ18O values of precipitation are partially controlled by climate parameters, which is of major importance for paleoclimate studies. We found that the main climate control on Postojna δ18O values of precipitation is the surface temperature. Amount effect was not recorded at this site, and the winter North Atlantic Oscillation (NAO) does not impact the δ18O values of precipitation. The Western Mediterranean Oscillation (WeMO) was correlated to oxygen stable isotope composition, although this atmospheric pattern was not a control. Instead we found that the link to δ18O values results from synoptic scenarios affecting WeMO index as well as temperature. Therefore, interpretation of δ18O values of precipitation in terms of climate is limited to surface temperature, although at least half of the variability observed still depends on unknown controls of the hydrological cycle.
Rock weathering is a key component of the rock cycle that transforms rocks in sediments, soils and/or dissolves minerals to ions, and plays an important role in landforms development and landscape ...evolution. The quantification of weathering is often challenging due to the impact of measuring techniques on the rock itself and/or the assumptions required to perform the calculations. Among different methods used to quantify rock weathering, rock tablet method is one of the most widely used. The rock tablet method consists of cutting rock samples in a shape that allows precise calculation of their volume and surface. These rock pieces, often named tablets when they are flat, are dried, precisely weighed and exposed to the natural environment for certain amount of time. After retrieval, the same laboratory procedure is repeated, and from the weight loss, the weathering rate is calculated. When the density of the rock is known and it is reasonable to assume that the lost material is removed from the ground surface, the weight loss can be used to calculate denudation rates. This is the case in karst regions, where rock tablets are one of the most commonly used methods to calculate denudation. This simple and versatile method requires only a limited amount of laboratory equipment to be implemented which makes it very popular. However, experimental design, preparation of samples and calculations are not homogeneous among researchers, sometimes making the results of this method challenging to be evaluated and compared. In this paper we describe the rock tablet method, include its historical overview, review the most significant studies that have used this technique, highlight its limitations, introduce correction factors to minimize some of those limitations, and provide good practice recommendations to help other researchers to obtain comparable results around the world.
The 5-day reconstruction of air mass history for the days with precipitation at Eagle Cave (central Spain), together with the determination of moisture uptake locations along back trajectories, is ...used to identify the moisture sources of precipitation for this site from 2009 to 2011. During this 3-year period, around 30% of precipitation at Eagle Cave originated from the moisture recycled within the Iberian Peninsula (IP), with the Proximal Atlantic being also a main source region of moisture, whereas the Mediterranean Sea and the Distal Atlantic have large variability during the studied period and other source regions are minor precipitation contributors. The comparison of monthly oxygen isotope composition of precipitation at Eagle Cave with the monthly percentage of precipitation originated in source regions shows a significant negative correlation for the IP region. Thus, the moisture recycling process in the IP region explains 12% of the variability of the monthly oxygen isotope composition of precipitation (p-value<0.1). However, when temperature, amount of precipitation and all source regions are considered, 74% of the variability of the monthly oxygen isotope composition of precipitation is explained (p-value<0.05). Therefore, although for Eagle Cave amount of precipitation and temperature are main contributors of the precipitation oxygen isotope variability (r2=0.54; p-value<0.001), the moisture uptake distribution among source regions is a substantial control and should be considered when interpreting oxygen isotope speleothem records from this cave. This research highlights the importance of moisture sources in controlling the oxygen isotope composition of precipitation in mid-latitudes, and the need of quantification of this effect in order to improve the interpretation of oxygen isotope paleo-records that depend on isotope composition of precipitation.
This paper focuses on the evolution and patterns of microscale weathering forms and dissolution rates of “standard” (Lipica) limestone tablets. Analysis of carbonate weathering using combination of ...methods (quantitative analysis by the weight loss of "standard" tablets, and qualitative analysis of the weathered surfaces by stained acetate peels and SEM imaging) showed that dissolution takes place not only at the surface of limestone tablets, but also along voids and cavities in limestone tablets which makes total weathering surface larger than the area of the tablet surface. Dissolution is more pronounced on the micritic calcite surfaces (due to different dissolution kinetics of carbonate minerals), resulting in lowering of the surface (calcite matrix) which causes gradual unburial and removal of authigenic dolomite grains.
•A novel soil hydrological reservoir cascade scheme model was developed.•Model simulates soil water content (SWC) following various configurations.•Model considers a general matrix flow and two ...preferential flow regimes.•Simulated SWC values are compared with measured SWC values.•Preferential flows significantly contribute to recharge in karst areas.
Models provide quantitative solutions that enable better understanding of karst soils hydrology. A location in central Spain was selected to implement a 1-D hydrological model that uses a reservoir cascade scheme (RCS) approach to simulate soil water content (SWC) at daily resolution. The model simulates SWC for six soil layers and includes additional modules to reproduce preferential flow regime. The soil profile in the model is 0.5 m deep, developed in silt loam soil on top of dolomite marbles. Three different configurations of the model were considered. Configuration 1 considers a basic RCS module, whereas other configurations include preferential flows. Thus, Configuration 2 considers RCS module together with a continuous preferential flow module, where 1 to 5% of available SWC from each layer is drained every day along preferential pathways. Configuration 3 contains a discontinuous preferential flow module in addition to previous modules. The discontinuous preferential flow is only activated during rainfall events occurring after long dry periods. Simulated SWC values are compared with measured SWC values, so model parameters are iteratively adjusted to optimize the model results. The model produces the best simulations implementing the Configuration 3, when matrix flow and two preferential flow regimes are considered. The model shows that preferential flows significantly contribute to recharge and should not be neglected in soil hydrological models and recharge estimations in karst areas.
•Continuous Terra Rossa soil monitoring provided data for a thermodynamic model.•Dissolution and precipitation of calcite in the soil was modelled at daily resolution.•The model suggests pedogenic ...carbonates being dissolved and precipitated seasonally.•Calcite nodules showing precipitation and dissolution events support the simulations.•Simulations suggest a variable threshold for inhibition of calcite precipitation in soils.
Red Mediterranean Soils developed over carbonate rocks can precipitate pedogenic carbonates at the base of their profiles, forming an evolutionary stage of calcretes. We studied a 0.6 m deep soil profile in Dalmatia (Croatia) where the lowest section of the soil is a calcic horizon formed by diffuse calcite particles and small nodules. These nodules record different events of dissolution and precipitation. Using a 3-month period of soil environmental monitoring, where soil temperature, soil water content, soil bulk electrical conductivity and soil air CO2 were measured, we implemented a thermodynamic model for dissolution and precipitation of calcite in the soil. The simulation shows a stage dominated by calcite dissolution during spring and early summer followed by a stage dominated by calcite precipitation. Soil air CO2 is the main control of the calcite reactions, with concentration of solutes also being of some importance during the stage of calcite precipitation. Soil water content and temperature, both affect soil air CO2. Precipitation events also have high-frequency impact on soil air CO2, but the response is complex. The model enables soil water solutions to be supersaturated in relation to calcite without precipitation of that mineral. Simulated soil water reached variable values of supersaturation in relation to calcite before its precipitation was triggered, suggesting that there is no fixed threshold value for the precipitation of calcite in a particular soil. During the 3-month simulated period, 83% of the calcite dissolved was reprecipitated as pedogenic carbonate. Most of the initially dissolved calcite, was expected to be diffuse pedogenic carbonate particles, although nodules and bedrock at the base of the soil should also have contributed to the solutes dissolved. Therefore, karst landscapes where soils have pedogenic carbonates are expected to record less denudation rates than those regions without calcretes.
The relatively stable temperature of most cave interiors is caused by the transfer of the surface atmosphere temperature signal to the karst underground by thermal conduction. Transferring ...underground thermal signals by conduction implies the amplitude attenuation of the external thermal anomaly as well as a signal delay. The magnitude of these attenuation and delay effects is proportional to the thickness of bedrock cover above the cave and is controlled by the bedrock thermal diffusivity. Here we present 5-year temperature record of Los Pilones Cave, in central Spain, where advection has a limited control on thermal anomalies all year round. Thus, thermal conduction dominates the thermal variability in the cave, with simulations of a 1-D thermal conduction model explaining up to 94% of the variability observed. Thermal attenuation and signal delays are highly correlated with the thickness of bedrock cover (r2 = 0.95 in both cases), enabling the calculation of a thermal diffusivity of 5.07*10−7 ±1.27*10−7 m2/s. The calculated thermal diffusivity is in the range of available values measured in laboratory for carbonate lithologies, although differs beyond uncertainty from the limited number of studies that calculated this value from field observations in karst. Thus, local rock properties significantly impact thermal diffusivities, and more experimental studies are required to show a complete distribution of this value in karst settings. The reported uncertainty, that was calculated using different time windows within the studied period, represents a variability of 23% on the thermal diffusivity. Karst terrains are mostly composed of three phases, rock, air and water, that have different thermal properties. Thus, variability of the water content in the rock porosity through time is likely a significant control on the measured uncertainty. Since temperature in the studied cave has a minor component affected by advection, this cave represents a paradigmatic location to observe the important role of thermal conduction in caves. Therefore, conclusions from this study can be useful to better understand thermal regimes and interannual trends of most cave interiors despite occurrence of variable impacts of seasonal ventilation dynamics in cave temperature records. In addition, the mechanism of thermal conduction results in a thermal decoupling between the surface and cave atmospheres affecting cave microclimate, which has important implications for multiple disciplines interested in karst underground environments.
•The transfer of external temperature to caves is dominated by thermal conduction.•Multiple cave thermal signals were used to calculate thermal diffusivity in a cave.•Thickness of bedrock cover above caves is a key control on cave temperature.•The transfer of thermal signals by conduction implies a delay of thermal anomalies.•Thermal conduction results in decoupling between external and cave air signals.
PDF
