Carbon mass-balance modelling and carbon isotope exchange processes in dynamic caves (original) (raw)
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Modeling speleothem δ13C variability in a central Sierra Nevada cave using 14C and 87Sr/86Sr
Geochimica et Cosmochimica Acta, 2010
Carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate. Progressing downward from the soil zone overlying the cave, these processes include soil respiration, fluid-rock interaction in the host limestone, degassing of CO 2 and precipitation of calcite upflow from the speleothem drip site, and calcite precipitation at the drip site. Here we develop a new approach to independently constrain the roles of water-rock interaction and soil processes in controlling stalagmite d 13 C. This approach uses the dead carbon proportion (dcp) estimated from coupled 14 C and 230 Th/U measurements, in conjunction with Sr isotope analyses on stalagmite calcite from a central Sierra Nevada foothills cave in California, a region characterized by a highly seasonal Mediterraneantype climate, to determine the roles of water-rock interaction and soil processes in determining stalagmite d 13 C. Increases in stalagmite dcp between 16.5 and 8.8 ka are coincident with decreased d 13 C, indicating a varying yet substantial contribution from the soil organic matter (SOM) reservoir, likely due to significantly increased average age of SOM in the soil veneer above the cave during wet climatic intervals.
Modeling speleothem d 13 C variability in a central Sierra Nevada cave using 14 C and 87 Sr/ 86 Sr
Carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate. Progressing downward from the soil zone overlying the cave, these processes include soil respiration, fluid–rock interaction in the host limestone, degassing of CO 2 and precipitation of calcite upflow from the speleothem drip site, and calcite precipitation at the drip site. Here we develop a new approach to independently constrain the roles of water–rock interaction and soil processes in controlling stalagmite d 13 C. This approach uses the dead carbon pro-portion (dcp) estimated from coupled 14 C and 230 Th/U measurements, in conjunction with Sr isotope analyses on stalagmite calcite from a central Sierra Nevada foothills cave in California, a region characterized by a highly seasonal Mediterranean-type climate, to determine the roles of water–rock interaction and soil processes in determining stalagmite d 13 C. Incr...
Earth and Planetary Science Letters, 2008
Keywords: stalagmite carbon dioxide growth rates paleoclimate oxygen isotopes cave atmosphere climate proxies calcite deposition stable isotopes seasonality Cave air P CO 2 at two Irish sites varied dramatically on daily to seasonal timescales, potentially affecting the timing of calcite deposition and consequently climate proxy records derived from stalagmites collected at the same sites. Temperature-dependent biochemical processes in the soil control CO 2 production, resulting in high summer P CO 2 values and low winter values at both sites. Large Large-amplitude, high-frequency variations superimposed on this seasonal cycle reflect cave air circulation. Here we model stalagmite growth rates, which are controlled partly by CO 2 degassing rates from drip water, by considering both the seasonal and high-frequency cave air P CO 2 variations. Modeled hourly growth rates for stalagmite CC-Bil from Crag Cave in SW Ireland reach maxima in late December (0.063 μm h − 1 ) and minima in late June/early July (0.033 μm h − 1 ). For well-mixed 'diffuse flow' cave drips such as those that feed CC-Bil, high summer cave air P CO 2 depresses summer calcite deposition, while low winter P CO 2 promotes degassing and enhances deposition rates. In stalagmites fed by well-mixed drips lacking seasonal variations in δ 18 O, integrated annual stalagmite calcite δ 18 O is unaffected; however, seasonality in cave air P CO 2 may influence non-conservative geochemical climate proxies (e.g., δ 13 C, Sr/Ca). Stalagmites fed by 'seasonal' drips whose hydrochemical properties vary in response to seasonality may have higher growth rates in summer because soil air P CO 2 may increase relative to cave air P CO 2 due to higher soil temperatures. This in turn may bias stalagmite calcite δ 18 O records towards isotopically heavier summer drip water δ 18 O values, resulting in elevated calcite δ 18 O values compared to the 'equilibrium' values predicted by calcite-water isotope fractionation equations. Interpretations of stalagmite-based paleoclimate proxies should therefore consider the consequences of cave air P CO 2 variability and the resulting intra-annual variability in calcite deposition rates.
Simulated oxygen isotopes in cave drip water and speleothem calcite in European caves
Interpreting stable oxygen isotope (18O) records from stalagmites is still one of the complex tasks in speleothem research. Here, we present a novel model-based approach, where we force a model describing the processes and modifications of 18O from rain water to speleothem calcite (Oxygen isotope Drip water and Stalagmite Model – ODSM) with the results of a state-of-the-art atmospheric general circulation model enhanced by explicit isotope diagnostics (ECHAM5-wiso). The approach is neither climate nor cave-specific and allows an integrated assessment of the influence of different varying climate variables, e.g. temperature and precipitation amount, on the isotopic composition of drip water and speleothem calcite. First, we apply and evaluate this new approach under present-day climate conditions using observational data from seven caves from different geographical regions in Europe. Each of these caves provides measured 18O values of drip water and speleothem calcite to which we compare our simulated isotope values. For six of the seven caves modeled 18O values of drip water and speleothem calcite are in good agreement with observed values. The mismatch of the remaining caves might be caused by the complexity of the cave system, beyond the parameterizations included in our cave model. We then examine the response of the cave system to mid- Holocene (6000 yr before present, 6 ka) climate conditions by forcing the ODSM with ECHAM5-wiso results from 6 ka simulations. For a set of twelve European caves, we compare the modeled mid-Holocene-to-modern difference in speleothem calcite 18O to available measurements. We show that the general European changes are simulated well. However, local discrepancies are found, and might be explained either by a too low model resolution, complex local soil-atmosphere interactions affecting evapotranspiration or by cave specific factors such as non-equilibrium fractionation processes. The mid-Holocene experiment pronounces the potential of the presented approach to analyse 18O variations on a spatially large (regional to global) scale. Modelled as well as measured European 18O values of stalagmite samples suggest the presence of a strong, positive mode of the North Atlantic Oscillation at 6 ka before present, which is supported by the respective modelled climate parameters.
Journal of Cave and Karst Studies, 2015
Stable-isotope analyses are valuable in karst settings, where characterizing biogeochemical cycling of carbon along groundwater flow paths is critical for understanding and protecting sensitive cave and karst water resources. This study quantified the seasonal changes in concentration and isotopic composition (d 13 C) of aqueous and gaseous carbon species-dissolved inorganic carbon (DIC) and gaseous carbon dioxide (CO 2 )-to characterize sources and transfer of these species along a karst flow path, with emphasis on a cave environment. Gas and water samples were collected from the soil and a cave in northwestern Arkansas approximately once a month for one year to characterize carbon cycling along a conceptual groundwater flow path. In the soil, as the DIC concentration increased, the isotopic composition of the DIC became relatively lighter, indicating an organic carbon source for a component of the DIC and corroborating soil DIC as a proxy for soil respiration. In the cave, a positive correlation between DIC and surface temperature was due to increased soil respiration as the organic carbon signal from the soil was transferred to the cave environment via the aqueous phase. CO 2 concentration was lowest in the cave during colder months and increased exponentially with increasing surface temperature, presumably due to higher rates of soil respiration during warmer periods and changing ventilation patterns between the surface and cave atmosphere. Isotopic disequilibrium between CO 2 and DIC in the cave was greatest when CO 2 concentration was changing during November/ December and March/April, presumably due to the rapid addition or removal of gaseous CO 2 . The isotopic disequilibrium between DIC and CO 2 provided evidence that cave CO 2 was a mixture of carbon from several sources, which was mostly constrained by mixture between atmospheric CO 2 and soil CO 2 . The concentration and isotopic composition of gaseous and aqueous carbon species were controlled by month-to-month variations in temperature and precipitation and provided insight into the sources of carbon in the cave. Stable carbon isotope ratios provided an effective tool to explore carbon transfer from the soil zone and into the cave, identify carbon sources in the cave, and investigate how seasonality affected the transfer of carbon in a shallow karst system. -Using stable isotopes of carbon to investigate the seasonal variation of carbon transfer in a northwestern Arkansas cave.
Concentration and stable carbon isotopic composition of CO2 in cave air of Postojnska jama, Slovenia
2013
Partial pressure of CO 2 (pCO 2 ) and its isotopic composition (δ 13 C airCO2 ) were measured in Postojnska jama, Slovenia, at 10 locations inside the cave and outside the cave during a one-year period. At all interior locations the pCO 2 was higher and δ 13 C airCO2 lower than in the outside atmosphere. Strong seasonal fluctuations in both parameters were observed at locations deeper in the cave, which are isolated from the cave air circulation. By using a binary mixing model of two sources of CO 2 , one of them being the atmospheric CO 2 , we show that the excess of CO 2 in the cave air has a δ 13 C value of -23.3 ± 0.7 ‰, in reasonable agreement with the previously measured soil-CO 2 δ 13 C values. The stable isotope data suggest that soil CO 2 is brought to the cave by drip water.
Journal of Cave and Karst Studies
Cave air pCO 2 (carbon dioxide partial pressure) is, along with drip rate, one of the most important factors controlling speleothem carbonate precipitation. As a consequence, pCO 2 has an indirect but important control on speleothem proxy data (e.g., elemental concentrations, isotopic values). The CO 2 concentration of cave air depends on CO 2 source(s) and productivity, CO 2 transport through the epikarst and karst zone, and cave air ventilation. To assess ventilation patterns in the Bunker-Emst Cave (BEC) System, we monitored the pCO 2 value approximately 100 m from the lower entrance (Bunker Cave) at bi-hourly resolution between April 2012 and February 2014. The two entrances of the BEC system were artificially opened between 1860-1863 (Emst Cave) and 1926 (Bunker Cave). Near-atmospheric minimum pCO 2 dynamics of 408 ppmv are measured in winter, and up to 811 ppmv are recorded in summer. Outside air contributes the highest proportion to cave air CO 2 , while soil, and possibly also ground air, provide a far smaller proportion throughout the whole year. Cave air pCO 2 correlates positively with the temperature difference between surface and cave air during summer and negatively in winter, with no clear pattern for spring and autumn. Dynamic ventilation is driven by temperature and resulting density differences between cave and surface air. In summer, warm atmospheric air is entrained through the upper cave entrance where it cools. With increasing density, the cooled air flows toward the lower entrance. In winter, this pattern is reversed, due to cold, atmospheric air entering the cave via the lower entrance, while relatively warm cave air rises and exits the cave via the upper entrance. The situation is further modulated by preferential south-southwestern winds that point directly on both cave entrances. Thus, cave ventilation is frequently disturbed, especially during periods with higher wind speed. Modern ventilation of the BEC system-induced by artificially openings-is not a direct analogue for pre-1860 ventilation conditions. The artificial change of ventilation resulted in a strong increase of δ 13 C speleothem values. Prior to the cave opening in 1860, Holocene δ 13 C speleothem values were significantly lower, probably related to limited ventilation due to the lack of significant connections between the surface and cave. Reduced ventilation led to significantly higher pCO 2 values, minimal CO 2 degassing from drip water and low kinetic isotope fractionation. Both modern and fossil speleothem precipitation rates are driven by water supply and carbonate saturation, and not by cave air pCO 2. Today, pCO 2 variability is too small to affect carbonate precipitation rates and the same is likely true for pCO 2 variability prior to artificial opening of the cave. Thus, fossil speleothems from BEC System are likely more sensitive to temperature and infiltration dynamics. The Bunker-Emst Cave System, therefore, represents different ventilation patterns and their influence on speleothem proxy data in an exemplary manner, and it may serve as a template for other cave systems.
2018
This study provides the results of an investigation of the carbon isotopic composition in a CO2 cave atmosphere at Havranická Cave, Malé Karpaty Mts., Western Carpathians. The measured concentrations and δC values range between near-atmospheric (~ 612 ppmV and –7.38 ‰ PDB) values and depleted (~1039 ppmV and –13,5 ‰) values suggesting mixing of two contrasting CO2 sources. Other measured characteristics such as temperature, humidity and CO2 concentrations show trends related to depth, where temperature and humidity remain constant and the CO2 content increases with depth. We conclude that the carbon isotopic composition of cave air is a result of mixing of atmospheric and soil-derived CO2, degassed from dripwater. The contribution of carbon from limestone bedrock is possible, however, it cannot be confirmed with the current dataset.
Climate of the Past, 9, 99-118 (2013), 2013
""The interpretation of stable isotope ratios in speleothem calcite is complex, and only in a few cases, unequivocal relationships with palaeoclimate parameters have been attained. A major issue is temperature, which has an effect on both the isotope incorporation into calcite and on environmental processes. Here, a field approach is taken, by studying the isotopic composition of calcites from monitored caves located in steep altitudinal topography in the northern Italian Alps. These create a thermal gradient (3–12 °C) apt to study the effects of temperature on the speleothem isotope record. Our data indicate that the magnitude of oxygen isotope disequilibrium effects, calculated as an offset from the experimentally determined equilibrium, decreases with increased elevation (cooler temperatures) and faster drip rate. Carbon isotope values exhibit 13C enrichment at high altitudes (colder temperatures) and slow drip rates. The results obtained support modelling and laboratory cave analogue experiments that indicate temperature, drip rate, pCO2 and supersaturation are important factors controlling stable isotope fractionation, but also stress the significance of ventilation and evaporation in the cave environment. It is proposed that the effects on stable isotope ratios observed along the altitudinal gradient can be analogues for glacial to interglacial temperature changes in regions which were extensively glaciated in the past.