Oliver Kost

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Kost

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Oliver

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0000-0001-6781-3557

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Publications1 - 10 of 10

Distinguishing the combined vegetation and soil component of δ13C variation in speleothem records from subsequent degassing and prior calcite precipitation effects
Stoll, Heather M.; Day, Chris; Lechleitner, Franziska; et al. (2023)
Climate of the Past
The carbon isotopic signature inherited from soil and epikarst processes may be modified by degassing and prior calcite precipitation (PCP) before its imprint on speleothem calcite. Despite laboratory demonstration of PCP effects on carbon isotopes and increasingly sophisticated models of the governing processes, to date, there has been limited effort to deconvolve the dual PCP and soil-epikarst components in measured speleothem isotopic time series. In this contribution, we explore the feasibility, advantages, and disadvantages of using trace element ratios and δ⁴⁴Ca to remove the overprinting effect of PCP on measured δ¹³C to infer the temporal variations in the initial δ¹³C of drip water prior to degassing and PCP. In nine examined stalagmites, the most widely utilized PCP indicators Mg/Ca and δ⁴⁴Ca covary as expected. However, Srg/gCa does not show consistent relationships with δ⁴⁴Ca so PCP is not the dominant control on Srg/gCa. From δ⁴⁴Ca and Mg/Ca, our calculation of PCP as f_Ca, the fraction of initial Ca remaining in solution at the time the stalagmite layer is deposited, yields multiple viable solutions depending on the assumed δ⁴⁴Ca fractionation factor and inferred variation in DMg. Uncertainty in the effective fractionation of δ¹³C during degassing and precipitation contributes to uncertainty in the absolute value of estimated initial δ¹³C. Nonetheless, the trends in initial δ¹³C are less sensitive to these uncertainties. In coeval stalagmites from the same cave spanning the 94 to 82 ka interval, trends in calculated initial δ¹³C are more similar than those in measured δ¹³C and reveal a common positive-Anomaly initial δ¹³C during a stadial cooling event. During deglaciations, calculated initial δ¹³C implies a trend of greater respiration rates and higher soil CO₂, although the higher interglacial drip water saturation favors more extensive PCP. Initial δ¹³C can be estimated for active and fossil speleothems from a range of settings, wherever there is confidence that Mg/Ca and/or δ⁴⁴Ca provides a quantitative indication of past changes in PCP. Further study of Mg partitioning in speleothems will improve the robustness of Mg/Ca as a PCP proxy.
Rapid northern hemisphere ice sheet melting during the penultimate deglaciation
Stoll, Heather M.; Cacho, Isabel; Gasson, Edward; et al. (2022)
Nature Communications
The rate and consequences of future high latitude ice sheet retreat remain a major concern given ongoing anthropogenic warming. Here, new precisely dated stalagmite data from NW Iberia provide the first direct, high-resolution records of periods of rapid melting of Northern Hemisphere ice sheets during the penultimate deglaciation. These records reveal the penultimate deglaciation initiated with rapid century-scale meltwater pulses which subsequently trigger abrupt coolings of air temperature in NW Iberia consistent with freshwater-induced AMOC slowdowns. The first of these AMOC slowdowns, 600-year duration, was shorter than Heinrich 1 of the last deglaciation. Although similar insolation forcing initiated the last two deglaciations, the more rapid and sustained rate of freshening in the eastern North Atlantic penultimate deglaciation likely reflects a larger volume of ice stored in the marine-based Eurasian Ice sheet during the penultimate glacial in contrast to the land-based ice sheet on North America as during the last glacial.
Relationship of seasonal variations in drip water δ13CDIC, δ18O, and trace elements with surface and physical cave conditions of La Vallina cave, NW Spain
Kost, Oliver; González-Lemos, Saúl; Rodríguez-Rodríguez, Laura; et al. (2023)
Hydrology and Earth System Sciences
Cave-monitoring studies clarify the climatic, surface vegetation, and karst processes affecting the cave system and lay the foundation for interpreting geochemical stalagmite records. Here we report the monitoring of cave air, bedrock chemistry, and drip water δ13CDIC, δ18O, and δD, as well as 16 trace elements, covering a full annual cycle spanning the 16 months between November 2019 and March 2021 in La Vallina cave in the northwestern Iberian Peninsula. While decreased rainfall and increased evapotranspiration in the summer months lead to a strong reduction in drip rates, there is little seasonal variation in δ18O and δD in a given drip, likely reflecting the discrete moderately mixed to well-mixed karst water reservoirs. Small differences in δ18O and δD between drip sites are attributed to variable evaporation intensity and/or transit times. The carbon isotope signature of the dissolved inorganic carbon of drip water (δ13CDIC) is likely driven by seasonal changes in the temperature controlling biological processes (vegetation and microbial soil activity), resulting in minimum δ13CDIC in summer and autumn months. Increased bedrock dissolution due to higher soil pCO2 in summer and autumn results in increased trace element concentrations of congruently dissolved elements. Cave air measurements (pCO2, δ13Cair, and temperature) indicate the seasonal ventilation (winter) and stagnation (summer) of cave air. The opposite effects of reduced cave air pCO2, seasonally variable biological activity, and increased drip rate limit the extent of the seasonal variation in degassing and prior calcite precipitation (PCP) supported by trace elements ( index). Estimated stalagmite growth rates using monitoring data suggest biannual phases of potential calcite precipitation in summer and winter and growth cessation during spring and autumn, depending on cave and drip water conditions and the location within the cave, which has important implications for the proxy interpretation of stalagmite records.
Iberian Margin surface ocean cooling led freshening during Marine Isotope Stage 6 abrupt cooling events
Zhang, Hongrui; Huang, Yongsong; Wijker, Reto; et al. (2023)
Nature Communications
The high-resolution paleoclimate records on the Iberian Margin provide an excellent archive to study the mechanism of abrupt climate events. Previous studies on the Iberian Margin proposed that the surface cooling reconstructed by the alkenone-unsaturation index coincided with surface water freshening inferred from an elevated percentage of tetra-unsaturated alkenones, C37:4%. However, recent data indicate that marine alkenone producers, coccolithophores, do not produce more C37:4 in culture as salinity decreases. Hence, the causes for high C37:4 are still unclear. Here we provide detailed alkenone measurements to trace the producers of alkenones in combination with foraminiferal Mg/Ca and oxygen isotope ratios to trace salinity variations. The results indicate that all alkenones were produced by coccolithophores and the high C37:4% reflects decrease in SST instead of freshening. Furthermore, during the millennial climate changes, a surface freshening did not always trigger a cooling, but sometimes happened in the middle of multiple-stage cooling events and likely amplified the temperature decrease.
Climatic and cave settings influence on drip water fluorescent organic matter with implications for fluorescent laminations in stalagmites
Endres, Laura; Jacquin, Céline; González-Lemos, Saúl; et al. (2024)
Quaternary Research
Speleothem fluorescence can provide insights into past vegetation dynamics and stalagmite chronology. However, its origin and especially the formation of fluorescent laminations in stalagmites are poorly understood. We conducted a year-long monthly monitoring of drip water fluorescence in La Vallina Cave (northern Iberian Peninsula) and compared the results to drip water chemistry and active speleothems from the same sites. Drip waters were analyzed using fluorescence spectroscopy and parallel factor analysis (PARAFAC). The resulting five-component model indicates contributions from vegetation, microbial activity, and bedrock. Intra-site fluorescence variability is mainly influenced by changes in overlying vegetation, water reservoir time, and respiration rates. Contrary to prevailing views, we find no systematic increase in drip water fluorescence during rainy conditions across drip sites and seasonal variations in drip water fluorescence are absent at a location where present-day speleothem layers form. Our findings challenge the notion of a higher abundance of humic-like fluorescence during the rainy season as the primary cause for layer formation and suggest additional controls on drip water fluorescence, such as bedrock interaction and microbial reprocessing. We also propose that growth rate may control the dilation of the fluorescence signal in stalagmites, indicating other potential mechanisms for fluorescent layer formation.
Stalagmite carbon isotopes suggest deglacial increase in soil respiration in western Europe driven by temperature change
Lechleitner, Franziska A.; Day, Christopher C.; Kost, Oliver; et al. (2021)
Climate of the Past
The temperate region of western Europe underwent significant 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 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 the NW Iberian Peninsula 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. The coupling of soil gas pCO2 and δ13C via a mixing line describing diffusive gas transport between an atmospheric and a respired end-member allows the 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 gas 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.
Marine aerosols in coastal areas and their impact on cave drip water – A monitoring study from Northern Spain
Kost, Oliver; Stoll, Heather (2023)
Atmospheric Environment
Marine aerosols affect coastal environments and human infrastructure, hence assessing the marine aerosol deposition in coastal areas is important. Field observations of marine aerosol deposition in coastal regions is commonly limited to a single station at high temporal resolution, which does not provide strong constraints on the spatial changes in the rate of aerosol delivery over the first 10 km distance from the coast, a gradient with the potential to constrain scavenging terms in aerosol models. Here, we provide data of monthly marine aerosol deposition (wet and dry) within the first 10 km from the coast during a 15-month monitoring study. Along a transect, a strong decrease of marine aerosols (Na, Mg, S and K) in the first 3 km from the coast but little decrease between 3 and 10 km is found. Based on fluxes of Na, the most conservative component measured and most completely attributed to marine aerosol sources, overall dry aerosol deposition decreases by nearly 20 to 30-fold from the coast to the station 10 km inland. Wet and dry marine aerosol deposition is significantly increased during the windy season (October until March), which is also the season of higher precipitation. The wind effect on aerosol deposition was greatest at the coast and diminished inland: between October and November 2019, the 3-fold increase in cumulative wind speed above the whitecap threshold caused a nearly 10-fold increase in the most proximal site, a 4-fold increase in 3.2 km sites, but only a 2-fold increase in dry aerosol deposition in the most distal sites. Independent of the season, the aerosol deposition rate >3 km from the coast drops to less than 20% of the most coast proximal station. Comparison of elemental ratios with seawater composition confirms the marine origin of Na and Mg in aerosols, but indicates additional sources and sinks for other elements (S and K) which do not correspond with expected marine sources. Furthermore, we test the time-integrated aerosol deposition in shallow groundwaters intercepted as cave drip waters from 13 caves, which confirm the decreasing contribution with distance from the coast but also reveal additional effects on the natural capture of wet and dry aerosols such as wind shielding effects and vegetation cover. The Na/Mg ratio of drip waters can be used to assess the significance of marine aerosols vs. bedrock to cave waters in the present day.
The influence of fluid inclusions, organics, and calcite fabric on trace element distributions in stalagmites
Kost, Oliver; Sliwinski, Jakub; Gies, Nils B.; et al. (2023)
Frontiers in Earth Science
Trace elements and calcite fabrics in stalagmites are increasingly employed as indicators of past environmental conditions. The drivers of trace element concentrations and fabrics reflect a complex interplay of factors during crystal growth, and accurate interpretation of geochemical proxies requires a clear understanding of the incorporation of the proxy signal in the stalagmite. We employ a series of non-destructive techniques to compare the spatial distribution of water, organic components, and trace elements and in relationship with calcite fabrics, and evaluate if these distributions are consistent with recent hypotheses about controlling factors on crystal fabrics, fluid inclusions, and element incorporation. We use for the first time Fourier Transformation Infrared Focal-Plane Array (FTIR-FPA) imaging to show the distribution of molecular water (3,400 cm-¹ absorbance) and organic matter (2,983 cm-¹) in calcite. To test if entrapped water in speleothem calcite affects the abundance or distribution of trace elements, the element distribution (Na, Mg, Sr, U, Al, Y, and Zn) is mapped by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Transmitted Light Microscope imaging constrains crystal fabrics. We find that stalagmite areas with strong Organic Matter fluorescence via Confocal Laser Scanning Microscopy coincide with areas of high Fourier Transformation Infrared absorbance at 2,983 cm-¹, suggesting common co-location of multiple organic matter types in the stalagmite. While microscopic fluid inclusions visible in transmitted light are also detected by FTIR 3400 cm-¹ absorbance, the FTIR additionally detects submicroscopic fluid inclusions, but not evident in transmitted light microscopy. In most samples, Na concentration is elevated in regions of high fluid inclusion density, suggesting that a significant portion of Na may be hosted in entrapped water rather than in the calcite. Only samples with 100-fold higher Y, feature Na concentrations which appear dominated by coupled Y-Na substitution and little influenced by fluid inclusion hosted Na. Our analysis provides new evidence of the influence of fluid inclusions on trace element content of stalagmites and illustrates the utility of several non-destructive imaging techniques to improve interpretations of proxy signals based on trace elements.
Exploring soluble and colloidally transported trace elements in stalagmites: The strontium-yttrium connection
Sliwinski, Jakub; Kost, Oliver; Endres, Laura; et al. (2023)
Geochimica et Cosmochimica Acta
While seasonality in speleothem trace element signatures is well-documented, the parameters that control the emergence of laminations vary between elements and tend to be multi-factorial. Here, we examine a series of active and fossil stalagmites from Asturias, Spain, with a particular focus on strontium and yttrium co-variations and fluorescent laminations. Coupled confocal fluorescence scanning light microscopy (layer counting) and time scales derived from accelerated mass spectrometry (F14C) in active stalagmites confirm that fluorescent banding is annual. This banding is coincident with Y peaks and Sr troughs, which are among the most robust trace element markers of seasonality. Strontium concentrations (in particular, the strontium partition coefficient, DSr) are positively correlated with stalagmite growth rate and are likely controlled by solution supersaturation, which is in turn controlled by seasonal variations in cave ventilation. DSr can be estimated after correcting for prior calcite precipitation using coeval Mg/Ca ratios, and is consistent with both empirical and experimental values. Meanwhile, yttrium is a proxy for colloidal organic input, and its concentration in stalagmites is likely controlled by a combination of Y drip water flux, surface retention time (i.e., how long a drip and its associated organic matter are in contact with the stalagmite surface), and dilation within the matrix (hereafter referred to as “dilation”). Persistent Sr-Y anti-correlation can be explained as an interplay between the individual controls on each element, and a breakdown in this relationship may be indicative of past changes in cave ventilation and/or drip hydrology.
Assessing marine aerosol delivery to infer paleo-wind changes using mid-latitude stalagmites from Northern Spain
Kost, Oliver (2023)
Wind is a pivotal component of the atmospheric system and plays a key role in the local and global weather and climate system. For example, wind delivers air masses loaded with humidity, which can rain out when elevated along a mountain range. Depending on the wind strength it can result in a mild breeze or a severe storm. Wind distributes dust, ash, pollen, contaminants and many other particles in the atmosphere, which all play their role in the Earth System. Furthermore, wind erosion or loess deposition are examples of the interaction with the geosphere. There are countless examples of interactions between wind and other components of the Earth system. However, little is known about wind in the past since it is not recorded directly in any geological climate archive. How can we reconstruct the climate of the past or infer wind related processes in the past if we do not know how wind actually behaved? Therefore, indirect approaches are necessary to reconstruct the wind in the past. In this doctoral thesis we assess the potential of using marine contributions recorded in speleothems (in particular stalagmites) as a novel paleowind proxy. It may sound counterintuitive to use cave deposits, which are protected by wind, however, stalagmites do record signals from outside the caves. A classical source to sink approach is applied where the fate of marine aerosols is investigated. Marine aerosols are primarily produced on the ocean by wind (the stronger the wind, the higher the marine aerosol load in the atmosphere) and are transported on land where they deposit. They are infiltrated into caves by drip water where they can form speleothems. Therefore, stalagmites might record changes in wind over the oceans. This hypothesis is tested by investigating each step starting with the marine aerosol transport and deposition on land (Chapter 1), arrival in the cave (Chapter 1 & 2), incorporation into stalagmites (Chapter 3) and finally brought into context with climate on longer time scales (Chapter 4). Hereby we focus on Sodium (Na) as a marine aerosol tracer due to its chemical conservation, low potential of contamination and favorable capability to be measured. Stalagmites are increasingly used as paleoclimate archives due to their conservation potential of climate proxies, absolute and precise chronologies by radiometric dating methods (radiocarbon and Uranium decay series), relatively easy accessibility in caves (no costly drilling techniques or expeditions to remote places) and detectability of geochemical signals in the laboratory. However, it is necessary to understand all processes affecting the geochemical signal in detail before a proxy can be used to reconstruct climate. With multiproxy approaches complex mechanisms can be brought into perspective to each other and interrelationships can be studied. Mid-latitude stalagmites from Northern Spain grown in caves within the first few kilometers from the coast provide a great target to test the feasibility of a wind proxy due to its influence by the westerly wind belt, low potential of contamination since wind primarily derives from the undisturbed open Atlantic Ocean and geochemical conservation of local and global climate signals allowing multiproxy approaches to contrast wind under different climate conditions. Monitoring the marine aerosol delivery (Chapter 1) confirms the marine origin and reveals a strong relationship with wind intensity. Coastal caves within the first few hundred meters from the coast are most sensitive to changes in wind intensity over the seasonal cycle, however, the strong decrease of marine contributions with distance from the coast has to be considered when sea-level changes over time shifting the coastline. Marine aerosol deposition at cave locations situated >3 km from the coast is still controlled by wind intensity but a shifting coastline due to sea-level change should not impact the marine aerosol delivery since deposition remains constant. Therefore, unshielded caves situated a bit inland are most feasible targets to reconstruct paleowind intensity. An extensive cave monitoring study is performed in La Vallina cave to assess the infiltration of marine contributions into caves (Chapter 2). Drip water measurements confirm the arrival of marine contributions in the cave. Furthermore, potential stalagmite formation, hydrological behaviors, sources or sinks along the percolation flow path, the role of vegetation on top of the cave and other important karst and cave processes are studied with the obtained data set of the cave monitoring. Before a paleowind proxy can be applied the incorporation of marine contributions into stalagmites is investigated (Chapter 3). It turns out that trace element incorporation is a complex mechanism influenced by different processes and features in stalagmites. Therefore, we conclude that wind cannot (yet) be reconstructed with stalagmites. Fluid inclusions, crystal defects and other impurities such as detrital particles overprint the marine contribution making it impossible to extract the marine component ultimately controlled by wind. In the last chapter a classical multi-proxy climate reconstruction is performed to demonstrate how midlatitude stalagmites from Northern Spain resolve local and global climate signals. This reconstruction focuses on the Penultimate Glacial phase, also known as MIS 6, and reveals climate variations on different time scales. On orbital time scales the climate reacts to solar forcing and exhibits melt phases of the Eurasian Ice Sheet during high solar insolation phases. On millennial time scales abrupt climate events are recorded which are explained by melt water releases affecting the ocean circulation reducing the heat transport to Europe, thus triggering abrupt cold events. Such climate feedback mechanisms are revealed in other climate archives confirming our interpretation. We demonstrate how this new highresolution record provides an absolutely dated reference chronology and can be used as a target to tune other records from the North Atlantic region lacking absolute chronologies.

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Oliver Kost

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