In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of highprecision U-series and radiocarbon ( 14 C) dates. The data reveal an extreme case of very high and also climate-dependent DCF. For M1-5, an average DCF of 56.2 ± 3.4 % is observed between 27 and 18 kyr BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed-system carbonate dissolution conditions. Towards the end of the last glacial period, decreasing Mg/Ca ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and DCF as seen in stalagmites from temperate zones.

To achieve high-precision and reproducible results from radiocarbon (14C) dating of carbonate samples in paleoclimate research, a new CO2 extraction line was designed, constructed, and characterized at the Heidelberg Radiocarbon Lab of the Institute of Environmental Physics, Heidelberg. The setup includes a circular glass-tube design, which is operated at vacuum pressure levels of the order of 10–5 mbar. The efficiency of the extraction process was assessed, showing significantly favorable conditions for solid piece samples (99.58 ± 4.69)% over powdered samples (88.28 ± 10.03)%. Process blank values are below 0.2 pMC apparent 14C activity. Repeated measurements of IAEA C2 standards with an average value of (41.09 ± 0.23) pMC attest high accuracy and reproducibility of the instrument. Six consecutive samples of 6 to 12 mg carbonate mass can be processed in one run of roughly 2.5 hours. Thus, the new setup contributes to time-efficient and reproducible radiocarbon dating results for paleoclimate research at the Institute of Environmental Physics. In a first application, Dead Carbon Fraction (DCF) values of a Holocene alpine stalagmite from Schratten Cave are presented, revealing extraordinarily high offsets between atmospheric and stalagmite 14C with DCF values between (49.4 ± 0.4)% and (61.6 ± 0.4)%.

No abstract

No abstract

Understanding how stalagmites grow under changing climate conditions is of great significance for their application as a paleoclimate archive. In this study, we present a shape modeling approach to stalagmite growth by combining three existing models accounting for climate variables, karst water chemistry, and speleothem deposition. The combined model requires only four input parameters: calcium concentration of the water drop, drip interval, cave temperature, and cave carbon dioxide (CO2) concentration. Using the output of the coupled atmosphere–ocean–land surface model MPI-ESM1.2 and the CaveCalc model for speleothem chemistry, we simulated stalagmite growth at Sofular Cave, Northern Turkey, (in the last 25 kyr) and compared the results to those of the existing So-1 stalagmite from the same cave. This approach allows simulating, completely independent of measured boundary conditions, a stalagmite geometry that follows the trend of the experimental data for the growth rate, with input parameters within the respective error ranges. When testing the sensitivity of the individual model parameters, the model suggests that the stalagmite radius mainly depends on the drip interval, whereas the growth rate is driven by the calcium concentration of the water drop. The model is also capable of showing some basic phenomena, like a decrease in growth rate (as observed in the real stalagmite), as CO2 concentration in the cave increases. The coupling of input parameters for the model to climate models represents the first attempt to understand an important climate archive in its shape and isotope content and opens the possibility for a new inverse approach to paleoclimate variables and model constraints.

<p>Speleothems have become a cornerstone in atmospheric <sup>14</sup>C reconstruction. In particular, the part of the IntCal20 calibration curve before 34 ka BP (Reimer et al., 2020) heavily relies on a set of speleothems from Hulu Cave in China (Cheng et al., 2018). The interpretation of speleothem <sup>14</sup>C archives, however, is often exacerbated by the so-called dead carbon fraction (DCF) in speleothem carbonate. It quantifies the percentage of old, <sup>14</sup>C-free carbon from dissolved bedrock carbonate or aged soil organic matter, and is controlled by various parameters. Modelling efforts to disentangle these parameters have already been made by previous studies.</p><p>Here, we present forward-modelled DCF time series obtained by coupling CaveCalc, a numerical model for speleothem chemistry and isotopes (Owen et al., 2018), with IntCal20 and results from paleoclimate modelling. To compare our coupled model with an extensive DCF measurement record from Sofular Cave in Northern Turkey, we convert time-dependent soil respiration output from the Max Planck Institute Earth System Model version 1.2 (MPI-ESM1.2) to soil pCO<sub>2</sub> via a simplistic soil respiration model and use it as input for CaveCalc. The resulting forward-modelled DCF is in very good agreement with the long-term trends of the measurement record and demonstrates that soil respiration has been the main driver of DCF variability in the Last Glacial Maximum and the Early Holocene at Sofular Cave.</p><p>Further, we show that, holding soil respiration and all other climate parameters constant, adding only 10 % of 1000 year old carbon to the soil CO<sub>2</sub> can cause variations of up to 200 years in the DCF. This finding suggests that the DCF variability of only 50 years, which is assumed for Hulu Cave by Reimer et al. (2020), might be significantly higher, and underlines the importance of including additional records, like the one from Sofular Cave, to the next generation of calibration curves.</p><p> </p><p><strong>References:</strong></p><p>Reimer, P. J., Austin, W. E. N., Bard, E., Bayliss, A., et al.: The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 cal kBP), Radiocarbon, 62(4), 725-757, doi:10.1017/RDC.2020.41, 2020.</p><p>Cheng, H., Lawrence Edwards, R., Southon, J., et al.: Atmospheric <sup>14</sup>C/<sup>12</sup>C changes during the last glacial period from Hulu Cave, Science, 362(6420), 1293–1297, doi:10.1126/science.aau0747, 2018.</p><p>Owen, R., Day, C. C., and Henderson, G. M.: CaveCalc: A new model for speleothem chemistry & isotopes, Computers & Geosciences, 119, 115–122, doi:10.1016/j.cageo.2018.06.011, 2018.</p>

No abstract

<p>In the tropical Americas, extreme precipitation events such as hurricanes are responsible for enormous damage and numerous fatalities each year. However, projections of hydro-climatic change and tropical cyclone (TC) activity in Central America and the Caribbean for the next decades are still challenging, requiring more reconstructions of past precipitation and TC activity. In tropical speleothems, stable oxygen isotope values (δ<sup>18</sup>O) are an often used proxy for precipitation amount, and in some cases TC activity, but may be masked by various effects such as evaporation or kinetic effects inside the cave, temperature, or variable moisture sources and trajectories.</p><p>Here we investigate the potential of trace metals in speleothems and drip waters from Larga Cave, Puerto Rico, as complementary proxies for past effective infiltration, and hence precipitation amount. The analysis of transition metal ratios in drip waters from 2014 to 2019 reveal a seasonal variation, with peaks in the Cu/Ni (and Cu/Co) ratios potentially reflecting the intensity of the prior wet season. The suggested imprint of Hurricanes Bertha (2014) and Maria (2017) in the drip water suggests that transition metal ratios might be even indicators of (past) tropical cyclone activity.</p><p>Laser ablation ICPMS analyses of speleothems from the same cave support the interpretation of a potential climate signal in the transition metal ratios. Both higher Cu/Ni and Cu/Co values are found during presumably warmer and wetter phases, such as e.g. during the late Holocene, as well as at the onsets of Dansgaard/Oeschger interstadials including the Bolling/Allerod (14.6-12.8 ka BP). Replicated records of the past 400 years combined with stable isotope values of oxygen and carbon (δ<sup>13</sup>C) will provide a test of the underlying mechanisms driving the observed variability on different timescales. Comparison with other reconstructions highlights the potential of Cu/Ni (and Cu/Co) ratios in speleothems for hydro-climate and past precipitation variability reconstruction.</p>