Bomb ¹⁴C Recorded in Laminated Speleothems: Calculation of Dead Carbon Proportion

Abstract

We performed radiocarbon measurements using accelerator mass spectrometry (AMS) on 6 stalagmites, 3 stalactites and 7 seepage waters from four different caves in Southwest France and Belgium in order to calculate the dead carbon proportion (dcp). All the speleothems studied are modern and annually laminated, which offers the advantage of an accurate chronology, with better than one-year resolution. Coupled with the fact that very little calcite is necessary for an AMS measurement (between 1.5 and 7 yr of calcite deposit), we obtained dead carbon values within an uncertainty limit of ± 1.5%.Results show that the dead carbon proportion varies from 9.2% to 21.9% for calcite deposits and from 3.6% to 21.9% for water. In each sampling site, the dcp is homogeneous. Although the inter-site dcp varies by >11%, its average value of 15.5% ± 4.4 still lies within the uncertainty range of the accepted value of 15% ± 5 (dilution factor of 0.85 ± 0.5). We compare the average dcp of each site with the local geology, vegetation and climate. Given similar geology and temperature, the highest dcp values are found under forest cover; dcp difference is up to 9%. However, the Belgian site, which is also under a forest, shows a dcp very close to the dcp found under grassland sites of Southwest France, which proves that other unknown factors may play an important role in dissolution processes. Secondary calcite deposition and redissolution in the soil zone or more likely in the fracture system before reaching the cave itself could also explain the inter-site differences. The IAEA isotopic model (Pearson model adapted for open systems) is in good agreement with the measured activities.

The Influence of Soil Organic Matter Age Spectrum on the Reconstruction of Atmospheric 14C Levels Via Stalagmites

“…This value is in agreement with the one found for the younger part (1800-1900) of the nearby stalagmite ER-76 (unpublished data). In addition, this value is consistent with Genty et al 1997;Beck et al 2001). The best-parameterized age spectrum of the SOM to fit the stalagmite 14 C measurements is then found to be:…”

Assessing influences on speleothem dead carbon variability over the Holocene: Implications for speleothem-based radiocarbon calibration

“…In this manuscript the term referred to as the DCP is simply the percentage of old 14 Cfree "dead carbon" incorporated in the speleothem at the time of formation. The percentage is calculated via the procedure of Genty and Massault (1997):…”

“…To date, the offset between speleothem 14 C and contemporaneous atmospheric 14 C has been referred to in a variety of different ways across the literature, often using the same acronym to refer to different metrics, which has been a source of some confusion. In this manuscript we will refer to the dead carbon proportion (DCP) as a percentage as defined by Genty and Massault (1997), but we also refer to the DCP as a "correction" or "offset" with units of 14 C years. Hendy (1971) considered two end member scenarios under which dissolution of carbonate bedrock can occur: open and closed system dissolution.…”

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

“…Genty and Massault (1997) referred to the soil a 14 CO 2 measurements of D€ orr and Münnich (1986) which suggested that under forested soil in Germany decomposition of SOM with t ¼ 100 years composes 25% of soil CO 2 flux in the summer and 75% of the flux in the winter. Under the assumption that the majority of infiltration in temperate settings occurs during the winter months, when photosynthesis is at a minimum, previous studies have argued that model parameters that suggest that speleothem carbon is derived from soil CO 2 that is primarily derived from decomposition of old SOM are in good agreement with observations of winter soil a 14 CO 2 .…”