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

Abstract

ABSTRACT. The imprint of the radiocarbon bomb peak was detected in the top of stalagmite ER-77 from Grotta di Ernesto (NE Italy). This recently grown stalagmite reveals a reservoir age, also known as dead carbon fraction (dcf), of ~1050 14 C yr, or 12%. By applying a 14 C soil-karst model, the age spectrum of soil organic matter (SOM) as well as the CO 2 contribution of the single SOM reservoirs to the total soil CO 2 can be derived. Under the assumption of constant vegetation, meaning both vegetation density and the age spectrum of SOM, it is possible to derive the soil-air 14 C activity of the past using the 14 C calibration curve (IntCal04). Hence, it is also possible to calculate an artificial stalagmite 14 C data set covering the last 25,000 yr with parameters determined for stalagmite ER-77. With this artificially constructed data set, we derived the hypothetical atmospheric 14 C activity by using the common method of applying a constant dcf on the modeled 14 C data set of the stalagmite. This theoretical approach allows to analyze the impact of a constant and variable SOM age spectrum on atmospheric 14 C reconstructions performed with real stalagmite 14 C measurements. We observe deviations between IntCal04 and the atmospheric 14 C activity as derived with our modeled 14 C data set, which are larger for older SOM than for younger SOM and vary in time up to 2 pMC, depending on the strength of the variations in the atmospheric 14 C level. This value is comparable with the 1- uncertainty given by IntCal04 for the last glacial. For a varying SOM age spectrum, the deviations between the calibration curve and 14 C level of the atmosphere reconstructed with a stalagmite exceed 3 pMC, which is larger than the 1- uncertainty of IntCal04. In general, the SOM has smoothing, shifting, and 14 C-depleting effects on the stalagmite 14 C record and, therefore, on the stalagmite-derived atmospheric 14 C activity. In this study, changes in soil-air pCO 2 and carbonate dissolution conditions, which have also an important impact on the 14 C record of a stalagmite, are not accounted for.

Hydrological control of the dead carbon fraction in a Holocene tropical speleothem

“…Although, given both the relatively short turnover time of SOM (Trumbore, 2000) and the relatively small 14 C fractionation from gaseous CO 2 to DIC, the 14 C in the soil-water DIC should reflect that of the atmosphere (Fohlmeister et al, 2011a), except during periods of large and abrupt changes in atmospheric 14 C (Trumbore, 2000). The degree to which each of these soil reservoirs (i.e.…”

“…Finally, fractionation occurs between the calcite-saturated solution and precipitating calcite forming the stalagmite. For a more detailed description of the model, the reader is referred to Fohlmeister et al (2011a). The model parameters used to derive the SOM age spectrum were determined using an iterative process consisting of two steps.…”

“…Following this, it was necessary to re-evaluate the host rock dead carbon due to changes in the vegetation-induced dead carbon. Next, the vegetation parameters were adjusted again, and the iteration was continued until the deviations of the model output were minimized with respect to the 14 C values measured in the stalagmite (Fohlmeister et al, 2011a).…”

“…climate, overlying vegetation, karst architecture). Moreover, DCF may fluctuate according to local changes in soilwater-rock interaction Genty et al, 2001) and variations in the age spectrum of soil organic matter (SOM) above the cave (Oster et al, 2010;Fohlmeister et al, 2011a), both of which are controlled by climate.…”

“…We use a well-defined uranium-series age model and the Southern Hemisphere 14 C calibration curve (SHCal04; McCormac et al, 2004) to derive a DCF time series through this interval, then compare this series to stable-isotope (d 13 C and d 18 O) and trace-element (Mg/Ca) data from the same speleothem (previously reported in Griffiths et al, 2009Griffiths et al, , 2010a to determine the forcing mechanisms. In addition, we analysed 14 C through the bomb pulse to provide input parameters to a soil-karst model Fohlmeister et al, 2011a), which was used to explore the initial 14 C activity (a 14 C) in greater detail between the soil-water DIC above Liang Luar and the speleothem within the cave. Coupling the stalagmite 14 C measurements with the soil-karst C model allowed us to explore the degree to which each of the known C sources influenced the DCF in our speleothem.…”