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

Abstract

Recently, it has been shown that U-Th dated speleothems may provide a valuable archive of atmospheric radiocarbon ( 14 C), but the reliability of these records is dependent upon the stability of the dead carbon proportion (DCP) derived from the soil and bedrock. In order to assess climatic influences on speleothem DCP, we have investigated DCP variability over the Holocene interval where atmospheric 14 C is well known based on dendrochronologically dated tree rings by conducting 14 C measurements on a U-Th dated stalagmite (HS4) from Heshang Cave, Hubei Province, China (30 • 27 N, 110 • 25 E; 294 m) spanning 0.5-9.6 ka. We investigated climatic controls on DCP, and found that DCP in HS4 has an average value over the Holocene of 10.3 ± 1.5%, with an average age offset from atmospheric radiocarbon of 875 ± 130 years, and displays a response to both precipitation increases and decreases. HS4 DCP increases during the wetter mid-Holocene interval (∼5.5-7.1 ka), likely reflecting a shift to more closed-system dissolution in response to increased soil moisture. DCP decreases during the 8.2 ka event, a time period of dry conditions at Heshang Cave, though the lower amplitude of this shift indicates that DCP may be less sensitive to dry events. Speleothems are potentially valuable archives of atmospheric radiocarbon, especially in older portions of the 14 C calibration curve where knowledge of atmospheric 14 C is limited, however minor climatic influences on DCP could introduce uncertainties of several hundred years to calibrated ages.

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

“…The HS4 site is located~150 m from the entrance of the cave, and was still actively dripping when this study was undertaken. Records of d 18 O, d 13 C, and a 14 C covering the Holocene measured on HS4 have been previously reported by Hu et al (2008b), Liu et al (2013), and Noronha et al (2014). The age model for the final 150 years of stalagmite growth is well-defined by annual layer counting (Hu et al, 2008b).…”

“…Changes in hydrology appear to be the most important driver of speleothem DCP (Griffiths et al, 2012, Noronha et al, 2014, with a working hypothesis that higher precipitation increases soil moisture, therefore limiting CO 2 diffusion and open-system dissolution, and leading to a higher proportion of carbon derived from the a 14 Cfree bedrock. In a system where most drip water DIC is acquired during open-system dissolution occurring at depth in the vadose zone, open-system dissolution, which relies on the assumption of a functionally infinite supply of CO 2 could only occur when voids in the bedrock are filled with more gas than water.…”

How Deep Can Surface Signals Be Traced in the Critical Zone? Merging Biodiversity with Biogeochemistry Research in a Central German Muschelkalk Landscape

“…However, less information is available for groundwater obtained from drilled boreholes within a karstic limestone setting (Gray and Engel, 2013). Studies of carbon cycling in kart environments have often focused on using isotopes to quantify the dissolution and precipitation of carbonates (models reviewed in Han and Plummer, 2013) or to understand the degree to which C derived in soils can be recorded in cave deposition (Noronha et al, 2014). The Hainich CZE is one of just a few observatories located in this important lithology.…”

A novel approach for construction of radiocarbon-based chronologies for speleothems

“…The model was tested using a previously published stalagmite 14 C dataset from 393 Heshang Cave (China) (Noronha et al, 2014). Stalagmite HS4 is 250 cm tall and 394 was dated using U-Th techniques, which suggest continuous growth with an 395 almost constant growth rate over the past ~8,000 years (Hu et al, 2008).…”

“…Stalagmite HS4 is 250 cm tall and 394 was dated using U-Th techniques, which suggest continuous growth with an 395 almost constant growth rate over the past ~8,000 years (Hu et al, 2008). DCF is 396 stationary but varied between 7 and 14% (Noronha et al, 2014); therefore thesample is ideal for testing the reliability of the 14 C chronology development 398 technique presented here. 399…”