Exploring the dating of “dirty” speleothems and cave sinters using radiocarbon dating of preserved organic matter

Abstract

Speleothems and other carbonate deposits such as tufa containing high proportions of detrital material can be difficult to chemically date due to detrital thorium levels causing a high level of error in conventional U-Th disequilibrium dating. Here we investigate the use of an alternative technique centring on radiocarbon dating of organic matter preserved within the detrital fraction. Non-acid soluble humic, particulate and detritally absorbed organic matter was recovered from eight samples from a flowstone sinter formed within a roman aqueduct at Trento in Italy with a maximum age of 100 CE (1850 cal yr BP), and two repeat samples from a dripstone formed within the 20 th Century on a wire fence at Lilly-Pilly Cave, Buchan Caves Reserve in Victoria, Australia. In the aqueduct samples the median calibrated 14 C ages ranged from 2232 to 2889 cal yr BP, with 95.4% probability age range in the youngest and oldest samples of 2153-2337 and 2342-3449 cal yr BP respectively. The median age of the more modern dripstone was 336 cal yr BP, with a 95.4% probability age range of 148 to 486 cal yr BP. These results provide very approximate ball-park estimates of the age of the sample, but are consistently too old when compared to the known maximum ages of formation. It is hypothesised that this offset is due to a combination of the nature of the organic carbon transported from the source organic matter pools, and reworking of both modern and old organic carbon by in situ microbial communities.

Integrating U-Th, 14C, and 210Pb methods to produce a chronologically reliable isotope record for the Belize River Valley Maya from a low-uranium stalagmite

“…A closed-system speleothem may prove difficult to date by U-Th methods, if it is contaminated with detrital 230 Th (common in ‘dirty’ stalagmites) (e.g. Blyth et al, 2017) or if it has a very low initial 238 U concentration (e.g. Fensterer et al, 2010).…”

“…Railsback et al, 2013). However, many speleothems that hold valuable paleoenvironmental information are difficult or impossible to date by U-Th methods because of low uranium concentrations, excess detrital contamination, or open system geochemical behavior (Bajo et al, 2016; Blyth et al, 2017; Fensterer et al, 2010; Lachniet et al, 2012). Alternative methods of dating speleothems can potentially produce successful paleoenvironmental reconstructions from specimens otherwise ignored or discarded, while also offering new opportunities in regions currently underutilized because of perceived dating difficulties.…”

Towards Organic Carbon Isotope Records from Stalagmites: Coupled δ13C and 14C Analysis Using Wet Chemical Oxidation

“…The conventional understanding is that OC incorporated in stalagmites is not directly affected by the addition of 14 C-dead carbon from limestone dissolution (Blyth et al, 2017), and may thus provide important constraints on drivers of the karst carbon cycle, and on (past) surface conditions.…”

“…Their results show that accurate and reproducible δ 13 C measurements are possible from stalagmites, using relatively small sample amounts (100-200 mg CaCO3). To our knowledge, the only published studies on stalagmite OC 14 C to date concentrate on the use of speleothems with high OC concentrations for dating purposes (e.g., Blyth et al, 2017;Borsato et al, 2000;Genty et al, 2011). Thus far, however, there have been no reported 14 C measurements on stalagmite NPOC, likely because sample size requirements are substantially greater, with attendant increases in procedural blanks and other analytical uncertainties.…”

Molecular Fingerprinting of14C Dated Soil Organic Matter Fractions from Archaeological Settings in NW Spain

“…Due to the intrinsic heterogeneity of SOM, 14 C ages have often been inconsistent with the stratigraphic sequence (Wang et al 1996; Schmidt et al 2015). Possible causes of heterogeneity in the 14 C activity of SOM fractions include bioturbation, leaching, incorporation of carbonates or deposition of windblown particles containing allochtonous C (Olsson 1974; Fowler et al 1986; Martin and Johnson 1995; Blyth et al 2017; Liu et al 2017).…”