Carbonate Sample Preparation for 14C Dating Using an Elemental Analyzer (original) (raw)
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Mollusk shells are commonly present in a broad array of geological and archaeological contexts. The shell carbonate can serve for numerical age determination (Δ 14 C) and as a paleoenvironmental indicator (δ 18 O, δ 13 C). Shell carbonate recrystallization in soils, however, may re-equilibrate the carbon (C) isotopic signature with soil CO 2. The equilibration dynamics remain poorly understood because of the absence of suitable experimental approaches. Here we used the artificial 14 C-labeling technique to study the process of shell carbonate recrystallization as a function of time. Organic-free and organic-containing shell particles of Protothaca staminea were mixed with loess or a carbonate-free loamy soil. The mixtures were placed in airtight bottles, where the bottle air containing 14 CO 2 (pCO 2 = 2%). The 14 C activity of shells was measured over time and related to the recrystallization of shell carbonate. Recrystallization of shell carbonate already began after one day. The recrystallization rates were 10 − 3 % day − 1 in organic containing shell embedded in soil and 1.6 · 10 − 2 % day − 1 in organic-free shells in loess. Removal of organic compounds increased shell porosity, and so, increased the contact surface for exchange with soil solution. Organic-free shells recrystallized much faster in loess (0.56% in 56 days) than in other treatments. Recrystallization was 2 to 7 times higher in loess (in the presence and absence of organic compounds, respectively) than in carbonate-free soil. Loess carbonate itself can recrystallize and accumulate on shells, leading to overestimation of shell carbonate recrystallization. A model for shell carbonate recrystallization as a function of time was developed. The model considers the presence or absence of organic compounds in shell structure and geogenic carbonates in the embedding matrix. The model enabled all results to be fitted with R 2 = 0.98. The modelled time necessary for nearly full recrystallization (95% of shell carbonate) was 88 years for organic-free shells in loess and up to 770 years for organic-containing shells in carbonate-free soil. After this period, the original isotopic signature will vanish completely and will be replaced by a new δ 13 C and Δ 14 C signature in the shell structure. Thus, shell carbonate recrystallization may proceed relatively rapidly in terms of geologic time. This is necessary to consider in the interpretation of dating results and paleoenvironmental reconstructions.
Stable isotope variability and the selection of terrestrial mollusc shell samples for 14C dating
Quaternary International, 2002
Assessment of stable isotopic measurements on shell carbonate as a guide to contamination by non-biogenic carbon requires information on variation within individual shells, within a species and between species occupying the same site. The stable carbon and oxygen isotopic composition of living and fossil shells of the land snails Cepaea nemoralis, C. hortensis, Helix pomatia and Pomatias elegans indicates that variations in the d 13 C of the organic matrix are not significantly influenced by habitat, and that microenvironment and climate override the effects of CO 2 exchange across the body. It follows that, as in marine molluscs, the d 13 C of unrecrystallised shell carbonate can serve as a useful indicator of errors in 14 C dates caused by recrystallisation or the uptake of old carbon, and that it can be used to screen samples especially if it is determined on CO 2 derived from an entire shell rather than a fragment of shell.
The dating of impure carbonates with decay-series isotopes
Nuclear Instruments and Methods in Physics Research, 1984
North-Holland. Amsterdam THE DATING OF IMPURE CARBONATES WITH DECAY-SERIES ISOTOPES Teh-Lung KU and Zhuo-Cheng LIANG * l)et~arlment o! Geological Sctence.~. Uni~ersi b' of Southern ('a/iJbrnia, Los Angeles, ('a/~lbrnia 90089-0741. USA The uranium-series disequilibrium methods have been successfully applied to the dating of clean carbonate precipitates such as coralline and speleothem materials. Similar success has yet to be achieved for the widely occurring inorganically precipitated impure carbonates (e.g., travertine, tufa. and calcrete), of which there is an ever-increasing need for determining their chronology m Quaternary and archaeological studies. The main problem involves the presence of detrita[ materials which cannot be isolated from the carbonate fraction by simple physical means. Chemical separation using dilute acid leaching has been often attempted. This process may solubilize some of the uranium and thorium isotopes from the detrital component, and this detrital contamination must be corrected for. This paper reviews the principles and assumptions of the detrital correction schemes, discusses the theoretical and e,~perimental aspects of the mixing-line plots as suggested by Rosholt and Szabo, and recommends analytical procedures pertinent to the use of such plots.
a b s t r a c t 8 19 The radiocarbon method has been frequently used to date mollusk shell carbonate. The accuracy of estimated 20 ages, however, depends on the degree and completeness of shell carbonate recrystallization. Although the effect 21 of contamination of the shell CaCO 3 with environmental carbon (C) is well known, the role of Ca 2+ in diagenetic 22 processes remains unclear. Addition of young C to shells during diagenesis occurs in soil solution, where the Ca 2+ 23 concentration is in equilibrium with exchangeable Ca 2+ and/or weathering of Ca-bearing minerals. While the ex-24 change process takes place within seconds, the dissolution equilibrium requires longer timescales (on the order 25 of months). It has therefore been hypothesized that the dissolution and recrystallization of shell carbonate in soils 26 with higher cation exchange capacity (CEC) should proceed slower compared to those with low CEC. The objec-27 tive was to determine the effects of soil CEC and exchangeable cations on shell carbonate recrystallization using 28 the 14C labeling approach. Shell particles of the bivalve Protothaca staminea were mixed with carbonate-free 29 sand (CEC = 0.37 cmol + kg −1) (Sand), a loamy soil (CEC = 16 cmol + kg −1) (Loam) or the same loamy soil 30 saturated with KCl, where exchangeable cations were replaced with K + (Exchanged). The high-sensitivity 14C 31 labeling/tracing approach was used to determine carbonate recrystallization rates. Shell carbonate recrystalliza-32 tion after 120 days in Loam and Exchanged (0.016 and 0.024 mg CaCO 3 , respectively) showed one order of mag-33 nitude lower recrystallization than in Sand (0.13 mg CaCO 3). A high level of soil exchangeable Ca 2+ decreased the 34 solubility of shell carbonate and consequently its recrystallization because the exchange is faster than dissolution. 35 Therefore, soil CEC and cation composition are determinant factors of shell carbonate recrystallization. Shells in 36 soils with low CEC may undergo more intensive recrystallization; hence they may need further pretreatments be-37 fore the dating procedure. 38
DATING THACH LAC: CRYPTIC CaCO3 DIAGENESIS IN ARCHAEOLOGICAL FOOD SHELLS AND IMPLICATIONS FOR 14C
Radiocarbon
ABSTRACTIn many locations around the world, shell radiocarbon dates underpin archaeological research. The dating of shell brings the chronological relationship between the sample and target event (e.g., hunting and food preparation) into congruence, while shells are valuable geochemical proxies for understanding past climate dynamics and environments. However, this information can be lost as the shell, composites of biopolymers and carbonate minerals (mostly calcite and or aragonite), undergo diagenetic alteration. While studies into Pleistocene-age carbonates are common in the radiocarbon literature, there has been little research into the impact of alteration on Holocene-age shells used to interpret recent societal developments. The limits of our understanding of these diagenetic changes became evident when dating Placuna placenta (naturally calcitic) and Tegillarca granosa (naturally aragonitic) shells from the site of Thach Lac in Vietnam. These shells returned ages significantl...
A Secondary Standard for Radiocarbon Dating
Radiocarbon, 1983
The preparation and calibration of a secondary standard for the INGEIS Radiocarbon Dating Laboratory are presented. This standard is barium carbonate with a specific activity almost twice that of NBS oxalic acid. It was prepared from BaCO3 with high specific activity and commercial potassium carbonate by an isotopic dilution technique. The advantages of this standard are: 1) the preparation is simple and can be achieved with ordinary labware; 2) the production of CO2 by acid attack from this carbonate shows minimum isotopic fractionation. At least, it has less fractionation than wet oxidation of oxalic acid, the problems of which are described in the literature. This standard ensures better reproducibility in activity measurements; 3) despite some problems of activity exchange with atmospheric CO2 concerning carbonates, measurements of activity over a period of about two years have shown no significant deviation from the mean value. A tentative explanation of this phenomenon is also...
Radiocarbon dating of small terrestrial gastropod shells in North America
Quaternary Geochronology, 2010
Fossil shells of small terrestrial gastropods are commonly preserved in wetland, alluvial, loess, and glacial deposits, as well as in sediments at many archeological sites. These shells are composed largely of aragonite (CaCO 3 ) and potentially could be used for radiocarbon dating, but they must meet two criteria before their 14 C ages can be considered to be reliable: (1) when gastropods are alive, the 14 C activity of their shells must be in equilibrium with the 14 C activity of the atmosphere, and (2) after burial, their shells must behave as closed systems with respect to carbon. To evaluate the first criterion, we conducted a comprehensive examination of the 14 C content of the most common small terrestrial gastropods in North America, including 247 AMS measurements of modern shell material (3749 individual shells) from 46 different species. The modern gastropods that we analyzed were all collected from habitats on carbonate terrain and, therefore, the data presented here represent worst-case scenarios. In sum, w78% of the shell aliquots that we analyzed did not contain dead carbon from limestone or other carbonate rocks even though it was readily available at all sites, 12% of the aliquots contained between 5 and 10% dead carbon, and a few (3% of the total) contained more than 10%. These results are significantly lower than the 20-30% dead carbon that has been reported previously for larger taxa living in carbonate terrain. For the second criterion, we report a case study from the American Midwest in which we analyzed fossil shells of small terrestrial gastropods (7 taxa; 18 AMS measurements; 173 individual shells) recovered from late-Pleistocene sediments. The fossil shells yielded 14 C ages that were statistically indistinguishable from 14 C ages of well-preserved plant macrofossils from the same stratum. Although just one site, these results suggest that small terrestrial gastropod shells may behave as closed systems with respect to carbon over geologic timescales. More work on this subject is needed, but if our case study site is representative of other sites, then fossil shells of some small terrestrial gastropods, including at least five common genera, Catinella, Columella, Discus, Gastrocopta, and Succinea, should yield reliable 14 C ages, regardless of the local geologic substrate.