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Osteocalcin As the Recommended Biopolymer for 14C Age Dating of Bone and S=C and 015N Paleodietary Reconstruction

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Osteocalcin As the Recommended Biopolymer for 14C Age Dating of Bone and S=C and 015N Paleodietary Reconstruction Stable Isotope Geochemistry: A Tribute to Samuel Epstein © The Geochemical Society, Special Publication No.3, 1991 Editors: H. P. Taylor, Jr., J. R. O'Neil and I. R. Kaplan Osteocalcin as the recommended biopolymer for 14C age dating of bone and s=c and 015N paleodietary reconstruction HENRYO. AJIEand ISAACR. KAPLAN Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567, U.S.A. Abstract-Osteocalcin, a gamrna-carboxyglutamic acid containing bone protein is tightly bound to the hydroxyapatite matrix of bone and is relatively more stable than the dominant collagen. Its distribution in nature is limited to vertebrates. Osteocalcin and collagen have been isolated from modern and fossil bone samples of different organisms in different depositional environments for analysis of their Ol3C, OI5N, and 14Ccontent. We present evidence suggesting that osteocalcin is a more suitable protein fraction for obtaining accurate 14Cage estimates and/or Ol3C and Ol5N for paleodietary reconstruction from bone samples. INTRODUCfION ganisms. Different classes of plants have different THE EARLIESTDESCRIPTIONSof diet in prehistoric carbon isotope ratios, depending on the type ofbio- humans were based upon inferences from artifact chemical reaction they utilize to obtain their carbon. assemblages or anecdotal accounts of midden con- C3 plants use a different enzymatic pathway to fix stituents. Column sampling performed with proper atmospheric carbon during photosynthesis than do statistical controls is adequate for measuring quan- C4 plants. The enzyme responsible for the C3 path- tities of small, well-preserved, and evenly distributed way discriminates l3C02 to a greater extent than food remains such as seed, shell, or fish bone (TRE- that for C4 plants (BENDER,1968, 1971), resulting GANZAand COOKE,1948; MEIGHAN,1972). It is in lower Ol3Cvalues ranging from -22 to -33 less accurate, however, at predicting quantities of (mean -25), whereas those of C4 plants fix carbon larger, unevenly distributed components such as with a Ol3Cof -9 to -16 (mean -12). CAM plants, animal bone. Similar problems are encountered which have the ability to use both Cj-like and C4- with faunal analyses where spotty distribution pat- like photosynthetic pathways, tend under natural terns are complicated by non-representation of conditions to resemble C4 plants in their isotopic bones due to diagenesis, butchering patterns, or composition (BENDERet aI., 1973; OSMONDet al., scavenger activity. Plant remains pose unique 1973). These differences have been exploited by problems. They infrequently are preserved in ar- many researchers to document the introduction of chaeological sites and when present, mostly repre- plants from one isotopic group into an environment sent uneaten foodstuffs. where the natural vegetation consists of plants of In contrast to standard midden analyses, trace another isotopic type. This has been particularly element and stable isotope analysis of human bone useful in documenting the progression of maize (a collagen provide a direct means for estimating the C4 plant) agriculture in the New World (VANDER contribution of broad categories of food items to MERWEand VOOEL, 1977;BENDER,1968;DENIRO the diet. This is possible because the slight, but de- and EpSTEIN, 1981; VAN DER MERWE, 1982; tectable differences in the isotopic composition of SCHWARCZet al., 1985). different food types are ultimately reflected in bone Carbon isotopes can also discriminate between collagen (DENIRO and EpSTEIN, 1978, 1981; marine and terrestrial organisms (TAUBER,1981; SCHOENINGERand DENIRO, 1984; CHISHOLMet CHISHOLMet al., 1982, 1983). This is possible due al., 1982; MINAGAWAet aI., 1986). The following to the different isotopic composition of the carbon sections discuss the basis for using stable carbon source in each environment. Marine organisms use and nitrogen isotope ratios for dietary reconstruc- seawater bicarbonate, which is approximately 7 per tion. mil more positive than atmospheric carbon dioxide. Thus, the mean Ol3Cvalue of organic carbon in Carbon isotope ratios marine organisms is -18 compared to the mean Ol3Cvalue of -25 for terrestrial C3 plants. Unfor- Carbon isotope ratios discriminate C3 plants from tunately, marine Ol3Cvalues may be mimicked in C4 and CAM (Crassulacean acid metabolism) plants a terrestrial animal that feeds on a mixture of C3 and also differentiate marine from terrestrial or- and C4 land plants. Due to this complication, it is 235 236 H. O. Ajie and I. R. Kaplan sometimes impossible to assess the marine contri- isotope ratios may resolve discrepancies resulting bution to the diet in any human population that from the use of only one isotope (SCHOENINGERet utilizes both marine foods and large quantities of al., 1983). However, there are certain conditions maize or other C4plants, by use of the l3C/l2Ciso- when the application of both isotopic techniques tope method alone. will still be inconclusive. For example, combined Ol3Cand Oi5Nanalyses sometimes do not permit a Nitrogen isotope ratios thorough dietary reconstruction of the native in- habitants of tropical island such as the Bahamas, Studies on animals raised on diets of known ni- where a diet consisting of C4 agricultural products trogen composition demonstrate that the Oi5Nof and marine resources are gathered from both coral animal tissues is determined by the Oi5Nof their reefs and the open ocean (KEEGANand DENIRO, diets (DENIROand EpSTEIN,1981). Oi5Nvalues of 1988). A similar situation is likely to be found in bone collagen was shown to be about 3 per mil coastal Peru, where archaeological populations are more positive than that of the diet. The nitrogen known to have cultivated both maize and non-le- isotopic composition of diet ultimately depends on gumes while also exploiting marine resources. Un- the composition of nitrogen compounds available der such conditions, resolution of dietary contri- for uptake by plants at the base of the food chain. butions becomes difficult. Subsequently, it was shown that nitrogen isotope ratios distinguish between marine and terrestrial Radiocarbon dating foods in the diet (SCHOENINGERet al., 1983; SCHOENINGERand DENIRO, 1984). Animals uti- Radiocarbon measurement is usually achieved lizing marine food sources are more enriched in by measuring the indigenous carbon extracted and i5N than animals feeding on land plant sources, purified from a suitable fraction of the material. because the nitrogen sources utilized during protein This measurement is then used to derive the age of synthesis in the two systems have a different nitro- the material. Radiocarbon measurement for bones gen isotope composition. Marine plants have Oi5N has conventionally been achieved by scintillation values which are about 8 per mil heavier than those measurement of the iJ-disintegration of i4C atoms of terrestrial plants. These differences are then re- in the mineral phase (usually hydroxyapatite) or flected in the food chain, with organisms feeding the organic fraction (usually collagen). Each of these directly on either phytoplankton or land plants phases has been observed to have its associated showing about a 3 per mil enrichment in i5N over problems for radiocarbon measurements (HEDGES the starting i5N!'4N ratio at each trophic level (DE- and LAW,1989). The exchange of carbon atoms of NIROand EpSTEIN,1981; MINAGAWAet al., 1986; the mineral phase with ground water poses a serious SCHOENINGERand DENIRO, 1984; SCHOENINGER, problem in the use of hydroxyapatite. Contami- 1985). This has the effect of magnifying the differ- nation through secondary deposition and recrys- ences between marine and terrestrial foods, as hu- tallization of indigenous carbonate further compli- mans tend to feed on marine organisms higher in cates the use of hydroxyapatite. An indigenous or- the food chain (fish) than on land animals (herbi- ganic fraction, such as collagen, which constitutes vores). >90% of bone organic matter is considered to be As with carbon isotopes, overlap in Oi5Nratios more suitable for radiocarbon measurements of of marine and land animals may be observed under bone. certain conditions. Organisms inhabiting estuaries The development of the Accelerator Mass Spec- or coral reef environments are likely to have Oi5N troscopy (AMS) now makes it feasible to utilize values similar to those of terrestrial organisms, due milligram amounts of organic materials for i4C to high nitrogen fixation rates (CAPONEand CAR- dating and to refine existing, or develop new, pre- PENTER,1982; SCHOENINGERand DENIRO,1984). treatment strategies for the i4Cdating of bone, in- If food from these environments is incorporated cluding the direct dating of Holocene and Pleisto- into the diet, there will be an apparent under-rep- cene hominid skeletal samples. This has been es- resentation of the marine components. Consump- pecially useful in dating bone fragments where only tion of large quantities of oi5N-depleted legumes small quantities are available. In cases where the (VIRGINIA and DELWICHE, 1982) might also majority of the intact collagen remains, it is widely skew marine/terrestrial contrasts, although this has agreed that accurate i4C age estimates can be ob- yet to be documented in human populations tained on this purified organic extract. However, (SCHWARCZet al., 1985). accepted methods used to chemically pretreat bone The combined application of carbon and nitrogen samples can, in some cases, yield unreliable
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