Soil radiocarbon analysis and soil dating

Abstract

Abstract. Based on 14C dating principles, techniques of measuring 14C concentrations in pedogenic organic materials have been well established. Basic lines of soil sample pretreatment for radiocarbon analysis by liquid scintillation are shown. Possible influences of lithogenic carbon on the results obtained are discussed.Soil organic matter 14C analysis results can-according to the radiocarbon dating formula-be interpreted as minimum ages of the soil under investigation if strong evidence for autochthonous pedogenic origin of the material is given. In case of fossil soils, minimum age is maximum estimate for period of burial. The gap existing between minimum age and true age can be reduced by special techniques of fractionation, as well as by taking account of additional pedological information on turnover features of the material. Discussion of possible soil organic matter turnover mechanisms leads to a theory of rates of decomposition high in the initial stages and slowing down to nearly zero with time.Application of 14C techniques to humus dynamics until now yielded apparently contradictory results on relative ages of different humic matter fractions. Enrichment of old material in the HC1 hydrolysis residue fraction, however, has been observed in all instances.Mathematical modelling of soil radiocarbon response on varying turnover parameters results in estimates of turnover time and migration speed of soil organic matter in total as well as fractions thereof. Zusanmlenfassung. Ausgehend von den Grundlagen der 14C Datierung wird das Verfahren der 14CMessung an organischem Bodenmaterial mit Hilfe der Flfissigkeits Szintillations Methode kurz erl~iutert. Ernste meJ3technische Probleme liegen heute nicht mehr vor. Als Mangel wird die uneinheitliche Probenvorbereitung bei CaCO 3 haltigen und CaCO3 freien Proben empfunden.Die bodendynamisch bedingte altersm~J3ige Inhomogenitfit der Proben erm6glicht es nicht, das 'wahre Alter' einer Probe, d. h. das Alter des filtesten Tells der in der Probe enthaltenen organischen Stofte nach der Datierungsformel zu berechnen. Eine genaue Analyse zeigt vielmehr, daJ3 der nach der Formel berechnete Wert nur als Minimalabsch~itzung flit das Alter der Probe gesichert ist. Das so ermittelte Minimalalter der untersuchten organischen Substanz ist gleichzeitig ein Minimalalter fiir den zugeh6rigen Boden, falls feststeht, da~ das erfaJ3te Material autochthon pedogen ist und weder aus dem Ausgangsmaterial stammt noch durch Lateraltransport in den Boden gelangte. Das Minimalalter fossiler B6den ist gleichzeitig eine Maximalabschfitzung ihrer Uberdeckungszeit.Dutch Isolierung einer an filterem Material angereicherten Fraktion der organischen Substanz und ihre gesonderte Untersuchung kann ein Minimalalter ffir den untersuchten Boden gewonnen werden, das dem 'wahren Alter' n~iher kommt. Effolge wurden bisher mit der Rfickstandsfraktion von HC1 Hydrolysen erzielt. Die Frage nach der Fraktion h6chsten Alters ist noch often.Ober das durch Fraktionierungsmethoden gewonnene MaJ3 hinaus kann Aufschlu...

Late Holocene solifluction lobe activity in the Mount Rae area, southern Canadian Rocky Mountains

“…In most pedogenic settings the organic matter present is of mixed age because the upper part of a soil profile is biologically active and in dynamic equilibrium with the 14C of the atmosphere (Paul 1969;Scharpenseel and Schiffmann 1977b;Ellis and Matthews 1984). This results in relatively low radiocarbon ages for thin youthful soils (Benedict 1966;Ellis 1979) and for the near-surface layers of deeper organic horizons (Matthews 1981, 1984: Matthews and Dresser 1983.…”

“…Cretaceous coals are common in this region (Allan and Carr 1947) and are a major problem in 14C dating (see Scharpenseel and Schiffmann 1977~). Unfortunately, as is characteristic of virtually all of the southem Canadian Rocky Mountains, the calcareous nature of most of the local drift meant there was still the possibility of sample contamination by inorganic carbonates (see Scharpenseel and Schiffmann 1977b;Ellis and Matthews 1984).…”

The Loess/Paleosol record and the nature of the younger Dryas climate in central China

“…This is related to a general problem associated with dating soils everywhere; the accurate dating of soil carbon is notoriously complex due to a variety of potential contamination vectors (Chichagova and Cherkinsky, 1993;Evans, 1985;Liu, 1985;Scharpenseel, 1971;Scharpenseel and Schiffmann, 1977). Generally, "in soil 14 C dating, 'soil age' is definable and applicable for absolute dating in a limited way, only in the case of well-protected paleosols, charcoal or wood relics" (Scharpenseel and Becker-Heidmann, 1992:548).…”

Radiocarbon ages of pedogenic carbonate nodules from Coimbatore region, Tamil Nadu

“…The addition of old carbon from these sources may alter the calcic nodule's radiocarbon age because geological limestone carbon used to form CaCO 3 contains significantly less radiocarbon than the atmosphere. The net effect is that 14 C dates on calcic nodules can be "older" than the nodules true geological age (Scharpenseel and Schiffmann, 1977;Stafford Jr. 1998;Chen et al 2002).…”

“…In contrast, upon formation, nodules from the carbonatebearing sediments or from sediments where pedogenic or groundwater carbonate are common are susceptible to postmortem exchange of their indigenous carbonate with foreign (exogenous) carbonate (Scharpenseel and Schiffmann, 1977;Stafford Jr. 1998;Chen et al 2002;Deutz et al 2002). This secondary carbonate can have radiocarbon contents ranging from modern values to undetectable, the latter representing carbon from geologically ancient carbonates.…”