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  Radiocarbon and Uranium-Series Dating of the Plitvice Lakes Travertines

  Srdoč, Dušan; Osmond, J. K.; Horvatinčić, Nada; Dabous, Adel A.; Obelić, Bogomil (Department of Geosciences, The University of Arizona, 1994-01-01)

  Radiocarbon and uranium-series ages of the calcareous deposits of the Plitvice Lakes show that travertines were deposited during three warm, humid, interglacial oxygen isotope stages. According to our measurements, only calcite crystals or crystal aggregates represent reliable material for both 230Th/234U and 234U/238U dating. Compact old travertine in the form of sandstone is less reliable; it can be dated by both methods provided that its detrital contamination is not significant, demonstrated by very low 14C activity (<1.5-2.0 pMC) and a high 230Th/232Th ratio. Old porous travertine contaminated with recent carbonates and Th-bearing clay (pMC > 5, 230Th/232Th < 5) gives erroneous results by both methods. Stage 1(Holocene) deposition is shown primarily by 14C dating corroborated by sedimentological and palynologic studies as well as by both 230Th/ 234U and 234U/238U disequilibrium methods. The intensive growth of travertine barriers coincided with significant climate warming in the Holocene. Stage 5 deposition is confirmed by the 230Th/234U dating of crystalline calcite aggregates embedded in the travertine matrix and by concordant 230Th/234U and 234U/238U ages, assuming that the 234U/238U activity ratio of 1.88 observed in modern streams and in Holocene deposits can be extended to past epochs. The travertine deposition period was very short, peaking ca. 120 +/- 10 ka BP. Stage 11 deposition is indicated by 234U/238U dating only, the period being within the 234U decay range, but not that of 230Th. Stage 11 travertine was deposited ca. 420 +/- 50 ka BP. We did not find travertine samples with U-series ages indicating a growth period during relatively warm Stages 7 and 9; due to the scarcity of old travertine outcrops, these and possibly other stages cannot be excluded on the basis of presented data. All of these isotopic dating results concur with the field relation of the travertine complex of the Plitvice Lakes.
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  A Method for Quantifying Deep-Sea Carbonate Dissolution Using 14C Dating

  van Kreveld, S. A.; Ganssen, G. M.; van der Plicht, J. E.; Melkert, M. M.; Troelstra, S. R.; van der Borg, K.; de Jong, Arie (Department of Geosciences, The University of Arizona, 1995-01-01)

  We quantified the rate of carbonate dissolution with increasing water depth by taking the difference in the carbonate mass accumulation rate of deep (3393-4375 m) core top sediments from the shallowest one (3208 m), which we assumed was unaffected by dissolution. This method depends on high quality 14C dates that we calibrated to calendar years for calculating sedimentation rates. Our results show low (ranging from 0 to 0.3 g cm-2 ka-1) and high (ranging from 1.5 to 1.7 g cm-2 ka-1) carbonate dissolution rates, above and below 4000 m, respectively. Therefore, we interpret the sudden increase in the carbonate dissolution rate at 4000-m water depth to mark the lysocline.
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  Carbon Isotope Analysis of Land Snail Shells: Implications for Carbon Sources and Radiocarbon Dating

  Goodfriend, Glenn A.; Hood, Darden G. (American Journal of Science, 1983-01-01)

  13C and 14C analyses were performed on a series of modern Jamaican land snails in order to quantitatively determine the sources of shell carbon. A model of these carbon sources, the pathways by which carbon reaches the shell, and the fractionation processes involved are presented. The contribution of limestone to shell carbonate is variable but may comprise up to 33% of the shell. About 25-40% of shell carbonate is derived from plants and about 30-60% from atmospheric CO2. Variation among populations and species with respect to 13C and 14C is attributed to the effects of limestone incorporation, snail size (as it affects CO2 exchange rate), physiological characteristics (presence of urease, respiration rate), and activity patterns of the snails. A formula for correction for isotopic fractionation of 14C of shell carbonate, based on "C measurements, is derived. Bicarbonate-aragonite fractionation is apparently very minimal. Shell organic carbon appears to be derived largely from plants but also to a lesser extent from inorganic hemolymph carbon. This introduces the possibility of a small age anomaly of shell organic 14C due to limestone incorporation.