• 14C and 234U-Excess Dating of Groundwater in the Haifa Bay Region, Israel

      Rogojin, Vasily; Carmi, Israel; Kronfeld, Joel (Department of Geosciences, The University of Arizona, 1998-01-01)
      Radiocarbon activities and uranium isotopic disequilibria were measured in water samples from both the sandy Pleistocene coastal aquifer and within the upper Cretaceous Judea Group carbonate aquifer of northwestern Israel. The samples in both aquifers exhibit a decrease in 14C activity that is concomitant to the growth in 234U-excess. This suggests that under specific conditions, 234U-excess dating of groundwater can be used to corroborate 14C dates, while offering the possibility of extending the range of dating of groundwater beyond that of 14C.
    • 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.
    • Radiocarbon Calibration and Comparison to 50 kyr BP with Paired 14C and 230Th Dating of Corals from Vanuatu and Papua New Guinea

      Cutler, K. B.; Gray, S. C.; Burr, G. S.; Edwards, R. L.; Taylor, F. W.; Cabioch, G.; Beck, J. W.; Cheng, H.; Moore, J. (Department of Geosciences, The University of Arizona, 2004-01-01)
      We calibrated portions of the radiocarbon time scale with combined 230Th, 231Pa, 14C measurements of corals collected from Espiritu Santo, Vanuatu and the Huon Peninsula, Papua New Guinea. The new data map 14C variations ranging from the current limit of the tree-ring calibration (11,900 calendar years before present [cal BP], Kromer and Spurk 1998, now updated to 12,400 cal BP, see Kromer et al., this issue), to the 14C-dating limit of 50,000 cal BP, with detailed structure between 14 to 16 cal kyr BP and 19 to 24 cal kyr BP. Samples older than 25,000 cal BP were analyzed with high-precision 231Pa dating methods (Pickett et al. 1994; Edwards et al. 1997) as a rigorous second check on the accuracy of the 230Th ages. These are the first coral calibration data to receive this additional check, adding confidence to the age data forming the older portion of the calibration. Our results, in general, show that the offset between calibrated and 14C ages generally increases with age until about 28,000 cal BP, when the recorded 14C age is nearly 6800 yr too young. The gap between ages before this time is less; at 50,000 cal BP, the recorded 14C age is 4600 yr too young. Two major 14C-age plateaus result from a 13 ppm drop in Delta-14C between 14-15 cal kyr BP and a 700ppm ppm drop in Delta-14C between 22-25 cal kyr BP. In addition, a large atmospheric Delta-14C excursion to values over 1000 ppm occurs at 28 cal kyr BP. Between 20 and 10 cal kyr BP, a component of atmospheric Delta-14C anti-correlates with Greenland ice delta-18O, indicating that some portion of the variability in atmospheric Delta-14C is related to climate change, most likely through climate-related changes in the carbon cycle. Furthermore, the 28-kyr excursion occurs at about the time of significant climate shifts. Taken as a whole, our data indicate that in addition to a terrestrial magnetic field, factors related to climate change have affected the history of atmospheric 14C.