• Soil Organic Matter Decomposition and Turnover in a Tropical Ultisol: Evidence from delta-13C, delta-15N and Geochemistry

      Krull, Evelyn S.; Bestland, Erick A.; Gates, Will P. (Department of Geosciences, The University of Arizona, 2002-01-01)
      Soil organic matter (SOM), leaf litter, and root material of an Ultisol from the tropical rainforest of Kakamega, Kenya, were analyzed for stable carbon (delta-13C) and nitrogen (delta-15N) isotopic values as well as total organic carbon (TOC) and total nitrogen (TN) contents in order to determine trends in SOM decomposition within a very well-developed soil under tropical conditions. In addition, we quantified mineralogy and chemistry of the inorganic soil fraction. Clay mineralogical variation with depth was small and the abundance of kaolin indicates intense weathering and pedoturbation under humid tropical conditions. The soil chemistry was dominated by silica, aluminium, and iron with calcium, potassium, and magnesium as minor constituents. The relative depletion of base cations compared with silica and aluminium is an indicator for intense weathering and leaching conditions over long periods of time. Depth profiles of delta-13C and delta-15N showed a distinct enrichment trend down profile with a large (average 13Delta-C = 5.0 per mil average 15Delta-N = 6.3 per mil) and abrupt offset within the uppermost 10-20 cm of the soil. Isotopic enrichment with depth is commonly observed in soil profiles and has been attributed to fractionation during decomposition. However, isotopic offsets within soil profiles that exceed 3 per mil are usually interpreted as a recent change from C4 to C3 dominated vegetation. We argue that the observed isotopic depth profiles along with data from mineralogy and chemistry of the inorganic fraction from the Kakamega Forest soil are a result of intense weathering and high organic matter turnover rates under humid tropical conditions.
    • Are the 14C Dates of the Dead Sea Scrolls Affected by Castor Oil Contamination?

      Carmi, Israel (Department of Geosciences, The University of Arizona, 2002-01-01)
      The paper "The effects of possible contamination on the radiocarbon dates of the Dead Sea Scrolls I: castor oil" by Rasmussen et al. (2001) is discussed. Detailed analysis of the extant dates of the Dead Sea Scrolls suggests that the pretreatment of the samples was adequate. Errors and omissions in the paper are discussed and the implications of the experiment of Rasmussen et al. (2001) are questioned.
    • A Simple Procedure for Evaluating Global Cosmogenic 14C Production in the Atmosphere Using Neutron Monitor Data

      Lowe, D. C.; Allan, W. (Department of Geosciences, The University of Arizona, 2002-01-01)
      Radiocarbon (14C) produced by cosmogenic processes in the atmosphere reacts rapidly with atomic oxyen to form 14CO. The primary sink for this species is oxidation by the OH radical, the single most important oxidation mechanism for pollutants in the atmosphere. Hence, knowledge of the spatial and temporal distribution of 14CO allows important inferences to be made about atmospheric transport processes and the distribution of OH. Because the chemical lifetime of 14CO against OH attack is relatively short, 1-3 months, its distribution in the atmosphere should show modulations due to changes in 14C production caused by variations in the solar cycle. In this work we present a simple methodology to provide a time series of global 14C production to help interpret time series of atmospheric 14CO measurements covering the whole of solar cycle 23. We use data from neutron monitors, a readily available proxy for global 14C production, and show that an existing 6-year time series of 14CO data from Baring Head, New Zealand, tracks changes in global 14C production at the onset of solar cycle 23.
    • WOCE Radiocarbon IV: Pacific Ocean Results; P10, P13N, P14C, P18, P19 & S4P

      Key, Robert M.; Quay, Paul D.; Schlosser, Peter; McNichol, A. P.; von Reden, K. F.; Schneider, Robert J.; Elder, Kathy L.; Stuiver, Minze; Östlund, H. Göte (Department of Geosciences, The University of Arizona, 2002-01-01)
      The World Ocean Circulation Experiment, carried out between 1990 and 1997, provided the most comprehensive oceanic study of radiocarbon to date. Approximately 10,000 samples were collected in the Pacific Ocean by U.S. Investigators for both conventional large volume B counting and small volume accelerator mass spectrometry analysis techniques. Results from six cruises are presented. The data quality is as good or better than previous large-scale surveys. The 14C distribution for the entire WOCE Pacific data set is graphically described using mean vertical profiles and sections, and property-property plots.
    • Marine Radiocarbon Reservoir Corrections for the Mediterranean and Aegean Seas

      Reimer, P. J.; McCormac, F. G. (Department of Geosciences, The University of Arizona, 2002-01-01)
      Radiocarbon measurements of nine known age shells from the Mediterranean and the Aegean Seas combined with previous measurements provide an updated value for Delta-R, the local variation in the reservior correction for marine samples. Comparison of pre-1950s samples from the Algerian coast, with one collected in 1954, indicates early incorporations of nuclear weapons testing 14C into the shallow surface waters of the Mediterranean. Comparisons between different basins indicate the surface waters of the Mediterranean are relatively homogenous. The recommended Delta-R for calibration of the Mediterranean marine samples with the 1998 marine calibration dataset is 58 +/85 14C yr, but variations in the resevoir age beyond 6000 cal BP should be considered.