• Tucson, One Year Later…

      Mook, W. G. (Department of Geosciences, The University of Arizona, 1998-01-01)
    • Associate Editors

      Department of Geosciences, The University of Arizona, 1998-01-01
    • Using 14C as a Tracer of Carbon Accumulation and Turnover in Soils

      Milton, G. M.; Kramer, S. J. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Three very different Canadian soils—clay soils of the St. Lawrence Lowlands, sandy forest soils of the Ottawa Valley, and organic-rich sediments from a wetland on the Canadian Shield—have been cored, sliced and separated into different density fractions, and the radiocarbon content of these soil fractions measured. In two of the areas sampled, cores were obtained close to operating nuclear reactors, as well as from beyond their region of influence. As a consequence, it has been possible to ascertain the depths of penetration of both the weapons-testing pulse (peaking in 1963), and a 25-50-yr chronic reactor input of 14C. The percentage of carbon stored in different density fractions varied with soil type. Turnover times for bulk soil organic carbon, estimated from soil degassing rates, have been compared with those predicated on the residual "bomb" 14C in background cores, and/or on the ratio of reactor-emitted 14C retained in the soils to the total deposited during the lifetime of operation. Residence times for the heavy carbon fraction present at depths below the influence of anthropogenic inputs have also been estimated. The accumulated data will be incorporated in a revised soil model, adjusted for the parameters deemed to be most important to carbon turnover rates under Canadian conditions.
    • The 14C Age of Palsas in Northern Eurasia

      Vasil, Chuk K.; Vasil, Chuk C. (Department of Geosciences, The University of Arizona, 1998-01-01)
      We discuss results of 14C dating peat of palsa of different regions of Northern Eurasia. We apply these dates to determine the age of active palsa growth during different periods of the Holocene in permafrost zone.
    • The 14C Age of Humic Substances in Paleosols

      Alexandrovskiy, Alexander L.; Chichagova, Olga A. (Department of Geosciences, The University of Arizona, 1998-01-01)
      By comparing the radiocarbon age of the soils under burial mounds of known archaeological age with the surface-exposed (background) soils of the surrounding landscapes, we may evaluate the rates of humus renewal in these soils. In the cold climate of the mideastern areas of the Russian plains, the value of humus rejuvenation coefficient decreases. This shows that humus renewal is 5-10 times slower than in the warmer climate of the southern regions. Using the obtained data on the rejuvenation rate of humus substances, we can determine the age of paleosols and study the dynamics of the carbon exchange processes in the biosphere.
    • Temporal Changes of the 14C Reservoir Effect in Lakes

      Geyh, Mebus A.; Schotterer, U.; Grosjean, M. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Conventional radiocarbon dates for sediment samples from aquatic systems and of coeval terrestrial samples deviate from each other due to the reservoir effect. The reservoir correction is usually assumed to be constant with time for a specific aquatic system. Our studies confirm that seasonal and secular changes are frequent and are governed by the limnological conditions. Lakes have two principal sources of 14C: atmospheric CO2 and the total dissolved inorganic carbon (TDIC) of the entering groundwater and runoff. The former has values of ca. 100 pMC; the latter usually has a 14C value well below 100 pMC. Atmospheric CO2 enters the lake by exchange via its surface. The proportions of these two kinds of input determine the magnitude of the reservoir correction in freshwater lakes. It is mainly a function of the volume/surface ratio of the lake and, consequently a function of the water depth. The surface of lakes with outflow does not change when sedimentation decreases the depth of the water. The depth of Schleinsee Lake in southern Germany has decreased from 30 to 15 m since ca. 9000 BP. As a result, the reservoir correction has decreased from ca. -1550 to -580 yr. In contrast, the depth of Lake Proscansko in Croatia increased with growth of the travertine dam and the reservoir correction changed from ca. -1790 to -2650 yr during the last 8800 yr. The largest fluctuations of lake levels occur in closed lakes in arid regions when the climate changes from humid to arid and vice versa. As a result, the reservoir correction of the 14C dates for the total organic fraction from Lejía Lake in the Atacama Desert of Chile varied between <-1800 yr and -4700 yr over a period of only 1800 yr between 11,500 and 9700 BP. The corresponding reservoir correction for the marl fraction is much higher. In summary, accurate and reliable 14C dating of lake sediments requires a study of the temporal changes of the reservoir effect by analysis of both the organic and marl fractions. The most reliable 14C dates are obtained from terrestrial plant remains.
    • Study of the Effect of Fossil Organic Carbon on 14C in Groundwater from Hvinningdal, Denmark

      Boaretto, E.; Thorling, L.; Sveinbjörnsdóttir, Á. E.; Yechieli, Y.; Heinemeier, J. (Department of Geosciences, The University of Arizona, 1998-01-01)
      The carbonate hydrochemistry of groundwater from the Hvinningdal aquifer (Denmark) was studied by radiocarbon (accelerator mass spectrometry (AMS)) and delta-13C measurements as a preliminary step towards 14C groundwater dating. The 14C concentrations varied between 30 and 100 percent modern carbon (pMC) in apparent contradiction with tritium (3H) data, which in most cases indicate a post-bomb date. The dilution of 14C can be explained as being due to the combined effect of dissolution of old soil carbonate and oxidation of old organic carbon. The last effect proved to be essential. To calculate this correction the dissolved oxygen concentration was used together with the delta-13C values. The combined corrections bring the 14C concentrations up to post-bomb levels in better agreement with the 3H data.
    • Soil Organic 14C Dynamics: Effects of Pasture Installation on Arable Land

      Römkens, Paul F. A. M.; Hassink, Jan; van der Plicht, Johannes (Department of Geosciences, The University of Arizona, 1998-01-01)
      In a study addressing composition and recovery of soil carbon following pasture installation on arable land, radiocarbon isotope ratios were measured in sizeand density-separated soil organic matter (SOM) fractions in a pasture and maize plot. The average soil carbon age increased with depth from 444 yr in the 0-30-cm layer to 2456 yr in the 60-80-cm layer in the pasture soils, and from 42 to 1625 yr in the maize-cultivated soil. Weight fractionation of the macro-organic matter (size >150 micrometers) in a light (density <1.17 g cm-3) intermediate (1.17 g cm-3 < density < 1.37 g cm-3), and heavy fraction (density >1.37 g cm-3) resulted in markedly different ages for different fractions with ages increasing from 2 yr in the light fraction to >3000 yr in the heavy fractions. 13C and 14C (accelerator mass spectrometry (AMS)) isotope ratios in the <20 micrometers fraction in the 60-80-cm layer indicated that vertical displacement of colloidal organic material occurred during maize cropping. The physical fractionation method, in combination with natural level 13C and 14C analysis, resulted in a better insight in carbon dynamics that occur after the conversion of arable land to pasture.
    • Reappraisal of Chinese Loess Plateau Stratigraphic Sequences Over the Last 30,000 Years: Precursors of an Important Holocene Monsoon Climatic Event

      Zhou, Weijian; An, Zhisheng; Jull, A. J. T.; Donahue, D. J.; Head, M. J. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Through the establishment of radiocarbon chronozones relating common geological events within lacustrine and eolian sediments from five profiles representative of loess yuan (tablelands), river valley and northwest margin features of the Loess Plateau, we propose a series of stratigraphic divisions within the last 30 ka. The focus of this detailed study involves stratigraphic relationships contributing to evidence of Younger Dryas events, with the recognition of cold-dry, cool-wet and cold-dry periods represented within the Heiheze silt, Midiwan peat and Liushuwan eolian sand. The stratigraphic profiles reflect century-scale fluctuations of the East Asian monsoons. The precursor events enable us to place the Pleistocene/Holocene boundary at 10,000 BP.
    • Reconstruction of Microenvironmental Changes in the Kopasz Hill Loess Area at Tokaj (Hungary) Between 15 and 70 ka BP

      Sümegi, Pál; Hertelendi, Ede (Department of Geosciences, The University of Arizona, 1998-01-01)
      We collected 11 Kopasz Hill loess profiles for paleoecological and geochronological analysis. The loess accumulation and development formed during the last (Weichselian) glacial period between 70 and 15 ka BP. We found that the majority of the loess profiles were composed of three typical loess strata and two well-developed paleosol horizons. Based on vertebrate remains, the lowest loess layer formed between 70-50 ka BP, during the first cool and dry climatic phase of the last glacial period, when forest steppe vegetation dominated in the Kopasz Hill area. On the surface of the lowermost layer, a paleosol developed between 50 and 40 ka BP as an indication of a more humid and warmer climatic phase. This paleosol layer was buried by a new loess layer that developed between 40 and 32 ka BP. The upper paleosol horizon developed between 32 and 26 ka BP. Molluscs preferring a mild climate were found in this layer, suggesting that this phase was wet and relatively temperate. A number of fired macrocharcoal remains can be found on the top of this paleosol layer. Charcoal samples from nine sites were dated by radiocarbon analyses. These results reflect the presence of a charcoal-rich horizon that developed 28-26 ka BP. Ca. 26 ka BP, loess formation resumed. We analyzed 14 samples from 6 sites by the 14C method. Based on 14C data, the uppermost part of loess profiles developed between 26 and 15 ka BP.
    • Radiometric Dating of Young and Old Calcrete

      Geyh, Mebus A.; Eitel, Bernhard (Department of Geosciences, The University of Arizona, 1998-01-01)
      To obtain a better understanding of the relationship between calcrete genesis and the results of different absolute dating methods, thermoluminescence (TL), radiocarbon (14C) and uranium/thorium (U/Th) were applied to coeval samples taken from a very young calcrete profile in Namibia. The methodically different ages reflect the characteristics of the applied dating methods, the genetics of calcrete and different events of calcrete genesis. The conventional 14C ages and the TL dates cover the last 50 ka, while the corresponding U/Th dates of coeval samples are many times larger. Uranium-series dates are not related to the deposition of the host material or to its cementation if the ages are smaller than ca. 120 ka. The TL clock is set to zero during eolian transport and the corresponding radiometric ages of the quartz and feldspar grains date the time of their deposition. The 14C ages of the cement correspond, on the other hand, to a time shortly after the onset of the cementation and long before its termination. In the case of very old calcrete, the mixture of young and old cement results in ambiguous ages if they cannot be confirmed by an independent technique.
    • Minimal Extension Phases of Unteraarglacier (Swiss Alps) During the Holocene Based on 14C Analysis of Wood

      Hormes, Anne; Schlüechter, Christian; Stocker, Thomas F. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Tree trunks and wood fragments in minerotrophic fen peat that accumulated as the result of a jökulhlaup in the outwash plain of Unteraarglacier were radiocarbon-dated using conventional beta -counting. Different pretreatment methods were tested on two wood samples to determine the reliability of our dates. We dated the wood compounds after extended acid-alkali-acid treatment, as well as extraction of cellulose and lignin. The results of the samples Picea (B-6687) and Pinus cembra (B-6699) show insignificant differences of <1 sigma. The 14C dates represent retreat of Unteraarglacier due to warmer and/or drier phases in the Holocene compared to modern climate conditions. The glacier was at least several hundred meters smaller in extent than today ca. 8100-7670 BP, 6175-5780 BP, 4580-4300 BP, 4100-3600 BP and 3380-3200 BP. The 14C dates suggest a ca. 2000-yr cyclicity of tree growth in the area covered by the present Unteraarglacier. The most intense warm and dry period occurred between 4100 BP (probably extending back to 4580 BP) and 3600 BP, with growth of fen peat between 3800 and 3600 BP attributed to wetter conditions.
    • Molecular, Radioactive and Stable Isotope Characterization of Estuarine Particulate Organic Matter

      Megens, Luc; van der Plicht, Johannes; D Leeuw, J. W. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Organic matter in sediments and suspended matter is a complex mixture of constituents with different histories, sources and stabilities. To study these components in a suspended matter sample from the Ems-Dollard Estuary, we used combined molecular analysis with pyrolysis/gas chromatography/mass spectrometry and stable and radioactive carbon isotope analyses of the bulk and separated chemical fractions. Carbohydrates and proteins, ca. 50% of the total organic carbon (TOC), are much younger than the bulk sample and have a somewhat higher delta-13C value. Lipids and the final residue are considerably older and have lower delta-13C values. The final residue, ca. 17% of the total carbon, consists mainly of aliphatic macromolecules that could be derived from algae or terrestrial plants. The delta-13C value points to a marine origin.
    • Isotopic Analysis and Cycling of Dissolved Inorganic Carbon at Lake Biwa, Central Japan

      Nakamura, Toshio; Kojima, Sadao; Ohta, Tomoko; Oda, Hirotaka; Ikeda, Akiko; Okuno, Mitsuru; Yokota, Ki-Ichiro; Mizutani, Yoshihiko; Kretschmer, Wolfgang (Department of Geosciences, The University of Arizona, 1998-01-01)
      This paper reports on concentrations and carbon isotopic results of dissolved inorganic carbon (DIC) in water samples collected at four locations and from several depths in Lake Biwa, central Japan, covering every season of the year, starting in the spring of 1995. Depth profiles of DIC concentration and DIC delta-13C showed a strong seasonal pattern, as a result of vertical mixing of the lake water in winter and early spring, or lack of mixing in the other seasons. No seasonal change in DIC Delta-14C depth profiles was recognizable, mainly owing to the wide scatter of DIC Delta-14C. Values typically ranged from 0.47 to 0.65 mmol kg-1 for DIC concentration, and from -4 to -8 per mil from +10 to +80 per mil for DIC delta-13C and DIC Delta-14C, respectively, for the Lake Biwa water.
    • Holocene Paleoenvironmental Changes in the Lower Mahi Basin, Western India

      Kusumgar, Sheela; Raj, Rachna; Chamyal, L. S.; Yadav, M. G. (Department of Geosciences, The University of Arizona, 1998-01-01)
      Evidence of paleoenvironmental changes during the Holocene from the Lower Mahi basin of Western India have been documented. The unpaired S2 surface all along the estuarine zone of the Mahi basin has been identified as an uplifted marine terrace. The terraces have preserved in their lithosections fairly distinct horizons of grayish brown clays rich in marine microfauna. The intervening silty-sand horizons are indicative of freshwater origin. The sedimentary structure and faunal assemblage indicate that these units have been deposited in a marginal marine environment. The 14C ages obtained on these marine mud horizons show that between 3600 and 1700 BP the sea level was higher than at present. The geomorphic evidence suggests that a late Holocene uplift has played a significant role in lowering the relative sea level to its present position.
    • Carbon Isotope Variations and Chronology of the Last Glacial-Interglacial Transition (14-9 ka BP)

      Turney, Chris M.; Harkness, Douglas D.; Lowe, J. J. (Department of Geosciences, The University of Arizona, 1998-01-01)
      We present delta-13C data from both bulk organic sediment samples and terrestrial plant macrofossils from five high-resolution sedimentary sequences from the United Kingdom from which extensive multiproxy data sets have been obtained. These span the last glacial-interglacial transition. Chronological control has been provided by radiocarbon dating and/or tephrochronology. The results demonstrate that significant shifts in bulk organic delta-13C can be identified at key climatic transitions in most of the sites. The data are affected by site-specific influences that restrict their use as chronological markers. However, terrestrial plant macrofossil records are more consistent and reveal shifts that appear to be synchronous and which therefore offer a basis for interregional correlation as well as significant paleoenvironmental information.
    • Apparent 14C Ages of Marine Mollusk Shells from a Greek Island: Calculation of the Marine Reservoir Effect in the Aegean Sea

      Facorellis, Yorgos; Maniatis, Yannis; Kromer, Bernd (Department of Geosciences, The University of Arizona, 1998-01-01)
      The excavation of the Cyclope cave, situated on the deserted island of Youra in the Northern Sporades (39 degrees 22'N, 24 degrees 10'E), revealed material of marine and terrestrial origin in undisturbed layers, suitable for radiocarbon dating. In some cases, material from different reservoirs was found together in the same archaeological layer. This research had two aims. The first was the dating of charcoal-seashell pairs in order to determine the marine reservoir effect in this region, based on samples with ages spanning from the end of the 8th millennium to the beginning of the 7th millennium BC. The second aim was dating the stratigraphy of this site, by using the calculated Delta-R value in conjunction with the marine calibration curve. This enabled the accurate calibration of the 14C ages of marine samples found in layers without charcoal pieces. The results show that this is the oldest human settlement ever found on an island in the Aegean Sea.
    • Application of 14C Data for the Estimation of Sphagnum Peat Increment in Estonian Ombrotrophic Mires

      Punning, J.-M.; Koff, Tiiu (Department of Geosciences, The University of Arizona, 1998-01-01)
      We calculated apparent increment values based on the radiocarbon dates of 21 Estonian ombrotrophic mires (raised bogs). For short periods, the values vary significantly, but the integrated increment for the total complex of ombrotrophic peat shows a strong increasing tendency with decreasing peat age. This is probably due to the decay of accumulated organic matter. Our hypotheses concern the mechanisms of decay and methods for increasing the reliability of the interpolation and extrapolation of 14C data.
    • AMS 14C Dating Historic Eruptions of the Kirishima, Sakurajima and Kaimondake Volcanoes, Southern Kyushu, Japan

      Okuno, Mitsuru; Nakamura, Toshio; Kobayashi, Tetsuo (Department of Geosciences, The University of Arizona, 1998-01-01)
      In the historic period, several large eruptions were recorded from Kirishima, Sakurajima and Kaimondake volcanoes in southern Kyushu, Japan. Estimated dates of volcanic activity were established on these volcanoes through historical documentation of major eruption events. This study presents the correspondence between these documents and the records of AMS 14C dating of soils underlying tephra layers. We conclude that AMS 14C dates of soil materials can be useful in correlating tephra layers with documentary records of eruption.