• Major Recent Tectonic Uplift in Iskenderun Bay, Turkey

      Koral, H.; Kronfeld, J.; Avsar, N.; Yanko, V.; Vogel, J. C. (Department of Geosciences, The University of Arizona, 2001-01-01)
      Radiocarbon dating was carried out in the sediment profiles of four marine sediment cores taken from Iskenderun Bay, Turkey. The bay is quite shallow in the present day, and a previous tectonic study had considered that the bay floor might have been subsiding. However, this cannot be so, for the 14C ages would thereby lead to the apparent paradox of normal marine sedimentation having taken place during times when glacio-eustatic sea level lowering would have exposed the bay floor. Rather, we conclude that the floor of Iskenderun Bay on the whole has been experiencing rapid uplift since the end of the Last Glacial, due to a combination of tectonic factors linked to the compression between the Anatolian and African plates.
    • Radiocarbon Age Profiles and Size Dependency of Mixing in Northeast Atlantic Sediments

      Brown, Louise; Cook, Gordon T.; MacKenzie, Angus B.; Thomson, John (Department of Geosciences, The University of Arizona, 2001-01-01)
      In recent years, the most common technique for radiocarbon dating of deep-ocean sediments has been accelerator mass spectrometry (AMS) analysis of hand-picked planktonic forminifera (forams). Some studies have exposed age offsets between different sediment size fractions from the same depth within a core and this has important implications when establishing a chronological framework for palaeoceanographic records associated with a particular sediment component. The mechanisms generating the age offsets are not fully understood, a problem compounded by the fact that the fraction defined as "large"varies between different studies. To explore this problem, we dated samples of hand-picked forams from two Biogeochemical Ocean Flux Study (BOFS) cores, for which the presence of an offset between the bulk carbonate and >150 micrometers foraminiferal calcite had already been demonstrated. The presence of a constant age offset between bulk carbonate and coarse fraction material at the two BOFS sites has been confirmed, but the magnitude of the offset is dependent on whether a simple size-separation technique or hand-picking of well-preserved forams is applied. This may be explained if the selection of well preserved forams biases the sample towards those specimens that have spent least time in the surface mixed layer (SML) or have undergone less size selective mixing. Modeling of the 14C profiles demonstrates that SML depth and sediment accumulation rates are the same for both the bulk and coarse sediment fractions, which is consistent with the hypothesis that size-selective mixing is responsible for the age offset.
    • The Late Quaternary Sedimentary Record of Reykjanes Ridge, North Atlantic

      Prins, M. A.; Troelstra, S. R.; Kruk, R. W.; van der Borg, K.; de Jong, A. F. M.; Weltje, G. J. (Department of Geosciences, The University of Arizona, 2001-01-01)
      Variability in surface and deep ocean circulation in the North Atlantic is inferred from grain-size characteristics and the composition of terrigenous sediments from a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of grain size data shows that deep-ocean circulation in this area decreased significantly during periods of maximum iceberg discharge. The episodes of reduced circulation correlate with the cold and abrupt warming phases of the Dansgaard-Oeschger cycles as recognized in the Greenland ice cores.