Ryan William B. F.

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Ryan
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William B. F.
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  • Preprint
    The co-evolution of Black Sea level and composition through the last deglaciation and its paleoclimatic significance
    ( 2006-01-28) Major, Candace O. ; Goldstein, Steven L. ; Ryan, William B. F. ; Lericolais, Gilles ; Piotrowski, Alexander M. ; Hajdas, Irka
    The strontium and oxygen isotopic compositions of carbonate shells are a measure of the water delivered to the Black Sea lake since the last glacial maximum. Commencing at ~18 ka BP cal with the arrival of substantial meltwater from the Alpine and northern European ice sheets and overflow via the Caspian Sea from the disintegrating Siberian ice cover, the 87Sr/86Sr ratio rose rapidly from a glacial minima around 0.7087 to reach a set of peaks near 0.7091 in layers of conspicuous reddish-brown clay with a mineralogy of Eurasian provenance. The 87Sr/86Sr ratio oscillates between high in the red-brown layers to low in interbedded gray clays with glacial era mineralogy, indicative that the meltwater came in pulses. On the other hand, the rise of the δ18O ratio from glacial low values of -7 per mil was delayed until15.2 ka BP cal, after the last meltwater pulse. The rising δ18O of the Black Sea lake corresponds with two episodes of calcite precipitation whose interruption corresponds to the Younger Dryas cold period. During each interval of calcite precipitation the δ18O increased a further 2 per mil, without variation in the 87Sr/86Sr composition. During cooling the 87Sr/86Sr ratio trended back toward its glacial value with little change in the δ18O. The disparity between the Sr and O isotope behavior demonstrates that δ18O is not simply a signal of end-member mixing, but instead the δ18O record reflects changes in atmospheric moisture delivered to the Black Sea watershed. At 9.4 ka BP cal the 87Sr/86Sr composition shifted to that of the global ocean and remained there to the present. Since lake water is significantly depleted in strontium relative to seawater, any earlier leakage from the Mediterranean should have left a corresponding signal.
  • Preprint
    The last reconnection of the Marmara Sea (Turkey) to the World Ocean : A paleoceanographic and paleoclimatic perspective
    ( 2008-07) McHugh, Cecilia M. G. ; Gurung, Damayanti ; Giosan, Liviu ; Ryan, William B. F. ; Mart, Yossi ; Sancar, Ummuhan ; Burckle, Lloyd H. ; Cagatay, M. Namik
    During the late glacial, marine isotope Stage 2, the Marmara Sea transformed into a brackish lake as global sea level fell below the sill in the Dardanelles Strait. A record of the basin’s reconnection to the global ocean is preserved in its sediments permitting the extraction of the paleoceanographic and paleoclimatic history of the region. The goal of this study is to develop a high-resolution record of the lacustrine to marine transition of Marmara Sea in order to reconstruct regional and global climatic events at 24 a millennial scale. For this purpose, we mapped the paleoshorelines of Marmara Sea along the northern, eastern, and southern shelves at Çekmece, Prince Islands, and Imrali, using data from multibeam bathymetry, high-resolution subbottom profiling (chirp) and ten sediment cores. Detailed sedimentologic, biostratigraphic (foraminifers, mollusk, diatoms), X-ray fluorescence geochemical scanning, and oxygen and carbon stable isotope analyses correlated to a calibrated radiocarbon chronology provided evidence for cold and dry conditions prior to 15 ka BP, warm conditions of the Bolling-Allerod from ~15 to 13 ka BP, a rapid marine incursion at 12 ka BP, still stand of Marmara Sea and sediment reworking of the paleoshorelines during the Younger Dryas at ~11.5 to 10.5 ka BP, and development of strong stratification and influx of nutrients as Black Sea waters spilled into Marmara Sea at 9.2 ka BP. Stable environmental conditions developed in Marmara Sea after 6.0 ka BP as sea-level reached its present shoreline and the basin floors filled with sediments achieving their present configuration.