Towards an event stratigraphy for Baltic Sea sediments deposited since AD 1900: approaches and challenges

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• 作者：Matthias Moros ; Thorbjø ; rn J. Andersen ; Detlef Schulz-Bull ; Katharina Hä ; usler ; Dennis Bunke ; Ian Snowball ; Aarno Kotilainen ; Lovisa Zillé ; n ; Jø ; rn B. Jensen ; Karoline Kabel ; Ines Hand ; Thomas Leipe ; Bryan C. Lougheed ; Bernd Wagner and Helge W. Arz
• 刊名：Boreas
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：46
• 期：1
• 页码：129-142
• 全文大小：2532K
• ISSN：1502-3885

文摘

Reconstructions of environmental changes at sub-decadal to decadal resolution based on central Baltic Sea sediments rely on accurate and precise high-resolution sediment depth/age relationships. A model chronology for Baltic Sea sediments is presented here based on established historical records of anthropogenic radionuclides (¹³⁷Cs/²⁴¹Am/bomb¹⁴C), polychlorinated biphenyls (PCBs), lead (Pb) and stable lead isotope (^206/207Pb ratios), and radionuclide ²¹⁰Pb and ¹⁴C decay dating methods. Marker horizons consisting of chemical precipitates formed by documented Major Baltic Inflow (MBIs) events and an extended diatom bloom period were also integrated into the model. The main time markers in Baltic Sea sediments that formed during the last 120 years were the following: (i) the deepest observation of ²¹⁰Pb_unsupp. (marking the ²¹⁰Pb dating horizon) and departure of Hg from natural background levels at c. AD 1900; (ii) first detectable presence of PCBs at AD 1935; (iii) radionuclide production (i.e. ²⁴¹Am) due to nuclear weapons testing between AD 1954 and AD 1975, with a peak in AD 1963; (iv) maximum heavy metal and PCB concentrations in the AD 1960s/1970s; (v) the Chernobyl nuclear accident in AD 1986 as a sharp ¹³⁷Cs increase; (vi) exceptionally strong diatom blooms with a massive diatom layer found in the Eastern Gotland Basin in AD 1988–1990; and (vii) characteristic manganese-carbonate layers in the deeper central basins formed by MBIs in AD 1993 and AD 2003. A precise and accurate sediment depth/age relationship can only be achieved in restricted areas of the Baltic Sea where continuous sedimentation has prevailed and there has been limited postdepositional disturbance. We demonstrate that parallel Hg and ¹³⁷Cs measurements can be used to assess the quality of sediment sequences for high-resolution environmental reconstructions. We show examples of sediment profiles that conform to the historical record, and examples from Western Baltic Sea areas where it appears to be impossible to establish accurate geochronologies.