Lead-210 sediment geochronology in a changing coastal environment |
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| Authors: | Jeffrey P Chanton Christopher S Martens George W Kipphut |
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| Institution: | Marine Sciences Program, University of North Carolina, Chapel Hill. NC 27514 U.S.A. |
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| Abstract: | Sediment accumulation rate studies utilizing excess 210Pb and 137Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm?2 · yr?1 or approximately 8.4 to 11.8 cm · yr?1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the 210Pb profile in order to establish a geochronology. Grain size 210Pb distribution measurements at three depths reveal that the specific activity (dpm · g?1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess 210Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events.Sixteen 137Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm?2 · yr?1. |
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