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Wang F.,Key Laboratory of Muddy Coast Geoenvironment | Wang F.,Tianjin Institute of Geology and Mineral Resources | Zong Y.Q.,University of Hong Kong | Li J.F.,Key Laboratory of Muddy Coast Geoenvironment | And 9 more authors.
Journal of Coastal Research | Year: 2016

To determine sedimentation rate changes across an open marine area along the west coast of Bohai Bay, more than 50 short sediment cores were collected and analyzed with their sedimentation rates estimated by excess 210Pb (210Pbexc) and 137Cs in the sediment. The surveys enabled us to explore the relationship between sediment supply changes from the rivers and the spatial/temporal deposition patterns in the shallow marine areas. Several core records show that sedimentation rates can be overestimated by the conventional constant initial concentration model based on the 210Pbexc curves alone. The overestimation was caused by a change of 210Pbexc in the top layers of these cores and this is confirmed by comparing 210Pbexc curves with the respective 137Cs curves. This phenomenon is likely caused by sediment erosion or redeposition of the surface sediment at the seabed, implying present-day active sedimentary processes as a result of the drastic decrease in sediment supply from the rivers during the last 50 years. The sedimentary boundary where sedimentation rate changed is largely comparable with the timing of sediment supply change according to the river discharge data, which can be used as a local time marker for sedimentation rate comparison. © Coastal Education and Research Foundation, Inc. 2016.

Shang Z.,Tianjin Institute of Geology and Mineral Resources | Shang Z.,Key Laboratory of Muddy Coast Geoenvironment | Wang F.,Tianjin Institute of Geology and Mineral Resources | Wang F.,Key Laboratory of Muddy Coast Geoenvironment | And 11 more authors.
Journal of Asian Earth Sciences | Year: 2016

Shelly cheniers and shell-rich beds found intercalated in near-shore marine muds and sandy sediments can be used to indicate the location of ancient shorelines, and help to estimate the height of sea level. However, dating the deposition of material within cheniers and shell-rich beds is not straightforward because much of this material is transported and re-worked, creating an unknown temporal off-set, i.e., the residence time, between the death of a shell and its subsequent entombment. To quantify the residence time during the Holocene on a section of the northern Chinese coastline a total 47 shelly subsamples were taken from 17 discrete layers identified on the west coast of Bohai Bay. This material was AMS 14C dated and the calibrated ages were systematically compared.The subsamples were categorized by type as articulated and disarticulated bivalves, gastropod shells, and undifferentiated shell-hash. It was found that within most individual layers the calibrated ages of the subsamples got younger relative to the amount of apparent post-mortem re-working the material had been subject to. For examples, the 14C ages of the bivalve samples trended younger in this order: shell-hash→split shells→articulated shells. We propose that the younger subsample age determined within an individual layer will be the closest to the actual depositional age of the material dated. Using this approach at four Holocene sites we find residence times which range from 100 to 1260cal yrs, with two average values of 600cal yrs for the original 14C dates older than 1kacal BP and 100cal yrs for the original 14C dates younger than 1kacal BP, respectively. Using this semi-empirical estimation of the shell residence times we have refined the existing chronology of the Holocene chenier ridges on the west coast of Bohai Bay. © 2015 Elsevier Ltd.

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