Historical documents provide a general chronological overview of the environmental evolution of the Yangtze River delta (YRD) during the last ca. 2000 years; however, absolute dating of the region’s late Holocene sediment is relatively rare. Optically stimulated luminescence (OSL) dating has been increasingly applied to the age determination of Holocene deposits in deltaic environments. In this study, three 23–27 m long drill cores running from south to north were collected from the Qihai plain of the northern YRD in order to reconstruct the history of this region’s formation since the late Holocene. A total of 24 samples from the three cores were subjected to OSL dating using coarse silt-sized (45–63 μm) quartz. The OSL ages range from approximately 190–3490 a revealing that the age of the delta front and delta plain facies in the coring sites are younger than 500 a while the sediments in the underlying prodelta facies are older than 2000 a. On the basis of the large age gap between the two set of deposits, we suspect that the coring sites remained submerged from 2000 to 500 years ago. As the central core has older and coarser sandy deposits than the neighbouring cores, we infer that the central core was located on a sandy mouth bar, while other cores sat within distributary channels within the estuary. The OSL ages are consistent with both the chronology implied by historical documents and other stratigraphic records in the area. This study enhances the chronological framework of land formation and delta evolution in the Qihai plain area of the YRD and thereby consolidates the conclusions derived from the application of a single technique alone.
Ferrimagnetic greigite (Fe 3 S 4) is commonly regarded as a precursor to pyrite (FeS 2), or a product of pyrite oxidation, in sedimentary environments (
<p>Delta deposits show large spatial heterogeneity in terms of depositional rate and age, which is critical to the study of delta erosion in response to the declining fluvial sediment load observed at many river mouths in the world. In this study, we demonstrate the magnetic susceptibility (&#967;) as a rough indicator to reveal age variations and stratigraphic heterogeneity in the Yangtze River subaqueous delta. Ages of three short sediment cores (<2 m) collected at 20-32 m water depth from the Yangtze River subaqueous delta were determined using <sup>210</sup>Pb, <sup>137</sup>Cs, and optically stimulated luminescence (OSL) dating. In addition, depth variation of &#967;, which is influenced by post-depositional diagenesis and hence age, was used to roughly estimate sediment ages among the cores in a quick way. The profiles of <sup>210</sup>Pb, <sup>137</sup>Cs, and OSL results indicate the spatial variability of ages, ranging from the last 100 years to more than 2000 years. Cores at shallow water depths are younger than those from deeper sites. Modern deposits (i.e., <100 years old) occur primarily at water depths shallower than ca. 30 m, which can be explained by the trapping depth of bottom plumes. The Core in the northern part of the subaqueous delta shows much older ages than the core at the southern site with similar water depth, which is caused by their distance relative to the mouth of active sediment discharge distributary. Profile of &#967; confirms such spatial variation of ages in terms of depth distribution pattern and &#967; value. Older sediments show lower and uniform &#967; values due to the reductive dissolution of ferrimagnetic minerals, while younger sediments show higher &#967; values in the top layer but they decline with increasing depth. Considering the quick way of magnetic measurement, stratigraphic correlation based on &#967; can be used first to screen for cores before they are subjected to more detailed dating. This study shows that the methodological approach of combining sediment dating with magnetic measurement has great potential in revealing heterogeneous deltaic deposits, which could be easily neglected in morphodynamical and biogeochemical study.</p>
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