No abstract

AMS 14C ages of post-glacial core sediments from the subaqueous Yangtze delta, along with sedimentary structures and distributions of grain size, pollen spores, and dinoflagellate cysts, show an estuarine depositional system from 13 to 8.4 cal ka BP and a deltaic system from 5.9 cal ka BP to the present. The estuarine system consists of intertidal to subtidal flat, estuarine, and estuarine-front facies, characterized by sand"mud couplets and a high sedimentation rate. The deltaic system includes nearshore shelf and prodelta mud featured by lower sedimentation rate, markedly fewer coastal wetland herbaceous pollens, and more dinoflagellate cysts. We explain the extremely high sedimentation rate during 9.2–8.4 cal ka BP at the study site as a result of rapid sea-level rise, high sediment load due to the unstable monsoonal climate, and subaqueous decrease of elevation from inner to outer estuary. A depositional hiatus occurred during 8.2–5.9 cal ka BP, the transition from estuarine to deltaic system, caused possibly by a shortage of sediment supply resulting from delta initiation in paleo-incised Yangtze valley and strong tidal or storm-related reworking in offshore areas. The subsequent development of deltaic system at the study site indicates accelerated progradation of Yangtze delta post-5.9 cal ka BP.

To examine the applicability of C/N and organic carbon stable isotope (δ13C) in studies of the Holocene sea level and freshwater discharge in the large river mouth of Yangtze, we observed the distribution of carbon, nitrogen and δ13C in a late-Quaternary core (ZK9) collected from the present subaqueous delta. We also collected published data of the two proxies for the suspended particulate matter (SPM) and surficial sediments from the lower Yangtze River to the adjacent East China Sea. The results show that the estuarine front is an important boundary for terrestrial and marine contribution of the organic component in the modern sedimentary environment. In the core ZK9, sediments deposited during c. 13–9 cal. ka BP are characterized by high values of TOC (0.54–1.16%), CaCO3 (0.35% on average), and C/N (>12), which reflect an inner tidal estuarine environment dominated by C3 terrestrial organic carbon input. During c. 9–0.7 cal. ka BP, both TOC content (0.57% on average) and C/N ratio (<10) decrease remarkably while TN increases, indicating a lower estuarine or shallow marine environment. An abrupt sea level rise from c. 9 cal. ka BP resulted in a deeper water environment and reduced terrestrial input at the core location. The low δ13C values (−24.23‰ on average) before c. 6 cal. ka BP reflect a dominantly terrestrial source of organic matter associated with increased freshwater discharge into the estuary during that time. The sediments since c. 6 cal. ka BP are characterized by increasing δ13C up to −24.1 to −23.39‰, reflecting more contribution from marine algae as freshwater discharge fell. We suggest that in the Yangtze River mouth the C/N ratio indicates an abrupt sea level rise at c. 9 cal. ka BP, while δ13C is more useful in reflecting freshwater discharge.

No abstract

Development of the Yangtze delta during the late Holocene, and its relationship to human activities in the drainage basin, was analyzed using data from 16 cores collected from distributaries to the prodelta. We used AMS 14C dating and digital elevation model (DEM) data from marine charts from 1864 through 2005 to determine ages and estimate sediment accumulation rates. The results demonstrate that the latest major subaqueous delta front formed within the past c. 0.8 cal. ka and features remarkably high accumulation rates (1—4 cm/yr) in comparison with those of previous delta fronts. We also examined the temporal distribution of grain size and magnetic susceptibility in all 16 cores. Results show soil-derived superparamagnetic (SP) minerals generally occur, and even dominate, in the recent ( c. 1.7 cal. ka) Yangtze delta fine-grained sediment, as shown by high values of frequency-dependent magnetic susceptibility (both χFD and χFD%). Rock-derived magnetite dominates generally in the river channel and delta front sand bodies as a result of hydrodynamic sorting, but is also enriched in both fine and coarse-grained sediment formed more recently ( c. 0.8 cal. ka), as evidenced by rising values of mass specific magnetic susceptibility (χLF). SP grains were deposited as early as the late Neolithic, possibly indicating local deforestation associated with the use of fire at that time. We suggest major deforestation in the drainage basin started c. 1.7 cal. ka BP, and intensified after c. 0.8 cal. ka BP when both χLF and χFD show the highest values. We therefore conclude that upland deforestation and cultivation as a result of the migration of human populations from northern China since c. 1.7 cal. ka BP resulted in increased sediment discharge of the Yangtze and played an important role in recent delta construction.