The Provenance Evolution Of West Taiwan Strait And Its Response To The Ocean Current During The Middle-Late Holocene

Sediments transport from source to sink has become a hot topic in the recent years. This process is not only limited in the river system, but also existing in the marine system, especially in the inner shelf region. Riverine sediments start a new "source-sink" tour when they are discharged to sea, which is transported by the oceanic current system. The current affects transport and deposition of terrestrial material, further marine ecosystem.The present study aims to reveal sediment provenance differences between the mainland (Yangtze, Qiantang, Ou and Min) and west Taiwan (Choshui, Daan, Dachia, Wu) rivers and they will help determine sediment provenance of F15 core in the west Taiwan Strait. The detailed analysis included magnetic properties and geochemical elements properties (acid-leached residues) of fine-grained sediments (a fraction of less than 45um).The results show the obvious differences of magnetic properties among mainland and west Taiwan rivers. The mainland rivers are characteristic of higher χlf (85×1-8m3/kg in average, all data in abstract is in average) and magnetite as the dominant magnetic minerals. In contrast, west Taiwan rivers have lower χlf (19×10-8m3/kg), but with abnormalously higher SIRM/χlf (>60kA/m; maximum of 136kA/m) in half samples from Choshui River which shows pyrrhotite exsiting. A provenance indicator of SIRM/χlf and HIRM is developed here to identify the sediment source from these rivers. However, the indicator is not effective when applied to identify sediment provenance of core F15 in the west Taiwan Strait. Magnetic properties of core F15 are influenced by strong early diagenesis which is indicated by rapidly decreasing of all magnetic parameters with core depth. Only surficial core sediments within the top of 50cm could be used for provenance identification reluctantly due to their magnetic properties changed little by weaker early diagenesis and it shows a big sediment contribution of the west Taiwan rivers.In order to eliminate the effect of particle size, all minor elements are treated using Al homogenization method in the paper. The Yangtze River is rich in most minor elements, especially Sc, V, Cr, Co, Cu, Ni, Sr etc; the Zhejiang and Fujian rivers are rich in Zn, Rb, Pb, Ba etc, the West Taiwan rivers are rich in Y, Zr, Hf, Th, U and rare earth elements (REE), the other elements are relatively low. Some element ratios or REE parameters are selected to identify the sources of these rivers due to their stable features during weathering, transporting and depositing processes, including Cr/Th, Sc/Th, Ba/Th, Eu/Eu*,δEu, (Eu/Sm)N. High values of these indicators occur in the Yangtze River and low values in the west Taiwan and Min Rivers. Cr/Th-Sc/Th and Eu/Eu*-Ba/Th are developed to identify the sources of the Yangtze River and west Taiwan rivers. The geochemical results of core F15 show that a significant change occurs at ～100cm in the core (～4.0cal. kyr BP). Before ～4.0cal. kyr BP, most of the minor elements keep high values, such as Sc, Co, V, Cr etc. and Cr/Th, Sc/Th, Eu/Eu* and Ba/Th are also relatively high, while the values of Zr, Hf, Th, Y, REE are relatively low. Such geochemical characteristics are similar to those of Yangtze River, which also supported by the provenance idenfication of Cr/Th-Sc/Th and Eu/Eu*-Ba/Th developed from the modem riverine sediments. Thereafter, most minor elements rapidly reduce in the core and Cr/Th, Sc/Th, Eu/Eu*, Ba/Th are relatively low, but Zr, Hf, Th, REE are increasing upcore especially on the top of 60cm. All of these reveal that the sediment contribution of west Taiwan rivers (mainly Choshui River) is increasing, which is also witnessed by the diagrams of Cr/Th-Sc/Th and Eu/Eu*-Ba/Th. Note that many heavy metals present very high contents on the top of core F15, thinking of influence by modern human activities in the last millennium.The sediment provenance changes of F15 core reveal that sediment contribution from the Yangtze River is reducing and that of west Taiwan rivers is increasing since-4.0 cal. kyr BP. Meanwhile, it is found that a decrease of mean grain size occurs in the sediment fraction <61μm in core F15, which fraction is believed related to the Zhejiang-Fujian Coastal Current (South part of Chinese Coastal Current). All above imply a decline of Zhejiang-Fujian coastal current since ～4.0 cal. kyr BP, which has been recorded in the other core in the inner shelf mud area and even in the route of Subei Coastal Currents (north part of Chinese Coastal Current). Such a decline is thought to be related to weakened East Asian winter monsoon during the late Holocene.