The Sedimentary Records Of Hole ZK2 In Liaohe River Deltasince Late Pleistocene And The Paleoclimate During Holocene Transgression

As one of four major deltas in China, the Liaohe River Delta is recognized as climate sensitive area for acrossing the temperate and warm-temperate zone and controlled by temperate monsoon climate.Based on the study of Stratigraphic framework and sediments provenance, we described in this paper the geological evolution history of the Bohai Sea since the late Pleistocene. And the paleoclimate reconstruction and comparative research of climate events during the Holocene transgression have provided a clue for better understanding of historical climatic changes and prediction of future climatic changes.The coring hole of ZK2 was drilled at the Liaohe River Delta for the purpose of geological research. It is 41.4m long with a core recovery up to 90.22%. The deposits formed during the Holocene transgression are thick enough and continuous. It provides excellent materials for the study of environmental and climatic changes during the period of Holocene transgression. Based on the lithology, grain size, foraminifera abundance and molecular markers of the cores, we first established the stratigraphic framework for the region and restored the environmental changing history of the Liaohe River Delta since late Pleistocene 33 kyr BP by integration of precise dating data, sea level change information and other research data from neighboring holes. Sediment provenance and parent rocks were then studied using REE geochemistry. Paleoclimatic evolution of the Liaohe River Delta since the Holocene transgression was reconstructed using paleoclimatic indexes of geochemical elements, organic carbon and nitrogen, pollen and molecular markers for the deposits of U3. Finally, we discussed the response of climatic indexes to the paleoclimatic changes through correlation with the data from surrounding regions.According to the depositional characteristics(lithology, grain size, foraminifera and BIT) and calibrated with dating data, the ZK2 core hole could be divided into four depositional units. From top to bottom, the U1 is a continental facies(river phase), the U2 also a continental facies(floodplain phase), the U3 a marine facies(Holocene transgression) and the U4 again a continental facies(upper delta plain facies). The Unit U3, i.e. the marine segment, is further divided into five layers, namely U31, U32, U33, U34, U35 from bottom to top, corresponding to a sequence of estuarine facies – neritic facies- predelta facies- delta front facies- lower delta plain facies(tidal flat). Grain size, foraminifera and BIT index all show obvious vertical changes in different depositional environments, revealing the history of sea level change of the delta since 33 kyr BP. Having continuous deposits without missing layers, complete stratigraphic sequence and reliable dating data, the U3(15.75m-2.65m) could well reflect the Holocene transgression history of the Liaohe River Delta since 9100 cal yr BP.The REE elements of ZK2 sediments change vertically in a complex and periodical manner. The content of REE is mainly controlled by provenance and chemical weathering. Effect of grain size is obvious. Fe-Mn oxides and TOC have some impact on REE; even the influence of heavy minerals is not obvious.(La / Sm) UCC and(Gd / Yb) UCC, the two characteristic parameters of REE fractionation, could eliminate the effect of sediment grain size, Fe-Mn oxides, TOC and chemical weathering. Their correlation with heavy minerals is not obvious either. They may keep the REE pattern of parent rocks. Therefore, the ratio of(La / Sm) UCC and(Gd / Yb) UCC is used as an effective indicator of sediment sources in this paper. The results also reveal that, in the study area, characteristics of source rocks from 23.75 m to 41.4m are similar with some modern sediment in surrounding rivers. The provenance characteristics of sediments from 15.75 m to 23.75 m are relatively close to those of the three modern rivers of Liaohe River, Daliaohe River; and formed during Holocene transgression with relatively stable source materials, the provenance for the sediment from 0m to 15.75 m is very close to that of modern Daliaohe River and Dalinghe River.We adopted several kinds of climate indexes, such as macro and trace elements, organic carbon / nitrogen, pollen and biomarker of GDGTs, and analyzed their vertical change and environmental implications for the sediments of segment U3(2.65-15.75m) deposited during the Holocene transgression period. The two indexes, CIA, a geochemical index and MAAT, a biomarker index, which are most sensitive to climate change, were chosen to reconstruct the paleoclimatic change since 9100 yr BP. The results show that the region had experienced four climatic stages: the rapid warming stage in early Holocene, the warm and humid stage within early mid-Holocene, the cold and dry stage in late mid-Holocene, and the warm and dry stage in late Holocene. There were at least six distinct dry cooling events discovered in the depth of 14.2m, 12.25 m, 10.65 m, 9.35 m, 8.15 m and 5.65 m respectively.Going through the published research results both at home and abroad, we found that the paleoclimatic change of this region since Holocene was in good agreement with published results, except for the timing of climate fluctuations and the intensity of cooling events.It was revealed that the CIA and MAAT were more sensitive climatic indexes than others, thus could be used as sensitive indexes to reconstruct paleoclimate. MAAT is better than the index of CIA before the mid-Holocene paleoclimate response.