The Inversion Of The Songhua River Systerm In The Eraly Pleistocene:Implication From Sr-Nd Isotopic And Heavy Minerals In The Haerbin Huangshan Cores

The study on the drainage evolution reconstruction is of great significance to the understanding global change and regional response.But the study of the evolution of the Songhua River system is extremely weak.Whether the water system of the Songhua River reversed in the early Pleistocene has been especially controversial and without clear evidence.River sediment is the most direct evidence of the water system evolution.Therefore,the core sediments of Huangshan in Harbin were selected as the research object,and the magnetic susceptibility,paleomagnetism,Sr-Nd isotope composition and heavy mineral composition were analyzed.The results showed that:(1)The core sedimentary facies of Huangshan profile reflects the change from lacustrine facies(95.18?101.11 m)to fluvial facies(30.4?95.18 m)to eolian loess accumulation(0.98?30.4 m),and fluvial facies deposition can be further divided.It is the first sedimentary cycle from the accumulation of the riverbed to the point bar deposits and then to the floodplain,and the second sedimentation cycle from the floodplain to the sedimentary environment of the floodplain.(2)Take 62.3 m as the limit,the river and lake sediments magnetic susceptibility of the upper and the lower sections are distinctly differences.The magnetic susceptibility value of the assise is low below 62.3m,with small fluctuations,and zero values appear in many places;the magnetic susceptibility of the formation above 62.3 m rises rapidly,with large fluctuations,and no obvious change rule.(3)The B/M limit of the Huangshan core is located at 51.4 m,and the time limit is0.78 Ma.The positive polarity zone of Jaramillo,Cobb Mountain and Gilsa are included below 51.4 m,and the geological age at 62.3 m of the Huangshan core is 0.94 Ma B.P.By linear interpolation of the core polar column.(4)The Huangshan core take 62.3 m as the boundary,and the Sr-Nd isotope composition of the strata below 62.3 m in the upper and lower magnetospheres changes drastically.The Sr isotope composition as a whole is lower than the core average,and the Nd isotope as a whole is higher than the core average.The Sr-Nd isotope composition of the 30.4?57.1 m section of the core has a small variation range,and the Sr isotope is higher than the average value of the core,and the Nd isotope is lower than the average value of the core.(5)The heavy mineral content and ferromagnetic mineral content in the stratum below 62.3 m of the Huangshan core is high,and the stable heavy mineral content is low;the heavy mineral content of the 30.4?57.1 m section of the core is low.The content of ferromagnetic minerals is almost 0%,the content of stable heavy minerals increases significantly,and the content of heavy minerals such as apatite and epidote in this layer also shows a significant increase trend.(6)The change in the source area is mainly controlling factor of the magnetic susceptibility,Sr-Nd isotope and heavy mineral composition of the Huangshan core sediments.The magnetic susceptibility,Sr-Nd isotope and heavy mineral composition of the 62.3 m upper and lower layers of the core are exist extremely significant difference,this difference reflects that the sediment source of the Huangshan area is not consistent before and after 0.94 Ma,and the source area has a large change.(7)The Sr-Nd isotopic composition and heavy mineral composition of the Huangshan core under 62.3 m are similar to those of the river sediments in the Yilan direction,while the Sr-Nd isotopic composition and heavy mineral composition of the strata above 62.3 m are similar to those of the Songyuan river sediments.It shows that the sediment source and river flow direction have changed significantly at the 62.3 m(0.94 Ma B.P).(8)Before the Early Pleistocene,the ancient Songhua River was cut off by the Jiayi Watershed,the upstream ancient Songhua River(Zhaoyuan-Yilan section)merged into the Songnen ancient lake,and the downstream ancient Sanjiang flowed eastward along the Sanjiang Plain.In the late Early Pleistocene,under the combined action of structure(tectonic uplift of Jiayi Watershed)and climate(climate transition of Mid-Pleistocene),Jiayi Watershed was cut through at 0.94 Ma BP,and the ancient Songhua River(Zhaoyuan-Yilan Section)Was captured by the ancient Sanjiang,the flow was reversed,and discharged from the Songnen Plain into the Sanjiang Plain,forming a single modern Songhua River system flowing eastward.