서북극 척치해의 제4기 후기 해양퇴적층에 대한 퇴적환경 연구

Abstract

In the Arctic Ocean, sediments are deposited by various sedimentary processes including hemipelagic settling, mass-flow, meltwater plume, iceberg, and bottom current. Therefore, defining the origins and depositional processes of the sediments plays an important role in understanding the paleoenvironment closely associated with paleoclimate changes. In this study, high-resolution subbottom profile (SBP) and gravity core data are analyzed to understand origins and depositional processes of the Arctic sediments in the Chukchi Sea and East Siberian Sea. Based on clarity and continuity of reflectors (echoes) and seafloor geometry, 5 echo facies are classified. Echo facies IA is characterized by continuous sharp bottom echoes and parallel to sub-parallel subbottom echoes. This echo facies is subdivided into IA-1 and IA-2 types depending on occurrence of iceberg scours. Echo facies IA-1 is generally attributed to hemipelagic settling with an influence of fine-grained turbidity current. It occurs in a great part of the Chukchi Borderland including the Chukchi Shelf, Chukchi Spur and the deep-sea floor. Echo facies IB includes acoustically prolonged and relatively flat bottom echo and fuzzy subbottom echo. This echo facies is restricted to the northern part of the Chukchi Spur, and interpreted to represent moraines planated by bottom current during the deglaciation. Echo facies IIB is characterized by overlapping hyperbolae with hummocky reflectors. This echo facies is further divided into IIB-1 and IIB-2 depending on occurrence of scoured reflectors by iceberg plowing. Type IIB occurs in the shallow part of the Chukchi Spur (200∼300 m in water depth), central part of the Chukchi Plateau, and topographic high in the KucherovTerrace. Distribution of echo facies IIB indicates that grounding zone moraines were formed extensively along the base of the ice sheet (grounding ice) during the glacial retreat. Echo facies IIC shows tightly overlapping hyperbolic echoes with acoustically transparent subbottom echoes of lens or wedge shape. Major part of the steep slopes including the southeastern part of the Mendeleev Ridge are dominated by echo facies IIC, which is interpreted as mass-flow deposits or debrites. In the deep-sea floor, lateral change from IIC to IA-1 indicates that mass flow, debris flow and turbidity current are formed by influence of ice advance during the glacial stage. Echo facies IIIB is characterized by acoustically transparent or partially prolonged subbottom echoes with a high relief (tens of meters). It mainly occurs in the northern part of the Chukchi Spur. This type echo is interpreted to be characteristic of the hard rock basement, and the high-relief echoes are interpreted as scour marks by floating iceberg during the deglaciation. In the gravity core sediments, 5 sedimentary facies are classified on the basis of sedimentary structures on X-radiographs and grain texture. Couplets of thinly laminated mud (TLM) and homogeneous mud (HM) are generally indicative of fine-grained turbidity current. In the ice-proximal zone (grounding line), these sedimentary facies can result from subglacial meltwater stream during the deglaciation. On the other hand, bottom current and turbid meltwater plume give rise to irregular and indistinct lamination with bioturbation (ILM). The sedimentary facies ILM/TLM indicate a gradual change from deglaciation to interglacial stage in the Chukchi Spur. Structureless or disorganized (gravelly) mud (DM) consists of many ice rafted debris (IRDs) supported by fine-grained matrix. This sedimentary facies is interpreted to represent sediments settled from the basal debris zone under thelarge and wide ice shelf during the deglaciation. The depositional model integrated from the SBP and sediment core analyses indicates that the Chukchi Borderland was influenced by ice sheet (grouding ice) and ice shelf during the LGM, which is useful for further paleo-climate and paleo-oceanographic researches in the Chukchi Sea. The sedimentary facies suggested in this study will be also helpful to enhancement of convenience and accuracy for paleoenvironmental researches.