동해 울릉분지의 코어 퇴적물에 대한 환경자기연구 및 전자현미경 관찰

Abstract

동해(East sea)의 과거 3만년동안의 고환경/고기후 변화를 해석하기 위해 울릉분지 코어퇴적물에 대한 환경자기학 연구를 수행하였다. 획득한 코어퇴적물 시료에 들어있는 자성광물의 함량, 입자크기 및 종류 변화를 관찰하기 위한 자기특성시험과 특성잔류자화 방향측정을 수행하였다. 기존 테프라층서(tephrastratigraphy) 연대와 본 연구에서 실시한 AMS 14C 연대측정결과를 기반으로 코어퇴적물의 형성시기를 결정하였다. 특성잔류자화방향, 암석자기변수 (χ, NRM, ARM, SIRM), 암석자기변수의 비 (χARM/χ, ARM/SIRM, SIRM/χ), 그리고 등온잔류자화, 무자기이력잔류자화 획득 실험, 잔류항자기력(S-ratio)의 결과들을 바탕으로 코어시료에 기록된 간빙기(interglacial), 빙하기(LGM), 해빙기(deglacial), 홀로세(Holocene) 동안의 고기후 및 퇴적환경의 변화를 해석하였다. 그 결과 본 연구의 울릉분지 코어퇴적물은 3만 5천년전 간빙기의 중-후반시기부터 홀로세 퇴적물로 이루어져 있으며 퇴적시기에 따라 자성광물의 함량 및 입자크기와 종류에 뚜렷한 변화를 보임이 드러났다. 해수면이 증가하고 온도가 높은 간빙기 동안에는 자성광물 함량이 감소하고 자성광물의 입자크기가 큰 특징을 보였으며, 빙하기 동안에는 자성광물 함량이 증가하고 입자크기가 매우 작으며 순수한 자철석 계열의 자성광물이 지배적인 것으로 관찰되었다. 전자현미경 관찰을 통해 발견된 빙하기 동안의 세립질 자철석은 박테리아에 의한 생물적 작용에 기인한 것으로 판단되며, 당시 동아시아 겨울몬순이 강화되어 대기를 통한 유입물질이 많아진 과정 속에서 동해의 고환경적 특징에 의해 야기된 수괴내의 성층화가 환원성 환경을 유도해 박테리아의 활동을 증대시켰던 것으로 해석된다. 한편, 위치에 상관없이 동해내의 모든 코어의 표층에서 자성광물의 함량이 급격히 증가하고 입자의 크기가 작아지며, 전자현미경 관찰결과 퇴적물 내에 자철석이 지배적으로 존재하는 것으로 나타난 특징은 빙하기와 유사하게 동해 표층 퇴적물 내에 쇄설성/자생성을 비롯한 생물기원 자성기록 매체의 가능성이 있음을 시사한다. 간빙기-빙하기-간빙기로의 기간 동안 뚜렷한 변화를 보이는 본 연구의 자기특성 결과는 기타 동해 연구에서의 산소동위원소 및 지화학적 지시자들과 높은 상관성을 보이며 동해의 입체적인 고해양/고기후 해석을 함에 있어 유용함을 드러낸다.| Magnetic characterization of core sediments (site 8, 10) from the Ulleung basin in East Sea was carried out in order to decipher paleo-environmental and climate changes of the basin for the last 35,000 years. Directions of characteristic remanent magnetization (ChRM), magnetic susceptibility (χ), intensities of natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and saturation isothermal remanent magnetization (SIRM) were measured as rock magnetic parameters. In addition, ratio of rock magnetic parameters such as χARM/χ and ARM/SIRM as well as S-ratio were calculated for identifying grain size and types of magnetic mineral. Down core variations of ChRM directions and rock magnetic parameters for the 520cm-long core sediment were compared with already published results of tephrastratigraphy and AMS 14C data from the Ulleung basin core, revealing the changes in composition, grain size, and concentration of magnetic minerals coincide with environmental changes from middle MIS 3 to Holocene. Glacial periods are characterized as high concentration and fine magnetic mineral possibly due to biological effect of iron-reducing bacteria, being affected by strong stratification of East sea and East Asia winter monsoon, while interglacial periods with warm temperature and high sea level are distinguished by low concentration and coarse magnetic minerals in the core sediment. High values of NRM, ARM, SIRM, and χ are observed in top several tens of centimeters of sediment cores regardless of sampling localities, possibly indicating that a certain type of chemical diagenesis and/or biological activities; including iron-reducing and/or magnetotactic bacteria may play an important role in the iron reduction regime for this interval. Interestingly, very small grains of magnetite presumably formed by bacteria is actually found in LGM and top sediment through TEM observations based on the similarities between the magnetic minerals extracted from sediments of LGM and top and those of cultured bacterial medium from top sediments. This might be evidence for possibility that geochemical conditions of sediment and that magnetotactic bacteria activity could contribute to the strong magnetic intensity of LGM and top sediment. These changes in magnetic signals of core sediments faithfully reflect the variations in source material, transportation methods of sediments, sedimentation processes and physio-chemical conditions of the basin, all of which have been influenced by paleoclimate changes such as glacial-interglacial transitions. This study provides new interpretation of paleoclimate or paleoceanography changes for East Sea and shows that the magnetic proxy can be powerful tool for paleoenvironmental research.; Magnetic characterization of core sediments (site 8, 10) from the Ulleung basin in East Sea was carried out in order to decipher paleo-environmental and climate changes of the basin for the last 35,000 years. Directions of characteristic remanent magnetization (ChRM), magnetic susceptibility (χ), intensities of natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and saturation isothermal remanent magnetization (SIRM) were measured as rock magnetic parameters. In addition, ratio of rock magnetic parameters such as χARM/χ and ARM/SIRM as well as S-ratio were calculated for identifying grain size and types of magnetic mineral. Down core variations of ChRM directions and rock magnetic parameters for the 520cm-long core sediment were compared with already published results of tephrastratigraphy and AMS 14C data from the Ulleung basin core, revealing the changes in composition, grain size, and concentration of magnetic minerals coincide with environmental changes from middle MIS 3 to Holocene. Glacial periods are characterized as high concentration and fine magnetic mineral possibly due to biological effect of iron-reducing bacteria, being affected by strong stratification of East sea and East Asia winter monsoon, while interglacial periods with warm temperature and high sea level are distinguished by low concentration and coarse magnetic minerals in the core sediment. High values of NRM, ARM, SIRM, and χ are observed in top several tens of centimeters of sediment cores regardless of sampling localities, possibly indicating that a certain type of chemical diagenesis and/or biological activities