Journal of the Korean earth science society (한국지구과학회지)

The Korean Earth Science Society (한국지구과학회)
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Elemental Composition of Authigenic Siderites in the Early Holocene Coastal Sediments, Western Coast of Korea and Their Depositional Implication

한국 서해 초기현세 퇴적물중 자생 능철석의 원소 성분과 퇴적학적 의미

  • Cho, J.W. (Department of Earth Science, Chosun University) ;
  • Lim, D.I. (Marine Enviroment and Climate Change Lab., Korea Ocean Research and Development Institute)
  • 조주환 (조선대학교 지구과학교육학과) ;
  • 임동일 (한국해양연구원 해양환경기후연구본부)
  • Published : 2002.12.31
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Abstract

Authigenic siderite grains, ranging 100 to 250-${\mu}$m in diameter, are abundant in an about 8,600-year-old sediment layer in Namyang Bay, west coast of Korea. The siderites exhibit the aggregated spherulitic morphology with well-developed rhombs on the grain surfaces. They consist mostly of FeCO$_3$ (average, 65%) and MnCO$_3$ (average, 22%) with low Mg/Ca ratio (less than 0.4) in their bulk composition. A series of compositional ternary discrimination diagrams, together with high Mn and low Mg contents, show that only meteoric porewater was involved in siderite precipitation, assuming that depositional environment of host sediment is an organic-rich freshwater system. Considering a series of results such as radiocarbon age, authigenic Mn-rich siderite and lithological features, siderite-hosting sediment (unit Tl) is interpreted as freshwater swamp or bog deposition, infilling the topographic depressions that locally existed before the formation of mid-to-late Holocene tidal deposits. Center-to-margin compositional variation within individual grain is very systematic; Mn and Ca decrease towards the margin of a siderite grain, while Fe and Mg increase. It suggests that the spherulitic siderites were precipitated in this sedimentary layer in a series during the early diagenesis of MnOx-FeOx reduction under steady-state.

한국 서해 남양만 조간대의 초기현세 (ca. 8,600yr BP) 니질 퇴적물에서 특징적으로 조립사 (100-250${\mu}$m) 크기의 자생 능철석 입자들이 풍부하게 발견된다. 이들 갈색의 능철석 입자들은 표면에 잘 발달된 육면체 결정을 가진 구형의 집합체 형태를 보이며, 주성분 원소는 철과 망간으로 각각의 평균 함량이 각각 65%와 22%에 이른다. 높은 망간함량과 낮은 마그네슘 함량 그리고 성분삼각도표 (compositional ternary discrimination diagram) 결과들은 이들 능철석 입자들이 담수환경에서 형성되었음을 보여준다. 결과적으로 능철석 입자들을 함유한 초기현세 퇴적물 (unit Tl)은 해침의 초기 단계에서 해수면과 일정 거리를 가진 해안선 근처의 육지에 발달한 소규모 담수 습지 (freshwater swamp or bog) 퇴적층으로 해석된다. 한편, 능철석 입자 내에서 뚜렷한 화학적 성분 변화가 나타나는 바, 입자의 중심에서 바깥쪽으로 갈수록 철과 마그네슘의 함량은 증가하는 반면 망간과 칼슘의 함량은 감소하는 경향을 보인다. 이러한 결과는 자생 능철석 입자들이 퇴적층의 공극내에서 일련의 초기속성작용 (early diagenetic process) 동안 순차적으로 형성되었음을 반영한다.

Keywords

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