Diagenesis in Eocene claystones, ODP site 647, Labrador Sea formation of complex authigenic carbonates, smectites, and apatite

Bohrmann, Gerhard and Thiede, Jörn (1989) Diagenesis in Eocene claystones, ODP site 647, Labrador Sea formation of complex authigenic carbonates, smectites, and apatite Proceedings of the Ocean Drilling Program: Scientific Results, 105 . pp. 137-154.

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Abstract

Authigenic carbonates were recovered in lower to middle Eocene claystones at Ocean Drilling Program Site 647 in the Labrador Sea. Detailed chemical, petrographic, and X-ray investigations reveal that these diagenetic carbonates have a complex mineralogical composition. At least five different carbonate phases are identified: calcium-rich rhodochrosite, rhodochrosite, manganosiderite, siderite, and calcite. Manganese carbonates are the dominant carbonate phases formed throughout the section. Textural analyses show two major generations of carbonate formation. Early cementation of micritic carbonate in burrow structures was followed by carbonate cementation forming microsparry to sparry crystals. At approximately 620 meters below seafloor (mbsf), three concretions of iron carbonates occur, which indicates a special pore-water chemistry. Thin section analyses from this level show (1) several generations of diagenetic carbonates, (2) widespread secondary cavity formation in burrow structures, and (3) various cement precipitations in voids. We suggest that this level represents a hiatus or highly condensed sequence, as indicated by (1) the low carbonate content in host sediments, (2) carbonate dissolution reflected by the high ratio of benthic to planktonic foraminifers, and (3) complex diagenetic alteration in the carbonate concretions. Iron and manganese enrichments observed in lithologic Unit IV may have been derived from a hydrothermal source at the adjacent, then active, Labrador Sea mid-ocean ridge. Authigenic smectites forming numerous pseudomorphs of siliceous microfossils are precipitated in burrow structures. We propose that diagenetic smectite formation from biogenic opal and iron oxyhydroxide (analogous to smectite formation in surface sediments of the East Pacific area) occurred in the Labrador Sea during the early and middle Eocene.

Document Type: Article
Keywords: Claystones, Eocene, Labrador Sea, ODP Site 647
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-P-OZ Paleo-Oceanography
Refereed: No
ISSN: 1096-7451
Projects: ODP
Date Deposited: 08 May 2019 08:55
Last Modified: 08 May 2019 08:55
URI: http://eprints.uni-kiel.de/id/eprint/46637

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