Bohrmann, Gerhard and Stein, Ruediger (1989) Biogenic Silica at ODP Site 647 in the Southern Labrador Sea: Occurrence, Diagenesis, and Paleoceanographic Implications. Proceedings of the Ocean Drilling Program: Scientific Results, 105 . pp. 155-170. DOI 10.2973/odp.proc.sr.105.121.1989.

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Abstract

Eocene to Holocene sediments from Ocean Drilling Program (ODP) Site 647 (Leg 105) in the southern Labrador
Sea, approximately 200 km south of the Gloria Drift deposits, were investigated for their biogenic silica composition.
Three sections of different diagenetic alteration products of primary siliceous components could be distinguished: (1)
opal-A was recorded in the Miocene and the early Oligocene time intervals with strongly corroded siliceous skeletons in
the Miocene and mostly well preserved biogenic opal in the early Oligocene; (2) opal-CT precipitation occurs between
250-440 meters below seafloor (mbsf) (earliest Oligocene to late Eocene); (3) between 620-650 mbsf (early/middle Eocene),
biogenic opal was transformed to clay minerals by authigenesis of smectites. Using accumulation rates of biogenic
opal, paleoproductivity was estimated for the early Oligocene to late Eocene interval. A maximum productivity of
biogenic silica probably occurred between 35.5 and 34.5 Ma (early Oligocene). No evidence for opal sedimentation during
most of middle Eocene was found. However, at the early/middle Eocene boundary (around 52 Ma), increased opal
fluxes were documented by diagenetic alteration products of siliceous skeletons.

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