Coated phosphate grains; proxy for physical, chemical, and ecological changes in seawater

Irrespective of geologic age, Phanerozoic coated phosphate grains deposited beneath productive surface waters in organic-rich paleoenvironments are of only two types. Unconformity-bounded grains contain internal discordances and erosional surfaces, attributable to multiple episodes of phosphogenesis...

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Bibliographic Details
Published in:Geology (Boulder) 2003-09, Vol.31 (9), p.801-804
Main Authors: Pufahl, Peir K, Grimm, Kurt A
Format: Article
Language:English
Subjects:
carbon
chemically precipitated rocks
depositional environment
diagenesis
Ecology
encrustations
geochemistry
global
Grain
grains
marine environment
natural analogs
Oceans
organic carbon
paleo-oceanography
paleoecology
Phanerozoic
phosphate rocks
Phosphates
physicochemical properties
residence time
sed rocks, sediments
sediment-water interface
Sedimentary petrology
sedimentary rocks
sedimentation rates
unconformities
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Summary:Irrespective of geologic age, Phanerozoic coated phosphate grains deposited beneath productive surface waters in organic-rich paleoenvironments are of only two types. Unconformity-bounded grains contain internal discordances and erosional surfaces, attributable to multiple episodes of phosphogenesis and sedimentary reworking during periods of stratigraphic condensation. Redox-aggraded grains consist of concordant concentric phosphate laminae that are intimately interlayered with circumgranular layers containing pyrite, chamosite, or barite, recording in situ diagenetic mineralization driven by changes in pore-water redox potential. Such changes can be attributed to variations in biological oxygen demand within suboxic pore-water environments resulting from fluctuations in sedimentation rate of organic carbon. Redox-aggraded grains are thus sensitive indicators of variations in organic carbon export and record changes in primary productivity and/or ecological dynamics of the surface ocean. This concept of coated-grain formation necessitates a long residence time just below the sediment-water interface. If sedimentation rate is too high, grains are rapidly buried and so removed from the zone of active phosphate precipitation. Coated phosphate grains can therefore be considered the granular equivalents of condensed beds. These concepts are equally applicable to the interpretation of other types of coated grains and concretions that contain Eh-sensitive minerals, such as iron-bearing ooids and polymineralic concretions.
ISSN:0091-7613
1943-2682
DOI:10.1130/G19658.1