Environmental Engineering Reference
In-Depth Information
18.2.2 Framboidal pyrite
The presence and size distribution of framboidal pyrite grains can be particularly
informative regarding ancient ocean chemistry because exclusively small framboi-
dal pyrite is expected when the overlying water column is euxinic (anoxic and
sulfidic) and pyrite framboids form in the water column (Wilkin
et al
.,
1996
).
Pyrite framboids have been widely reported from PTB sections, particularly in
strata overlying the main extinction horizon (
Figure 18.1
; e.g. Wignall and Hallam,
1992
; Bond and Wignall,
2010
). Small framboids occur in abundance above
the main extinction horizon on shallow-marine, tropical carbonate platforms
(e.g. Bond and Wignall,
2010
; Liao
et al
.,
2010
); in shallow-marine, tropical,
mixed carbonate and clastic systems (Wignall
et al
.,
2005
); in deep-water, tropical
carbonates (e.g. Shen
et al
.,
2007
); and in high-latitude clastic deposits (e.g.
Nielsen and Shen,
2004
; Bond and Wignall,
2010
; Nielsen
et al
.,
2010
). Abundant
framboids occur below the mass extinction horizon at some localities, suggesting
euxinic conditions developed prior to the main extinction event in some regions
(e.g. Nielsen and Shen,
2004
). In deep-water sections, the co-occurrence of small
framboidal pyrite grains with redox-sensitive metal abundance patterns, indicative
of oxic to dysoxic bottom water conditions, suggests that a euxinic oxygen
minimum zone existed within a water column oxygenated at the surface by wind
mixing and in deep water via large-scale circulation (Algeo
et al
.,
2010
).
The frequency of framboidal pyrite in stratigraphic sections and trends toward
smaller size within some sections point toward a greater prevalence of euxinic
conditions above the end-Permian extinction horizon.
18.3 Geochemical evidence for anoxia
18.3.1 Organic biomarkers
Biomarker studies have primarily focused on the immediate PTB, with very
limited data for the Changhsingian and Griesbachian. Isorenieratane and aryl
isoprenoid derivatives occur in late-Permian and PTB rocks from the Perth
Basin (Western Australia; Grice
et al
.,
2005
)andMeishan(China;Cao
et al
.,
2009
) in the Tethys region, re
ecting the presence of green sulfur bacteria and
indicating euxinic conditions within the photic zone (
Figure 18.1
). In addition,
Panthalassic sediments from the Peace River Basin (Canada; Hays
et al
.,
2007
)
andBorealOceansedimentsfromKapStosch(Greenland;Hays
et al
.,
2012
)
also contain biomarker evidence for photic-zone euxinia (PZE) during late-
Permian time.
Following the extinction, evidence for PZE persists into Griesbachian strata in
all three of the major ocean basins. Again, isorenieratane occurs in the Perth Basin
(Grice
et al
.,
2005
), at the GSSP (Global Boundary Stratotype Section and Point)
