Environmental Engineering Reference
In-Depth Information
18.3.3 Cerium and REEs
The concentration of cerium relative to other rare earth elements (REEs) is redox
(Ce/Ce
*
) recorded in carbonate sediments is interpreted to reflect suboxic
-
anoxic
conditions in the local water column (Alibo and Nozaki,
1999
).
There is spatial variation in Ce/Ce
throughout the Changhsingian and
Griesbachian (
Figure 18.1
). Records from Panthalassa have been interpreted as
a shift from oxic conditions in the middle Permian to suboxic
-
anoxic conditions
during the Changhsingian (Kato
et al
.,
2002
). In the Tethys region, some sections
indicate anoxic conditions at the extinction horizon (Kakuwa and Matsumoto,
2006
;Algeo
et al
.,
2007
), whereas others indicate oxygenated conditions
(e.g. Dolenec
et al
.,
2001
;Fio
et al
.,
2010
;Brand
et al
.,
2012
). These PTB
records could either re
*
ect spatially heterogeneous bottom-water redox condi-
tions or suggest that Ce/Ce
records have been misinterpreted; Loope
et al
.
(
2013
) argue that, in some sections, the discrepancy between geochemical
evidence for anoxia and very fossiliferous strata indicates that the Ce/Ce
*
*
records
have been compromised by silicilastic in
uence or dolomitization. Following the
extinction, Ce/Ce
measured in conodont apatite from south China
suggests multiple pulses of anoxia occurring from the PTB to the earliest
Dienerian, the basal Smithian to the earliest Spathian, and in the mid Spathian
(Song
et al
.,
2012
).
*
18.3.4 Uranium
The marine uranium cycle offers the potential to constrain the global extent of
seafloor anoxia, as the uranium concentration and the
238
U/
235
U isotope ratio are
each sensitive to redox conditions at the sediment
-
water interface (Stirling
et al
.,
2007; Weyer
et al
.,
2008
). Changes in uranium concentrations and isotope ratios
may represent global conditions because the residence time of uranium in the
modern ocean (3.2
-
5.6
10
5
yr) is much longer than the ocean mixing time
(Dunk
et al
.,
2002
).
The majority of Permian
-
Triassic uranium data are con
ned to the extinction
horizon. Uranium concentrations decrease abruptly at the extinction horizon in
shallow-marine carbonates in Saudi Arabia (Ehrenberg
et al
.,
2008
), Iran (Tavakoli
and Rahimpour-Bonab,
2012
) and south China (Brennecka
et al
.,
2011
; Song
et al
.,
2012
), indicating a global drawdown of uranium concentrations in seawater
caused by ocean anoxia (
Figure 18.1
). The only sections that show an increase in
uranium concentration are localities where deposition switches to black shales
and other dysaerobic facies (e.g. British Columbia and Italy), where uranium
would be concentrated due to local anoxia (Wignall and Twitchett,
2002
). Records
