Environmental Engineering Reference
In-Depth Information
each is likely to have occurred. To illustrate the disparity in precision between the
most precise published dates on the timing of biotic crises and the majority of dates
characterizing LIP eruption/emplacement,
Figure 4.5
compares the analytical
uncertainty (at the 2
σ
percent level) on the weighted mean
206
Pb
-
238
n-
ing cessation of the end-Permian mass extinction with recently-published U
-
Pb
weighted mean
206
Pb
-
U date de
39
Ar plateau dates for four Phanerozoic
LIPs that have been temporally associated with, and implicated as triggers for,
mass extinction: The Siberian Traps, CAMP, Karoo
-
Ferrar, and Deccan Traps.
It is clear that the precision on dates constraining the age and duration of mass
extinction events, e.g. the end-Permian biotic crisis (Reichow
et al
.,
2009
; Burgess
et al
.,
2014
) using both zircon and sanidine are more than one order of magnitude
more precise than the majority of dates available on ma
238
U dates and
40
Ar
-
c LIP rocks.
39
Ar dates has improved over the past decade,
illustrated in
Figure 4.5
using dates for the Permian
-
Triassic boundary. Published
dates have evolved over the past 16 years as analytical capabilities and adoption of
protocols including chemical abrasion have led to improvements in accuracy and
precision (Mattinson,
2005
;Schoene
et al
.,
2006
; Kuiper
et al
.,
2008
;Renne
et al
.,
2014
). A similar trend is expected with application of these advances to LIP dating.
The accuracy of U
-
Pb and
40
Ar
-
4.3.1 Case study
Central Atlantic Magmatic Province (CAMP)
The CAMP LIP and the end-Triassic extinction serve as an example of the
complex relationship between eruption, environmental effects and extinction.
Estimates on the total duration of CAMP magmatism provided by astrochronology
(Whiteside
et al
.,
2007
) and
40
Ar
-
39
Ar dates (e.g. Nomade
et al
.,
2007
; Hames and
Renne,
2000
; Beutel
et al
.,
2005
; Marzoli
et al
.,
2011
; Knight
et al
.,
2004
) differed
by an order of magnitude. Most of the available
40
Ar
-
39
Ar geochronologic data
for CAMP in the eastern USA and Morocco permit a causal relationship by
overlapping within uncertainty with both the end-Triassic extinction (201.56
0.015 Ma) and the subsequent biologic recovery (Triassic
-
Jurassic boundary at
201.46
0.040 Ma (Blackburn
et al
.,
2013
;
Figure 4.6a
). Problematically, this
includes geochronologic data from rocks that lie stratigraphically
above
the extinc-
tion horizon. Though these lower-precision
40
39
Ar dates permit a causal
relationship with CAMP and the extinction, they prevent a detailed comparison
of the eruption, extinction and recovery histories and permit an emplacement
interval of up to several million years
-
durations that far exceed the effusion rates
required to explain the inferred increase in the partial pressure of CO
2
(
p
CO
2
)in
the atmosphere (Rampino and Caldeira,
2011
).
Ar
-
