Timing of recovery from the end-Permian extinction:
Geochronologic and biostratigraphic constraints from south China
Daniel J. Lehrmann* Department of Geology, University of Wisconsin–Oshkosh, Oshkosh, Wisconsin 54901, USA
Jahandar Ramezani
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology,
Samuel A. Bowring
Cambridge, Massachusetts 02139, USA
Mark W. Martin†
Paul Montgomery#
Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA
Paul Enos
Jonathan L. Payne Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305,
USA
Michael J. Orchard Geological Survey of Canada, Vancouver, BC V6B 5J3, Canada
Wang Hongmei
Guizhou Bureau of Geology and Mineral Resources, Guiyang, Guizhou, People’s Republic of China
Wei Jiayong
ABSTRACT
Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb
dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian)
boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new
dates constrain the Early Triassic interval characterized by delayed biotic recovery and
carbon-cycle instability to ⬃5 m.y. This time constraint must be considered in any model
for the end-Permian extinction and subsequent recovery.
Keywords: U-Pb geochronology, conodont, biostratigraphy, Permian, Triassic, extinction.
INTRODUCTION
The greatest extinction since the evolution
of multicellular life occurred at the end of the
Permian. Despite intensive research on
Permian-Triassic boundary sections, the cause
of the end-Permian extinction remains controversial. However, recent studies have narrowed potential causes and environmental circumstances associated with the extinction
(Erwin, 2005). Crucial to models for the extinction and subsequent recovery is a precise
chronology.
It has long been recognized that full recovery from the end-Permian extinction event
was primarily a Middle Triassic phenomenon
and that evidence of meaningful biotic recovery during the Early Triassic is largely absent
(e.g., Bottjer, 2001; Flügel, 2002; Erwin,
2005). Hallam (1991) and many subsequent
workers suspected that delayed Early Triassic
recovery was longer than could be accounted
for merely by the magnitude of the extinction
and was best explained by continuing environmental stress. Early Triassic strata are characterized by low diversity, reduced size of organisms, reduced abundance of skeletal
animals, reduced levels of bioturbation, and
carbon-cycle instability (e.g., Schubert and
Bottjer, 1995; Woods et al., 1999; Wignall and
Twitchett, 2002; Payne, 2005; Payne et al.,
*E-mail: lehrmann@uwosh.edu.
†Current address: Shell International Exploration
and Production, Houston, Texas 77099, USA.
#Current address: Chevron Australia Pty Ltd.,
Perth, 6000, Australia.
2004, 2006). However, the duration of the
Early Triassic has been poorly constrained. Interpolating between radiometric dates near the
Permian-Triassic boundary and dates within
the Middle and Late Triassic suggested the
Early Triassic lag in recovery lasted between
5 and 10 m.y. (Erwin, 1998; Bottjer, 2001;
Flügel, 2002). The absence of geochronology
at the Early-Middle Triassic (OlenekianAnisian) boundary, however, left a great deal
of uncertainty regarding the duration the Early
Triassic.
We present new geochronologic and biostratigraphic data from the Guandao section of
south China that place the end of the Early
Triassic at 247.2 Ma, indicating that the Early
Triassic had a duration of ⬃5 m.y. Improved
constraints on the duration of the Early Triassic can be used to constrain models for the
delay in biological recovery until the beginning of the Middle Triassic.
OLENEKIAN-ANISIAN BOUNDARY
SECTION AT GUANDAO
The Guandao section occurs on the slope of
an isolated carbonate platform called the Great
Bank of Guizhou, in the Nanpanjiang Basin
of south China (Fig. 1). The Guandao section
(Fig. 2) is advantageous for establishing a
chronostratigraphy for the Olenekian-Anisian
boundary because (1) it occurs in deep-marine
facies without significant unconformity, (2) it
is physically correlated with an adjacent
shallow-marine platform (Fig. 1), and (3) it
contains abundant conodonts, volcanic-ash
horizons, and a primary magnetic signature
(Fig. 2). The geochronology, biostratigraphy,
and paleomagnetic data presented herein, in
addition to the detailed carbon-isotope record
(Payne et al., 2004) and the recent discovery
of ammonoids, highlight the value of the
Guandao section as a reference for the
Olenekian-Anisian boundary.
Details of the facies at the Guandao section
and the evolution of the Great Bank of Guizhou are found in Lehrmann et al. (1998). The
Great Bank of Guizhou evolved from a lowrelief bank with oolite shoals and exceedingly
low biodiversity (Payne et al., 2006) in the
Early Triassic, to a steep Tubiphytes-reefrimmed platform in the Middle Triassic (Fig.
1). The Tubiphytes reefs contain subordinate
calcisponges, scleractinian corals, echinoderms, mollusks, foraminifera, and problematic encrusters (Lehrmann et al., 1998). The
evolution of the Great Bank of Guizhou from
a bank with low biodiversity to a rimmed platform with biologically diverse reefs is reflected in the Guandao section by a change in the
composition of allochthonous material shed
from the adjacent platform (Fig. 2). The Lower Triassic facies at Guandao contain ooids
and mollusks, whereas more diverse fauna including Tubiphytes and echinoderms first occurs in the uppermost Olenekian and Middle
Triassic (Fig. 2).
CONODONT BIOSTRATIGRAPHY
Conodont biostratigraphy constrains the position of the Olenekian-Anisian boundary at
245.6 m (Fig. 2). We defined the OlenekianAnisian boundary to coincide with the first appearance of the conodont Chiosella timorensis
(Fig. 2). Cs. timorensis has been recognized
as a key index fossil for definition of the
boundary, as it has a narrow stratigraphic
range and global distribution (Orchard, 1995;
Orchard and Tozer, 1997). Moreover, the International Commission on Stratigraphy has
informally agreed that the appearance of Cs.
timorensis at the Desli Caira section in Dobrogea, Romania, could serve as a GSSP for
the Olenekian-Anisian boundary (Internation-
䉷 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology; December 2006; v. 34; no. 12; p. 1053–1056; doi: 10.1130/G22827A.1; 2 figures; Data Repository item 2006229.
1053
Figure 1. A: Early Triassic paleogeographic map of the Nanpanjiang Basin. B: Stratigraphic
cross section of the Lower-Middle Triassic strata of the Great Bank of Guizhou. The location
of the Guandao section is indicated as GD.
al Commission on Stratigraphy, 2004). At
Desli Caira, the first occurrence of Cs. timorensis closely corresponds with the occurrence
of biostratigraphically important ammonoids
Japonites, Paradanubites, and Paracrochordiceras (Gradinaru et al., 2001).
Additional constraints on placement of the
boundary include last occurrences of Neospathodus abruptus and Ns. triangularis (Orchard, 1995) well below the boundary, occurrences of Ns. symmetricus and Ns. homeri
below and extending slightly above the
boundary, and first occurrences of Gladiogondolella tethydis, Nicoraella germanica, and
Ni. kockeli above the boundary (Fig. 2). Ns.
abruptus, Ns. triangularis, Ns. symmetricus,
and Ns. homeri are typical Olenekian forms;
the latter two extend upward into the base of
the Anisian (Orchard, 1995; Gradinaru et al.,
2001). Gd. tethydis, Ni. germanica, and Ni.
kockeli are Anisian species; the latter two approximate respectively the beginning of the
Bithynian and Pelsonian substages (Kozur,
2003; see discussion in the GSA Data
Repository1).
1GSA Data Repository item 2006229, analytical
methods and additional data for U-Pb geochronology, magnetic-reversal stratigraphy, and conodont
biostratigraphy, is available online at www.
geosociety.org/pubs/ft2006.htm, or on request from
editing@geosociety.org or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
1054
U-Pb ZIRCON GEOCHRONOLOGY
Zircons were separated from 1 to 2 kg
volcanic-ash samples using conventional techniques. Acicular, colorless, doubly terminated
crystals lacking obvious xenocrystic cores,
mineral inclusions, or fractures were selected
for analysis. The analyses reported herein
were obtained over a period of five years, during which time the analytical procedures
evolved significantly. Among the most notable
improvements are lower laboratory Pb blanks
(⬃0.3 pg) and the adoption of the chemical
abrasion technique (CA-TIMS, chemical
abrasion–thermal ionization mass spectrometry) of Mattinson (2005). A full description of
U-Pb analytical procedures can be found in
the GSA Data Repository (see footnote 1),
with additional details reported in Schoene et
al. (2006).
It has become clear that high-precision zircon analyses are characterized by a systematic
discrepancy between the 207Pb/206Pb, 207Pb/
235U, and 206Pb/238U dates and that the latter
is the most accurate (Mattinson, 2000;
Schoene et al., 2006). The discrepancy is most
likely to do with uncertainties in one or both
of the U decay constants. For late Paleozoic
and younger zircons, we regard the 206Pb/238U
date as the most precise and reliable for timescale calibration purposes. There is systematic
discrepancy between U/Pb and 40Ar/39Ar
dates that can be as much as 1% (e.g., Min et
al., 2000; Schoene et al., 2006). Caution must
be used when comparing U/Pb and 40Ar/39Ar
dates, or dates from either system from different labs, or when calculating durations using different systems/labs, and in these cases,
systematic errors for both methods should be
included. U/Pb errors are reported here as
⫾X(Y)[Z] Ma (Fig. 2), where X is the internal
or analytical error in the absence of all systematic errors, Y includes the tracer calibration error, and Z includes both the latter and
decay constant errors of Jaffey et al. (1971).
The MSWD (mean square of weighted deviates; York, 1966, 1967) is calculated prior to
addition of systematic errors.
High-precision ash-bed geochronology is
often challenging. Despite careful grain selection and pretreatment by either mechanical
abrasion or CA-TIMS, analyses exhibit more
scatter than can be accounted for by purely
analytical errors (MSWD ⬎ 1). These include
subtle effects due to either Pb-loss or inheritance of zircon that is only slightly older than
the eruption age. The latter can reflect debris
incorporated into the eruption column from a
long-lived eruption center. In all samples, we
have used a subset of the data to arrive at the
best estimate for the eruption/depositional age
of individual ash beds. Our choices were
based on isolating the largest group of data
points that overlap within 2 (analytical) uncertainties. While admittedly subjective in
some instances, our experience with dozens of
ashes of similar age range gives us confidence
in the validity of this approach. A somewhat
similar approach was adopted by Ludwig and
Mundil (2002) and implemented in Ludwig
(2005), but calculates a median age with
asymmetric uncertainties. Both methods yield
ages that overlap within error.
An interval rich in volcanic ash, named the
‘‘green-bean rock’’ by Chinese geologists, has
been mapped across south China and is widely
interpreted to mark the Olenekian-Anisian
boundary on the basis of macrofossils (Guizhou Bureau of Geology and Mineral Resources, 1987). At the Guandao section, this
interval consists of several volcanic-ash beds
separated by pelagic carbonate (Fig. 2). We
report 98 single-zircon analyses from four
volcanic-ash horizons above and below the
Olenekian-Anisian boundary at the Guandao
section (Table DR1; Fig. DR1 [see footnote
1]). Analyses of zircons from each sample
contain evidence for both Pb-loss and
inheritance.
The lowest volcanic-ash horizon dated,
PGD-1, occurs at 238.8 m, 6.8 m below the
Olenekian-Anisian boundary (Fig. 2). Twentythree single-zircon analyses are reported in Table DR1, of which we use eight analyses to
define a weighted mean 206Pb/238U date of
247.38 ⫾ 0.10(0.13)[0.40] Ma with a MSWD
of 1.4. Six of the used data points were analyzed
by the CA-TIMS technique (nCA-TIMS ⫽ 6).
GEOLOGY, December 2006
the GSA Data Repository (see footnote 1).
Magnetic-reversal stratigraphy from the Guandao section (Fig. 2 and Fig. DR3) correlates
with the reversal zonation of the OlenekianAnisian boundary in western Tethys (Muttoni
et al., 2000; Nawrocki and Szulc, 2000) and
the global compilation of Ogg (2004). In all
of these zonations, normal polarity occurs in
the Middle Spathian followed by a predominantly reversed zone with a few brief reversals
in the uppermost Spathian and Aegean and
predominantly normal polarity in the Bithynian to Lower Pelsonian (Fig. 2 and Fig.
DR3). The Guandao zonation differs from the
western Tethys and global compilations by
Olenekian-Anisian boundary placement on the
basis of Cs. timorensis slightly lower than the
boundary placed on the basis of ammonoids,
and by apparent lack of a longer normal zone
that occurs within the Aegean, or that brackets
the Olenekian-Anisian boundary, on the basis
of Olenekian-Anisian boundary definition by
conodonts or ammonoids respectively (Fig. 2
and Fig. DR3).
Figure 2. Biostratigraphy, lithofacies, magnetostratigraphy, and U-Pb age constraints on the
Olenekian-Anisian boundary interval in the Guandao section. Mab denotes meters above
the base of the section. The biostratigraphic ranges of conodonts are shown with vertical
bars; dots on the bars indicate conodont collections. Ng. denotes the genus Neogondolella.
Black is normal magnetic polarity; white is reversed. Histograms depict the distribution of
206
Pb/238U ages for the analyzed zircon grains from each sample, including those selected
for weighted mean age calculation (inside dashed boxes). Horizontal shaded lines represent
the calculated (weighted mean 206Pb/238U) age with their thickness proportional to the age
uncertainty.
This date is interpreted to represent the eruption age of the volcanic ash (Figs. 2 and DR1).
The next higher dated horizon, PGD-2, occurs at 239.3 m, 6.3 m below the OlenekianAnisian boundary (Fig. 2). From the 29 analyses reported, 11 were selected (nCA-TIMS ⫽
6) that yield a weighted mean 206Pb/238U date
of 247.32 ⫾ 0.08(0.11)[0.39] Ma (MSWD ⫽
0.93) (Figs. 2 and DR1), slightly younger but
within error of PGD-1.
Sample PGD-3 occurs closest to, and 2.3 m
above, the boundary at 247.9 m (Fig. 2).
Twenty zircons were analyzed, from which a
depositional age of 247.13 ⫾ 0.12(0.15)[0.43]
Ma (MSWD ⫽ 2.2) was calculated based on
a subset of eight CA-TIMS analyses (Figs. 2
and DR1).
The highest dated sample, GDGB-110, occurs in the Anisian (Pelsonian) in the uppermost Cs. timorensis biozone at 260.25 m,
GEOLOGY, December 2006
14.65 m above the Olenekian-Anisian boundary (Fig. 2). Twenty-six analyses are reported,
from which 11 (nCA-TIMS ⫽ 7) yield a weighted mean 206Pb/238U date of 246.77 ⫾
0.13(0.16)[0.44] Ma (MSWD ⫽ 1.4) (Figs. 2
and DR1). Thus, the four ash beds yield statistically significant calculated dates that are
consistent with their relative stratigraphic order. Linear interpolation between the ashes
closest to the boundary (PGD-2 and PGD-3)
allows an estimate for the age of the
Olenekian-Anisian boundary at ca. 247.2 Ma.
MAGNETIC-REVERSAL
STRATIGRAPHY
Magnetostratigraphic data collected at the
Guandao section defined ten normal and ten
reverse magnetozones for the Lower Triassic
and Lower Anisian (Figs. DR2 and DR3 [see
footnote 1]). Techniques used are provided in
IMPLICATIONS
U-Pb zircon dates from the Guandao section bracket the biostratigraphically constrained Olenekian-Anisian boundary. Further,
the stratigraphic sequence of dates ascending
across the boundary provides an independent
test of our approach. From these results we
conclude that the Olenekian-Anisian boundary
is older than 247.13 ⫾ 0.12 Ma and younger
than 247.32 ⫾ 0.08 Ma. Linear interpolation,
assuming constant sediment accumulation
rates, yields a boundary age estimate of
247.18 Ma.
Previous estimates of ca. 240, 242, and 245
Ma of the Olenekian-Anisian boundary were
made on the basis of interpolation from Permian and Middle Triassic dates (Gradstein et al.,
1995; Ogg, 2004). Our new age for the
Olenekian-Anisian boundary is consistent
with U-Pb dates that place the AnisianLadinian boundary at ca. 241 Ma (Mundil
et al., 1996). Given that dates for the endPermian extinction horizon from independent
labs are converging near 252.2–252.6 Ma
(Bowring et al., 1998, personal commun.;
Mundil et al., 2001, 2004), the minimal duration of the Early Triassic epoch and interval of delayed biotic recovery from the endPermian extinction is now constrained to be
⬃5 m.y. Recent estimates for the age of the
O-A boundary and a minimal duration of the
Early Triassic of 4.5 ⫾ 0.6 m.y. (Ovtcharova
et al., 2006) were based on a single new date
from the Upper Olenekian and citation of a
basal Anisian age from our preliminary report
(Lehrmann et al., 2005). The OlenekianAnisian dates presented herein provide a more
robust estimate of the age of the boundary and
supersede those reported in Lehrmann et al.
(2005).
1055
Two end-member possibilities might explain the Early Triassic lag in biotic rediversification: (1) A long time for recovery resulted from the great magnitude of the
extinction (Erwin, 1998) or (2) adverse environmental conditions persisted in the aftermath and thus prevented diversification until
the end of the Early Triassic. The shortened
time frame for biotic recovery indicated by the
results presented above could be seen to lend
more support for the former possibility. However, persistent carbon-cycle instability (Payne
et al., 2004) and facies evidence for persistent
or intermittent marine anoxia (Wignall and
Twitchett, 2002) are suggestive of continuing
disturbance. If delayed recovery reflects persistent environmental disturbance, then mechanisms such as the eruption of the Siberian
Traps may have influenced the recovery. If, on
the other hand, extinction resulted from bolide
impact (Becker et al., 2004), then carboncycle instability and evidence of continuing
anoxia must reflect environmental and ecological feedbacks, as yet poorly understood, following the initial disturbance. Similarly, Coxall et al. (2006) have suggested that full
recovery following the end-Cretaceous extinction took as long as 3 m.y. The chronostratigraphic framework presented herein constrains the time frame needed for evaluation
of models of postextinction rediversification
and provides correlation tools needed to constrain geographic patterns of biotic recovery
(cf. Erwin, 1998). Unraveling these patterns
will be essential to resolving the mechanisms
that shaped repopulation of life on Earth following the greatest mass extinction.
ACKNOWLEDGMENTS
This research was supported by the National Science
Foundation (EAR-9804835 to Lehrmann, EAR9805731 to Enos, and EAR-0451802 to Bowring), by
the American Chemical Society (ACS-40948-B2 to
Lehrmann, ACS-34810-AC8 and ACS-37193-AC8 to
Enos and Montgomery), and by NASA Astrobiology
(NCC2-1053 to Bowring). David Bottjer, Roland Mundil, and James Ogg provided thoughtful reviews.
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Manuscript received 26 March 2006
Revised manuscript received 4 July 2006
Manuscript accepted 14 July 2006
Printed in USA
GEOLOGY, December 2006