Introduction
The Permian–Triassic boundary mass extinction (PTBME) is considered the
largest mass extinction within the Phanerozoic. About 90 % of all marine
species suffered extinction (Raup, 1979; Stanley and Yang, 1994; Erwin et
al., 2002; Alroy et al., 2008) and terrestrial plant communities underwent
major ecological reorganization (Hochuli et al., 2010). This major caesura
in global biodiversity marked the end of the Palaeozoic faunas and the
inception of the modern marine and terrestrial ecosystems (e.g., Benton,
2010; Van Valen, 1984). Several kill mechanisms have been proposed, such as
global regression (e.g., Erwin 1990; Yin et al., 2014), marine anoxia (e.g.,
Feng and Algeo, 2014), ocean acidification (e.g., Payne et al., 2010) or a
combination thereof. Rapid global warming (e.g., Svensen et al., 2009), high
nutrient fluxes from continent into oceans (Winguth and Winguth, 2012) and
increased sediment accumulation rates (Algeo and Twitchett, 2010) also came
into the play, but their respective relations with the global regression
near the PTB and the main extinction peak at the PTB remain unclear. In
spite of the rapidly growing amount of data, the detailed timing of
available diversity estimates and environmental proxies is still lacking,
and the ultimate triggers of the PTBME remain elusive. The most likely cause
derives from the temporal coincidence with massive and short-lived volcanism
of the Siberian Traps (e.g., Burgess and Bowring, 2015) that injected
excessive amounts of volatiles (H2O, CO2, SO2, H2S) into
the atmosphere. Accompanying destabilization of gas hydrates (CH4) and
contact metamorphism of organic carbon-rich sediments (Retallack and Jahren,
2008; Svensen et al., 2009) are likely to have contributed additional
volatiles into the atmosphere, thus substantially altering the climate and
the chemical composition of the ocean. This presumably close chronological
association has led many authors to support a cause–effect relationship
between flood basalt volcanism and mass extinctions. Constraining the timing
and duration of the PTBME in a precisely and accurately quantified model
that combines relative (i.e., biostratigraphy, environmental changes) and
sequences of absolute (zircon geochronology) ages is key to reveal the
cascading causes and effects connecting rapid environmental perturbations to
biological responses.
The South China block provides a few exceptional marine successions with a
continuous stratigraphic record across the PTB (e.g., Yin et al., 2014).
Among these is the Global Stratotype Section and Point (GSSP) in Meishan D
(Yin et al., 2001), where the PTB is defined by the first occurrence (FO) of
the Triassic conodont Hindeodus parvus. Additionally, these South
Chinese sections reflect intense regional volcanic activity during late
Permian and Early Triassic times as manifested by many intercalated
zircon-bearing ash beds (Burgess et al., 2014; Galfetti et al., 2007;
Lehrmann et al., 2015; Shen et al., 2011). High-precision U-Pb zircon
geochronology can be applied to these ash beds by assuming that the age of
zircon crystallization closely approximates the age of the volcanic eruption
and ash deposition. Earliest U-Pb geochronological studies (e.g., Bowring et
al., 1998; Mundil et al., 2004; Ovtcharova et al., 2006; Shen et al., 2011)
do not reach decamillennial resolution, which is necessary to resolve biotic
events, such as extinction or recovery. Recent improvements in the U-Pb
dating technique by the development of the chemical abrasion–isotope
dilution–thermal ionization mass spectrometry (CA-ID-TIMS; Mattinson, 2005),
by the revision of the natural U isotopic composition (Hiess et al., 2012),
by the development of data reduction software (Bowring et al., 2011; McLean
et al., 2011) and by the calibration of the EARTHTIME
202Pb-205Pb-233U-235U tracer solution (Condon et al.,
2015) now provide more accurate weighted mean zircon population dates at the
< 80 ka level (external uncertainty) for a PTB age, which allow for
more precise calibration between biotic and geologic events during mass
extinctions and recoveries. Two of the cases benefiting from this improved
technique are the highly condensed GSSP defining the PTB at Meishan (Burgess
et al., 2014) and the expanded Early–Middle Triassic boundary in Monggan (Ovtcharova
et al., 2015).
The aim of this work is (1) to date the PTB in two sedimentary sections that
are continuous with a conformable PTB and with higher sediment accumulation
rates over the same duration than in Meishan, using the highly precise and
accurate dating technique of CA-ID-TIMS, and (2) to test the age consistency
between the PTB as defined paleontologically in Meishan and as recognized by
conformable formational boundaries in the deeper water sections of Dongpan
and Penglaitan. Our high-precision dates provide a future test for the
synchronicity of conodont biozones, chemostratigraphic correlations, and
other proxies involved in the study of the PTBME. Moreover, applying
Bayesian age modeling (Haslett and Parnell, 2008) based on these
high-precision data sets allows us to model variations in sediment
accumulation rate and to directly compare other proxy data across different
PTB sections, inclusive of the Meishan GSSP.
(a) Locality map showing the position of Dongpan,
Penglaitan and Meishan Global Stratotype Section and Point (GSSP), China.
(b) Late Permian paleogeographic reconstruction after Ziegler et
al. (1997), indicating the location of the South China block in the
peri-Gondwana region. Beneath the paleogeographic map of the Nanpanjiang
Basin in South China showing the position of the investigated Dongpan and
Penglaitan section during late Permian times (base map modified after Wang
and Jin, 2000).
Our data demonstrate that the PTB, as recognized in our sections by
conformable boundaries between late Permian and basal Triassic formations,
is synchronous within analytical uncertainty of ca. 40 ka. We also show that
Bayesian age models produce reproducible results from different sections,
even though U-Pb datasets originate from different laboratories. We
construct a coherent age model for the PTB in Dongpan and Penglaitan, which
is also in agreement with the PTB age model from Meishan (Burgess et al.,
2014). These results further demonstrate that 206Pb / 238U dates
produced in two different laboratories using the EARTHTIME tracer solution
provide reproducible age information at the 0.05 % level of uncertainty.
Stratigraphy, geochronology and Bayesian age–depth modeling for the
Dongpan section from late Changhsingian to Griesbachian.
(a) Weighted mean 206Pb / 238U dates of the volcanic ash
beds and volcanogenic sandstones are given in Ma. U-Pb data of DGP-21 are
taken from Baresel et al. (2016). Investigated radiolarian samples (DGP-1 to
DGP-5) are shown in their stratigraphic positions. (b) The rescaled
Bayesian Bchron age–depth model is presented with its median (middle grey
line) and its 95 % confidence interval (grey area). Radioisotopic dates
together with their uncertainty (red horizontal bars) are presented as
206Pb / 238U weighted mean dates of the dated volcanic ash beds in
their stratigraphic positions. Predicted dates for the onset of the
radiolarian decline (RD) and the Permian–Triassic boundary (PTB) are
calculated with their associated uncertainty using the rescaled Bayesian
Bchron age–depth model assuming stratigraphic superposition.
Geological setting
Regional context
The newly investigated volcanic ash beds were sampled from two PTB sections:
Dongpan in southwestern Guangxi Province and Penglaitan in central Guangxi
Province in South China (Fig. 1a, exact sample locations are given in
Sect. S1 in the Supplement). Both sections are within the Nanpanjiang Basin (Lehrmann et
al., 2015), a late Permian–Early Triassic pull-apart basin in a back arc
context located on the present-day southern edge of the South China block.
This deep-marine embayment occupied an equatorial position in the eastern
paleo-Tethys Ocean (e.g., Golonka and Ford, 2000; Lehrmann et al., 2003;
Fig. 1b). The basin was dominated by a mixed carbonate–siliciclastic regime
during Permian and Early Triassic times and underwent a major change to a
flysch-dominated regime in later Triassic times (e.g., Galfetti et al.,
2008; Lehrmann et al., 2007). Decimeter- to meter-thick beds of mixed volcanic and
clastic material as well as millimeter- to centimeter-thick volcanic ash beds are locally
abundant and especially well preserved in down-thrown blocks recording deep
water records in low energy environments and, to a lesser degree, on
up-thrown blocks recording shallow water to outer platform settings.
Volcanic ash beds are, however, usually not preserved in traction-dominated
slope deposits. Genetically related volcanic rocks crop out in the southwestern part of the basin towards Vietnam, suggesting the proximity of a
volcanic arc related to the convergence between Indochina and South China
(Faure et al., 2016). The volcanism produced by this convergence is the most
likely source of the analyzed volcanic ashes (Gao et al., 2013).
Stratigraphy, geochronology and Bayesian age–depth modeling for the
Penglaitan section from late Changhsingian to Griesbachian.
(a) Weighted mean 206Pb / 238U dates of the volcanic ash
beds and volcanogenic sandstones are given in Ma. U-Pb data of PEN-28 and
PEN-22 are taken from Baresel et al. (2016). Investigated conodont samples
(PEN-23 and PEN-24; see also Sect. S5) and first occurrence of Triassic
conodonts are shown in their stratigraphic positions. A poorly preserved
Permian nautiloid is indicated in its stratigraphic position ca. 1.3 m below
the Permian–Triassic boundary (PTB). (b) The rescaled Bayesian
Bchron age–depth model is presented with its median (middle grey line) and
its associated 95 % confidence interval (grey area). Radioisotopic dates
together with their uncertainty (red horizontal bars) are presented as
206Pb / 238U weighted mean dates of the dated volcanic ash beds in
their stratigraphic positions. Predicted date for the PTB is calculated with
its associated uncertainty using the rescaled Bayesian Bchron age–depth model
assuming stratigraphic superposition.
In Dongpan and Penglaitan, the PTB is manifested by a sharp and conformable
transition from the late Permian Dalong Formation (= Talung Formation) to the basal
Triassic Ziyun Formation. Late Permian rocks in these two sections are classically
assigned to the Dalong Formation of Changhsingian age. However, we note that there
are substantial facies differences between these two late Permian records.
The Dalong Formation in Dongpan is composed of thin-bedded siliceous mudstones,
numerous ash layers and minor limestone beds (Fig. 2a). This facies
association is in agreement with the vast majority of reported occurrences
of this formation within the South China block. The Dalong Formation is
interpreted as a basinal depositional environment with restricted
circulation and an estimated water depth of 200 to 500 m (He et al., 2007;
Yin et al., 2007). In Guangxi and Guizhou, the thickness of the typical
Dalong Formation is highly variable and ranges from a couple of meters to ca. 60 m. Rocks assigned to the Dalong Formation in Penglaitan markedly diverge from
those of the typical Dalong Formation. In Penglaitan, the Dalong Formation reaches an
unusual thickness of ca. 650 m and is lithologically much more
heterogeneous, with a marked regressive episode in its middle part (Shen et
al., 2007). Moreover, in Penglaitan the Dalong Formation contains numerous
volcanogenic sandstones distributed within the entire succession, a
distinctive feature when compared to other sections. Only the lower part of
the “Dalong Formation” in Penglaitan can be unambiguously assigned to this
formation. The middle and upper part of this section are notably shallower,
showing cross bedding and ripple marks in the uppermost 30 m of the Permian,
which are underlain by upper shoreface to foreshore facies deposits
containing coal seams and abundant plant fossils (Shen et al., 2007). The
uppermost part of the Dalong Formation was deposited in relatively deep water
settings that comprise thin-bedded dark-grey limestone intercalated with
thick volcanogenic sandstones and thin volcanic ash beds (Fig. 3a). All
associated volcanogenic sandstones were deposited by geologically
instantaneous turbidites, mainly reflecting the basal part (Bouma A–B
sequence) of such gravity flow deposits. Gradual accumulations and sediment
mixing are restricted to sands bars occurring in the middle part of the
section, in association with coal seams during an intervening regressive
episode. Hence, the volcanogenic sandstones from the top of the Dalong Formation
in Penglaitan may not suffer from substantial sediment reworking and mixing
and do not represent substantial cumulative amounts of time relative to the
interlayered shales and thin-bedded limestones. The depositional setting of
Penglaitan is interpreted as that of a fault-bounded block successively
thrown down and up. Hence, Penglaitan stands in marked contrast with the
homogenous deeper water facies of the typical Dalong Formation in other sections.
The conformably overlying Early Triassic rocks have been previously assigned
to the Luolou Formation in both Penglaitan and Dongpan (Feng et al., 2007; He et
al., 2007; Shen et al., 2012; Zhang et al., 2006). At its type locality and
elsewhere in northwestern Guangxi and southern Guizhou, the base of the
Luolou Formation is invariably represented by shallow water microbial limestone.
In contrast, the onset of the Triassic at Dongpan and Penglaitan is
represented by ca. 30 m of laminated black shales overlain by several
hundred meters of thin-bedded, mechanically laminated, medium- to light-grey
limestone. In Dongpan, edgewise conglomerates and breccias are occasionally
intercalated within the platy, thin-bedded limestone unit. This succession
of facies illustrates a change from basinal to slope depositional
environments and is identical to that of the Ziyun Formation at its type locality
3 km east of Ziyun, Guizhou Province (Guizhou Bureau of Geology
and Mineral Resources, 1987). Therefore, Early Triassic rocks in Dongpan and
Penglaitan are here reassigned to the Ziyun Formation, whose base is of
Griesbachian age. In most sections in Guangxi and Guizhou, where latest
Permian rocks are represented by the Dalong Formation, these are consistently and
conformably overlain by basal black shales of the Early Triassic Ziyun Formation
or the Daye Formation (e.g., Feng et al., 2015). In these downthrown blocks, the
effects of the Permian–Triassic global regression were negligible in comparison to those observed in adjacent, up-thrown blocks
that recorded pronounced unconformities or condensation.
The Dongpan section
Numerous litho-, bio- and chemo-stratigraphic studies (e.g., Feng et al.,
2007; He et al., 2007; Luo et al., 2008; Zhang et al., 2006) have been
published on the Dongpan section during the last two decades. However, the
volcanic ash beds of this continuous PTB section have never been dated. The
classic lithostratigraphic subdivisions of the Dongpan section (bed 2 to 13;
indicated in Fig. 2a) (Meng et al., 2002) can easily be recognized in the
field. Based on the conodont alteration index (CAI), Luo et al. (2011)
established that the section shows only a low to moderate thermal overprint
equivalent to a maximal burial temperature of 120 ∘C. Our own
estimation of the CAI of conodont elements obtained from the same beds
points toward values around 3, thus confirming the estimation of Luo et al. (2011).
Beds 2 to 5 consist of thin (dm to cm) siliceous mudstones, mudstones, minor
lenticular limestone horizons and numerous intercalated volcanic ash beds.
These beds yield radiolarians, foraminifera (Shang et al., 2003), bivalves
(Yin, 1985), ammonoids (Zhao et al., 1978), brachiopods (He et al., 2005),
ostracods (Yuan et al., 2007), and conodonts (Luo et al., 2008) of
Changhsingian age. Chinese authors have provided very detailed studies of
radiolarian occurrences from the top of the Dongpan section, documenting
about 160 species belonging to 50 genera (Feng et al., 2007; Wu et al.,
2010; Zhang et al., 2006). Most of these radiolarians belong to the
Neoalbaillella optima assemblage zone of late Changhsingian age
(Feng and Algeo, 2014), although it is unclear whether some of the Permian taxa
reported from the top of the section by previous authors (i.e. above bed 6;
Feng et al., 2007) still belong to this assemblage or to a provisional
ultimate Permian biozone (Xia et al., 2004).
We collected five samples with visible radiolarians (DGP-1 to DGP-5; see
Fig. 2a) for this study. Our goal was not to duplicate the detailed faunal
studies performed at Dongpan by previous authors but essentially to
correlate these previous results with our U-Pb ages using own radiolarian
data. A selection of well-preserved taxa is illustrated in Sect. S4. We
also report the occurrence of morphotypes belonging to the genus
Hegleria, which was previously reported from the section but not
illustrated. Our data confirm that radiolarians of the Dongpan section
belong to the Neoalbaillella optima assemblage zone.
The conodont fauna obtained from beds 3 and 5 was assigned to the
Neogondolella yini interval zone by Luo et al. (2008).
Neogondolella yini is also a characteristic species of the UAZ1
zone, which is the oldest zone of a new high-accuracy zonation around the
PTB constructed by means of unitary associations (Brosse et al., 2016). Bed 6 is composed of a yellow fine-grained volcanic ash bed and thin-bedded
siliceous mudstone. Beds 7 to 12 contain more frequent mudstone and yield a
diverse Permian fauna (Feng et al., 2007; He et al., 2007; Yin et al.,
2007). Additionally, He et al. (2007) showed that end-Permian brachiopods
underwent a size reduction in the uppermost beds of the Dalong Formation, which
they linked with a regressive trend.
The sharp and conformable base (bed 13) of the Early Triassic Ziyun Formation
consists of brown-weathering black shales containing a few very thin
(mm to cm) volcanic ash beds and volcanogenic sandstones. Previous studies
did not recognize how recent weathering superficially altered these black
shales. Bed 13 contains abundant bivalves and ammonoids of Griesbachian age
(Feng et al., 2007; He et al., 2007), which are also known from other
sections where the equivalent black shales are not weathered. Therefore, the
formational boundary placed between beds 12 and 13 is reasonably well-constrained in terms of paleontological ages. Even in the absence of any
close conodont age control, this boundary has been unanimously acknowledged
as the PTB in all previous contributions, thus emphasizing the significance
of this formational change.
The Penglaitan section
The Penglaitan section is well known for its Guadalupian–Lopingian boundary
(Capitanian–Wuchiapingian GSSP; Jin et al., 2006; Shen et al., 2007).
However, the part of the section that straddles the PTB has not been the
focus of any detailed published work. Shen et al. (2007) report
Changhsingian Peltichia zigzag–Paryphella brachiopod assemblage
from a volcanogenic sandstone bed at ∼ 28 m below the PTB. In
addition, Palaeofusulina sinensis is abundant in the uppermost
limestone units of the Dalong Formation and conodonts in the topmost part were
assigned to the Clarkina yini Zone. A poorly preserved Permian
nautiloid was recovered from the volcanogenic sandstone 1.3 m below the PTB
(Fig. 3a). About 0.3 m above the PTB, concretionary, thin-bedded micritic
layers intercalated within the basal black shales of the Ziyun Formation yielded
one P1 element of Hindeodus parvus (Fig. 3a; see also Sect. S5).
Pending the age confirmation of new paleontological data, and in full
agreement with Shen et al. (2007), we place the PTB at this sharp but
conformable formational boundary.
Single-grain zircon analysis and 206Pb / 238U weighted mean
dates for (a) Dongpan and (b) Penglaitan volcanic ash beds and volcanogenic
sandstones. U-Pb data of DGP-21, PEN-28 and PEN-22 are taken from Baresel et
al. (2016). Each horizontal bar represents a single zircon grain analysis
including its 2σ analytical (internal) uncertainty, whereas grey bars
are not included in the weighted mean calculation. Vertical lines represent
the weighted mean age, with the associated 2σ uncertainty (in grey).
Uncertainty in the weighted mean dates is reported as 2σ internal,
2σ external uncertainty including tracer calibration and 2σ
external uncertainty including 238U decay constant uncertainty; MSWD –
mean square of weighted deviates.
Methods
CA-ID-TIMS analysis
Sample preparation, chemical processing and U-Pb CA-ID-TIMS zircon analyses
were carried out at the University of Geneva. Single zircon grain dates were
produced relative to the EARTHTIME 202Pb-205Pb-233U-235U
tracer solution (Condon et al., 2015). All uncertainties associated with
weighted mean 206Pb / 238U ages are reported at the 95 %
confidence level and given as ±x/y/z, with x as analytical
uncertainty, y including tracer calibration uncertainty, and z including
238U decay constant uncertainty. The tracer calibration uncertainty of
0.03 % (2σ) has to be added if the calculated dates are to be
compared with other U-Pb laboratories not using the EARTHTIME tracer
solution. The 238U decay constant uncertainty of 0.11 % (2σ)
should be used if compared with other chronometers such as Ar-Ar. All
206Pb / 238U single-grain ages have been corrected for initial
230Th-238U disequilibrium assuming Th/Umagma of 3.00 ± 0.50 (1σ). This should best reflect the Th/U of the whole rock and
is identical to the Th/Umagma used by Burgess et al. (2014) for the
Meishan ash beds in order to provide an unbiased comparison of the Dongpan,
Penglaitan and Meishan chronology. Th-corrected 206Pb / 238U dates
are on average 80 ka older than the equivalent uncorrected dates when
applying this correction, but changes in the Th/Umagma have only minor
effects on the deposition ages of the Dongpan and Penglaitan volcanic beds.
Compared to the Th/Umagma of 3.00 ± 0.50 (1σ) used in
this study, they would become max. 11 kyr younger with Th/Umagma of 2.00 ± 0.50 (1σ) and max. 7 kyr older
with Th/Umagma of 4.00 ± 0.50 (1σ). All Th-corrected 206Pb / 238U dates are
presented as mean ages of selected zircon populations and their associated
±2σ analytical uncertainties in Figs. 2 and 3, and as single-grain 206Pb / 238U age ranked distribution plots in Fig. 4. The full
data table and analytical details are given in Sect. S2.
Bayesian chronology
In this study we use Bayesian interpolation statistics to establish a
probabilistic age model based on our high-precision U-Pb zircon dates of
each individual ash bed and its stratigraphic position, as it is
incorporated in the free Bchron R software package (Haslett and Parnell,
2008; Parnell et al., 2008) to constrain the chronological sequence and
sedimentation history of the investigated sections. By assuming normal
distribution of our U-Pb dates within one sample and based on the principle
of stratigraphic superposition, which requires that any stratigraphic point
must be younger than any point situated below in the stratigraphic sequence,
it models the age and its associated 95 % confidence interval for any
depth point within the studied sedimentary sequence. The model is based on
the assumption of random variability of sediment accumulation rate, yielding
a family of dispersed piecewise monotonic sediment accumulation models
between each dated stratigraphic horizon. The number of such accumulation
models is inferred by a Poisson distribution, and the size of the sediment
accumulation rates by a gamma distribution. The strength of this approach is
its flexibility that allows changes in sediment accumulation rate from zero
(hiatus in sedimentation) to very large values (sedimentation event at high
rate). In contrast to standard linear regression models, this approach leads
to more realistic uncertainty estimates, with increasing uncertainty at
growing stratigraphic distance from the dated layers. The model also detects
and excludes outliers, which conflict with other evidence from the same
sequence in order to produce a coherent and self-consistent chronology; no
predefined outlier determination is required from the user. One of the
drawbacks of this Bayesian approach is that a change in the sediment
accumulation rate is assumed to occur at each dated stratigraphic position,
though it is unlikely that the change in sedimentation occurs exactly at the
depth of a dated bed. Another drawback is that the sedimentation parameters
are shared across the whole sequence. In consequence, Bchron does not allow
much opportunity for users to individually influence the chronology
behavior.
In this study we use the Bayesian Bchron model as it is part of the Bchron
package (http://cran.r-project.org/web/packages/Bchron/index.html). This
model outperforms other Bayesian age–depth models, as shown by an extensive
comparison conducted on radiocarbon dates from Holocene lake sediments
(Parnell et al., 2011). It provides a non-parametric chronological model
according to the compound Poisson–gamma model defined by Haslett and Parnell (2008), requiring the weighted mean 206Pb / 238U age and the
stratigraphic position of the investigated ash beds as input parameters.
Since the Bchron model was initially coded for radiocarbon dating with a
commonly unknown duration of accumulation for a radiocarbon-dated bed, it
also allows for the input thickness of such a horizon to be defined. However, the
thickness of the geologically instantaneously deposited volcanic ashes was
reduced to zero and the lithostratigraphy has been rescaled in order to
remove the thickness of the volcanic horizons and to produce a more accurate
age–depth model (Figs. 2a and 3a; see also Sect. S3). The technical
details were given in Haslett and Parnell (2008). The Bchron model uses a
Markov chain Monte Carlo (Brooks et al., 2011) rejection algorithm which
proposes model parameters and accepts or rejects them in order to produce
probability distributions of dates for a given depth that match likelihood
and do not violate the principle of stratigraphic superposition. In order to
create an adequate number of accepted samples, the model was run for 10 000 iterations. The Bchron R scripts of Dongpan, Penglaitan and Meishan are
provided in Sect. S3.
Samples
In total, 12 volcanic ash beds and volcanogenic sandstones were sampled from
the Dalong Formation of late Permian age and from the overlying Ziyun Formation of Early
Triassic age at Dongpan and Penglaitan (see Sect. S1). Most of the dated
samples exhibit 206Pb / 238U age dispersions that exceed the
acceptable scatter from analytical uncertainty and are interpreted as
reflecting magmatic residence or a combination of the latter with
sedimentary recycling. Only in two cases (DGP-16, PEN-22) do we find single-grain analyses younger than our suggested mean age and interpret them as
unresolved Pb loss since they violate the stratigraphic order established by
the chronology of the volcanic ash beds.
At Dongpan, six fine- to medium-grained volcanic ash beds (DGP-10, DGP-11,
DGP-12, DGP-13, DGP-16 and DGP-17) in the uppermost 10 m of the Dalong Formation,
one fine-grained ash bed (DGP-21) just 10 cm above the base of the Ziyun
Formation, and one thin-bedded volcanogenic sandstone (DGP-18) 40 cm
stratigraphically higher were collected for geochronology. At Penglaitan,
the basal part of a 25 cm thick volcanogenic sandstone (PEN-6), one
thin-layered volcanic ash bed (PEN-70) and the base of a 30 cm thick
volcanogenic sandstone (PEN-28), all together representing the uppermost
1.1 m of the Dalong Formation, were dated. A single fine-grained and extremely
thin (2–3 mm) volcanic ash bed (PEN-22) was sampled 50 cm above the base of
the Ziyun Formation and thus closely brackets the formational boundary. U-Pb
CA-ID-TIMS geochronology following procedures described above and in the
Appendix was applied to a number of single crystals of zircon extracted from
these volcanic beds. Trace element and Hf isotopic compositions of these
dated zircons will be presented elsewhere. Stratigraphic positions of
volcanic ash beds at Dongpan and Penglaitan and weighted mean
206Pb / 238U dates of individual zircon grains for the samples below
are given in Figs. 2 and 3.
Results
The U-Pb isotopic results are presented in Fig. 4 as 206Pb / 238U
age ranked plots for each individual sample and in Table S1 (Sect. S2).
U-Pb age determinations from the Dongpan section
Sample DGP-10
This volcanic ash bed was sampled 9.7 m below the formational boundary. All
10 dated zircons are concordant within analytical error, where the seven
youngest grains define a cluster with a weighted mean 206Pb / 238U
age of 252.170 ± 0.055/0.085/0.28 Ma (mean square of weighted deviates
(MSWD) = 1.18) for the deposition of DGP-10.
Sample DGP-11
This volcanic ash bed was sampled 7.9 m below the formational boundary.
Eleven zircon crystals were analyzed, resulting in scattered
206Pb / 238U dates of 251.662 ± 0.263 to 252.915 ± 0.352 Ma. The six youngest zircons yield a weighted mean
206Pb / 238U age of 251.924 ± 0.095/0.12/0.29 Ma (MSWD = 1.80) that is too young with respect to the stratigraphic sequence defined
by over- and underlying ash beds. Therefore, we have to assume that abundant
unresolved lead loss affected these zircons, despite application of the same
chemical abrasion procedure as for all other samples. It is worth noting
that all zircons from DGP-11 were almost completely dissolved after chemical
abrasion and show elevated 206Pb / 238U age uncertainties of
∼ 0.30 Ma compared to other volcanic ash beds from Dongpan.
Sample DGP-12
This volcanic ash bed was sampled 7.3 m below the formational boundary. The
weighted mean age of 252.121 ± 0.035/0.074/0.28 Ma (MSWD = 1.04) is
derived from eight concordant grains representing the youngest zircon
population of this ash bed.
Sample DGP-13
This volcanic ash bed was sampled 6.4 m below the formational boundary.
Analyses of seven individual zircons yield a statistically significant
cluster with a weighted mean 206Pb / 238U age of 251.101 ± 0.037/0.075/0.28 Ma (MSWD = 0.67) representing the youngest zircon
population of this ash bed.
Sample DGP-16
This volcanic ash bed was sampled 3.2 m below the formational boundary. Nine
zircons yield a weighted mean 206Pb / 238U age of 251.978 ± 0.039/0.076/0.28 Ma (MSWD = 0.66). The youngest grain shows unresolved
lead loss and was discarded because it violates the stratigraphic
superposition. Incorporating this zircon into the mean age calculation would
also lead to a statistically flawed MSWD of 4.80.
Sample DGP-17
This volcanic ash bed was sampled 2.7 m below the formational boundary. A
total of 11 zircons define a weighted mean 206Pb / 238U age of
251.956 ± 0.033/0.073/0.28 Ma (MSWD = 0.96). One single zircon
displays inheritance with an 206Pb / 238U age of 252.896 ± 0.108 Ma and was consequently excluded from the weighted mean age
calculation.
Sample DGP-21
This volcanic ash bed was sampled 0.1 m above the formational boundary. Fourteen zircons were dated, among which the eight youngest yield a cluster with a
weighted mean 206Pb / 238U age of 251.953 ± 0.038/0.075/0.28 Ma (MSWD = 0.26). The six oldest grains display an inherited component as
suggested by their scattered 206Pb / 238U dates ranging from 252.145 ± 0.120 to 252.715 ± 0.084 Ma. The U-Pb data of DGP-21
have already been published in a companion study (Baresel et al., 2016) that deals
with the stratigraphic correlation of ash beds straddling the PTB in deep-
and shallow-marine successions of the Nanpanjiang Basin.
Sample DGP-18
This bed was sampled 0.5 m above the formational boundary. The re-sedimented
nature of this volcaniclastic bed is reflected in the 206Pb / 238U
zircon ages ranging from 252.559 ± 0.261 to 257.274 ± 0.689 Ma. This sample was excluded from the age–depth model because it clearly
violates the stratigraphic superposition.
U-Pb age determinations from the Penglaitan section
Sample PEN-6
PEN-6 comes from the base of a volcanogenic sandstone. It was sampled 1.1 m
below the formational boundary. Fifteen zircon grains were dated. The three
youngest grains define a weighted mean 206Pb / 238U age of 251.137 ± 0.082/0.11/0.29 Ma (MSWD = 0.13). Because zircon dates from this
bed spread over almost 2 Myr, recycling of older volcanic material via
sedimentary processes appears more likely than via magmatic recycling.
Sample PEN-70
This volcanic ash bed was sampled 0.6 m below the formational boundary.
Eighteen zircon grains were analyzed. As in the case of PEN-6, they yield a
scatter of 206Pb / 238U dates spanning 1.5 Myr, ranging from 251.994 ± 0.169 to 253.371 ± 0.165 Ma. The weighted mean age of
252.125 ± 0.069/0.095/0.29 Ma (MSWD = 0.59) for the deposition of
this ash bed is calculated by using the seven youngest concordant grains.
Sample PEN-28
This sample was taken 0.3 m below the formational boundary. It is derived
from the base of a 30 cm thick volcanogenic sandstone which represents the
youngest Permian bed in Penglaitan. Analyses of seven zircon grains yield a
cluster with a weighted mean 206Pb / 238U age of 252.062 ± 0.043/0.078/0.28 Ma (MSWD = 0.49), reflecting the last crystallization
phase of this zircon population. Six older grains, ranging from 252.364 ± 0.156 to 253.090 ± 0.375 Ma, indicate either magmatic or
sedimentary recycling. The U-Pb data of PEN-28 have already been published in
Baresel et al. (2016).
Sample PEN-22
This 2 mm thick volcanic ash bed was sampled 0.5 m above the formational
boundary. Eight zircons yield a weighted mean 206Pb / 238U age of
251.907 ± 0.033/0.073/0.28 Ma (MSWD = 0.10). One zircon grain shows
a significantly younger age suggesting lead loss. Two slightly older grains
reflect noticeable pre-eruptive crystallization. Incorporation of these
grains into the weighted mean calculation would lead to an excessive MSWD of
3.6 and 1.9, respectively.
Comparison of the different age–depth models based on linear
interpolation, cubic spline fit and Bayesian statistics for
(a) Dongpan and (b) Penglaitan. Each age–depth model is
presented with its median (middle grey line) and its associated 95 %
confidence interval (grey area). Radioisotopic dates, used in the age–depth
models, together with their uncertainty (red horizontal bars) are presented
as 206Pb / 238U weighted mean dates of the Dongpan and Penglaitan
volcanic ash beds and volcanogenic sandstones in their stratigraphic
positions. U-Pb data of DGP-21, PEN-28 and PEN-22 are taken from Baresel et
al. (2016). Predicted dates (blue horizontal bars) for the onset of the
radiolarian decline (RD) and the Permian–Triassic boundary (PTB) in Dongpan
and Penglaitan are calculated with their associated uncertainty using the
different age–depth models.
However, we noticed that some volcanic ash beds and volcanogenic sandstones
in these sections show a large age dispersion of up to 2 Myr, incompatible
with recycling of zircon that previously crystallized within the same
magmatic system and became recycled into later melt batches, leading to dispersion of dates of a few
100 kyr (e.g., Broderick et al., 2015; Samperton et al.,
2015), but pointing to sedimentary reworking. The U-Pb data of PEN-22 have
already been published in Baresel et al. (2016).
Age–depth models
Figure 5 shows a comparison of three different age–depth models based on
linear interpolation, cubic spline interpolation and Bayesian statistics,
each applied to exactly the same U-Pb dataset of Dongpan (Fig. 5a) and
Penglaitan (Fig. 5b). As discussed in the Methods section, the Bayesian
Bchron model leads to more realistic uncertainty estimates, producing an
increased uncertainty of the model age with increasing distance from the
stratigraphic position of a U-Pb dated sample. Due to the well constrained
U-Pb dates of Dongpan and Penglaitan, all three age–depth models predict
(within uncertainty) similar ages for the PTB in Dongpan (Fig. 5a) and
Penglaitan (Fig. 5b). Given that the Bayesian Bchron model evaluates the age
probability distribution of each U-Pb date with respect to the other dates
of the sequence, it provides a more robust and better constrained
chronology, which even results in smaller uncertainties of the predicted
model dates compared to the standard linear regression models (as indicated
by the smaller uncertainty of the Bchron model age for the PTB in Dongpan
and Penglaitan). In contrast to the other models, the Bayesian Bchron model
can identify U-Pb dates that violate the principle of stratigraphic
superposition, as shown for the Dongpan ash beds DGP-11 (outlier probability
of 67 %) and DGP-18 (outlier probability of 100 %). Including them
into the age–depth chronology of Dongpan results in unrealistic negative
sediment accumulation rates, as reflected by the linear and cubic
interpolation models for the interval between DGP-11 and DGP-12, and for the
interval between DGP-21 and DGP-18 (Fig. 5a).
In all Bchron models, the thickness of the geologically instantaneously
deposited volcanic ash beds was virtually removed and the lithostratigraphy
has been rescaled in order to create accurate deposition rate models for the
investigated sedimentary successions. This approach has only minor effects
in the Bchron age–depth model of Dongpan, where the changes in the calculated
age of the PTB and the radiolarian decline (RD) are negligible (see Sect. S3), mainly due to the limited thickness (max. 8 cm) of the volcanic horizons
in Dongpan. The Bchron model of Penglaitan would be much more affected
by such a rescaling if the thickness of the volcanogenic sandstones
were also removed, but it is not clear whether each volcanogenic sandstone
represents only one “instantaneous” turbidity current event or might
reflect a series of several turbidite deposits over a certain time. Hence,
also in Penglaitan only the thickness of the volcanic ash beds was removed.
However, the relative substantial thickness of “instantaneously” deposited
turbiditic volcanogenic sandstone at the top of the Penglaitan section may
indeed induce some distortions in the Bchron model. Facies analysis did not
reveal any signs of an omission surface at the formational boundary, but the
strong contrast in sediment accumulation rates between the “instantaneous”
deposition of the last Permian bed and the much slower accumulation of next
overlying black shales likely generates a distortion of the Bchron model at
the formational boundary. Hence, the Bchron model derived from Dongpan is
certainly more reliable than that derived from Penglaitan.
The aim for applying Bayesian age modeling to the dated volcanogenic beds
from these two sections was to obtain an age model for the PTB. The
age–depth models yield ages of 251.939 ± 0.030 Ma (Dongpan; Figs. 2a
and 5a) and of 251.984 ± 0.031 Ma (Penglaitan; Figs. 3b and 5b) for
the lithological boundary between the Dalong and Ziyun Formation in both sections.
These two ages overlap within uncertainties and thus demonstrate the
synchronicity of the PTB in the two sections. Making the reasonable
assumption of absence of significant gaps in these two sections, the new
U-Pb dates can be used to infer sediment accumulation rates. The age–depth
model of Dongpan suggests increased sediment accumulation rates in the
uppermost part of the Dalong Formation from bed 6 (DGP-17) upwards. Below bed 6,
calculated sediment accumulation rates appear to be relatively constant with
3.6 ± 1.2 cm kyr-1, but above bed 6 they jump to 6.0 ± 2.4 cm kyr-1. In Penglaitan, the sediment accumulation rate of the
uppermost Dalong Formation and basal-most Ziyun Formation is significantly lower than
in Dongpan with 0.7 ± 0.3 cm kyr-1. Previously published U-Pb
zircon geochronology from Penglaitan (Shen et al., 2011), including a
weighted mean date of 252.16 ± 0.09 Ma from a volcanogenic sandstone
at 26.7 m below the PTB, was not considered in our age model, since
substantial improvements in the analytical protocol hamper comparing these
dates with our U-Pb results.
Comparison of Bayesian Bchron age models for Dongpan, Penglaitan and
the Meishan GSSP. Predicted dates together with their uncertainty
for the lithological boundaries and the first occurrence of the conodont
Hindeodus parvus at Dongpan and Penglaitan are calculated using U-Pb
ages of this study and of Baresel et al. (2016), and the U-Pb ages of Burgess
et al. (2014) for Meishan. The durations of the conodont unitary association zones (UAZs) UAZ1 and UAZ2
(Brosse et al., 2016) are inferred from the Bchron age model of Meishan and
projected to the model of Dongpan and Penglaitan, respectively.
Comparison of the Dongpan and Penglaitan sections with Meishan GSSP
results
The change of the PTB age through analytical improvement of U-Pb
dating
The first geochronological studies in the GSSP at Meishan D have been carried
out on a volcanic ash in bed 25, whose base starts 4 cm below the
formational PTB, by U-Pb sensitive high-resolution ion microprobe (SHRIMP)
analysis of zircons yielding a 206Pb / 238U age of 251.2 ± 3.4 Ma (Claoué-Long et al., 1991) and by 40Ar/39Ar dating of
sanidine at 249.91 ± 0.15 Ma (Renne et al., 1995). However, these
dates are either not sufficiently precise to allow calibrating magmatic and
biological timescales at resolution adequate for both groups of processes
or are biased by a systematic age offset between the U-Pb and Ar-Ar systems
of ∼ 1.0 % (Schoene et al., 2006). In order to properly
compare the two systems, all older 40Ar/39Ar data have to be
corrected for the revised age of the standard Fish Canyon sanidine of 28.201 ± 0.046 Ma (Kuiper et al., 2008) and the decay constant uncertainty
has to be added to U-Pb and Ar-Ar ages, which would drastically expand the
40Ar/39Ar age error and recalculate the 40Ar/39Ar age of
Renne et al. (1995) to 251.6 ± 0.6 Ma. In a first detailed ID-TIMS
study, U-Pb ages of mechanically abraded zircons were published by Bowring
et al. (1998) for six volcanic ash beds at Meishan, placing the PTB at 251.4 ± 0.3 Ma. Though much more precise than former studies, these ages are
mainly based on multi-grain zircon analyses. It was shown by Mundil et al. (2001), by confining data selection to single-crystal analyses of the same
horizons, that the multi-grain approach might disguise complexity of zircon
population ages which are caused by pervasive lead loss and inheritance. In
a second attempt, driven by further improvements in the U-Pb ID-TIMS
technique (e.g., chemical abrasion of zircon grains by hydrofluoric acid
exposure to remove zircon domains with lead loss; reduced procedural common
Pb blanks), the PTB extinction horizon in Meishan and Shangsi (China) was
dated at 252.6 ± 0.2 Ma by Mundil et al. (2004). Unlike previous
studies, Shen et al. (2011) dated larger number of zircon grains per ash bed
in order to overcome inheritance, magmatic residence, and lead loss
phenomena of zircon population ages. They determined the duration of the PTB
extinction interval (bed 25 to bed 28 in Meishan) at 200 ± 100 kyr
starting at 252.28 ± 0.08 Ma in bed 25 together with a sharp negative
δ13C excursion. By using the same mineral separates from
identical ash beds as in Shen et al. (2011), the extinction period at
Meishan was determined by Burgess et al. (2014) between 251.941 ± 0.037 Ma (bed 25) and 251.880 ± 0.031 Ma (bed 28). The differences in
age and precision compared to Shen et al. (2011) reflect significant
progress of the EARTHTIME community in data acquisition and reduction such
as refined tracer calibration, new error propagation algorithms, and the
development of the EARTHTIME 202Pb-205Pb-233U-235U
tracer solution.
Figure 6 illustrates the three Bayesian age–depth models based on our U-Pb
dates from Dongpan and Penglaitan compared to the latest generation of U-Pb
ages from Meishan GSSP (Burgess et al., 2014). Such a comparison is possible
because all the dates from these three sections were obtained with the same
analytical procedures, including identical data reduction procedures, error
propagation and Th correction, thus leading to closely comparable precision
and accuracy of the ages. This tight temporal framework allows us to perform
a quantitative comparison of the Dongpan, Penglaitan and Meishan sections in
terms of lithostratigraphy, biostratigraphy and chemostratigraphy via the
Bayesian statistics.
Comparison of lithostratigraphy
All three interpolated ages of the formational boundary in Dongpan (251.939 ± 0.030 Ma), Penglaitan (251.984 ± 0.031 Ma) and Meishan
(251.956 ± 0.035 Ma) are in agreement within errors (Fig. 6). They
support the synchronicity of the conformable boundary between the Dalong Formation
and the Ziyun Formation in Dongpan and Penglaitan, and also demonstrate their
temporal coincidence with the conformable boundary in Meishan between the
Changhsing Formation and Yinkeng Formation. The age model also confirms the extreme
condensation around the PTB in Meishan, with a maximal sediment accumulation
rate of 0.4 cm kyr-1 as reported by Burgess et al. (2014) for the 26 cm
thick interval between the base of bed 25 and the base of bed 28. In this
respect, Dongpan and Penglaitan offer a greater potential for higher-resolution studies of environmental proxies around the PTB with maximal
sediment accumulation rates for the same interval of 8.4 and
1.0 cm kyr-1, respectively. The increased sediment accumulation rate
above bed 6 in Dongpan is in agreement with the previously inferred
sedimentary fluxes deduced from elemental chemical analyses (Shen et al.,
2012). From bed 7 upward, He et al. (2007) showed a clear increase in
Al2O3 and TiO2, indicating increased fluxes of terrestrial
input into this trough. The accompanying size reduction of brachiopods (He
et al., 2007) led them to infer a regressive trend in the upper part of the
Dongpan section. The ecological consequences of any regressive trend or
increased clastic input might conceivably impact the diversity of marine
species, but distinguishing between increased fluxes and regression remains
difficult because both causes may have converging consequences.
Comparison of biostratigraphy
The PTB is defined by the FO of H. parvus in the Meishan GSSP in
bed 27c. This definition is complicated by the suggested existence of a
hardground within bed 27, which is at the position of the previously defined
PTB (Chen et al., 2009). Others have suggested that the FO of H. parvus at Meishan is not the timing of the true evolutionary origination of
this species (Jiang et al., 2011; Yuan et al., 2015). In Meishan, the FO of
H. parvus in bed 27c is interpolated at 251.892 ± 0.045 Ma
(Fig. 6) and is located 21 cm above the formational boundary which occurs
between beds 24 and 25. Temporally coincident, the FO of H. parvus
in Penglaitan is interpolated at 251.929 ± 0.032 Ma (Fig. 6) and is
located 33 cm above the formational boundary. With respect to formational
boundaries, the higher stratigraphic position of the FO of H. parvus in Penglaitan than in Meishan is to be expected because of the
higher sediment accumulation rate. However, in Meishan H. parvus
first occurs 64 ± 56 kyr and in Penglaitan 53 ± 46 kyr after the
formational boundary, indicating perfect synchronicity within our temporal
resolution. In Dongpan, the lack of conodont bearing beds around the PTB
hampers testing the synchronicity of the FO of H. parvus between
Dongpan and the two other sections.
Brosse et al. (2016) established a new and robust conodont zonation based on
unitary associations around the PTB in South China that includes the Meishan
GSSP. This zonation contains six unitary association zones (UAZs), with the
PTB falling into the separation interval between UAZ2 (bed 25) and UAZ3
(bed 27a–d). This new zonation also places the UAZ-based PTB in Meishan
closer to the conformable boundary between the Changhsing Formation and the Yinkeng
Formation than the FO of H. parvus (bed 27c) does. Available conodont data from
Meishan allow the assignment of bed 24a–e to UAZ1 (UAZ1 might reach further
down as indicated by a dashed line in Fig. 6), bed 25 to UAZ2, bed 27a–d to
UAZ3 and bed 28 to UAZ4 (Brosse et al., 2016). The stratigraphic thickness
comprised between the base of UAZ1 and the top of UAZ4 amounts to 1.22 m.
By using the three-section age–depth models, we attempted to project the
respective thickness corresponding to the UAZ1–UAZ4 interval in Meishan onto
the two other sections. This projection resulted into a pronounced,
artificial lengthening of UAZs in Dongpan and Penglaitan. UAZ1 is the
penultimate Permian conodont UAZ in Meishan (Brosse et al., 2016). When
projected onto the age–depth models of Dongpan and Penglaitan, this UAZ1 is
artificially expanded and even crosses the PTB in Penglaitan (Fig. 6). In
Penglaitan, the last Permian UAZ2 projects correctly above UAZ1 without
overlap but is completely within the Triassic. The cause of these
contradictions stems from the irreconcilable conjunction of (i) extreme
condensation in Meishan, (ii) high evolutionary rates of conodonts, and
(iii) the ∼ 40 ka precision of the last generation of U-Pb dates.
Comparison of the organic carbon isotope chemostratigraphy of
Dongpan (Zhang et al., 2006) with that of Meishan (Cao et al., 2002). Dates
and their associated uncertainty for the negative carbon isotope excursions
in both sections are revealed from the Bayesian Bchron age models of Dongpan
and Meishan, respectively. Abundance of spores and pollen, as well as algae
and acritarchs (i.e., spores per 100 cm2 of soil) in Dongpan, is from Shen et al. (2012).
Stratigraphic positions of the first radiolarian crisis (FRC) and second
radiolarian crisis (SRC) (after Feng et al., 2007), the latest Permian
extinction event (LPME) in Dongpan and the Permian–Triassic boundary (PTB) in
both sections are indicated as well. Meishan section thickness is not to
scale with the Dongpan section.
In Dongpan, the onset of a protracted radiolarian diversity decline in bed 5
reported by Feng and Algeo (2014) is here interpolated at 251.990 ± 0.029 Ma, occurring
51 ± 42 kyr before the formational boundary (Fig. 2). Excess SiO2 values of this bed (Shen et al., 2012) suggest a
genuine diversity pattern at the local scale, which seems to be unrelated to
any substantial change or trend in the local redox conditions (as shown by
Co / Al, Cr / Al, Cu / Al and V / Al measurements of He et al., 2007).
Comparison of chemostratigraphy
Organic carbon isotope chemostratigraphy of Dongpan (Fig. 7) extending from
the Permian bed 5 to the Triassic bed 13 was provided by Zhang et al. (2006)
and for Meishan (Fig. 7) extending from the Permian bed 24 to the Triassic
bed 29 by Cao et al. (2002). The correlation of these δ13Corg records by Zhang et al. (2006), based on the
occurrence of ash beds in both sections, is largely over-interpreted. With
the exception of a short negative excursion followed by a more prominent
positive excursion between beds 9 and 11, the Permian part of the δ13Corg record in Dongpan is relatively stable and oscillates
between -28 and -27 ‰. With the exception of a negative
excursion culminating in beds 25 and 26, the Permian part of the
δ13Corg record in Meishan shows a sustained positive
trend from -30 to -26 ‰. The basal Triassic part of these two
records is also incompatible in that they display opposite trends. With the
possible exception of the Xinmin section (J. Shen et al., 2013), the
δ13Corg record of Dongpan does not correlate with that
of any other South Chinese section, but even Xinmin shows a
∼ 3 ‰ offset of the base trend in comparison to Dongpan.
However, we note that in Meishan an abrupt decline in
δ13Ccarb occurs in bed 24e at 251.950 ± 0.042 Ma
(Burgess et al., 2014) and slightly above in bed 26 in
δ13Corg at 251.939 ± 0.032 Ma, which is
temporally coincident with the main negative δ13Corg
excursion in bed 9 in Dongpan at 251.956 ± 0.030 Ma. The second
smaller negative δ13Corg excursion at the PTB in
Dongpan at 251.941 ± 0.030 Ma and in Meishan at
251.892 ± 0.045 Ma cannot be distinguished within uncertainty from the
main excursion, which hampers the correlation of the
δ13Corg records based on U-Pb ages. However,
interpreting organic carbon records requires the simultaneous analysis of
palynofacies, which are not documented in Dongpan. Shen et al. (2012) also
showed that the total organic carbon (TOC) never exceeds 0.2 %, thus
indicating a generally poor preservation of the organic matter in this
section. As shown by Shen et al. (2012), this preservation bias is further
supported by coincident peaks in both terrestrial (spore and pollens) and
marine (algae and acritarchs) organic material (see Fig. 7). This uneven
preservation of the organic matter further hampers the understanding of the
δ13Corg signal in Dongpan. More generally, the
consistency and lateral reproducibility of the late Permian carbonate and
organic carbon isotope records in South China remain equivocal (e.g.,
S. Z. Shen et al., 2013). These records are probably influenced by the local
graben and horst paleotopography that hampered efficient circulation of water
masses with the open ocean, thus reflecting more local than global changes.
Comparison of astrochronology
Sedimentary cycles driven by orbital forcing (100 kyr eccentricity cycles)
were inferred by Peng et al. (2008) on the basis of Ce/La fluctuations in
Dongpan. These cycles were also used by Feng and Algeo (2014) to calibrate
their radiolarian extinction and survival intervals. The duration of these
two intervals amounts to ∼ 260 kyr (see their Fig. 5). For the
same stratigraphic interval, our U-Pb ages (interval from DGP-16 to DGP-21)
indicate a much shorter duration of max. 75 kyr. It is not clear whether this
chemical cyclicity might represent either precession instead of eccentricity
cycles or rather a local signal of the sedimentary-chemical system. Huang et
al. (2011) produced an astrochronological timescale across the PTB in China
and Austria with an estimated duration of 700 kyr for the extinction
interval. However, the extinction interval in their study is too long and
ranges from the start of the Neogondolella meishanensis conodont zone to the
base of the Isarcicella isarcica zone, defining a prolonged extinction
interval stretching from the top of bed 24e to the base of bed 29 in
Meishan. Wu et al. (2013) reported Milankovitch cycles from late Permian
strata at Meishan and Shangsi, South China, indicating a 7.793 Myr duration
for the Lopingian epoch based on 405 kyr orbital eccentricity cycles. Their
inferred duration of 83 kyr for the extinction interval in Meishan between
the base of bed 25 and the top of bed 28 is in good agreement with the
radioisotopically dated duration of 61 ± 48 kyr for the same interval
(Burgess et al., 2014). This is consistent with the study of Li et al. (2016), whose astronomical-cycle tuning of spectral gamma-ray logs constrains
the extinction interval in Meishan to less than 40 % of a 100 kyr
eccentricity cycle (i.e., < 40 kyr).