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Australia: The Land Where Time Began |
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Prominent Isotope Excursions on a Global Scale Just Prior to the
Cambrian Explosion
Precambrian/Cambrian
(Pc/C) boundary sections from around the world have been correlated by
the use of carbon isotope chemostratigraphy, the results of which have
elucidated significant changes in the carbon cycle that occurred during
the rapid diversification of skeletal Metazoa - the
Cambrian Explosion.
Mainly as the result of the lack of a continuous stratigraphic record,
the standard δ13C curve of the Early Cambrian has been poorly
established. In this paper the authors1 report high
resolution δ13C chemostratigraphy of a sample from a drill
core that crosses the Pc/C boundary in the area of the Three Gorges,
South China. The section extends from the Dengying Formation, the
uppermost dolostone of the
Ediacaran
through muddy limestones of the lowermost Early Cambrian Yanjiahe
Formation to a calcareous black shale of Middle Early Cambrian age, the
Shuijingtuo Formation. Ishikawa et
al.1 identified 2
positive and 2 negative isotope excursions within this interval. The δ13Ccarb
increases moderately from 0 to +2‰ (positive excursion: P1) near the
Pc/C boundary then drops dramatically down to -7/‰ (negative excursion1:
N1). The δ13Ccarb subsequently increases
continuously up to about +5‰ at the upper part of the Nemakit-Daldynian
stage. The δ13Ccarb then decreases sharply, down
to about -9‰ (N2), after the positive excursion, just below the basal
Tommotian Unconformity. A primary origin of the record is suggested as
these continuous patterns of the δ13C shifts are irrespective
of lithotype. The δ13C profile that was obtained is also
comparable to records of other sections within and outside the Yangtze
Platform, except for the sharp excursion N2. Ishikawa et
al. conclude that the general
feature of their δ13C profile best represents the seawater
chemical change that occurred on a global scale. The minimum δ13C
of the N1 (-7‰) is below the input of carbon from the mantle, implying
an enhanced flux of carbon that is 13C-depleted just across
the Pc/C boundary. At that time the ocean therefore probably became
anoxic, and this may have affected sessile or benthic
Ediacaran biota.
The subsequent δ13C rise up to +5‰ indicates an increase of
primary productivity, or an enhanced carbon burial rate, which should
have lowered the pCO2 and resulted in the following
global cooling. The cause of sea level fall at the base of the Tommotian
Stage, which occurred on a global scale, is accounted for by this
scenario. Subsequently, the very short term, and exceptionally low δ13C
(-9‰) in N2 could possibly have been associated with methane release
from gas hydrates as a result of falling sea level. It appears the
inferred environmental changes, such as ocean anoxia, increasing
productivity, global cooling and subsequent sea level fall with the
release of methane, coincided with, or occur just prior to the Cambrian
Explosion. Ishikawa et al.
suggest that this may indicate synchronism between the environmental
changes and the rapid diversification of skeletal Metazoa.
Across the Precambrian-Cambrian boundary soft-bodied Ediacaran biota
declined. Animals evolved at a phylum level in the Early Cambrian, and
by that time all the animal phyla that are extant at the present had
appeared in the fossil record (e.g. Knoll & Carroll, 1999; Conway
Morris, 2000; Budd, 2003; Shu, in press). This interval is believed,
therefor, to be one of the most significant periods in the evolution of
animals. According to Ishikawa et
al. it is essential to correlate the terminal Ediacaran with those
of the Early Cambrian, which are distributed to different parts of the
Earth, to understand the temporal link between the evolution of early
animals and global environmental changes.
The first appearance of the trace fossil
Treptichnus pedum at
Fortune Head, southeast Newfoundland defines the boundary between the
Ediacaran and the Cambrian (Narbonne et
al., 1987; Landing, 1994;
Brasier et al., 1994b). This
index fossil does not always occur globally, however. A substitute that
has been used are small shelly fossils (SFFs) for biostratigraphic
correlation, especially in Siberia and South China. The
Nemakit-Daldynian and Tommotian stages of the Early Cambrian were
defined by the presence of some SSFs (Khomentovsky & Karlova, 1993;
Knoll et al., 1995; Qian,
1999; Jensen, 2003). At the base of the Tommotian the diversity of the
SSFs increases suddenly (Khomentovsky & Karlova, 1993), which implies
that environmental change occurred across the
Nemakit-Daldynian/Tommotian boundary (Budd, 2003). In the subsequent
Atdabanian stage there appears to have been a major increase in the
diversity of metazoans, determined by the first appearance of trilobites
(Knoll & Carroll, 1999). In the case of the Early Cambrian these
evolutionary innovations are useful benchmarks, though it is sometimes
difficult to achieve stratigraphic correlation because of the scarcity
of fossil records.
Carbonate carbon isotope composition (δ13Ccarb),
on the other hand, has developed as an alternative tool for the
correlation of the Precambrian-Cambrian boundary sections (e.g. Tucker,
1986, 1991; Brasier et al.,
1990; Kirschvink et al.,
1991; Narbonne et al, 1994;
Kaufman et al., 1997). To
date variations in δ13Ccarb across the
Precambrian-Cambrian boundary have been reported from many sections
around the Earth e.g.:
·
Morocco: Margaritz et al.,
1991; Kirschvink et al.,
1991;
·
Canada: Narbonne et al.,
1994;
·
Siberia: Margaritz et al.,
1996; Brasier et al., 1994a;
Brasier & Sukhov, 1998;
Kaufman et
al., 1995; Pelechaty et
al., 1996;
·
Mongolia: Brasier et al.,
1996;
·
Iran: Kimura et al., 1997;
·
Oman: Amthor et al., 2003;
·
China: Hsu et al., 1985;
Lambert et al., 1987; Shen &
Schidlowski, 2000; Shen, 2002.
It was revealed by these results that δ13Ccarb
shows a negative anomaly across the Precambrian-Cambrian boundary, which
accordingly can be a useful indicator for global correlation.
A significant change in the carbon cycle is indicated at the time by the
negative carbon isotope excursion. Ishikawa et
al. suggest that the
anomalous 13C depletion in carbonate from the
Precambrian-Cambrian boundary could possibly have been derived from:
1)
A decline in primary productivity coupled with mass extinction (e.g. Hsu
et al., 1985; Lambert et
al., 1987; Shen & Schidlowski,
2000; Amthor et al., 2003),
2)
Enhanced export of particulate organic matter to the deep sea by the
appearance of a plankton-producing faecal pellet (Logan et
al., 1995; Rothman et
al., 2003),
3)
An influx of carbon that was isotopically light as a result of a
stratified ocean (Kimura et al.,
1997, 2005), and
4)
Short term input of methane that was released by destabilised gas
hydrates (Bartley et al.,
1998).
According to Ishikawa et al.
it is essential that more detailed δ13C from the
Ediacaran-Cambrian be obtained, in order to evaluate these possibilities
and their impact on the evolution of animals. Nonetheless, there have
been only rare reports of marine carbonate δ13C, as there are
often intervals of depositional hiatus and siliciclastic sedimentation
free of carbonate in the early sections of the Cambrian. A standard
13C curve for the early Cambrian has, therefore, been poorly
established.
Sediments dominated by carbonate in South China from the Early Cambrian
were deposited on the continental shelf of the Yangtze Platform. The
tectonic setting, i.e. subsidence by rifting (Li, 1998; Li et
al., 2003a; Wang & Li, 2003)
may be responsible for the relatively good preservation of these
sediments. The presence of the negative excursion near the
Sinian-Cambrian boundary that was demonstrated by the previous isotope
analysis (Hsu et al., 1985;
Lambert et al., 1987; Chu et
al., 2003), though the
details of the Early Cambrian system have not been focused on in most
studies. The precise position of the isotope excursion has, therefore,
not been identified adequately (Zhu et
al., 2001).
Ishikawa et al. therefore
conducted an on-land drilling in Aijiahe region, Three Gorges Area,
South China, where carbonated shelf sediments that are very well
preserved crop out. They obtained pristine drill core samples that
extended from dolostone of Ediacaran age through limestone of Lower
Cambrian age, to carbonate-bearing shale of middle Early Cambrian age.
Geological setting
Strata of Neoproterozoic age are distributed widely in China. Coincident
with the breakup of the Rodinia supercontinent these rocks were
deposited in a basin related to rifting of the Yangtze and Cathaysia
Blocks (Li, 1998). Basaltic and rhyolitic volcanic rocks, carbonates,
clastic sediments, black shales and tillites from the Cryogenian
comprise the lower half of the strata, which were deposited in the
Nanhua Rift Basin, that trends N-S and NE-SW, from 750 Ma (Li, 1998; Li
& Powell, 2001; Li et al.,
2003b). Dolomitic limestone, dolostone, black shale, limestone and
mudstone, which had been deposited on the southern flank of the Yangtze
Block, i.e. the Yangtze platform, comprise the upper half of the strata.
The strata from the
Cryogenian to the Cambrian in the Three Gorge Area were deposited on
the Western Hubei platform. They occur around the dome of a
granite-gneiss complex dating to the mid-Archaean to the late
Proterozoic. The section that was studied, Wuhe-Aijiahe, is one of the
best known sections of the Three Gorge Area. This section is comprised
of 7 formations; Liantuo, Nantuo, Doushantuo, Dengying, Yanjiahe,
Shuijingtuo, and Shipai Formations, in ascending order (e.g. Zhang,
1981; Zhao et al., 1985; Zhou
and Xu, 1987; Chen, 1987; Zhao & Xu, 1987; Zhao et
al., 1988; Ding et
al., 1996). The Liantuo
formation is about 400 m thick, and is composed of sandstone from the
Cryogenian that is about 759 Ma (Ma et
al., 1984).
a)
The Nantuo Formation is comprised of diamictite which is about 40 m
thick, and which is equivalent to the Marinoan tillites.
b)
The Doushantuo Formation is comprised of 4 members in ascending order;
I)
A dolostone that is 5 m thick, and
II)
Alternating dolostone and black shale containing abundant siliceous
nodules, and
III)
Massive dolostone ~80 m thick, and
IV)
Black shale that is 20 thick (Zhou & Xu, 1987).
c)
The Dengying is about 275 m thick and is subdivided into 3 members: the
Hamajing, Shibantan, and Baimatuo Members in ascending order, which are
characterised by light grey dolostone, black limestone, and white
dolostone, respectively.
a)
The Yanjiahe Formation consists of dolomitic muddy limestone, calcareous
black shale, and minor sandstone and chert, is about 35 m thick.
b)
The Shuijingtuo Formation consists of black shale with many carbonate
nodules, is about 100 m thick.
c)
The Shipai Formation is comprised of calcareous black shale, limestone,
and minor sandstone, and is >50 m thick.
Zircons that were separated from acidic tuff layers have U-Pb ages of
635.2 ± 0.6 or 621 ± 7 Ma in cap dolostone at the base of the Doushantuo
Formation and 551.1 ± 0.7 or 555.2 ± 6.1 Ma at the top of the Doushantuo
Formation (Condon et al.,
2005; Zhang et al., 2005).
Protohertzina anabarica
and
Anabarites trisulcatus,
according to paleontological studies in this area (Chen, 1984; Qian,
1999) are key fossils of the Nemakit-Daldynian stage, that appeared
first in a horizon 11.7 m above the Degying-Yanijiahe boundary, then at
2.7 m below the Yanjiahe-Shuijingtuo boundary, Tommotian type Small
Shelly Fossils (e.g.
Aldanella). Also, in the
upper part of the Shuijingtuo Formation (Ding et
al., 1992; Zhu et al.,
2003).
Ishikawa et al. obtained a
151 m long drill core in the Aijiahe, the Three Gorge area, which
extends from the Baimatuo Member of the Dengying Formation, through the
Yanjiahe Formation and Shuijingtuo Formation, to the bottom of the
Shipai Formation. Included in this core sample, the 30 m thick uppermost
part of the Baimatuo Member is dominated by laminated grey dolostone
with more clastic materials. The Yanjiahe Formation is 42 m thick, of
which the lower 15 m contain alternating dolomitic limestone and
laminated calcareous black shale with minor grey dolostone, whereas the
upper 27 m consist of black shale and black limestone that alternated
rhythmically in about 10-20 cm intervals. The Yanjiahe-Shuijingtuo
boundary is a discontinuity. The basal part of the Shuijingtuo
Formation, that is about 60 m thick, consists of flat pebble
conglomerate that is poorly sorted, sandy limestone, and dolostone with
a fan structure. The Shuijingtuo Formation, that is about 69 m thick,
consists of black shale with many black limestone nodules, in the lower
10 m in particular. The bottom part of the Shipai Formation that
consists of laminated muddy limestone form the upper part of the drill
core.
Conclusion
The new high resolution profile of drill core samples obtained by
Ishikawa et al. provides a
record of the δ13Ccarb profile that is almost
complete from the latest Ediacaran to the Early Cambrian. According to
Ishikawa et al. their δ13Ccarb
profile is comparable to that previously reported, though there are less
complete records of other sections within and outside of the Yangtze
Platform. They concluded, therefore, that the general feature of their δ13Ccarb
profile best represents a global change in seawater chemistry.
They identified 2 positive as well as 2 negative isotope excursions
within the interval. The δ13Ccarb increases
moderately near the Precambrian-Cambrian boundary, from 0 - +2‰
(positive excursion 1: P1), and then drops dramatically down to -7‰
(negative excursion 1: N1). An enhanced carbon flux that is 13C
depleted originated from organic matter just across the
Precambrian-Cambrian boundary is implied by the minimum 13C
of N1 (-7‰) that is slightly lower than that of juvenile carbon.
Therefore, the ocean at that time probably became anoxic, and it is this
that may have affected the survival of the sessile or benthic biota of
the Ediacaran.
It is indicated, on the other hand, by the subsequent rise up to +5‰
(P2), that there was an increase in primary productivity or an enhanced
rate of carbon burial, which should have resulted in the lowering of the
ρCO2 following the global cooling. The cause of the sea level
fall on a global scale at the base of the Tommotian stage is accounted
for by glaciation.
Also, the subsequent very sharp, exceptionally significant 13C
drop to -9‰ (N2) can be explained by the release of methane hydrates as
a result of the lowering of sea level.
The dramatic environmental changes that are inferred (e.g., ocean
anoxia, increasing productivity, global cooling and subsequent sea level
fall with the release of methane) appear to be coincident with, or occur
just prior to the Cambrian Explosion. Ishikawa et
al. suggest this may indicate
synchronism between the environmental changes and the rapid
diversification of skeletal Metazoa.
Ishikawa, Tomoko, Yuichiro Ueno, Tsuyoshi Komiya, Yusuke Sawaki, Jian
Han, Degan Shu, Yong Li, Shigenori Maruyama, and Naohiro Yoshida. "Carbon
Isotope Chemostratigraphy of a Precambrian/Cambrian Boundary Section in
the Three Gorge Area, South China: Prominent Global-Scale Isotope
Excursions Just before the Cambrian Explosion."
Gondwana Research 14, no. 1–2 (8// 2008): 193-208.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |