Australia: The Land Where Time Began
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.
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.
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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