Organic molecular evidence in the Late Neoproterozoic Tillites for a palaeo-oceanic environment during the snowball Earth era in the Yangtze region, southern China

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Precambrian Research 162 (2008) 317–326

Organic molecular evidence in the Late Neoproterozoic Tillites for a palaeo-oceanic environment during the snowball Earth era in the Yangtze region, southern China T.-G. Wang ∗ , Meijun Li, Chunjiang Wang, Guangli Wang, Weibiao Zhang, Quan Shi, Lei Zhu Key Laboratory for Hydrocarbon Accumulation Mechanism of Education Ministry, China University of Petroleum, 18 Fuxue Road, Changping, Beijing 102249, China Received 1 April 2007; received in revised form 28 September 2007; accepted 28 September 2007

Abstract The Upper Neoproterozoic Nantuo and Jiangkou Formations in southern China have commonly been identified as the deposits of glaciations dating back to ∼600 Ma and ∼720 Ma, belonging to the Marinoan and Sturtian ice-ages, respectively, or the so-called snowball Earth events. In the present study, 44 typical Upper Neoproterozoic to Lower Cambrian sedimentary rock samples, including 16 diamictites, have been collected from 12 stratigraphic sections in Hubei, Hunan, Shanxi, Chongqing, Guizhou and Yunnan Provinces in the Yangtze region, southern China, and analyzed by routine organic geochemical techniques. Except for the diamictites and cap carbonates, most of these rocks have middle to high TOC values of up to 0.16–6.50% in the Cambrian and 0.12–9.37% in the Upper Neoproterozoic rock samples. By contrast, the diamictites and cap carbonates have a TOC range only from 0.01% to 0.13%, 1–2 orders of magnitude lower than those in most of the non-glacial sedimentary rocks, indicating very limited organic productivity in the palaeo-oceanic environment during the snowball Earth era. Phytane and pristane, which are derivatives of Chlorophyll-a, and various steranes, which are biomarkers of eukaryotes, were detected in all the rock samples. There is a tendency for positive correlation between TOC and the concentrations of phytane and/or pristane, therefore, it implies that photosynthetic autotrophs should have been the primary organisms contributing to the sedimentary organic matter in the Yangtze palaeo-ocean. Moreover, the concentrations of phytane plus pristane range from 0.024 ng/g to 0.446 ng/g in most non-glacial sedimentary rocks, and from 0.005 ng/g to 0.076 ng/g in diamictites. The unusually low concentrations of phytane and pristane reveal a very weak photosynthetic process, a restricted euphotic zone and quite limited sunlight within the palaeo-oceanic water column. Even though the productivity of photosynthetic organisms was very low, it can still be inferred that photosynthetic process never ceased in the Late Neoproterozoic palaeo-oceanic environment. This evidence suggests that the palaeo-ocean in the Yangtze region during the snowball Earth was either covered by a thin sea-ice sheet, or was covered by a thicker sea-ice sheet that included polynyas or areas of open water. Both these possibilities would have allowed the survival of photosynthetic eukaroytes and other organisms. © 2007 Elsevier B.V. All rights reserved. Keywords: Tillite; Late Neoproterozoic; Snowball Earth; Chlorophyll-a; Eukaryotes; Photosynthesis

1. Introduction The Late Neoproterozoic global glaciation has been considered as one of the important constraints on the Cambrian biological explosion (Hoffman et al., 1998a; Hoffman and Schrag, 2002 and references therein), and the global glaciation events are thought to have occurred at least twice in the Late Neoproterozoic, termed the Sturtian and Marinoan iceages (Hoffman et al., 1998a,b; Kennedy et al., 1998; Hyde et

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al., 2000). Based on geological, palaeomagnetic and geochemical studies, it is suggested that the glacial ice limits reached sea level close to the ancient equator, and the world’s oceans were entirely ice covered during the Neoproterozoic snowball Earth era (e.g. Kirschvink, 1992; Schmidt and Williams, 1995; Evans et al., 2000; Walker, 2001; Hoffman and Schrag, 2002; Porter et al., 2004; Fairchild et al., 2006; Zhan et al., 2007). The snowball Earth hypothesis has explained most of the puzzling observations in the Neoproterozoic sedimentary record, including low-latitude glaciation, banded iron-formations, cap carbonates, and carbon isotopic excursions (Hoffman et al., 1998a,b). Climate model simulations indicate that such a snowball state for the earth depends on anomalously low atmospheric carbon diox-

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ide concentrations, in addition to the sun being 6% fainter than it is today, although the mechanisms producing such low carbon dioxide concentrations remain controversial (Donnadieu et al., 2004). However, this hypothesis has also been questioned, with particular uncertainties as to whether the tropical ocean could have ever become entirely sea-ice covered, and whether life (especially photosynthetic organisms) could have survived in such a cold palaeo-oceanic environment. Regarding the thickness of sea-ice sheet over the palaeoocean, many researchers have proposed different points of view principally based on speculation and/or various numerical modeling attempts. Hoffman and Schrag (2002) suggested that the global mean thickness of the ice sheet depends strongly on sea-ice albedo, e.g., ∼1.4 km for an albedo of 0.6. Using a spectral model, Warren et al. (2002) found that if the tropics did freeze, the ice would be too thick (>100 m) to allow photosynthesis, and therefore a refuge for photosynthetic organisms under widespread thin ice was unlikely. Kirschvink (1992) speculated that areas of open water (polynyas) would still remain over some parts of the ocean during a snowball Earth event. With a coupled climate/ice-sheet model, Hyde et al. (2000) proposed that sea-ice extended to only about 25◦ palaeolatitude, with a sea-ice thickness ranging from ∼1 m near the ice edge to 10 m at higher latitudes, open water could exist in the equatorial oceans, which could have allowed the survival of metazoans. By means of general circulation and ice-sheet models (GCM and ISM), Donnadieu et al. (2003) suggested a dynamic glacial environment consistent with the “hard” snowball Earth scenario, in which the ice accumulated and eventually took the form of dynamic ice sheets, and in which the GCMpredicted equatorial temperature was warm enough to prevent sea-ice thicker than 10 m. This thickness would permit the survival of photosynthetic organisms. Condon et al. (2002) observed rock records and explained their findings by the existence of locally and seasonally unfrozen water in the snowball Earth era.

Regarding the fate of early life such as photosynthetic organisms in the snowball Earth event, photosynthesis could have continued beneath bare ice if the ice sheet was sufficiently thin and sufficiently clear (Warren et al., 2002), or if open water or polynyas were available in the tropical ocean (Kirschvink, 1992). Photosynthetic eukaryotic algae appear to have been present both before and after the times of the snowball events (Knoll, 1985). In particular, silicified microfossils from pre- and syn-glacial units in the Death Valley region, California, reveal little change during the glacial interval; in fact, the syn-glacial microbiota is slightly more diverse and contains more putative autotrophic and heterotrophic eukaryotes than underlying strata (Corsetti et al., 2006). Several lines of evidences suggested that metazoans (multicellular animals) may have also evolved before the final “Varanger/Sturtian” glaciation at about 600 Ma (Baum and Crowley, 2001). Based on organic geochemical data, especially biological marker characteristics, we have already reported an initial investigation on the Late Neoproterozoic sedimentary rocks to verify that photosynthetic processes occurred in the palaeo-oceanic environment during the snowball Earth era in the Yangtze region in southern China (Wang et al., 2003). Since Olcott et al. (2005) examined biomarker evidence for photosynthesis during the Neoproterozoic glaciation on the Sao Francisco craton in southeastern Brazil, we would like to further provide the organic molecular evidence for photosynthesis as well as photosynthetic organisms and eukaryotes in tillites of the Nantuo and Jiangkou Glaciations in the Yangtze region, and also to discuss the palaeo-oceanic environment during the snowball Earth era. 2. Geological settings and samples The Yangtze region is located in southern China and is tectonically referred to as the Yangtze Block or South China Block (Fig. 1). The stratigraphy of the Yangtze region is characterized by well developed successions of Upper Neoproterozoic to

Fig. 1. Distribution of the Late Neoproterozoic glacial deposits in the Yangtze region, southern China and the sampling sites in this study.

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Table 1 Stratigraphy, sample location and numbering, Ro and TOC data of Upper Neoproterozoic–Lower Cambrian sediments in Yangtze region, China Stratigraphy Erathem

Section, location System

Series

Hezi’ao, Hubei Wangjiaping, Hubei

Cambrian

Lower

Lithologies

TOC (%)

Romv (%)

Ro (%)

1 2 3 4

Black shale Black shale Black shale Black shale

1.45 6.50 4.52 3.73

2.34 2.87 2.36 2.27

2.14 2.63 2.16 2.07

5 6 7 8 9 10 11 12

Black platy limestone Black shale Black siliceous limestone Silty ferromanganese rock Silty ferromanganese rock Silty ferromanganese rock Ferromanganese mudstone Phosphoric limestone

2.29 2.43 0.16 1.42 3.08 1.03 1.72 0.40

2.54 2.31 2.23

2.33 2.11 2.04

2.48

2.27

13

Blackish lamellose dolomite Black recrystallized dolomite Dark gray laminal dolomite Blackish argillaceous dolomite Blackish argillaceous dolomite Blackish gray carbonaceous and argillaceous dolomite Gray laminal dolomite Black argillaceous dolomite Black shale Cap domolite Cap domolite Phosphoric and argillaceous silt-crystal-like mudstone and domolite Tillite

0.42

2.14

1.95

0.26

2.33

2.13

0.12 9.37

2.64 2.61

2.42 2.39

1.82

2.91

2.66

3.63

2.94

2.70

0.08 0.40

2.11 5.59

1.93 5.15

1.74 0.13 0.23 0.18

5.28 2.40 3 07

4.86 2.20 2.81

0.10

5.09

4.68

Diamictite Diamictite Diamictite Diamictite Diamictite Diamictite/tillite Diamictite/tillite Diamictite/tillite Diamictite/tillite Diamictite Diamictite Diamictite Siliceous and phosphoric ferromanganese rocks Siliceous and phosphoric ferromanganese rocks Rhodochrosite Rhodochrosite Diamictite Diamictite Diamictite

0.05 0.02 0.07 0.06 0.06 0.11 0.09 0.13 0.13 0.12 0.08 0.10 0.18

2.25 2.73

2.06 2.50

3.76

3.45

5.84 5.31

5.37 4.88

2.55

2.33

2.51

2.37

2.17

5.41 3.10 0.01 0.01 0.01

2.32 3.10 8.22 8.32 8.16

2.12 2.84 7.57 7.66 7.52

Formation

Shuijingtuo

Palaeozoic (Pz)

No.

Yanjiahe

Sandouping, Hubei

Yu’anshan

Chengjiang, Yunnan

Meishucun

Ningqiang, Shanxi

Dengying (Nhdy)

Sandouping, Hubei

14 15 16 17

Doushantuo

Jijiawan, Hiihei

18

19 20 Neoproterozoic (Pt3 )

Nanhua Weng’an, Guizhou

Jijiawan, Hubei Ningqiang, Shanxi Nantuo

Liuchapo, Hunan Anhua, Hunan

Yangjiaping, Hunan

Datangpo

Jiangkouqun

Xiushan, Chongqing

Jiangkou, Hunan

Romv : measured marine vitrinite reflectance; Ro: equivalent vitrinite reflectance.

21 22 23 24

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

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Paleozoic strata, among which, the Upper Neoproterozoic strata principally consist of diamictites in the Jiangkou and Nantuo Tillites, Si, P, Fe, Mn-containing rocks in the Datangpo Formation and dolomites in the Doushantuo and Dengying Formations. By contrast, the Lower Cambrian sediments are mainly composed of phosphoric limestones in the Meishucun Formation, Fe, Mn-containing rocks in the Yu’anshan Formation, limestones and black shales in the Yanjiahe Formation and shales in other formations (Table 1). The Late Neoproterozoic glacial units (the Nantuo and Jiangkou Tillites), dated at ∼600 Ma and ∼720 Ma, respectively, and linked to the Marinoan and Sturtian ice-ages (Zhang et al., 2002), are overlaid by a 1–2 m thick layer of cap dolomite (the cap dolomite for the Jiangkou Tillite is not shown in Table 1). Forty-four typical Upper Neoproterozoic to Lower Cambrian sedimentary rock samples were collected from 12 stratigraphic sections, distributed across west Hubei, northwest Hunan, east Chongqing, south Shanxi, south Guizhou and east Yunnan Provinces, in the Yangtze region, southern China (Fig. 1). The lithologies of the rock samples are shown in Table 1. Among the rock samples, 16 are diamictites (13 for Nantuo and 3 for Jiangkou Tillites), and 2 are cap dolomite associated with the Nantuo Tillite.

72 h, with extended times used for low organic level samples (such as diamictites). After removing the sulfur with copper turnings and deasphalting with petroleum ether, the extract was quantitatively separated into aliphatic, aromatic and N,S,Ocompound fractions by routine column chromatography on a silica gel plus alumina (9:1, v/v) column.

3. Experimental methods

Each of the aliphatic fractions was analyzed on an Agilent Model 6890 gas chromatograph with an SE-54 fused silica capillary column (30 m × 0.25 mm i.d. × 0.25 ␮m). The oven temperature program was as follows: initially at 100 ◦ C for 1 min, ramped to 300 ◦ C at 4 ◦ C/min, and then isothermal for 30 min. Helium was used as the carrier gas. An internal standard, D-substituted tetracosane (C24 D50 ) was co-injected for quantitative GC analysis of phytane and pristane. GC–MS analyses of the aliphatic fractions were performed on a Finnigan Model SSQ-710 quadrupole analytical system coupled with a HP-5 fused silica capillary column (30 m × 0.32 mm i.d. × 0.25 ␮m). The temperature program was from 80 ◦ C to 250 ◦ C at 3.5 ◦ C/min, then from 250 ◦ C to 310 ◦ C at 2 ◦ C/min, with a final hold of 20 min. Helium was used as the carrier gas. MS conditions were as follows: full scan mode, scan rate 1.4 s/scan, scan range from 50 a.m.u. to 500 a.m.u. and electron energy 70 eV.

3.1. Clean laboratory conditions Some of the rock samples analyzed, especially diamictites in the Jiangkou and Nantuo Tillites, show an extremely low level of sedimentary organic matter (OM), and are very easily contaminated by external organics either in the field or in the laboratory. In order to prevent these rock samples from organic contamination, all the organic geochemical pretreatments are strictly performed under following clean laboratorial conditions: 1. All the glassware and aluminum foil used were successively washed by chromic acid mixtures, distilled water and dimethylchloride, and then baked in an electric furnace at 450 ◦ C for 6 h in order to clean out all traces of artificial organic pollutants. 2. All the solvents, i.e., petroleum ether, dimethylchloride, methanol and toluene, etc., were double-distilled, and chemicals, i.e., silica gel, alumina, absorbent cotton, filter paper and copper turnings, were extracted with dichloromethane to ensure solvents and chemicals were pure and clean. 3. Any potential pollutants on the surface of rock samples were removed by a grinding machine, and then, the surfaces of the remaining rocks were cleaned using distilled water and dimethylchloride in turn in order to ensure all the rock samples were free from organic pollution. 3.2. Sample preparation Each of the cleaned and dried rock samples was ground into powder (<100 mesh). Three hundred grams were extracted with 500 ml of dichloromethane:methanol (83:17, v/v) at 50 ◦ C for

3.3. Ro measurement and TOC analysis All the rock samples were prepared into polished blocks. The marine vitrinite (also called “vitrinite-like macerals”) reflectance Romv was measured on each polished block using a Leica Model MPV-SP microscopic photometer, and the measured Romv was then converted to the equivalent vitrinite reflectance Ro value so that the thermal maturity of OM in rock samples could be determined (Buchardt and Lewan, 1990; Kisch, 1980; Van Gijzel et al., 1992; Zhong and Qin, 1995). Total organic content (TOC) was analyzed on a WR-112 carbon analyzer to determine the OM abundance in each rock sample. 3.4. GC and GC–MS analyses

4. Results and discussion 4.1. Possible contamination sources and arguments for indigenity Since contamination in Precambrian rock extracts is a common phenomenon, indigenity tests would have to be conducted in any organic geochemical research on Precambrian rocks. In order to protect these Precambrian rocks from external organic compound contamination, strict clean laboratory procedures were followed during sample analysis (e.g. Brocks et al., 2003; Li et al., 2003; Peng et al., 1998). In this study, the analytical procedure we designed followed recommendations in the above literature and other sources, and was even stricter in some aspects (Li et al., 2006).

T.-G. Wang et al. / Precambrian Research 162 (2008) 317–326

Since there were no terrestrial organisms in existence on the Earth before the Devonian age, no terrestrial organic matter should be found within sedimentary rocks dating from the Neoproterozoic to Cambrian ages. In addition, it is well known that organic matter in old sedimentary rocks would be efficiently destroyed by weathering and erosion, especially soluble organic matter, such as lipids and various hydrocarbons. To date, only traces of insoluble type IV kerogen have been found as recycled organic matter. Although this may contribute to TOC, these traces have rarely been found and occur only in very small quantities. However, no hydrocarbons have been considered as being recycled in organic geochemical studies; neither in the present study nor in the literature.

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4.3. Molecular composition Even though these Upper Neoproterozoic–Lower Cambrian sedimentary rocks are of high maturity, some biomarker compounds could still be well preserved, and are as follows.

4.2. OM abundance and maturity

4.3.1. n-Alkanes A complete homologous series of C11 –C35 n-alkanes has been detected from the aliphatic fractions in all rock samples analyzed. n-Alkanes in diamictite of the Nantuo Tillite are shown in Fig. 2; the distribution of the n-alkane series shows a bimodal pattern with the first major peak at nC17 and second at nC25 , without any odd to even carbon-number-predominance. This distribution probably reflects a source derived from different organisms.

TOC can serve as an abundance parameter for the OM preserved in sedimentary rocks. The typical Upper Neoproterozoic–Lower Cambrian rocks, except for diamictites, were analyzed and generally show a high TOC range from 0.08% to 9.37%, with a 2.08% average. However, the diamictites in the Nantuo and Jiangkou Tillites have much lower TOCs ranging from 0.01% to 0.13%, with averages only 0.086% and 0.01%, respectively, 1–2 orders of magnitude lower than those in most of the non-glacial sedimentary rocks (Table 1), and indicating a very low level of organic productivity in the palaeo-oceanic environment during the snowball Earth era. Based on the equivalent Ro values, ranging from 1.95% to 7.57%, the OM in the sedimentary rocks analyzed is mostly post-mature (Table 1). In comparison, the Ro values of Lower Cambrian OM seem to be relatively lower, ranging from 2.04% to 2.63% with an average of 2.43%, whereas diamictites in the Nantuo Tillites and its cap dolomite have Ro values of 2.07–5.37%, and in Jiangkou Tillites 7.52–7.66% (Table 1). It can be seen from the data that Ro maturity shows a decreasing trend from the older to the younger strata. However, a few samples are abnormally higher than others, for example tillites from Jiajiawan, Hubei and Anhua, Hunan, which may be due to different thermal history or tectonic activity history.

4.3.2. Regular isoprenoid hydrocarbons A pseudohomologous series of iC15 –iC20 (no iC17 ) regular isoprenoid hydrocarbons is also detected in the aliphatic fractions of all rock samples, even in diamictites (e.g., in Fig. 2). Phytane (iC20 ) and pristane (iC19 ) are primarily derived from the phytyl side-chain on the chlorophyll skeleton in phototrophic organisms, such as photosynthetic bacteria, cyanobacteria, most algae and higher plants. Therefore, these compounds can be commonly used as biomarkers of photosynthetic organisms (Peters and Moldowan, 2005), with the exception of extreme niches where archaobacteria would be another precursor of phytane (Chappe et al., 1982). In addition, the Pr/Ph ratio can act as a molecular parameter indicative of redoxy and salinity in the sedimentary aquatic environments, for example, low Pr/Ph ratios (<0.6) typify anoxic, commonly hypersaline environments (Peters and Moldowan, 2005). The Upper Neoproterozoic–Lower Cambrian sedimentary except for diamictite rocks generally show higher Pr/Ph ratios, ranging from 0.69 to 1.45 with a 0.95 average, while the Pr/Ph ratios of diamictites range from 0.33 to 1.11 with an average of 0.66 in the Nantuo Tillite and range from 0.51 to 0.64 with an average of 0.56 in the Jiangkou Tillite (Table 2). These higher

Fig. 2. Total ion current of the aliphatic fraction from diamictite in the Nantuo Tillite. nC11 –nC35 : n-alkanes; iC15 , iC16 ,iC18 : regular isoprenoid hydrocarbons; Pr: pristane (iC19 ); Ph: phytane (iC20 ).

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Table 2 The concentrations of pristane and phytane in Upper Neoproterozoic–Lower Cambrian sedimentary rocks in the Western Yangtze region Formation

Sample no.

TOC (%)

Pr (ng/g rock)

Ph (ng/g rock)

Pr + Ph (ng/g rock)

Pr/Ph

Shuijingtuo

1 2 3 4

1.45 6.50 4.52 3.73

0.071 0.027 0.034 0.031

0.065 0.026 0.030 0.034

0.136 0.053 0.064 0.066

1.10 1.01 1.13 0.91

Yanjiahe

5 6 7

2.29 2.43 0.16

0.042 0.035 0.029

0.035 0.024 0.029

0.076 0.059 0.058

1.21 1.45 0.98

Yu’anshan

8 9 10 11

1.42 3.08 1.03 1.72

0.046 0.011 0.019 0.186

0.051 0.013 0.025 0.260

0.097 0.024 0.044 0.446

0.89 0.90 0.74 0.72

Meishucun

12

0.40

0.038

0.034

0.072

1.13

Dengying

13 14 15

0.42 0.26 0.12

0.025 0.016 0.017

0.032 0.021 0.024

0.057 0.037 0.041

0.79 0.75 0.69

Doushantuo

16 17 18 19 20 21 22 23 24

9.37 1.82 3.63 0.08 0.40 1.74 0.13 0.23 0.18

0.020 0.056 0.059 0.018 0.029 0.096 0.043 0.016 0.055

0.021 0.068 0.082 0.020 0.026 0.072 0.034 0.016 0.058

0.041 0.124 0.141 0.039 0.056 0.167 0.077 0.032 0.113

0.95 0.83 0.72 0.89 1.11 1.34 1.29 1.01 0.95

Nantuo

25 26 27 28 29 30 31 32 33 34 35 36 37

0.10 0.05 0.02 0.07 0.06 0.06 0.11 0.09 0.13 0.13 0.12 0.08 0.10

0.017 0.025 0.011 0.008 0.012 0.010 0.012 0.014 0.008 0.007 0.009 0.002 0.007

0.036 0.051 0.023 0.023 0.029 0.015 0.012 0.014 0.007 0.007 0.019 0.003 0.009

0.054 0.076 0.034 0.031 0.041 0.025 0.023 0.028 0.015 0.014 0.029 0.005 0.016

0.48 0.50 0.46 0.33 0.42 0.70 1.03 0.94 1.11 0.93 0.49 0.50 0.71

Datangpo

38 39 40 41

0.18 2.51 5.41 3.10

0.017 0.055 0.055 0.035

0.019 0.048 0.051 0.034

0.035 0.102 0.106 0.069

0.91 1.15 1.08 1.02

Jiangkou

42 43 44

0.01 0.01 0.01

0.013 0.003 0.002

0.020 0.006 0.004

0.033 0.010 0.007

0.64 0.53 0.51

values imply that there must have been no typical extreme niches available in the palaeo-oceanic environments of the Yangtze region during the Late Neoproterozoic–Early Cambrian periods. However, the palaeo-oceanic environment characterizing the deposition of the Nantuo and Jiangkou Tillites during the snowball Earth era would be more anoxic and more saline than that of non-glacial deposition. Generally, phytane, pristane and their pseudohomologues detected in the sedimentary rocks in the Yangtze region, should be clear indicators of the photosynthetic process within the palaeo-oceanic water column during the Late

Neoproterozoic–Early Cambrian periods. There is a tendency for positive correlation between TOC and the concentrations of phytane and/or pristane (Table 2), which implies that photosynthetic autotrophs should have been primary organisms contributing to sedimentary OM in the Yangtze palaeo-ocean. However, it is clear in Table 2 that both TOCs and concentrations of phytane and pristane in diamictites are present at an unusually low level: the concentrations of phytane plus pristane are 0.005–0.076 ng/g with an average of 0.030 ng/g in the Nantuo Tillite, and 0.007–0.033 ng/g with an average of 0.016 ng/g in the Jiangkou Tillite. On the other hand, the non-glacial sedi-

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323

Fig. 3. The concentration distribution of phytane and pristine in the Upper Neoproterozoic–Lower Cambrian sedimentary rocks in the Yangtze region.

mentary rocks have concentrations of 0.024–0.446 ng/g with an average of 0.087 ng/g, much higher than that in the diamictites (Fig. 3 and Table 2). The unusually low concentrations of phytane and pristane in diamictites reveal a very weak photosynthetic process, a restricted euphotic zone and quite limited sunlight within the palaeo-oceanic water column during the snowball Earth event. Nevertheless, it can still be inferred that, even though the productivity of photosynthetic organisms is very low, the photosynthetic process never ceased in the palaeo-oceanic environment during the snowball Earth event. Therefore, the palaeo-ocean in the Yangtze region during snowball Earth was either covered by a thin sea-ice sheet, or by a thicker sea-ice sheet that included polynyas or areas of open water. Considering the palaeolatitude of the Yangtze region, as determined by palaeomagnetic data, several authors have suggested that the Yangtze region was situated at a moderate palaeolatitude in the snowball Earth era. For instance, Li (1996) suggested the position of the South China Block was between just ∼10◦ and ∼20◦ . Zhang and Piper (1997) indicated that deposition of the diamictites (of the Nantuo Tillite) took place at

a palaeolatitude of ∼37◦ . Evans et al. (2000) proposed moderate palaeolatitudes (30–40◦ ) for the basal Sinian deposits across the South China Block. Recently, however, Macouin et al. (2004) obtained a palaeolatitude of 3.0 ± 4.5◦ for the Doushantuo carbonates, suggesting the underlying Nantuo Tillite were deposited close to the equator. No matter whether the diamictites were deposited at a moderate or a low palaeolatitude, it is difficult to envisage how the palaeo-ocean could have been entirely sea-ice covered during the snowball Earth era, especially at the equator, based on the organic molecular evidence for the photosynthetic process described above. 4.3.3. Steranes Various steroid hydrocarbons, C21 –C22 pregnanes, C27 –C29 diasteranes and regular steranes, were detected in all the rock samples. As an example, Fig. 4 shows diasteranes and regular steranes detected in diamictite in the Nantuo Tillite. Steranes are derived from sterols, which occur in all eukaryotes (Ourisson et al., 1984). Therefore, the detection and identification of steranes from the Upper Neoproterozoic Nantuo and Jiangkou Tillites should indicate the presence of eukaroytes

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Fig. 4. m/z 217 mass chromatogram showing the C27 –C29 diasteranes and regular steranes detected from diamictite in the Nantuo Tillite. 27St, 28St and 29St: C27 , C28 and C29 regular steranes; 27Dia: C27 Diasteranes.

(probably algae) in the palaeo-oceanic environment. In other words, eukaryotes were able to survive in such a cold, anoxic and saline palaeo-oceanic environment as would have occurred during the snowball Earth event.

4.3.4. Terpanes Various terpanoid hydrocarbons, i.e., C19 –C29 tricyclic terpanes, C27 –C35 (no C28 ) hopanes, C27 neohopanes and C30 gammacearane, were found in all rock samples (e.g., Fig. 5).

Fig. 5. m/z 191 mass chromatograms showing the composition and distribution of triterpanes in the Upper Neoproterozoic–Lower Cambrian sedimentary rocks in the Yangtze region. T.T.: tricyclic terpanes; Ts: 18␣(H)-22,29,30-trisnorhopane; Tm: 17␣(H)-22,29,30-trisnorhopane; H: hopane; Ga: gammacerane.

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As the biomarker of prokaryotes, hopanes are derived from bacteriohopanetetrol, a constituent of cell membranes in prokaryotes such as eubacteria (Ourisson et al., 1984, 1987). Gammacearane may be derived from the reduction of tetrahymanol, a lipid in the membranes of certain eukaryotic protozoa (Ourisson et al., 1987; Peters and Moldowan, 2005; Song and Wang, 2004). Tricyclic terpanes have an uncertain microbial origin (Aquino et al., 1982). Therefore, the detection of various terpanoids and steroids could reveal that the source inputs of sedimentary OM comprise numerous microbial contributions, such as algae, eubacteria, protozoa, etc., in the Late Neoproterozoic–Early Cambrian periods, even in the snowball Earth era. This provides additional evidence for the survival of numerous eukaryotic organisms in the snowball Earth event. 5. Conclusions Numerous biomarkers, n-alkanes, regular isoprenoid hydrocarbons (phytane, pristane and their pseudohomologues), various steranes, tricyclic terpanes, hopanes and neohopanes, gammacerane, etc., have been detected from the Upper Proterozoic–Lower Cambrian sedimentary rocks, including diamictites, in the Yangtze region, southern China. These biomarkers indicate multiple organic source inputs to the OM in Neoproterozoic and Cambrian sedimentary rocks (including diamictites in the Nantuo and Jiangkou Tillites) from early life, such as prokaryotic eubacteria, eukaryotic algae and protozoa, and other microbes. Therefore, it is these biomarkers, acting as organic molecular evidence, that confirm the survival of photosynthetic eukaryotes in the palaeo-oceanic environment during the snowball Earth era. Based on the organic molecular evidence of phytane, pristane and their pseudohomologues, which are derivatives of Chlorophyll-a, and various steranes, which are biomarkers of photosynthetic eukaryotes, in diamictites of the Nantuo and Jiangkou Tillites, it could be concluded that the photosynthetic process never ceased in the palaeo-oceanic environment during the snowball Earth era. Therefore, the palaeo-ocean in the Yangtze region during the snowball Earth was either covered by a thin sea-ice sheet, or was covered by a thicker sea-ice sheet that included polynyas or areas of open water. Acknowledgements This work is supported by the National Science Foundation of China (Grant no. 40172049). We thank Profs. Chen Junyuan, Zhu Maoyan and Zhang Junming (Nanjing Institute of Geology & Palaeontology, CAS), Zhang Oirui and Chu Xuelei (Institute of Geology & Geophysics, CAS) for providing rock samples, field work support and constructive comments throughout this study. References Baum, S.K., Crowley, T.J., 2001. GCM response to late Precambrian (∼590 Ma) ice-covered continents. Geophys. Res. Lett. 28, 583–586.

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