25th International Meeting on Organic Geochemistry IMOG 2011

P-051
Kerogen sulphur, hydrogen and carbon isotope variation across
the Permian-Triassic boundary in the South China
Chunfang Cai, Lei Xiang, Kaikai Li, Lei Jiang
Key Lab of Petroleum Resource, Institute of Geology and Geophysics, Chinese Academy of Sciences,
Beijing, China (corresponding author:cai_cf@mail.iggcas.ac.cn)
The biogeochemical cycle of organic and inorganic
carbon and pyrite and sulphate sulphur across the
Permian-Triassic boundary (PTB) has been widely
studied to determine the cause of mass extinction and
environment change in the PTB. However, organically
bound sulphur and hydrogen in kerogen have been
seldom characterised. Kerogen δ 34 S and δD values
are measured to reflect the change cross-checked
δ 13 CKerogen values from four sections nearby the PTB.
Broadly parallel changes of Kerogen δ 34 S, δD and
δ 34 C values occur across the PTB for the all four
sections (Fig.1)
In the Meishan section (Changxing, Zhejiang)
deposited in an upper slope environment, δ 34 Skerogen
values range from -20.5 to -8.3‰ (n=7) below PTB
with the heaviest δ 34 Skerogen and δ 34 Spyrite values and
the lightest δ 13 Ckerogen measured in the bed 22 and
bed 24, where abundant aryl isoprenoids were
detected (at the bottom of bed 24, mass extinction
occurred). δDkerogen values show dramatic change with
the lightest value (-174‰) above the PTB at bed 28.
Shangsi and Guangzi sections, NW Sichuan were
deposited in a deeper lower slope environment. In
Shangsi and Guangzi section, δ 34 Skerogen values range
from -42.3 to -10.6‰ (n=11) and δ 34 Spyrite from -35.9
to -5.0‰ below the PTB with the heaviest δ 34 Skerogen
and δ 34 Spyrite values measured from the uppermost
Changxing Fm (P3ch). Much heavier δ 34 Skerogen value
of -12.6‰ was measured from the below Longtan Fm
(P3l). In Guangzi section, Changxing Fm δ 34 Skerogen
values are -30.5‰ and -27.1‰, and δ 34 Spyrite from -
32.6‰ and -34.5‰. Longtan Fm δ 34 Skerogen values
show a wide range from -32.2‰ to -13.1‰ (averaged
-24.2‰, n=9), and δ 34 Spyrite from -37.2‰ to -22.5‰
(averaged -30.5‰, n=5). Abundant aryl isoprenoids
were detected nearby the PTB in the both section.
In Wanyuan section, NE Sichuan, much heavier
δ 34 Skerogen values (+5.5‰ and -1.7‰) were detected
from the uppermost Changxing Fm (P3ch) (Cai et al.,
2010) and similar values from -6.6 to +4.1‰ in the
below Longtan Fm (P3l) (averaged -0.5‰). No aryl
isoprenoids were detected from the Longtan Fm.
From the above, there exists large variation in δ 34 S
values of kerogen and pyrite with geographical
location: the heaviest values in NE Sichuan and the
lightest values in NW Sichuan and Meishan section
ranging in between. A shallower environment for the
Meishan during the deposition than the NW sections
may indicate water depth may have controlled the
δ 34 S values. This is likely due to euxinic and stratified
water column and recycling of H2S by green and
purple sulphur bacteria to greater degree in deeper
water. Decades of reefs found in the NE Sichuan
suggest a shallow water and oxygen is not too limited
before the end of Permian in this area.
Interestingly, those samples with abundant aryl
isoprenoids show the heaviest δ 34 Skerogen and δ 34 Spyrite
values in the all three sections. Possible reasons
among others include 1) much lower sulphate
concentration of seawater during the period; 2)
bacterial sulphate reduction in the sediments not in
water columns, i.e., in a relatively closed system with
a limited supply of sulphate (Cai et al., 2009).
A significant negative shift in δDkerogen value above
PTB is associated with high Pr/Ph ratio, indicating an
influx of more freshwater carrying organic matter as a
result of terrestrial ecological collapse into the ocean.
P d
Strata T f
1
3
P l
3
Shangsi
Kerogen
δ C
13
δ S
-40 -10
34
-28 -25
Py OS
Strata
T f
1
P d
3
P w
3
Guanzi
δ S
34 Kerogen
δ C
13
-34 -16 -29 -26
Py OS
Strata
T f
1
P d
3
P l
Wanyuan
3
Kerogen
δ C
-27 -3 -28 -22
13
S δ34
Py OS
Beds
Formation
Strata
T f
1
P d
3
P l
3
YinKeng
Changxing
Meishan
Kerogen
δ S
δD
-165 -125
34 Kerogen
δ C
-22 -2
13
-29 -25
31
29- 30
28
Py OS
27
26
25
24
23
22
21
20
18-19 17
15-16 12-13 10-11 8-9 7
2-6 1
Py—Pyrite
OS—Organically bound sulfur
Fig. 1. Variation of kerogen � 34 S,� 13 C and �D and
pyrite � 34 S values across the PTB from four sections
from NW and NE Sichuan and Changxing, Zhejiang.
Acknowledgement
This work is financially supported by NSFC 40839906.
References
[1] Cai, C.F. (2009) Organic Geochemistry 40, 755-
768.
[2] Cai, C.F. (2010) Organic Geochemistry 41, 871-
878.
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