STABLE ISOTOPE INDICATION OF SEAWATER/FRESHWATER EVENTS:
BALTIC’S GOTLAND DEEP AND THAI MANGROVE PEAT PROFILE
M. O. JĘDRYSEK
University of Wrocław, Lab.Isot.Geol.&Biogeochem., Dept. Applied Geol.&Geochem., 50-205 Wrocław
st Cybulskiego 30, Poland; Tel. 48-71-3288924
e-mail: morion@ing.uni.wroc.pl
Isotope analysis of carbonates and organic
matter in sediments potentially offers a valuable
tool for paleoenvironmental reconstructions.
Carbon and oxygen isotope analysis of authigenic
Ca-rhodochrosite and organic matter occurring in
organic-rich sediments from Gotland deep in
Baltic sea has been carried out (Fig. 1).
The analysed 2m thick profile represents the
0
present marine stage following the Ancylus
freshwater event (9,200 to 7,700 yr. BP).
Moreover, carbon and sulphur isotope analysis
and organic sulphur concentration measurements
have been carried out in a vertical peat profile
(Bacho 1) from Southern Thailand, Narathiwat
Province (Fig2).
5
Depth in the sediments [cm]
24
30
35
-50
53
77
-100
100
120
129
-150
150
165
Gotland Deep profile
Baltic sea
199
-200
freshwater-brackish-marine
-28
-27
-26
organic matter
d
-25 -9
13
C [PDB ‰]
-8
-7
rhodochrosite
d 13C
-6 -4
-2
rhodochrosite
0
18
d O [PDB ‰]
18
[PDB ‰]
Fig. 1 Vertical variation in the d C of organic matter and rhodochrosite and d O in rhodochrosite in Gotland
deep sediments. Data points labels show depth in cm below recent sediments surface.
13
Peat profile from Bacho,
Narathiwat, SE Thailand
1200
1100
850+/-130
890+/-70
1000
1020+/-130
1060+/-90 seawater
Year [Anno Domini]
900
ingression
800
700
1330+/-100
600
500
400
1610+/-100
300
1730+/-50
200
-34 -32 -30 -28 -26 -24
d
13
C [PDB, ‰]
12 13 14 15 16 17 18 19 20
d 34 S
[CDT, ‰]
0.00
0.02
0.04
0.06
S org. [%, wt]
Fig.2. Vertical variation in the d13C, d34S and in Sc (organic sulphur concentration) value in the peat profiles
from Bacho I, Southern Thailand. Data point labels show 14C dates.
13
The d13C value of organic matter in Gotland
sediments varies from to -28.9 to –25.9 ‰.
The d13C and d18OPDB values in carbonate
(rhodochrosite) in the same sediments vary from 8.4 to –6.5 ‰ and from –3.4 to –1.1 ‰,
respectively. It has been shown increasing
reduction
processes
(bacterial
oxidation
of methane) in the recent history of Baltic sea
(Fig3). This proves that carbon isotope analysis in
organic matter and carbonates is a valuable tool to
reconstruct
variations
red-ox
conditions.
Variations in oxygen isotope ratios in the Gotland
deep rhodochrosite reflects less thermal
fluctuations in precipitation of that carbonate and
dominantly
invasion
of
seawater
i.e.
opening/closing the Baltic sea to the North sea.
More advanced isolation, reflected by lower d18O
value, results in increase in d13C(DIC) and
20
b
m rac
et ki
ha sh
no w
ge ate
ne r
sis
5
-26
77
34
24
120
-27
18
36.5cm
17
53
35
16
100 129
-27
150
-28
Gotland Deep profile
Baltic sea
-28
-9
-8
-7
rhodochrosite
d
199
-6
17.5 cm
12.5 cm
7.5 cm
50.0cm
2.5 cm
14
13
-31
-30
-29
-28
-27
13
d C [‰]
13
C [PDB ‰]
Fig. 3. d C of rhodochrosite and organic matter
in Gotland deep sediments. Data points labels
show depth in profile in cm below recent
sediments surface.
13
22.5 cm
42.5 cm
47.5 cm
15
165
m fre
e t sh
ha w
no at
g e er
ne
si s
organic matter
30
d13 C [PDB ‰]
27.5cm 32.5cm
19
d S [‰]
-26
anaerobic conditions followed by an active
methanogenesis (reflected with certain inertia by
higher d13C value in organic matter and lower
d13C value in carbonates). It has been suggested
that recently, Baltic sea drives in general into
more brackish conditions with strong oxygen
deficit. This is probably resulted in part from
anthropogenic impact (Fig1).
d13C profiles in peat from Thailand range from
-30.7 to -27.6‰, and the d34S value ranges from
7.4‰ to 19.4‰. It was shown that sulphur and
carbon isotope variations in organic-rich profiles
consist reliable record of source of water, water
level and temperature variations (Fig 4).
A short ingression (not more than about 30 years)
of seawater has been documented in Southern
Thailand about 900 A.D (Fig.2).
13
34
Fig.4. d C - d S correlation for peat profile
Bacho I, Narathiwat airport, Southern Thailand.
Data point labels show depth in the core profile.
Samples at the depth 27.5 and 32.5 cm represent the
maximal stage of the seawater presence in the
Bacho wetland.
14