Environmental Engineering Reference
In-Depth Information
CH
4
[nM]
13
δ
CH
4
(per mil)
1 10
4
1.5 10
4
0
5000
-60
-55
-50
-45
-40
0
0
500
500
1000
1000
1500
1500
2000
2000
2500
2500
Figure 5.
Depth profile of methane concentrations of reference site 064 (light circles) and a
composite profile of seep sites 068 and 075 that were sampled very close to station 072 (dark
circles). Carbon isotopic composition of the dissolved methane from the water column above
the reference site 064. The depletion in the isotopic values from approximately 800 to 500 m
water depth indicates an additional source of methane from the upper to middle slope.
13
C
CH
4
increased to -20 ‰. At the north-western station (7617, Fig. 4) the samples
from 300 to 165 m hadδ
occurred just below the chemocline at 100 m water depth, Here, δ
13
C
CH
4
values around - 56‰followed by a significant
increase to -51‰. The top sample at 50 m water depth had δ
13
C
CH
4
values of
-52 ‰. The stable carbon isotopic composition of methane at site 064 (Fig. 5)
varied between -49 and -51 ‰ VPDB. Above 800 mδ
13
C
CH
4
values decreased
to a minimum of -58 ‰ VPDB at 500 m and subsequently increased again
above 160 m to δ
13
C
CH
4
values of -46 ‰ VPDB near the surface. The isotopic
composition of the dissolved methane in the oxic water column near the seafloor
at the two shelf stations (Fig. 3) differed with a δ
13
C
CH
4
value of -67 ‰ VPDB
13
C
CH
4
value of -58 ‰ VPDB at
the bottom of the reference site (055). A constant increase in δ
at the bottom of the seep site (038) from the δ
13
C
CH
4
values
from -67 ‰ VPDB to -49 ‰ VPDB from the bottom to the surface could be
noted at the seep station (038).
