Erlenkeuser and von Grafenstein: Stable Oxygen Isotope Ratios ill Benthic Carbonate Shells
505
sediments retrieved by spade box coring and have been stored unfrozen in plastic bags at 4°C.
Staining to mark living organisms was used for the IK-series only. Some of the species, like
Cytheropteron, showed poor staining only or none.
The bulk sediments were oven-dried at 60°C, weighed, and soaked with a mixture of 10 %
hydrogenperoyide, water and ammonia for 1 hr to remove organic compounds. Care was taken
to keep foaming low. The soaked samples were gently wet-sieved on a 63Ilm-mesh, using tap
water as washing liquid, the coarse fraction washed in ethanol, dried at room temperature and
the fraction> 125 !lm separated. The fine fraction in the soakage was concentrated for further
studies by reverse filtration and was repeatedly washed with ammonia water to remove residual
tap water and peroxide. From the> I 25!lm-fraction, the fragile shells of the ostracods were
picked under the binocular and were gently though carefully cleaned, using a fine brush,
ethanol, and a steel needle, from adhered sedimentary particles which easily settle behind the
inner calcified lamella. Other taxa such as bivalvia and foraminifera (Elphidium) were separated
as well.
From 49 stations, 316 ostracod carapaces of 13 species were separated for isotope analysis
on the species level, 48 samples of bivalvia of 3 species yet undetermined, and 143 samples of
foraminifera of the genus Elphidium (3 species). A summary of sample statistics is given in
Table 2. Reference specimens were archivated separately and the bulk of the shells splitted into
single shell samples for isotope analysis to learn and possibly quantify the isotopic variability in
this environment of the Laptev Sea. The carapaces of the ostracods were well sufficient for
reliable isotope measurements on single specimens. Similarly Elphidium and bivalvia species
could be measured as single shells. The present work discusses the average isotope figures as
calculated for each station for each taxon rsp. species if determined.
For isotope analysis, the carbonate sample was reacted with an individual aliquot of (4 drops
ot) 100% Orthophosphoric acid under vacuum at 73°C in the Kiel carbonate device which is online-coupled to a Finnigan MAT 251 gas isotope mass-spectrometer. Minimum sample size,
normally at 12 !lg of CaC0 3 , was reduced for this study to 61lg applying a specially designed
valve/ cold finger gas provider assembly at the gas inlet capillary of the spectrometer (Cordt and
Erlenkeuser, in prep.). The external error amounts to less than 0.08 %0 on the olga-scale (1sigma value).
The isotope results are given in the usual o-notation quoting the relative difference in perrnille
of the sample's isotope abundance ratio from that ratio of the PDB-standard. The PDB-standard
is represented by the NBS 20 secondary isotope standard. (Recently NBS 19 was said to have
the better isotopic reproducibility and was chosen to represent the international isotope scale,
then termed VPDB (Vienna PDB; Coplen, 1995). There is no systematic offset between the
PDB and VPDB measures).
The effect of river discharge to the Laptev Sea relates the 0 18 0 of the water and its salinity
with a coefficient of 0.6 %o/psu (Erlenkeuser, unpub!.), with the Lena water showing 8 180
about 20 %0 lower than the open ocean source waters advecting onto the shelf (33.6 psu at 40 m
water depth on the outer Laptev shelf and slope; Karpiy et aI., 1994). The effect of water
temperature on 8 18 0 of a calcite precipitate is about -0.25 %%K (Shackleton, 1974).
Results and discussion
The sampling stations are listed in Tables la, 1 b and are shown in Figure 1. They cover the
southern and middle Laptev Sea and represent benthic environments with widely differing
oceanographic disposition, ranging from the prominent seasonally scheduled fresh water
regime in the southeast to almost full permanent marine conditions in the northwest. The
505
sediments retrieved by spade box coring and have been stored unfrozen in plastic bags at 4°C.
Staining to mark living organisms was used for the IK-series only. Some of the species, like
Cytheropteron, showed poor staining only or none.
The bulk sediments were oven-dried at 60°C, weighed, and soaked with a mixture of 10 %
hydrogenperoyide, water and ammonia for 1 hr to remove organic compounds. Care was taken
to keep foaming low. The soaked samples were gently wet-sieved on a 63Ilm-mesh, using tap
water as washing liquid, the coarse fraction washed in ethanol, dried at room temperature and
the fraction> 125 !lm separated. The fine fraction in the soakage was concentrated for further
studies by reverse filtration and was repeatedly washed with ammonia water to remove residual
tap water and peroxide. From the> I 25!lm-fraction, the fragile shells of the ostracods were
picked under the binocular and were gently though carefully cleaned, using a fine brush,
ethanol, and a steel needle, from adhered sedimentary particles which easily settle behind the
inner calcified lamella. Other taxa such as bivalvia and foraminifera (Elphidium) were separated
as well.
From 49 stations, 316 ostracod carapaces of 13 species were separated for isotope analysis
on the species level, 48 samples of bivalvia of 3 species yet undetermined, and 143 samples of
foraminifera of the genus Elphidium (3 species). A summary of sample statistics is given in
Table 2. Reference specimens were archivated separately and the bulk of the shells splitted into
single shell samples for isotope analysis to learn and possibly quantify the isotopic variability in
this environment of the Laptev Sea. The carapaces of the ostracods were well sufficient for
reliable isotope measurements on single specimens. Similarly Elphidium and bivalvia species
could be measured as single shells. The present work discusses the average isotope figures as
calculated for each station for each taxon rsp. species if determined.
For isotope analysis, the carbonate sample was reacted with an individual aliquot of (4 drops
ot) 100% Orthophosphoric acid under vacuum at 73°C in the Kiel carbonate device which is online-coupled to a Finnigan MAT 251 gas isotope mass-spectrometer. Minimum sample size,
normally at 12 !lg of CaC0 3 , was reduced for this study to 61lg applying a specially designed
valve/ cold finger gas provider assembly at the gas inlet capillary of the spectrometer (Cordt and
Erlenkeuser, in prep.). The external error amounts to less than 0.08 %0 on the olga-scale (1sigma value).
The isotope results are given in the usual o-notation quoting the relative difference in perrnille
of the sample's isotope abundance ratio from that ratio of the PDB-standard. The PDB-standard
is represented by the NBS 20 secondary isotope standard. (Recently NBS 19 was said to have
the better isotopic reproducibility and was chosen to represent the international isotope scale,
then termed VPDB (Vienna PDB; Coplen, 1995). There is no systematic offset between the
PDB and VPDB measures).
The effect of river discharge to the Laptev Sea relates the 0 18 0 of the water and its salinity
with a coefficient of 0.6 %o/psu (Erlenkeuser, unpub!.), with the Lena water showing 8 180
about 20 %0 lower than the open ocean source waters advecting onto the shelf (33.6 psu at 40 m
water depth on the outer Laptev shelf and slope; Karpiy et aI., 1994). The effect of water
temperature on 8 18 0 of a calcite precipitate is about -0.25 %%K (Shackleton, 1974).
Results and discussion
The sampling stations are listed in Tables la, 1 b and are shown in Figure 1. They cover the
southern and middle Laptev Sea and represent benthic environments with widely differing
oceanographic disposition, ranging from the prominent seasonally scheduled fresh water
regime in the southeast to almost full permanent marine conditions in the northwest. The