510
Land-Ocean SYstems in the Siberian Arctic: Dynamics and History
considered the marine source waters advected onto the Laptev Sea shelf. This marine source
applied here does not represent the possible ultimate high marine water, as its salinity is
significantly lower at about 33.6 psu than in the core of Atlantic Water found at greater depths
in the boundary current along the upper Laptev Sea slope (Schauer et aI., 1995).
With regard to the prominent presence of Atlantic water, the 40 m-shelf-water' may be
considered to recruit from this Atlantic realm and be mixed up with Siberian riverine waters.
Given 0 18 0 = 0.26 %0 SMOW and S = 35.2 psu for Atlantic water far ,upstream' off MidNorway (Craig and Gordon, 1965) and accounting for the effect of Siberian riverine waters on
0 18 0 as 0,6 %o/psu, 0 18 0 = -0,7 %0 SMOW is expected for the shelfwater salinity of S = 33.6
psu. This value is corroborated by measured data of -0.6 to -0.7 %0 SMOW (Stein, 1996;
profile 4, Laptev Sea shelf at ca. 40 m; S = 33.5 psu). The calcite equilibrium 0 18 0 (termed oc
in Shackleton 1974) is calculated as oc = 3.8 %0 PDB for t = -1.5 DC from Shackleton's (1974)
paleotemperature equation, and we refer to this value to quantify the isotopic vital offsets.
As the seasonal variation in temperature and salinity presumably plays a minor role at the deeper
stations of transect N, the remaining isotope differences among the studied species ofthe
carbonate benthos likely reflect vital offsets, i.e. species-dependent isotopic compositions other
than in a calcite precipitated in thermodynamic isotopic equilibrium with the surrounding water.
The phenomenon of , vital offsets' in the benthic isotopic composition of biogenic carbonates
is known for long for foraminiferal carbonates (e.g. Duplessy et ai, 1970) and is commonly
corrected for, deducing environmental parameters from isotope results. For ostracods, vital
effects are largely unknown.
In this work, the vital offsets were roughly quantified, for the present state of evaluation, in
units of 0.5 %0 (Table 2), for the deep samples from transect N by refering to the calculated
equilibrium-o l8 0 of the ,40m-shelf-water' and for a few more species from other, shallower
stations by inter-species isotope comparison. Directly calculated offsets, i.e. differences
between measured and expected isotopes values, and resulting average figures are indicated in
Table I a, 1 b through specimen number and species key each labeled by an asterisk. Reference
species and counterpart in inter-species comparison are marked by a dot and underlined dot,
respectively. Occasionally, with too poor a data basis, a uniform offset on the species level was
hypothesized, e.g. for species C2 (Cytheropteron elaeni). Of the ostracoda, 7 of the 11 species
analysed reveal isotopic equilibrium, while the other 4 show 180/160-ratios too light by 0.5 %0.
The bivalvia species appear to calcify out of equilibrium as well, by about -0.5 %0, while the
benthic foraminfer Elphidium is even more biased in 0 18 0, by about -1 %0.
Correcting for vital offsets greatly reduces the inter-species isotope scatter for the deep
stations of the northern transect N (Figure 4). This is no longer true for the shallower stations
and the stations along the eastern transects Land Y, in particular its southern stations in the
Yana Bight (Figure 4). This finding probably relates to the interference of the species' life
rhythm, i.e. the seasonal schedule of shell growth, and the seasonal variation of salinity and
temperature. Ostracods can calcify their carapaces in a short time, possibly within days, thereby
flash-lighting the isotopic situation in the water. On the other hand, we expect bivalvia to build
their shells in a more gradual way. In the Baltic, for instance, isotope studies on Mytilus edulis
revealed calcification throughout the year, though at a rate varying with the season (Erlenkeuser
et aI., 1980). The bivalve shells from the Yana Bay stations, where the riverine regime is
dominant, surprisingly reflect the seasonal low salinity phase (or higher summer temperature)
much more pronounced than the ostracoda. Offset values gained by interspecies comparison in
particular are sensitive to differences in life rhythms, and these values, such as for the bivalve
species (Table 2), still have an unknown reliability.
The different oceanographic disposition of the benthic localities studied shows in the
Land-Ocean SYstems in the Siberian Arctic: Dynamics and History
considered the marine source waters advected onto the Laptev Sea shelf. This marine source
applied here does not represent the possible ultimate high marine water, as its salinity is
significantly lower at about 33.6 psu than in the core of Atlantic Water found at greater depths
in the boundary current along the upper Laptev Sea slope (Schauer et aI., 1995).
With regard to the prominent presence of Atlantic water, the 40 m-shelf-water' may be
considered to recruit from this Atlantic realm and be mixed up with Siberian riverine waters.
Given 0 18 0 = 0.26 %0 SMOW and S = 35.2 psu for Atlantic water far ,upstream' off MidNorway (Craig and Gordon, 1965) and accounting for the effect of Siberian riverine waters on
0 18 0 as 0,6 %o/psu, 0 18 0 = -0,7 %0 SMOW is expected for the shelfwater salinity of S = 33.6
psu. This value is corroborated by measured data of -0.6 to -0.7 %0 SMOW (Stein, 1996;
profile 4, Laptev Sea shelf at ca. 40 m; S = 33.5 psu). The calcite equilibrium 0 18 0 (termed oc
in Shackleton 1974) is calculated as oc = 3.8 %0 PDB for t = -1.5 DC from Shackleton's (1974)
paleotemperature equation, and we refer to this value to quantify the isotopic vital offsets.
As the seasonal variation in temperature and salinity presumably plays a minor role at the deeper
stations of transect N, the remaining isotope differences among the studied species ofthe
carbonate benthos likely reflect vital offsets, i.e. species-dependent isotopic compositions other
than in a calcite precipitated in thermodynamic isotopic equilibrium with the surrounding water.
The phenomenon of , vital offsets' in the benthic isotopic composition of biogenic carbonates
is known for long for foraminiferal carbonates (e.g. Duplessy et ai, 1970) and is commonly
corrected for, deducing environmental parameters from isotope results. For ostracods, vital
effects are largely unknown.
In this work, the vital offsets were roughly quantified, for the present state of evaluation, in
units of 0.5 %0 (Table 2), for the deep samples from transect N by refering to the calculated
equilibrium-o l8 0 of the ,40m-shelf-water' and for a few more species from other, shallower
stations by inter-species isotope comparison. Directly calculated offsets, i.e. differences
between measured and expected isotopes values, and resulting average figures are indicated in
Table I a, 1 b through specimen number and species key each labeled by an asterisk. Reference
species and counterpart in inter-species comparison are marked by a dot and underlined dot,
respectively. Occasionally, with too poor a data basis, a uniform offset on the species level was
hypothesized, e.g. for species C2 (Cytheropteron elaeni). Of the ostracoda, 7 of the 11 species
analysed reveal isotopic equilibrium, while the other 4 show 180/160-ratios too light by 0.5 %0.
The bivalvia species appear to calcify out of equilibrium as well, by about -0.5 %0, while the
benthic foraminfer Elphidium is even more biased in 0 18 0, by about -1 %0.
Correcting for vital offsets greatly reduces the inter-species isotope scatter for the deep
stations of the northern transect N (Figure 4). This is no longer true for the shallower stations
and the stations along the eastern transects Land Y, in particular its southern stations in the
Yana Bight (Figure 4). This finding probably relates to the interference of the species' life
rhythm, i.e. the seasonal schedule of shell growth, and the seasonal variation of salinity and
temperature. Ostracods can calcify their carapaces in a short time, possibly within days, thereby
flash-lighting the isotopic situation in the water. On the other hand, we expect bivalvia to build
their shells in a more gradual way. In the Baltic, for instance, isotope studies on Mytilus edulis
revealed calcification throughout the year, though at a rate varying with the season (Erlenkeuser
et aI., 1980). The bivalve shells from the Yana Bay stations, where the riverine regime is
dominant, surprisingly reflect the seasonal low salinity phase (or higher summer temperature)
much more pronounced than the ostracoda. Offset values gained by interspecies comparison in
particular are sensitive to differences in life rhythms, and these values, such as for the bivalve
species (Table 2), still have an unknown reliability.
The different oceanographic disposition of the benthic localities studied shows in the