Reconstruction of Surface Ocean Nitrate Utilization
451
Untreated samples (n = 6) exhibited a mean
8 1 SN of7.8%0 (s.d. = 0.4%0), mean 8 15 N of decalcified samples (n = 8) was 7.7%0 (s.d. = 0.2%0) and
decalcified, rinsed samples (n = 5) had a mean
8 1 SN value of8.0%0 (s.d. = 0.4%0). A single classification Analysis of Variance (Anova) tesfl)
(P < 0.05) indicated thatthe means of treated samples were not significantly different from the untreated control or from each other.
For the experiments at the lOW, sediment from
the mud flats ofthe North Sea ("North Sea Standard"; 0.12% organic carbon, 0.01 % nitrogen) was
dried at 60°C and homogenized. The same three
treatments as listed above were performed on the
''North Sea Standard" (using 2N HCl) and in addition one set of samples was rinsed, but not decalcified. These samples were measured with a
Finnigan Mat Delta S mass spectrometer connected
via a split-interface to a Carlo Erba 1108 CHNAnalyzer. 8 15 N measurements of the untreated
replicate samples (n=4) exhibited a mean value of
9.7%0 and a lower standard deviation than any of
the treated ''North Sea Standards" (s.d. = 0.3%0).
The mean isotopic compositions of the decalcified,
unrinsed samples (n = 5; mean = 10.1%0; s.d. =
0.5%0), the decalcified and rinsed samples (n = 4;
(I) Anova is based on the F distribution (Sokal and Rbolf 1981)
a)
9
-
L'm
ui 8>
0
t
z 7Ln
~
-
-
6
b)
11
'2"
'm 10 -
ui
>
0
0
~
z 9Ln
~
8
untreated
0
0
oI e
•
::=E
•
decalcified
~I
o
o
• •
·1 •
•
decalcified,
rinsed
o
•
•
±
•
rinsed
•
:1
•
•
Fig. 2. Results of replicate measurements of "watt standard" samples after different preparation methods in two
laboratories: (a) Universitat Bremen (b) Institut fUr Ostseeforschung.