4.3 Shore-Based Replicate Measurements

The replicate samples from 16 Niskin bottles at 8 stations were collected for shore-based reference analyses during Section P16A/P17A, from only 2 bottles at 1 station during Section P17E/P19S, and from 18 Niskin bottles at 10 stations during Section P19C. The analyses were performed at the laboratory of Dr. C. D. Keeling of SIO. The TCO₂ measurements were produced by vacuum extraction/manometric analysis in controlled laboratory conditions using standards. Samples were collected from the same Niskin bottles used to collect samples for shipboard analyses of TCO₂. The shore-based analyses employed a precise and proven methodology to provide information on the quality of shipboard analyses (Guenther et al. 1994).

For all shore-based analyses during Section P16A/P17A, a replicate (standard deviation of shore-based analyses calculated from bottle pair agreement) of 1.19 µmol/kg for 14 unflagged pairs was calculated, with no deltas greater then 3. The average difference for 14 comparisons of single replicate samples comparing ship and shore TCO₂ values (LDEOSIO) was -3.4 ±1.8 µmol/kg (Table 1) (Fig. 5). This compares with the (LDEO SIO) difference for the CRM analyses of -1.0 ± 1.7 µmol/kg. The observed difference of 2.4 µmol/kg is somewhat greater than the standard deviations for the respective measurements, and suggests an addition of CO₂ during the storage period of the SIO seawater samples.

For all shore-based analyses during Section P19C, a replicate of 0.48 µmol/kg for 17 unflagged pairs was calculated, with no deltas greater than 3. The average difference for 15 comparisons of single replicate samples comparing ship and shore TCO₂ values (LDEOSIO) was -1.1 ± 1.9 µmol/kg (Table 2) (Fig 6). This compares with the (LDEO - SIO) difference for the CRM analyses of 0.2 ± 2.0 µmol/kg. The observed difference is within the standard deviations for the respective analytical methods. Hence, the results of the SIO analyses of the stored samples are in agreement with the shipboard TCO₂ data measured by LDEO.

Only two water samples were compared for Section P17E/P19S. These represent too few data points for meaningful comparative analyses.