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FIGURE 4 | Cruise tracks of four SFB 754 crusies in the Atlantic Ocean in the third fundina period of the project (2016-2019) 
on the focus of the cruise) were collected and measured 
covering most CTDO casts (Tanhua and Mehrtens, 2021; 
see Table 2 and Supplementary Table 10). Samples were 
collected in 100 ml wide-necked WOCE glass bottles 
with pre-defined volumes. Oxygen samples were collected 
immediately after the CTDO cast was finished and always 
directly after the sampling of transient tracers. The sample 
bottles were flushed with at least three times its volume 
and the samples were free of air-bubbles. Immediately 
after sampling, the seawater samples were spiked from the 
bottom with the fixation solutions (NaIl/NaOH and MnCl,). 
Some 10 to 20% of the discrete samples were collected as 
duplicates or triplicates in order to quantify sampling and 
titration uncertainties. 
The oxygen concentration was determined by Winkler 
titration within a minimum of 40 min and a maximum of 
L6 h after sampling following GO-SHIP best practices (Langdon, 
2010). Details of oxygen measurements can be found in the 
cruise reports (Table 1) of the individual cruises. For all 
cruises, we followed the standard procedures for compensating 
for impurities in the reagents and oxygen in the fixation 
solution. On cruises in the Southeast Tropical Pacific very low 
oxygen concentrations were found (well below 5 uWmol/kg) 
and on the third set of cruises in the area (M135 to M138, 
see Table 1) we additionally compensated for a presumed 
sampling blank, i.e., contamination from air during sampling 
and fixation, and outgassing from the PVC Niskin bottles. 
The blank was determined by comparing titrated oxygen 
concentrations in the central depths of the oxygen minimum 
-rontiers in Marine Science | www.frontiersin.ore 
layer where we had previously determined that there was 
NO Oxygen present. 
Nutrient Measurements 
Nutrients were measured on-board for a sub-set of the cruises, 
and on another sub-set the samples were frozen for post-cruise 
processing in Kiel (Tanhua and Mehrtens, 2021; see Table 2 
and Supplementary Table 10). Nutrient measurements on-board 
were performed with QuAAtro gas-segmented continuous flow 
analyzers (AutoAnalyzers) from SEAL Analytical. The exact 
methods used are listed in the cruise reports (see Table 1) and 
were normally: Nitrite and Nitrate - Q-068-05 Rev 11; Nitrite - 
Q-070-05 Rev 6; Phosphate - Q-064-05 Rev 8; Silicate - Q-066-05 
Rev 5. The precision of the nutrient measurements was calculated 
as the average of the standard deviation from the replicate 
measurements of samples, and are recorded in the cruise reports. 
For the majority of the cruises where nutrients were measured 
on-board, reference Material for Nutrients in Seawater (RMNS) 
from the General Environmental Technos (KANSO) Co., Ltd., 
Osaka/Japan were used. Normally, reference material samples 
were measured as triplicates at least once in every sampling run. 
For nutrient analysis we followed the GO-SHIP best practices for 
nutrient measurements (Hydes et al., 2010). 
Transient Tracer Measurements 
Three tracer release experiments were conducted during the 
SFB 754 using the artificial tracer CF3SFs; two in the tropical 
North Atlantic, and one in the tropical South Pacific (Tanhua 
and Mehrtens, 2021; see Table 2 and Supplementary Table 10). 
3eptember 2021 | Volume 8 | Article 72330«