Dataset: Bottle-calibrated dissolved oxygen profiles
Deployment: AR21

SeaBird processing 

CTD data are processed using SeaBird data processing software. The raw 24 Hz CTD data are converted from HEX to ASCII, lag corrected, edited for large spikes, smoothed according to sensor, and pressure averaged for final data quality control and analysis. Table 1 summarizes the order in which SeaBird Modules were processed and the inputs applied during each module. 

• DATCNV: Convert the raw data (.hex) to pressure, temperature, conductivity, and dissolved oxygen (V) to a file with a .cnv extension. Use default hysteresis correction
• BOTTLESUM: Writes out a summary of the bottle data to a file with a .btl extension
• ALIGNCTD: Advance Oxygen raw [V] by time determined by processing relative to pressure
• WILDEDIT: Checks for and marks ‘wild’ data points: first pass 2.0 standard deviations; second pass 20 standard deviations
• CELLTM: Conductivity cell thermal mass correction; alpha = 0.03 and 1/beta = 7.0
• FILTER: Low pass filter pressure and depth (DO [V]) with a time constant of 0.15 seconds to increase pressure resolution for LOOPEDIT
• LOOPEDIT: Mark scans where the CTD is moving less than the minimum velocity (0.25 m/s) or traveling backward due to ship roll.
• BINAVG: Average data into 1 or 2 db pressure bins to match bottle-calibrated salinity files (when available)
• SPLIT (u or d): Split .cnv file into upcast and downcast files. Files are appended automatically with leading u or d

Post-processing SBE oxygen calibrations 

To produce the best possible oxygen measurements, post-processing procedures were modeled after methods described in Uchida et al. 2010. The following Seabird-recommended calibration equation was used to calibrate the SBE-43 oxygen sensor data:

O2= SOC(V+ Voff)* Oxysol(T,S)*(1 + A*T + B*T2 + C*T3) * eE*p/(273.15 +T) (Equation 1)

where O2 is the CTD oxygen [μmol/kg], V is the output voltage signal processed with the SBE default hysteresis correction [volts], Oxysol is the oxygen saturation [μmol/kg], T is temperature [deg C], S is salinity [psu], and P is pressure [dbar] (SBE Application Note 64-2). Coefficients for the oxygen calibration slope (SOC); voltage offset at 0 (Voff); temperature-related calibration coefficients of A, B and C; and pressure-related E term were determined initially from an 18-point factory calibration and provided by the manufacturer (Appendix A). Voff, A, B, and C are constant over the sensor life while values for SOC and E can be optimized using discrete water samples measured analytically for dissolved oxygen, and commonly referred to as Winklers. 

Calibration coefficients for SOC and E were optimized by applying a non-linear least-squares fit to Winkler samples while calibration coefficients for A, B, C, and Voff were held constant. Oxygen sensor hysteresis due to pressure effects on the sensor membrane was improved by enabling the Seabird default hysteresis correction (Edwards et al. 2010; SBE Application Note 64-3). The optional response time correction, or tau correction, was determined to add excessive signal noise in relatively stable, deep portions of the water column; and, therefore, was not applied in the calibration equation.

While using this approach, a non-linear functional fit of Equation 1 was first attempted using one set of coefficients for the entire data set (whole cruise). Model fits were iterative with outliers discarded. Outliers were determined as values more than three scaled median absolute deviations from the median.

With outliers removed, residuals between CTD values and water sample values were then examined as a function of pressure, temperature, oxygen concentration, and cast number (≈ cruise time). An examination of the residuals as a function of cast number was used to 1) identify episodic events resulting in abrupt changes in SOC values, and 2) determine potential drift in SOC over the course of the cruise. After examining the residuals, cast numbers were grouped if necessary to minimize the residuals with an attempt to limit the number of groups used per cruise. New calibration coefficients were then determined for each group.

If a linear drift as a function of cast number (cn) or cruise time (dt) was determined for a group, a linear correction of the SOC drift was applied to the group while keeping other coefficients constant. The linear drift as a function of cast number/cruise time was then incorporated into the calibration equation, replacing SOC in Equation 1 as a function of cruise number/cruise time as:

SOCcn/dt = SOC1 + F*cn/dt            (Equation 2)

where cn or dt is the cast number [-] or time since first cast [d], respectively. SOC1 is the initial SOC value, and F is the rate of SOC change per cast number or day since first cast.  

A nonlinear functional fit including the cast or time-dependent SOCcn/dt was then fit to the group determining coefficients for E, SOC1, and F while holding A, B, C, and Voff constant. After iterative fitting and outliers removed, residuals were examined again as a function of pressure, temperature, oxygen concentration and cast number to ensure no linear dependence of residuals as a function of time. Outliers were determined as values more than three scaled median absolute deviations from the median.

For each cruise, CTD-derived measurements of salinity and oxygen solubility were calculated using the TEOS-10 Gibbs-SeaWater Oceanographic Toolbox (McDougall and Barker, 2011). Measurements from primary CTD sensors recorded in line with the SBE43 oxygen sensors were used to calculate oxygen concentrations unless collected data was poor. In these instances, measurements from the secondary CTD sensors were used. Bottle-calibrated CTD salinity measurements were used to produce oxygen concentrations when available (Years 5 - 9). The measurements of temperature, salinity, pressure and oxygen voltage (processed with SBE default hysteresis correction) used to produce oxygen concentration profiles are included in files for downcasts (indicated with ‘d’ appended) or upcasts (indicated with ‘u’ appended).  

Throughout this documentation, oxygen profiles calibrated using a constant SOC value are indicated by SOCk while oxygen profiles calibrated using an SOC term that varies with time or cast (station) number are indicated by SOCdt and SOCcn, respectively. Lastly, oxygen sensor gain is determined as the Winkler-determined SOC value over the factory-determined SOC to assess changes in oxygen calibration slope since factory calibration. Note, the manufacturer recommends factory service inspection and calibration for a SBE43 DO sensor with a gain correction greater than 1.2 from the original factory value (Application Note 64-2).  

Post-processing Aanderaa oxygen calibrations

Oxygen concentration was calculated from raw Aanderaa oxygen optode output in volts following the methods detailed in Bittig et al. (2018). Raw voltage outputs are converted to calphase, which is used to calculate oxygen concentration using the optode-specific calibration coefficients from factory calibration, the Stern Volmer equation, and compensation equations to correct for salinity and pressure effects. The measurements of temperature, salinity (bottle-calibrated), pressure, and raw Aanderaa voltage used to produce oxygen concentration profiles are included in files for downcasts (indicated with ‘d’ appended) or upcasts (indicated with ‘u’ appended).

Aanderaa optodes are not routinely used for profiling applications on shipboard CTD casts, since oxygen optodes have a significantly slower response time than Clark electrodes such as the SBE43. This complicates or precludes the ability to resolve strong oxygen gradients at the speed at which CTD rosettes are routinely raised and lowered. Aanderaa optode data were used in place of SBE43 oxygen sensor on cruises where the SBE43 was malfunctioning (AR35-05; August 2019) or was unable to be calibrated with shipboard discrete oxygen measurements (AR46; August 2020). The time lag was quantified by empirically determining the optode’s temperature-dependent boundary layer thickness based on paired upwards and downwards profiles (Gordon et al. 2020) and corrected for following the methods of Bittig and Kortzinger (2017), using the optode-response-time code provided by Gordon et al.

Bottle dissolved oxygen samples were used to calculate a gain correction for the Aanderaa optode after all other corrections were applied. Because of the optode’s inability to resolve strong oxygen gradients at CTD profiling speeds, only Winkler samples collected in relatively stable sections of the water column (below 500 m) were used to determine the Aanderaa optode gain correction.

Irminger Sea 1 Deployment Cruise KN221-04

A complete cruise report and results of water sample analyses for KN221-04 are available on Alfresco. A total of nine CTD casts were performed on-board the R/V Knorr. Bottle-calibrated CTD salinity measurements are not available for this cruise; however, the shipboard measurements of salinity were used to validate the CTD-derived salinity measurements. The oxygen sensor was switched from SN 1646 to SN 1679 prior to Station 2. The hysteresis between upcast and downcast oxygen data due to sensor response time was removed by advancing the oxygen sensor data 4 seconds relative to the pressure sensor data. Data from the primary CTD sensor package was used to determine SBE oxygen concentrations. 

Oxygen calibration Results

Calibration coefficients for SOC and E in Equation 1 were attempted using a non-linear least squares fit between CTD oxygen values and 44 Winkler samples from Casts 2-9. The model fit using all Winklers was poor with a RMSE of 9.95 μmol/kg, R2 value of -0.543 and no outliers determined. An inspection of residuals (Winkler - SOCk model) determined a negative correlation between residuals and CTD DO concentrations. Negative correlation between residuals and CTD DO concentrations in conjunction with the determined high oxygen sensor gain value of 1.368 suggests that fouling of the oxygen sensor membrane greatly impacted the oxygen data collected. In order to salvage interpretable oxygen data from this cruise, a non-linear functional fit of the calibration equation was performed using 4 Winkler samples collected at Station 4 (RMSE = 2.98 μmol/kg, R2 = 0.922, n = 4). The calibration coefficients determined from Station 6 were then applied to data collected at Stations 2-9. 

Irminger Sea 2 Deployment Cruise AT30-01

A complete cruise report and results of water sample analyses for AT30-01 are available on Alfresco. A total of 13 CTD casts were performed on-board the R/V Atlantis. Bottle-calibrated CTD salinity measurements are not available for this cruise; however, the shipboard measurements of salinity were used to validate the CTD-derived salinity measurements. The hysteresis between upcast and downcast oxygen data due to sensor response time was removed by advancing the oxygen sensor (SN 0712) data 8 seconds relative to the pressure sensor data. Data from the primary CTD sensor package was used to determine SBE oxygen concentrations.     

Oxygen calibration Results

Calibration coefficients for SOC and E in Equation 1 were determined using a non-linear least squares fit between CTD oxygen values and 106 Winkler samples from Station 1-13. Residuals (Winkler - SOCk model) were found to be linearly dependent on station number. Application of the non-linear regression model using a station-dependent SOC term (Equation 2) was found to further minimize the residuals between Winkler oxygen and CTD oxygen. No relationship between residuals (Winkler- SOCcn model) and pressure, temperature, station or oxygen concentration remained. The SOCcn model (RMSE = 1.03 μmol/kg, R2 = 0.999, n = 106) flagged 1 Winkler sample (0.94%) as an outlier.See the supplemental data docs for the summary of the Winkler-optimized calibration coefficients and model results.

Irminger Sea 3 Deployment Cruise AR07-01

A complete cruise report and results of water sample analyses for AR07-01 are available on Alfresco. A total of 10 CTD casts were performed on-board the R/V Armstrong. Bottle-calibrated CTD salinity measurements are not available for this cruise; however, the shipboard measurements of salinity were used to validate the CTD-derived salinity measurements. The hysteresis between upcast and downcast oxygen data due to sensor response time was removed by advancing the oxygen sensor (SN 0338) data 4 seconds relative to the pressure sensor data. Data from the primary CTD sensor package was used to determine SBE oxygen concentrations. 

Oxygen calibration Results

Calibration coefficients for SOC and E in Equation 1 were determined using a non-linear least squares fit between CTD oxygen values and 69 Winkler samples from Station 5-10. Residuals (Winkler - SOCk model) were minimized by creating two groups. Group 1 consisted of Stations 5 and 6 and Group 2 consisted of Stations 7- 10. Residuals in both groups were found to be linearly dependent with cruise time. Application of the non-linear regression model using a time-dependent SOC term (Equation 2) was found to further minimize the residuals between Winkler oxygen and CTD oxygen. No relationship between residuals (Winkler- SOCdt model) and pressure, temperature, station or oxygen concentration remained with the SOCdt model fit. The SOCdt model for Group 1 (RMSE = 1.3 μmol/kg, R2 = 0.956, n = 23) flagged no outliers.The SOCdt model for Group 2 (RMSE = 0.806 μmol/kg, R2 = 0.994, n = 46) flagged 2 Winklers (4.3%) as outliers. See the supplemental data docs for the summary of the Winkler-optimized calibration coefficients and model results.

Irminger Sea 4 Deployment Cruise AR21

A complete cruise report and results of water sample analyses for AR21 are available on Alfresco. A total of 12 CTD casts were performed on-board the R/V Armstrong. Salinity and oxygen data at Stations 1-8 experienced issues consistent with clogging of the CTD pump. The primary CTD package pump was switched prior to Station 9. Bottle-calibrated CTD salinity measurements are not available for this cruise; however, the shipboard measurements of salinity were used to validate the CTD-derived salinity measurements. The hysteresis between upcast and downcast oxygen data due to sensor response time was minimized by not advancing the oxygen sensor (SN 0930) data relative to the pressure sensor data. Data from the primary CTD sensor package was used to determine SBE oxygen concentrations for Casts 9 –12.

Oxygen calibration Results

Calibration coefficients for SOC and E in Equation 1 were determined using a non-linear least square fit between CTD oxygen values and the 35 Winkler samples collected at Stations 10-12. Residuals (Winkler - SOCk model) determined using a constant SOC revealed no relationship between residuals and pressure, temperature, station or oxygen concentration. The SOCk model (RMSE = 0.968 μmol/kg, R2 = 0.988, n = 35) flagged 2 Winkler samples (5.7%) as outliers.See the supplemental data docs for the summary of the Winkler-optimized calibration coefficients and model results.

Irminger Sea 5 Deployment Cruise AR30-03

A complete cruise report and results of water sample analyses for AR30-03 are available on Alfresco. A total of 23 CTD casts were performed on-board the R/V Armstrong. CTD-derived salinity data were calibrated by Leah McRaven using discrete bottle samples, and results are summarized in the AR30-03 CTD Calibration Report. The secondary CTD package is recommended for calibration use on this cruise; therefore, the temperature, bottle-calibrated salinity and depth data from the secondary CTD package were used in the oxygen sensor calibration equation to calibrate SBE43 (SN 0113) oxygen sensor data. The hysteresis between upcast and downcast oxygen data due to sensor response time was removed by advancing the oxygen sensor data 6 seconds relative to the pressure sensor data.  

Oxygen calibration Results

Calibration coefficients for SOC and E in Equation 1 were first attempted using a non-linear least squares fit between CTD oxygen values and all 271 Winkler samples. Examination of residuals (Winkler - SOCk model) revealed higher residuals associated with Casts 10 and 21, perhaps due to sensor fouling. Therefore, calibration coefficients for SOC and E in Equation 1 were determined for all casts but 10 and 21 using a non-linear least squares fit between CTD oxygen values and 179 Winkler samples while calibration coefficients for A, B, C, and Voff were held constant at their factory-determined values. The SOCk model (RMSE = 1.31 μmol/kg, R2 = 0.989, n = 172) flagged 7 Winkler samples (4.1%) as outliers. 

Calibration coefficients for SOC were then determined for casts 10 and 21 (both < 1000 m) while using pressure-associated E values determined from Casts 1-9, 11-20, 22-23 (included samples greater than 1000 m). The SOCk model for Cast 10 (RMSE = 0.62 μmol/kg , R2 = 0.998, n =42 ) flagged 10 Winkler samples (19.2%) as outliers, while the SOCk model for Cast 21 (RMSE = 0.80 μmol/kg, R2 = 0.996 , n = 34 ) flagged 6 Winkler samples (15.0%) as outliers. After optimizing calibration coefficients, no relationship between residuals (Winkler- SOCk model) and pressure, temperature, station or oxygen concentration remained. See the supplemental data docs for the summary of the Winkler-optimized calibration coefficients and model results.

Irminger Sea 6 Deployment Cruise AR35-05

A complete cruise report and results of water sample analyses for AR35-05 are available on Alfresco. A number of issues affected the quality of CTD-measured hydrographic and oxygen data on this cruise. Briefly, major biofouling issues were observed for all primary CTD sensors (temperature, salinity, and oxygen) resulting in no usable CTD-DO oxygen measurements for AR35-05. Therefore, an Aanderaa optode (SN 502) was integrated into the CTD rosette prior to Station 3. Additionally, failure of the pylon on the CTD rosette resulted in no bottle samples fit for calibration purposes prior to Station 11. Salinity data collected by the secondary CTD package were bottle calibrated for Stations 11-13 by Leah McRaven and results are summarized in AR35-05 CTD Calibration Report. Temperature, salinity, and pressure data collected by the secondary CTD package were used to calculate Aanderaa optode oxygen concentrations on this cruise for Stations 3-12.

Oxygen calibration Results

Oxygen concentrations were calculated from raw Aanderaa oxygen optode output in volts following the methods detailed in Bittig et al. (2018). Raw voltage outputs were converted to oxygen concentration using the optode-specific calibration coefficients from factory calibration, the Stern Volmer equation, and compensation equations to correct for salinity and pressure effects. Using the approach of Gordon et al. (2020), paired downcasts and upcasts were used to determine and then correct an optode time lag of 13 sec at a reference temperature of 4℃. A total of 55 Winklers (sampled at greater than 500 m) were used to calculate a gain correction of 1.0023 for the Aanderaa optode (SN 502) integrated into the CTD rosette. After applying the gain correction, absolute bias between Winklers and Aanderaa DO was 1.1 ± 0.96 μmol/kg (n = 55). Residuals (Winkler - Aanderaa DO) showed no relationship with pressure, temperature, station or oxygen concentration.        

Irminger Sea 7 Deployment Cruise AR46

A complete cruise report and results of water sample analyses for AR46 are available on Alfresco. A total of 19 CTD casts were performed on-board the R/V Armstrong. CTD-derived salinity data were calibrated by Leah McRaven using discrete bottle samples, and results are summarized in the AR46 CTD Calibration Report. Temperature, bottle-calibrated salinity and depth data from the primary CTD package were used to calculate oxygen concentrations at Stations 1-10 and 18-19 while bottle-calibrated data from the secondary CTD package were used to calculate oxygen concentrations at Stations 12-17 . 

The COVID-19 pandemic presented a number of challenges for sea-going operations in 2020. Traditionally, Winkler measurements are made at sea by trained technicians; however, in 2020 discrete samples for oxygen were collected and stored until subsequent analysis back on land to minimize shipboard personnel. The storage of discrete oxygen samples according to methods described by Zhang et al. (2002) allowed for quality measurements from the previous cruise (AR45) but not from AR46. As a result, there are no discrete oxygen measurements for calibration purposes for AR46. Additionally, the SBE43 oxygen sensor was changed between AR45 and AR46 excluding the possibility of using the SBE43 oxygen calibration coefficients determined on AR45 on oxygen data collected on AR46. However, the same Aanderaa optode (SN 277) was integrated into the CTD rosette for both AR45 and AR46. Aanderaa optodes are relatively stable in their calibration, allowing the gain correction determined on AR45 to be applied to optode data collected during AR46. 

Oxygen calibration Results

Oxygen concentrations were calculated from raw Aanderaa oxygen optode output in volts following the methods detailed in Bittig et al. (2018). Raw voltage outputs were converted to oxygen concentration using the optode-specific calibration coefficients from factory calibration, the Stern Volmer equation, and compensation equations to correct for salinity and pressure effects. Using the approach of Gordon et al. (2020), paired downcasts and upcasts were used to determine and then correct an optode time lag of 13.1 sec at a reference temperature of 4℃ 

A total of 17 Winklers (sampled at greater than 500 m) from AR45 were used to calculate a gain correction for the Aanderaa optode (SN 277) integrated into the CTD rosette. Upon inspection, residuals (Winklers - Aanderaa DO) revealed a linear relationship between residuals and pressure using the standard pressure correction coefficient of 0.032 (Uchida et al. 2008). The pressure correction coefficient was then adjusted to 0.027 in order to remove the residual relationship with pressure, and resulted in a gain correction of 1.1386. After application of the gain correction, absolute bias between Winklers and Aanderaa DO was 0.74 ± 0.58 μmol/kg (n = 17) for AR45. The gain and adjusted pressure correction value D determined during AR45 were then applied to optode data collected during AR46.          

Irminger Sea 8 Deployment Cruise AR60-01

A complete cruise report and results of water sample analyses for AR60-01 are available on Alfresco. A total of 12 CTD casts were performed on-board the R/V Armstrong. CTD-derived salinity data were calibrated by Leah McRaven using discrete bottle samples, and results are summarized in the AR60-01 CTD Calibration Report. This temperature, bottle-calibrated salinity and depth data from the primary CTD package were used in the oxygen sensor calibration equation to calibrate SBE43 (SN 0444) oxygen sensor data from this cruise. The hysteresis between upcast and downcast oxygen data due to sensor response time was removed by advancing the oxygen sensor data 8 seconds relative to the pressure sensor data.  

Oxygen calibration Results

Calibration coefficients for SOC and E in Equation 1 were attempted using a non-linear least square fit between CTD oxygen values and the 134 Winkler samples collected (RMSE = 5.85 μmol/kg, R2 = 0.906, n = 79). Residuals were relatively large and more than half the Winklers were flagged as outliers. Examination of Winklers and CTD-measured oxygen values revealed issues with Winklers collected from casts 5 -12. Therefore, calibration coefficients for SOC and E in Equation 1 were determined using a non-linear least squares fit between CTD oxygen values and the 30 Winkler samples from casts 2 and 4. Residuals (Winkler - SOCk model) determined using a constant SOC revealed no relationship between residuals and pressure, temperature, station or oxygen concentration for the two stations used. The SOCk model (RMSE = 1.41 μmol/kg, R2 = 0.997, n = 23) flagged 7 Winkler samples (23.3%) as outliersSee the supplemental data docs for the summary of the Winkler-optimized calibration coefficients and model results. These calibration coefficients were applied to CTD oxygen data collected at Stations 1 through 12.  

Irminger Sea 9 Deployment Cruise AR69-01

A complete cruise report and results of water sample analyses for AR69-01 are available on Alfresco. A total of 23 CTD casts were performed on-board the R/V Armstrong. The CTD experienced hardware issues that were sorted prior to Cast 6. CTD-derived salinity data were calibrated by Leah McRaven using discrete bottle samples, and results are summarized in the AR69-01 CTD Calibration Report (in prep). Bottle-calibrated salinity data from the primary CTD package were used in the oxygen sensor calibration equation to calibrate SBE43 (SN 1960) oxygen sensor data from this cruise. The hysteresis between upcast and downcast oxygen data due to sensor response time was removed by advancing the oxygen sensor data 6 seconds relative to the pressure sensor data.    

Oxygen calibration Results

Calibration coefficients for SOC and E in Equation 1 were determined using a non-linear least squares fit between CTD oxygen values and 157 Winkler samples while calibration coefficients for A, B, C, and Voff were held constant at their factory-determined values (Appendix A). Residuals (Winkler - SOCk model) determined using a constant SOC were found to be linearly dependent on station number. Application of the non-linear regression model using a time-dependent SOC term (Equation 2) was found to further minimize the residuals between Winkler oxygen and CTD oxygen, and no relationship between residuals (Winkler- SOCdt model) and pressure, temperature, station or oxygen concentration remained. The SOCdt model (RMSE = 0.94 μmol/kg, R2 = 0.994) flagged 12 Winkler samples (7.6%) as outliers. See the supplemental data docs for the summary of the Winkler-optimized calibration coefficients and model results.

Problems/Issues

Quality flags applied to this dataset follow the recommendations of Jiang et al. 2022:

    1 = not evaluated/quality unknown
    2 = acceptable
    3 = questionable
    4 = known bad
    6 = median of replicates
    9 = Missing value

Year 1 KN221-04: Use of CTD oxygen data data from KN221-04 is generally poor due to suspected fouling and should be approached with caution. An oxygen sensor gain of 1.3675 was determined for this cruise; SBE recommends sensor maintenance for oxygen sensor gains greater than 1.2. All oxygen data has been flagged as questionable with the high gain correction likely resulting in oxygen concentrations that are biased low. For discrete water samples, there are discrepancies between bottle numbers (Niskin bottles) created by the SBE data processing software and bottle numbers (Niskin bottles) associated with shipboard Winkler analysis in OOI Alfresco datasheets. 

Year 2 AT30-01: None

Year 3 AR07-01: Oxygen data collected at Stations 1-4 were not calibrated since casts at Stations 1-4 measured a very different water mass in the Western Labrador Sea, and these casts were not associated with any discrete oxygen samples. Surface oxygen measurements in the upper ~50 m of the downcast profile at Station 10 have been flagged as questionable.

Year 4 AR21: Oxygen and salinity sensors experienced issues consistent with clogging of the pump at Stations 1-8 and data was not salvageable. The pump was changed prior to Station 9. An oxygen sensor gain of 1.2218 was determined for this cruise and applied to Stations 9 -12, which is slightly over the recommended sensor maintenance gain value of 1.2.

Year 5 AR30-03: CTD data collected at Stations 7-12 experienced issues associated with biofouling. Portions of Casts from stations 7-8 and 11-12 have been flagged as questionable as they exhibit behavior indicative of biofouling.

Year 6 AR35-05: Oxygen profiles were collected using an Aanderaa optode because of issues with the SBE43-DO sensor. All data has been flagged as questionable because of the inability for Aanderaa optodes to resolve strong oxygen gradients at CTD rosette profiling speeds. A broken pylon resulted in no discrete water samples for calibration prior to Station 11. 

Year 7 AR46: COVID-affected cruise. No discrete oxygen measurements for AR46. Oxygen profiles from AR46 were collected using an Aanderaa optode that was calibrated during the previous cruise (AR45) using water samples that were stored and not measured onboard. All data has been flagged as questionable because of these issues and also because of the inability for Aanderaa optodes to resolve strong oxygen gradients at CTD rosette profiling speeds.

Year 8 AR60-01: Examination of Winklers and CTD-measured oxygen values revealed issues with Winklers collected from casts 5 -12. Residuals from casts 5-12 were relatively large and more than half of the Winklers were flagged as outliers. Also, oxygen data collected on the upcast from Cast 10 between ~750 - 920 db is consistent with pump clogging and has been flagged as questionable.
Year 9 AR69-01: None