Acceleration of ocean warming, salinification, deoxygenation and acidification in the surface subtropical North Atlantic Ocean

Sampling methods

Ocean sampling at hydrostation S and BATS started in 1954 and 1988, respectively. Hydrostation S began with the pioneering efforts of Hank Stommel (Woods Hole Oceanographic Institution) and colleagues⁵⁰ at a site approximately 26 km southeast of Bermuda (32° 10′N, 64° 30′W, Fig. 1). The first water–column sampling occurred on the 7th June 1954 from the 61′ R.V. Panularis with more than 1381 cruises conducted up to the present time from the R.V. Panularis II (1967–1983), R.V. Weatherbird (1983–1989); R.V. Weatherbird II (1989–2006) and R.V. Atlantic Explorer (2006–present).

Since the arrival of the R.V. Weatherbird, Hydrostation S has been occupied at near biweekly intervals, with multiple CTD–hydrocasts through the water-column to ~2600 m, and to 4500 m at BATS (more than 450 cruises to the site). Nansen bottles were used for water sampling at first, then 5 L Niskin samplers until October 1988. Thereafter sampling has been conducted with a Seabird 9/11 CTD equipped with 12 L Niskin and Ocean Test Equipment (OTE) samplers.

Water sampling, temperature and CTD measurements

The sampling format has remained substantially consistent for the past 65 years, but with the introduction of CTD–hydrocast sampling in October 1988. From 1954 to 1988, reversing mercury thermometers were used for measurements of temperature until replaced by CTD measurements using a Sea-Bird 9/11 system. Bottle samples before 2012 were taken down to 2600 m at hydrostation S. Since the addition of reliable altimeters to the CTD package, sampling was extended to full ocean depth at both the hydrostation S (~3400 m) and BATS (~4500 m) sites. Numerous sensor configurations have been used on the CTD package (e.g., dual temperature, dual conductivity, dual DO sensors, transmissometer, fluorometer, PAR and altimeter). In contrast, the CTD sampling system has predominately been a Seabird 24-place rosette using 12 L Ocean Test bottles. Before profiling, the CTD is allowed to stabilise at 10 m and once stable, the CTD returns to the surface to start the profile with typical descent rates of 0.5–1.0 m s⁻¹, depending on weather conditions. Water samples are collected on the upcast, whereby the OTE bottles are closed at the target depth after a waiting period of 45 s. The CTD is held at the target depth for another 10 s to allow the SBE35-RT sensor to take an 8 s average. The CTD continues with the upcast at an ascent rate of 0.7–1.0 m s⁻¹. Temperature, conductivity and DO sensors are routinely returned to SeaBird every 6–9 months for routine calibration. The differences between primary and secondary temperature sensors in the deep ocean at BATS (>3000 m) were 0.002–0.006 °C regardless of time since most recent factory calibration.

Determination of salinity

Salinity samples are typically taken from the OTE bottles at all depths. These samples are collected immediately following DO and CO₂ sampling. Samples are taken in 125–250 ml borosilicate glass bottles (Ocean Scientific, UK) that use plastic thimbles to form a better seal. The sample remaining from the previous use is left in the bottles between cruises to prevent salt crystal buildup due to evaporation. When drawing a new sample, the old sample is first discarded over the sampling spigot, and the bottle is rinsed three times with water from the new sample. The bottle is then filled to the shoulder with the sample, and the thimble inserted into the container. The neck of the bottle and the inside of the cap are dried, then the thimble is inserted, and the cap is replaced and firmly tightened. These samples are stored in a temperature-controlled laboratory for later analysis (typically within 1–2 weeks of their collection).

Salinity measurements have been made with a Guildline salinometer at BIOS from 1981 to present (calibrated with IAPSO standard water⁵¹) for both BATS and Hydrostation S samples. At present, samples for salinity are analysed on a Guildline Autosal 8400B laboratory salinometer using the manufacturer’s recommended techniques. All readings (10 s average) are taken using the Ocean Scientific interface box and PC software. The salinometer drift during and between successive runs tends to be zero (room temperature carefully controlled and monitored). Bottle salinities are used to calibrate the profiling CTD SBE-04 sensors, and additionally, they are also compared with the downcast CTD profiles to search for possible outliers. Deep-water samples (>2000 m) are replicated for precision estimates (typically < 0.002 salinity units).

Determination of DO

Early samples for DO were analysed by manual endpoint detection (1954–1988) though more recently, automated titration systems have been used (1988–present). On each hydrostation S and BATS cruise, the first samples from the rosette are taken for DO, immediately following the opening of the valve on the OTE bottle and confirmation that the OTE bottle has not leaked. The sample flasks used for this measurement are Pyrex iodine determination flasks of 140 ml nominal capacity with ground glass barrel stoppers. The precise volume of each stopper/bottle pair is determined gravimetrically.

Seawater is carefully sampled to avoid the introduction of air bubbles with a minimum of five bottle volumes overflow. The samples are then immediately fixed with 1 ml of manganous chloride and 1 ml of sodium iodide/sodium hydroxide solution. With the stopper in place, and after vigorously shaking, the necks of the flasks are sealed with surface seawater. The samples are stored upright in the dark at 21–24 °C. The temperature of the water from the OTE bottle is measured to allow the conversion to units of mass.

The samples are analysed after 6–8 h, based on the method proposed by Winkler (1888; modified by Strickland and Parsons 1968⁵²). For this analysis, BIOS/BATS currently uses an automated temperature-controlled titration system (developed by SIO) using an ultra-violet light endpoint detection system with a Metrohm 665 Dosimat burette for precise delivery of the sodium thiosulfate (0.18 M: reagent grade).

Before running the samples, the precise concentration of the thiosulfate and the chemical blanks are determined. Typically, six to eight standards are run for determination of a mean value. Blanks are prepared in the same manner as the standards. However, they are based on the 1 ml standard addition of KIO₃. The samples are run following the sodium thiosulphate normality determination. The water in the neck of the sample is carefully removed, taking care to avoid disturbing the precipitate. The stopper is then removed, and 1 ml of 50% sulphuric acid added slowly, then a stir bar, and the sample titrated immediately. A 30% triplication is performed for estimates of precision (typically < 0.4 µmoles kg⁻¹; ~0.1%).

Determination of DIC and TA

The description of seawater CO₂–carbonate chemistry used in this paper follows long-established knowledge of the marine carbon cycle^53,55,55. In this paper, DIC and TA were directly determined, and the chemical description of DIC is as follows^53,54:

DIC = [CO₂*] + [HCO₃⁻] + [CO₃²⁻] [equation S1]

The term [CO₂*] denotes the summed concentration of dissolved H₂CO₃ and CO₂. The chemical description of alkalinity of seawater (TA) is defined^53,54 as:

TA = [HCO₃⁻] + 2[CO₃²⁻] + [B(OH)₄⁻] + [OH⁻] + [HPO₄²⁻] + 2[PO₄³⁻] + [SiO(OH)₃⁻] + [HS⁻] + [NH₃] + minor species − [H⁺] − [HSO₄⁻] − [HF] − [H₃PO₄] − minor species [equation S2].

In this second chemical definition, the summed concentrations of [HCO₃⁻] + 2[CO₃²⁻] + [B(OH)₄⁻] represent the major chemical components of TA and the proton/charge balance of seawater. Other chemical contributors to alkalinity are typically considered minor constituents in seawater^54,56,56. Given DIC and TA are both expressed as µmoles kg⁻¹ following long-established guidelines (Dickson et al., 2007⁵⁴ and references therein).

Initially, at hydrostation S, samples for DIC and TA were collected into 1 l Pyrex bottles with a change to 500 ml bottles in the early 1990s, poisoned with Hg₂Cl, sealed with ground glass stoppers and then shipped to Scripps Institution of Oceanography (SIO) for analysis^56,57 (1983–1988). Storage time for samples before analysis ranged from a few months to several years. Similar sampling protocols were established at BIOS for sampling at the BATS^36,58,59, but in the early 2000s, smaller Pyrex bottles (350 ml) were used. Samples for DIC and TA have been typically analysed within a few months of collection at BIOS since 1988.

The sampling frequency of the combined dataset from hydrostation S and BATS was not uniform in time. In the 1980s, samples were collected 9–12 times a year, while since 1992, sampling increased to 14–15 times a year^7,40. The increase in sampling since the middle 1990s was due to supplemental BATS bloom cruises (1–4 in number) conducted during the January to April period in addition to BATS core cruises. The increase in sampling frequency over time weights the time-series to springtime when determining non-seasonally aliased trend^7,40.

Potentiometric titration methods were also used for determination of TA at BIOS⁵⁸. At the beginning of the 1990s, a manual alkalinity titrator was used for determination of TA at BIOS. This was replaced by an automated VINDTA 2S (Versatile Instrument for the Determination of Titration Alkalinity) in the early 2000s⁴⁰. For both manual and automated TA systems, 15–20 titration points past the carbonic acid endpoint were determined for each sample, with TA computed from these titration data using nonlinear least-squares methods⁵⁴. Surface samples of Sargasso Sea water were also analysed each day before sample analyses, and certified reference materials (CRMs) were used routinely to calibrate the TA measurements.

At BIOS, DIC was determined using coulometric methods with a SOMMA system^7,40,54. During the first 2 years of sampling, DIC samples were analysed at WHOI (e.g., BATS cruise 1–21), and subsequently at BIOS. DIC measurements were calibrated with known volumes of pure CO₂ gas while CRM’s⁵ were routinely analysed each day of analysis from 1991. Hydrostation S samples were analysed for DIC at SIO from 1983 to 1988 using manometric methods⁵⁶. Potentiometric titration methods were used for determination of TA⁵⁶ at SIO, and after that at BIOS. No significant bias has been noted between these earlier methods (also analysed by A.G. Dickson to present)⁴⁰. Analytical precision for DIC and TA at BIOS was typically <0.03% and <0.05%, respectively for within the bottle and between bottle replicate analyses of more than 5,000 samples⁴⁰. Analytical accuracy for both DIC and TA using CRM analyses is better than 0.1%.

Seawater Co₂–carbonate chemistry computations and errors

The suite of seawater CO₂–carbonate parameters (i.e., [CO₂], [H₂CO₃], [HCO₃⁻], [CO₃²⁻], [H⁺]) for each sample can be determined from the measurement of any two parameters (i.e., DIC, TA, fCO₂, pCO₂ and pH), along with temperature and salinity^53,54,55 Seawater fCO₂ (fugacity of CO₂) is used here rather than the partial pressure of CO₂ (i.e., pCO₂; note that this parameter was also computed in data files), and with units of µatm. pH, Ω_calcite, _{c a} and Ω_aragonite in seawater are dimensionless and pH expressed on the total scale⁵⁴. All parameters are computed at in situ temperature and salinity.

Here, seawater fCO₂, pH, [CO₃²⁻], mineral saturation states for Ω_calcite and Ω_aragonite, the Revelle factor (β) were computed from DIC, TA, temperature and salinity data using the programme CO₂calc⁶⁰. Carbonic acid dissociation constants (i.e., pK₁ and pK₂)⁶¹, as refit by Dickson and Millero (1987)⁶² were used for the computation, as well as dissociation constants for KHSO₄⁻⁶³, borate⁶⁴. The computation error estimates for calculation of pCO₂, pH, Ω_calcite and Ω_aragonite were undertaken using standard procedures for the propagation of uncertainty⁵⁴. Given the analytical uncertainty of the two seawater carbonate chemistry parameters (i.e., DIC and TA; ± 1 µmoles kg⁻¹) from replicate measurements of CRM’s, the error of the computation is estimated at 3 µatm, 0.003, 0.018, and 0.012, for pCO₂, pH, and Ω_calcite and Ω_aragonite for surface waters, respectively⁶⁵.

Use of the tracer TrOCA

TrOCA and TA is a method first proposed by Touratier and Goyet (2004a, b)^66,67, and updated by Touratier et al. (2007)²⁹ to quantify anthropogenic carbon (i.e., C_ANT) in seawater. The quasi-conservative tracer, TrOCA in water masses accounts for changes in carbonate chemistry due to biological influences (e.g., remineralization of organic matter) and abiotic processes (e.g., dissolution of CaCO₃), and as such, is also useful for following ocean chemistry changes in the surface ocean. Here, the formulation proposed by Touratier et al. (2007²⁹; their equation 11) is used as a water tracer:

C_ANT = ([O₂] + 1.279 x (DIC − TA/2) − EXP (7.511 − 1.087 × 10⁻² × Φ − 7.81 × 10⁵/TA²))/1.279)) [equation S3]

Φ is the potential temperature. The uncertainty of the estimate is ~6 µmoles kg^{−1 (29)}. The C_ANT estimates determined using the TrOCA method for the mixed layer are modified by euphotic zone biological activity and air–sea gas exchange²³, but, over time, provide additional evidence for changes in ocean chemistry.

Trend analyses

Trend analyses were conducted of the time-series of surface temperature and salinity, seawater carbonate chemistry (DIC, TA, pCO₂ and Revelle factor) and OA indicators (pH, [CO₃²⁻], Ω_calcite and Ω_aragonite). Here, trend analysis of salinity normalised DIC (nDIC), and TA (nTA) data were also made to account for local evaporation and precipitation changes. These data were normalised to salinity of 36.6, as this represents the mean salinity observed at the BATS site^37,40. Trend analysis was performed with observed data, and seasonally detrended data is given in Table 1) Statistics generated from least-squares regression approaches were slope, coefficients, error, multiple r, r², p value and n. Trends with p values greater than 0.01 were deemed statistically not significant at the 99% confidence level.

Trend analysis for the period from 1983 to 2020, and for each decade (i.e., the 1980s, 1990s, 2000s, 2010s) was determined using linear regression methods with physical and biogeochemical data are the dependent variable and time as independent, and p values < 0.05 used to determine statistical significance of trend. Trend analyses with observed data exhibit seasonal aliasing due to sampling weighting to spring conditions^12,13. To account for seasonal weighting, the data were also seasonally detrended⁶⁸. Seasonal detrending of the BATS/Hydrostation S data was accomplished by binning data into the appropriate month, with mean values calculated from two or more cruises conducted within a representative month each year. This provides a uniform time step of approximately 1 month (i.e., 365 or 366 days/12) throughout the time-series, thereby removing any potential seasonal weighting especially to springtime conditions. Secondly, a mean and standard deviation is then determined each month for the 1983–2020, and anomalies computed from monthly data minus mean values. Trends and regression statistics anomaly data determined from seasonally detrended data are given in Table 1. Removing any potentially seasonal weighting allowed trend analysis of data that had any non-temporal uniformity reduced as much as possible.

BATS and Hydrostation S data

The BATS and Hydrostation S sites have been sampled on a monthly and twice-monthly basis, respectively, since October 1988. The data is publicly and permanently available at http://bats.bios.edu/data/ with a transition to BCO-DMO (Biological and Chemical Oceanography Data Management Office; Woods Hole, USA). The data used are primarily fully processed CTD profiles and bottle data with both dataset available to the end of 2019. The surface data is also available as Supplemental Data accompanying this paper. Information regarding CTD processing and QC routines are detailed at http://bats.bios.edu/wp-content/uploads/2017/07/report_methods.pdf.

Source: Ecology - nature.com