Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Five locations were sampled once, only location 5 got sampled twice.

Samples for pH analysis were collected first in the rosette sampling sequence. Seawater samples were collected from the Niskin bottles directly into 10-cm glass cylindrical optical cells (~30 mL volume) using a section of silicone tubing (~15 cm long). One end of the silicone tubing was first attached to the nipple of the Niskin bottle. The nipple was pushed in to initiate flow, and the silicone tubing was squeezed to eliminate air bubbles. The other end of the silicone tubing was attached to the optical cell, which was agitated to eliminate any residual bubbles. After ~15 seconds of sample flow, the cell was capped at one end with a Teflon stopper. The silicone tubing was then detached from the optical cell, and, with the water still flowing, the other cap was rinsed and used to seal the optical cell. Samples collected this way are not exposed to the atmosphere, and each cell was flushed with at least three cell volumes of seawater. The samples were collected and taken into the lab, where the outside of the cell was rinsed with tap water to eliminate salt build-up. The cells were dried thoroughly, and the optical windows were cleaned with Kimwipes immediately before measurement. Samples were thermostatted at 25 ºC (±0.05 °C) in a custom-made, 36-position cell warmer.

Seawater samples for total alkalinity (AT) were collected directly after pH samples from the Niskin bottles into clear 300 mL BOD borosilicate glass (Fisher Scientific) bottles as described in Section 3.4 of Liu et al. (2015). 

Silicone tubing, connected directly to the Niskin, was inserted into the bottom of the bottle. Each bottle was rinsed three times with seawater with approximately one-third of the sample bottle volume. Sample bottles were overflowed for at least one complete volume (300 mL).  The silicone tubing was squeezed to eliminate air bubbles, and care was taken to ensure no air bubbles were trapped in the bottle. While filling the bottle, the sample temperature was measured (ts) using a Fisher Scientific Traceable Digital Thermometer (±0.05℃). Sample temperature is required to ascertain sample volume and convert it to sample mass. Once all bubbles dissipated, sample caps were rinsed, and the tubing, while water still flowed, was slowly removed, and the sample was sealed by inserting the bottle cap. Seawater in the flange area was eliminated, and the bottleneck area was dried with a Kimwipe. 

In the ship’s laboratory, the outside of each bottle was carefully rinsed with tap water, and the bottle surface was wiped clean with Kimwipes to remove any salt build-up. Samples were left to equilibrate to room temperature. Samples were not poisoned with HgCl2. When the samples warmed, seawater was forced into the flanged bottleneck. Care was taken to ensure no sample loss—the bottle was maintained upright, and water within the flanged neck was not wiped away as the sample warmed.

DIC and delta13C of DIC were analyzed with a Picarro Cavity Ring-Down Spectrometer (G2131-i) with Liaison autosampler; the detailed methodology is described in Subhas et al. (2015). These measurements were made on board the ship. Dickson seawater CRM was used as the standard for DIC; pre-weighed optical calcite powder was used as the standard for delta13C. Exetainer vials were pre-acidified and pre-weighed in the lab prior to the cruise. After the vials were filled with 3-5 mL of porewater and analyzed on the cruise, the stored vials were weighed again in the lab to obtain the sample mass. Results using this methodology were corrected by normalizing to measured values of Dickson seawater CRM. Uncertainty (1sigma) for replicate DIC and delta13C measurements were ±23 umol/kg and ±0.15 per mille (VPDB), respectively. DIC uncertainty was higher than reported in our previous studies (e.g., Subhas et al., 2015) likely due to mass determination: ship-board analysis necessitated weighing after analysis, but there was uncertainty in precisely how much sample mass may be removed during the analysis. Additionally, lower sample volume (5 mLs, instead of 7-8 mLs in our previous studies) may have added error.

The pH of each sample was determined on an Agilent 8453 spectrophotometer setup with a custom-made temperature-controlled cell holder. Only the tungsten lamp was turned on. The UV lamp was turned off to prevent photodegradation of organic matter in the samples by UV light. A custom macro program running on Agilent UV-Visible ChemStation Software Rev. B.04.01 was used to guide the measurements and data processing. The macro automated the procedures of sample input information, blank and sample scans, quality control, and data archiving. The quality control steps included checking the baseline shift after dye injection and monitoring the standard deviation of multiple scans. Absorbance blanks were taken for each sample, and 10 μL of purified m-cresol purple (10 mmol kg-1) was added for the analysis. pHT(total scale) was calculated according to Müller and Rehder (2018).

The salinity and temperature dependence is provided in Table 1 of Müller and Rehder (2018). The temperature and salinity dependence of e1 and e3/e2 are given in Eq. (6-7) of Muller and Rehder (2018). Sample temperature was then measured using a Fisher Scientific Traceable Digital Thermometer (±0.05℃). These equations are applicable for samples between temperature (278.15 ≤ T ≤ 308.15) and salinity (0 ≤ S ≤ 40). In all our measurements at sea, T ≈ 298.15 K. Water column salinity was measured using a Sea-Bird Electronics CTD sensor. Duplicate pH samples, collected from discrete samples taken from Niskin bottles (N=12), displayed a standard deviation of 0.001.

The automated spectrophotometric alkalinity system consists of a stir plate with an anchoring bottle holder and a Metrohm Dosimat 665. The bottle holder is configured so that optical fibers, connected either to an Ocean Optics LS-1 tungsten halogen light source with a blue filter or an Ocean Optics USB4000 spectrophotometer, are positioned about 1 cm above the bottom of the bottle (configuration shown in Liu et al. 2015, Fig 1). The spectrophotometer and Dosimat are connected to a portable computer equipped with custom software that controls acid delivery from the titrator and monitors absorbances. The software is programmed to control the rate and sequence of acid titration and record sample input, absorbance ratios, and acid volumes.  

DIC and delta13C of DIC were analyzed with a Picarro Cavity Ring-Down Spectrometer (G2131-i) with Liaison autosampler.

The absorbance ratio of the indicator dye used was adjusted to R = 1.6 (pH ∼ 7.9) in the laboratory before the cruise. Small changes in sample pH (measurement perturbations; Clayton and Byrne, 1993) are created by the addition of titrant to samples. Dye perturbation was previously quantified using samples collected from profiles over a range of pH values. For each perturbation determination, ∆pH was defined as ∆pH = pHfinal – pHinitial, where pHinitial is the total scale pH taken after a single titrant addition, and pHfinal is the total scale pH after a second titrant addition.

An equation developed using this perturbation data was used to correct pH measurements: 

                                                                                            (1.3)

where pH is the raw pHT measurement and pH0 is the perturbation-corrected pHT measurement.

All spectrophotometric pH measurements were tentatively flagged if the baseline shifted more than 0.002 absorbance units. A series of five spectra were averaged for each determination, and samples were rerun if the overall standard deviations were higher than 0.0004. This process was repeated until the standard deviation of multiple readings was within 0.0004. Absorbance values were saved so that the quality criteria could be evaluated in the future.

Perturbation-corrected pHT measurements were reported along with their associated quality-control flags. pHT was reported at the measurement temperature of approximately 25 ºC, with the exact measurement temperature specified as an average of temperature measurements at the beginning and end of each set of samples.

The spectrophotometric AT measurement titration procedure is outlined in Liu et al. (2015).

The sample bottle thermostatted to room temperature (∼24℃) was wiped clean and placed inside the bottle holder on the stir plate. The bottle cap was removed, and excess seawater that may have moved into the flanged bottleneck was rinsed into the sample bottle with a small amount (1–2 mL) of MilliQ water. A small stir bar was added to the bottle, and the acid tube from the Dosimat titrator was inserted just below the sample liquid’s surface to ensure that the HCl was well mixed with the seawater before reaching the light path at the lower part of the bottle. 

After a reference absorbance was recorded, 0.1 mL of 10 mM bromocresol purple BCP stock solution (R ∼ 0.3) was added and mixed. Acid was then delivered to the sample bottle, with the rate of acid addition being guided by continuous spectrophotometric monitoring of the seawater pH. The titrations were terminated at R = 0.070 (pHT ~ 4.2).  

A plastic syringe was used to remove ∼60 mL of the acidified sample so that the sample did not overflow and to decrease purging time. A fritted glass purging tube was placed into the bottle, positioning it above the light path, and the sample was purged of CO2 with N2 until R stabilized (i.e., ± 0.002 units in 1 min). The final solution temperature was measured and used with the absorbance ratio to calculate the final pHT of the sample.

Spectrophotometric measurements of AT were calculated using the following equation:

where [HCl]AV is the HCl concentration of the acid titrant in volumetric units, and VA is the volume of the added acid titrant (mL). 

MSW is the sample mass obtained from volumetric measurements and solution densities (SOP 12, Dickson et al., 2007). Sample bottles were calibrated (as a function of temperature) in the laboratory prior to the cruise following methods outlined in Section 3.2 in Liu et al. (2015). On board the ship, sample fill temperature was used to calculate the mass of the collected seawater (SOP 12, Dickson et al., 2007):

                                                                                                     (2.2)

where seawater density (ρ(sw, ts)) as a function of salinity and temperature was calculated according to Millero and Poisson (1981), and V(ts) is the volume of the sample at fill temperature (SOP 12, Dickson et al., 2007, Liu et al., 2015, Eqs. 7 – 11).

• The gravimetric MA term (MASW = MSW + MA) was calculated by measuring the added volume and density (σA) of the acid (MA = VA σA). 

The acid concentration ([HCl]AV) was determined by titrating the acid with a seawater Certified Reference Material (CRM) with known AT and solved for [HCl]AV in Eq. (2.1). CRM Batch #197 was obtained from Dr. Andrew Dickson of the Scripps Institution of Oceanography and used throughout this cruise (Salinity = 33.529; AT = 2256.77 ± 0.90 μmol kg-1). 

The titrant acid (0.2 N) was prepared from concentrated HCl and adjusted to an ionic strength of 0.7 M using NaCl. [HCl]AV was calculated to be 0.1934 N at 23°C (N = 7).

The [H+]ASW term in Eq. (2.1) provides proton concentrations directly in terms of total moles per kg of seawater (pHT = −log[H+]ASW). Solution pHT was measured spectrophotometrically from absorbance ratio measurements (R) at λ2A and λ1A (Liu et al., 2015; Hudson-Heck et al., 2021): 

                                                                (2.3) 

•  is the indicator’s equilibrium constant, and the symbols e1, e2, and e4 refer to indicator molar absorbance ratios at wavelengths λ1 and λ2.        Sample pH was calculated using the indicator bromocresol purple (BCP). Absorbances were measured using at λ1 = 432 nm and λ2 =589 nm, and an additional wavelength of 700 nm (λ3) to correct for baseline changes ().  The remaining terms in Eq. (2.3) can be calculated using equations from Hudson-Heck et al. (2021) over a range of salinity from 0 to 40 and temperatures from 278.15 to 308.15 K. 

                                                                                                                       (2.4a)

                                                  (2.4b)

 
            (2.4c)

The sample pH was calculated using the recorded R value and the final measured temperature.

The acid concentration of the titrant was calibrated using a CRM (Batch 197, certified AT value: 2256.77 ± 0.90 μmol kg-1). At the start of the cruise and for each CTD cast, a CRM was titrated before or after analyzing collected samples to check acid concentration. A total of 7 CRMs were measured, with an average [HCl]AV of 0.1934 ± 0.0001 N and measured AT value of 2256.77 ± 1.03 μmol kg-1.  

For each CTD cast, duplicates were collected from two Niskin bottles. The average standard deviation of the duplicates was 0.53 μmol kg-1 (N=12). All sample data was saved so that the quality criteria could be evaluated in the future. AT measurements were reported along with their associated quality-control flags.

Omega_calcite was calculated from pH and Total Alkalinity using CO2SYS (MATLAB v3.1.1) (Lewis and Wallace, 1998), using the total pH scale and K1, K2 constants from Hansson (1972, 1973) and Mehrbach et al. (1973), refit by Dickson and Millero (1987).

NSF Award Abstract:
The uptake of anthropogenic CO2 by the ocean will eventually be mitigated by the dissolution of CaCO3 on the sea floor. Dissolution is an important component of the carbon cycle in models used for climate projections though the relative importance of where it occurs (water column versus sediments) and the rates and processes involved are not fully understood. This ambitious field and laboratory study is designed to advance our knowledge of the important factors that control carbonate dissolution/ preservation in deep ocean sediments. Using a novel tracer approach and multiple in situ sampling strategies, the project will investigate sea floor dissolution rates, their kinetic controlling factors, the depth in sediments at which dissolution occurs, the role that oxidation of particulate organic carbon plays, and the ramifications of solid phase alteration for the use of geochemically-based paleoceanographic proxies. The project will foster further development of benthic lander technology and yield key information relating sea floor conditions to carbonate dissolution rate, thereby helping to constrain the rate at which the ocean can neutralize the impacts of ocean acidification. Graduate and undergraduate students will be trained and the research team will use film and animation to bring this work to a broader audience through a collaboration with the Los Angeles Natural History Museum.

The research team has developed a new approach to quantify calcium carbonate dissolution rates based on 13-C labeled carbonate substrates, a technique which is significantly more sensitive than traditional approaches based on alkalinity and/or calcium measurements. This has opened a range of new opportunities and insights into the governing mechanisms and rates of calcium carbonate dissolution, a challenging and long-standing geochemical problem. Carbonate dissolution rates on the sea floor will be directly assessed by benthic chamber flux measurements of alkalinity and calcium as well as pore water models of TCO2 and alkalinity and their isotopic composition. The potential impacts of organic carbon remineralization will be measured through oxygen and nutrient flux determinations, pore water gradients and modeling. Labeled 13C-enriched calcite will serve as a tracer of near surface dissolution processes when added to benthic chambers and of down-core dissolution processes using 13C-labeled rods inserted into the sediment column. These in situ experiments of labeled carbonate dissolution will be the first of their kind. To complement these measurements, the team will continue development of a rhizon-based pore water sampler that works on a multi-corer at all ocean depths. Field experiments will be conducted at sea at 4-6 sites in a transect through water column supersaturation to undersaturation between Panama and the Galapagos. Dissolution rate measurements, coupled with analyses of cation/Ca ratios in CaCO3 foraminiferal shells will help calibrate the impact of dissolution on paleo-proxy interpretations. Further, analyses of sediment calcite and aragonite fractions will help explain net dissolution and sediment response with time. The results from this study should help to better parameterize sediment variables in ocean-climate models (GCMs), which has important implications for predicting the consequences of ocean acidification and the modeling of paleoceanographic records. The methodologies and new techniques will surely be adopted by other researchers, therefore impacting the larger geochemical community.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.