Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Experiments were conducted during a research expedition aboard the R/V Atlantic Explorer in the Sargasso Sea (31º40'00''N, 64º10'00''W) at the Bermuda Atlantic Time Series site (BATS) from September 5 to 11, 2023. The experiments were performed in opaque 5L Cali-5-Bond(TM) multi-layer foil bags (Calibrated Instruments), placed in a flow-through incubator where surface seawater was continuously flowed to maintain a constant temperature of approximately 27 degrees Celsius (ºC). Each bag was filled with approximately 3 ± 0.02 liters (L) of unfiltered seawater using a rubber hose that was flushed with water to remove air bubbles, ensuring that no air entered the bags. Bags were rinsed 3 times with seawater before filling and sealing with the Luer-fitted stopcock. The mixed layer at the study site was approximately 40 meters (m) thick, and seawater for the experiment was collected from the upper 10 m. The bags were allowed to float freely in the incubator and they moved continuously due to the boat movement.

Experiments involved the addition of NaOH solution prepared by weighing ACS grade NaOH in the lab prior to the cruise in a plastic Falcon tube that was capped and sealed with parafilm tape. During the cruise, DI water was added to make up stock NaOH solutions with a final concentration of 1 M. The NaOH solution was pipetted into the seawater-filled bags through the Luer-fitted stopcock. Because NaOH contributes only alkalinity but not DIC, seawater in the experiments was out of equilibrium with the atmosphere, which was intended to simulate conditions immediately following alkalinity addition to seawater during OAE deployments.

In total, 5 experiments were conducted. The first experiment (experiment A) was a control with no alkalinity addition. In the second, third, and fourth experiments (B, C, and D), alkalinity was enhanced by 500, 1000, and 2000 micromoles per kilogram (µmol/kg) respectively. The fifth experiment (E) represents a set of "sacrificial" time series experiments whereby 9 bags were prepared similar to other experiments and alkalinity was enhanced by 1000 µmol/kg in each one of them, but each bag was sequentially opened and filtered in order to evaluate the precipitate mineralogy through time. In experiment E, water samples for TA and DIC measurements were taken only at the end of the experiment. The experiments were run for approximately 5 days.

Experiments were quenched by filtering all remaining seawater through 47 mm 0.8 µm polycarbonate filters using a peristaltic pump. Filters were rinsed with deionized and purified water (18.2 MΩ), dried at 55ºC, and stored cool and in the dark. The precipitates were then scraped off the filters and analyzed for mineralogy with X-ray diffraction.

Data were processed using the fundamental-parameters Rietveld refinement program TOPAS V7 (Coelho, 2018). The data were corrected for an instrument zero error determined using the NIST standard LaB₆ (660c), which has a certified unit-cell parameter of 4.156826 Å, crystallite size (L(vol)) of 500 nanometers (nm), and no micro-strain related peak broadening.

- Imported sheet 2 of original file "Hashim et al OAE resaerch data_XRD raw and results.xlsx" into the BCO-DMO system.
- Flagged "NA" as a missing data identifier (missing data are blank/empty in the final CSV file).
- Renamed fields to comply with BCO-DMO naming conventions.
- Converted date columns to YYYY-MM-DD format.
- Saved the final file as "963717_v1_xrd_results.csv".
- Converted sheet 1 of original file "Hashim et al OAE resaerch data_XRD raw and results.xlsx" to CSV format and saved as "963717_v1_xrd_raw_data.csv".

NSF Award Abstract:
OCE-PRF Towards Quantifying Calcium Carbonate Sediment Dissolution During Marine Diagenesis The goal of the project is to investigate dissolution of calcium carbonate (CaCO3) in sediments below the seafloor and determine its importance to the chemistry of seawater. This project uses sediment samples and chemical data collected from different parts of the ocean during the past five decades by scientific ocean drilling programs. Sediment dissolution of carbonate can lessen the impact of ocean acidification, the process that causes the pH of the ocean to decrease due to the uptake of carbon dioxide (CO2) from the atmosphere. Ocean acidification threatens the survival of marine organisms, such as oysters, clams, and coral reefs, which could alter marine food chains and food supply to humans. By improving understanding of carbonate dissolution in the ocean, results from this project will enable better predictions of the effects of ocean acidification on marine organisms. This will advance the progress of science and contribute to the knowledge that can inform public policy. In addition, understanding carbonate sediment dissolution serves other important purposes. For example, dissolution can create small spaces between sediments that may get filled with groundwater once sediments convert to rocks over millions of years. Thus, understanding the occurrence and spatial distribution of spaces within rocks may help determine the volume and movement of groundwater in subsurface aquifers. This project provides support for a postdoctoral research fellow and research training opportunities for students through the Summer Student Fellowship and Woods Hole-wide Partnership Education Programs at the Woods Hole Oceanographic Institution.

Carbonate mineral dissolution is an integral part of the alkalinity and carbon cycles in the ocean and is expected to play an increasingly significant role in mediating changes in ocean chemistry as atmospheric CO2 continues to rise. The goal of this project is to provide thermodynamic constraints necessary for quantifying carbonate sediment dissolution in marine diagenetic environments. Specifically, the CaCO3 saturation state of pore fluids will be calculated in 365 globally distributed sites from previous scientific ocean drilling expeditions using a specially developed Pitzer ion activity model which is particularly useful for calculating activity coefficients in high ionic strength solutions such as those that characterize most diagenetic environments. These calculations will be substantiated with geochemical and textural analyses of sediment samples from four representative sites to identify the specific diagenetic processes (e.g., dissolution, precipitation, and recrystallization) and document the conditions responsible for their occurrence and prevalence. The immediate advantage of calculating the saturation state of pore fluids is that such data can be used to estimate carbonate sediment dissolution below the seafloor and quantify its contribution to the alkalinity and carbon cycles, which will lead to more accurate predictions of the consequences of ocean acidification. Another benefit of the global saturation state dataset is that it will improve our understanding of authigenic carbonate precipitation and its link to the carbon cycle over Earth history, which has been proposed as a significant sink for carbon. Furthermore, by complementing the thermodynamic calculations with textural and geochemical analyses, this project will parse out various diagenetic processes and identify the sedimentological and geochemical conditions responsible for their occurrence. Such knowledge is crucial for evaluating the impact of diagenesis on the carbonate-hosted paleoenvironmental proxies. Collectively, this project will pave the way towards a mechanistic understanding of carbonate diagenesis. This will provide important constraints on the oceanic alkalinity cycle, carbon burial rates, and geochemical proxies, which ultimately help us better understand the future of our ocean system in the context of climate change.

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.