Table of Contents

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

Water sampling details

Water samples were gravity filtered from Niskin bottles through 0.2 μm Whatman Polycap 36 AS nylon membrane cartridge filters, directly into combusted borosilicate glass bottles. Filtered sample waters were stored in the dark at 4°C until measurements. Sample waters were left in the dark at room temperature (~20°C) so they were back to room temperature before absorbance measurements.

These seawater samples were collected for photochemical degradation experiments, and water samples at the various BATS stations had similar optical properties, for example, similar absorbance at 325 nm. Therefore, which stations we collected samples from were largely determined by the cruise schedule, scientific tasks performed by the core BATS crew, and weather, i.e., where CTDs can be cast that allowed us ample time to directly filter water from the Niskin bottles.

We also wanted to compare the photoproduction of dissolved inorganic carbon (DIC) between seawater at the surface and at depth, as the deep water may have different photodegradation history and different dissolved organic carbon composition, and thus have different DIC photoproduction rates and efficiencies. Therefore, we collected water samples from the surface (1 m) and at depth (3000 m).

Measurement details

Absorbance (A(λ)) of the water samples were measured at 250–800 nm at 1.0 nm intervals, in triplicate. May 13th, 2017 samples were measured in 10-cm-pathlength quartz spectrophotometric cuvettes with an Agilent 8453 UV-visible spectrophotometer with ChemStation software; July 17th, 2018 samples were measured in a World Precision Instruments UltraPath capillary cell set at 50 cm pathlength using a Maya Pro 2000 spectrophotometer with SpectraSuite software, and the rest of the samples were measured in the same UltraPath capillary cell set at 200 cm pathlength using the same Maya spectrophotometer.

Milli-Q water was similarly measured to use as blanks. The absorbance spectra of the sample water were first corrected by subtracting the absorbance spectra of blanks (Milli-Q water) from the absorbance spectra of the sample water; the absorbance spectra of the sample water were further corrected for potential offsets and instrument drift by subtracting the average absorbance at 690–710 nm. The corrected absorbance spectra (A(λ)corr) of the sample water generated from these corrections were converted to Napierian absorption coefficients (ag(λ); m−1; represented in this dataset as CDOM_absorption_a_g), using the following equation: ag(λ)= A(λ)corr ln10/L, where L (m) is the pathlength.

The May 13th, 2017 samples were measured in 10-cm-pathlength cuvettes with the Agilent spectrophotometer because these samples were T0 samples of a photoirradiation experiments, the water was photoirradiated in these 10-cm-pathlength cuvettes, and the absorbance were also measured in these cuvettes, which only works on the Agilent spectrophotometer, which has a 10-cm-pathlength cuvette holder.

Data Corrections and Conversions

The average absorbance spectra of blanks were subtracted from the absorbance spectra of the sample water.

Absorbance spectra were further corrected for potential offsets and instrument drift by subtracting the average absorbance at 690–710 nm.

A(λ) was converted to Napierian absorption coefficients (ag(λ); m−1), using the following equation: ag(λ)= A(λ) ln10/L, where L (m) is the pathlength.

Data Processing Software

• Importing and correcting absorbance measurements (A(λ))
• Converting A(λ) to Napierian absorption coefficients (ag(λ))
• R-3.3.3, R-3.4.4, R-3.5.3, R-3.6.2, R-3.6.3, R-4.2.1
• Package ‘stringr’:
  • Wickham H (2023). _stringr: Simple, Consistent Wrappers for Common String Operations_. R package version 1.5.1, <https://CRAN.R-project.org/package=stringr>.

- Latitude and longitude values were joined to the dataset for BATS station sites

NSF Award Abstract:
Photoproduction describes chemical reactions which use the energy of sunlight to change the molecular structure of one product to another. This process has been shown to play an important role in the coastal ocean changing dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC); however, minimal information is available on the extent to which this process takes place in the open ocean. The main obstacle to obtaining DIC production data for open ocean waters is the inability to measure the small DIC production rates against the large background of seawater DIC. Currently the most common method for high precision measurement of DIC production has unacceptable detection limits to resolve this difference. Researchers from the University of Georgia have developed MoDIE (Moderate DI13C Isotope Enrichment), a novel means of determining low levels of DIC produced in the open ocean with high precision. As part of this project, they plan to refine their MoDIE method and obtain measurements of DIC photoproduction in open ocean waters (i.e, South Atlantic Bight, North Atlantic Ocean and North Pacific Ocean subtropical gyres). Results from this work will provide improved estimates of DIC photoproduction in the open ocean which in turn will improve existing models of carbon cycling.

The broader impacts of this project are particularly exciting. The proponents plan to create an exhibit at the University of Georgia Marine Extension public aquarium to teach the general public about ocean color and marine carbon cycling. In addition, they plan to participate in the Skidaway Institute for Oceanography ?Marine Science Day? open house to share their science with the public and incorporate results into their class curricula. One of the researchers is a first-time investigator who plans to incorporate education and training of undergraduate and graduate students in the project. Finally, undergraduate interns from underrepresented groups will be involved in the project to give them research experience and help with retention in a science discipline and career.

Photoproduction of dissolved inorganic carbon (DIC) from dissolved organic carbon (DOC) has been shown to be a non-negligible process in the marine carbon cycle; however, it is still unclear the extent and importance of this process to the overall carbon cycle in the open ocean (blue water chemistry). The major issue causing this uncertainty is the lack of available methods with acceptable detection limits. The high background of DIC in the open ocean (2mM) makes it difficult to measure the photoproduced portion of DIC when the detection limits are high. Additionally, the current best method requires pre-stripping samples of background DIC, which could introduce unnecessary errors by over manipulating samples. This project will develop a new method called MoDIE (Moderate DI13C Isotope Enrichment) to get accurate measurements of DIC photoproduction in the open ocean. MoDIE works by enriching a sample in 13C and monitoring the dilution of DI13C under irradiation using liquid chromatography-isotope ratio mass spectrometry (LC-IRMS). By removing the need for pre-sample treatment and introducing a highly precise measurement technique, the researchers will be able to compare their techniques with published methods of photochemical DIC detection and put forth more accurate estimate of marine DIC photoproduction in the global ocean. This will greatly improve marine carbon cycle models.