The >5000-year radiocarbon age (14C-age) of much of the 630±30 Pg C oceanic dissolved organic carbon (DOC) reservoir remains an enigma in the marine carbon cycle. The fact that DOC is significantly older than dissolved inorganic carbon (DIC) at every depth in the ocean forms the basis of our current framing of the marine DOC cycle where some component persists over multiple cycles of ocean mixing. As a result, 14C-depleted, aged DOC is hypothesized to be present as a uniform reservoir with a constant 14C signature and concentration throughout the water column. However, a key requirement of this model would be direct observations of DOC with similar 14C signatures in the surface and deep ocean. To address this data gap, we applied a thermal fractionation method to investigate DOC in water samples collected from a range of water column depths during the R/V Western Flyer cruise to Station M off the coast of central California in April 2018. This dataset presents carbon isotope data for both bulk samples and samples processed using ramped pyrolysis oxidation (RPO) procedures.

Table of Contents

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

Field collection of samples
Samples were collected during a cruise aboard the R/V Western Flyer to Station M off the coast of central California in April 2018 (34° 50’N, 123° 00’W; 4100 meters depth).  30 to 40 Liters of seawater were collected at five depths ranging from 45 to 3700 meters. Water was then acidified to pH 2 using trace metal grade hydrochloric acid (HCl). Solid phase extraction columns (1g Bond Elut PPL cartridges by Agilent) were first activated overnight with liquid chromatography/mass spectrometry (LC/MS) grade methanol and then rinsed with LC/MS grade water, LC/MS grade methanol, and LC/MS grade water adjusted to pH 2 using trace metal grade HCl. The acidified seawater was passed through the PPL cartridges under gravitational pressure over 1-2 days. This PPL solid phase extraction method has been used extensively to isolate marine DOM (Dittmar et al., 2008; Petras et al., 2017 and more) and provides yields of 35-60%. Following acidification, the method preferentially extracts hydrophobic compounds but is also able to isolate some semi-polar compounds (Johnson et al., 2017; Petras et al., 2021) whose overall concentrations may change with depth. In addition to the relatively high yield, previous work has shown that Δ¹⁴C of PPL-DOC is similar to that of the bulk (Lechtenfeld et al., 2014; Lewis et al., 2021). Typically, 5L of seawater was extracted with one 1g PPL cartridge.

Before eluting the PPL resin, and immediately following the extraction, the cartridge was rinsed with three column volumes of pH 2 LC/MS grade water and dried under ultra-high purity nitrogen gas. Organic matter retained on the cartridge was then eluted with 10 mL of methanol/1 g PPL cartridge (PPL-DOC) and stored at -20 °C. Seawater for total organic carbon concentration [TOC] and dissolved organic carbon concentration [DOC] analysis (i.e., seawater pre-filtered through combusted Whatman corporation GF/F filters) was collected into combusted 40-mL borosilicate vials, immediately acidified to pH 2 using trace metal grade 12 N HCl (Fisher Scientific), capped with acid-washed vial caps with septa, and stored at room temperature until measurement. 

A fraction of the extracted PPL-DOC sample was dried extensively to remove methanol and then subjected to a 2 M acid hydrolysis. Briefly, the organic matter was transferred to an ampoule with 2 mL of 2 M hydrochloric acid and sealed under a nitrogen atmosphere before being placed in a drying oven at 80°C for 18 hours. Following hydrolysis, the sample was diluted with LC/MS grade water to pH 2 and then re-extracted onto a PPL cartridge as described above. The organic matter retained by the cartridge was then eluted in methanol and dried down for frozen storage. 

Laboratory sampling
Ramped pyrolysis/oxidation (RPO) was performed at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility in Woods Hole, MA, USA. Samples were transferred while dissolved in a small amount of methanol into quartz cups (pre-combusted at 850 °C, 1 hour) and then dried down under ultra-high purity nitrogen gas. Samples were further dried in an oven at 50°C for multiple days in an attempt to remove all methanol. Each quartz cup containing a single sample was then placed inside the RPO reactor via a quartz insert tube. The RPO instrument continuously heats the sample at a set rate, monitors evolved CO₂ during the pyrolysis/oxidation reactions as “thermograms” (Rosenheim et al., 2008), and can be used to trap CO₂ for isotope analysis. (For Station M thermogram data, see BCO-DMO dataset here: https://www.bco-dmo.org/dataset/892564). The RPO methodology has been described previously and further information describing the detailed protocol can be found in other publications (Hemingway et al., 2017; Rosenheim et al., 2008). In summary, all samples were operated in oxidation mode (carrier gas 92% He, 8% O₂) with a flow rate of 35 mL min⁻¹ and a ramp rate of either 20 or 5 degrees Celsius per minute. Samples analyzed at 20°C min⁻¹ ranged in size from 6.4 to 36.8 µmol C and all resulting CO₂ was collected for isotope analysis. Samples analyzed at 5°C min⁻¹ ranged in size from 48.2 to 185.2 µmol C, where these amounts represent the sums of six fractions collected during each 5°C min⁻¹ ramp, with each fraction ranging in size from 4.4 to 43.6 µmol C.  In total we ran ten 20°C min⁻¹ ramps and six 5°C min⁻¹ ramps of samples from Station M.

Isotope measurements
All bulk and RPO-fraction isotope measurements were performed at NOSAMS. Stable carbon isotope compositions were measured on the resultant CO₂ gas using a dual-inlet isotope ratio mass spectrometer (IRMS), with resulting ¹³C content expressed in δ¹³C per mil (‰) notation relative to Vienna Pee Dee Belemnite (VPDB). Radiocarbon measurements were completed via accelerator mass spectrometry (AMS) following standard graphitization methods (McNichol et al., 1994). The amount of CO₂ for each RPO fraction, their δ¹³C values, and Fm (framction modern) values were corrected for blank carbon contributions from the RPO system (resulting in corrections of no more than 3 ‰ for any sample). δ¹³C was additionally corrected to ensure ¹³C mass balance.

 

The amount of CO₂ for each RPO fraction, their δ¹³C values, and Fm values were corrected for blank carbon contributions from the RPO system following Hemingway et al. 2017a (resulting in corrections of no more than 3‰ for any sample). δ¹³C was additionally corrected to ensure ¹³C mass balance, using the ‘ramped pyrox’ python package developed by Hemingway et al., 2017b and available online at http://pypi.python.org/pypi/rampedpyrox.

Problems/Issues
Residual methanol may be contributing to the first fraction collected for some of the samples when this fraction was small (~50 µg). This would result in a lower Δ¹⁴C signature for the first fraction, potentially masking the presence of younger carbon. This contamination would not have affected the bulk isotope measurements which were made on larger samples, and which did not have anomalous δ¹³C values. The potential for residual methanol to linger even after several drying steps, should be kept in mind when interpreting unexpectedly depleted δ¹³C and Δ¹⁴C values for PPL DOC samples.

Coverage: Eastern North Pacific Ocean

NSF Award Abstract:

This research project will identify biological sources and chemical structures that are responsible for the long-term storage of carbon in the ocean. Each year, microscopic marine plants remove about as much carbon dioxide from the atmosphere as do land plants. Respiration returns much of this carbon to the ocean as carbon dioxide, but some is locked in the remnants of living organisms. These remaining compounds are modified by pathways that involve bacteria, sunlight, chemical reactions, and other processes that lead to storage of carbon for thousands to millions of years. Some compounds eventually contribute to the petroleum reservoir. Building on previous results, this project will study the reactions and oceanic lifetime of a particular set of biochemicals, called carotenoids, as a possible organic carbon storage pathway. Carotenoids are abundant in very many marine organisms, increasing the likelihood that they are part of this long-term carbon storage and petroleum formation. These compounds also have unique chemical properties that make them subject to specific chemical reactions. For this reason, they have been marketed as powerful antioxidants. Therefore, scientific outcomes from this research on carotenoid chemistry will not only inform ocean carbon cycles but could also benefit studies of their properties as antioxidants. The project will determine the lifetime of carotenoids and their degradation products in seawater to provide new insights into pathways that transfer carbon from the atmosphere through biota and into long-term storage reservoirs. Graduate students and underrepresented undergraduate students will be engaged in the research.

Previous work has identified specific chemical backbones of compounds that are broadly distributed within the marine dissolved organic matter (DOM) reservoir. A high-resolution analytical approach that combines nuclear magnetic resonance (NMR) spectroscopy with comprehensive gas chromatography-mass spectrometry (GC-MS) has detected DOM compounds with unique structures closely related to carotenoids. Photochemical reactions of a representative carotenoid in laboratory experiments has further linked compounds detected in seawater to carotenoid degradation products (CDP). These preliminary studies show promise that the work funded here will be able to identify specific CDP structures and establish the quantitative significance, lifetimes, and timescales of CDP accumulation in seawater. The project will combine laboratory experiments, high resolution analyses, and chemical synthesis methods to determine the chemical composition of CDP and their abundance in seawater. The novel analytical methods developed through this research will be relevant for other carotenoid-focused studies in petroleum formation, soil chemistry, as well as food chemistry. Ramped pyrolysis oxidation (PyrOx) coupled to radiocarbon measurements will be used to determine the radiocarbon content of CDP-enriched DOM and seek to estimate the accumulation timescale of these dissolved molecules in the environment, and it is hypothesized that deeper, older ocean water will contain a relatively higher proportion of radiocarbon-depleted CDP. Collecting samples from different depths in the North Pacific Ocean where CDP-enriched DOM will be isolated following established sample processing methods will provide insights and new information on the mechanisms that control the amount and timescale of carbon redistribution among Earth's various reservoirs.

Resources:
Ramped pyrolysis oxidation (RPO) data is currently accessible through the RPO database maintained on GitHub by Jordon  Hemingway (http://github.com/FluvialSeds/RPO_Database).

J.D. Hemingway et al. A compiled database of published ramped pyrolysis/oxidation results, 2018- , http://pypi.python.org/pypi/RPO_Database, doi:10.5281/zenodo.1158742 [online; accessed 2022-07-10]