Isotopic and optical heterogeneity of solid phase extracted marine dissolved organic carbon
Introduction
The oceanic dissolved organic carbon (DOC) pool contains ~662 Pg of carbon (Hansell et al., 2009) and is comprised of thousands of individual molecules. Identifiable compound classes include carbohydrates, proteins, lipids, and black carbon (Dittmar and Paeng, 2009; Ziolkowski and Druffel, 2010; Coppola and Druffel, 2016; Repeta, 2015). Early radiocarbon (∆14C) measurements revealed that the age of deep Pacific Ocean DOC is ~6000 14C years (∆14C = −525‰), indicating that bulk DOC is much older than dissolved inorganic carbon (2000 14C years, −240‰) (Williams and Druffel, 1987). This implies that DOC survives multiple mixing cycles of the deep ocean. Understanding the processes responsible for this extended residence time of DOC is important for our understanding of the oceanic carbon cycle. A large effort has been underway to characterize DOC on a molecular level, but large gaps remain in our understanding of the biogeochemical cycling of this complex carbon pool.
Solid-phase extraction (SPE) is a common method for isolating DOC from seawater salts for further molecular and isotopic analysis. This technique sorbs the hydrophobic fraction of DOC onto a resin that is eluted in a concentrated aliquot of solvent, usually methanol. Bond Elut PPL (styrene-divinylbenzene polymer) SPE resins have been widely used by the marine DOC community for their ease of use and relatively high recoveries of DOC from seawater and freshwater (>40% and 60%, respectively; Dittmar et al., 2008).
DOC extracted from PPL resins (SPE-DOC) can be processed for stable isotope (δ13C) and radiocarbon (∆14C) analyses, which are powerful tools used to investigate the sources and cycling of DOC. The PPL method has been used to study changes in SPE-DOC ∆14C with depth in the open ocean (Flerus et al., 2012; Broek et al., 2017; Zigah et al., 2017), and in the molecular level characterization of DOC using FT-ICR-MS (Li et al., 2017). Radiocarbon analysis paired with FT-ICR-MS of SPE-DOC extracted using PPL resins helped refine our understanding of how DOM degradation is linked to molecular formulae (Lechtenfeld et al., 2014; Flerus et al., 2012). DOC isolated using SPE is usually the low molecular weight (LMW) fraction of DOC, and typically has either the same or lower δ13C and ∆14C values than bulk seawater (Broek et al., 2017; Coppola and Druffel, 2016; Druffel et al., 1992), indicating that PPL resins selectively isolate certain compound groups. This result is echoed in CDOM and FDOM measurements that found less variability in SPE-DOM than bulk DOM (Wünsch et al., 2018). These studies provide evidence that SPE-DOC is different from bulk DOC.
Although PPL SPE-DOC extracts are used for isotopic analyses, there are little detailed carbon mass and isotopic blank assessments for seawater SPE-DOC protocols. A measured ∆14C value represents the weighted-average ∆14C value of all compounds in a sample, as well as that of the process-blank. Therefore, the mass and the isotopic value of the process-blank must be determined to accurately assess the true ∆14C value of the sample. In addition, changes in seawater SPE-DOC isotopic and chemical composition during elution remain undetermined.
The first objective of this study was to optimize the PPL resin protocol (Dittmar et al., 2008) for maximum SPE-DOC recoveries for future compound specific radiocarbon analyses. Our second objective was to assess extraneous‑carbon blanks in our SPE-DOC ∆14C and δ13C measurements. Extraneous carbon refers to the amount of carbon added to the sample from the resin and other processes, and must be quantified in order to correct for true environmental isotope values. Our third objective was to determine whether the isotopic and optical composition of SPE-DOC changes with increasing elution volume, and if so, how this impacts final ∆14C and δ13C measurements of SPE-DOC.
We present evidence that PPL resins elute marine-derived SPE-DOC first, followed by terrestrial SPE-DOC, based on changing stable isotopic (δ13C) and radiocarbon (∆14C) signatures of SPE-DOC captured at different elution volumes. We also present a detailed blank assessment of PPL SPE-DOC using the applied background correction method (Santos et al., 2010).
Section snippets
Sample collection
Seawater samples were collected from the Newport Beach Pier, CA (33° 36′ 21″ N, 117° 55′ 52″ W). We collected large volume samples (12.5 to 15 L) to match the requirements needed for compound specific radiocarbon analysis of dissolved black carbon. A flowchart to clarify sampling dates, volumes, and subsequent analyses is shown in Fig. 1. On March 13, 2017, one 15 L sample (herein called sample 1) was collected for CDOM (chromophoric dissolved organic matter) absorbance analyses. On April 20,
SPE-DOC concentrations
Fig. 2a shows cumulative elution volume versus the blank corrected carbon mass of the SPE-DOC fractions. Distinct peaks in carbon mass appear at 1.6, 1.8 and 2.0 mL for samples 2B, 2C, and 3 respectively. Within the initial 2 mL elution, 60 ± 1% of cumulative carbon by mass was recovered for samples 2B and 2C, and 16 ± 1% was recovered for sample 3. Although sample 3 eluted more gradually, all samples reached 96 ± 1% cumulative SPE-DOC by 6 mL eluted methanol. Extending the elution to 32 mL
Discussion
Most SPE-DOC in our samples is contained within the first 2 mL of elution, whose ∆14C and δ13C values closely resemble those of phytoplankton-derived, open ocean DOC (Druffel et al., 1992; Williams and Gordon, 1970). Over the next ~5 mL of eluted SPE-DOC, the δ13C values are lower, similar to those of terrestrial C3 plants (Williams and Gordon, 1970), and ∆14C values reach maximum values. Our ∆14C and δ13C datasets together imply that there is a compositional shift in the eluted SPE-DOC, from
Implications for further work
One objective of this study was to optimize the PPL protocol for SPE-DOC isotopic analysis. We found that an extra 5% of SPE-DOC can be extracted from coastal seawater using increased elution volume. Another objective of this study was to assess changes in SPE-DOC isotopic composition with increased elution volume. The isotopic heterogeneity indicates gradients in composition and polarity of the SPE-DOC. This implies that SPE-DOC that elutes early is relatively polar, while SPE-DOC that elutes
Funding
This work was supported by the U.S. American Chemical Society Petroleum Research Fund New Directions grant (55430-ND2 to ERMD and BDW), U.S. NSF (OCE–141458941 to ERMD), the Fred Kavli Foundation, and the Keck Carbon Cycle AMS Laboratory (U.S.). This research was undertaken, in part, thanks to funding from the Canada Research Chairs program (to BDW).
Declaration of Competing Interest
The authors declare no conflicts of interest.
Acknowledgements
We thank Jennifer Walker, Xiaomei Xu and Dachun Zhang for their help with the stable carbon isotope measurements, Sheila Griffin for her help with methodology and laboratory assistance, and John Southon and staff of the Keck Carbon Cycle AMS Laboratory for their assistance and advice.
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