Composition of Dissolved Organic Matter in Pore Waters of Anoxic Marine Sediments Analyzed by 1H Nuclear Magnetic Resonance Spectroscopy

Abstract

Marine sediments are globally significant sources of dissolved organic matter (DOM) to the oceans, but the biogeochemical role of pore-water DOM in the benthic and marine carbon cycles remains unclear due to a lack of understanding about the molecular composition of DOM. To help fill this knowledge gap, we used 1 H nuclear magnetic resonance (NMR) spectroscopy to examine depth variability in the composition of pore-water DOM in anoxic sediments of Santa Barbara Basin, California Borderland. Proton detected spectra were acquired on whole samples without pre-concentration to avoid preclusion of any DOM components from the analytical window. Broad unresolved resonance (operationally assigned to carboxyl-rich alicyclic molecules, or CRAM) dominated all spectra. Most of the relatively well-resolved peaks (attributed to biomolecules or their derivatives) appeared at chemical shifts similar to those previously reported for marine DOM in the literature, but at different relative intensities. DOM composition changed significantly within the top 50 cm of the sediment column, where the relative intensity of CRAM increased, and the relative intensity of resolved resonances decreased. The composition of CRAM itself also changed throughout the entire length of the 4.5-m profile, as CRAM protons became increasingly aliphatic at the expense of functionalized protons. Given that pore-water DOM is generated from sedimentary organic matter that includes pre-aged and degraded material, and that DOM is theoretically subject to microbial reworking in the pore waters for centuries to millennia, these data suggest that marine sediments may be sources of CRAM that are compositionally unique from CRAM generated in the upper ocean.

Abiotic formation of dissolved organic sulfur in anoxic sediments of Santa Barbara Basin

“…In a recent study, Pohlabeln et al (2017) estimated that the flux of DOS from sulfidic sediments could range from 30 to 200 Tg DOS per year, and concluded that it may be one order of magnitude larger than the riverine DOS flux. While we did not measure the benthic flux of DOS compounds from SBB sediments, our past work has shown that SBB sediments are a source of CRAM-like material to the water column (Fox et al, 2018). As the majority of the DOS formulas we detected fall in the CRAM region, it is likely that this S-CRAM material is a part of the CRAM-like pool of compounds that escape from SBB sediments into the water column.…”

“…The lower percentage of such DOS formulas near the sediment surface may be attributed to the high concentration of reactive iron near the surface that competes with reactive DOM compounds for reduced inorganic sulfur species (Canfield et al, 1992(Canfield et al, , 1996Werne et al, 2003). If CRAM is indeed an important reactant in DOS formation, low levels of CRAM near the sediment-water interface (Fox et al, 2018) could also explain this observation. Sulfurization of DOM in anoxic sediment pore waters is expected to alter the geochemical reactivity of the precursor compounds.…”

Molecular Composition of Dissolved Organic Matter in Sediment Porewater of the Arctic Deep-Sea Observatory HAUSGARTEN (Fram Strait)

“…± SD of 134 ± 34 and 212 ± 78 µmol/L, respectively; Rossel et al, 2016). Compared to other marine systems, DOC concentrations in Arctic porewaters of Fram Strait were in the lower range, and those of the Central Arctic substantially lower than that of other oceans (Burdige and Komada, 2015;Schmidt et al, 2017;Fox et al, 2018;Loginova et al, 2020). Nonetheless, the gradients between bottom water and porewater are in line with previous reports indicating at least one magnitude higher DOC concentration in porewater compared to bottom water, supporting the role of POM diagenesis in surface sediments for the production of DOC and flux to overlying bottom water (Schmidt et al, 2011;Burdige and Komada, 2015;Loginova et al, 2020).…”

Analytical and Computational Advances, Opportunities, and Challenges in Marine Organic Biogeochemistry in an Era of “Omics”

“…Therefore, 1 H NMR has typically required extraction, drying, and redissolution in D 2 O (e.g., Repeta et al, 2002). However, recent advances in high magnetic field NMR (currently up to 900 MHz or 21 Tesla) and the development of water suppression techniques allow analysis of marine DOM using 1 H NMR techniques without any isolation or pretreatment on open ocean and sediment pore water samples (Lam and Simpson, 2008;Zheng and Price, 2012;Fox et al, 2018).…”

“…This is because higher magnetic fields require a higher spinning speed for the sample rotor to remove the spinning sideband, which is not currently feasible (see Mopper et al, 2007 for a more complete explanation). However, higher magnetic field NMR and NMR Cryoprobes may allow the use of water suppression 1 H NMR techniques to analyze the entire marine DOM composition even in the deep ocean (Lam and Simpson, 2008;Fox et al, 2018). Diffusion ordered spectroscopy (DOSY) and 2D-NMR techniques ( 1 H-1 H-TOCSY and 1 H-1 H-COSY) estimate the molecular weight of DOM components, resolve overlapping peaks, confirm the interpretation of the chemical shifts, and help to identify the structures of specific DOM components.…”