Elsevier

Marine Chemistry

Volume 92, Issues 1–4, 1 December 2004, Pages 107-121
Marine Chemistry

Examining marine particulate organic matter at sub-micron scales using scanning transmission X-ray microscopy and carbon X-ray absorption near edge structure spectroscopy

https://doi.org/10.1016/j.marchem.2004.06.020Get rights and content

Abstract

Marine sinking particulate organic matter (POM) represents the link between surface primary production and burial of organic matter in marine sediments. As such, the nature of this material has been the subject of numerous studies attempting to characterize its composition. The results of these studies have shown that a significant proportion of POM is not recognizable as known compounds, and that the proportion of uncharacterized material increases with age/depth/diagenesis. However, few studies have examined the spatial heterogeneity of this material. This study uses a new tool, scanning transmission X-ray microscopy (STXM), together with carbon X-ray absorption near edge structure (C-XANES), to examine POM collected from sediment traps deployed at one location in the Arabian Sea as part of the JGOFS program. The results indicate that POM is composed primarily of four distinct phases: protein, an aliphatic rich phase, a carboxylic-acid-rich phase, and a phase with complex unsaturated and quinone character. This last phase may be a condensation product between carbohydrates and proteins, or from degraded plant pigments. Many particles consisted of a single chemical phase; however, in particles with mixed compositions, individual domains retained distinctive chemical signatures at the instrument's resolution limit (50 nm). All major chemical phases were observed in sediment trap particles from 531 to 3369 m depth, supporting the hypothesis that non-selective degradation dominates particle remineralization, and that overall particle compositions are determined by near surface processes. Only one particle, out of more than 60 examined, exhibited soot-like composition. The lack of a significant black carbon/soot component may be attributable to sampling during the winter monsoon period.

Introduction

The nature of organic matter leaving the euphotic zone and its fate have been the subject of much debate (Lee et al., in press). Wakeham et al. (1997) were able to identify many compounds directly by HPLC, GC, and GC/MS, yet, their work pointed to an inability to identify the majority (>50%) of carbon in sinking POM. The presence of “analytically uncharacterized” material in sinking POM (Hedges et al., 2000) has led to a number of hypotheses about the nature of this material. Sources may include: algaenan or waxy insoluble material (Hwang and Druffel, 2003), soot or ash-derived black carbon (Deuser et al., 1983), transparent exopolymers formed from polysaccharides (Chin et al., 1998, Passow et al., 2001, Lee et al., in press), or organic matter that has been rendered unidentifiable by internal polymerization/crosslinking (Hedges, 1978, Qian et al., 1992). Attempting to reconcile between these competing hypotheses is difficult as different methods examine only selective fractions of POM. There is also the unstated assumption that compounds are homogenously distributed within samples, but there is very little information to support or refute this.

One reason that spatial information content of samples has been largely ignored in biogeochemistry is that the analytical techniques most commonly used in this field do not lend themselves to acquiring this information. Studies of cells using various microscopy techniques (Mitchell, 2001, Sommer and Franke, 2002, Jamin et al., 2003) serve as reminders that organic matter is produced with chemical heterogeneity on nanoscales. Although the composition of individual structures may be tremendously altered by diagenesis, it is not a given that these structures will be homogenized. In special cases, plant cell structures have been shown to survive burial for hundreds of millions of years, with some features still identifiable by their distinct chemical signatures (Boyce et al., 2002).

Electromagnetic-radiation-based micro-spectroscopy methods such as infrared and X-ray microscopy are increasingly used as tools to investigate problems in environmental chemistry and biochemistry (Myneni, 2002, Dumas and Miller, 2003, Miller et al., 2003). These spectromicroscopy methods can generate information on functional group distributions within samples with enough spatial and chemical resolution to minimize “signal averaging” problems, and with minor sample handling (e.g., no derivatization or extraction). Infrared microscopy is diffraction limited to spatial resolutions of roughly 3–10 μm, while X-ray microscopy has a theoretical resolution limit of less than 10 nm, with current instruments limited to about 30 nm (Jacobsen and Kirz, 1998).

Using marine particulate organic matter (POM) from the Arabian Sea, we have applied scanning transmission X-ray microscopy (STXM) together with concurrent measurements of carbon X-ray absorption near edge structure (C-XANES) spectroscopy to examine the potential for identifying particle compositions and mapping particle heterogeneity. This approach allows us to examine the composition of POM on a particle-by-particle basis. Although there are limitations in examining large particles (>200 μm diameter particles were not analyzed), and particles need to be dried and briefly heated to embed them in S° prior to sectioning, our approach examines all particles without extraction or derivatization and thus provides an overall view of particle compositions.

Section snippets

Background on X-ray microscopy

X-ray absorption edges arise when an incident X-ray photon exceeds the threshold energy needed to completely remove (ionize) an electron from an inner-shell orbital. The result is a step-like rise (the “edge” in X-ray absorption near edge structure spectroscopy) in the absorption cross section. Photons with energies just below the ionization edge (280–300 eV in the case of carbon 1s electrons) can promote core electrons into a variety of bound states that correspond to unoccupied or partially

Methods

Archived sediment trap samples from the Arabian Sea JGOFS program were analyzed from station M4 (531, 814, 2222, and 3369 m). The collection methodologies are described in Lee et al. (1998) and Wakeham et al. (2002). Small subsamples (∼1 mg) were embedded in reagent grade elemental sulfur by briefly heating (∼120 °C) a mixture of fine S° grains with the samples on a clean glass slide until the mixture melted into a drop. The drop was cooled, removed from the slide with a razor, and glued to an

Results

Rather than present spectral libraries of possible model organic compounds present in sinking POM, we present below representative spectra found in an investigation of more than 60 POM particles analyzed. Although material from four traps was analyzed, the number of particles observed ranged from 7 (531 m trap) to 20 (2222 m trap). The limited number of observations, necessitated by an average analysis time of 2 h per observation (one to five particles per observation), makes combining all

Overall distributions

The roughly 60 particles from Arabian Sea sediment traps examined by C-XANES were dominated by acid-rich (∼25%), aliphatic (15%), protein-rich (15%), and quinone/aromatic (10%) compositions (Fig. 3). In many cases, particles consisted of adjoining regions with different chemical compositions, such as carbohydrate/acid, protein/acid, and algaenan/acid; in several cases, particles consisted of three or more dominant chemical types distributed non-homogenously (denoted as Mix, Fig. 3). Our

Conclusions

The use of synchrotron-radiation-based soft X-ray spectromicroscopy has enabled the examination of individual particle compositions from sediment trap samples with minimal sample alteration. Particles collected in traps from 531, 814, 2222, and 3369 m depths from station M4 in the Arabian Sea JGOFS program (Lee et al., 1998) showed a variety of individual particle phases, but were dominated by four types. Particles with strong 288.5 eV peaks characteristic of carboxylic acids formed the highest

Acknowledgements

This work was supported by NSF grants OCE-0221295 and OCE-0118036 (JAB), OCE-9310364 (SGW), and OCE-9312694 (CL). The National Synchrotron Light Source is a Department of Energy supported facility. The authors wish to thank Patty Garlough and Paul Haberstroh (UT) for assistance in sample handling and sample analysis, and Mirna Lerotic (SUNY) for assistance in PCA and cluster analysis. This work is dedicated to the memory of John Hedges, who provided encouragement, stimulating discussions and

References (59)

  • J.J. Middelburg et al.

    Black carbon in marine sediments

    Marine Chemistry

    (1999)
  • U. Passow et al.

    The origin of transparent exopolymer particles (TEP) and their role in the sedimentation of particulate matter

    Continental Shelf Research

    (2001)
  • Y. Qian et al.

    Stable isotope fractionation of biomonomers during protokerogen formation

    Chemical Geology

    (1992)
  • M. Tsapakis et al.

    Evaluation of atmospheric transport as a nonpoint source of polycyclic aromatic hydrocarbons in marine sediments of the Eastern Mediterranean

    Marine Chemistry

    (2003)
  • S.G. Wakeham et al.

    Molecular indicators of diagenetic status in marine organic matter

    Geochimica et Cosmochimica Acta

    (1997)
  • S.G. Wakeham et al.

    Lipid biomarker fluxes in the Arabian Sea, with a comparison to the equatorial Pacific Ocean

    Deep-Sea Research. Part 2. Topical Studies in Oceanography

    (2002)
  • R.A. Armstrong et al.

    A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals

    Deep-Sea Research. Part 2. Topical Studies in Oceanography

    (2002)
  • C.K. Boyce et al.

    Organic chemical differentiation within fossil plant cell walls detected with X-ray spectromicroscopy

    Geology

    (2002)
  • J. Cazaux

    A physical approach to the radiation damage mechanisms induced by X-rays in X-ray microscopy and related techniques

    Journal of Microscopy-Oxford

    (1997)
  • V. Cherezov et al.

    Too hot to handle? Synchrotron X-ray damage of lipid membranes and mesophases

    Journal of Synchrotron Radiation

    (2002)
  • W.C. Chin et al.

    Spontaneous assembly of marine dissolved organic matter into polymer gels

    Nature

    (1998)
  • W.G. Deuser et al.

    Fly-ash particles intercepted in the deep Sargasso Sea

    Nature

    (1983)
  • R.R. Dickerson et al.

    Analysis of black carbon and carbon monoxide observed over the Indian Ocean: implications for emissions and photochemistry

    Journal of Geophysical Research, D: Atmospheres

    (2002)
  • P. Dumas et al.

    Biological and biomedical applications of synchrotron infrared microspectroscopy

    Journal of Biological Physics

    (2003)
  • M. Feser et al.

    Instrumentation advances and detector development with the Stony Brook scanning transmission X-ray microscope

  • M. Feser et al.

    Scanning transmission soft X-ray microscopy at beamline X-1A at the NSLS—advances in instrumentation and selected applications

  • G.F. Foster et al.

    Investigation of radiation-damage to biological specimens at water window wavelengths

    Review of Scientific Instruments

    (1992)
  • J.T. Francis et al.

    Inner-shell spectroscopy of para-benzoquinone, hydroquinone, and phenol—distinguishing quinoid and benzenoid structures

    Journal of Physical Chemistry

    (1992)
  • J.I. Hedges et al.

    Evidence for non-selective preservation of organic matter in sinking marine particles

    Nature

    (2001)
  • Cited by (0)

    View full text