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Chemical Reference Materials: Setting the Standards for Ocean Science 4 Chemical Reference Materials for the Analysis of Particulate and Sediment Samples RATIONALE FOR SEDIMENT AND PARTICULATE MATTER ANALYSES Many of the analytes of interest for solid phase chemical reference materials are the same as those in seawater, but the need for and the preparation of reference materials for suspended particulate matter and sediments is quite different. The low concentrations of many seawater species and the presence of the salt matrix create particular difficulties for seawater analyses. However while sediments frequently have higher component concentrations than seawater, they also have more complicated matrices that may require unique analytical methods. A number of particulate inorganic and organic materials are employed as paleoceanographic proxies, tracers of terrestrial and marine input to the sea, measures of carbon export from the surface waters to the deep sea, and tracers of food-web processes. Some of the most important analytes are discussed below as they relate to important oceanographic research questions. Major Bio-organic Elements As discussed in Chapter 3, measurements of the major elements composing particulate organic matter are among the longest established, most fundamental, and broadly informative analyses in the marine sciences. This tradition is based on the fact that six elements (carbon, hydrogen,
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Chemical Reference Materials: Setting the Standards for Ocean Science oxygen, nitrogen, sulfur, and phosphorus) make up essentially all the mass of marine organic matter and largely govern the processes by which these elements are cycled in the ocean. The rationale for the analysis of specific elements is presented below. Carbon Carbon is the most abundant element common to all organic substances (approximately 55 wt percent of average marine plankton) (Hedges et al., 2002) and has been used as the practical currency for measuring total organic matter concentrations in dissolved and particulate marine samples (MacKinnon, 1981). Carbon comprises two stable isotopes (12C and 13C), and one useful radioactive isotope (14C), which together provide indicators of source, reaction history, age, and dynamics of the organic substances in which they occur (Raymond and Bauer, 2001). Carbon constitutes 27 wt percent of CO2, a potent greenhouse gas and driver of climate change (Siegenthaler and Sarmiento, 1993; Sarmiento et al., 1998). At present, about 10 percent (3.5 Gt, 1 Gt = 1015 g) of all carbon actively cycling at the Earth’s surface occurs in organic substances. The global rate of net photosynthesis (approximately 100 Gt C/yr) is sufficient to pass the entire mass of carbon currently in atmospheric CO2 through living biomass in less than 10 years. Thus, any process or perturbation that affects the formation or remineralization rate of organic carbon can substantially influence the global carbon cycle and the potential for climate change within a human life span. Carbon Isotopes As mentioned above, the three isotopes of carbon are powerful tools for studying ocean processes, the carbon cycle and paleoceanography. 13C and 14C in biological materials and sediments are most commonly measured in bulk organic matter and in either bulk CaCO3 or isolated foraminiferal shells. With the advent of isotope-ratio-monitoring gas chromatography mass spectrometry (irm-GCMS), sophisticated analytical isolation techniques, and accelerator mass spectrometry (AMS), carbon isotopes can now be measured in specific organic compounds isolated from sediments and particulate material. In coastal areas, measurements of δ13C in bulk organic matter can help identify the origins of organic material in sediments. In general, material produced using the dominant C3 photosynthetic pathway has a value of δ13C around −27 ‰ for terrestrial matter and around −20 ‰ for marine matter (Deines, 1980). The interpretation of such results is complicated because some plants use the C4 photosynthetic pathway, which
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Chemical Reference Materials: Setting the Standards for Ocean Science produces terrestrial matter with a δ13C value near −12‰, and other plants use both pathways (CAM plants)1 (Deines, 1980). In the open ocean, δ13C values of particulate organic matter in the surface ocean vary greatly. Isotopic variations have been linked to changes in temperature and latitude (Freeman and Hayes, 1992; Goericke and Fry, 1994). Fractionation is also a function of the concentration of CO2 in the aqueous form, the δ13C of the CO2 (a function of temperature), and the fraction of the dissolved inorganic carbon pool taken up by an organism (Hayes, 2001; Laws et al., 2001). In addition to its use in studying organic matter, the measurement of δ13C in the CaCO3 of foraminifera is helping to evaluate the role of the deep ocean in global climate change. Long-term changes (i.e., over glacial cycles) in the mean ocean δ13C and the isotopic relationship to nutrients over time can be similarly studied. Radiocarbon measurements provide the accurate chronologies required to pinpoint causal events, either by determining sedimentation rates or by assigning an actual date to a specific event. AMS now provides the capability to measure radiocarbon routinely in small (0.5-1 mg) samples, making it possible to add chronologies to paleoceanographic studies of sediments as well as to use the natural 14C cycle as a tracer of the transfer of carbon among the different global carbon pools. Measurement of radiocarbon in bulk organic matter establishes time histories within sediment columns and the rates of sediment accumulation in carbonate-poor areas of the ocean. The presence or absence of bomb-produced radiocarbon in bulk particulate and sedimentary matter provides clues as to how recently the material was actively involved in the carbon cycle. Calibrations of ∆14C in corals with U/Th series isotopes are extending the calendar chronology of 14C back 20,000 to 40,000 years, allowing researchers to better study the events causing glacial/interglacial cycles (Bard et al., 1999). Radiocarbon measurements of bulk CaCO3 can help provide information on the role of carbonate dissolution within sediments in the global carbon cycle (Broecker et al., 1980; Keir and Michel, 1993; Martin et al., 2000). Measurement of ∆14C in coral bands provides a record of ∆14C in the surface ocean (Druffel, 1995) and can be used to track the oceanic uptake of the atmospheric bomb pulse described in Chapter 3. Studies of the uptake of the bomb pulse provide useful constraints to ocean mixing models (Toggweiler et al., 1989a,b). The very detailed records of seasonal and annual fluctuations of δ 14C in corals provide 1 There are three major photosynthetic pathways: C3, C4 and CAM. Please see glossary for further detail. (Appendix C).
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Chemical Reference Materials: Setting the Standards for Ocean Science tighter constraints on ocean mixing in global carbon cycle models (Guilderson et al., 1998, 2000). Isotopes in specific organic compounds will be discussed separately. Nitrogen, Phosphorus, and Sulfur Nitrogen and phosphorus, in addition to being potentially limiting nutrients, provide an indication of the source and history of the organic materials in which they occur. Both elements are typically lost in preference to carbon during the digestion and bacterial degradation of plankton biomass, and therefore have potential use as indicators of substrate freshness. Because marine organic nitrogen occurs predominantly in protein, carbon to nitrogen (C:N) ratios can be indicative of the food quality of different organic mixtures. Since increased nitrogen richness is characteristic of marine mixtures, C:N ratios have also been used to discriminate plankton- from land-derived organic matter in coastal ocean zones. Such C:N-based assessment of organic source and food quality, however, must take into account that nitrogen can become elevated versus carbon in the course of biodegradation (Suess and Müller, 1980). Neither nitrogen nor phosphorus has a long-lived naturally occurring radioisotope, and phosphorus has only one stable isotope. However, 15N to 14N ratios can be used under appropriate circumstances to detect nitrogen fixation, denitrification, cumulative uptake of upwelled nutrients, and passage of nitrogen up trophic levels (Goericke et al., 1994). Such stable isotope analyses have the potential to become a powerful tool in ecosystem studies that exploit the isotopic fractionation associated with the transfer of carbon and nitrogen between trophic levels (Gannes et al., 1997). Organic sulfur accounts for less than 1 wt percent of average marine plankton, is not a limiting nutrient in the ocean, and is difficult to quantify in marine samples due to interference by seawater sulfate. Hydrogen and Oxygen Hydrogen and oxygen, although major components of marine organic matter with multiple stable isotopes, have remained largely “orphan elements” with respect to biogeochemical studies. In large part, this analytical avoidance results from the challenge of measuring these elements in a sea of potentially interfering water in living cells, hygroscopic salts, and hydrous minerals. Current estimates of the elemental composition of marine plankton biomass based on measured biochemical compositions (Anderson, 1995) and spectroscopic analysis (Hedges et al., 2002) indicate a compositional formula near C106H177O37N17S0.4. This study did not consider phosphorus (or its associated oxygen), as it undergoes no
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Chemical Reference Materials: Setting the Standards for Ocean Science change in redox state during photosynthesis and remineralization and thus does not affect respiration demand. This revised formula corresponds to much lower hydrogen (8 wt percent) and oxygen (26 wt percent) contents than given in the Redfield-Ketchum-Richards equation (see Chapter 3) and requires approximately 10 percent more oxygen for complete respiration. Because the polysaccharides, proteins, and lipids that compose marine biomass have uniquely different elemental composition, their weight percent in plankton (and their remains) could be directly determined if the content of oxygen and hydrogen were measured in addition to carbon and nitrogen. The hydrogen content of organic matter is also of broad interest because it is positively related to the petroleum formation potential of marine sediments (Pederson et al., 1992; Gélinas et al., 2001a) and rocks (Demaison and Moore, 1980). Accurate measurements of the major elements composing marine organic matter offer a wealth of useful biogeochemical information, if appropriate methods can be developed and compared for widely available reference materials. Specific Organic Compounds The multitude of organic compounds present in the environment makes it impossible to discuss each compound, or even compound class. A general rationale for the matrix approach to preparation of reference materials is presented below, as well as a specific discussion of several classes of particular importance. The last 50 years have seen a steadily increasing fraction of ocean measurements directed toward organic substances. This growing emphasis reflects the importance of organic molecules as the fundamental components of all living organisms and as key agents for the transfer and storage of the partially degraded remains of plants and animals in ocean waters and sediments. Organic molecules occur in a multitude of structures that are chemically unique and often persistent, and which carry a wealth of information about their biological sources and subsequent reaction histories. Almost all the organic matter in the ocean occurs as the incompletely degraded nonliving remains of marine organisms—living biomass is relatively minor. On average, organic substances account for about one in thirty carbons dissolved in seawater, four of five carbons sinking in particulate debris from the surface to the interior ocean, and one in five of all carbons preserved in marine sediments. Lipid and Pigment Biomarkers The synthesis, destruction, and vertical transport of organic matter in the marine environment all have important implications for the produc-
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Chemical Reference Materials: Setting the Standards for Ocean Science tivity of higher trophic levels (e.g., micronekton and fish) in coastal, pelagic, and benthic habitats. The major components of the marine planktonic food web include bacteria, Archaea, phytoplankton, and zooplankton. Knowledge of plankton community structure and its variability is therefore essential for evaluating the predictive ability of complex food web models and assessing ecological response to climate change (e.g., the El Niño Southern Oscillation, the Pacific Decadal Oscillation, and the North Atlantic Oscillation). Historically, the determination of the biomass and diversity of the microbial component (bacteria, Archaea, and phytoplankton) within planktonic food webs posed a difficult problem for oceanographers. A range of uncertainties, including the selectivity of various culturing methods, overlapping size distributions, poor preservation properties, and the subjectivity of microscopic methods have encouraged the development of lipid biomarker approaches for characterizing microbial communities. Lipids are major constituents of all living cells and include a wide range of functional and storage biomolecules, such as chlorophylls, porphyrins, carotenoids, hopanoids, sterols, fatty alcohols, diterpenes, ubiquinones, fatty acids, and waxes. This enormous structural diversity reflects, in part, phylogenetic relationships, and consequently, specific lipid compounds are frequently employed as biomarkers (Table 4.1). Lipid biomarkers have varying degrees of taxonomic specificity. The carotenoids and sterols of microalgae, for example, are typically used to provide taxonomic distinction at the class level. Individual fatty acids, hydrocarbons and sterols reflect their different plankton sources (Wakeham and Lee, 1993) and help distinguish terrestrial from marine origins. Bacteria add diagnostic branched-chain fatty acids to particles they colonize (de Baar et al., 1983; Wakeham and Canuel, 1988). Chlorophyll a (Chl a) functions as the primary light harvesting pigment in marine oxygenic phototrophs. Even though the C:Chl a ratio of photoautotrophic cells varies considerably as a function of environmental conditions and growth rate (Laws et al., 1983), measurements of Chl a have been used extensively to estimate the biomass of photoautotrophic microorganisms in the sea. Cells are typically concentrated by filtration and extracted into an organic solvent (usually acetone) after which, pigments are detected by fluorescence or absorption spectroscopy, sometimes after chromatographic separation (Bidigare and Trees, 2000). The application of HPLC to phytoplankton pigment analysis has lowered the uncertainty in the measurement of Chl a and accessory carotenoids, since compounds are physically separated and individually quantified. Pigment distribution is useful for quantitative assessment of phytoplankton community composition, phytoplankton growth rate and
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Chemical Reference Materials: Setting the Standards for Ocean Science TABLE 4.1 Examples of Microbial Biomarkers and Potential Source Organisms (Volkman, 1986; Ourisson et al., 1987; Ratledge and Wilkinson, 1988; Mayer et al., 1989; Conte et al., 1994; Jeffrey et al., 1997; Béjà, et al., 2000: Kolber et al., 2000; Madigan et al., 2000) Biomarker Potential Source Organism(s) Tetrapyrroles Divinyl chlorophylls a and b Prochlorococcus spp. Monovinyl chlorophyll b Chlorophytes, prasinophytes Chlorophylls c1, c2 and c3 Chromophyte microalgae Bacteriochlorophyll a Anoxygenic photosynthetic bacteria Carotenoids Peridinin Dinoflagellates Fucoxanthin Diatoms 19’-butanoyloxyfucoxanthin Pelagophytes 19’-hexanoyloxyfucoxanthin Haptophytes Alloxanthin Cryptophytes Prasinoxanthin Prasinophytes Lutein Chlorophytes Zeaxanthin Cyanobacteria, chlorophytes C20 isoprenoids Phytol Photoautotrophs All trans-retinal Proteobacteria Ether-linked lipids Archaea Sterols Dinosterol Dinoflagellates 24-methylcholesta-5,22E-dien-3β-ol Diatoms, Haptophytes 24-methylcholesta-5,24(28)-dien-3 β-ol Diatoms 24-methyl cholest-5-en-3β-ol Chlorophytes C37-39 alkenones Emiliania huxleyi and Gephyrocapsa oceanica Hopanoids Diploptene, hopanoic acids Prokaryotes, including cyanobacteria Lipopolysaccharides (LPS) β-hydroxy-acids Gram-negative bacteria Polar lipid fatty acids Branched-chain C15 and C17 acids Bacteria, especially Bacillus spp. Peptidoglycan D-amino acids Bacteria, mainly gram-positive strains
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Chemical Reference Materials: Setting the Standards for Ocean Science zooplankton grazing activity. Global maps of Chl a distribution are now available daily from satellite-based ocean color sensors such as SeaWiFS and MODIS. These high resolution (less than or equal to 1 km) Chl a images have provided new insight into the effect of physical and climate driven forcing on phytoplankton biomass and productivity at regional-to-global scales (Chavez et al., 1999; Behrenfeld et al., 2001; Seki et al., 2001). Biomarker distributions can also provide important information on microbial metabolism. For example, the presence of Chl a, bacteriochlorophyll a, or trans-retinal in a culture or environmental sample would indicate microbial growth driven by oxygenic photosynthesis, anoxygenic photosynthesis, or photo-heterotrophy, respectively (Takaichi et al., 1990, 1991; Béjà et al., 2000; Kolber et al., 2000). Diagenetic products for many of the lipid biomarkers have been characterized structurally and used as “molecular fossils” to study ancient rocks, sediments, and petroleum (Lee and Wakeham, 1988; Wakeham and Lee, 1989; Peters and Moldowan, 1993; Kenig et al., 1994; Summons et al., 1999; Moldowan et al., 1996). These studies have demonstrated that biomarkers have utility for tracing the evolution of life in the geologic record, recreating the structure of ancient microbial communities, and determining which organisms contribute to hydrocarbon deposits. The haptophyte microalga Emiliania huxleyi produces biomarkers in the form of long-chain (C37, C38, and C39) alkenones (Brassell, 1993). Alkenones are well preserved in marine sediments and their molecular distributions and isotopic composition have been used to infer paleotemperatures (Brassell, 1993) and pCO22values (Jasper et al., 1994), respectively. Unsaturation patterns in the alkenone series are related to the growth temperature of the haptophyte algae that produce these compounds (Brassell et al., 1986; Prahl and Wakeham, 1987), and hold great promise as indicators of absolute ocean paleotemperature. A variety of molecular indicators of the freshness (and perhaps nutritional quality) of organic matter exists. For example, certain labile phytoplankton constituents, such as polyunsaturated fatty acids, are readily degraded in the environment or in herbivore guts, and are thus depleted in more degraded particles (de Baar et al., 1983; Wakeham and Canuel, 1988). Preferential loss of labile algal fatty acids resulting in the enrichment of more stable components in the products of heterotrophic metabolism has been observed in both field studies and laboratory feeding experiments (Prahl et al., 1985; Wakeham and Canuel, 1988; Harvey et al., 2 Partial pressure of CO2. When partial pressure in one medium is higher thatn another there will be a net flow of gas from that phase to the other.
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Chemical Reference Materials: Setting the Standards for Ocean Science 1987). A resistant organic matrix may protect otherwise labile lipids. For example, the resistant waxy coating common to land plants appears to physically shield terrestrial biomarkers, which are then selectively preserved, as opposed to their marine-derived counterparts in abyssal sediments (Gagosian et al., 1983; Wakeham et al., 1984; Volkman , 1986). The freshness of organic matter can also be influenced by an intimately associated mineral or organic matrix (Gordon and Millero, 1985; Hedges and Keil, 1995; Mayer, 1994). Amino Acids and Sugars Amino acids are structural components of proteins and compose the largest reservoir of organic nitrogen in most organisms. They make up a major fraction of characterized carbon in marine particulate matter and are useful indicators of decomposition and transport in the marine environment. Their natural occurrence in marine plankton (Degens and Mopper, 1976; Tanoue et al., 1982; Cowie and Hedges, 1996), suspended and sinking particles (Lee and Cronin, 1982, 1984; Ittekkot et al., 1984a, b; Cowie and Hedges, 1992; Cowie et al., 1992), and sediments (Henrichs and Farrington, 1984; Henrichs et al., 1984; Dauwe and Middleburg, 1998) is frequently used to elucidate the biogeochemical behavior of marine organic matter, particularly the diagenetic state. Chromatographically measurable carbohydrates constitute a maximum of 20 to 40 percent of plankton carbon, 13 percent of sinking particulate organic carbon (POC), 15 percent of suspended POC, and 13 percent of sedimentary carbon (Hernes et al., 1996). Amino acids and carbohydrates can be used to elucidate the fate of organic carbon and to infer past marine sources and oceanic conditions. Although generally not as species-specific as lipids and pigments, compositional differences among carbohydrates and amino acids can also reflect biological sources. For example, some siliceous diatoms are characterized by elevated concentrations of glycine and fucose while organisms with carbonate tests often exhibit unusually high concentrations of aspartic acid and arabinose (Hecky et al., 1973; Carter and Mitterer, 1978; Ittekkot et al., 1984a,b). Although both plankton and bacteria are characterized by high relative abundances of ribose and fucose (Cowie and Hedges, 1984), O-methyl sugars and uronic acids may provide a means of discriminating between these two biological sources (Mopper and Larsson, 1978; Bergamaschi et al., 1999). In addition, muramic acid can serve as a specific marker for bacteria (Lee et al., 1983). High relative concentrations of nitrogen and carbon in the form of amino acids (Whelan, 1977; Lee and Cronin, 1984; Cowie and Hedges, 1992) and carbohydrates (Cowie et al., 1992; Cowie and Hedges, 1992,
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Chemical Reference Materials: Setting the Standards for Ocean Science 1984) indicate relatively undegraded organic matter. Conversely, high relative concentrations of characteristic diagenetic products, such as the non-protein amino acids ornithine and β-alanine can indicate the presence of bacterially degraded material (Lee and Cronin, 1982, 1984; Ittekkot et al., 1984a,b). The combined use of multiple amino acid- and carbohydrate-based diagenetic indicators provides a sensitive and consistent means of comparing the state of organic matter in aquatic environments (Hedges et al., 1999). Preservation of certain amino acids and sugars occurs in both siliceous and carbonate tests. These can be sensitively detected in part because amino acid composition of fresh materials is so uniform. Carbon and Nitrogen Isotopes in Specific Compounds Although carbon isotopic measurements of bulk organic material can be very informative, much more information can be obtained from the isotopic characterization of specific compound classes or individual organic compounds isolated from bulk material. Such compound-specific isotope analyses (CSIA) can target individual molecules known to be associated primarily with specific processes (e.g., photosynthesis) or with specific sources. The stable isotopic compositions of algal organic matter can provide important insight into the environmental conditions under which carbon and nitrogen fixation occur (Hayes et al., 1990; Goericke et al., 1994). Determination of δ13C and δ15N provides valuable tools for assessing the rates of ancient biological processes such as phytoplankton growth and NO3 uptake (Altabet and Francois, 1994; Bidigare et al., 1999). Application of stable isotopes to the modern ocean and sedimentary record, however, is confounded by the presence of non-algal carbon and nitrogen biomass. To resolve this problem, CSIA has been employed as a means of estimating the δ13C and δ15N of modern and ancient phytoplankton populations (Jasper et al., 1994; Bidigare et al., 1997, 1999; Sachs and Repeta, 1999). Researchers are now investigating the potential for dating targeted compound classes and individual compounds to understand the true chronology of sediments. Pearson et al. (2000, 2001) measured the radiocarbon content of individual sterols and other lipids isolated from sediments. While most of the compounds were derived from marine photosynthesis or subsequent remineralization, data from the archeal isoprenoids were consistent with chemoautotrophic growth below the euphotic zone. Eglinton et al. (1997) found significant variation in the age of individual compounds isolated from the same sediment, indicating that older components either have a different inorganic carbon source or are stored for appreciably longer time periods in “upstream” reservoirs.
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Chemical Reference Materials: Setting the Standards for Ocean Science Radionuclides Measurements of radionuclides and metals in marine sediments and particulate matter are conducted for a variety of purposes, including the determination of sedimentation rates, trace metal and radionuclide fluxes through the water column, enrichment of metals in specific phases of the sediments, and examination of new sedimentary phases produced after sediment deposition. Such studies address fundamental questions concerning the chronology of deep-sea and near-shore sedimentary deposits, removal mechanisms and cycling of metals in the ocean, and diagenesis within deep-sea sediments. Numerous radionuclides have been applied to marine sedimentary problems. These are generally grouped into cosmic-ray produced (cosmogenic) nuclides (14C, 10Be, 7Be, 26Al), nuclear bomb-produced (fallout) nuclides (137Cs, plutonium isotopes, 241Am), and naturally occurring nuclides ultimately derived from the decay of 238U, 235U, and 232Th parents. 230Th and 231Pa are ubiquitous components of recently deposited deep-sea sediments because they are produced uniformly throughout the ocean from the decay of dissolved uranium isotopes and they are actively collected onto sinking particles. The distribution with depth of these nuclides in deep-sea sediments may be modeled to estimate rates of sedimentation extending over the past 200 to 300 thousand years. These techniques complement 14C dating methods that only extend to about 40 thousand years before the present. Other techniques are used for shorter time scales, including the measurement of the 226Ra:Ba ratio in barite extracted from sediments (Paytan et al., 1996). This technique has a time scale of about five thousand years. Alternately, assessments of rapid sedimentation and bioturbation on time scales of days to centuries require shorter half-life nuclides such as 210Pb, 228Th, 234Th, and 222Rn. INFLUENCE OF MATRIX COMPOSITION ON CHEMICAL DETERMINATIONS Several major matrix types are found in marine particles and sediments. Marine organisms surround themselves with tough polymeric organic cell walls and/or with opal or calcium carbonate tests. These contrasting matrices respond differently to various analytical methods. In sediments, the remains of these organisms combine with clay minerals to form a heterogeneous mixture. In this section, the influence of these matrices on analyte quantification are discussed.
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Chemical Reference Materials: Setting the Standards for Ocean Science technique involves direct measurement of gamma rays emitted during radionuclide decay. The interaction of these gamma rays with the sample is a function of the energy of the gamma-ray and the composition of the sample; low energy gamma rays are strongly absorbed by sedimentary minerals. For example, 210Pb is often measured by direct counting of low energy gamma rays (46.5 keV) that are partially absorbed by minerals within the sample matrix. The fraction of the gamma rays absorbed by the sample is a function of the sample matrix. There are methods to account for self-absorption, but there are no widely distributed reference materials to test these corrections. Similar problems exist for measuring gamma rays from other radionuclides. Measurement of radionuclides by destructive analyses can involve combustion, fusion, leaching, or acid-dissolution of the samples. The choice of technique depends on both the radionuclide of interest and the sample matrix. Techniques developed for one matrix may not work for other matrices. In addition, many radionuclides in sediments exist both as surface-bound components adsorbed by particles sinking through the water column, and also as a structural component within physically intact mineral grains. These detrital structural components generate a background of supported activity that does not change with time. Thus, to use nuclides such as 230Th and 231Pa for dating, the radioactivity derived from uranium decay in the solid (supported activity) must be subtracted from that derived from adsorbed 230Th and 231Pa (unsupported activity). Different analytical techniques for these physically dissimilar phases may lead to different corrections for supported activity, which has yet to be separated with convincing conclusions. For the 226Ra:Ba ratio in barite technique, the results depend on the isolation of a pure barite sample and the assumption that the separation techniques do not alter the 226Ra:Ba ratio of the barite. Applying different techniques to reference sediments could strengthen this technique. REFERENCE MATERIALS CURRENTLY AVAILABLE FOR THE ANALYSIS OF SEDIMENT AND PARTICULATE SAMPLES Solid Matrix Reference Materials A selected list of reference materials (sediments as well as biological tissues) distributed by several Canadian, U.S., and E.U. sources shows a wide range of solid samples that could be used for comparative analysis of major organic elements (Table 4.2). These materials are widely available and have been analyzed for at least some constituents. In addition, these materials are homogeneous and can be expected to exhibit stable compositions over time. All of the thirty or so listed reference materials,
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Chemical Reference Materials: Setting the Standards for Ocean Science however, lack precise measurements of the major bio-organic element compositions of the materials. The trace organic components that are certified, such as PAH, PCBs, pesticides, and hydrocarbons, do not constrain the concentrations of the organic elements. These samples do have potential for use as consensus reference materials, but only after elemental data are provided by different labs for selected samples using contrasting and well-defined analytical procedures. The matrices and sources of the sediments listed in Table 4.2 are sometimes unclear. Those that are known are highly weighted toward clastic (quartz- and aluminosilicate-rich) marine sediments from coastal environments. Some of these reference materials, such as MESS-3 (NRC-Canada), MAG-1 (USGS) and the Arabian Sea and Pacific Ocean samples (IAEA 315, and 368), could provide excellent examples of clastic marine sediment representing the main repositories of organic matter in the ocean (Hedges and Keil, 1995). The listed materials fail to include both open-ocean opal and carbonate oozes, as well as pelagic red clays. The biological reference materials listed in Table 4.2 represent a small subset of all the biomass samples available for analysis, though it is representative of the emphasis placed on animals versus macrophytes. Reference materials for plankton are particularly limited, although samples of the freshwater green alga, Chlorella, are available for trace element analysis from the Slovak Institute of Radioecology and Japan’s National Institute of Environmental Studies. Notably, no reference materials appear to be available for marine phytoplankton of any type. This dearth of marine biological material for elemental analysis is unfortunate because it fails to represent the opal and carbonate matrices that can greatly complicate carbon, oxygen, and hydrogen analyses. Carbon Isotopes Organic and carbonate reference materials for δ13C and ∆14C are both available and widely used for isotopic analysis. The stable carbon isotope reference materials currently available are useful for instrument calibration and limited methods verification. These materials are available as powders for carbonates and in a wide range of forms for organic materials (e.g., oil, graphite, etc.) (Table 4.3). There is one certified primary standard for radiocarbon (NIST SRM 4990C) and a number of additional reference materials are available from the International Atomic Energy Agency (IAEA) in Vienna, and from Marian Scott at the University of Glasgow (Table 4.4, Appendix E). The FIRI samples represent a study in progress by an international group of radiocarbon researchers (Bryant et al., 2001). Samples from these sources are available primarily as discrete organic or carbonate materials and do not represent materials (such as
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Chemical Reference Materials: Setting the Standards for Ocean Science TABLE 4.2 Selected Solid Reference Materials Currently Available from Canadian, U.S., and European Sources with Potential Utility for Marine Organic Studies (See Appendix E for information about obtaining these materials.) Identifier Type Matrix Selected materials distributed by the National Research Council of Canada (NRC) MESS-3 Sed clastic marine sediment PACS-2 Sed clastic marine sediment HISS-1 Sed clastic marine sediment HS-3B Sed clastic marine sediment HS-4B Sed clastic marine sediment HS-5 Sed clastic marine sediment HS-6 Sed clastic marine sediment SES-1 Sed clastic estuarine sediment CARP-1 Bio whole carp residue DOLT-1 Bio dogfish muscle DORM-2 Bio dogfish muscle LUTS-1 Bio lobster hepatopancreas TORT-2 Bio defatted lobster pancreas Selected materials distributed by the U.S. National Institute of Standards and Technology (NIST) SRM 1648 Dust urban particulate matter SRM 1649A Dust urban dust SRM 1650A Soot diesel particulate matter SRM 1939 Sed clastic river sediment SRM 1941B Sed clastic marine sediment SRM 1944 Sed clastic marine sediment SRM 1945 Bio whale blubber Selected materials distributed by the International Atomic Energy Agency (IAEA), Austria IAEA-384 Sed marine sediment IAEA-315 Sed clastic marine sediment IAEA-368 Sed marine sediment IAEA-356 Sed clastic marine sediment IAEA-383 Sed clastic marine sediment IAEA-408 Sed clastic marine sediment Selected material distributed by the U.S. Geological Survey (USGS) MAG-1 Sed clastic marine sediment Abbreviations: %OC = weight percent organic carbon, Sed = sediment, Bio = biological, E = elemental, TE = trace element, PAH = polynuclear aromatic hydrocarbons, PP = PAH + PCB, PPP = PAH + PCB + pesticides, H = hydrocarbons, S = sterols, and ND = not determined, U = Unknown.
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Chemical Reference Materials: Setting the Standards for Ocean Science Source Form Analyte %OC Beaufort Sea dry powder TE ~2 Esquimalt BC harbor dry powder TE ~3.3 Hibernia shelf, NFL dry powder TE ND Nova Scotia harbor dry powder PAH ND Nova Scotia harbor dry powder PAH ND Nova Scotia harbor dry powder PAH ND Nova Scotia harbor dry powder PAH ND U dry powder PAH ND Lake Huron water slurry PCB ND U dry powder TE ND U dry powder TE ND U water slurry TE ND U dry powder TE ND St. Louis atmosphere dry powder PAH ND Washington DC air dry powder PAH ND heavy duty engine dry powder PAH ND U dry powder PCB ND U dry powder PP ND U dry powder PPP ND U frozen tissue PP ND Fangatufa Atoll dry powder RN ND Arabian Sea dry powder RN ND Pacific Ocean dry powder RN ND U dry powder TE ND U dry powder PHS ND Targus Estuary dry powder PHS ND Gulf of Mexico dry powder E 2.15
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Chemical Reference Materials: Setting the Standards for Ocean Science TABLE 4.3 Carbon Isotopic Composition of Selected Carbon−bearing Isotopic Reference Materials (Coplen et al., 2001) Identifier Substance δ13C Reference NBS 18 CaCO3 (carbonate) −5.01 ± 0.06 Stichler, 1995; Coplen et al., 1983 NBS 19 CaCO3 (calcite) +1.95 Hut, 1987 IAEA−CO−1 CaCO3 (marble) +2.48 ± 0.03 Stichler, 1995 IAEA−CO−8 (IAEA-KST) CaCO3 −5.75 ± 0.06 Stichler, 1995 L−SVEC Li2CO3 −46.48 ± 0.15 Stichler, 1995 IAEA−CO−9 (IAEA-NZCH) BaCO3 −47.12 ± 0.15 Stichler, 1995 USGS24 C (graphite) −15.99 ± 0.11 Stichler, 1995 NBS 22 Oil −29.74 ± 0.12 Gonfiantini et al., 1995; Coplen et al., 1983 RM 8452 (Sucrose ANU) Sucrose −10.43 ± 0.13 Gonfiantini et al., 1995 NGS1 CH4 in natural gas −29.0 ± 0.2 Hut, 1987 NGS1 C2H6 in natural gas −26.0 ± 0.6 Hut, 1987 NGS1 C3H8 in natural gas −20.8 ± 1 Hut, 1987 NGS2 CH4 in natural gas −44.7 ± 0.4 Hut, 1987 NGS2 C2H6 in natural gas −31.7 ± 0.6 Hut, 1987 NGS2 C3H8 in natural gas −25.5 ± 1 Hut, 1987 NGS2 CO2 in natural gas −8.2 ± 0.4 Hut, 1987 NGS3 CH4 in natural gas −72.7 ± 0.4 Hut, 1987 NGS3 C2H6 in natural gas −55.6 ± 5 Hut, 1987 IAEA−CH−7 (PEF1) Polyethylene −31.83 ± 0.11 Gonfiantini et al., 1995 NOTE: Values for δ13C given in per mille relative to VPDB, defined by assigning a δ13C value of +1.95‰ to NBS 19 carbonate. (See Appendix E for information about obtaining these materials.) sediment) that have small amounts of organic matter present in a complex mineral matrix. Individual Organic Compounds A wide range of high quality, non-certified carotenoid and chlorophyll chemical standards are commercially available (e.g., Sigma-Aldrich, DHI, and Roth). The availability of a mixed pigment reference standard and biological matrix reference materials would improve analytical performance in individual laboratories, facilitate method and laboratory in-
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Chemical Reference Materials: Setting the Standards for Ocean Science TABLE 4.4 Reference Materials Available to the Radiocarbon Community (Rozanski et al., 1992; Bryant et al., 2001) Standard Material Consensus fm IAEA C-1 Carbonate Carrara marble 0.0002 IAEA C-2 Carbonate freshwater travertine 0.4114 IAEA C-3 Cellulose 1989 growth of tree 1.2941 IAEA C-5 Subfossil wood E. WI forest 0.2305 IAEA C-6 ANU sucrose 1.5061 FIRI A, B Wood 0.0033 FIRI C Turbidite Carbonate 0.1041 FIRI D, F* Dendro-dated wood (Belfast Scots Pine) 0.5705 FIRI E St. Bees Head humic acid 0.2308 FIRI G, J Barley mash 1.1069 FIRI H Dendro-dated wood (Hohenheim Oak) 0.7574 FIRI I Wood cellulose (Belfast Scots Pine) 0.5722 FIRI L* Whalebone 0.2035 *No longer available NOTE: The fraction modern (fm) listed is the consensus value reported by the directors of the studies. (See Appendix E for information about obtaining these materials.) ter-comparisons, and help resolve inter-method variability during oceanographic expeditions. A variety of high quality, non-certified organic chemical standards are commercially available, e.g., from Alltech (fatty acids), Pierce Chemical Co. (amino acids), Sigma-Aldrich (phytol and a wide range of fatty acids and amino acids), Steraloids, Inc. (a wide range of marine and terrestrial sterols), and Chiron AS (diagenetic products of key lipid biomarkers, including deuterated internal standards). Radionuclides Several organizations (e.g., NIST, NRC-Canada, and IAEA) provide sediment reference materials containing radionuclides, many of which are only certified for artificial radionuclides (137Cs, 90Sr, 241Am, and 239Pu). Certain specific radionuclides have no certified natural matrix materials, including ocean, lake, and river sediments. Although these sediments are certified for a few naturally occurring and artificial radionuclides, the extent of radioactive equilibrium of the uranium and thorium decay series in these environmental materials is not provided. NIST currently offers an ocean sediment Standard Reference Material3 (SRM 4357) in 3 The term “Standard Reference Material” is a trademark of NIST.
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Chemical Reference Materials: Setting the Standards for Ocean Science which 230Th, 226Ra, 232Th, 228Th, and numerous artificial radionuclides are certified, and which give non-certified activities of uranium isotopes, 210Pb, 228Ra, and additional artificial radionuclides. This reference material is a blend of sediments collected from both the Chesapeake Bay and from the seafloor off of the British Nuclear Fuels Sellafield facility in the United Kingdom. The IAEA supplies reference materials in the form of a number of marine and terrestrial sediments, soils, and ores, some of which are certified for long-lived members of the uranium and thorium decay series, as well as trace elements. The IAEA reference materials are prepared in limited quantity and replaced frequently. Radiochemists require reference materials with matrices that reflect sediments with widely different composition. For example recently deposited deep-sea sediments contain 230Th and 231Pa primarily as surface-adsorbed components, whereas sediments collected on (or close to) continental margins contain a higher proportion of structurally-bound components. No reference materials exist that can be used to test selective leaching procedures designed to identify how these radionuclides are incorporated into the matrix. Additionally, no reference materials exist with which to evaluate matrix-correction procedures in gamma-ray analyses or to compare analytical results of radionuclide measurements from different laboratories. To correct this problem, radiochemists need solid reference materials containing known amounts of the longer-lived radionuclides with an estimate of the degree of radioactive equilibrium through the series. Reference materials that represent the primary deep-sea and coastal depositional environments and biological materials would solve many of the problems that radiochemists face in analysis of sediments from these settings. Radiochemists require reference materials comprising the primary end member sediment and biological types (calcium carbonate, opal, and red clay from the deep-sea and carbonate-rich, silicate-rich, and clay mineral-rich sediments from coastal environments and representative biological materials). Additional sediment reference material from a river delta would be valuable to test the release of radionuclides that occurs as riverine particles contact seawater. RECOMMENDED REFERENCE MATERIALS Given that analyses of particulate and sedimentary elements and compounds are highly matrix-dependent, the committee recommends that reference materials be made available for ten important matrix types, covering three biological and seven sedimentary forms described below, which also cover a large variety of diagenetic states. Rather than recommend that these reference materials be certified for a wide variety of
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Chemical Reference Materials: Setting the Standards for Ocean Science constituents, the greatest value will be derived by producing stable, homogeneous materials that can be used to derive community consensus values for many specific analytes. Further certification could then be accomplished as needed. Recommended Biological Matrices The diversity of marine photosynthetic microalgae and bacteria is extensive, making it impractical to develop reference materials for all of the species currently in culture. For the three biological matrices, it is recommended that diatom (Thalassiosira pseudonana), dinoflagellate (Scrippsiella trochoidea), and haptophyte (Emiliania huxleyi) mass cultures be developed as reference materials in order to improve analytical performance and facilitate inter-laboratory comparison. These three organisms provide a wide range of oceanographically relevant mineral, trace metal, and organic analytes for the establishment of a comprehensive list of consensus values (Table 4.5). They represent three major matrices—opal, carbonate, and organic matter. They also include the two mineral phases thought to be important “ballast” materials for facilitating preservation and vertical export of POM to the sediments (Armstrong et al., 2002). Consensus analyte values (e.g., isotope ratios) can be obtained for both the inorganic (calcium carbonate, opal) and organic (POC, PN, and biomarkers) phases of these biological matrices. Biological reference materials can also be used for the preparation of both C37:2 and C37:3 alkenone standards (isolated from the E. huxleyi) and a mixed pigment standard containing a wide range of individual chlorophylls and carotenoids (prepared by mixing acetone extracts obtained from the three phytoplankton reference materials). Recommended Sedimentary Matrices Recommended sedimentary reference materials include three separate carbonate-, opal- and clay mineral- rich open-ocean sediments; three coastal sediments containing the same three mineral types; and a deltaic sediment that has not been in contact with seawater. To include relatively young material, each of these seven samples should be taken from within the top 10 cm of the sedimentary column. Table 4.6 suggests possible locations to sample the types of material matrices of interest. The combination of both coastal and open-ocean sediments would provide a wide range of diagenetic states, with the previously described phytoplankton reference materials providing fresh organic matrices and open-ocean redclay material providing some of the most degraded sedimentary material that exists in the marine environment. The mineral types represented in
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Chemical Reference Materials: Setting the Standards for Ocean Science TABLE 4.5 Distribution of Mineral, Trace Metal, and Organic Analytes in the Target Biological Matrices Recommended as Reference Materials (+ = present and − = absent). Analyte T. pseudonana S. trochoidea E. huxleyi Particulate Organic Carbon + + + Particulate Nitrogen + + + Minerals CaCO3 − − + Opal + − − Trace metals Fe + + + Zn + + + Tetrapyrroles Monovinyl chlorophyll a + + + Chlorophyll c1 + − − Chlorophyll c2 + + + Chlorophyll c3 − − + Carotenoids Dinoxanthin − + − Peridinin − + − Fucoxanthin + − − 19’-hexanoyloxyfucoxanthin − − + Diadinoxanthin + + + Diatoxanthin + + + β,β-carotene + + + Phytol + + + Sterols + + + C37-39 alkenones − − + Fatty acids + + + Amino acids + + + Nucleic acids (RNA and DNA) + + + Carbohydrates + + + Cellulose − + −
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Chemical Reference Materials: Setting the Standards for Ocean Science TABLE 4.6. Examples of Locations Where the Recommended Sedimentary Reference Materials Could be Obtained Matrix Types Open-Ocean Locations Coastal Locations Carbonate Mid-Atlantic Ridge Florida Bay Opal Southern Ocean Peru Upwelling Clastic detrital North Pacific Gyre Gulf of Mexico Deltaic clastic detrital Atchafalaya River Table 4.6 (opal, carbonate and aluminosilicate) were chosen to provide end members for matrix analysis. These samples also could be blended to produce any mixture desired. Sampling sediments from well-studied depositional regions such as MANOP Site R in the North Pacific (for open-ocean red clay) and Florida Bay (for carbonate sediments) would offer a variety of supplemental information that is already available. In addition, the (Atchafalaya) river sediment would provide a useful link between those studying terrestrial and marine processes. As riverine particulate matter encounters seawater, a variety of changes may occur to the terrestrial particles. Some surface-bound ions are released to solution as they exchange with dissolved species. Such exchange reactions are especially important in understanding the geochemistry of radium. Evaluation of these exchange reactions is often approximated by collecting sediment from the freshwater region of an estuary and exposing it to seawater. However, such experiments may contain artifacts due to a lack of control of the simulated conditions. A freshwater deltaic reference sediment could be employed to help refine these experiments and eliminate artifacts. POTENTIAL LONG-TERM NEEDS FOR ADDITIONAL REFERENCE MATERIALS Some primary standards exist for the uranium to thorium series isotopes currently used extensively in the field. There is a need, however, to produce a certified reference material for uranium and thorium decay series isotopes with masses greater than 226Ra, using a natural material such as uraninite. Sediment dating/mixing studies would benefit from a marine sediment reference material containing 210Pb. The committee recognizes that producing this material in different matrices would be difficult, though not impossible. Few samples contain background levels for the cosmogenic and bomb-produced radionuclides 10Be, 36Cl, 26Al,
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Chemical Reference Materials: Setting the Standards for Ocean Science 241Am, 90Sr, 239Pu, 240Pu, 137Cs, 129I, or 14C, which also lack solution standards. NIST has an active program to address the development of all of these radionuclide standards except 14C. Long-term needs for organic reference materials include ether-linked lipids, hopanoids, pheopigments, and mycosporine-like amino acids (MAA). Members of the Archaea possess diagnostic ether-linked isoprenoid lipids that are used in the construction of monolayer or bilayer cell membranes. These ether-linked lipids occur as glycerol diethers or diglycerol tetraethers. Hopanoids are chemical components found in bacterial cell membranes and are thought to play an important role in membrane stabilization. Ether-linked lipids and hopanoids are useful biomarkers for detecting the presence of Archaea and Bacteria in environmental samples. Pheopigments are magnesium-free chlorophyll degradation products that are useful as tracers of heterotrophic processes such as zooplankton grazing. MAAs are imino-carbonyl derivatives of mycosporine cyclohexenone. They are structurally diverse and widely distributed among marine cyanobacteria, microalgae, invertebrates and vertebrates. MAAs absorb strongly in the 310-360 nm waveband and are thought to protect marine organisms from the damaging effects of ultraviolet radiation. It is likely that the need for quantification of these organic compounds will grow in the future. Consequently, future matrix-based reference materials should be prepared from materials that contain these analytes.
Representative terms from entire chapter: