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Molecular indicators of diagenetic status in marine organic matter
Geochimicaet CosmochimicaActa, Vol. 61, No. 24, pp. 5363-5369, 1997 Copyright © 1997 ElsevierScienceLtd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00
Pergamon
P I I S0016-7037(97)00312-8
Molecular indicators of diagenetic status in marine organic matter STUART G. WAKEHAM,1 CINDY LEE, 2 JOHN I. HEDGES,3 PETER J. HERNES,3 and MICHAEL L. PETERSON3 Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, Georgia 31411, USA 2 Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794-5000, USA 3School of Oceanography, P.O. Box 357940, University of Washington, Seattle, Washington 98195-7940, USA
(Received February 17, 1997; accepted in revised form August 19, 1997)
Abstract--The fluxes of individual carbohydrates, amino acids, lipids and pigments have been determined in net-plankton, particulate matter and sediments from three sites (9°N, 5°N, and 0°N) in the central equatorial Pacific to evaluate sources and reactivities of organic compounds. Although primary production rates vary markedly across this 9 ° swath, vertical trends in biochemical compositions remained remarkably parallel. Together these one hundred plus biochemicals account for 80% of the total organic carbon (C,,rg) in net-plankton and particles sinking from the euphotic zone, but represent only 24 and 20% of the organic carbon in deep-water particles and surface sediments, respectively. Scaled profiles of relative abundances, clearly illustrate (a) exponential losses of plankton remains and increases in heterotroph biomarkers throughout the water column, (b) elevated proportions of bacterial markers near the sediment surface, and (c) preservation of selected remains of bacteria, phytoplankton and vascular land plants deeper in the sediments. In spite of one of the most comprehensive analyses of major biochemicals yet applied to marine particulate samples, percentages of molecularly uncharacterized organic carbon increase progressively down the water column to values near 80% in the underlying sediments. The composition, formation pathway and information potential of this uncharacterized fraction are among the most fascinating questions in marine organic geochemistry. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION
the underlying sediments. These stages include (a) the planktonic source of particulate matter in the euphotic zone (net-plankton), (b) particles exiting the euphotic zone (floating sediment traps at 105 m), (c) particles transiting the ocean's interior (moored sediment traps at 1000 m below the sea surface and 1000 m above the sea floor), and (d) material being buried at 0 - 0 . 5 cm and the underlying 1 0 12 cm in the sediment. Molecular distributions of individual sugars, amino acids, lipids, and pigments were determined in this set of samples. Our choice of these biochemicals is based on their quantitative importance in living organisms and utility as tracers of sources and alteration processes. The EqPac study afforded a novel opportunity to observe molecular alteration across a broad suite of biochemicals in a comprehensive series of samples collected throughout the water column and sediments at a single open ocean site.
Particulate organic matter is the major vehicle for vertical redistribution of bioactive elements within the ocean. The rain of plankton-derived material out of surface waters supports essentially all life in deeper waters, transfers carbon and nutrients to depth, and imprints a record of water column processes into underlying sediments. Sinking organic particles are extensively (usually > 9 9 % ) and selectively degraded during transit to the deep-sea floor, leading to highly altered sedimentary records (Wakeham and Lee, 1993). Individual biochemicals or their ratios have been used to measure this diagenetic status of organic matter (Lee and Cronin, 1984; Ittekkot et al., 1984a,b; Cowie and Hedges, 1992; Wakeham, 1996). Knowing the diagenetic status of organic matter is essential for employing biomarkers in source studies and understanding carbon preservation. Organic matter reactivity, however, is a function of material matrix as well as inherent lability (Wakeham and Lee, 1989; Cowie and Hedges, 1992, 1994), so that diverse chemical reactivities do not correspond simply with compound class or structure in consistently predictable patterns. As part of the U.S. Global Ocean Flux Study (U.S. JGOFS) (Murray et al., 1992), we determined the composition and flux of over 100 organic compounds in the central equatorial Pacific Ocean (EqPac). The equatorial zone extending 5 ° north and south of the equator from New Guinea to South America is characterized by elevated levels of productivity (Barber et al., 1996) and represents an important oceanic region of organic carbon production and cycling. We intercepted particulate organic material from three sites (9°N, 5°N, and 0 ° at 140°W) at six sequential stages of increasing depth and diagenesis from the surface ocean to
2. SAMPLES AND EXPERIMENTAL TECHNIQUES Samples were collected on four cruises of the EqPac program along the transect from 12°N to 5°S along 140°W and extended to 12.5°S, 135°W during February-March, 1992, and August-September, 1992. Net-plankton was collected using oblique tows of a 26 #m mesh net. Material exiting the photic zone (the export flux) was collected in floating sediment traps deployed for 1.5-3 days at 105 m along the transect. The floating sediment traps were equipped with Indented Rotating Sphere (IRS; Peterson et al., 1993) valves to minimize collection of zooplankton swimmers and to prevent washout of poison and preservatives. Particulate material sinking through the ocean's interior was collected at 9°N, 5°N, and the equator using moored sediment traps deployed at about 1000 m below the sea surface and at about 1000 m above the sea floor. A full description of the trap moorings is given in Honjo et al. (1995). Prior to deployment of both floating and moored traps, 500 mL of a dense solution of 50 mg HgC12 and 50 g NaC1 in filtered seawater 5363
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was introduced into the collection cups. Upon retrieval, sedimented material was passed through a 850 #m stainless steel sieve, and split for multiple analysis, where one-half was used for elemental and carbohydrate analyses, one-fourth for lipids, and one-sixteenth each for amino acids, pigments, archive, and other miscellaneous analyses. If any "swimmers" <850 #m in size were present, they were not picked from the trap samples. The elemental/carbohydrate split was centrifuged and the pellet frozen. The other splits were collected onto glass fiber filters and frozen. Sediment cores were collected at sites from 9°N to 12°S using a multi-corer (Barnett et al., 1984). Cores were sectioned aboard ship and stored frozen until thawed and split for analysis. Organic carbon, total nitrogen, and inorganic carbon (by difference) were determined in duplicate using a Carlo Erba CHN analyser according to the method of Hedges and Stern (1984), as modified (Cowie and Hedges, 1991 ) for analysis of the carbonate-rich sediments. Neutral carbohydrates were analyzed using the technique of Cowie and Hedges (1984) as described in detail by Hernes et al. (1996). Samples were pretreated with 72 wt% H2SO4 for 2 h at room temperature, diluted to 1.2 M H2804 and hydrolysed for 3 h at 100°C. Samples were then neutralized with Ba(OH)2 and desalted with an ion exchange column. Dried isolates were equilibrated in a 0.2% LiCIO4 in pyridine solution, derivatized with BSTFA + 1% TMCS, and analyzed by gas chromatography using adonitol as an internal standard. Amino acids were measured by fluorescence-high pressure liquid chromatography after hydrolysis according to Lee and Cronin (1984) and Lee et al. (1998). Samples were hydrolysed with 6 N HC1 under N2 at 110°C for 19 h. Hydrolysates were dried in vacuo, taken up in water, and the free amino acids analyzed by HPLC using a modification of the Lindroth and Mopper (1979) ophthaldialdehyde derivative HPLC technique. Analytical techniques for lipids were described in Wakeham et al. (1998). Lipids were extracted with CHzClz:MeOH (2:1). An aliquot of the extract was saponified with 0.5 N KOH in methanol and the neutral and acidic fractions were extracted from basic and acidic solutions, respectively. Neutral lipids, (sterols, fatty alcohols, hydrocarbons, and long-chain alkenones) were treated with BSTFA to convert hydroxyl functions to trimethylsilyl ethers. Acids were methylated with diazomethane. Both fractions were analyzed by gas chromatography and gas chromatography-mass spectrometry. In the following discussion, "total lipid" refers to the sum of chromatographically resolved and identified components of the neutral and acidic lipid components (not a gravimetric fraction). The major chlorophyll pigments and their degradation products were analyzed by HPLC (Lee et al., 1998) following extraction with 100% acetone. Chlorophyll and its degradation products were separated by gradient reverse-phase chromatography using a methanol/acetone/ion pairing solution (tetrabutylammonium acetate-ammonium acetate; Mantoura and Llewellyn, 1983), followed by fluorescence detection. 3. RESULTS AND DISCUSSION
Net fluxes of Corg and the major biochemical classes in the equatorial Pacific were greatly attenuated (Fig. 1 ) during transit of particles through the water column and into surface sediment. Detailed molecular results for individual biochemical classes in this EqPac study are available elsewhere (Hernes et al., 1996; Wakeham et al., 1998; Lee et al., 1998). Fluxes varied among the three study sites studied in response to differences in primary productivity (Barber et al., 1996), sediment type (carbonate vs. red clay) (Murray and Leinen, 1993) and sedimentation rate (DeMaster et al., 1998). In general, fluxes in sediment traps at 105 m were 1 - 2 orders of magnitude lower than rates of carbon and biochemical production, where biochemical production rates were estimated by multiplying net carbon primary production by the biochemical/carbon ratio of net plankton from the sites. Most particulate organic matter was therefore degraded in the upper 100 m of the water column. Fluxes between traps
at 105 m, at 1000 m depth, and at 1000 m above the sea floor decreased comparatively little. A second major stepdecrease in flux occurred between traps deployed 1000 m above the sea floor and the sediment surface ( 0 - 0 . 5 cm). Although the respective samples represent very different timescales, ranging from minutes for net tows to 1.5-3 days for floating traps, and from 1 year for moored traps to centuries for sediments, it is nevertheless evident that only 0 . 1 0.01% of organic material originating in surface waters accumulates in underlying sediments. In spite of the unavoidable uncertainty involved with flux comparisons over contrasting timescales, this conclusion is consistent with the general understanding of the fate of organic matter in the ocean (Wakeham and Lee, 1993). The fraction of total organic matter that was not measurable by our molecular techniques increased steadily as particles sank through the water column to the sediment (Fig. 2). In net-plankton, individually measured amino acids (originally proteins), carbohydrates (originally polysaccharides), and lipids (neutral and polar) together accounted for 82% of Corg, with amino acids clearly contributing 67% of the total carbon. Chlorophyll, while an essential constituent of phytoplankton, represented less than 0.2% of planktonic carbon. Material uncharacterized at the molecular level composed the remaining 18% of planktonic Corg, and likely included other major biochemical classes such as nucleic acids, amino sugars, uronic acids, and nonchlorophyll pigments. In the 105-m sediment traps, major biochemical classes accounted for 79% of Corg, leaving 21% of Corg uncharacterized. There was relatively little difference in bulk biochemical composition between plankton and the 105-m traps. Below 105-m traps, however, the amount of characterized organic carbon decreased dramatically. Amino acids and lipids accounted for most of the loss, with the remaining C,,rg comprised of 24% amino acid and 3% lipid. Carbohydrates were generally 5% of Corg, similar to their percentage in both net-plankton and floating traps. The result of this preferential loss of amino acids and lipids was that only 32% of Co,.g in particulate matter sinking through the lower water column could be characterized. Even less Corg(20%) can be identified in sediments, where contributions from amino acids and lipids again decrease most relative to deep trap material. The compositions of biochemical classes, and hence the turnover of organic substances in general, were markedly more dynamic than would be perceived from measurements of bulk organic carbon and nitrogen alone. From this compositional information, it is possible to assign the following order of overall relative lability for biochemical classes in the equatorial Pacific: pigments ~> lipids > amino acids > carbohydrates. In part, the relative stability of carbohydrates may be due to their function as structural components, as well as more labile energy storage compounds (Ittekkot et al., 1984a,b; Hernes et al., 1996). Much of the uncharacterized Cor~ in deep traps ( - 7 0 % of Corg) and in sediments ( ~ 8 0 % ) may be complex macromolecular material that is intractable to our molecular analyses. However, this does not imply that this material cannot be metabolized by mesopelagic, bathypelagic and benthic organisms. Molecular analyses within individual biochemical classes in the EqPac sample set reveal a variety of compositional
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Fig. I. Fluxes (mg/m2d) of organic carbon and biochemical classes at 9°N, 5°N, and the equator. Compound class fluxes are in mg of compound, not mg C. Fluxes for net-plankton are derived from primary production rates (Barber et al., 1996) and our measurements of biochemical content of net-plankton. Floating sediment trap fluxes at 5°N and the equator are means of two surveys (there was only 1 measurement at 9°N). Fluxes into surface sediments were calculated using the sediment Cofg content and accumulation rates (DeMaster et al., 1997) and our biochemical measurements. There are no accumulation rate data for 10-12 cm sediments.
changes diagnostic of sequential organic input and removal processes. No measured compound consistently increased in absolute flux below the ocean surface to an extent that clearly indicates net secondary (heterotrophic) production. Many compounds with multiple sources or average reactivities (e.g., many sugars, amino acids and fatty acids) changed little in composition throughout the depth sequence. Selected compounds within various classes, however, exhibited consistent patterns in relative abundance with depth. Trends were similar for all three latitudes where the entire water column was sampled. Scaled profiles of these dynamic biochemicals (Fig. 3) provide a new approach to evaluating diagenetic status with minimal mathematical interdependence and without reliance on absolute fluxes which are difficult to measure accurately. Most vertical trends in biochemical composition fell into four general groups, which we categorized solely on the basis of their behavior in the sample set, without consideration
of preconceptions of apparent sources or expected relative reactivities. Group I compounds were characterized by maximal concentrations in surface plankton and relative abundances that decrease downward to varying extents within their compound classes. Most pronounced in this behavior were chlorophyll a (bar 1 in Fig. 3) and polyunsaturated C22 (2) and C20(3) fatty acids, which decreased exponentially down the water column and were not found at detectable concentrations within deeper ( 1 0 - 1 2 cm) sediments. These plankton-derived biochemicals are subject to extensive alteration by zooplanktonic and microbial heterotrophs during passage through the water column (Welschmeyer and Lorenzen, 1985; Wakeham and Canuel, 1988). Their steady decreases indicate continued selective degradation throughout the ocean and into surficial sediments. Glucose (4) decreased in the upper 100 m of the ocean but persisted to depth, as might be expected for a major component of storage polysaccharides in phytoplankton with multiple other
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Fig. 2. Cumulative biochemical class distributions (compound class-C as percent of total Corg). Values at 9°N, 5°N, and the equator have been averaged; values for the 105-m traps represent means for the two surveys. Carbon contents were calculated directly for amino acids and sugars, while the measured lipids were estimated to be 85% C. Chlorophyll and its degradation products are included with lipids but are quantitatively negligible. Residual "uncharacterized" carbon was obtained as the difference between total Co,g and the sum of carbon in amino acid + sugar + lipid.
sources (Ittekkot et al., 1984a,b; Cowie and Hedges, 1984; Hernes et al., 1996). Interestingly, the large ( ~ 9 0 - 9 9 % ) decreases in flux between plankton production and particle sinking through 100 m (Fig. 1) are accompanied by relatively small compositional changes (Figs. 2 and 3). Degradation (and resynthesis) appear to be less selective in the early stages of heterotrophic utilization, than later below the euphotic zone. Group II compounds characteristically exhibited maximal relative concentrations within the water column. This pattern is typical of resistant components of phytoplankton, or of biochemicals produced at depth by heterotrophs. The most extreme example of this category is the pigment, phaeophorbide a (6), which increased from a small fraction of netplankton pigments to maximal abundance in 1000 m trap samples, and then decreased to zero concentration in 10-12 cm sediments. Such behavior would be expected of this major chlorophyll a grazing product which is subject to further degradation in the water column (Welschmeyer and Lorenzen, 1985). The growing percentage of galactose (7) among sugars might result either from stability within structural polysaccharides, or possibly from sorption of galactose-rich
dissolved organic matter (McCarthy et al., 1996) by particles sinking between 100 and 1000 m. Evidence for sorption of DOC by particles is seen in isotopic analyses of compound classes (Wang et al., 1996). The C28-sterol, 24-methylcholesta-5,24(28)-dien-3/3-ol (5), which peaked at 105 m, appears to be a selectively preserved, largely phytoplanktonderived (Volkman, 1986), component. Oleic acid ( 18:1 ~9; 8) was most abundant in the deep water column. Although oleic acid derives from phytoplankton, zooplankton, and to a lesser extent bacteria (Sargent, 1976), its deep-water flux and abundance maximum has been attributed to deep-dwelling zooplankton (Wakeham et al., 1984). Compounds within Group III had highest relative abundances in surficial ( 0 - 0 . 5 cm) sediments. For example, this pattern was characteristic of cis-vaccenic acid (18:1~7; 9) and iso- and anteiso-C~5 and C~7 fatty acids (e.g., anteisoC~5, 10), all of which have bacterial sources (Perry et al., 1979; Kaneda, 1991). These lipids occurred throughout the water column, possibly due to production by bacteria in zooplankton guts or on particles. Bisnorhopane (12) is also of bacterial derivation (Ourisson et al., 1987), but occurred at detectable levels only in sediments. The C29-sterol, 24ethylcholest-5-en-3/3-ol (11), although traditionally assigned a vascular land plant origin (Huang and Meinschein, 1979), also has an algal source in marine systems (Volkman, 1986). All Group III biochemicals appeared to be selectively preserved in surface sediments. The reduced abundances of Group III biochemicals in 10-12 cm versus surficial sediments, however, suggests either diagenetic alteration at depth or, less likely, reduced input to the sediment in the past. Compounds that occurred at maximal abundance within the 10-12 cm sediments (Group IV) appear to be characteristically resistant to microbial degradation. Biochemicals with this behavior include three separate homologous suites of high-molecular-weight, straight-chain fatty acids (13), fatty alcohols, and alkanes (data not shown). Compounds comprising these series exhibit carbon-number predominance patterns characteristic of cuticles from vascular land plants and likely were transported to the central Pacific by wind (Gagosian and Peltzer, 1986; Prahl et al., 1989). A suite of four polyunsaturated C~7 and C3~ methyl- and ethylalkenones (14), derived primarily from marine haptophytes (Volkman et al., 1980), demonstrated an almost identical abundance pattern. Long-chain fatty acids, alcohols, alkanes and alkenones thus represent refractory constituents of water column particles that become magnified at depth by selective preservation as >99% of the surface produced Cor~ was respired. The approximate doubling between the surface and deep sediment horizons characteristic of all 4 Group 1V compounds would be expected for passively concentrated constituents without in situ source and is consistent with enhanced preservation in the sediment. The stability of longchain acids, alcohols, and alkanes may be enhanced by their high molecular weight and a protective waxy matrix (Wakeham et al., 1984). The longer chain alkenones also may be difficult to metabolize due to the unusual form (trans) and position of their carbon double bonds (Rechka and Maxwell, 1988). The corresponding predominance of the isoprenoid hydrocarbon, squalene, (16) is surprising for a cis-polyunsaturated molecule and suggests a sedimentary source, possi-
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Fig. 3. Relative abundances of selected biochemicals in net-plankton, floating (105-m) sediment traps, moored sediment traps (at 1000 m below the sea surface and 1000 m above the sea floor), and surface (0-0.5 cm) and deep (10-12 cm) sediment samples. Normalized abundances for each compound represent averages of samples at 9°N, 5°N, and the equator. Compound identities: 1, chlorophyll a; 2, 22:6•3 polyunsaturated fatty acid; 3, 20:5~3 polyunsaturated fatty acid; 4, glucose; 5, 24-methylcholesta-5,24(28)-dien-3fl-ol;6, phaeophorbide a; 7, galactose; 8, oleic acid (18: l w9); 9, cis-vaccenic acid (18:1~7); 10, anteiso-15:0 fatty acid; 11, 24-ethylcholest-5-en-3fl-ol; 12, bisnorhopane; 13, summed C24 + C26 + C28 + C30 fatty acids; 14, summed C37 + C38 methyl and ethyl, di- and triunsaturated alkenones; 15, glycine and 16, squalene. Groupings of compounds are based on similar diagenetic behaviors (see text).
bly from bacteria and meiofauna. The increasing relative abundance of glycine (15) may result either from selective preservation or from in situ production as a degradation product of other compounds (Lee and Cronin, 1984). General diagenetic fates of many of these biochemicals have been inferred previously from widely disparate studies of plankton, particulate material and sediments (see reviews by Lee and Wakeham, 1988, and Wakeham and Lee, 1993). The present study, however, is the first systematic compila-
tion of such a wide range of compounds in samples collected throughout the water column and into the sediments of a single oceanic region. This type of broad characterization of biochemical abundances allows tests of oceanographic consistency, inspections of parallel sources and reactivities, and demonstrates the varying strengths of coupling between different levels of the surface ocean and underlying sediment. Similar abundance patterns between source-specific lipids and more abundant, but widely distributed, amino
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acids or sugars (e.g., glycine and galactose) can point toward otherwise obscure origins or physical relationships. In EqPac, parallel patterns of biochemical degradation and resynthesis at depth overshadow other variables in primary productivity and mineralogy and thus largely determine deep-water-column compositions and the imprinted sedimentary record. These trends are not explained simply by c o m m o n sources or shared structural characteristics, making sweeping treatments of reaction kinetics or other bulk properties problematic. The preserved biochemical assemblages sensitively record inputs of such varied materials as haptophyte lipids, terrigenous dust and sedimentary bacterial biomass, but underrepresent the abundances and activities of most water column organisms. The processes leading to diagenetic selection of organic c o m p o u n d s for preservation must be investigated in oceanic water columns and diagenetically active surface sediments within a variety of settings before extremely rich, but highly edited, biochemical records can be applied to their full potential in paleoenvironmental reconstructions. Our inability to characterize the bulk of the particulate organic matter at depth reinforces the general paradigm of a transition from highly labile and relatively well-characterized organic matter in organisms and surface waters to biologically and analytically recalcitrant material in the deep-water column and sediments. Detailed study of the uncharacterized organic fraction that becomes increasingly pred o m i n a n t with depth is especially needed in addition to conventional biochemical measurements. In particular, the question of whether most sedimentary organic matter is largely remnant biochemical, versus structurally complex " h e t e r o p o l y c o n d e n s a t e s " of metabolic intermediates, is key to assessing the potential of the as-yet-uncharacterized fraction as a source of paleoceanographic information. To this end, pyrolysis-MS, pyrolysis G C - M S , solid state 13C-NMR analyses, and compound-specific carbon isotope analyses are underway on these same EqPac samples and should prove vital steps towards structurally elucidating the nature of this presently uncharacterized c o m p o n e n t of marine organic carbon. Acknowledgments--This paper is dedicated to Neil Anderson who has been a major source of encouragement and support for marine organic geochemical research. We are also grateful to the captains and crews of R/V Thompson and R/V Wecoma and to our JGOFS colleagues for sampling support. We are especially grateful to S. Honjo, J. Dymond, S. Manganini, and C. Moser for including our traps in their mooring deployments, and to S. Kadar for her impeccable logistical help. This research was supported by the U.S. National Science Foundation. U.F. JGOFF contribution 440. REFERENCES
Barber R. T., Sanderson M. P., Lindley S. T., Chai F., Newton J., Trees C. C., Foley D. G., and Chavez F. P. (1996) Primary productivity and its regulation in the equatorial Pacific during and following the 1991-92 E1 Nifio. Deep-Sea Res. II 43, 933-969. Barnett R.P.O., Watson J., and Connelly D. (1984) A multiple corer for taking virtually undisturbed samples from shelf, bathyal, and abyssal sediments. Oceanol. Acta 7, 399-408. Cowie G.L. and Hedges J.I. (1984) Carbohydrate sources in a coastal marine environment. Geochim. Cosmochim. Acta 48, 2075-2087. Cowie G. L. and Hedges J. I. (1991) Organic carbon and nitrogen geochemistry of Black Sea surface sediments from stations span-
ning the oxic:anoxic boundary. In Black Sea Oceanography (ed. E. Izdar and J. W. Murray), pp. 343-349. Kluwer. Cowie G.L. and Hedges J.I. (1992) Sources and reactivities of amino acids in a coastal marine environment. Limnol. Oceanogr. 37, 703-724. Cowie G. L. and Hedges J. I. (1994) Biochemical indicators of diagenetic alteration in natural organic matter mixtures. Nature 369, 304-307. DeMaster D. J., Pope R. H., Rageuneau O. and Smith C. R. (1998) Burial rates of biogenic material along the EqPac transect: Holocene variability and paleoflux indicators. Deep-Sea Res. I1 (in prep. ). Gagosian R. B. and Peltzer E. T. (1986) The importance of atmospheric input of terrestrial organic matter to deep-sea sediments. In Advances in Organic Geochemistry 1985 (ed. D. Leythaueser and J. Rullk6tter); Org. Geochem. 10, 661-669. Hedges J. I. and Stern J. H. (1984) Carbon and nitrogen determinations of carbonate containing solids. Limnol. Oceanogr. 29, 657663. Hernes P. J., Hedges J. I., Peterson M. L., Wakeham S. G., and Lee C. (1996) Neutral carbohydrate geochemistry of particulate matter in the central Equatorial Pacific. Deep-Sea Res. H 43, 1181-1204. Honjo S., Dymond J., Collier R., and Manganini S. J. (1995) Export production of particles to the interior of the equatorial Pacific Ocean during the 1992 EqPac experiment. Deep-Sea Res. 11 42, 831-870. Huang W.-Y. and Meinschein W. G. (1979) Sterols as ecological indicators. Geochim. Cosmochim. Acta 43, 739-745. Ittekkot V., Deuser W. G., and Degens E. T. (1984a) Seasonality in the fluxes of sugars, amino acids, and amino sugars to the deep ocean: Sargasso Sea. Deep-Sea Res. 31, 1057-1069. Ittekkot V., Deuser W. G., and Degens E. T. (1984b) Seasonality in the fluxes of sugars, amino acids, and amino sugars to the deep ocean: Panama Basin. Deep-Sea Res. 31, 1071-1083. Kaneda T. ( 1991 ) lso- and anteiso-fatty acids in bacteria: biosynthesis, function, and taxonomic significance. Microbiol. Rev. 55, 288-302. Lee C. and Cronin C. (1984) Particulate amino acids in the sea: effects of primary productivity and biological decomposition. J. Mar. Res. 42, 1075-1097. Lee C. and Wakeham S.G. (1988) Organic matter in seawater. In Chemical Oceanography, Vol. 9 (ed. J. P. Riley), pp. 1-51. Academic Press. Lee C., Wakeham S. G., and Hedges J. I. (1998) Composition and flux of particulate amino acids and pigments in equatorial Pacific seawater and sediments. (in prep). Lindroth P. and Mopper K. (1979) High performance liquid chromatographic determination of subpicomole amounts of amino acids by precolumn fluorescence derivatization with o-phthaldialdehyde. Anal. Chem. 51, 1667-1674. Mantoura R. F. C. and Llewellyn C. A. ( 1983 ) The rapid determination of algal chlorophyll and carotenoid pigments and their breakdown products in natural waters by reverse-phase high-performance liquid chromatography. Analyt. Chim. Acta 151, 297-314. McCarthy M., Hedges J. I., and Benner R. (1996) Major biochemical composition of dissolved high molecular weight organic matter in seawater. Mar. Chem. 55, 281-297. Murray R. W. and Leinen M. W. (1993) Chemical transport to the seafloor of the equatorial Pacific across a latitudinal transect at 135°W: Tracking sedimentary major, trace, and rare earth element fluxes at the equator and the Intertropical Convergence Zone. Geochim. Cosmochim. Acta 57, 4141-4163. Murray J.W., Leinen M.W., Feely R.A., Toggweiler J.R., and Wanninkof R. (1992) EqPac: a process study in the central equatorial Pacific. Oceanography 5, 134-142. Ourisson G., Rohmer M., Poralla K. (1987) Prokaryotic hopanoids and other polyterpenoid sterol surrogates. Annu. Rev. Microbiol. 41, 301-333. Peterson M. L., Hernes P. J., Thoreson D. S., Hedges J. I., Lee C., and Wakeham S. G. (1993) Field evaluation of a valved sediment trap. Limnol. Oceanogr. 38, 1741-1761. Perry G.A., Volkman J. K., and Johns R.B. (1979) Fatty acids
Molecular indicators of diagenetic status in marine organic matter of bacterial origin in contemporary marine sediments. Geochim. Cosmochim. Acta 43, 1715-1725. Prahl F. G., Muelhausen L. A. and Lyle M. (1989) An organic geochemical assessment of oceanographic conditions at MANOP Site C over the past 26,000 years. Paleoceanogr. 4, 495-510. Rechka J. A. and Maxwell J. R. (1988) Characterization of alkenone temperature indicators in sediments and organisms. In Advances in Organic Geochemistry 1987 (ed. L. Mattavelli and L. Novelli) ; Org. Geochem. 13, 727-734. Sargent J.R. (1976) The structure, function, and metabolism of lipids in marine organisms. In Biochemical and Biophysical Perspectives in Marine Biology, Vol. 3 (ed. D. C. Malins and J. R. Sargent), pp. 149-212. Academic Press. Volkman J. K. (1986) A review of sterol markers for marine and terrigenous organic matter. Org. Geochem. 9, 83-99. Volkman J. K., Eglinton G., Corner E. D. S., and Forsberg T. E. V. (1980) Long-chain alkenes and alkenones in the marine coccolithophoprid, Emiliania huxleyi. Phytochem. 19, 2619-2622. Wakeham S. G. (1996) Lipid biomarkers for heterotrophic alteration of suspended particulate organic matter in oxygenated and anoxic water columns of the ocean. Deep-Sea Res. 42~ 1749-1771.
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Wakeham S. G. and Canuel E. A. (1988) Organic geochemistry of particulate matter in the eastern tropical North Pacific Ocean: Implications for particle dynamics. J. Mar. Res. 46, 183-213. Wakeham S. G. and Lee C. (1993) Production, transport, and alteration of particulate organic matter in the marine water column. In Organic Geochemistry' (ed. M. Engel and S. Macko), pp. 145169. Plenum Press. Wakeham S. G., Gagosian R. B., Farrington J. W. and Lee C. (1984) Biogeochemistry of particulate organic matter in the oceans-results from sediment trap experiments. Deep-Sea Res. 31, 509528. Wakeham S. G., Hedges J. I., Lee C., Peterson M. L. and Hernes P. J. (1998) Compositions and fluxes of lipids through the water column and surficial sediments of the equatorial Pacific Ocean. Deep-Sea Res. 11 (in press). Wang X. C., Druffel E. R. M., and Lee C. (1996) Radiocarbon in organic compound classes in particulate organic matter and sediment in the deep northeast Pacific Ocean. Geophys. Res. Letts. 23, 3583-3586. Welschmeyer N. A. and Lorenzen C. J. (1985) Chlorophyll budgets: Zooplankton grazing and phytoplankton growth in a temperate fjord and the Central Pacific gyres. Limnol. Oceanogr. 30, 1-21.
Pergamon
P I I S0016-7037(97)00312-8
Molecular indicators of diagenetic status in marine organic matter STUART G. WAKEHAM,1 CINDY LEE, 2 JOHN I. HEDGES,3 PETER J. HERNES,3 and MICHAEL L. PETERSON3 Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, Georgia 31411, USA 2 Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794-5000, USA 3School of Oceanography, P.O. Box 357940, University of Washington, Seattle, Washington 98195-7940, USA
(Received February 17, 1997; accepted in revised form August 19, 1997)
Abstract--The fluxes of individual carbohydrates, amino acids, lipids and pigments have been determined in net-plankton, particulate matter and sediments from three sites (9°N, 5°N, and 0°N) in the central equatorial Pacific to evaluate sources and reactivities of organic compounds. Although primary production rates vary markedly across this 9 ° swath, vertical trends in biochemical compositions remained remarkably parallel. Together these one hundred plus biochemicals account for 80% of the total organic carbon (C,,rg) in net-plankton and particles sinking from the euphotic zone, but represent only 24 and 20% of the organic carbon in deep-water particles and surface sediments, respectively. Scaled profiles of relative abundances, clearly illustrate (a) exponential losses of plankton remains and increases in heterotroph biomarkers throughout the water column, (b) elevated proportions of bacterial markers near the sediment surface, and (c) preservation of selected remains of bacteria, phytoplankton and vascular land plants deeper in the sediments. In spite of one of the most comprehensive analyses of major biochemicals yet applied to marine particulate samples, percentages of molecularly uncharacterized organic carbon increase progressively down the water column to values near 80% in the underlying sediments. The composition, formation pathway and information potential of this uncharacterized fraction are among the most fascinating questions in marine organic geochemistry. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION
the underlying sediments. These stages include (a) the planktonic source of particulate matter in the euphotic zone (net-plankton), (b) particles exiting the euphotic zone (floating sediment traps at 105 m), (c) particles transiting the ocean's interior (moored sediment traps at 1000 m below the sea surface and 1000 m above the sea floor), and (d) material being buried at 0 - 0 . 5 cm and the underlying 1 0 12 cm in the sediment. Molecular distributions of individual sugars, amino acids, lipids, and pigments were determined in this set of samples. Our choice of these biochemicals is based on their quantitative importance in living organisms and utility as tracers of sources and alteration processes. The EqPac study afforded a novel opportunity to observe molecular alteration across a broad suite of biochemicals in a comprehensive series of samples collected throughout the water column and sediments at a single open ocean site.
Particulate organic matter is the major vehicle for vertical redistribution of bioactive elements within the ocean. The rain of plankton-derived material out of surface waters supports essentially all life in deeper waters, transfers carbon and nutrients to depth, and imprints a record of water column processes into underlying sediments. Sinking organic particles are extensively (usually > 9 9 % ) and selectively degraded during transit to the deep-sea floor, leading to highly altered sedimentary records (Wakeham and Lee, 1993). Individual biochemicals or their ratios have been used to measure this diagenetic status of organic matter (Lee and Cronin, 1984; Ittekkot et al., 1984a,b; Cowie and Hedges, 1992; Wakeham, 1996). Knowing the diagenetic status of organic matter is essential for employing biomarkers in source studies and understanding carbon preservation. Organic matter reactivity, however, is a function of material matrix as well as inherent lability (Wakeham and Lee, 1989; Cowie and Hedges, 1992, 1994), so that diverse chemical reactivities do not correspond simply with compound class or structure in consistently predictable patterns. As part of the U.S. Global Ocean Flux Study (U.S. JGOFS) (Murray et al., 1992), we determined the composition and flux of over 100 organic compounds in the central equatorial Pacific Ocean (EqPac). The equatorial zone extending 5 ° north and south of the equator from New Guinea to South America is characterized by elevated levels of productivity (Barber et al., 1996) and represents an important oceanic region of organic carbon production and cycling. We intercepted particulate organic material from three sites (9°N, 5°N, and 0 ° at 140°W) at six sequential stages of increasing depth and diagenesis from the surface ocean to
2. SAMPLES AND EXPERIMENTAL TECHNIQUES Samples were collected on four cruises of the EqPac program along the transect from 12°N to 5°S along 140°W and extended to 12.5°S, 135°W during February-March, 1992, and August-September, 1992. Net-plankton was collected using oblique tows of a 26 #m mesh net. Material exiting the photic zone (the export flux) was collected in floating sediment traps deployed for 1.5-3 days at 105 m along the transect. The floating sediment traps were equipped with Indented Rotating Sphere (IRS; Peterson et al., 1993) valves to minimize collection of zooplankton swimmers and to prevent washout of poison and preservatives. Particulate material sinking through the ocean's interior was collected at 9°N, 5°N, and the equator using moored sediment traps deployed at about 1000 m below the sea surface and at about 1000 m above the sea floor. A full description of the trap moorings is given in Honjo et al. (1995). Prior to deployment of both floating and moored traps, 500 mL of a dense solution of 50 mg HgC12 and 50 g NaC1 in filtered seawater 5363
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was introduced into the collection cups. Upon retrieval, sedimented material was passed through a 850 #m stainless steel sieve, and split for multiple analysis, where one-half was used for elemental and carbohydrate analyses, one-fourth for lipids, and one-sixteenth each for amino acids, pigments, archive, and other miscellaneous analyses. If any "swimmers" <850 #m in size were present, they were not picked from the trap samples. The elemental/carbohydrate split was centrifuged and the pellet frozen. The other splits were collected onto glass fiber filters and frozen. Sediment cores were collected at sites from 9°N to 12°S using a multi-corer (Barnett et al., 1984). Cores were sectioned aboard ship and stored frozen until thawed and split for analysis. Organic carbon, total nitrogen, and inorganic carbon (by difference) were determined in duplicate using a Carlo Erba CHN analyser according to the method of Hedges and Stern (1984), as modified (Cowie and Hedges, 1991 ) for analysis of the carbonate-rich sediments. Neutral carbohydrates were analyzed using the technique of Cowie and Hedges (1984) as described in detail by Hernes et al. (1996). Samples were pretreated with 72 wt% H2SO4 for 2 h at room temperature, diluted to 1.2 M H2804 and hydrolysed for 3 h at 100°C. Samples were then neutralized with Ba(OH)2 and desalted with an ion exchange column. Dried isolates were equilibrated in a 0.2% LiCIO4 in pyridine solution, derivatized with BSTFA + 1% TMCS, and analyzed by gas chromatography using adonitol as an internal standard. Amino acids were measured by fluorescence-high pressure liquid chromatography after hydrolysis according to Lee and Cronin (1984) and Lee et al. (1998). Samples were hydrolysed with 6 N HC1 under N2 at 110°C for 19 h. Hydrolysates were dried in vacuo, taken up in water, and the free amino acids analyzed by HPLC using a modification of the Lindroth and Mopper (1979) ophthaldialdehyde derivative HPLC technique. Analytical techniques for lipids were described in Wakeham et al. (1998). Lipids were extracted with CHzClz:MeOH (2:1). An aliquot of the extract was saponified with 0.5 N KOH in methanol and the neutral and acidic fractions were extracted from basic and acidic solutions, respectively. Neutral lipids, (sterols, fatty alcohols, hydrocarbons, and long-chain alkenones) were treated with BSTFA to convert hydroxyl functions to trimethylsilyl ethers. Acids were methylated with diazomethane. Both fractions were analyzed by gas chromatography and gas chromatography-mass spectrometry. In the following discussion, "total lipid" refers to the sum of chromatographically resolved and identified components of the neutral and acidic lipid components (not a gravimetric fraction). The major chlorophyll pigments and their degradation products were analyzed by HPLC (Lee et al., 1998) following extraction with 100% acetone. Chlorophyll and its degradation products were separated by gradient reverse-phase chromatography using a methanol/acetone/ion pairing solution (tetrabutylammonium acetate-ammonium acetate; Mantoura and Llewellyn, 1983), followed by fluorescence detection. 3. RESULTS AND DISCUSSION
Net fluxes of Corg and the major biochemical classes in the equatorial Pacific were greatly attenuated (Fig. 1 ) during transit of particles through the water column and into surface sediment. Detailed molecular results for individual biochemical classes in this EqPac study are available elsewhere (Hernes et al., 1996; Wakeham et al., 1998; Lee et al., 1998). Fluxes varied among the three study sites studied in response to differences in primary productivity (Barber et al., 1996), sediment type (carbonate vs. red clay) (Murray and Leinen, 1993) and sedimentation rate (DeMaster et al., 1998). In general, fluxes in sediment traps at 105 m were 1 - 2 orders of magnitude lower than rates of carbon and biochemical production, where biochemical production rates were estimated by multiplying net carbon primary production by the biochemical/carbon ratio of net plankton from the sites. Most particulate organic matter was therefore degraded in the upper 100 m of the water column. Fluxes between traps
at 105 m, at 1000 m depth, and at 1000 m above the sea floor decreased comparatively little. A second major stepdecrease in flux occurred between traps deployed 1000 m above the sea floor and the sediment surface ( 0 - 0 . 5 cm). Although the respective samples represent very different timescales, ranging from minutes for net tows to 1.5-3 days for floating traps, and from 1 year for moored traps to centuries for sediments, it is nevertheless evident that only 0 . 1 0.01% of organic material originating in surface waters accumulates in underlying sediments. In spite of the unavoidable uncertainty involved with flux comparisons over contrasting timescales, this conclusion is consistent with the general understanding of the fate of organic matter in the ocean (Wakeham and Lee, 1993). The fraction of total organic matter that was not measurable by our molecular techniques increased steadily as particles sank through the water column to the sediment (Fig. 2). In net-plankton, individually measured amino acids (originally proteins), carbohydrates (originally polysaccharides), and lipids (neutral and polar) together accounted for 82% of Corg, with amino acids clearly contributing 67% of the total carbon. Chlorophyll, while an essential constituent of phytoplankton, represented less than 0.2% of planktonic carbon. Material uncharacterized at the molecular level composed the remaining 18% of planktonic Corg, and likely included other major biochemical classes such as nucleic acids, amino sugars, uronic acids, and nonchlorophyll pigments. In the 105-m sediment traps, major biochemical classes accounted for 79% of Corg, leaving 21% of Corg uncharacterized. There was relatively little difference in bulk biochemical composition between plankton and the 105-m traps. Below 105-m traps, however, the amount of characterized organic carbon decreased dramatically. Amino acids and lipids accounted for most of the loss, with the remaining C,,rg comprised of 24% amino acid and 3% lipid. Carbohydrates were generally 5% of Corg, similar to their percentage in both net-plankton and floating traps. The result of this preferential loss of amino acids and lipids was that only 32% of Co,.g in particulate matter sinking through the lower water column could be characterized. Even less Corg(20%) can be identified in sediments, where contributions from amino acids and lipids again decrease most relative to deep trap material. The compositions of biochemical classes, and hence the turnover of organic substances in general, were markedly more dynamic than would be perceived from measurements of bulk organic carbon and nitrogen alone. From this compositional information, it is possible to assign the following order of overall relative lability for biochemical classes in the equatorial Pacific: pigments ~> lipids > amino acids > carbohydrates. In part, the relative stability of carbohydrates may be due to their function as structural components, as well as more labile energy storage compounds (Ittekkot et al., 1984a,b; Hernes et al., 1996). Much of the uncharacterized Cor~ in deep traps ( - 7 0 % of Corg) and in sediments ( ~ 8 0 % ) may be complex macromolecular material that is intractable to our molecular analyses. However, this does not imply that this material cannot be metabolized by mesopelagic, bathypelagic and benthic organisms. Molecular analyses within individual biochemical classes in the EqPac sample set reveal a variety of compositional
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changes diagnostic of sequential organic input and removal processes. No measured compound consistently increased in absolute flux below the ocean surface to an extent that clearly indicates net secondary (heterotrophic) production. Many compounds with multiple sources or average reactivities (e.g., many sugars, amino acids and fatty acids) changed little in composition throughout the depth sequence. Selected compounds within various classes, however, exhibited consistent patterns in relative abundance with depth. Trends were similar for all three latitudes where the entire water column was sampled. Scaled profiles of these dynamic biochemicals (Fig. 3) provide a new approach to evaluating diagenetic status with minimal mathematical interdependence and without reliance on absolute fluxes which are difficult to measure accurately. Most vertical trends in biochemical composition fell into four general groups, which we categorized solely on the basis of their behavior in the sample set, without consideration
of preconceptions of apparent sources or expected relative reactivities. Group I compounds were characterized by maximal concentrations in surface plankton and relative abundances that decrease downward to varying extents within their compound classes. Most pronounced in this behavior were chlorophyll a (bar 1 in Fig. 3) and polyunsaturated C22 (2) and C20(3) fatty acids, which decreased exponentially down the water column and were not found at detectable concentrations within deeper ( 1 0 - 1 2 cm) sediments. These plankton-derived biochemicals are subject to extensive alteration by zooplanktonic and microbial heterotrophs during passage through the water column (Welschmeyer and Lorenzen, 1985; Wakeham and Canuel, 1988). Their steady decreases indicate continued selective degradation throughout the ocean and into surficial sediments. Glucose (4) decreased in the upper 100 m of the ocean but persisted to depth, as might be expected for a major component of storage polysaccharides in phytoplankton with multiple other
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Fig. 2. Cumulative biochemical class distributions (compound class-C as percent of total Corg). Values at 9°N, 5°N, and the equator have been averaged; values for the 105-m traps represent means for the two surveys. Carbon contents were calculated directly for amino acids and sugars, while the measured lipids were estimated to be 85% C. Chlorophyll and its degradation products are included with lipids but are quantitatively negligible. Residual "uncharacterized" carbon was obtained as the difference between total Co,g and the sum of carbon in amino acid + sugar + lipid.
sources (Ittekkot et al., 1984a,b; Cowie and Hedges, 1984; Hernes et al., 1996). Interestingly, the large ( ~ 9 0 - 9 9 % ) decreases in flux between plankton production and particle sinking through 100 m (Fig. 1) are accompanied by relatively small compositional changes (Figs. 2 and 3). Degradation (and resynthesis) appear to be less selective in the early stages of heterotrophic utilization, than later below the euphotic zone. Group II compounds characteristically exhibited maximal relative concentrations within the water column. This pattern is typical of resistant components of phytoplankton, or of biochemicals produced at depth by heterotrophs. The most extreme example of this category is the pigment, phaeophorbide a (6), which increased from a small fraction of netplankton pigments to maximal abundance in 1000 m trap samples, and then decreased to zero concentration in 10-12 cm sediments. Such behavior would be expected of this major chlorophyll a grazing product which is subject to further degradation in the water column (Welschmeyer and Lorenzen, 1985). The growing percentage of galactose (7) among sugars might result either from stability within structural polysaccharides, or possibly from sorption of galactose-rich
dissolved organic matter (McCarthy et al., 1996) by particles sinking between 100 and 1000 m. Evidence for sorption of DOC by particles is seen in isotopic analyses of compound classes (Wang et al., 1996). The C28-sterol, 24-methylcholesta-5,24(28)-dien-3/3-ol (5), which peaked at 105 m, appears to be a selectively preserved, largely phytoplanktonderived (Volkman, 1986), component. Oleic acid ( 18:1 ~9; 8) was most abundant in the deep water column. Although oleic acid derives from phytoplankton, zooplankton, and to a lesser extent bacteria (Sargent, 1976), its deep-water flux and abundance maximum has been attributed to deep-dwelling zooplankton (Wakeham et al., 1984). Compounds within Group III had highest relative abundances in surficial ( 0 - 0 . 5 cm) sediments. For example, this pattern was characteristic of cis-vaccenic acid (18:1~7; 9) and iso- and anteiso-C~5 and C~7 fatty acids (e.g., anteisoC~5, 10), all of which have bacterial sources (Perry et al., 1979; Kaneda, 1991). These lipids occurred throughout the water column, possibly due to production by bacteria in zooplankton guts or on particles. Bisnorhopane (12) is also of bacterial derivation (Ourisson et al., 1987), but occurred at detectable levels only in sediments. The C29-sterol, 24ethylcholest-5-en-3/3-ol (11), although traditionally assigned a vascular land plant origin (Huang and Meinschein, 1979), also has an algal source in marine systems (Volkman, 1986). All Group III biochemicals appeared to be selectively preserved in surface sediments. The reduced abundances of Group III biochemicals in 10-12 cm versus surficial sediments, however, suggests either diagenetic alteration at depth or, less likely, reduced input to the sediment in the past. Compounds that occurred at maximal abundance within the 10-12 cm sediments (Group IV) appear to be characteristically resistant to microbial degradation. Biochemicals with this behavior include three separate homologous suites of high-molecular-weight, straight-chain fatty acids (13), fatty alcohols, and alkanes (data not shown). Compounds comprising these series exhibit carbon-number predominance patterns characteristic of cuticles from vascular land plants and likely were transported to the central Pacific by wind (Gagosian and Peltzer, 1986; Prahl et al., 1989). A suite of four polyunsaturated C~7 and C3~ methyl- and ethylalkenones (14), derived primarily from marine haptophytes (Volkman et al., 1980), demonstrated an almost identical abundance pattern. Long-chain fatty acids, alcohols, alkanes and alkenones thus represent refractory constituents of water column particles that become magnified at depth by selective preservation as >99% of the surface produced Cor~ was respired. The approximate doubling between the surface and deep sediment horizons characteristic of all 4 Group 1V compounds would be expected for passively concentrated constituents without in situ source and is consistent with enhanced preservation in the sediment. The stability of longchain acids, alcohols, and alkanes may be enhanced by their high molecular weight and a protective waxy matrix (Wakeham et al., 1984). The longer chain alkenones also may be difficult to metabolize due to the unusual form (trans) and position of their carbon double bonds (Rechka and Maxwell, 1988). The corresponding predominance of the isoprenoid hydrocarbon, squalene, (16) is surprising for a cis-polyunsaturated molecule and suggests a sedimentary source, possi-
Molecular indicators of diagenetic status in marine organic matter
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Fig. 3. Relative abundances of selected biochemicals in net-plankton, floating (105-m) sediment traps, moored sediment traps (at 1000 m below the sea surface and 1000 m above the sea floor), and surface (0-0.5 cm) and deep (10-12 cm) sediment samples. Normalized abundances for each compound represent averages of samples at 9°N, 5°N, and the equator. Compound identities: 1, chlorophyll a; 2, 22:6•3 polyunsaturated fatty acid; 3, 20:5~3 polyunsaturated fatty acid; 4, glucose; 5, 24-methylcholesta-5,24(28)-dien-3fl-ol;6, phaeophorbide a; 7, galactose; 8, oleic acid (18: l w9); 9, cis-vaccenic acid (18:1~7); 10, anteiso-15:0 fatty acid; 11, 24-ethylcholest-5-en-3fl-ol; 12, bisnorhopane; 13, summed C24 + C26 + C28 + C30 fatty acids; 14, summed C37 + C38 methyl and ethyl, di- and triunsaturated alkenones; 15, glycine and 16, squalene. Groupings of compounds are based on similar diagenetic behaviors (see text).
bly from bacteria and meiofauna. The increasing relative abundance of glycine (15) may result either from selective preservation or from in situ production as a degradation product of other compounds (Lee and Cronin, 1984). General diagenetic fates of many of these biochemicals have been inferred previously from widely disparate studies of plankton, particulate material and sediments (see reviews by Lee and Wakeham, 1988, and Wakeham and Lee, 1993). The present study, however, is the first systematic compila-
tion of such a wide range of compounds in samples collected throughout the water column and into the sediments of a single oceanic region. This type of broad characterization of biochemical abundances allows tests of oceanographic consistency, inspections of parallel sources and reactivities, and demonstrates the varying strengths of coupling between different levels of the surface ocean and underlying sediment. Similar abundance patterns between source-specific lipids and more abundant, but widely distributed, amino
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acids or sugars (e.g., glycine and galactose) can point toward otherwise obscure origins or physical relationships. In EqPac, parallel patterns of biochemical degradation and resynthesis at depth overshadow other variables in primary productivity and mineralogy and thus largely determine deep-water-column compositions and the imprinted sedimentary record. These trends are not explained simply by c o m m o n sources or shared structural characteristics, making sweeping treatments of reaction kinetics or other bulk properties problematic. The preserved biochemical assemblages sensitively record inputs of such varied materials as haptophyte lipids, terrigenous dust and sedimentary bacterial biomass, but underrepresent the abundances and activities of most water column organisms. The processes leading to diagenetic selection of organic c o m p o u n d s for preservation must be investigated in oceanic water columns and diagenetically active surface sediments within a variety of settings before extremely rich, but highly edited, biochemical records can be applied to their full potential in paleoenvironmental reconstructions. Our inability to characterize the bulk of the particulate organic matter at depth reinforces the general paradigm of a transition from highly labile and relatively well-characterized organic matter in organisms and surface waters to biologically and analytically recalcitrant material in the deep-water column and sediments. Detailed study of the uncharacterized organic fraction that becomes increasingly pred o m i n a n t with depth is especially needed in addition to conventional biochemical measurements. In particular, the question of whether most sedimentary organic matter is largely remnant biochemical, versus structurally complex " h e t e r o p o l y c o n d e n s a t e s " of metabolic intermediates, is key to assessing the potential of the as-yet-uncharacterized fraction as a source of paleoceanographic information. To this end, pyrolysis-MS, pyrolysis G C - M S , solid state 13C-NMR analyses, and compound-specific carbon isotope analyses are underway on these same EqPac samples and should prove vital steps towards structurally elucidating the nature of this presently uncharacterized c o m p o n e n t of marine organic carbon. Acknowledgments--This paper is dedicated to Neil Anderson who has been a major source of encouragement and support for marine organic geochemical research. We are also grateful to the captains and crews of R/V Thompson and R/V Wecoma and to our JGOFS colleagues for sampling support. We are especially grateful to S. Honjo, J. Dymond, S. Manganini, and C. Moser for including our traps in their mooring deployments, and to S. Kadar for her impeccable logistical help. This research was supported by the U.S. National Science Foundation. U.F. JGOFF contribution 440. REFERENCES
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