John I. Hedges

Sedimentary organic matter preservation: an assessment and speculative synthesis

Throughout Earth history, almost all preserved organic matter has been incorporated in marine sediments deposited under oxygenated waters along continental margins. Given modern oceanic productivity and sediment burial rates of 50 × 1015 and 0.16 × 1015 gC yr−1, respectively, organic preservation in the marine environment is < 0.5% efficient. Although correlative information is often used to suggest that productivity, sediment accumulation rate, bottom water oxicity, and organic matter source are key variables, the mechanisms governing sedimentary organic matter preservation have remained unclear. The factors which directly determine preservation vary with depositional regime, but have in common a critical interaction between organic and inorganic materials over locally variable time scales. More than 90% of total sedimentary organic matter from a wide variety of marine depositional environments cannot be physically separated from its mineral matrix. This strongly associated organic component varies directly in concentration with sediment surface area and thus appears to be sorbed to mineral grains. Sediments accumulating outside deltas along continental shelves and upper slopes characteristically exhibit mineral surface area loadings approximately equivalent to a single molecular covering. These monolayer-equivalent coatings include a fraction of reversibly bound organic molecules that are intrinsically labile, but resist appreciable mineralization as they pass rapidly through oxygenated surface sediments and are preserved within underlying anoxic deposits. The delivery of mineral surface area is the primary control on organic matter preservation within these expansive coastal margin regions where roughly 45% of all organic carbon accumulates. Deltaic sediments account for roughly another 45% of global carbon burial, but often exhibit much less than monolayer-equivalent organic coatings. This pattern is seen in periodically oxygenated sediments off the mouth of the Amazon River, even though the component clastic minerals are discharged by the river with monolayer coatings. Comparably extensive losses of organic matter, including distinct particles such as pollen grains, occur in the surfaces of deep-sea turbidites in which long term reaction with O2 is clearly the causative factor. Sub-monolayer organic coatings also are observed in continental rise and abyssal plain sediments where slower accumulation rates and deeper O2 penetration depths result in increased oxygen exposure times and little (~ 5% of the global total) organic matter preservation. A transition zone between monolayer and sub-monolayer organic coatings apparently occurs on lower continental slopes, and is marked along the Washington coast by parallel offshore decreases in total organic matter and pollen between 2000–3000 m water depth. Sediments underlying highly productive, low-oxygen coastal waters such as off Peru and western Mexico are characteristically rich in organic matter, but account for only ~ 5% of total organic carbon burial. These sediments show a direct relationship between organic matter content and mineral surface area, but at organic loadings 2–5 times a monolayer equivalent. Organic materials sorbed in excess of a monolayer thus also may be partially protected. Such high sedimentary organic contents may result from equilibration with DOM-rich porewaters, or very brief O2 exposure times which allow preservation of extremely oxygen-sensitive organic materials such as pigments and unsaturated lipids. Thus organic matter preservation throughout much of the ocean may be controlled largely by competition between sorption at different protective thresholds and oxic degradation. Future research strategies should be specifically directed at delineating the mechanisms for organic matter preservation in marine sediments. In particular, special effort is needed to determine the amounts and types of sorbed organic materials and the nature of their bonding to mineral surfaces. The extent and dynamics with which organic molecules are partitioned between porewaters and solid phases also should be determined, as well as the effects of these phase associations on their reactivities toward chemical and biological agents. In addition, processes for slow oxic (and suboxic) degradation of organic materials bear investigation in deep-sea sediments, as well as in other extreme environments such as oxidizing turbidites, weathering shales, and soils. Such studies should include characterizations of hydrolysis-resistant organic materials and emphasize the complementary use of biochemical compositions with readily separable particles such as pollen to calibrate and typify the mechanisms and stages of sedimentary organic degradation.

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

Abstract In simulation studies of the oceans role in the global carbon cycle, predicting the depth-distribution for remineralization of particulate organic carbon (POC) is of particular importance. Following Sarmiento et al. (Global Biogeochemical Cycles 7 (1993) 417), most simulation models have the power-law curve of Martin et al. (Deep-Sea Research 34 (1987) 267) for this purpose. The Martin et al. curve is an empirical fit to data, most of which is from shallow floating sediment traps. Using such a fit implies that all the information necessary for prediction is contained in the carbon flux itself, so that the organic-carbon flux F OC ( z ) at any depth z can be predicted from the flux of organic carbon F OC ( z 0 ) at some near-surface depth z 0 . Here, we challenge this basic premise, arguing that fluxes of ballast minerals (silicate and carbonate biominerals, and dust) determine deep-water POC fluxes, so that a mechanism-based model of POC flux must simultaneously predict fluxes of both POC and ballast minerals. This assertion is based on the empirical observation that POC fluxes are tightly linked quantitatively to fluxes of ballast minerals in the deep ocean. Here, we develop a model structure that incorporates this observation, and fit this model to US JGOFS EqPac data. This model structure, plus the preliminary parameter estimates we have obtained, can be used to explore the implications of our model in studies of the ocean carbon cycle.

Bulk chemical characteristics of dissolved organic matter in the ocean.

Dissolved organic matter (DOM) is the largest reservoir of reduced carbon in the oceans. The nature of DOM is poorly understood, in part, because it has been difficult to isolate sufficient amounts of representative material for analysis. Tangential-flow ultrafiltration was shown to recover milligram amounts of >1000 daltons of DOM from seawater collected at three depths in the North Pacific Ocean. These isolates represented 22 to 33 percent of the total DOM and included essentially all colloidal material. The elemental, carbohydrate, and carbon-type (by 13C nuclear magnetic resonance) compositions of the isolates indicated that the relative abundance of polysaccharides was high (∼50 percent) in surface water and decreased to ∼25 percent in deeper samples. Polysaccharides thus appear to be more abundant and reactive components of seawater DOM than has been recognized.

The characterization of plant tissues by their lignin oxidation products

Abstract The cupric oxide oxidation products of 23 vascular and nonvascular plant tissues have been measured. Compositional data for vanillyl, syringyl, and cinnamyl phenols are presented in the form of five lignin parameters which are related to plant variety, lignin concentration, and tissue type. On the basis of these parameters, the 23 plant tissue samples are resolved into distinct compositional regions corresponding to: 1. (a) nonvascular plants, 2. (b) gymnosperm woods, 3. (c) non woody gymnosperm tissues, 4. (d) angiosperm woods, and 5. (e) nonwoody angiosperm tissues. The same five parameters also can be determined for organic materials in soils and sediments and used either to discriminate between compositionally different organic mixtures or to estimate the relative amounts of each of the above types of plant materials in the deposits.

Influence of oxygen exposure time on organic carbon preservation in continental margin sediments

Today, over 90% of all organic carbon burial in the ocean occurs in continental margin sediments. This burial is intrinsically linked to the cycling of biogeochemically important elements (such as N, P, S, Fe and Mn) and, on geological timescales, largely controls the oxygen content of the atmosphere. Currently there is a volatile debate over which processes govern sedimentary organic carbon preservation. In spite of numerous studies demonstrating empirical relationships between organic carbon burial and such factors as primary productivity, the flux of organic carbon through the water column, sedimentation rate,, organic carbon degradation rate, and bottom-water oxygen concentration,, the mechanisms directly controlling sedimentary organic carbon preservation remain unclear. Furthermore, as organic carbon burial is the process that, along with pyrite burial, balances O2 concentrations in the atmosphere, it is desirable that any mechanism proposed to control organic carbon preservation include a feedback buffering atmospheric oxygen concentrations over geological time. Here we compare analyses of sediments underlying two regions of the eastern North Pacific Ocean, one which has oxygen-depleted bottom waters and one with typical oxygen distributions. Organic carbon burial efficiency is strongly correlated with the length of time accumulating particles are exposed to molecular oxygen in sediment pore waters. Oxygen exposure time effectively incorporates other proposed environmental variables, and may exert a direct control on sedimentary organic carbon preservation and atmospheric oxygen concentrations.

The molecularly-uncharacterized component of nonliving organic matter in natural environments.

Molecularly-uncharacterized organic matter comprises most reduced carbon in soils, sediments and natural waters. The origins, reactions and fates of these ubiquitous materials are relatively obscure, in large part because the rich vein of geochemical information that typically derives from detailed structural and stereochemical analysis is yet to be tapped. This discussion highlights current knowledge about the origins and characteristics of molecularly uncharacterized organic matter in the environment and outlines possible means by which this structurally uncharted frontier might best be explored.

Comparative organic geochemistries of soils and marine sediments

Striking similarities and sharp contrasts exist between the geochemistries of organic matter in surface soils and marine sediments. The contrasts result in part from physical differences in the two environments and their indigenous biota. Vascular plants predominate on land, where soils are deeply leached by percolating water and receive organic matter from falling debris and penetrating roots. The large size of vascular plants, and their high concentrations of carbon-rich biomacromolecules such as cellulose, lignin and tannin, necessitate recycling by aggressive consortia of microorganisms, including fungi armed with O2-requiring oxidative enzymes. In the ocean, nitrogen-rich microorganisms produce and recycle most organic matter in the water column, from which degraded particles rain onto the underlying sea floor. Water saturation restricts O2 penetration into sediments accumulating along most continental margins to less than several centimeters, below which biomacromolecules must be broken down hydrolytically with nitrate and sulfate as the primary electron acceptors. In both soils and sediments, plant products are degraded extensively by microorganisms, leaving small organic remnants which are soluble in base and depleted in conventionally measurable biochemicals. Much of the surviving organic matter is intimately associated with mineral surfaces and enclosed within particle aggregates, and thus may be physically protected from microbial attack. Degradation under oxic conditions is severe both on land and within surface ocean deposits. As a result, even physically protected organic matter can slowly be mineralized, along with intrinsically resistant substrates such as lignin, pollen, kerogen and coal. The only long-term shelter from mineralization is within anoxic marine sediments which accumulate one mole of organic carbon for every 500–1000 fixed by photosynthetic organisms. The buried organic matter joins the geological cycle, surfacing again millions of years later as kerogen uplifted in continental rocks. Chemists investigating organic matter in soils or sediments employ distinct strategies and experimental methods for disparate purposes. Soil studies focus primarily on bulk properties linked to complex system functions such as fertility and erosion. Investigations of sedimentary organic matter are more molecularly-based and directed toward interpretations of water column processes and paleorecords. With the pressing need for more efficient large-scale research, the time is ripe for increased interchange between chemists studying subaerial and subaqueous systems. Combination of methods (such as preparative particle sorting and solid-state NMR) for determining the forms and physical distributions of organic matter in soils, with highly sensitive tracer techniques being developed in the aquatic field is a particularly promising crossover area. The aim of this review is to facilitate such interactions between soil and sedimentary organic geochemists by a comparative evaluation of conditions, concepts and opportunities in both fields.

Land-derived organic matter in surface sediments from the Gulf of Mexico

Abstract Lignin oxidation products and 13C/12C ratios were compared as indicators of land-derived organic matter in surface sediments from the western Gulf of Mexico. Whole sediments were reacted with cupric oxide to yield phenolic oxidation products that indicated the types and relative amounts of the lignins that were present. Measurements of lignin concentration and carbon isotope abundances both indicated a sharp offshore decrease of land-derived organic matter in most areas of the western Gulf. This decrease results primarily from mixing of terrestrial and marine organic matter. The terrestrially derived material in these sediments has a lignin content similar to that of grasses and tree leaves. Flowering plants contribute most of the sedimented lignin compounds. These lignins apparently occur in the form of well-mixed plant fragments that are transported to sea by rivers and deposited primarily on the inner continental shelf.

Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers

Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets. Amazonian rivers were recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon, but little agreement exists about the sources or turnover times of this carbon. Here we present results of an extensive survey of the carbon isotope composition (13C and 14C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than five years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in medium to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.

Mineralogical and textural controls on the organic composition of coastal marine sediments: Hydrodynamic separation using SPLITT-fractionation

Abstract SPLITT-fractionation was used to sort hydrodynamically surficial sediments from the Washington margin, USA, into sand- (>250, 63–250 μm), silt- (35–63, 17–35, 8–17, 3–8 μm), and clay-sized (1–3, 0.5–1, 64 μm) from the shelf, where terrestrially-derived vascular plant debris accounted for >95% of the organic matter. Organic matter that could not be separated from the inorganic sediment accounted for >90% of the total organic carbon in most fractions, and loadings of organic carbon increased as the surface area of the inorganic particles increased. For the sand- and silt-sized fractions, the observed relationship of 0.81 ± 0.04 mg C m−2 (r = 0.97) was consistent with the hypothesis that a monolayer of organic matter is sorbed to the mineral surfaces. Clay-sized particles had lower organic loadings (0.37 ± 0.07 mg C m−2, r = 0.85), probably because the large interlamellar area of expandable clays was inaccessible to most organic molecules. After correcting for interlamellar area, clay-sized particles have the same organic carbon: surface area relationship as sands and silts (0.78 ± 0.08 mg C m−2, r = 0.96). The relationship over all the particle sizes was 0.76 ± 0.03 mg C m−2, (r = 0.96). While total organic matter concentrations were largely controlled by sediment surface area, the elemental composition of the organic matter appears to be partially affected by sediment mineralogy, and shifted from carbon-rich material (atomic C:N ~ 18.0) in larger, quartz-dominated fractions to N-rich material (C:N ~ 9) in the smaller, clay mineral-dominated fractions. Nitrogen enrichment relative to carbon (atomic N:C) was correlated with the amount of total clay (r = 0.80), smectite (r = 0.79), and the iron content (r = 0.74) of the sediments. Measurements of stable carbon isotopes indicate that clay-sized particles preferentially transport sorbed soil organic matter to deep sites while sand-sized fractions contain terrestrial plant debris (discrete and sediment-associated) that is transported along the shelf. The concentrations of terrestriallyderived organic matter in organic matter from shelf and slope sediments was estimated to be 60–85% and 10–15%, respectively. The quantity, bulk chemical composition, and distribution of marine and terrestrially derived organic matter to Washington margin sediments are influenced by 1. (1) the surface area of the sediment minerals, 2. (2) the mineralogical composition of the sedimentary matrix, and 3. (3) the natural hydrodynamic sorting of sedimentary materials along the continental margin. The major fraction of organic material in these sediments is sorbed to mineral grains. Interactions between organic material and mineral surfaces strongly influence the distribution and elemental composition of the organic material present in marine sediments.