Lawrence M. Mayer

Surface area control of organic carbon accumulation in continental shelf sediments

The relationship between organic carbon (OC) and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments. Cores from many North American shelf environments show downcore decreases in OC to similar refractory background concentrations if expressed relative to the surface area of the sediments. This consistent concentration is 0.86 mg-OC m−2, which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces. A more global collection of sediment-water interface samples show that this relationship is even more extensive, with exceptions occurring in areas of very high riverine sediment input, organic pollution, or low-oxygen water columns. Density separations indicate that organic matter is largely adsorbed to mineral grains. The microtopography of surfaces was examined with N2 sorption and most surface area was found to be inside pores of <10 nm width. These data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay. Such protection would likely work in concert with other protection mechanisms such as humification. This consistent surface area correlation with OC concentration may explain control of spatial and temporal variations in OC burial rates by sedimentation rates; the pore protection hypothesis provides a causal mechanism for this observed control.

Relationships between mineral surfaces and organic carbon concentrations in soils and sediments

Abstract Relationships between mineral specific surface area and organic carbon (OC) concentration are examined for sediments and soil A-horizons from throughout the world. I found (published elsewhere) that continental shelf sediments from many different regions exhibit downcore loss of OC to a refractory background level which shows a consistent relationship with mineral surface area (slope = 0.86 mg m−2 OC). This trend is equivalent to a monolayer of organic matter over all surfaces, and is termed the monolayer-equivalent (ME) level. Sediments and soils from other environments are compared to this empirically derived relationship. Several continental slope areas show extension of this relationship to considerable depth. Marked excesses of OC above this trend, which persist downcore, were found in sediments with high carbonate mineral content or slope sediments with low dissolved oxygen concentrations in the overlying water column. About half of the soils examined also adhered to this relationship, while soils with high carbonate content, low pH, or poor drainage showed OC concentrations higher than the ME level. OC concentrations below the ME level are found in deltaic regions and areas with low organic matter delivery such as the deep sea or arid soils. The nature of mineral surfaces was examined using N2 adsorption-desorption isotherms, and most surface area was found to be present as pores with

Loss of organic matter from riverine particles in deltas

Abstract In order to examine the transport and burial of terrigenous organic matter along the coastal zones of large river systems, we assessed organic matter dynamics in coupled river/delta systems using mineral surface area as a conservative tracer for discharged riverine particulate organic matter (POM). Most POM in the rivers studied (n = 6) is tightly associated with suspended mineral material; e.g., it is sorbed to mineral surfaces. Average organic loadings in the Amazon River (0.67 ± 0.14 mg C m−2), the river for which we have the largest dataset, are approximately twice that of sedimentary minerals from the Amazon Delta (∼0.35 mg C m−2). Stable carbon isotope analysis indicate that approximately two-thirds of the total carbon on the deltaic particles is terrestrial. The combined surface-normalized, isotope-distinguished estimate is that >70% of the Amazon fluvial POM is not buried in the delta, consistent with other independent evidence (Aller et al., 1996). Losses of terrestrial POM have also been quantified for the river/delta systems of Columbia in the USA, Fly in New Guinea, and Huange-He in China. If the losses of riverine POM observed in these river/delta systems are representative of rivers worldwide, then the surface-constrained analyses point toward a global loss of fluvial POM in delta regions of ∼0.1 x 1015 g C y−1.

Extent of coverage of mineral surfaces by organic matter in marine sediments

Abstract Organic matter loading in continental margin sediments frequently occurs at a concentration equivalent to a monolayer coating of mineral grains, raising the question of whether adsorbed organic matter is indeed dispersed over all mineral surfaces. A method was developed to address this configurational issue using the energetics of gas adsorption on oxide surfaces. Enthalpies of gas adsorption were assessed using the C constant of the Brunauer-Emmett-Teller (BET) equation. Physisorption of nitrogen or argon gas involves higher enthalpies onto naked than onto organically coated oxide surfaces. Studies on model adsorbate-adsorbent systems provided an algorithm relating gas adsorption energetics to the fraction of surface coated with organic matter. Application of this algorithm to marine aluminosilicate sediments shows that those with low to moderate loadings of organic matter ( −2 ) have generally less than 15% of their surfaces coated. Most minerals in these sediments, which account for most sediments in the ocean, therefore present a largely naked aluminosilicate surface to aqueous solutions.

Deep-sea deposit-feeding strategies suggested by environmental and feeding constraints

The principle of lost opportunity from optimal foraging theory, coupled with recent information about fluxes in the deep sea, allows prediction of feeding behaviours potentially specific to deep-sea deposit feeders. One possible strategy, thus far documented only indirectly, is to ‘ squirrel ’ away rich food from the seasonal or episodic pulses that recently have been shown to fuel meiofaunal growth. Echiurans and sipunculids show morphological and faecal handling patterns consonant with this suggestion. Where it is prevalent, this foraging strategy can have profound effects on stratigraphy. Autocoprophagy is another expected behaviour across a wider taxonomic spectrum, but one that is especially difficult to document. The principle of lost opportunity also predicts highly selective ingestion, not necessarily accomplished by the assessment of individual particles but possibly through pit building in areas where fluids move near-bed material. Under many depositions regimes, small but abundant feeding depressions may be the primary sites where deposition occurs. Conversely, digestive utilization of heterogeneous refractory substrates like humic acids seems as unlikely as an effective municipal waste recycling system that starts with mixed garbage. High gut: body volume ratios in deep-sea deposit feeders, rather than representing an adaptation to use this heterogeneous and refractory end of the food spectrum, instead may allow (through greater residence time of ingested material) greater conversion and absorption of the labile fraction of sediments as it becomes scarcer. Intense natural selection for particle selection ability in fact is one possible reason for the prevalence of meiofauna in the deep sea, and for the diminutive size of macrofaunal taxa there. This selective pressure probably imposes a very restrictive bottleneck on the initial developmental stages of deposit feeders.

Late Precambrian Oxygenation; Inception of the Clay Mineral Factory

An enigmatic stepwise increase in oxygen in the late Precambrian is widely considered a prerequisite for the expansion of animal life. Accumulation of oxygen requires organic matter burial in sediments, which is largely controlled by the sheltering or preservational effects of detrital clay minerals in modern marine continental margin depocenters. Here, we show mineralogical and geochemical evidence for an increase in clay mineral deposition in the Neoproterozoic that immediately predated the first metazoans. Today most clay minerals originate in biologically active soils, so initial expansion of a primitive land biota would greatly enhance production of pedogenic clay minerals (the “clay mineral factory”), leading to increased marine burial of organic carbon via mineral surface preservation.

Dissolved protein fluorescence in two Maine estuaries

Abstract Fluorescence of dissolved proteinaceous materials was examined in two estuaries differing primarily in river input. Low-wavelength excitation (220–230 nm) was found to be more useful than the high-wavelength excitation (280 nm) usually reported in the literature. Levels of fluorescence in estuarine samples were of the order to be expected from the probable levels of dissolved amino acids. However, quantitation of protein levels by fluorescence, even in relative terms, is virtually impossible, due to positive interferences among the two amino acid peaks and humic material and negative interference by various types of quenching. Salinity has little or no effect on quantum yield. Proteinaceous fluorescence along estuarine transects was noisy, with some positive correlations with chlorophyll levels. Noisy data are consistent with the short lifetimes of proteins in seawater. Sediments appeared to provide a source of proteinaceous fluorescence. Seaward samples tended to show higher tyrosine peaks while upstream samples were richer in tryptophan emission.

Environmental impact of salmon net-pen culture on marine benthic communities in Maine: A case study

The environmental impacts of salmon net-pen aquaculture on the benthic environment were investigated at a commercial fish farm located in coastal Maine waters. This site has a sandy mud bottom and low current velocities, is subjected to episodic sediment resuspension, and way in production for 3 yr prior to this study: We examined both the increase in carbon flux to the benthos caused by the net-pen and the effects of the elevated flux on sediment biogeochemistry and the microbenthic communities. The experimental design involved the establishment of two study sites, an ambient site ca. 100 m from the net-pen and a treatment site around the pen. Sediment traps deployed 1 m above the sediment-water interface indicated that carbon flux to the benthos was increased 1-fold to 6-fold (to a maximum of 5 g m−2d−1) at the edge of the net-pen with little or no increase in carbon flux 10 m from the pen. Unlike carbon flux rates, sediment organic matter inventories showed a complex pattern of change over time. Mineral surface area, organic carbon and nitrogen, digestible protein, and sterol content were initially (April 1991) lower beneath the pen than in ambient sediments. During 1991 ambient sediment accumulated organic matter until July after which it decreased, to a low during November. In contrast, organic matter inventories of sediment beneath the pen remained low until July and then increased to a high during November. These latter gains were associated with the development of bacterial mats at the sediment-water interface. Beneath the pen, microbial and macrofaunal communities were shifted toward those commonly associated with organic enrichment but seasonal trends and storm-related resuspension events also significantly affected these sediment communities. When abundant, most epibenthic organisms were more numerous near the pen than in adjacent ambient areas. These results suggest that net-pen aquaculture can alter the benthic ecosystem in Maine Coastal waters but indicate that the effects are spatially limited.

Digestive environments of benthic macroinvertebrate guts: Enzymes, surfactants and dissolved organic matter

Hydrolytic enzyme activity, surfactancy, and dissolved organic matter in the digestive lumens of 19 benthic echinoderm and polychaete species were examined, using consistent and quantifiable methods. Enzyme activities were compared with those of extracellular enzymes from ambient sediments. Enzyme activities ranged over five orders of magnitude, with averages decreasing in the order polychaetes > echinoderms > sediment. Highest activities in animals were usually associated with the fluid phase in midgut sections, with posteriorward decreases indicating little export to the external environment. At some phyletic levels, activity correlated inversely with animal size. Hydrolase patterns reflected food type; for example, high 1ipase:protease ratios in carnivores reflected esterified lipids in their diets. High surfactant activity was found in gut sections having high enzyme activity. Deposit feeders had the most intense surfactancy, including evidence for micelles. While enzymes reflected the biochemical nature of the digestible food substrate regardless of feeding mode (e.g., deposit vs. suspension feeder), surfactants reflected dilution of this digestible substrate with mineral grains. Dissolved organic matter levels were high, with amino acids reaching levels > 1 M and lipids commonly 1 g L-r. Among polychaete deposit-feeders, low molecular weight amino acids reflected the composition of the food substrate, but were present at much higher concentrations than could be explained by sediment present in the gut-suggesting longer residence times for fluid than for transiting sediment particles. Deposit feeder digestive fluids are better able to solubilize sedimentary food substrates than are sedimentary extracellular enzymes, owing to either more powerful solubilizing agents or to their deployment in freely diffusing, dissolved form. Gut environments may lead to chemical condensation as well as solubilization reactions.

Importance of suspended participates in riverine delivery of bioavailable nitrogen to coastal zones

Total nitrogen (TN) loadings in riverine sediments and their coastal depocenters were compared for 11 river systems worldwide to assess the potential impact of riverine particulates on coastal nitrogen budgets. Strong relationships between sediment specific surface area and TN allow these impacts to be estimated without the intense sampling normally required to achieve such budgets. About half of the systems showed higher nitrogen loadings in the riverine sediments than those from the coastal depocenter. In spite of uncertainties, these comparisons indicate that large, turbid rivers, such as the Amazon, Huanghe, and the Mississippi, deliver sediments that in turn release significant or major fractions of the total riverine nitrogen delivery. Riverine particulates must therefore be considered an essential factor in watershed nutrient loading to coastal ecosystems and may affect delivered nutrient ratios as well as total nutrient loading. The relative importance of particulate versus dissolved delivery has decreased over recent decades in the Mississippi as a result of damming and fertilizer use in the watershed.