Neal E. Blair

The persistence of memory: the fate of ancient sedimentary organic carbon in a modern sedimentary system

The cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales. The burial of organic carbon occurs predominantly at sea in association with clay-sized particles derived from the erosion of uplifted continental rocks. It follows that the history of the fine-grained particles on land may bear on the nature of the organic carbon buried. In this study, the evolution of clay-associated organic matter was followed from bedrock source to the seabed in the Eel River sedimentary system of northern California using natural abundance 13C and 14C tracers. Approximately half of the fine-grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted Mesozoic-Tertiary Franciscan Complex of the watershed. The short residence time of friable soils on steep hill slopes, coupled with rapid sediment accumulation rates on the shelf-slope, act to preserve the ancient organic carbon. A comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope. The bimodal mixture of ancient and modern C in soils and sediments may be characteristic of many short, mountainous rivers. If the Eel River chemistry is typical of such rivers, more than 40 Tg of ancient organic C may be delivered to the world’s oceans each year. A flux of that magnitude would have a significant influence on marine and global C-cycles.

Rapid subduction of organic matter by maldanid polychaetes on the North Carolina slope

In situ tracer experiments conducted on the North Carolina continental slope reveal that tube-building worms (Polychaeta: Maldanidae) can, without ingestion, rapidly subduct freshly deposited, algal carbon ( 13 C-labeled diatoms) and inorganic materials (slope sediment and glass beads) to depths of 10 cm or more in the sediment column. Transport over 1.5 days appears to be nonselective but spatially patchy, creating localized, deep hotspots. As a result of this transport, relatively fresh organic matter becomes available soon after deposition to deep-dwelling microbes and other infauna, and both aerobic and anaerobic processes may be enhanced. Comparison of tracer subduction with estimates from a diffusive mixing model using 234 Th-based coefficients, suggests that maldanid subduction activities, within 1.5 d of particle deposition, could account for 25-100% of the mixing below 5 cm that occurs on 100-day time scales. Comparisons of community data from the North Carolina slope for different places and times indicate a correlation between the abundance of deep-dwelling maldanids and the abundance and the dwelling depth in the sediment column of other infauna. Pulsed inputs of organic matter occur frequently in margin environments and maldanid polychaetes are a common component of continental slope macrobenthos. Thus, the activities we observe are likely to be widespread and significant for chemical cycling (natural and anthropogenic materials) on the slope. We propose that species like maldanids, that rapidly redistribute labile organic matter within the seabed, probably function as keystone resource modifiers. They may exert a disproportionately strong influence (relative to their abundance) on the structure of infaunal communities and on the timing, location and nature of organic matter diagenesis and burial in continental margin sediments.

Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments

Diagenetic reactions and redox properties of Amazon shelf sediments are characterized by extensive vertical and lateral regions of Fe and Mn cycling. This is in contrast to many temperate estuarine and shelf deposits where S can dominate early diagenesis, but may be typical of wet-tropical regions draining highly weathered terrain with energetic coastlines. Although the major pathways of Corg remineralization in surfical sediments apparently differ from previously studied areas, the absolute magnitude and relative importance of benthic decomposition on the Amazon shelf are comparable to many shallow water regions of equivalent depth range (10–40 m). Net ΣC02 production over the upper ∼1–2 m of deposits is >50 mmol m−2 d−1 and has a predominantly planktonic isotopic composition (δ13C∼−21to−22%‰), indicating that marine organic matter largely drives diagenesic reactions and that >20% of average water-column primary production is metabolized on the seafloor. The ΣCO2 production rates in the upper 0–5 cm of sediment tend to increase slightly alongshelf away from the turbid river mouth, but are relatively uniform within cross-shelf transects any given season and independent of net sedimentation rate. Near uniformity in surface decomposition rates, despite substantial offshore increases in water-column productivity and net accumulation at the delta front, implies rapid cross-shelf particle exchange by estuarine circulation and tidal currents. Build-up patterns of pore-water ΣCO2 indicate in some cases that the upper ∼20 cm was deposited only a few days prior to core collection. Benthic ΣC02 production is highest during periods of low or falling river flow, but no dramatic seasonality occurs. O2 penetrates ∼2–4 mm into sediments and diffusive OZ uptake averages ∼13 mmol m−2 d−1 annually. Anaerobic metabolism accounts for >75% of sedimentary remineralization, but C/S burial ratios are usually >6 (average world shelf |2.8). Seasonal patterns in sedimentary Fe oxidation states indicate that sediments can be partially reoxidized to depths ∼0.6–1 m during erosion/redeposition and that subsequent Fe reduction can account for much of the anaerobic decomposition. Diffusive ΣC02 fluxes and pore-water inventories imply substantial loss of remineralized C to authigenic sedimentary-carbonate formation or flux imbalances due to nonsteady-state ingrowth of disturbed pore-water profiles. Reoxidation of metabolites and nutrient release to the water column occur during massive physical remobilization of sediments. The total benthic N remineralization flux (recycled) is comparable to external riverine and shelf-upwelling fluxes. During stable seabed periods, however, little or no co-remineralized N (N4+, N03−, NO2− or P escapes diffusively into overlying water, indicating the potential for loss of up to ∼100% of the benthic remineralized N (by denitrification) and P (during authigenic mineral precipitation and adsorption). Overall, the shelf apparently acts as an efficient, fluidized-bed denitrification processor (∼50% recycled N) but an inefficient burial sink for riverine and upwelled N. In contrast to the atmospheric sink for N, rapidly regenerated P is eventually lost to the open ocean during sediment resuspension and desorption into the overlying water. Approximately 90% of the remineralized ΣCO2 production flux escapes the sediment and ∼10% is permanently buried as authigenic carbonate. Despite reoxidation and carbonate dissolution during reworking, net burial of C is ∼5 x 1012 g Ctotal per year, of which ∼25–30% is carbonate from remineralized organics (∼70% of this fraction is marine) ∼20% is residual marine Corg, and the remaining (∼50% is residual terrestrial Corg. “Refractory” terrestrial POC is apparently subject to repetitive co-oxidation and redox cycling, resulting in remineralization of ∼65–70% of input and leaving less than ∼30–35% of the riverine POC flux stored on the shelf.

Methane emission from rice: Stable isotopes, diurnal variations, and CO2 exchange

The importance of vegetation in supporting methane production and emission within flooded rice fields was demonstrated. Methane emission from Lousiana, United States, rice fields was correlated to the quantity of live aboveground biomass and the rate of CO2 exchange. The quantity of belowground methane was greater in vegetated plots relative to plots maintained free of vegetation. The diurnal maximum in the rate of methane emission was coincident with the release of the most 13C-enriched methane and a maximum in transpiration rate rather than stomatal conductance, suggesting that diurnal variations in methane emission rate are linked with transpiration, in addition to temperature. Results of isotopic measurements of belowground, lacunal, and emitted methane indicate that methane is transported from rice predominantly via molecular diffusion with a small component due to transpiration-induced bulk flow. Samples of methane collected from air-filled internal spaces within the rice culm were 13C-enriched (−53.1 ± 0.3‰) relative to emitted (−64.5 ± 1.0‰) and belowground methane (−59 ± 1.0‰) . Reproduction of these observed 13C values with a numerical model required isotopic fractionation effects associated with transport of methane into and from rice plants. The model could not conclusively confirm rhizospheric methane oxidation. However, 13C-enriched methane was observed in the floodwater overlying the flooded soil (−44.4 ± 2.2‰), consistent with the oxidation of substantial quantities of methane as it diffused across the soil-water interface.

The carbon isotope biogeochemistry of acetate from a methanogenic marine sediment

The δ13C value of porewater acetate isolated from the anoxic sediments of Cape Lookout Bight, North Carolina, ranged from −17.6% in the sulfate reduction zone to −2.8% in the underlying methanogenic zone. The large 13C-enrichment in the sulfate-depleted sediments appears to be associated with the dissimilation of acetate to CH4 and CO2. Fractionation factors for that process were estimated to be 1.032 ±0.014 and 1.036 ±0.019 for the methyl and carboxyl groups. A subsurface maximum in δ13C of the total acetate molecule, as well as the methyl and carboxyl carbons at 10–15 cm depth within the sediment column indicates that changes in the relative rates of acetate cycling pathways occur in the methanogenic zone. The methyl group of the acetate was depleted in 13C by 7–14% relative to the carboxyl moiety. The intramolecular heterogeneity may be the result of both synthetic and catabolic isotope effects.

Anaerobic methane oxidation on the Amazon shelf

Anaerobic methane oxidation on the Amazon shelf is strongly controlled by dynamic physical sedimentation processes. Rapidly accumulating, physically reworked deltaic sediments characteristic of much of the shelf typically support what appear to be low rates of steady state anaerobic methane oxidation at depths of 5–8 m below the sediment-water interface. Methane oxidation in these cases is responsible for 10 m) methane-charged sediment directly to seawater. In this nonsteady-state situation, methane is a major source of recently produced ΣCO2 and an important reductant for sulfate. These observations suggest that authigenic sedimentary carbonates derived from anaerobic methane oxidation may sometimes reflect physically enhanced nonsteady-state exposure of methane to sulfate in otherwise biogeochemically unreactive deposits. The concentration profiles of CH4, SO 4= , and ΣCO2 in the eroded deposit were reproduced by a coupled reaction-transport model. This area of the shelf was reexposed to seawater approximately 5–10 years ago based on the model results and the assumption that the erosion of the deposit occurred as a single event that has now ceased. The necessary second order rate constant for anaerobic methane oxidation was ≥0.1 mM −1 d −1.

Bacillus thuringiensis-toxin resistance management: Stable isotope assessment of alternate host use by Helicoverpa zea

Data have been lacking on the proportion of Helicovera zea larvae that develop on noncotton host plants that can serve as a refuge from selection pressure for adaptation to transgenic cotton varieties that produce a toxin from the bacterium Bacillus thuringiensis. We found that individual H. zea moths that develop as larvae on cotton and other plants with C3 physiology have a different ratio of 13C to 12C than moths that develop on plants with C4 physiology, such as corn. We used this finding in determining the minimum percentage of moths that developed on noncotton hosts in two cotton-growing areas. Our results indicate that local corn can serve as a refuge for H. zea in midsummer. Our results contrast dramatically with the prevailing hypothesis that the large majority of late-season moths are produced from larvae feeding on cotton, soybean, and other C3 plants. Typically, <50% of moths captured in August through October have isotope ratios indicative of larval feeding on C3 plants. In one October sample, 100% of the moths originated from C4 hosts even though C4 crops were harvested at least 1 mo earlier, and no common wild C4 hosts were available. These findings support other research indicating that many late-season H. zea moths captured in Louisiana and Texas are migrants whose larvae developed on corn in more northern locations. Our isotope data on moths collected in Texas early in the season indicate that the majority of overwintering H. zea do not originate from cotton-feeding larvae and may be migrants from Mexico. Non-Bt corn in Mexico and the U.S. corn belt appears to serve as an important refuge for H. zea.

BENTHIC FLUXES AND POREWATER CONCENTRATION PROFILES OF DISSOLVED ORGANIC CARBON IN SEDIMENTS FROM THE NORTH CAROLINA CONTINENTAL SLOPE

Abstract Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO2 and SO42−) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.

Factors that control the stable carbon isotopic composition of methane produced in an anoxic marine sediment

The carbon isotopic composition of methane produced in anoxic marine sediment is controlled by four factors: (1) the pathway of methane formation, (2) the isotopic composition of the methanogenic precursors, (3) the isotope fractionation factors for methane production, and (4) the isotope fractionation associated with methane oxidation. The importance of each factor was evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Methane did not accumulate during the initial 42-day period when sediment contained sulfate, indicating little methane production from “noncompetitive” substrates. Following sulfate depletion, methane accumulation proceeded in three distinct phases. First, CO2 reduction was the dominant methanogenic pathway and the isotopic composition of the methane produced ranged from −80 to −94‰. The acetate concentration increased during this phase, suggesting that acetoclastic methanogenic bacteria were unable to keep pace with acetate production. Second, acetate fermentation became the dominant methanogenic pathway as bacteria responded to elevated acetate concentrations. The methane produced during this phase was progressively enriched in 13C, reaching a maximum δ13C value of −42‰. Third, the acetate pool experienced a precipitous decline from >5 mM to <20 μM and methane production was again dominated by CO2 reduction. The δ13C of methane produced during this final phase ranged from −46 to −58‰. Methane oxidation concurrent with methane production was detected throughout the period of methane accumulation, at rates equivalent to 1 to 8% of the gross methane production rate. Thus methane oxidation was too slow to have significantly modified the isotopic signature of methane. A comparison of microcosm and field data suggests that similar microbial interactions may control seasonal variability in the isotopic composition of methane emitted from undisturbed Cape Lookout Bight sediment.

Watershed control on the carbon loading of marine sedimentary particles

Abstract Previous investigations of the factors governing organic carbon burial on continental margins have pointed toward the important, apparently protective association of carbon with mineral particles. These studies have also revealed dramatic transformations of carbon-particle relationships at the land-sea interface. Riverine particles in some settings lose a large portion of their loads of sorbed terrestrial carbon upon discharge to the ocean and gradually reload to similar levels with marine carbon. The Eel River in northern California and the adjacent continental shelf were selected as an ideal system to investigate the rates of these processes. The river is episodically subject to large floods, and the shelf stratigraphy preserves a record of the resultant large pulses of sediment and carbon input to the marine environment. Carbon isotopic, carbon to nitrogen, and carbon to surface area ratios of particles in flood deposits were expected to reflect the rapid unloading of terrestrial carbon from discharged particles, whereas nonflood sediments that have accumulated at slower rates on the shelf were expected to carry higher loads of marine carbon. Our results indicate, however, that particles on the Eel shelf have retained their loads of terrigenous carbon, and that a significant portion of the particle-sorbed carbon buried on the shelf is kerogen derived from the Mesozoic-Tertiary Franciscan Complex. We hypothesize that rates of uplift and mass wasting in the Eel watershed and rates of particle delivery to and burial on the continental shelf, are so rapid that kerogen is not completely oxidized and is recycled instead. The loading of carbon on clay-sized particles delivered to the shelf, moreover, is dependent on river discharge and may reflect the relative importance of different mass wasting processes during precipitation events of varying intensity. The Eel River system is likely to be representative of other small, mountainous rivers and indicates that processes on land may play an important role in governing the amount and character of carbon being buried on the continental margins.