The effect of biogenic irrigation intensity and solute exchange on diagenetic reaction rates in marine sediments

The activities of infaunal macrobenthos strongly influence the pathways, rates, and extent of organic matter remineralization and associated reactions in marine sediments. Solute transport during irrigation is a particularly important process that stimulates microbial activity and net remineralization, both within and adjacent to the bioturbated zone. Part of the stimulation proximal to the bioturbated zone is due to redox oscillation and oxidant supply during transport, but part of both the near and far-field effects are a result of other factors. Experiments designed to simulate different degrees of diffusive exchange, and thus infaunal abundances or activity, demonstrate a regular and strong dependence of anaerobic remineralization on diffusive transport. For example, net production of NH4+, HPO4=, I-, and Mn++ increases as the effective distance between burrows becomes ≲2 cm (burrow abundance ≳ 800 m-2) in otherwise identical anoxic sediment. Corresponding changes in sedimentary bacterial numbers, exoenzyme activity, per cell growth rate (RNA), and solid phase properties (N, C/N, P) indicate that the increases in net rates are due in part to an absolute increase in total production. Transport-reaction models and experimental results demonstrate that relative decreases in the uptake of solutes into biomass, abiogenic precipitation reactions, and increased removal of inhibiting metabolites all occur simultaneously, enhancing both total and net remineralization. The phenonomenological first-order rate constant for organic matter decomposition is therefore a function not only of the reductant and oxidant pool properties, but also the environmental transport regime. Solid phase reaction products can differ substantially as a function of diffusive openness. For example, both organic P and the organic P/inorganic P ratio increase in more diffusively-open (irrigated) compared to diffusively-closed, anoxic sediment. The sensitivity of solute concentrations, microbial activity and diagenetic reaction balances to diffusive transport regime, indicates that macrofauna can functionally manipulate these properties through relatively small changes in burrow spacing patterns and individual burrow geometries

Conceptual models of early diagenetic processes: The muddy seafloor as an unsteady, batch reactor

Conceptual models of early diagenetic processes in sedimentary deposits guide interpretation and investigation of compositional patterns, elemental fluxes, and biogeochemical interactions. The ideas that sediments are open to exchange, are laterally homogeneous, and often accrete steadily upward underlie most diagenetic theory. Net accumulation rate of a deposit is thus a master variable controlling reactions, net fluxes, and sediment properties. These basic ideas and corresponding models have proved extraordinarily robust and useful. Large regions of the seafloor, however, can deviate significantly from some of the common assumptions of traditional diagenetic models, particularly along continent-ocean boundaries, where most sedimentary debris is processed. A spectrum of diagenetic facies representing a wide range of boundary conditions and internal transport-reaction regimes is typically present. Mobile muds are one of the major endmember diagenetic facies found in energetic, high sedimentation environments such as estuaries and deltas. These deposits often behave as episodically-mixed, fluidized batch reactors dominated by microbial biomass rather than,for example, classic advective plug flow reactors or geometrically complex, bioturbated bodies. Redox reaction patterns in mobile muds are unsteady. Suboxic conditions often dominate temporally, reflecting a balance between frequency of seafloor disturbance and the relative abundance and reactivity of recently entrained oxidants and reductants. Sedimentary dynamics, rather than net sedimentation, control the magnitude and nature of elemental fluxes and biogeochemical properties of mobile muds and the lateral exchange of material between diagenetic regimes. The understanding of elemental cycling in continental margins and their evolution as biogeochemical systems require consideration of the different dominant modes and the relative importance of diagenetic processing within and between individual facies

ERRATUM in The importance of the diffusive permeability of animal burrow linings in determining marine sediment chemistry, by R. C. Aller; Journal of Marine Research 41(2): 299-322.

On page 304 The roots, am of equation (3) are correctly defined as… The an preceding the cos function was left out by mistake in the original printed version

The sedimentary Mn cycle in Long Island Sound: Its role as intermediate oxidant and the influence of bioturbation, O2,and Corg flux on diagenetic reaction balances

The potential importance of sedimentary Mn as a secondary oxidant and redox intermediate between O2 and Corg is often discounted in nearshore sediments. Study of the Mn cycle at 19 stations in Long Island Sound (LIS) demonstrates that sedimentary Mn can be a significant redox intermediate, accounting in many cases for 30–50% of the benthic O2 flux (annual mean ≈40 ± 35%). At some sites and times, the import of solid oxidant as Mn in suspended matter is also apparently greater than the dissolved O2 flux to the bottom. The relative importance of the Mn cycle as redox intermediate varies substantially both seasonally (summer \u3e fall \u3e spring \u3e winter) and spatially as a function of biogenic reworking, Corg flux, and O2 concentration in the overlying water. During warmer seasons, the net flux of Mn++ from the bottom decreases (≈5–10x) generally from west to east, correlating directly with the benthic flux of planktonic debris and storage of residual Corg. Average fluxes are −0.003, 0.43–0.94, 2.2, and 0.43 mmol Mn/m2/d during winter, spring, summer, and fall respectively. Mn++ fluxes are relatively elevated during the spring bloom despite low temperatures. At most sites and times, surface sediments are enriched in excess Mn (4–17 μmol/g) above lithogenic background, with exponentially decreasing concentrations to 2–3 cm depth. A regular seasonal and spatial cycle of excess Mn occurs. Excess Mn inventories are often ≈5–10 μmol/cm2 but range from ≈0–25 μmol/cm2. The highest inventories are found in mid LIS, in the transition area between high Corg flux and seasonally low O2 regions of the western Sound and the lower Corg flux, better oxygenated regions of central LIS. Excess Mn decreases at most sites following the spring plankton bloom and is lost entirely from westernmost sediments of highest Corg flux, several months before noticeable O2 depletion in overlying water. The bloom is a major factor in mobilization of metals into suspended matter and promotes lateral redistribution of Mn. Destratification and oxygenation of the water column in the fall results in the capture and reestablishment of excess sedimentary Mn in all regions of the Sound. Bioturbation transports Mn and Corg into anoxic sediment zones. When overlying water is well-oxygenated, the resulting Mn++ is efficiently irreversibly adsorbed or reoxidized (≈80–90%, during summer–fall), closing the sedimentary Mn cycle and inhibiting net Mn++ fluxes. The internal Mn cycle is therefore most important as an intermediate oxidant during warm periods of high bioturbation, well-oxygenated overlying water, and moderate Corg flux. S species apparently dominate the direct reduction of Mn

The importance of the diffusive permeability of animal burrow linings in determining marine sediment chemistry

Many of the abundant burrows formed by animals in marine sediments are lined with thin layers of organic material. The permeability of these linings to solute diffusion can be an important determinant of the chemical composition of surrounding sediment and the burrow habitat. Measurement of the diffusive permeability of burrow linings from 8 species of marine invertebrates indicates that the diffusion coefficients of small inorganic solutes within linings are ∼ 10-40% that in free solution. The diffusion of anions, represented by Br−, is hindered somewhat relative to cations, represented by NH4+, indicating that linings have negatively charged regions at the pH of natural waters. The time dependence of diffusion through linings is affected by simple ion exchange within the lining material. Adsorption constants for solutes like NH4+ and Si(OH)4, are in the range of K ∼ 2-8 (relative to pore water volume). Silica shows evidence of interaction other than reversible ion exchange in some cases. The permeability of linings can affect sedimentary solute distributions differently depending on the types of reactions controlling a particular solute in surrounding sediment. The concentrations of solutes subject to zeroth-order reactions are greatly influenced by lining permeability but net fluxes (at steady state) across the lining are not. The opposite is true for solutes subject to first or higher order concentration-dependent reactions. Assuming burrow linings can act as molecular sieves, then some classes of solutes will be strongly influenced by the presence of irrigated burrows while others will be distributed in a deposit as though burrows were completely absent. This may greatly complicate sedimentary solute distributions

Oxic and anoxic decomposition of tubes from the burrowing sea anemone Ceriantheopsis americanus: Implications for bulk sediment carbon and nitrogen balance

Many marine infaunal animals form organic tube and burrow linings. The role of these materials in organic matter cycling and preservation in sediments is largely unknown. In the case examined here, the infaunal sea anemone, Ceriantheopsis americanus, (a common component of bottom communities along the east coast of North America) forms a leathery, fibrous tube lining 2–3 mm thick, ∼1 cm in diameter, and typically extending 20–30 cm into deposits. Tube fibers (∼2 mm long, 2–5 μm thick) formed from discharged specialized nematocyst cells, ptychocysts, are composed of a silk-like protein copolymer, cerianthin. Tubes incubated under oxic and anoxic conditions over a period of 122 days demonstrate that initial rates of whole tube decay are 10–100 times slower than usually found for fresh planktonic debris and aquatic macrophytes despite a relatively low molar C:N ratio of ∼5.1. First order decomposition rate constants in oxic water, anoxic water and anoxic sediment are ∼0.76, ∼0.41 and ∼0.22 yr–1 for particulate tube carbon and ∼0.2, ∼0.1 and ∼0.1 yr–1 for particulate nitrogen, respectively (20°C). There are no obvious (under SEM) morphological changes in tube fibers during initial tube decomposition, implying slower long term rates. Although slow, tube decomposition stimulates bacterial activity in sediments from below ∼10 cm depth where any organic matter present is even more refractory than the tubes themselves. In central Long Island Sound muds, tubes apparenlly account for a minimum of ∼0.6–1.8% and 2.8–8.4% of the steady state C and N detrital pools in the upper 10–30 cm of the sediment. C. americanus tube production apparently accounts for ∼9% of the average particulate carbon and ∼12% of the nitrogen fluxes to the benthos. Tube construction by infaunal benthos may thus represent an important pathway for refractory compound formation and organic matter preservation

Nitrogen cycling in muddy sediments of Great Peconic Bay, USA: Seasonal N reaction balances and multi-year flux patterns

To better understand the capacity of sediments to serve as both source and sink of nitrogen (N) and to identify any evidence of evolving changes in sedimentary N cycling, N2 production, N remineralization, and N2 fixation were studied over a multi-year period (2010–2015) in bioturbated mud of Great Peconic Bay, a temperate northeastern U. S. estuary. Benthic fluxes and rates of organic matter remineralization were measured using in situ and ex situ incubations. Net annual NH+ 4, NO–3/NO–2, and N2–N fluxes (μ = 1.1, 0.03, and 1.2 mmol m –2d –1) were close to averages for comparable sedi- mentary environments from surveys of published field studies. Net N2 fluxes (by membrane inlet mass spectrometry) were influenced in different periods by temperature, oxygenation of sediment, pulsed Corg, and the activity of benthic macrofauna and benthic microalgae, although no single physical or biogeochemical variable showed a strong, direct relationship with net N2 fluxes over all sampling periods. In situ measurements sometimes showed more dynamic and higher amplitude diurnal N flux cycles than did ex situ incubations, suggesting ex situ incubations did not fully capture impacts of bioirrigation or benthic photosynthesis.15 N tracer experiments indicated anammox was \u3c 7% of total N2 production. Acetylene reduction assays demonstrated C2 H4 production to depths ≥ 15 cm and suggested N2 fixation may have approached 25% of gross N2 production(3:1 C2 H4 : N2). Mass balances incorporating independently measured N remineralization estimates were consistent with measured levels of N2 fixation. Overall, complex balances of competing processes governed sedimentary N cycling seasonally, and N2 production dominated N2 fixation. Measured N2 fixation was consistent with constraints from N remineralization rates and net N fluxes except in episodic conditions (e. g., algal blooms). There was no indication of progressive changes in N cycling magnitudes or relative N reaction balances over the study period

Editor\u27s Commentary: On the oxidation of organic matter in marine sediments by bacteria

Selman Waksman is best known for his work on natural antibiotics, for which he originated the term and received a Nobel Prize. However, he and co-workers also extensively investigated soil microbes and their interactions with biogeochemical processes and elemental cycling..

Animal-Sediment Relations in a Tropical Lagoon: Discovery Bay, Jamaica

The distribution of many macrobenthic species in the back-reef lagoon of Discovery Bay, Jamaica can be related to a gradient in bottom stability. This gradient is defined by increasing rates of biogenic reworking and sediment resuspension in the western part of the lagoon. Infaunal diversity and coral growth decrease in the western, unstable areas. The infauna of the carbonate sand consists mainly of deposit feeders. In the western lagoon, the feeding activities of this group result in high biogenic reworking rates (up to 6-7 cm/week) producing loose surface sediment easily resuspended by waves. A maximum, mean resuspension rate of 19 mg/cm2/day was measured. Instability of the lagoon floor, resulting in high water turbidity, inhibits settlement and growth of most suspension feeders and reduces infaunal diversity and coral growth. Because stability of the soft-bottom is significantly influenced by deposit feeders, our observations represent an extension of the trophic group amensalism principle to tropical nearshore enviornments

Effects of the marine deposit-feeders Heteromastus filiformis (Polychaeta), Macoma balthica (Bivalvia), and Tellina texana (Bivalvia) on averaged sedimentary solute transport, reaction rates, and microbial distributions

Experimental studies of the deposit-feeders Heteromastus flliformis, Tellina texana, and Macoma balthica demonstrate that, at natural population abundances, each species has major effects on sediment overlying water solute transport, bulk sediment reaction rates, and microbial distributions. Using Cl− as a conservative tracer, one-dimensional transport models show that the effective diffusion coefficient, De, in the presence of these macrofauna is ∼2–5× the molecular diffusion value in the upper 8–12 cm of sediment. In general, the exact value of De is time dependent as shown by time-course experiments with Heteromastus and by comparison of one-dimensional model predictions with a transient-state, two-dimensional cylindrical coordinate model. This latter model takes into account changes in diffusion geometry caused by irrigated burrow structures. The magnitude of apparent time variation in De depends on burrow abundance, size, and depth of burrowing; larger values of De are measured at longer times of tracer transport. In contrast, the simple nonlocal parameter required to mimic the two-dimensional model distributions is essentially constant with time and can be related to different solutes by the ratio of their diffusion coefficients.Models of pore water distributions demonstrate that the production rates of NH4+ in sediments are increased by at least 20–30% in the presence of macrofauna compared to controls or anoxic incubations, regardless of the model used. This is presumably due to the overall lowering of inhibitory metabolite concentrations as well as stimulation of bacteria during grazing. Total bacterial numbers increase at depth in the presence of Heteromastus and Tellina relative to controls but are depleted in fecal material. A substantial increase in ATP/bacteria ratios occurs in fecal and surface sediment, presumably indicative of active growth and conversion from anaerobic to aerobic metabolism. Apparent elevated numbers and stimulation of metabolic activity is consistent with a microbial gardening effect by macrofauna.Most of the NH4+ produced in experimental sediment was apparently oxidized at the sediment-water interface and, along with the oxidation of sulfide and metals, results in a major surficial zone of low pH and HC03− consumption at a rate \u3e24 meq/m2/d. This should cause substantial dissolution of CaCO3 as shown in previous studies. A zone of elevated Si(OH)4 production is also associated with the redoxcline, but Si(OH)4 is otherwise produced at a sufficiently slow rate that detectable decreases in concentration occur in irrigated sediments. Although no measurable effects of infauna on Si(OH)4 reaction constants could be demonstrated from pore water profiles, lowered concentrations result in higher net production rates, and sediment-water fluxes of Si(OH)4 increased in the presence of macrofauna by ∼1.4–1.6×, in agreement with theoretical models. Despite their limitations, the transport-reaction models and the two-dimensional or nonlocal parameterization models in particular, provide a consistent basis for description of the effects of macroinfauna on bulk sediment properties, and allow for comparison of different species at similar population abundances