Decomposition Of Sedimentary Organic Matter Research Articles

Regeneration of sedimentary phosphorus and iron in a large seasonally hypoxic estuary deciphered by 224Ra/228Th disequilibria

Using a 224Ra/228Th disequilibrium approach, we demonstrate in this study that benthic fluxes of dissolved inorganic phosphorus (DIP) in the seasonally hypoxic Yangtze River Estuary were largely manipulated by two counteracting processes: the decomposition of sedimentary organic matter and adsorption of DIP onto iron (Fe) oxides. The decomposition rate of sedimentary organic matter rose exponentially with bottom water dissolved oxygen (DO) concentration multiplied by the amplification factor of sediment surface area (ξ), a variable used to describe the intensity of bio-irrigation and physical reworking in the sediment deposit. In the summer of 2020, the Yangtze River catchment encountered the largest flood event in the past 20 years. As a result of enhanced physical reworking of the seabed, dissolved inorganic carbon (DIC) flux increased by approximately 4-fold as compared to the summer of 2019 (657 vs. 154 mmol m−2 d−1). DIP flux exhibited similar inter-annual variations to DIC flux, indicating that changes in the decomposition rate of sedimentary organic matter were the main cause of the inter-annual differences in DIP flux (1.5 mmol m−2 d−1 in 2020 vs. 0.3 mmol m−2 d−1 in 2019). On the other hand, benthic dissolved Fe fluxes declined exponentially with rising DO concentration because of re-oxidation of Fe2+ in porewater into Fe oxides. As a consequence, approximately 90% of DIP sourced from the decomposition of sedimentary organic matter was retained within the sediment when the bottom water was well oxygenated (DO >125 μmol l−1). Our results imply that regeneration of sedimentary P and Fe may be efficient only within a very narrow redox window where DO concentrations are below a threshold value of ∼60 μmol l−1, but bio-irrigation or physical reworking is still active.

Winter mixing accelerates decomposition of sedimentary organic carbon in seasonally hypoxic coastal seas

Deltaic systems are characterized by the highest sedimentation rates in the globe. Meanwhile, sedimentary organic matter therein can be efficiently decomposed so that these depositional systems may deviate substantially from the oft-quoted correlation between net sediment accumulation and preservation of organic matter. The exact mechanisms that cause such a deviation in any given case, however, remain poorly understood. In this study, we utilize a novel 224Ra/228Th disequilibrium method to examine sediment oxygen consumption and the release of diagenetic products of organic matter along the major mud wedge system in the inner shelf of the East China Sea. Our sampling campaign was carried out in two contrasting seasons: the summer when seasonal hypoxia was at its peak and physical conditions were relatively quiescent, and the winter when the water column was well oxygenated by intense winter mixing and underlying deposits were subjected to reworking. Unexpectedly, during summer 2017 when the seafloor received the annual maximum supply of organic matter, sediment oxygen consumption rates and benthic fluxes of NH4+ were relatively low, ranging from 6 to 59 mmol O2 m−2 d−1 and from 1.6 to 13 mmol N m−2 d−1, respectively. In contrast, during winter 2018 sediment oxygen consumption rates and benthic fluxes of NH4+ surged to 44–690 mmol O2 m−2 d−1 and 22–58 mmol N m−2 d−1, respectively. We have also identified an exponential relationship between amplification factor of sediment surface area and oxygen concentration in the bottom water. This relationship suggests that kinetic energy dissipation in the water column not only controlled air-sea exchange and seawater mixing, but also intensified sediment–water interaction. Importantly, sediment oxygen consumption rates (FO2) in the mud wedge can be empirically described using a modified form of Michaelis-Menten kinetics, suggesting that FO2and the associated benthic consumption and production of chemicals are controlled by both the transport and reaction processes. We have further demonstrated that a large portion of the organic matter deposited over the seafloor in summer is likely decomposed in winter. Overall, this study highlights intense winter mixing as an important mechanism that causes the highly efficient decomposition of sedimentary organic matter in coastal seas.

Activity and structure of methanogenic microbial communities in sediments of cascade hydropower reservoirs, Southwest China

Freshwater reservoirs are an important source of the greenhouse gas methane (CH4). However, little is known about the activity and structure of microbial communities involved in methanogenic decomposition of sediment organic matter (SOM) in cascade hydropower reservoirs. In this study, we targeted on sediments of three cascade reservoirs in Wujiang River, Southwest China. Our results showed that the content of sediment organic carbon (SOC) was between 3% and 11%, and it's positively correlated with both C/N ratio and recalcitrant organic carbon content of SOM. Meanwhile, SOC content was positively correlated with CH4 production rates but had no significant correlation with total CO2 production rates of the sediments, when rates were normalized to sediment volume. Resultantly, the sediment anaerobic decomposition rates hardly significantly increase along with the SOC content. These results suggested that the terrestrial organic matter accumulated after damming stimulated CH4 production from the reservoir sediments even though its decomposition rate was limited. Meantime, high throughput sequencing of 16S rRNA genes indicated that not only the hydrogenotrophic and acetoclastic, but also the methylotrophic methanogens (Methanomassiliicoccus) are abundant in the reservoir sediments. Moreover, metagenomic sequencing also suggested that methylotrophic methanogenesis are potentially important in the sediment of cascade reservoirs. Finally, the hydraulic residence time of the reservoir could be the key controlling factor of the structures of bacterial and archaeal communities as well as the CH4 production rates of the reservoir sediments.

Enhanced organic matter decomposition in sediment by Tubifex tubifex and its pathway

The role of Tubifex tubifex in organic matter (OM) decomposition in aquatic ecosystems has been widely studied, but considerable uncertainties exist in terms of the effect mechanism. The effect of T. tubifex on sediment OM decomposition in laboratory-scale microcosms was quantified, and possible pathways were identified. In the first 7 days of the decomposition of OM mixed in sediment, no significant effect of T. tubifex on organic matter loss (OML) was observed for both low- and high-OM treatments; meanwhile, from day 7–60, T. tubifex addition significantly improved OML from 55.0%-57.5% to 71.8%–77.7% in the low-OM treatments and from 55.5%-56.6% to 64.1%–68.7% in the high-OM treatments. The enhanced OML observed with T. tubifex was mainly due to the promoted decomposition of refractory organic components, e.g., cellulose, hemicellulose and lignin. The proportion of refractory components in the gut of T. tubifex was significantly lower than that in the sediments (p < 0.01), indicating a pathway corresponding to the ingestion and digestion of refractory components by T. tubifex. Although T. tubifex reduced the water dissolved oxygen (DO) by increasing the water chemical oxygen demand (COD), the oxygen supply was improved by T. tubifex, and this could be affected by the increase in the relative abundance of aerobic to anaerobic bacteria in the sediments. T. tubifex significantly increased the diversity of the bacterial and fungal communities in the sediments. Moreover, the community structure of bacteria and fungi was substantially different between gut and sediment. Therefore, multiple pathways of the effect of T. tubifex on OM decomposition were established, and the results have great significance for the artificial manipulation of OM circulation using T. tubifex and the restoration of damaged aquatic ecosystems.

Tidal Freshwater Zones as Hotspots for Biogeochemical Cycling: Sediment Organic Matter Decomposition in the Lower Reaches of Two South Texas Rivers

While organic and inorganic nutrient inputs from land are recognized as a major driver of primary production in estuaries, remarkably little is known about how processes within the tidal freshwater zones (TFZs) of rivers modify these inputs. This study quantifies organic matter (OM) decomposition rates in surface sediment layers in the lower reaches of two south Texas river channels and identifies key parameters that influence sediment decomposition rates. Sediment cores were collected from non-tidal and tidal freshwater sites in the Mission and Aransas rivers during two summers (June 2015 and June 2016) and two winters (February 2016, January 2017). We measured oxygen consumption rates, organic carbon and nitrogen content, stable isotope ratios (δ13C and δ15N of OM), and sediment porosity. O2 consumption rates in TFZ sediments were 385 ± 88 μmol O2 m−2 h−1 (summer) and 349 ± 87 μmol O2 m−2 h−1 (winter) in the Aransas River and 767 ± 153 μmol O2 m−2 h−1 (summer) and 691 ± 95 μmol O2 m−2 h−1 (winter) in the Mission River. These rates in TFZs were similar to rates in estuaries and higher than rates at non-tidal riverine sites. Rates of sediment O2 consumption were primarily controlled by OM content and temperature. Sediment OM was dominated by algal biomass from in situ production in both TFZs. We hypothesize that algal production and sinking within TFZs is a major pathway for translocation of watershed-derived nutrients from the water column to the sediments within TFZs. Further work is needed to quantify linkages between decomposition, nutrient remineralization, and potential removal through processes such as denitrification.

Respiration and aeration by bioturbating Tubificidae alter biogeochemical processes in aquatic sediment

This study investigates the potential of bioturbating Tubificidae to alter biogeochemical processes by sediment aeration in order to enhance ecosystem development in eco-engineering projects. We introduced Tubificidae in three different densities (5000, 15,000, and 30,000 individuals m−2) in clay-rich sediment from lake Markermeer (The Netherlands). Redox potential, nutrients and major elements were measured from the water column and porewater at different depths. Mineral phase and redox transfers were chemically modelled and oxygen concentrations in bioturbated sediments for each density were mathematically predicted. The measured results of this experiment showed that Tubificidae oxygenated the upper 15 mm of the sediment. This resulted in decomposition of sedimentary organic matter with an associated sixfold increase in NH4 and NOx concentrations in the porewater and the water column. However, phosphorus concentrations were declining in the upper 16 mm, likely as a result of immobilization by pyrite oxidation and production of iron oxides. These bioturbation effects were highest in the treatment with an intermediate density of Tubificidae (15,000 worms m−2) as aeration effects in the treatment with the highest density of Tubificidae (30,000 worms m−2) was impeded by high respiration rates. Furthermore, with a two dimensional diffusion model, simulated effects of respiration and aeration on the oxygen concentration in the sediment suggest that the bioturbation effect is strongest at a density of 12,000 worms m−2. In ecological engineering projects where fast ecosystem development is important, introducing Tubificidae to aquatic sediments to optimal densities might enhance initial ecosystem development due to improved availability of nitrogen as nutrient.

Microbial decomposition of sedimentary organic matter in small temperate lakes

Microbial decomposition of sedimentary organic matter in small temperate lakes

Carbon Sequestration in a Pacific Northwest Eelgrass (Zostera marina) Meadow

Coastal wetlands are known to be efficient carbon sinks due to high rates of primary productivity, carbon burial by mineral sediments, and low rates of sediment organic matter decomposition. Of the three coastal wetland types: tidal marshes, tidal forests, and seagrass meadows, carbon burial by seagrasses is relatively under-studied, and reported rates range widely from 45 to 190 g C m-2 yr-1. Additionally, most of these seagrass rates are biased toward tropical and subtropical species, particularly Posidonia oceanica, with few focused on Zostera marina, the most widespread species in the northern hemisphere. We measured sediment organic content, carbon content, and long-term accretion rates to estimate organic carbon stocks and sequestration rates for a Z. marina meadow in Padilla Bay, a National Estuarine Research Reserve in Washington. We found rates of carbon sequestration to be quite low relative to commonly reported values, averaging 9 to 11 g C m-2 yr-1. We attribute this to both low sediment organic content and low rates of accretion. We postulate here that Padilla Bay's low carbon sequestration capacity may be representative of healthy Z. marina meadows rather than an anomaly, and that Z. marina meadows have an inherently low carbon sequestration capacity because of the species' low tolerance for suspended sediment (which limits light availability) and sediment organic content (which leads to toxic sulfide levels). Further research should focus on measuring carbon sequestration rates from other Z. marina meadows, particularly from sites that exhibit, a priori, the potential for higher rates of carbon sequestration.

Deepest and hottest hydrothermal activity in the Okinawa Trough: the Yokosuka site at Yaeyama Knoll.

Since the initial discovery of hydrothermal vents in 1977, these ‘extreme’ chemosynthetic systems have been a focus of interdisciplinary research. The Okinawa Trough (OT), located in the semi-enclosed East China Sea between the Eurasian continent and the Ryukyu arc, hosts more than 20 known vent sites but all within a relatively narrow depth range (600–1880 m). Depth is a significant factor in determining fluid temperature and chemistry, as well as biological composition. However, due to the narrow depth range of known sites, the actual influence of depth here has been poorly resolved. Here, the Yokosuka site (2190 m), the first OT vent exceeding 2000 m depth is reported. A highly active hydrothermal vent site centred around four active vent chimneys reaching 364°C in temperature, it is the hottest in the OT. Notable Cl depletion (130 mM) and both high H2 and CH4 concentrations (approx. 10 mM) probably result from subcritical phase separation and thermal decomposition of sedimentary organic matter. Microbiota and fauna were generally similar to other sites in the OT, although with some different characteristics. In terms of microbiota, the H2-rich vent fluids in Neuschwanstein chimney resulted in the dominance of hydrogenotrophic chemolithoautotrophs such as Thioreductor and Desulfobacterium. For fauna, the dominance of the deep-sea mussel Bathymodiolus aduloides is surprising given other nearby vent sites are usually dominated by B. platifrons and/or B. japonicus, and a sponge field in the periphery dominated by Poecilosclerida is unusual for OT vents. Our insights from the Yokosuka site implies that although the distribution of animal species may be linked to depth, the constraint is perhaps not water pressure and resulting chemical properties of the vent fluid but instead physical properties of the surrounding seawater. The potential significance of these preliminary results and prospect for future research on this unique site are discussed.

Increase in Nutrients, Mercury, and Methylmercury as a Consequence of Elevated Sulfate Reduction to Sulfide in Experimental Wetland Mesocosms

AbstractMicrobial sulfate reduction (MSR) in both freshwater and marine ecosystems is a pathway for the decomposition of sedimentary organic matter (OM) after oxygen has been consumed. In experimental freshwater wetland mesocosms, sulfate additions allowed MSR to mineralize OM that would not otherwise have been decomposed. The mineralization of OM by MSR increased surface water concentrations of ecologically important constituents of OM: dissolved inorganic carbon, dissolved organic carbon, phosphorus, nitrogen, total mercury, and methylmercury. Increases in surface water concentrations, except for methylmercury, were in proportion to cumulative sulfate reduction, which was estimated by sulfate loss from the surface water into the sediments. Stoichiometric analysis shows that the increases were less than would be predicted from ratios with carbon in sediment, indicating that there are processes that limit P, N, and Hg mobilization to, or retention in, surface water. The highest sulfate treatment produced high levels of sulfide that retarded the methylation of mercury but simultaneously mobilized sedimentary inorganic mercury into surface water. As a result, the proportion of mercury in the surface water as methylmercury peaked at intermediate pore water sulfide concentrations. The mesocosms have a relatively high ratio of wall and sediment surfaces to the volume of overlying water, perhaps enhancing the removal of nutrients and mercury to periphyton. The presence of wild rice decreased sediment sulfide concentrations by 30%, which was most likely a result of oxygen release from the wild rice roots. An additional consequence of the enhanced MSR was that sulfate additions produced phytotoxic levels of sulfide in sediment pore water.

Using isotope labeling to partition sources of CO2 efflux in newly established mangrove seedlings

AbstractCarbon dioxide (CO2) flux is a critical component of the global C budget. While CO2 flux has been increasingly studied in mangroves, better partitioning of components contributing to the overall flux will be useful in constraining C budgets. Little information is available on how CO2 flux may vary with forest age and conditions. We used a combination of 13C stable isotope labeling and closed chambers to partition CO2 efflux from the seedlings of the widespread mangrove Avicennia marina in laboratory microcosms, with a focus on sediment CO2 efflux in establishing forests. We showed that (1) above‐ground part of plants were the chief component of overall CO2 efflux; and (2) the degradation of sediment organic matter was the major component of sediment CO2 efflux, followed by root respiration and litter decomposition, as determined using isotope mixing models. There was a significant relationship between C isotope values of CO2 released at the sediment–air interface and both root respiration and sediment organic matter decomposition. These relative contributions of different components to overall and sediment CO2 efflux can be used in partitioning of the sources of overall respiration and sediment C mineralization in establishing mangroves.

Geochemistry of sedimentary organic matter and trace elements in modern lake sediments from transitional karstic land-sea environment of the Neretva River delta (Kuti Lake, Croatia)

This paper investigates recent sedimentation processes and geochemistry of sedimentary organic matter (SOM) and trace elements in a unique semi-enclosed lacustrine sedimentation system in a transitional land-sea environment of the Neretva River delta. The results have shown that the influence of the Neretva River on the lake sedimentation processes is limited to short-term flood events carrying terrigenous loads, while the authigenic formation of nanostructured calcite is the primary source of sediments in the recent past. The enhanced release of biogenic CO2 into the pore water, deriving from the decomposition of SOM, acts as a major contributor to this process. The occurrence of early formed syngenetic and diagenetic pyrite indicates the existence of oxygen-depleted conditions in the investigated system in the recent past. The input of terrigenous material was identified as the main source of trace elements (Pb, Cu, Zn, Sn) in the lake sediments. The distribution of redox sensitive elements (Mo, Tl, V) particularly depends on the oxygen deficiency and the formation of sedimentary pyrite. The human impact in the study area is negligible, although somewhat elevated concentrations of Sn were found in the surface sediment layers. Finally, the results obtained indicate that the Kuti Lake represents a unique lacustrine environment which can be viewed as an isolated sedimentological and biogeochemical system in the Neretva River delta.

Alteration of extracellular enzyme activity and microbial abundance by biochar addition: Implication for carbon sequestration in subtropical mangrove sediment

Biochar has attracted more and more attention due to its essential role in adsorbing pollutants, improving soil fertility, and modifying greenhouse gas emission. However, the influences of biochar on extracellular enzyme activity and microbial abundance are still lack and debatable. Currently, there is no information about the impact of biochar on the function of mangrove ecosystems. Therefore, we explored the effects of biochar on extracellular enzyme activity and microbial abundance in subtropical mangrove sediment, and further estimated the contribution of biochar to C sequestration. In this study, sediments were amended with 0 (control), 0.5, 1.0 and 2.0% of biochar and incubated at 25 °C for 90 days. After incubation, enzyme activities, microbial abundance and the increased percentage of sediment organic C content were determined. Both increase (phenol oxidase and β-glucosidase) and decrease (peroxidase, N-acetyl-glucosaminidase and acid phosphatase) of enzyme activities were observed in biochar treatments, but only peroxidase activity showed statistical significance (at least p < 0.01) compared to the control. Moreover, the activities of all enzymes tested were significantly related to the content of biochar addition (at least p < 0.05). On the other hand, bacterial and fungal abundance in biochar treatments were remarkably lower than control (p < 0.001), and the significantly negative relationship (p < 0.05) between bacterial abundance and the content of biochar was found. Additionally, the increased percentage of organic C gradually increased with biochar addition rate, which provided evidence for applying biochar to mitigate climate change. Given the importance of microorganisms and enzyme activities in sediment organic matter decomposition, the increased C sequestration might be explained by the large decrease of microbial abundance and enzyme activity after biochar intervention.

Particle-size fractions-dependent extracellular enzyme activity in sediments and implications for resource allocation in a subtropical mangrove ecosystem

The distribution of extracellular enzyme activities in particle-size fractions of sediments was investigated in a subtropical mangrove ecosystem. Five enzymes involved in carbon (C), nitrogen (N), and phosphorus (P) cycling were analyzed in the sand, silt, and clay of sediments. Among these fractions, the highest activities of phenol oxidase (PHO), β-D-glucosidase (GLU), and N-acetyl-glucosiminidase (NAG) were found in sand, and greater than bulk sediments of both intertidal zone (IZ) and mangrove forest (MG). This result implied that sand fractions might protect selective enzymes through the adsorption without affecting their activities. Additionally, the enzyme-based resource allocation in various particle-size fractions demonstrated that nutrients availability varied with different particle-size fractions and only sand fraction of MG with highest total C showed high N and P availability among fractions. Besides, the analysis between elemental contents and enzyme activities in particle size fractions suggested that enzymes could monitor the changes of nutrients availability and be good indicators of ecosystem responses to environmental changes. Thus, these results provided a means to assess the availability of different nutrients (C, N, and P) during decomposition of sediment organic matter (SOM), and thus helping to better manage the subtropical mangrove ecosystems to sequester C into SOM.

Changes in the adsorption of bisphenol A, 17 α-ethinyl estradiol, and phenanthrene on marine sediment in Hong Kong in relation to the simulated sediment organic matter decomposition

Marine sediment with an input of particulate organic matter was incubated to simulate the early aging process. On the sediment after various incubation periods, adsorption and desorption tests were conducted for three selected organic micropollutants: bisphenol A (BPA), 17α-ethinyl estradiol (EE2), and phenanthrene (Phe). The results showed significant sediment organic matter (SOM) decomposition during the incubation, and the SOM decay and transformation had a profound impact on the adsorption of organic compounds by the sediment. An increasing-delay-increasing pattern of change was observed for the SOM normalized partition coefficients of EE2 and Phe. This change was accordant to the transformation of SOM from labile organics into active biomass and its microbial products, and finally into more condensed and humic-like substances. Comparison between the 3 model micropollutants indicates that the chemical adsorption behaviors were mostly affected by their hydrophobic properties.

Origin of methane in high-arsenic groundwater of Taiwan – Evidence from stable isotope analyses and radiocarbon dating

Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH 4). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH 4 and its hydrological implications, stable carbon isotope ratios ( δ 13C) and radiocarbon ( 14C) ages of exsolved CH 4, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ 13 C CH 4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample ( δ 13 C CH 4 = 38.2 ‰ ) from the Ilan plain, all CH 4 samples analyzed were produced via microbially mediated CO 2 reduction. Many δ 13C DIC values are considerably greater than −10‰ and even up to ∼10‰ due to Rayleigh enrichment during CO 2 reduction. (2) Almost all the 14C ages of CH 4 samples from the shallow aquifer (I) (<60 m depth) are greater than the 14C ages of coexisting DIC and sediments, suggesting the presence of CH 4 from underlying aquifers. (3) The 14C ages of coexisting CH 4, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH 4 and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH 4 and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH 4 sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO 2 and CH 4 from underlying strata.

海底泥火山堆積物中の間隙水の起源

Submarine mud volcanoes are remarkable geological features on the seafloor, which are probably formed by mud breccia extruded from sub-seafloor sediment layers to the seafloor. Most of such volcanoes are found near the continental margin. The driving force of mud volcanism is thought to be unusually high pressure within the deep sedimentary layer and the release of that high pressure. It is important to know the origins of fluids in a mud volcano, because the production of low-density fluid and/or gas production in the deep sedimentary layer has been assumed to be one of the most probable sources of the pressure. Therefore, geochemical studies of pore fluids have been done at various mud volcanoes to identify the fluid origin. These studies revealed common chemical characteristics of the fluids, indicating the effects of dehydration of clay minerals. Also, the fluids contain hydrocarbon gases derived from thermocatalyte decomposition of sedimentary organic matter. These characteristics suggest that the mud volcano fluids must originate at a depth in the sedimentary layer greater than 2 km. In some mud volcano fields in the active continental margin, it is proposed that fluid in the mud volcano has migrated through faults from greater depths than the original depth of extruded sediments. Such fluid migration may be another source of high pressure in sedimentary layers.

Effects of bacterial dynamics on organic matter decomposition and nutrient release from sediments: A modeling study

Nutrient inputs to lakes, reservoirs, estuaries, and continental shelf waters are often dominated by nutrient release from sediments. Rates of nutrient release from the decomposition of sedimentary organic matter are determined by bacterial demands for food and energy. This dependence of decomposition kinetics on bacterial dynamics, however, is not explicitly considered in current models of early diagenesis. Here, we present a new model of early diagenesis that is based on soil decomposition models and that explicitly includes bacterial biomass as a state variable. Using the new model, we perform steady-state sensitivity analyses and integrate two time-dependent scenarios to determine the major controls on the abundance of bacteria in sediments and on sediment nutrient release. First, we test the sensitivity of bacterial biomass and nutrient release to substrate quantity and quality, and to bacterial growth parameters, growth efficiency and mortality. The model predicts that bacterial abundance in the sediments and nutrient release rates increase significantly with higher substrate inputs and higher quality of organic substrate. High growth efficiencies, on the other hand, reduce nutrient release rates and lead to more nutrient immobilization into bacterial biomass. Efficiency driven increases in bacterial abundances must be counteracted by higher mortality rates, if bacterial pool sizes are to remain within the range of values reported in the literature. Second, we simulate eutrophication and lake recovery over a 20-year period and the effect of a short-term peak in substrate input from an algal bloom. The model predicts that nutrient immobilization in the bacterial pool during eutrophication reduces nutrient release, while the decline of bacterial biomass in response to reduced substrate loadings prolongs lake recovery. The model further suggests that bacterial dynamics dampen the response of nutrient release to short-term peak inputs of organic matter. When bacterial biomass is allowed to vary over time, nutrient release rates are suppressed at their peak but remain elevated for a longer time. Models that do not consider the role of bacterial growth dynamics for organic matter decomposition are not able to simulate the potentially important effects of nutrient immobilization and variable decomposition rates on nutrient release.

Morphology of Molar-Tooth Structures in Precambrian Carbonates: Influence of Substrate Rheology and Implications for Genesis

Abstract Molar-tooth (MT) is an enigmatic carbonate fabric composed of variously shaped cracks and voids filled with a characteristically uniform, equant microspar. MT is both abundant and widespread in Mesoproterozoic and early Neoproterozoic strata, where void-filling microspar comprises up to 90% of individual beds and 5–25% of preserved carbonate. The temporal restriction of this fabric suggests a potential link between MT formation and the biogeochemical evolution of marine environments. Detailed petrographic relationships among MT crack morphology, distribution of MT microspar, and composition of the surrounding substrate suggest that crack formation and microspar precipitation are intimately linked to the decomposition of sedimentary organic matter in the presence of supersaturated Proterozoic seawater. Laboratory experiments have shown that gas generated within unconsolidated mud can reproduce a variety of MT crack morphologies, yet current gas expansion and migration models do not explicitly consider the role of substrate variability in determining morphologies of MT cracks. A detailed petrographic examination of MT structures from the Mesoproterozoic Belt Supergroup, Montana, permits interpretation of the microscale relationship between crack morphology and lithologic, and potentially rheologic, variability of the surrounding substrate by tracing the distribution of petrographically distinctive MT microspar. Observations of lateral offset of MT cracks at bedding planes or within coarser-grained siltstone or sandstone layers, termination of cracks beneath clay- or organic-rich horizons, grain collapse into underlying MT cracks, and the presence of MT microspar as a pore-filling precipitate suggest that grain size, substrate lithology, and substrate cohesion all play critical roles in the development of MT cracks. By contrast, the presence of a wide range of MT crack morphologies within petrographically homogeneous substrates, and an apparent relationship between crack diameter and sinuosity, suggest that the void-forming process itself also played a role in determining the final morphology of MT cracks. Together, these petrographic observations are used to define a model of microscale gas–sediment interactions that can be used to interpret crack morphology in terms of gas pressure and the strength of sedimentary substrates. The presence of characteristic, void-filling microspar is integral to preservation of MT structures. Cathodoluminescence (CL) identification of this characteristic microspar within MT voids, in pore space of coarse-grained facies, and interstitially within fine-grained facies adjacent to MT voids suggests that MT voids and cement share a common genesis. Because microspar cores are similar in size and morphology to vaterite precipitated experimentally in the presence of a variety of dissolved organic molecules, we suggest that precipitation of MT microspar was intimately linked with gas production during organic decomposition within the host substrate. In this scenario, gas production would result in pore fluids with elevated concentrations of dissolved organic molecules, which would initiate precipitation of MT microspar when the pore fluids come in contact with supersaturated Proterozoic seawater. Restriction of MT largely to Mesoproterozoic and early Neoproterozoic strata likely reflects a critical level of carbonate saturation that limited early substrate lithification, thereby allowing void production but remained high enough that organic catalysts were able to initiate precipitation of MT microspar.

Extreme13 C enrichments in a shallow hypereutrophic lake: Implications for carbon cycling

An analysis of stable carbon isotope (δ13C) ratios in Lake Apopka, Florida, reveals extreme 13C enrichments of dissolved inorganic carbon (DIC) pools in the water column and sediment pore water of this shallow polymictic and hypereutrophic lake. The sediment pore water had high averageδ13C of DIC (26.4‰), DIC (8.37 mmol L−1), and methane (CH4) concentrations (1.23 mmol L−1). The extreme 13C enrichment in the sediment pore‐water DIC pool is attributed to methanogenesis, which produces 13C‐rich carbon dioxide (CO2) and 13C‐poor CH4 during the bacterial fermentation of organic matter. The δ13C in the water‐column DIC pool ranged from 5‰ to 13‰ with an average of 9.0‰. The flux‐weighted δ13C from the DIC due to external loading and sediment respiration was estimated as −12‰, whereas the δ13C from particulate organic carbon (POC) due to water‐column production was −13‰. The 13C enrichment in the water column is attributed directly to the diffusion and advection of isotopically heavy DIC from the sediment and to the isotopic fractionation by phytoplankton photosynthesis and is attributed indirectly to the removal of isotopically light CH4 by ebullition and organic matter by sedimentation and outflow. Atmospheric invasion and sedimentation were the most important source and sink, respectively, in the carbon mass balance. CH4 oxidation, atmospheric invasion, anaerobic respiration, and sedimentation are the important flux terms affecting the isotopic mass balance. A combination of shallow water depth, frequent wind mixing, anoxic sediments with high rates of methanogenesis, high phytoplankton productivity, and lack of external loading dominated by terrestrial carbon led to the 13C enrichment of the water‐column DIC pool in Lake Apopka.