Yoko Furukawa

Bubble growth and rise in soft sediments

The mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance. We offer definitive evidence on the mode of bubble formation in the form of X-ray computed tomographic images and comparison with theory. Natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in response to growth stresses. We also present bubble-rise results from gelatin, a mechanically similar but transparent medium, that suggest that initial rise is also accomplished by fracture. Given that muddy sediments are elastic and yield by fracture, it becomes much easier to explain physically related phenomena such as seafloor pockmark formation, animal burrowing, and gas buildup during methane hydrate melting.

Bioirrigation modeling in experimental benthic mesocosms

Burrow irrigation by benthic infauna affects chemical mass transfer regimes in marine and estuarine sediments. The bioirrigation facilitates rapid exchange of solutes between oxygenated overlying water and anoxic pore water, and thus promotes biogeochemical reactions that include degradation of sedimentary organic matter and reoxidation of reduced species. A comprehensive understanding of chemical mass transfer processes in aquatic sediments thus requires a proper treatment of bioirrigation. We investigated bioirrigation processes during early diagenesis using laboratory benthic mesocosms. Bioirrigation was carried out in the mesocosms by Schizocardium sp., a funnel-feeding enteropneust hemichordate that builds and ventilates a U-shaped burrow. Interpretation of the laboratory results was aided by a two-dimensional multicomponent model for transport and reactions that explicitly accounts for the depth-dependent distribution of burrows as well as the chemical mass transfers in the immediate vicinity of burrow walls. Our study shows that bioirrigation significantly affects the spatial distributions of pore water solutes. Moreover, bioirrigation promotes burrow walls to be the site of steep geochemical gradients and rapid chemical mass transfer. Our results also indicate that the exchange function, a, widely used in one-dimensional bioirrigation modeling, can accurately describe the bioirrigation regimes if its depth attenuation is coupled to the depth-dependent distribution of burrows. In addition, this study shows that the multicomponent 2D reaction-transport model is a useful research tool that can be used to critically evaluate common biogeochemical assumptions such as the prescribed depth dependencies of organic matter degradation rate and C/N ratio, as well as the lack of macrofaunal contribution of metabolites to the pore water.

Aggregation of montmorillonite and organic matter in aqueous media containing artificial seawater

BackgroundThe dispersion-aggregation behaviors of suspended colloids in rivers and estuaries are affected by the compositions of suspended materials (i.e., clay minerals vs. organic macromolecules) and salinity. Laboratory experiments were conducted to investigate the dispersion and aggregation mechanisms of suspended particles under simulated river and estuarine conditions. The average hydrodynamic diameters of suspended particles (representing degree of aggregation) and zeta potential (representing the electrokinetic properties of suspended colloids and aggregates) were determined for systems containing suspended montmorillonite, humic acid, and/or chitin at the circumneutral pH over a range of salinity (0 – 7.2 psu).ResultsThe montmorillonite-only system increased the degree of aggregation with salinity increase, as would be expected for suspended colloids whose dispersion-aggregation behavior is largely controlled by the surface electrostatic properties and van der Waals forces. When montmorillonite is combined with humic acid or chitin, the aggregation of montmorillonite was effectively inhibited. The surface interaction energy model calculations reveal that the steric repulsion, rather than the increase in electronegativity, is the primary cause for the inhibition of aggregation by the addition of humic acid or chitin.ConclusionThese results help explain the range of dispersion-aggregation behaviors observed in natural river and estuarine systems. It is postulated that the composition of suspended particles, specifically the availability of steric polymers such as those contained in humic acid, determine whether the river suspension is rapidly aggregated and settled or remains dispersed in suspension when it encounters increasingly saline environments of estuaries and oceans.

HYPOXIA HOTSPOTS IN THE MISSISSIPPI BIGHT

Foraminiferal proxies of hypoxia indicate apparent low oxygen to hypoxic conditions in several hotspots in the Mississippi Bight. The foraminiferal hypoxia proxies, the Ammonia to Elphidium (A/E) index and the Pseudononion-Epistominella-Buliminella (PEB) index, were tabulated from three sets of core tops collected in 1951–1956. Additionally, the oxygenation history of a site near the Balize delta was evaluated over the past one hundred years based on A/E and PEB indices and size distributions of pyrite framboids in a gravity core dated by 210Pb geochronology. The results from the 1950’s core-top collections show apparent, recurrent low-oxygen to hypoxic bottom water on the inner shelf at hotspot locales seaward of the Mississippi-Alabama barrier islands and the eastern distributaries of the Balize delta. Specifically, the A/E index exceeds 90% on the inner shelf seaward of Horn and Dauphin islands, both of the Mississippi-Alabama barrier islands, and a center between Pass a Loutre and Main Pass of the Balize delta. In partial support of these results are reports of present-day low oxygen to hypoxic concentrations in bottom waters associated with seasonally high surface chlorophyll a and seasonal strengthening of a brackish-water cap at these locales. In contrast, the PEB index in core tops suggests good oxygenation at mid-shelf depths >30 m. The PEB index, size distributions of framboidal pyrite, and other indicators in a gravity core 44 km northeast of Pass a Loutre indicate no clear change in conditions over the past century, constraining the seaward extent of the hotspot near the Balize delta.

The Role of Biologically-Enhanced Pore Water Transport in Early Diagenesis: An Example from Carbonate Sediments in the Vicinity of North Key Harbor, Dry Tortugas National Park, Florida

Biologically enhanced pore water irrigation affects the course of early diagenesis in shallow marine sediments, as illustrated here for the carbonate sediments from North Key Harbor, Dry Tortugas National Park, Florida. Whereas macrofaunal activity at the study site extends approximately 15 cm below the water-sediment interface, measured O 2 microprofiles only show O 2 penetration to depths of a few mm. This apparent discrepancy can be explained by considering the 3-D O 2 distribution in the burrowed sediments. Calculations based on an idealized tube model for burrow irrigation show that measureable O 2 concentrations are limited to the immediate vicinity of burrows. Given the observed burrow density (705 ± 15 m -2 ), a randomly positioned O 2 microprofile has a high probability (>90%) to fall outside the reach of radial O 2 diffusion from burrows. Hence, the shallow penetration depths recorded at the site do not exclude a much deeper supply of O 2 in the sediment via the burrows. Other characteristic features observed in the upper 15-20 cm of the sediments, in particular, the absence of SO 4 2- depletion and the presence of subsurface maxima in the profiles of NH 4 + and TCO 2 , are also the result of pore water irrigation. These features are reproduced by the multicomponent reactive transport model STEADYSED1. Results of the model simulations indicate that bacterial SO 4 2- reduction is the dominant pathway of organic carbon degradation, but that consumption of SO 4 2- in the sediments is compensated by its enhanced transport by irrigation. Thus, depth profiles of SO 4 2- may be poor indicators of the importance of SO 4 2- reduction in irrigated sediments.

Effect of organic matter on estuarine flocculation: a laboratory study using montmorillonite, humic acid, xanthan gum, guar gum and natural estuarine flocs

BackgroundRiverine particles undergo a rapid transformation when they reach estuaries. The rapid succession of hydrodynamic and biogeochemical regimes forces the particles to flocculate, settle and enter the sediment pool. The rates and magnitudes of flocculation depend on the nature of the particles which are primarily affected by the types and quantities of organic matter (OM). Meanwhile, the OM characteristics vary widely between environments, as well as within a single environment due to seasonal climate and land use variability. We investigated the effect of the OM types and quantities through laboratory experiments using natural estuarine particles from the Mississippi Sound and Atchafalaya Bay as well as model mixtures of montmorillonite and organic molecules (i.e., biopolymers (guar/xanthan gums) and humic acid).ResultsBiopolymers promote flocculation but the magnitude depends on the types and quantities. Nonionic guar gum yields much larger flocs than anionic xanthan gum, while both of them exhibit a nonlinear behavior in which the flocculation is the most pronounced at the intermediate OM loading. Moreover, the effect of guar gum is independent of salinity whereas the effect of xanthan gum is pronounced at higher salinity. Meanwhile, humic acid does not affect flocculation at all salinity values tested in this study. These results are echoed in the laboratory manipulation of the natural estuarine particles. Flocculation of the humic acid-rich Mississippi Sound particles is unaffected by the OM, whereas that of biopolymer-rich Atchafalaya Bay particles is enhanced by the OM.ConclusionsFlocculation is positively influenced by the presence of biopolymers that are produced as the result of marine primary production. Meanwhile, humic acid, which is abundant in the rivers that drain the agricultural soils of Southeastern United States, has little influence on flocculation. Thus, it is expected that humic acid-poor riverine particles (e.g., Mississippi River, and Atchafalaya River, to a lesser degree) may be prone to rapid flocculation and settling in the immediate vicinity of the river mouths when mixed with biopolymer-rich coastal waters. It is also expected that humic acid-rich riverine particles (e.g., Pearl River) may resist immediate flocculation and be transported further away from the river mouth.

CHARACTERIZATION OF MICROBIALLY Fe(III)-REDUCED NONTRONITE: ENVIRONMENTAL CELL-TRANSMISSION ELECTRON MICROSCOPY STUDY

Microstructural changes induced by the microbial reduction of Fe(III) in nontronite by Shewanella oneidensis were studied using environmental cell (EC)-transmission electron microscopy (TEM), conventional TEM, and X-ray powder diffraction (XRD). Direct observations of clays by EC-TEM in their hydrated state allowed for the first time an accurate and unambiguous TEM measurement of basal layer spacings and the contraction of layer spacing caused by microbial effects, most likely those of Fe(III) reduction. Non-reduced and Fe(III)-reduced nontronite, observed by EC-TEM, exhibited fringes with mean d001 spacings of 1.50 nm (standard deviation, σ = 0.08 nm) and 1.26 nm (σ = 0.10 nm), respectively. In comparison, the same samples embedded with Nanoplast resin, sectioned by microtome, and observed using conventional TEM, displayed layer spacings of 1.0–1.1 nm (non-reduced) and 1.0 nm (reduced). The results from Nanoplast-embedded samples are typical of conventional TEM studies, which have measured nearly identical layer spacings regardless of Fe oxidation state. Following Fe(III) reduction, both EC- and conventional TEM showed an increase in the order of nontronite selected area electron diffraction patterns while the images exhibited fewer wavy fringes and fewer layer terminations. An increase in stacking order in reduced nontronite was also suggested by XRD measurements. In particular, the ratio of the valley to peak intensity (v/p) of the 1.7 nm basal 001 peak of ethylene glycolated nontronite was measured at 0.65 (non-reduced) and 0.85 (microbially reduced).

THE EFFECT OF MICROBIAL FE(III) REDUCTION ON SMECTITE FLOCCULATION

This study was undertaken to investigate the changes in flocculation properties of Fe-rich smectite (nontronite, NAu-1) suspensions, including settling velocity, aggregate size and floc architecture associated with microbial Fe(III)-reduction in the smectite structure. The dissimilatory Fe-reducing bacterium Shewanella oneidensis MR-1 was incubated with lactate as the electron donor and structural Fe(III) as the sole electron acceptor for 3, 12, 24 and 48 h in an anaerobic chamber. Two controls were prepared; the first was identical to the experimental treatments except that heat-killed cells were used (non-reduced control), and the second control was the same as the first except that the incubation was carried out in an aerobic environment. The extent of Fe(III) reduction for the 48 h incubation was observed to reach up to 18%. Neither the non-reduced control nor the aerobically inoculated sample showed Fe(III) reduction. Compared with the non-reduced control, there was a 2.7 μm increase in mean aggregate size and a 30-fold increase in average settling velocity in the bioreduced smectite suspensions as measured using a Micromeritics Sedigraph®. The aerobically inoculated smectite showed a similar aggregate-size distribution to that of the non-reduced control. Significant changes in physical properties of smectite suspensions induced by microbial Fe(III) reduction were measured directly using transmission electron microscopy. The floc architecture of bioreduced smectite revealed less open structures compared to those of a non-reduced control. The aspect ratio (thickness/length) of individual smectite particle increased from 0.11 for the non-reduced control to 0.18 on average for the bioreduced smectite suspensions. The effects of pH on the clay flocculation were minimal in this study because the value of pH remained nearly constant at pH = 7.0–7.3 before and after the experiments. We therefore suggest that the increase in net negative charge caused by microbial Fe(III) reduction significantly promoted clay flocculation by increasing the electrochemical attraction in the smectite suspensions.

Energy-filtering transmission electron microscopy (EFTEM) and electron energy-loss spectroscopy (EELS) investigation of clay–organic matter aggregates in aquatic sediments

High resolution (< 10 nm) transmission electron microscopy (TEM), energy-filtering TEM (EFTEM), and electron energy loss spectroscopy (EELS) have been used for the direct microstructural imaging and analysis of clay-organic matter aggregates in fine-grained aquatic sediments from Jourdan River Estuary, MS, USA. EFTEM and EELS allow rapid, high-resolution spatial mapping and analysis of light elements such as carbon. The study area sediments are comprised of discrete organic matter masses and aggregates of clay plates. Clay aggregates often include organic matter. The comparison of clay aggregate images obtained by the TEM bright-field technique and the EFTEM carbon mapping technique shows that carbon within clay domains has spatial features that are in the same size scale as the features of individual clay plates within the aggregates (< ∼20 nm). These intimate spatial associations suggest that organic matter in clay aggregates is either closely attached to the surfaces of individual clay plates or structurally incorporated into clay crystals. Organic matter within clay aggregates does not appear to exist as discrete or massive masses that fill the pore spaces within the clay aggregates. These intimate associations, whether they are due to chemical interaction or physical sequestration, should affect the reactivity of organic matter during early diagenesis. The EELS spectra of clay aggregates show that organic matter always coexists with calcium, suggesting that Ca-containing smectite, rather than Ca-poor clay minerals such as kaolinite or illite, is preferentially associated with organic matter in the study area sediments. Further research is required to determine whether this is due to the sources and depositional history or physico-chemical properties of different clay minerals.

Endocrine disrupting chemicals in New Orleans surface waters and Mississippi Sound sediments

Endocrine disrupting compounds (EDCs), represented by steroid hormones, organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), and bisphenol A have been determined in four sediment cores from the Gulf of Mexico, from New Orleans surface water (Lake Pontchartrain and Mississippi River), and from the influent and effluent of a New Orleans municipal sewage treatment plant. During the five-month monitoring of selected EDCs in the Mississippi River (MR) and Lake Pontchartrain (LP) in 2008, 21 of 29 OCPs in MR and 17 of 29 OCPs in LP were detected; bisphenol A was detected in all of the samples. Steroid hormones (estrone, 17β-estradiol and 17α-ethinylestradiol) were detected occasionally. Total EDC (OCPs + PCBs + steroid hormones + bisphenol A) concentrations in the two surface water samples were found to vary from 148 to 1112 ng L(-1). Strong correlation of the distribution of total OCPs, total PCBs and total EDCs between solid and water phases was found in LP, while moderate or no correlation existed in MR. OCPs, PCBs, steroid hormones, and bisphenol A were all detected in the ocean sediments, and total EDCs were measured in the range of 77 to 1796 ng g(-1) dry sediment weight. The EDCs were also found in untreated and treated municipal sewage samples with a removal efficiency of 83% for OCPs but no removal efficiency for 17α-ethinylestradiol.