Boudreau, Bernard P.
Permanent URI for this collectionhttps://hdl.handle.net/10222/22022
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Item Open Access Black Sea Salt(2021) Boudreau, Bernard P.Item Open Access Black Sea Flow Code(2021) Boudreau, Bernard P.Item Open Access Current CaCO3 dissolution at the seafloor caused by anthropogenic CO2(PNAS, 2018-10-29) Boudreau, Bernard P.Oceanic uptake of anthropogenic CO2 leads to decreased pH, carbonate ion concentration, and saturation state with respect to CaCO3 minerals, causing increased dissolution of these minerals at the deep seafloor. This additional dissolution will figure prominently in the neutralization of man-made CO2. However, there has been no concerted assessment of the current extent of anthropogenic CaCO3 dissolution at the deep seafloor. Here, recent databases of bottom-water chemistry, benthic currents, and CaCO3 content of deep-sea sediments are combined with a rate model to derive the global distribution of benthic calcite dissolution rates and obtain primary confirmation of an anthropogenic component. By comparing preindustrial with present-day rates, we determine that significant anthropogenic dissolution now occurs in the western North Atlantic, amounting to 40–100% of the total seafloor dissolution at its most intense locations. At these locations, the calcite compensation depth has risen ∼300 m. Increased benthic dissolution was also revealed at various hot spots in the southern extent of the Atlantic, Indian, and Pacific Oceans. Our findings place constraints on future predictions of ocean acidification, are consequential to the fate of benthic calcifiers, and indicate that a by-product of human activities is currently altering the geological record of the deep sea.Item Open Access Quantifying particle dispersal in aquatic sediments at short time scales: model selection(2008) Meysman, Filip J. R.; Malyuga, Volodymyr S.; Boudreau, Bernard P.; Middelburg, Jack J.In a pulse-tracer experiment, a layer of tracer particles is added to the sediment-water interface, and the down-mixing of these particles is followed over a short time scale. Here, we compared different models (biodiffusion, telegraph, CTRW) to analyse the resulting tracer depth profiles. The biodiffusion model is widely applied, but entails 2 problems: (1) infinite propagation speed-the infinitely fast propagation of tracer to depth, and (2) infinitely short waiting times-mixing events follow each other infinitely fast. We show that the problem of waiting times is far more relevant to tracer studies than the problem of propagation speed. The key issue in pulse-tracer experiments is that models should explicitly account for a finite waiting time between mixing events. The telegraph equation has a finite propagation speed, but it still assumes infinitely short waiting times, and, hence, it does not form a suitable alternative to the biodiffusion model. Therefore, we advance the continuous-time random walk (CTRW), which explicitly accounts for finite waiting times between mixing events, as a suitable description of bioturbation. CTRW models are able to cope with lateral spatial heterogeneity in reworking, which is a crucial feature of bioturbation at short time scales. We show how existing bioturbation models (biodiffusion model, telegraph equation, non-local exchange model) can be considered as special cases of the CTRW model. Accordingly, the CTRW model is not a new bioturbation model, but a generalization of existing models.Item Open Access Bioturbation and porosity gradients(1998-01) Mulsow, S.; Boudreau, Bernard P.; Smith, JNUbiquitous porosity gradients have a potentially important effect on the mixing of particle-bound tracers, such as Pb-210. Mass-depth coordinates cannot be used to deal with these effects if values of the traditional mixing coefficient, D-B, are required. This paper compares and evaluates three different means of dealing directly with porosity gradients while modeling bioturbation, i.e. mean constant porosity, interphase mixing (porosity mixed), and intraphase mixing (porosity not mixed). We apply these models to 11 different Pb-210 profiles collected at various depths and times on the eastern Canadian Margin. A statistical analysis of the resulting best fits shows that these models produce equivalent mixing coefficient values for 55% of the profiles. For the remaining 45% of the profiles, the interphase mixing model predicts the existence of well-mixed near-surface zones on the time scale of Pb-210 decay, a phenomenon not predicted by the other models. Unfortunately, our tracer dataset by itself cannot be used to establish which mixing mode is actually operative at each station.Item Open Access What controls the mixed-layer depth in deep-sea sediments? The importance of particulate organic carbon flux(2004-03) Boudreau, Bernard P.No abstract available.Item Open Access Rate of growth of isolated bubbles in sediments with a diagenetic source of methane(American Society of Limnology and Oceanography, Ann Arbor, MI, 2001-05) Boudreau, Bernard P.; Gardiner, Bruce S.; Johnson, Bruce D.Observation of bubbles in estuarine and coastal sediments indicates that bubbles at or below 10 cm depth grow on seasonal time scales (May-October). In order to determine the controls on this growth rate, we have constructed a diffusion-reaction model that accounts for the dynamics of methane formation, its diffusion through pore waters, its incorporation into a bubble, and the consequent growth of the bubble. The model produces an explicit equation for the radius of a growing bubble, R(t), with time using mean parameter values and under the assumption that the mechanics of the sediment response to growth can be neglected: R(t) = [phi D/2c (sub g) {SR (super 2) (sub 1) /3D + (c (sub 1) -c (sub 0) )}t + R (super 2) (sub 0) ] (super 1/2) where phi is the porosity, D is the tortuosity-corrected diffusivity, C (sub g) is the concentration of gas in the bubble, S is the rate of methanogenesis near the bubble, R (sub 1) is the half-separation distance between bubbles (R (sub 1) >>R), c (sub 1) is the ambient CH (sub 4) concentration, c (sub 0) is the pore-water CH (sub 4) concentration at R, t is time, and R (sub 0) is the initial bubble radius, if not zero. The effects of the source S and supersaturation (c (sub 1) -c (sub 0) ), thus, appear as separate contributing terms, and this formula can then be applied even in those cases where apparently c (sub 1) nearly equal c (sub 0) . The model is applied to three sediments where bubbles have been previously studied, i.e., Cape Lookout Bight (USA), White Oak River (USA) and Eckernforde Bay (Germany). In all three cases, using the site-specific time-averaged parameter values, the model predicts seasonal growth rates, consistent with the observations. Furthermore, the source term dominates the rate of growth at the first of these two sites, whereas diffusion from the ambient supersaturation dominates at the German location. Real bubbles may follow a more complicated growth history than predicted by the above equation because of the mechanical properties of sediments; nevertheless, the overall growth times are concordant with ultimate diffusion control. The effects of rectified diffusion, that is, the pumping of gas into a bubble by pressure oscillations, e.g., from waves and tides, were also examined. Existing models for that process suggest that it is negligible, due to the low frequency of these types of oscillations.Item Open Access Early diagenesis in a marine sapropel, Mangrove Lake, Bermuda(American Society of Limnology and Oceanography, Ann Arbor, MI, 1992-12) Boudreau, Bernard P.; Canfield, Donald E.; Mucci, AlfonsoNo abstract available.Item Open Access Mean mixed depth of sediments; the wherefore and the why(American Society of Limnology and Oceanography, Ann Arbor, MI, 1998-05) Boudreau, Bernard P.No abstract available.