Douglas E. Hammond

Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I. Hydrological provinces

Extensive deposits of methane hydrate characterize Hydrate Ridge in the Cascadia margin accretionary complex. The ridge has a northern peak at a depth of about 600 m, which is covered by extensive carbonate deposits, and an 800 m deep southern peak that is predominantly sediment covered. Samples collected with benthic instrumentation and from Alvin push cores reveal a complex hydrogeologic system where fluid and methane fluxes from the seafloor vary by several orders of magnitude at sites separated by distances of only a few meters. We identified three distinct active fluid regimes at Hydrate Ridge. The first province is represented by discrete sites of methane gas ebullition, where the bulk of the flow occurs through channels in which gas velocities reach 1 m s−1. At the northern summit of the ridge the gas discharge appears to be driven by pressure changes on a deep gas reservoir, and it is released episodically at a rate of ∼6×104 mol day−1 following tidal periodicity. Qualitative observations at the southern peak suggest that the gas discharge there is driven by more localized phenomena, possibly associated with destabilization of massive gas hydrate deposits at the seafloor. The second province is characterized by the presence of extensive bacterial mats that overlay sediments capped with methane hydrate crusts, both at the northern and southern summits. Here fluid typically flows out of the sediments at rates ranging from 30 to 100 cm yr−1. The third province is represented by sites colonized by vesicomyid clams, where bottom seawater flows into the sediments for at least some fraction of the time. Away from the active gas release sites, fluid flows calculated from pore water models are in agreement with estimates using published flowmeter data and numerical model calculations. Methane fluxes out of mat-covered sites range from 30 to 90 mmol m−2 day−1, whereas at clam sites the methane flux is less than 1 mmol m−2 day−1.

Geochemistry of barium in marine sediments : Implications for its use as a paleoproxy

Abstract Variations in the accumulation rate of barium in marine sediments are thought to be indicative of variations in marine biological productivity through time. However, the use of Ba as a proxy for paleoproductivity is partly dependent upon its being preserved in the sediment record in a predictable or consistent fashion. Arguments in favor of high Ba preservation are partly based on the assumption that sediment porewaters are generally at saturation with respect to pure barite. The idea is that because nondetrital sedimentary Ba predominantly exists as barite, porewater saturation would promote burial. We present sediment porewater, sediment solid phase, and benthic incubation chamber data suggesting that solid-phase Ba preservation may be compromised in some geochemical settings. We propose that under suboxic diagenetic conditions, characterized by low bottom water oxygen and high organic carbon respiration rates, Ba preservation may be reduced. Independent of the mechanism, if this assertion is true, then it becomes important to know when the Ba record is unreliable. We present evidence demonstrating that the sedimentary accumulation of authigenic U may serve as a proxy for when the Ba record is unreliable. We then provide an example from the Southern Ocean during the last glacial period where high authigenic U concentrations coincide with high Pa:Th ratios and high accumulation rates of biogenic opal, but we find low accumulation rates of sedimentary Ba. Thus, for the study sites presented here during the last glacial, we conclude that Ba is an unreliable productivity proxy.

Nutrient exchange across the sediment-water interface in the Potomac River estuary

Abstract The flux of ammonia, phosphate, silica and radon-222 from Potomac tidal river and estuary sediments is controlled by processes occurring at the sediment-water interface and within surficial sediment. Calculated diffusive fluxes range between 0·6 and 6·5 mmol m−2 day−1 for ammonia, 0·020 and 0·30 mmol m−2 day−1 for phosphate, and 1·3 and 3·8 mmol m−2 day−1 for silica. Measured in situ fluxes range between 1 and 21 mmol m−2 day−1 for ammonia, 0·1 and 2·0 mmol m−2 day−1 for phosphate, and 2 and 19 mmol m−2 day−1 for silica. The ratio of in situ fluxes to diffusive fluxes (flux enhancement) varied between 1·6 and 5·2 in the tidal river, between 2·0 and 20 in the transition zone, and from 1·3 to 5·1 in the lower estuary. The large flux enhancements from transition zone sediments are attributed to macrofaunal irrigation. Nutrient flux enhancements are correlated with radon flux enhancements, suggesting that fluxes may originate from a common region and that nutrients are regenerated within the upper 10–20 cm of the sediment column. The low fluxes of phosphate from tidal viver sediments reflect the control benthic sediment exerts on phosphorus through sorption by sedimentary iron oxyhydroxides. In the tidal river, benthic fluxes of ammonia and phosphate equal one-half and one-third of the nutrient input of the Blue Plains sewage treatment plant. In the tidal Potomac River, benthic sediment regeneration supplies a significant fraction of the nutrients utilized by primary producers in the water column during the summer months.

Benthic fluxes in San Francisco Bay

Measurements of benthic fluxes have been made on four occasions between February 1980 and February 1981 at a channel station and a shoal station in South San Francisco Bay, using in situ flux chambers. On each occasion replicate measurements of easily measured substances such as radon, oxygen, ammonia, and silica showed a variability (±1α) of 30% or more over distances of a few meters to tens of meters, presumably due to spatial heterogeneity in the benthic community. Fluxes of radon were greater at the shoal station than at the channel station because of greater macrofaunal irrigation at the former, but showed little seasonal variability at either station. At both stations fluxes of oxygen, carbon dioxide, ammonia, and silica were largest following the spring bloom. Fluxes measured during different seasons ranged over factors of 2–3, 3, 4–5, and 3–10 (respectively), due to variations in phytoplankton productivity and temperature. Fluxes of oxygen and carbon dioxide were greater at the shoal station than at the channel station because the net phytoplankton productivity is greater there and the organic matter produced must be rapidly incorporated in the sediment column. Fluxes of silica were greater at the shoal station, probably because of the greater irrigation rates there. N + N (nitrate + nitrite) fluxes were variable in magnitude and in sign. Phosphate fluxes were too small to measure accurately. Alkalinity fluxes were similar at the two stations and are attributed primarily to carbonate dissolution at the shoal station and to sulfate reduction at the channel station. The estimated average fluxes into South Bay, based on results from these two stations over the course of a year, are (in mmol m−2 d−1): O2 = −27 ± 6; TCO2 = 23 ± 6; Alkalinity = 9 ± 2; N + N = −0.3 ± 0.5; NH3 = 1.4 ± 0.2; PO4 = 0.1 ± 0.4; Si = 5.6 ± 1.1. These fluxes are comparable in magnitude to those in other temperate estuaries with similar productivity, although the seasonal variability is smaller, probably because the annual temperature range in San Francisco Bay is smaller.Budgets constructed for South San Francisco Bay show that large fractions of the net annual productivity of carbon (about 90%) and silica (about 65%) are recycled by the benthos. Substantial rates of simultaneous nitrification and denitrification must occur in shoal areas, apparently resulting in conversion to N2 of 55% of the particulate nitrogen reaching the sediments. In shoal areas, benthic fluxes can replace the water column standing stocks of ammonia in 2–6 days and silica in 17–34 days, indicating the importance of benthic fluxes in the maintenance of productivity.Pore water profiles of nutrients and Rn-222 show that macrofaunal irrigation is extremely important in transport of silica, ammonia, and alkalinity. Calculations of benthic fluxes from these profiles are less accurate, but yield results consistent with chamber measurements and indicate that most of the NH3, SiO2, and alkalinity fluxes are sustained by reactions occurring throughout the upper 20–40 cm of the sediment column. In contrast, O2, CO2, and N + N fluxes must be dominated by reactions occurring within the upper one cm of the sediment-water interface. While most data support the statements made above, a few flux measurements are contradictory and demonstrate the complexity of benthic exchange.

Benthic chamber and profiling landers in oceanography - A review of design, technical solutions and functioning

We review and evaluate the design and operation of twenty-seven known autonomous benthic chamber and profiling lander instruments. We have made a detailed comparison of the different existing lander designs and discuss the relative strengths and weaknesses of each. Every aspect of a lander deployment, from preparation and launch to recovery and sample treatment is presented and compared. It is our intention that this publication will make it easier for future lander builders to choose a design suitable for their needs and to avoid unnecessary mistakes.

Early diagenesis of biogenic opal: Dissolution rates, kinetics, and paleoceanographic implications

Abstract A study was undertaken to measure the rate of biogenic opal dissolution in equatorial Pacific sediments along the equator between 103 and 140°W, and across the equator between 12°S and 9°N. Along the equator, benthic incubation chamber measurements indicate a gradient in the opal dissolution rate, with rates decreasing from ∼0.7 mmol m−2 day−1 at 103°W to 0.4 mol m−2 day−1 at 140°W. Across the equator at 140°W, the pattern of opal dissolution is symmetrical, with dissolution rates of ∼0.4 mmol m−2 day−1 from 2°S to 2°N, decreasing to ∼0.1 mmol m−2 day−1 at the ends of the transect. Benthic fluxes calculated from pore water profiles of silicic acid are in good agreement with incubation chamber measurements. Each pore water profile fits with a function that exponentially approaches a constant value with depth (Cd), and Cd co-varies with the dissolution flux. At least three previously published models can explain this relationship: one in which Cd is regulated by the solubility of the opal present in the sediments; a second in which Cd depends on the availability of easily dissolvable opal; and the sediment mixing rate and a third rate in which Cd is controlled by the development of surface coatings. If the first model is correct, the data demonstrate that opal solubility varies spatially and that solubility is positively correlated with the opal rain rate, although the rate at which pore waters become saturated varies little among the stations between 5°N and 5°S. The implication of this model is that the opal burial rate depends on dissolution kinetics and sediment accumulation rate. If the second model is correct, fits to the pore water data and knowing the sediment mixing rate indicate that at least three types of solid phase opal must be present in the equatorial Pacific region, one that is essentially unreactive, one that has a dissolution rate constant between 0.27 ± 0.09 and 0.05 ± 0.02 year−1 , and another that has a dissolution rate constant of 6 ± 4 × 10−4 year−1. The more reactive phase dominates the dissolution flux between 5°S and 5°N, whereas the less reactive phase dominates the flux at the high latitude extremes of the transect. The implication of this second model is that sedimentary opal in equatorial Pacific sediments provides a record of only the non-reactive opal supply. If the third model is correct, surface coating development and opal preservation may depend upon the kinetics of the opal surface aging process or on the concentration of the coating material within the sediments. Storage experiments suggest that this third model may be the most realistic, but the implications of this model cannot be explored until the factors regulating coating growth are identified.

Biogenic budgets of particle rain, benthic remineralization and sediment accumulation in the equatorial Pacific

Abstract Budgets of organic C (Corg), CaC03 and opal have been constructed for the Palisades, NY Pacific equatorial region at 140°W between 5°N and 5°S. Measurements of the rain and benthic remineralization rate of biogenic materials have been adjusted and normalized to account for sampling biases. Sea surface temperature serves as a master variable in normalizing sediment trap and benthic remineralization data to average conditions. The rain and remineralization rates for Corg are nearly equal: 0.40±0.05 and 0.46±0.06 mmol m−2 d−1 respectively; thus only a minor fraction of this constituent is buried. Rain and dissolution rates for biogenic opal are similarly balanced (0.3±0.06 and 0.36±0.01 mmol m−2 d−1) and consistent with the value for opal burial (0.0±0.004). The CaC03 budget appears to have changed during the Holocene. The best estimates of modern CaC03 dissolution (0.58±0.03 mmol m−2 d−1) and rain rate (0.61±0.06) are consistent with230Th-normalized carbonate accumulation rates for the late Holocene (0.1 mmol m−2 d−1). However, the balance between dissolution and rain is not consistent with early Holocene carbonate accumulation (0.3 mmol m−2 d−1 ), and this imbalance suggests: 1) a recent increase in the rate of CaC03 dissolution on the sea floor, or 2) a decrease in the rain rate of carbonate particles. Modeling230Th profiles in sediments from this region define the last 3000 years as the duration of increased dissolution or decreased particle rain. 231Pa/230Th ratios in sediments indicate that particle rain rates have remained constant or possibly increased slightly through the Holocene. Two potential causes for increased dissolution were investigated; a change in deep water carbonate saturation or a change in Co,g/CaC03 rain ratios. A model describing carbonate dissolution as a function of the degree of undersaturation and the amount of organic carbon oxidation within sediments indicates that the recent increase in dissolution is more likely due to changes in bottom water chemical composition. We propose that Pacific Ocean bottom water carbonate ion concentration has decreased by 10–15 μM over the last 3000 years.

Barium cycling in the North Pacific: Implications for the utility of Ba as a paleoproductivity and paleoalkalinity proxy

Benthic incubation chambers have been deployed in a variety of geochemical environments that provide a comprehensive geochemical framework from which to address issues related to Ba geochemistry and the use of Ba as a paleoproxy. First order budgets for barium show that in the equatorial Pacific, present rates of Ba rain and benthic remobilization are nearly in balance, indicating that the rate of net accumulation is negligible and is clearly much less than the average for the Holocene; thus any paleoproxy algorithms built on the assumption of steady state are questionable. In contrast, budgets for sediments in the southern California Borderland indicate much higher burial efficiencies, in the range of 50–80%. The Ba:alkalinity (Alk) flux ratio is found to be remarkably constant throughout the environments studied and is indistinguishable from the deep water ratio used for paleoceanographic reconstructions. However, the Ba:organic carbon remobilization ratio is not constant. Combined, these results do not indicate a simple, first-order direct link between Ba and alkalinity remobilization via organic carbon oxidation; however, the similarities in the Ba and alkalinity source functions conspire to maintain the Ba:Alk ratio near the global water column average. This latter observation provides promise for the use of the Ba:Ca ratio in benthic foraminifera as a paleocirculation tracer.

Remobilization of barium in continental margin sediments

The rate of Ba release from California continental margin sediments has been measured, using an in situ benthic flux chamber, and the range of values (25–50 nmol cm−2 y−1) is larger than any previously published benthic flux estimate for this element. The magnitude of the Ba flux suggests that a significant fraction of the Ba raining from the euphotic zone is recycled at the seafloor. Ba:Si regeneration ratios from these margin sediments increase with depth, demonstrating that Ba is decoupled from Si during the earliest stages of diagenesis. On the other hand, Ba regeneration rates and CaCO3 dissolution rates covary; the coupling between these two constituents is supported by the observation that the Ba: CaCO3 dissolution flux ratio (1.7 ± 0.4 × 10−3) is independent of bottomwater depth—even in sediments underlying the oxygen minimum zone along the continental margin. Furthermore, this flux ratio is consistent with both the water column Ba:alkalinity ratio for the worlds ocean, as well as the Ba:CaCO3 ratio in sediment-trap solid phases from the Equatorial Pacific (1.1–2.2 × 10−3). However, the constancy of the Ba:alkalinity ratio over geologic time remains in question, because the mechanism that controls this relationship remains a mystery. Our flux measurements suggest that diagenesis does not significantly influence the Ba:Ca ratio in the upper 0.5 mm of Pacific sediments, thereby supporting the idea of using the Ba concentration in surface-dwelling benthic forams as a proxy for deep-water chemical conditions (Lea and Boyle, 1989, 1990). On the other hand, we predict that if a foraminifer lives 0.5 mm or more below this interface, then diagenetic effects could influence the Ba:Ca ratio that foram species would record. The carrier phase of the particulate Ba reactive during early diagenesis does not appear to be organic matter, oxyhydroxides, or calcium carbonate, but rather a mineral phase related to marine barite or perhaps celestite.

Dissolution kinetics of calcium carbonate in equatorial Pacific sediments

Benthic chambers were deployed in the equatorial eastern Pacific Ocean on a transect along the equator between 103°W and 140°W and on a transect across the equator at 140°W in order to establish the rate of calcium carbonate dissolution on the seafloor. Dissolution was determined from the rate of alkalinity increase within an incubation chamber, measured over an 80–120 hour incubation period. Dissolution rates were lowest at eastern Pacific sites (0.2-0.4 mmol CaCO3/m2/d) and highest at the equatorial, 140°W sites (0.5-0.7 mmol/m2/d). Both oxygen consumption rates and the degree of bottom water saturation govern dissolution rates. Measured dissolution and oxygen consumption rates are used with a numerical model to constrain the value of the dissolution rate constant k, formulated according to the equation developed by Keir [1980]: dissolution rate = kγ(1-Ω)n. The observed dissolution fluxes are predicted by the model when k = 5 to 100%/d and n = 4.5. This range of k values has important implications regarding the type of carbonate dissolving and its location within the sediment column. At low values of k, organic carbon rain rates to the seafloor become the dominant driving force of carbonate dissolution. At higher values of k, the degree of bottom water undersaturation becomes more important. Dissolution of carbonate within equatorial Pacific sediments can be adequately described with k = 20 ± 10%/d, a rate constant much lower than some previously used values. Dissolution rates do not vary significantly over chamber boundary layer thicknesses between 200 and 800 μm, indicating that dissolution is not controlled by hydrodynamic conditions. Chambers acidified with HCl yield very large dissolution rates, but for a given degree of acidification the dissolution rate was constant for sites ranging from water depths of 3300–4400 m. This implies that there are not more and less easily dissolved forms of CaCO3 arriving on the seafloor between these depths. A budget for alkalinity in the deep Pacific, predicted by the dissolution model and based on the assumption that all carbonate dissolution takes place in the sediments, is within 85% of the input required by a published box model alkalinity budget based on the distribution of nutrients and the water mass transport rates.