Francis J. Sansone

A large source of atmospheric nitrous oxide from subtropical North Pacific surface waters

Nitrous oxide (N2O), a trace gas whose concentration is increasing in the atmosphere, plays an important role in both radiative forcing and stratospheric ozone depletion,. Its biogeochemical cycle has thus come under intense scrutiny in recent years. Despite these efforts, the global budget of N2O remains unresolved, and the nature and magnitude of the sources and sinks continue to be debated despite the constraints that can be provided by characterizations of the gas,. We report here the results of dual-isotope measurements of N2O from the water column of the subtropical North Pacific Ocean. Nitrous oxide within the lower-euphotic and upper-aphotic zones is depleted in both 15N and 18O relative to its tropospheric and deep-ocean composition. These findings are consistent with a prediction, based on global mass-balance considerations, of a near-surface isotopically depleted oceanic N2O source. Our results indicate that this source, probably produced by bacterial nitrification, contributes significantly to the ocean–atmosphere flux of N2O in the oligotrophic subtropical North Pacific Ocean. This source may act to buffer the isotopic composition of tropospheric N2O, and is quantitatively significant in the global tropospheric N2O budget. Because dissolved gases in near-surface waters are more readily exchanged with the atmospheric reservoir than those in deep waters, the existence of a quantitatively significant N2O source at a relatively shallow depth has potentially important implications for the susceptibility of the source, and the ocean–atmosphere flux, to climatic influences.

Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge

We have located warm springs on an isolated basement outcrop on 3.5 Ma crust on the eastern flank of the Juan de Fuca Ridge in the northeast Pacific Ocean. These are the first ridge-flank hydrothermal springs discovered on crust older than 1 Ma. The springs are venting altered seawater at 25.0 °C along a fault near the summit of Baby Bare outcrop, a high point along a ridge-axis-parallel basement ridge that is otherwise buried by turbidite sediment. Baby Bare is a small volcano that probably erupted off-axis ca. 1.7 Ma; it is thermally extinct, but acts as a high-permeability conduit for venting of basement fluids. The springs have been sampled from the manned submersible Alvin . Compared with the ambient ocean bottom water, they are heavily depleted in Mg, alkalinity, CO 2 , sulfate, K, Li, U, O 2 , nitrate, and phosphate, and enriched in Ca, chlorinity, ammonia, Fe, Mn, H 2 S, H 2 , CH 4 , 222 Rn, and 226 Ra. The springs appear to support a community of thysirid clams. Although we saw no obvious bacterial mats, the surficial sediments contain the highest biomass concentrations ever measured in the deep sea, based on their phospholipid phosphate content. Areal integration of Alvin heat-flow and pore-water velocity data yields flux estimates of 4–13 L/s and 2–3 MW for the total (diffuse and focused) hydrothermal output from Baby Bare, comparable to that from a black smoker vent on the ridge axis. Warm springs such as those on Baby Bare may be important for global geochemical fluxes.

Volatile fatty acid cycling in organic-rich marine sediments

Volatile fatty acid (VFA) apparent turnover rates were determined by measuring whole sediment VFA concentrations and the corresponding reaction rate constants. The following ranges of VFA concentrations were measured in Cape Lookout Bight, N.C. sediments (μmole·ls−1): acetate 54–660, propionate 1–24, butyrate <0.5–22, iso-butyrate <0.5–6. Apparent turnover rates measured over a one-year period ranged from 18–600 μmole·ls−1·h−1 for acetate and 0.7–7 μmole·ls−1·h−1 for the carboxyl carbon of propionate. Methane production was observed only with acetate and only in sulfatedepleted sediments; total acetate turnover attained approximately the same maximum value in both sulfate-reducing and sulfate-depleted sediments. Apparent turnover rates for acetate and propionate appeared to be controlled by similar factors: in sulfate-reducing (surface) sediments the turnover rates were stimulated by autumn storm-mediated deposition/resuspension events; in deeper sulfate-depleted sediments the turnover rates followed changes in the ambient temperature. Changes in VFA poolsizes were proportionally much larger than changes in corresponding rate constants. The ratio of CO2 to CH4 produced from acetate vs. depth suggested that non-methanogenic bacteria accounted for 60% of the acetate turnover in sulfate-depleted sediments. VFA concentrations were much lower in N.C. continental slope mud than in Cape Lookout sediments; acetate was the only VFA detectable throughout the top 40 cm of the slope sediments. The estimated production rate of CO2 from acetate decreased rapidly with depth. The surface rate was approximately 20 times less than that measured at similar temperatures in sulfate-reducing Cape Lookout sediments.

Carbon kinetic isotope effect accompanying microbial oxidation of methane in boreal forest soils

Atmospheric methane (CH4) oxidation occurs in soils at sites in the Bonanza Creek L.T.E.R. near Fairbanks, Alaska, USA, at rates ⩽2 mg CH4 m−2 d−1; the maximum CH4 oxidizing activity is located in loess at a depth of ∼15 cm. Methane, carbon dioxide, and stable isotope (δ 13C-CH4, δ 13C-CO2) depth distributions were measured at two sites: South facing Aspen (AS2) and North facing Black Spruce (BS2). The combined effects of diffusion and oxidation are similar at both sites and result in a CH4 concentration decrease (1.8–0.1 ppm) and a δ 13C-CH4 increase (−48% to −43%) from the soil surface to 60–80 cm depth. Isotope flux ratio and diffusion-consumption models were used to estimate the kinetic isotope effect (KIE); these results agree with the observed top-to-bottom difference in δ 13C-CH4, which is the integrated result of isotope fractionation due to diffusion and oxidation. The KIE for CH4 oxidation determined from these measurements is 1.022–1.025, which agrees with previous KIE determinations based on changes in headspace CH4 concentration and δ 13C-CH4 over time. A much lower soil respiration rate in the North facing Black Spruce soils is indicated by fivefold lower Soil CO2 concentrations. The similarity in CH4 oxidation at the two sites and the differences in inferred soil respiration at the two sites suggest that soil CH4 oxidation and soil respiration are independent processes. The soil organic matter responsible for the CO2 flux has a δ 13C estimated to be −27 to −28%.

Stoichiometric modeling of carbon diagenesis within a coral reef framework

Abstract Water sampled from the interior framework of Checker Reef, Oahu, Hawaii, indicates that the aerobic and anaerobic oxidation of organic matter dominates diagenesis within the reef framework. Reef interstitial water chemistry shows clear deviations from surface seawater: oxygen is depleted while dissolved inorganic carbon, H + , inorganic nutrients, sulfide and methane concentrations are elevated. Dissolved calcium is also elevated in most interstitial waters, indicating net dissolution of calcium carbonates. A mass-balance model used to determine the extent to which major biogeochemical reactions occur reveals that sulfate reduction is the predominant anaerobic process.

Methane stable isotopic ratios and concentrations as indicators of methane dynamics in estuaries

Mixing diagrams of methane (CH4) concentration and stable isotopic ratio (δ13C-CH4) were used to examine the fate of river-borne CH4 as it crosses a variety of estuaries: Columbia River (Oregon/Washington), Parker River (Massachusetts), Great Bay (New Hampshire), Kaneohe Bay (Hawaii), and Elkhorn Slough (California). Unlike the surface of the open ocean, these systems are not in near atmospheric equilibrium with respect to concentration or δ13C-CH4 value. The range of observed CH4 concentrations and δ13C-CH4 values were 33–440 nM and −36 to −58 per mil, respectively, for the freshwater end-members for these systems, 12–330 nM and −48 to −60 per mil for water at the mouths of the estuaries, and 1.6–6 nM and −45 to −60 per mil for the seawater end-members. In the Kaneohe Bay estuary, CH4 concentration and δ13C-CH4 displayed near-conservative behavior. In the Columbia River estuary, there was loss of riverine CH4 coupled with shifts to heavier isotopic values, apparently the result of in situ CH4 oxidation; this oxidation exhibited an apparent kinetic isotopic fractionation factor of 1.0042−1.012. In contrast, the other estuaries showed elevated concentrations and more negative δ13C-CH4 values apparently resulting from inputs of biogenic CH4 from midestuary marshes and sediments. The upper reaches of all these systems were well out of equilibrium with the atmosphere on a concentration basis, indicating that they are atmospheric CH4 sources. However, these first δ13C-CH4 measurements show that there is a wide range of isotopic variation in these waters, which indicates that it will be difficult to estimate the collective isotopic contribution of estuaries to the global methane budget.

Continuous sampling of hydrothermal fluids from Loihi Seamount after the 1996 event

For at least 9 years prior to July 1996, hydrothermal fluids flowed from Peles Vents on Loihi Seamount, Hawaii. In July–August 1996 a tectonic-volcanic event occurred that destroyed Peles Vents, creating a pit crater (Peles Pit) and several sites with hydrothermal venting. In October 1996 we deployed two new continuous water samplers (OsmoSamplers) at two of these hydrothermal sites and collected fluids using traditional sampling techniques to monitor the evolution of crustal and hydrothermal conditions after the event. The samplers were recovered in September 1997, and additional discrete vent fluid samples were collected. The OsmoSampler located along the south rift at Naha Vents captured a change in composition from a low-chlorinity, high-K fluid (relative to bottom seawater) to a high-chlorinity, low-K fluid. These changes are consistent with the fluid cooling during ascent and being derived from several different sources, which include high- (>330°C) and low- ( 330°C) into which magmatic volatiles were added. During the deployment, thermal and fluid fluxes decreased. At Naha the transport of heat and chemicals was decoupled. The chemical and thermal evolution of hydrothermal fluids after the event on Loihi is consistent with previous models based on events that have occurred along mid-ocean ridges. The event at Loihi clearly had an effect on the local hydrography; however, the integrated effect of chemical fluxes to global budgets from similar events is uncertain. Chemical fluxes from similar events may have a global impact, if ratios of chemical (e.g., CO2, Fe/Mn, Mg, sulfate, and K) to thermal anomalies greatly exceed, or are in the opposite direction to, fluxes from mid-ocean ridge hydrothermal systems.

Highly elevated methane in the eastern tropical North Pacific and associated isotopically enriched fluxes to the atmosphere

During the May - June, 2000 Eastern Pacific Redox Experiment (EPREX) we examined the dynamics of methane (CH4) in the eastern tropical North Pacific (ETNP), a large region of high surface-ocean productivity fueled by coastal upwelling. We discovered that (I) the ETNPcontains by far the largest pool of CH4 yet discovered in the open ocean; (2) CH4 production in the upper half of this subsurface pool is associated with the decomposition of locally produced sinking particulate matter; (3) the deeper half of this pool is from a coastal source; (4) advection and oxidation of the upper pool leads to the heavy CH4 isotopic values seen at midwater in the North Pacific subtropical gyre; and (5) the ETNP is a source of isotopically enriched CH4 to the atmosphere. Our results suggest that other oceanic areas of upwelling-induced anoxia may be sites of significant atmospheric input of isotopically heavy CH 4 •

Christmas Island lagoonal lakes, models for the deposition of carbonate{evaporite{organic laminated sediments

The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Pacific) lies at about 28N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km 2 ) is occupied by numerous lakes in which carbonate, evaporite (calcium sulfate, halite) and organic layers are deposited. Observations suggest that deposition of these different laminae is controlled by climatic and biologic factors. It is thought that periodic climatic variations, such as El Nino- Southern Oscillations (ENSO) events which bring heavy rainfall to the atoll, result in the succession of the precipitation of carbonate minerals (during periods after dilution of hypersaline waters by heavy rains), followed by evaporitic minerals (carbonate, calcium sulfate, halite) when salinity increases through evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mats develop continuously on the lake bottom surfaces providing the sediment with an important (total organic carbon 2-5%) organic contribution in the form of an internal, geometrically structured, network in which the authigenic minerals precipitate. The high bioproductivity of these microbial populations is reflected in low d 13 C values of sedimentary organic carbon (214 to 217‰), interpreted as being the result of high atmospheric CO2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment. The high ratio of deposited carbonate vs. sulfate 1 chloride, when compared to low ratio in evaporitic salinas, results from both a lack of limitation of calcium, magnesium and carbonate ions (in a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization). q 2001 Elsevier Science B.V. All rights reserved.

Manganese and methane in hydrothermal plumes along the East Pacific Rise, 8°40′ to 11°50′N

Abstract In November, 1991, we surveyed the water column for hydrothermal plumes along 350 km of the East Pacific Rise axis from 8°40′ to 11°50′N, using a combination of physical and chemical measurements. Our survey included the two major ridge segments north and south of the Clipperton Transform Fault at about 10°10′N, both limbs of the overlapping spreading centers (OSCs) at 9°03′N and 11°45′N, and a 30-km section of the next ridge segment to the south. We found vigorous plumes along most of this ridge axis, in keeping with its magmatically robust cross-section, axial summit caldera, and shallow, magma-related seismic reflector. These plumes were detectable by both physical (temperature and light attenuation) and chemical (dissolved Mn and CH4) measurements, although the chemical measurements were more sensitive. The least active sections were the southern third of the northern segment from 10°20 to 52′N and the OSCs, especially the OSC at 11°45′N. Plumes there had weak Mn and CH4 signals and were barely detectable by physical methods. These axial sections were the only ones surveyed that lie deeper than 2600 m and appear to be magma starved. The most active sections on the northern segment gave stronger signals for Mn and temperature than for CH4 and light attenuation, whereas the opposite was true on the southern segment, which was the site of a volcanic eruption at 9°45–52′N only seven months prior to our cruise. On the northern segment the four physical and chemical plume tracers correlated positively and linearly with one another, suggesting that the segment was fed by relatively uniform end-member fluids with a mean CH 4 Mn molar ratio of 0.075. The southernmost section surveyed, from 8°42′ to 9°08′N, closely resembled the northern segment. The rest of the southern segment fell into three sections with different CH 4 Mn ratios: 9°39 to 53′N with CH 4 Mn as high as 10, 9°08 to 39′N with CH 4 Mn of 0.51, and 9°53′ to 10°07′N with CH 4 Mn of 0.85. The section with the highest CH 4 Mn was the site of the volcanic eruption, which produced high-temperature, low-salinity, gas-rich vent fluids carrying abundant bacterial particles. The high CH4 concentrations are clearly associated with the volcanic eruption, but the origin of the CH4 is unclear.