David Kadko

Oregon Subduction Zone: Venting, Fauna, and Carbonates

Transects of the submersible Alvin across rock outcrops in the Oregon subduction zone have furnished information on the structural and stratigraphic framework of this accretionary complex. Communities of clams and tube worms, and authigenic carbonate mineral precipitates, are associated with venting sites of cool fluids located on a fault-bend anticline at a water depth of 2036 meters. The distribution of animals and carbonates suggests up-dip migration of fluids from both shallow and deep sources along permeable strata or fault zones within these clastic deposits. Methane is enriched in the water column over one vent site, and carbonate minerals and animal tissues are highly enriched in carbon-12. The animals use methane as an energy and food source in symbiosis with microorganisms. Oxidized methane is also the carbon source for the authigenic carbonates that cement the sediments of the accretionary complex. The animal communities and carbonates observed in the Oregon subduction zone occur in strata as old as 2.0 million years and provide criteria for identifying other localities where modern and ancient accreted deposits have vented methane, hydrocarbons, and other nutrient-bearing fluids.

Degradation of Terrigenous Dissolved Organic Carbon in the Western Arctic Ocean

The largest flux of terrigenous organic carbon into the ocean occurs in dissolved form by way of rivers. The fate of this material is enigmatic; there are numerous reports of conservative behavior over continental shelves, but the only knowledge we have about removal is that it occurs on long unknown time scales in the deep ocean. To investigate the removal process, we evaluated terrigenous dissolved organic carbon concentration gradients in the Beaufort Gyre of the western Arctic Ocean, which allowed us to observe the carbons slow degradation. Using isotopic tracers of water-mass age, we determined that terrigenous dissolved organic carbon is mineralized with a half-life of 7.1 ± 3.0 years, thus allowing only 21 to 32% of it to be exported to the North Atlantic Ocean.

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.

Chemical reaction rates and entrainment within the Endeavour Ridge hydrothermal plume

The aging of the hydrothermal plume over the Endeavour segment of the Juan de Fuca Ridge was estimated by measuring the222Rn3He ratio in the plume as it dispersed. Despite uncertainties in the source function of hydrothermal input, it wa determined that the relative sequence of removal from the plume isH2 > Δc >222Rn>CH4 ≫Mn, whereΔc is a measure of particle concentration and the mean life of222Rn is 5.5 days. H2 is removed from the plume within hours of input while Mn is not removed within the two-week timescale of the radon-helium clock. Entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone. This is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient-like profile in the deep ocean. Thus we found that significant fractions of the Si and226Ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height.

Chlorophyll-a and pheopigments as tracers of labile organic carbon at the central equatorial Pacific seafloor

High rates of primary production and fast sinking rates lead to the deposition of phytodetritus and highly reactive organic matter to the sediments of the central equatorial Pacific. These substances are responsible for driving important chemical fluxes and fueling benthic organisms. Chloropigments have proven useful as tracers of similar highly reactive organic carbon components in lacustrine and nearshore marine sediments. In this study we investigate the degradation of chlorophyll-a and pheopigments-a at four abyssal sites on the JGOFS equatorial Pacific transect along 140°W and explore the usefulness of these chloropigments as tracers of the most-reactive component of the deep-sea particulate organic carbon (POC) rain. First-order reaction rate constants (k = 1–75 yr−1, half-life 3–250 d) and relative reactivities (chlorophyll-a > allomer and pheophorbide-a > pheophytin-a) derived from most of the sedimentary profiles are similar to those found in laboratory and other field (lake and coastal marine) studies. However, in some profiles, the rate constants determined by fitting data below 0.5 cm are slower by up to 3 orders of magnitude despite an apparent abundance of bacteria, macrofauna, and porewater oxygen. Model results assuming multi-G kinetics suggest that these chloropigments degrade as two components: one, which accounts for at least 99% of the degradation and 11–57% of the sediment inventory, degrades with a half-life of 4–120 days. The other component degrades with a half-life of up to 440 years. These results suggest that some otherwise labile POC may be protected and escape rapid degradation near the sediment-water interface. If phytodetritus is deposited continuously throughout the year, our model-calculated chlorophyll-a fluxes indicate that it could account for 25–100% of the annual POC flux at sites close to the equator.

Compositions, growth mechanisms, and temporal relations of hydrothermal sulfide-sulfate-silica chimneys at the northern Cleft segment, Juan de Fuca Ridge

Three active hydrothermal vents forming sulfide mounds and chimneys (Monolith, Fountain, and Pipe Organ) and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern Cleft segment, Juan de Fuca Ridge. The formation of zoned tubular Curich chimneys (type I) on the Monolith sulfide mound is related to focused flow of high-temperature (to 328°C) fluid. Bulbous chimneys (type II or “beehives”) at the Monolith and Fountain vents are products of diffuse high-temperature (to 315°C) discharge. A broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite-rich shells. Columnar Zn-sulfide-rich chimneys with narrow channelways (type III) are constructed where focused and relatively low-temperature (261°C) fluid vents directly from the basalt substrate. The bulk chemistry (low Cu; high Pb, Ag, and SiO2 contents), mineralogy (pyrite-marcasite-wurtzite-amorphous silica-anglesite), colloform and filamentous textures, and oxygen isotope characteristics of inactive (type IV) chimneys indicate a low-temperature (<250°C) origin involving diffuse and sluggish flow patterns and conductive cooling. Seafloor observations and 210Pb data indicate that (1) type IV chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet flow eruption, (2) the high-temperature Monolith and Fountain vents are manifestations of the same heating event (shallow emplacement of magma) that led to the sheet flow eruption and recent megaplumes, and (3) the Pipe Organ Vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption.

Radiochemical constraints on the crustal residence time of submarine hydrothermal fluids: Endeavour Ridge

The 210Pb/Pb and 228Ra/226Ra ratios measured in fluids and particles venting from the Endeavour Ridge are used to constrain the crustal residence time of the converting hydrothermal fluid from the initiation of basalt alteration where Mg+2 loss from seawatcr results in rapidly falling pH conditions, to termination at seafloor venting. The 210Pb/Pb ratios of hot, low Mg fluids are close to that of the basalts, suggesting a residence time of no greater than ten years. Particles associated with these vents have slightly higher ratios which may in part be due to scavenging of seawater 210Pb. The 228Ra/226Ra ratios of the fluids and an associated Ba-rich particle sample were also close to the basalt ratio, futher constraining the residence time to 3 years or less. These estimates indicate that the mass of fluid interacting with newly formed crust at any one time is less than 9 × 1013kg. if the axial heat flux is to be no greater than 30% of the total advective heat loss from the oceanic crust.

Insights into tide-related variability at seafloor hydrothermal vents from time-series temperature measurements

Abstract Thermocouple/thermistor array packages and an in situ gamma detector were deployed in 1994 at two vent sites on the northern Cleft Segment of the Juan de Fuca Ridge. Continuous records of fluid temperatures were obtained in four separate locations over a period of 5.5 months, and these data were supplemented by current meter observations made ∼2.5 km to the south within the axial valley. Temperatures measured at a location of focused high temperature flow showed that: (1) the maximum temperature in the chimney was stable and did not exhibit tide-related variability; (2) temperatures within the chimney wall were variable on time-scales of minutes, indicating rapid shifts in amounts of cold seawater or hot vent fluid flowing across chimney walls; and (3) the stable maximum temperature within the chimney conduit was ∼9°C less than the maximum fluid temperature recorded at the vent site during the same time interval, and thus stable high temperatures within chimneys are not necessarily indicative of the maximum temperature within the vent structure or the hydrothermal system. Time-series records from areas of diffuse flow indicate modulation of temperature and total radioactivity by tidally induced changes in bottom currents. Spectra of the current meter record and temperature records are similar, with spectral peaks observed at 12.4 h, 16–17 h (inertial peak) and 4–5 days. Phases between maxima in current, tide and temperature records are consistent with temperature changes resulting from periodic shifts in currents from north to south, and the subsequent northward or southward advection of warm fluids venting from multiple local sources. Periodic (12.4 h) variability of temperature was also recorded by a thermocouple buried ∼1 cm within one of the deposits and is likely a result of periodic variations in the temperature at the boundary of the highly conductive sulfide deposit. The time-series results presented demonstrate the need for measuring and considering the effects of local currents when investigating causes of temporal variability within seafloor hydrothermal systems.

Evidence of subduction within cold filaments of the northern California Coastal Transition Zone

Profiles of 222Rn, chlorophyll, nutrients, oxygen, temperature, and salinity taken within a cold filament observed in satellite images of the northern California coastal transition zone provide evidence that there is strong vertical transport out of the surface layer by subduction processes occuring within these features. Layers of chlorophyll found below the euphotic zone have associated with them other characteristics of water that at one time had been in contact with the sea surface, including deficiencies of 222Rn with respect to 226Ra, elevated oxygen concentrations, and warm water on plots of temperature and salinity. The data indicate that the subsurface maxima in chlorophyll are derived from subducted surface layers and are not produced by in situ phytoplankton growth nor derived by particle settling. The presence of a 222Rn deficiency at depth further suggests that the subducted water had been in contact with the surface within the past week and indicates that the rate of vertical transport is approximately 25 m/d. This has important implications for the fate of the filaments and for the high primary productivity within them.

An estimate of hydrothermal fluid residence times and vent chimney growth rates based on210Pb/Pb ratios and mineralogic studies of sulfides dredged from the Juan de Fuca Ridge

Abstract The210PbPb ratios across two sulfide samples dredged from the Juan de Fuca Ridge are used to estimate the growth rate of the sulfide material and the residence time of the hydrothermal fluid within the oceanic crust from the onset of basalt alteration.210Pb is added to the hydrothermal fluid by two processes: (1) high-temperature alteration of basalt and (2) if the residence time of the fluid is on the order of the 22.3-year half-life of210Pb, by in-situ growth from222Rn ( Krishnaswami and Turekian, 1982 ). Stable lead is derived only from the alteration of basalt. The210Pb/Pb ratio across one sample was ∼ 0.5 dpm/10−6 g Pb, and across the other it was ∼ 0.4 dpm/10−6 g Pb. These values are quite close to the238UP/b ratios of basalts from the area, suggesting that the residence time of the hydrothermal fluid from the onset of basalt alteration is appreciably less than the mean life of210Pb, i.e., the time required for ingrowth from the radon. An apparent growth rate of 1.2 cm/yr is derived from the slope of the210Pb/Pb curve for one of the samples. This is consistent with its mineralogy and texture which suggest an accretionary pattern of development. There is no obvious sequential growth pattern, and virtually no gradient in210Pb/Pb across the second sample. This is consistent with alteration of the original210Pb/Pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample.