Geomicrobial transformation of manganese in Gorda Ridge event plumes

Event plumes form as episodic discharges of large volumes of hydrothermal solutions in response to magmatic diking/eruptive events. In consequence, event plumes represent the sudden injection of exploitable reduced chemical substrates, as well as inhibitory constituents, and likely induce successional changes in the microbial community structure and activity within event plume waters. In response to a major seismic event detected beginning 28 February 1996 at the northern Gorda Ridge, a series of three rapid response and follow-up cruises (GREAT 1, 2 and 3) were mounted over a period of three months. This report focuses on time-series measurements of manganese geomicrobial parameters in the two event plumes found in association with this seismic event.Scanning transmission electron microscopy, elemental microanalysis, and radioisotope (⁵⁴Mn) uptake experiments were employed on samples collected from vertical and tow-yo casts from the three cruises. Numbers of bacteria and ratios of metal precipitating capsuled bacteria to total bacteria were greatest in the youngest (days old) plume, EP96A, found during GREAT 1; however, when normalized to the hydrothermal temperature anomaly, the greatest values were found in a second event plume, EP96B, discovered during GREAT 2 (up to 1 month old). Early capsule bacteria and particulate Mn distributions may have been influenced by entrainment of resuspended sediment, while those of the oldest (2–3 months) plume sample may have been subjected to preferential aggregation and particle settling.     

Manganese and iron in hydrothermal plumes resulting from the 1996 Gorda Ridge Event

We sampled hydrothermal plumes over the N. Gorda Ridge four times between March and August 1996 to document Mn and Fe discharge resulting from a magmatic intrusion/seafloor eruption. Two separate event plumes, EP96A and B, and chronic hydrothermal emissions lasting 6 months were characterized. Shipboard time-series measurements of an event plume sample were used to calibrate an Fe phase clock useful for estimating sample age up to 6 days after fluid discharge. Samples collected from EP96A and B had Mn/heat (<0.15 nmol J^-1) and Fe/Mn (>2 mol mol^-1) ratios similar to historical event plume observations. We suggest these “signature” ratio values are generally characteristic of event plumes and hypothesize that Mn and Fe may be supplied to event plumes by different processes: Mn by entrainment of fluids from an extant shallow subseafloor reservoir, and Fe by short-lived, high-temperature water-rock reaction coincident with dike emplacement. Calculations based on the Fe phase clock indicate that the two event plumes were released more than a month apart. The largest event plume, EP96A (2.3×10⁶ M Mn and 13×10⁶ M Fe), formed 7 March soon after seismic activity began. The smaller EP96B (0.49×10⁶ M Mn and 3.5×10⁶ M Fe) was not discharged until 11 April, 3 weeks after the cessation of seismic activity detectable by SOSUS T-phase monitoring. We hypothesize that the subseafloor disturbance that triggered EP96B also resulted in the episodic flushing of a reservoir of chronic-plume-like fluids. Total event plume inventories of Mn and Fe at N. Gorda Ridge are much smaller than those associated with the 1986 event at N. Cleft segment of the Juan de Fuca Ridge, but comparable to event plume inventories at N. Cleft segment in 1987 and CoAxial segment in 1993. Mn/heat values for chronic plumes over the eruption site underlying EP96A evolved from moderate (0.25 oonmol J^-1, reflecting probable admixture with event plume formation fluids) to high (0.7 nmol J^-1, typical of chronic plumes) to low (0.1 nmol J^-1, similar to diffuse vent fluid values), marking a complete episode of intrusion/eruption-induced hydrothermal discharge.     

Bacterial and viral abundances in hydrothermal event plumes over northern Gorda Ridge

This study presents first-time observations of bacterial and viral abundances in hydrothermal event plumes. Two water-column event plumes were formed in conjunction with seismic events and seafloor volcanic eruptions on the northern Gorda Ridge in February–March 1996. Epifluorescence counts of bacteria and viruses were performed on water samples from 3 successive cruises staged in the 10–90 days that followed the onset of seismicity. Relative to background seawater at these 1800–3200 m depths, bacterial abundance was enhanced by 2–3 fold within both event plumes. In contrast, viral numbers were below background seawater values in the younger and more intense of the two event plumes (EP96A), and enhanced in the other (EP96B). Changes in viral abundance may be a secondary response to that of plume bacteria as well as being influenced by particle formation and precipitation within the plumes. Lower bacteria/heat, virus/heat and virus/bacteria ratios in EP96A versus EP96B confirm distinct differences in the microbial response to event plume formation, possibly related to observed differences in plume chemistry.     

On the calculation of total heat, salt and tracer fluxes from ocean hydrothermal events

Total heat, salt, and other tracer masses released during a hydrothermal event are shown to be proportional to, but not necessarily equal to, volume integrals of resulting water column anomalies. Proportionality coefficients depend on anomaly definition, on background hydrographic and tracer profiles, on expansion coefficients of the equation of state at an appropriate pressure, and on tracer to heat anomaly ratios at the venting source. For Gorda Ridge event plumes, which are described in other papers of this issue, volume integrals of conventionally defined heat anomalies underestimate actual released heat by a factor of 2.4 if the discharge is not anomalously saline. Under certain combinations of hydrographic and source anomaly conditions, not unlike those found on the East Pacific Rise at 10°N, the apparent total heat released during an event can be deceivingly zero. This analysis also establishes a linear relationship between the ratio of tracer anomaly to heat anomaly at any point in the plume to the same ratio at the source. One consequence is that the ratio of anomalous ³He to heat in Gorda Ridge event plumes is approximately 2.4 larger in the water column than it is at the source. Results are independent of the entrainment process involved in event plume formation, and they are shown to hold true even for background hydrographic profiles that do not vary linearly with depth.     

Metal distributions and their fluxes at the coastal boundary of a semi-enclosed ria

Water column samples have been collected in the outer channel of the Ferrol Ria (NW Spain) during four occasions over a tidal cycle. The objective was to study the exchange of dissolved and particulate Cd, Cu, Pb and Zn and particulate Al, Fe and Si between the ria and the adjacent coastal waters. This study provides the first extensive dataset on dissolved and particulate metal concentrations in the water column of a Galician ria. Typical concentrations of dissolved Cd (96 ± 31 pM), Cu (8 ± 4 nM), Pb (270 ± 170 pM) and Zn (21 ± 10 nM) were similar than in other European Atlantic shelf and coastal waters. The fraction of metals in the particulate phase followed the trend: Pb > Cu Zn > Cd. The outgoing water from the ria was enriched in dissolved and particulate Cu, Pb and Zn compared with incoming waters, whereas Cd concentrations were similar for both waters. The suspended particulate matter was composed of a mixture of marine and continental material. The latter end-member was found to arise from the metal-rich ria bed sediments, which is diluted by the dominant metal-poor marine end-member. The net output flux of Cu from the channel is balanced by the freshwater inputs to the ria, and the net Zn flux gave a positive output to coastal waters. For Pb, the net flux to the coastal waters is less than that input from the rivers, as a result of its particle reactivity and deposition in sediments. On the contrary, a net input flux of dissolved Cd from coastal waters was observed, highlighting the oceanic source of this metal in the Galician rias. Results from the budget calculations are in agreement with the differential geochemical behavior of these elements in coastal waters.     

Thermophilic subseafloor microorganisms from the 1996 North Gorda Ridge eruption

High-temperature microbes were present in two hydrothermal event plumes (EP96A and B) resulting from the February–March 1996 eruptions along the North Gorda Ridge. Anaerobic thermophiles were cultured from 17 of 22 plume samples at levels exceeding 200 organisms per liter; no thermophiles were cultured from any of 12 samples of background seawater. As these microorganisms grow at temperatures of 50–90°C, they could not have grown in the event plume and instead most probably derived from a subseafloor environment tapped by the event plume source fluids. Event plumes are thought to derive from a pre-existing subseafloor fluid reservoir, which implies that these thermophiles are members of a native subseafloor community that was present before the eruptive event. Thermophiles also were cultured from continuous chronic-style hydrothermal plumes in April 1996; these plumes may have formed from cooling lava piles. To better understand the nutritional, chemical, and physical constraints of pre-eruptive crustal environments, seven coccoidal isolates from the two event plumes were partially characterized. Results from nutritional and phylogenetic studies indicate that these thermophiles are heterotrophic archaea that represent new species, and probably a new genus, within the Thermococcales.     

Heavy metals in sinking particles and bottom sediments from the eastern Turkish coast of the Black Sea

Concentrations of Cd, Cu, Cr, Co, Ni, Zn, Fe, Mn, Pb, As, and Sb were determined in sediment trap and bottom sediment samples collected seasonally from a station on the eastern Turkish coast of the Black Sea. Cd, Pb and Mn concentrations were highest in the sediment trap samples except during the summer period, whereas Co, Ni, Zn and Fe levels were much lower than corresponding levels found in the surface sediments. Cu, Cr, As and Sb levels showed no definite trend with sediment type. In general, with the exception of Cr, relatively lower metal concentrations in the sediment trap material were determined in the summer period. The highest mass flux, 56.5 g m⁻² day⁻¹, was measured during autumn. The highest flux of heavy metals also occurred during autumn and was strongly dependent on particle mass flux. Based on these results, we suggest that the downward vertical transport of particulate heavy metals in this region is related to the high degree of land erosion and the resultant particulate flux dynamics, which occur here. It was noteworthy that the highest concentrations of Cd, Cu, Co, Zn, Fe and Sb in particles were measured during winter a finding which suggests that enhanced fossil fuel combustion, which occurs during this period in adjacent urban and industrial areas plays an important role in the metal composition of sinking particles in nearshore waters.     

Hydroacoustic detection of volcanic activity on the Gorda Ridge, February–March 1996

Beginning at 0700 GMT on 28 February 1996, intense seismicity was detected in the northeast Pacific Ocean using the T-phase Monitoring System developed by NOAA/PMEL to access the U.S. Navy’s SOund SUrveillance System (SOSUS) in the North Pacific. The event was preliminarily located on the northernmost segment of the Gorda Ridge near 42.67°N and 126.8°W, in the vicinity of the ridge segment high (“narrowgate”). The nature of the seismicity was similar to that observed in June 1993 at the CoAxial segment of the Juan de Fuca Ridge, which was later documented to be a lateral magma injection with subsequent eruption. Due to several gaps in the data, the detection information was not as comprehensive as during the CoAxial event, but an initial migration of epicenters from the narrowgate area down rift is inferred based on arrival bearings from a single array; there is evidence for an additional diking event on the second and third day of activity. There is also indication of a concentration of epicenters located near 42.6°N, as occurred during the CoAxial episode at what was later determined to be an eruption site. Examination of T-wave rise times generally supports this interpretation. Based on the nature and duration of the activity, a response effort was initiated, which later confirmed hot-water plumes and fresh lava flows at the site. Based on both hydroacoustic information and field observations, it is proposed that the episode began with a lateral dike injection, possibly with eruptive activity in the summit region, followed by multiple magma pulses and eventual focusing of the seismic activity and extrusion near 42.6′N.     

Enriched H2, CH4, and He concentrations in hydrothermal plumes associated with the 1996 Gorda Ridge eruptive event

The rapid field response in March and April 1996 to diking-eruptive events on the Northern Gorda Ridge allowed the first detailed co-measurement of ³He, H₂, and CH₄ in plumes associated with a submarine eruption, and examination of the chemical transformation of these volatile species during aging of the plumes. The diking-eruptive event generated extensive event plumes with a low ³He/heat ratio of ∼0.4×10^-12 cm³ STP gm^-1 °C^-1, while chronic plumes had ³He/heat ratios of 2.1–6.2×10^-12 cm³ STP gm^-1 °C^-1. Hydrogen concentrations in the initial event plume (EP96A) reached a maximum value of 47 nM, 235 times that of ocean background concentration (0.2 nM). In contrast, roughly one month later H₂ concentrations, although still elevated, had drastically dropped to 40 times background values. Methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments, reaching a maximum value of 7 nM. The intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high, localized concentrations of H₂. The data indicate, however, that maximum rates of H₂ production during event plume formation could have lasted a few days at most.     

The distribution of Mn,Fe, Zn,Cd and Cu in Baltic seawater; a study on the basis of one anchor station

A total of 150 samples were collected at a 10-days' anchor station in the Bornholm basin (55° 31.1′N, 15° 32.1′E) and analyzed for dissolved (< 0.4 μm) and particulate trace metals. For dissolved Mn, large gradients have been found in the vertical distribution with minimum concentrations (< 0.2 μgl^?1) in the halocline zone and considerably higher values in the deep waters (up to 50 μgl^?1). Ultrafiltration studies indicate that dissolved Mn is probably present as Mn²⁺ in the oxygenated bottom layer. The primary production process was not evident in the particulate Mn profile; the suspended particulate material (SPM), however, shows a considerable enrichment with depth, apparently due to Mn-oxide precipitation.The distribution of dissolved Fe was rather homogeneous, with average concentrations throughout the water column between 0.86 and 1.1 μgl^?1, indicating that the oxidation of Fe²⁺ ions released from the sediments must already be complete in the very near oxidation boundary layer. Relatively high concentrations of particulate Fe were actually measured in the bottom layer, with the maximum mean of 11.2 μgl^?1 at 72 m. Similarly to Mn, the profile of particulate Fe does not reflect the SPM curve of the eutrophic layer. On average, about 70% of the total Fe in surface waters was found to be particulate.The average concentrations of dissolved Zn, Cd and Cu were found to be rather homogeneous in the water column but showed a relatively high variability with time. A simplified model on trace-metal uptake by phytoplankton indicates no significant change in dissolved metal concentrations during the period of investigation. On average, only 1.7% Zn, 3.3% Cd and 9.8% Cu of the total metal concentrations were found in particulate form. SPM analyses showed significant correlations of Zn, Cd and Cu with Fe, indicating that particulate iron is an important carrier for particulate trace metals in Baltic waters.     

Behaviors of dissolved and particulate Co,Ni, Cu,Zn, Cd and Pb during a mesoscale Fe-enrichment experiment (SEEDS II) in the western North Pacific

During mesoscale Fe enrichment (SEEDS II) in the western North Pacific ocean, we investigated dissolved and particulate Co, Ni, Cu, Zn, Cd and Pb in seawater from both field observation and shipboard bottle incubation of a natural phytoplankton assemblage with Fe addition. Before the Fe enrichment, strong correlations between dissolved trace metals (Ni, Zn and Cd) and PO₄³⁻, and between particulate trace metals (Ni, Zn and Cd) and chlorophyll-a were obtained, suggesting that biogeochemical cycles mainly control the distributions of Ni, Zn and Cd in the study area. Average concentrations of dissolved Co, Ni, Cu, Zn, Cd and Pb in the surface mixed layer (0–20 m) were 70 pM, 4.9, 2.1, 1.6, 0.48 nM and 52 pM, respectively, and those for the particulate species were 1.7 pM, 0.052, 0.094, 0.46, 0.037 nM and 5.2 pM, respectively. After Fe enrichment, chlorophyll-a increased 3 fold (up to 3 μg L⁻¹) during developing phases of the bloom (<12 days). Mesozooplankton biomass also increased. Particulate Co, Ni, Cu and Cd inside the patch hinted at an increase in the concentrations, but there were no analytically significant differences between concentrations inside and outside the patch. The bottle incubation with Fe addition (1 nM) showed an increase in chlorophyll-a (8.9 μg L⁻¹) and raised the particulate fraction up to 3–45% for all the metals, accompanying changes in Si/P, Zn/P and Cd/P. These results suggest that Fe addition lead to changes in biogeochemical cycling of trace metals. The comparison between the mesoscale Fe enrichment and the bottle incubation experiment suggests that although Fe was a limiting factor for the growth of phytoplankton, the enhanced biomass of mesozooplankton also limited the growth of phytoplankton and the transformation of trace metal speciation during the mesoscale Fe enrichment. Sediment trap data and the elemental ratios taken up by phytoplankton suggest that export loss was another reason that no detectable change in the concentrations of particulate trace metals was observed during the mesoscale Fe enrichment.     

Geochemistry of suspended matter from the Baltic Sea 2. Results of bulk trace metal analysis by AAS

In 1984, on a transect covering the whole Baltic Sea and parts of the adjacent North Sea, 160 water samples were taken and analysed for their concentrations of particulate and dissolved metals. In addition, the suspended materials were investigated for their elemental bulk composition.The particulate fractions represented from about 5% (Cd, Cu and Ni) to 50% (Fe and Pb) of the total (particulate plus dissolved) concentrations. For some elements (Ba, Cd, Cu, Pb and Zn), the particulate matter from the surface microlayer was enriched with respect to those suspended materials taken from 0.2 m depth. This could reflect the atmospheric input of metal-rich aerosols. In anoxic deep waters, maximum contents of Zn (6400 μg g⁻¹), Cu (1330 μg g⁻¹) and Cd (12 μg g⁻¹) were observed in the particulate matter, indicating sulphidic forms. On the other hand, under oxic conditions the distribution coefficients (K_d) decreased with the water depth (Cd, Fe and Pb).Relative to global background levels, the particulate matter contained metal “excesses” amounting to more than 90% of the total contents (Cd, Mn, Pb and Zn). Automated electron probe X-ray microanalysis (EPXMA) revealed that the elemental composition of sediments is mainly governed by post-depositional processes of early diagenesis and is only weakly related to the composition of suspended matter in the overlying water body. For instance, in relation to surface mud sediments of the central Baltic net-sedimentation basins, Zn, Cd, Cu and Mn had 30–100% higher levels in the suspended materials. The general pattern of metal contents of particulate matter taken from 10 m depth on a transect between the Bothnian Bay and the North Sea were—possibly as a result of anthropogenic inputs—rather similar for Pb, Zn and Cu. For Fe and Mn, the distribution patterns along the transect were probably governed by the natural loading characteristics and by the biogeochemistry of those elements.     

Emergence and petrology of the Mendocino Ridge

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a⁴⁰Ar/³⁹ Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.     

珠江口水域微表层中的痕量金属

采用自制滚筒式自动采样器采样，研究了珠江口水域微表层中Ｆｅ，Ｍｎ，Ｃｕ，Ｐｂ，Ｚｎ，Ｃｄ的含量、分布变化及富集状况。并通过颗粒痕量金属／颗粒Ｆｅ比值计算，探讨了痕量金属的来源。结果表明：Ｃｕ，Ｐｂ，Ｚｎ，Ｃｄ在微表层的平均富集系数分别为２．３，１．４，３．０和１．９。颗粒态Ｆｅ，Ｍｎ的平均富集系数为１．１和０．９。各种金属有明显的季节和空间变化，有各种不同的来源。     

The 1996 Gorda Ridge eruption: geologic mapping, sidescan sonar, and SeaBeam comparison results

As part of a response effort following the February 1996 T-wave swarm on the North Gorda Ridge, camera tows were conducted at the site in April and discovered that a lava flow had erupted onto the seafloor during the earthquake swarm. The lava flow is located on axis between 42.665° and 42.688°N, just south of the axial high of the ridge segment, near the northern extent of T-wave epicenters, and under the site where a hydrothermal event plume was found 2 weeks after the swarm began. Temperature sensors on the camera sled recorded anomalies up to 0.5°C over and near the new flow, showing that it was still actively cooling. Data from camera tows, remotely operated vehicle (ROV) dives, sidescan sonar imagery, and SeaBeam resurveys show that the new flow is 2.6 km long, 400 m wide, and up to 75 m thick, with a volume of 18×10⁶ m³. We interpret that this flow was erupted during the first half of the T-wave swarm. A combination of T-wave, plume, sidescan, and SeaBeam evidence also suggests that another lava flow (not imaged by camera or ROV) may have erupted 8 km to the south between 42.605° and 42.615°N, where the second half of the T-wave swarm was concentrated. However, this possible second eruption site remains unconfirmed.     

Outflow of trace metals into the Laptev Sea by the Lena River

Freshwater concentrations confirm the pristine character of the Lena River environment as already pointed-out in a previous study with a limited set of data (Martin et al., 1993). Total dissolved concentrations of the freshwater are 13.8 ± 1.6 nM, Cu, 4.4 ± 0.1 nM, Ni, 0.054 ± 0.047 nM, Cd, 642 ± 208 nM, Fe, 0.2–0.3 nM Pb and 1.2 ± 1.0 nM, Zn. For Zn and Pb, a simple mixing of the Lena River waters with the Arctic waters is observed. Relationships with salinity suggest that for Cu, Ni and Cd, there is a mobilization of the dissolved fraction from the suspended matter, with an increase of the dissolved concentration of 1.5, 3 and 6 times, respectively. For Fe, the total dissolved concentrations follow an exponential decrease in the mixing zone and 80% of the total “dissolved” Fe is removed from the solution. For Cu, Ni, Cd and Fe, the riverine end-members are 20 nM, 12 nM, 0.3 nM and 47 nM, respectively. When considering the input of total dissolved metals to the Arctic Ocean, the fraction attributed to the freshwaters from the Arctic rivers appears to be small (4% of the input of dissolved metal to the Arctic Ocean for Cd, 27% for Cu, 11 % for Ni and 2% for Zn). Metal concentrations in the Laptev Sea and Arctic Ocean are very similar, indicating a generally homogeneous distribution in the areas sampled.     

Relationship between metallothionein, copper and zinc in perch (Perca fluviatilis) environmentally exposed to heavy metals

Hepatic levels of Cu, Zn and metallothionein (MT) in perch, caught in a Cu/Zn gradient from a brassworks, reflected the water concentration of Cu (1·0–9·4 ppb) and Zn (0·56–59 ppb). Significant correlations were found between hepatic Cu and MT levels (r = 0·72), and between Zn and MT levels (r = 0·69). There was an increase of the amount of Cu and Zn in the cytosolic fraction of the liver with increased hepatic levels of the metals. When liver samples. from perch caught at the most contaminated location, were run on a gel filtration column (Sephadex G-75) 78% of the cytosolic Cu and 24% of the Zn in the cytosol eluted together with MT.     

Tectonic evolution of Gorda Ridge inferred from sidescan sonar images

Gorda Ridge is the southern segment of the Juan de Fuca Ridge complex, in the north-east Pacific. Along-strike spreading-rate variation on Gorda Ridge and deformation of Gorda Plate are evidence for compression between the Pacific and Gorda Plates. GLORIA sidescan sonographs allow the spreading fabric associated with Gorda Ridge to be mapped in detail. Between 5 and 2 Ma, a pair of propagating rifts re-orientated the northern segment of Gorda Ridge by about 10° clockwise, accommodating a clockwise shift in Pacific-Juan de Fuca plate motion that occurred around 5 Ma. Deformation of Gorda Plate, associated with southward decreasing spreading rates along southern Gorda Ridge, is accommodated by a combination of clockwise rotation of Gorda Plate crust, coupled with left-lateral motion on the original normal faults of the ocean crust. Segments of Gorda Plate which have rotated by different amounts are separated by narrow deformation zones across which sharp changes in ocean fabric trend are seen. Although minor lateral movement may occur on these NW to WNW structures, no major right-lateral movement, as predicted by previous models, is observed.     

Simplified synthesis of an 8-hydroxyquinoline chelating resin and a study of trace metal profiles from Jellyfish Lake, Palau

We report a simplified synthesis, and some performance characteristics, for 8-hydroxyquinoline (8-HOQ) covalently bonded to a chemically resistant TosoHaas TSK vinyl polymer resin. The resin was used to concentrate trace metals from stored, acidified seawater samples collected from Jellyfish Lake, an anoxic marine lake in the Palau Islands. The Mn, Fe, and Zn profiles determined from the 8-HOQ resin extraction were similar to those determined using Chelex-100 resin. The Zn and Cd profiles did not exhibit removal by sulfide “stripping” in contrast to other anoxic marine basins. The profiles of Co and Ni also exhibited elevated concentrations in the anoxic hypolimnion. The solution speciation and saturation states for the metals were calculated using revised metal-bisulfide stability constants. The calculations suggest that the MS(HS)⁻ species dominates the solution speciation for Mn, Co, Ni, Zn, Cd, and Pb. Cu(I) is modeled as the CuS⁻ or Cu(HS)₂⁻ species, while Fe(II) behaves as the free Fe²⁺ cation. The Mn, Co, Ni, Cu and Cd concentrations appeared to be at least 10-fold undersaturated, while the Fe(II), Zn, and Pb concentrations were close to saturation with respect to their metal sulfides.     

An investigation into the exchange of iron and zinc between soluble,colloidal, and particulate size-fractions in shelf waters using low-abundance isotopes as tracers in shipboard incubation experiments

A vertical mixing event was simulated in shipboard incubation experiments on the mid-continental shelf of the eastern Bering Sea to investigate Fe and Zn cycling between the soluble (< 0.03 μm or 200 kDa), colloidal (0.03–0.2 μm), and particulate (0.2–10 μm, > 10 μm) size-fractions. The particulate Fe and Zn were further separated into chemically labile (25% acetic acid-leachable) and refractory pools. The experiment employed ⁵⁷Fe (+ 0.90 nM) and ⁶⁸Zn (+ 0.99 nM) as stable, low-abundance isotope amendments to the soluble fraction, and the exchange of Fe and Zn between the different physico-chemical fractions was measured using high resolution-inductively coupled plasma-mass spectrometry (HR-ICP-MS). More than 50% of the added ⁵⁷Fe partitioned to the colloidal fraction within 45 min of adding the tracer. Both the ⁵⁷Fe and ⁵⁶Fe colloidal fraction were removed from the dissolved phase at a faster rate than the soluble Fe fraction. In contrast, the colloidal ⁶⁶Zn and ⁶⁸Zn concentrations remained constant over the 5-day experiment, suggesting a unique removal mechanism for colloidal Fe. The net removal of dissolved ⁵⁷Fe was observed to be 3 to 4 times more rapid than dissolved ⁵⁶Fe, which can be attributed to the regeneration of particulate Fe. Using a simple first-order model, it was determined that the net removal of 2.0 nM of dissolved Fe during the experiment was a consequence of dynamic cycling, whereby 2.9 nM of particulate Fe was regenerated and contributed to an overall removal of 4.9 nM of Fe from the dissolved phase. The amended ⁶⁸Zn tracer resided in the soluble fraction and was assimilated by the diatom biomass (> 10 μm size-fraction) at the same rate as ⁶⁶Zn. This similarity in rates suggests that nearly all of the net removal of Zn was due to assimilation and that regeneration did not play a significant role in Zn cycling within the incubation experiment. This research demonstrates the advantage of using low-abundance isotopes as tracers and the importance of particulate and colloidal Fe in the overall biogeochemical cycling of Fe in ocean surface waters.