Geochemical Fluxes Research Articles

Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico

Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone. 87Sr/ 86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 ± 700 mg cm −2 ka −1. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/ 86Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified.

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Dynamic topography and long-term sea-level variations: There is no such thing as a stable continental platform

Globally uniform (i.e., eustatic) sea-level trends with time scales approaching 100 Myr have been inferred from both seismic and backstripping stratigraphic analysis at a small set of geographic sites that are presumed to lie on stable continental platforms and passive continental margins characterized by simple thermal subsidence histories (e.g., the New Jersey margin, the western African margin). We demonstrate, using mantle flow simulations based on high resolution seismic tomography, that both the New Jersey margin and the conjugate western African margin have been subject to orders of 100 m of dynamic (i.e., flow induced) topography change over the last 30 Myr. We also show that the changing pattern of downwelling mantle flow associated with plate subduction is a significant contributor to the background eustatic sea-level trend, which is also of order 100 m during the 30 Myr time window. Therefore, Late-Cenozoic variations of dynamic topography on these passive margin sites are comparable to the eustatic sea-level changes and can partially mask the latter. Furthermore, even if the observed trend could be accurately corrected for local dynamic topography variation, the residual eustatic signal does not merely reflect changes in mean spreading rates at mid-ocean ridges. We conclude that the observed long term sea-level variations at so-called “stable” sites cannot be interpreted as eustatic. Moreover, previous analyses that have used long-term sea-level trends as a proxy for spreading rates and geochemical fluxes must be revisited.

Geochemical fluxes and weathering of volcanic terrains on high standing islands: Taranaki and Manawatu-Wanganui regions of New Zealand

Sediment fluxes from high standing oceanic islands (HSIs) such as New Zealand are some of the highest known [Milliman J. D. and Syvitski J. P. M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol. 100, 525–544]. Recent geochemical work has suggested that along with their extremely high physical weathering yields, many New Zealand watersheds also have very high chemical weathering yields. In New Zealand, the magnitude of both the physical and chemical weathering yields is related to the lithology of the watershed. Most of the previous work on this topic has been undertaken in Southern Alps watersheds of schist and greywacke and in East Cape watersheds of semi-consolidated marine sediments and greywacke. We recently sampled North Island watersheds in the Taranaki and Manawatu-Wanganui regions which have been subjected to volcanism since the Miocene. We sampled watersheds that contain both volcanic and sedimentary rocks. A series of water and sediment samples was collected and analyzed for major, minor and trace elements. This was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies. Our results reveal distinct chemical signatures for the different regions. Waters draining the Taranaki region volcanics are significantly enriched in K +, and depleted in Ca 2+ and Sr 2+ compared to waters draining the Manawatu-Wanganui region volcanics, which also traverse expanses of sedimentary siltstones and mudstones. The Ca 2+ and Sr 2+ depletions may reflect the relative absence of CaCO 3 in the Taranaki region watersheds. In addition, sediment samples from the Taranaki region show significant enrichment in Ti, Al, Ca, Fe, Mn, Mg, Ca, and P and depletion in Si and Rb compared to those of the Manawatu-Wanganui region. From total dissolved solids concentrations and mean annual water discharge, we calculate chemical weathering yields of 60–240 tons km −2 a −1. These weathering yields fall within the middle to upper range of those previously documented for the Southern Alps (93–480 tons km −2 a −1) and East Cape (62–400 tons km −2 a −1). Calculated silicate weathering yields of 12–33.6 tons km −2 a −1 and CO 2 consumption of 852–2390 × 10 3 mol km −2 a −1 for the rivers draining the Taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on HSIs. CO 2 consumption is found to be within the range previously measured for the basaltic terrains of the Deccan Traps (580–2450 × 10 3 mol km −2 a −1) and Réunion Island(1300–4400 × 10 3 mol km −2 a −1). Our calculated chemical weathering yields demonstrate the importance of HSIs, particularly those with volcanic terrains, when considering global geochemical fluxes.

海洋リソスフェアの熱水変質に伴う元素移動

The hydrothermal circulation of seawater in the oceanic lithosphere is an important factor controlling seawater chemistry, compositions of subducted materials returned to the mantle and microbial activity. We summarize the results of hydrothermally altered rocks taken directly from the ocean floor in terms of major and trace elements combined with petrographic descriptions. Hydrothermal circulation starts at the spreading axis where magmatic heat from a basaltic crustal formation is available (high temperature of > 350°C). Low-temperature alteration (< 150°C) may persist for > a million of years through the ridge flanks. Due to ridge flanks occupying large regions of the seafloor, changes in chemistry, mineralogy and physical properties of the oceanic lithosphere are accompanied by geochemical fluxes that may be even larger than those at the ridge axis. Two deep drill holes, DSDP/ODP 504B and 1256D, allow an examination of downhole variations of hydrothermal alteration in basaltic rocks, and dolerite in the extrusive and sheeted dike sequence. Recent direct sampling from the ocean floor reveals that gabbros and peridotites crop out in significant amounts on the ocean floor, particularly in the slow-spreading ridges. The chemical behavior of these originally deep-seated rocks during hydrothermal circulation thus has a large impact on global mass budgets for many elements. Previous studies on the ocean floor have been mainly conducted in the Atlantic Ocean and the Pacific Ocean. We present our results on hydrothermally altered basaltic rocks, gabbros and peridotites recovered from the Indian Ocean. Basaltic samples dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction are classified into three types—a fresh lavas, low-temperature altered rocks and high-temperature altered rocks. Petrological and geochemical features of these rocks are basically comparable to those of the basaltic rocks in DSDP/ODP Hole 504B, which suggests generalities in alteration processes and chemical exchange fluxes during hydrothermal activity across all world oceans. Gabbros and peridotites were sampled from an oceanic core complex, which was composed of tectonically exposed footwalls of detachment faults, from the Central Indian Ridge. Less deformed serpentinized and gabbros were recovered from the ridge-facing slope, whereas highly deformed schist-mylonites of a mixture of these rocks were recovered from the top of the surface (i.e., detachment fault). Efficient localization of strain was probably due to the formation of secondary minerals (e.g., talc, chlorite, serpentine) onto large, discrete shear zones where fluid was introduced locally. In-situ microanalysis of trace elements of the primary minerals and their secondary minerals revealed that selective elements, such as Rb, Sr, Ba, Pb and U, are enriched in the secondary minerals. Although oceanic core complexes are places that allow cross-sectional samplings of deep-seated rocks (i.e., gabbros and peridotites) in the oceanic lithosphere, we should keep in mind the implications of the results for the normal oceanic lithosphere. To understand the nature of the oceanic lithosphere, a close linkage between the ophiolite study and a number of deep holes in the oceanic lithosphere, including a deep hole through the crust-mantle boundary, is required.

Brief thermal pulses during mountain building recorded by Sr diffusion in apatite and multicomponent diffusion in garnet

We use the records of Sr diffusion preserved in apatite and corroborating multicomponent Fe-Mg–Ca–Mn diffusion preserved in garnet from the classic Barrovian metamorphic zones (Scotland) to quantify the timescale for the thermal peak of crustal heating c. 465 Ma. We show that Sr diffusion in apatite is a powerful means to determine thermal timescales, and provide the first set of diffusion-based timescale estimates across a wide range of metamorphic grades for Barrovian metamorphism. The results demonstrate that the thermal peak was extremely brief and lasted a few hundred thousand years. This timescale is one to two orders of magnitude shorter than peak timescales predicted by conventional models based on conductive relaxation of overthickened crust. The short peak thermal pulse or pulses probably involved advective heat transfer driven by magmas and associated fluid flow, followed by rapid exhumation. Peak thermal pulses may represent a very short part (< 1 Ma) of overall mountain building cycles lasting ∼ 10 7 yr or more, but play a dominant role in determining mineralogy, geochemical fluxes, and fluid production in mountain belts.

Recognizing hydrothermal alteration through a granulite facies metamorphic overprint at the challenger Au deposit, South Australia

The Challenger Gold Mine, South Australia, is hosted in granulite-facies migmatitic gneisses. Lack of hydrous alteration minerals, evidence for remobilization of Au-rich sulfide melts, and intense stretching of the ore package parallel to the regional gneissic fabric all point to a pre-metamorphic mineralization event. A combination of textural, mineralogical, and lithogeochemical criteria have been used to recognize the effects of partial melting during high-grade metamorphism and mass changes during pre-metamorphic hydrothermal alteration. Accounting for these superimposed processes allows us to document the style of pre-metamorphic hydrothermal alteration that affected the deposit. Alteration in a ∼ 50 m halo surrounding Au-mineralization is characterised by losses of Si, Na, Ca, and Sr and an increase in Al, K, Fe and Mg relative to the mean composition of unaltered distal gneisses. These geochemical fluxes significantly reduced the melting potential of these domains such that the altered rocks were transformed during metamorphism into mottled, granoblastic aluminous gneisses. In contrast, Au-rich zones host abundant relict quartz veins that underwent variable assimilation in voluminous K-feldspar-rich granitic melts generated through dehydration melting of quartz-and mica-rich layers. Geochemical trends attributable to this period of anatexis are distinct from those arising from pre-metamorphic hydrothermal alteration. These physical and geochemical observations are consistent with feldspar-destructive alteration and chloritization of proximal host rocks accompanied by silicification and sericitization (quartz veins + micas) alteration in the Au-rich horizons prior to prograde metamorphism. U–Pb geochronology of pristine magmatic zircon preserved in granoblastic proximal gneisses demonstrate that hydrothermal alteration occurred after deposition of the host volcaniclastic sequence (∼ 2520 Ma) and before the onset of regional metamorphism (∼ 2470 Ma). The evidence gathered here points to an epithermal-style Au deposit that was reworked during collisional orogenesis. The short time interval between Au-deposition and subsequent basin inversion prevented erosive forces from erasing the deposit from the rock record prior to metamorphism.

Geochemical fluxes related to alteration of a subaerially exposed seamount: Nintoku seamount, ODP Leg 197, Site 1205

Hole 1205A was drilled on Nintoku Seamount, which lies in the midportion of the Emperor Seamount Chain. This seamount was emergent ∼56 Myr ago but was submerged by 54 Ma, so the lavas have endured weathering in both subaerial and submarine environments. We have studied the petrology, mineralogy, and geochemistry of intercalated altered basalts, breccias, and soil samples recovered at Hole 1205A to quantify the chemical exchanges between the seamount and seawater and/or meteoric fluids. The secondary mineralogy is relatively uniform throughout the section and comprises smectite, Fe‐oxyhydroxides, iddingsite, and Ca‐carbonates. Soils are composed of variably altered basaltic clasts in a matrix of kaolinite, smectite, and vermiculite with minor goethite, hematite, and magnetite. Throughout the basement section, altered basalts, breccias, and soils are depleted in Si, Mg, Ca, Na, Sr, Rb, and Ba and enriched in Fe. Fe3+/FeT (up to ∼1), δ18O (up to ∼+20‰), and 87Sr/86Sr ratios are strongly elevated relative to primary igneous values. Differences in the 87Sr/86Sr ratios define an Upper Alteration Zone with 87Sr/86Sr close to 56 Ma seawater (∼0.7077) from a Lower Alteration Zone where 87Sr/86Sr are less elevated (∼0.704). The Lower Alteration Zone likely reflects interaction with a subaerial oxidizing fluid at low temperature. This zone probably retained most of the original subaerial weathering signature. The Upper Alteration Zone was altered through circulation of large quantities of cold oxidizing seawater that partially overprinted the subaerial weathering chemical characteristics. Altered samples were compared to estimated protolith compositions to calculate chemical gains and losses. Global chemical fluxes are calculated for the entire basement section using different lithological proportions models and different rates of oceanic island emplacement. Although the global construction rate of ocean islands is small compared to igneous accretion at mid‐ocean ridges, the magnitude of the chemical changes indicates that ocean islands and seamounts may be a significant contributor to the chemical budget of the oceans.

Watershed Nitrogen and Mercury Geochemical Fluxes Integrate Landscape Factors in Long-term Research Watersheds at Acadia National Park, Maine, USA

This paper is an overview of this special issue devoted to watershed research in Acadia National Park (Acadia NP). The papers address components of an integrated research program on two upland watersheds at Acadia NP, USA (44 degrees 20' N latitude; 68 degrees 15' E longitude). These watersheds were instrumented in 1998 to provide a long-term foundation for regional ecological and watershed research. The research was initiated as part of EPA/NPS PRIMENet (Park Research and Intensive Monitoring of Ecosystems Network), a system of UV-monitoring stations and long-term watershed research sites located in US national parks. The initial goals at Acadia NP were to address research questions about mercury, acid rain, and nitrogen saturation developed from prior research. The project design was based on natural differences in forests and soils induced by an intense wildfire in one watershed in 1947. There is no evidence of fire in the reference watershed for several hundred years. We are testing hypotheses about controls on surface water chemistry, and bioavailability of contaminants in the contrasting watersheds. The unburned 47-ha Hadlock Brook watershed is 70% spruce-fir mature conifer forest. In contrast, burned 32-ha Cadillac Brook watershed, 4 km northeast of the Hadlock watershed, is 20% regenerating mixed northern hardwoods and 60% shrub/rocky balds. Differences in atmospheric deposition are controlled primarily by forest stand composition and age. The watersheds are gauged and have water chemistry stations at 122 m (Cadillac) and 137 m (Hadlock); watershed maximum elevations are 468 and 380 m, respectively. The stream water chemistry patterns reflect, in part, the legacy of the intense fire, which, in turn, controls differences in forest vegetation and soil characteristics. These factors result in higher nitrogen and mercury flux from the unburned watershed, reflecting differences in atmospheric deposition, contrasting ecosystem pools of nitrogen and mercury, and inferred differences in internal cycling and bioavailabilty.

Transient features of the erosion of shales in the Mackenzie basin (Canada), evidences from boron isotopes

In order to refine our knowledge of the mechanisms that drive the weathering reactions and the rate at which elements are exported out of a shale basin, we have analyzed the boron concentrations and isotopic compositions of 24 rivers contributing to the Mackenzie basin (Canada). Boron has been chosen to conduct this study because of the relatively high sensitivity of its isotopic system to water/rock interactions. After examination of the chemical composition of the Mackenzie River samples, boron appears to be regulated, in the largest part, by reactions involving silicate rocks. The other contributions of rainwaters and the dissolution of carbonate and evaporite rocks can locally control the B budget but they remain of second importance at the regional to continental scales. It also appears that dissolved B in rivers is regulated by input of groundwaters. The comparison between the isotopic composition of the B being exported by rivers (soluble and particulates) and of its bulk source indicates that it is behaving out of steady state in most of the Mackenzie tributaries and particularly in rivers draining the plain. A model of transport/reaction is proposed in this study, which demonstrates that B is controlled by rapid ion exchanges at the mineral/water interface and by long-term dissolution of shales. The responses of the model to tested conditions reveal that the B isotopic ratio of river waters is highly sensitive to hydrological conditions and to changes of the weathering rate of shales, of its partition coefficient with adsorbing surface, and of the water velocity. The departure of the B geochemical cycle from a steady state is interpreted as the record of changes in the weathering regime in the Mackenzie basin. The temporal evolution of the B cycle modeled here implies an increase of the weathering rate of shales in the Mackenzie basin and/or an increase of the partition coefficient between soluble B and adsorbing surfaces. It also appears that these changes are consistent with timescales of the recent deglaciation of the basin. This study demonstrates that the present B fluxes are actually responding to past changes of the water/rock interactions and that the weathering history is a key parameter to interpret the modern geochemical fluxes. This conclusion is valid for any other element that has similar chemical properties with solid surfaces.

Microbes and volcanoes: A tale from the oceans, ophiolites, and greenstone belts

Submarine volcanic glass alteration displays two easily discernable types of textures, one that is best interpreted as the result of an abiotic diffusive exchange process and another that involves microbial activity. Glass bioalteration textures dominate in the upper 300 m of the oceanic crust and have been found in nearly all ocean basins and in many ophiolites and greenstone belts back to 3.5 Ga. Bioalteration may involve a globally significant biomass and may influence geochemical fluxes from seafloor alteration. Glass bioalteration creates an entirely new discipline of research that involves microbiologists and volcanologists working in active volcanic systems and in the geologic record. Submarine volcanoes exposed on the ocean floor are studied along with ophiolites and greenstone belts to understand Earth not only as a physical and chemical heat engine but also as a bioreactor.

Seafloor spreading, sea level, and ocean chemistry changes

High Cretaceous ocean crust production rates have been causally linked to high global sea level and global CO2 due to increased outgassing. However, recent studies have questioned the empirical basis for high Cretaceous global seafloor spreading rates, high Cretaceous sea level (230–320 m above present), and the relationship between geochemical fluxes and spreading rates.Although this topic has been discussed at several recent international meetings, there has been little opportunity for the protagonists in the debate of constant versus variable global seafloor spreading rates to interact. However, a group of tectonophysicists, stratigraphers, and geochemists recently met at Rutgers, The State University of New Jersey (Piscataway N.J.) to discuss global seafloor spreading changes and their possible relationships to sea level and geochemical variations.

Venting formation fluids from deep‐sea boreholes in a ridge flank setting: ODP Sites 1025 and 1026

During ODP Leg 168, two of ten boreholes, ODP Holes 1025C and 1026B, were cased through the sediment section, penetrated basaltic crust that is overpressured, and sealed. In 1999 and 2000 the seals were removed, allowing crustal formation fluids to vent and be sampled. The composition of these fluids is compared to those of basal deep‐sea pore waters, which have been the basis for estimating geochemical fluxes from low‐temperature ridge flank hydrothermal systems. Estimates for the composition of the major ions in formation fluids based on basal pore waters are within 5% of the values measured in borehole fluids. Similar comparisons for minor and trace elements are not as good; some are reactive in the sediment section, resulting in large uncertainties in the pore water extrapolation, while others are influenced by a variety of contaminants, including steel, grease, drilling muds, and basal sediment. Evidence for contamination includes high dissolved and particulate concentrations of several metals (e.g., Fe, Cu, Co, Zn, and Pb) and measurable changes in concentration during the past four years in response to reaction with basal sediment. This new confidence in estimating the primary composition of formation fluids, coupled with advances in thermodynamic and kinetic models, reveals the possibility of anhydrite precipitation in ridge flank hydrothermal systems at temperatures of ∼70°C. Such new insights allow us to address the timing and conditions under which seawater‐crustal reactions occur, leading to more accurate models of crustal evolution.

An experimental study of alteration of oceanic crust and terrigenous sediments at moderate temperatures (51 to 350°C): insights as to chemical processes in near-shore ridge-flank hydrothermal systems

Studies of hydrothermal circulation within partly buried basement on the eastern flank of the Juan de Fuca Ridge (JFR) have shown that ridge-flank geochemical fluxes are potentially important for the global budgets of some elements. There are major uncertainties in these flux calculations, however, because the composition of these basement fluids is strongly dependent on temperature and because they may be modified by interaction with the overlying terrigenous sediments, either by diffusive exchange with basement or during upwelling to the seafloor. To better understand the nature and temperature control of basalt-fluid and sediment-fluid reactions at the JFR flank, we have conducted laboratory experiments between 51 and 350°C and at 400 bars pressure. K, Rb, and Si are leached from basalt between 150 and 351°C, and Sr and U are taken up. The direction of exchange of Li and Ca with basalt varies with temperature. Li and Sr are removed from fluid at 150°C, but isotope studies show that there is simultaneous release of both elements from basalt, indicating that uptake is controlled by the formation of secondary minerals. Moreover, our experiments confirm that Sr isotope exchange with oceanic crust occurs at moderate temperature and is not confined to high-temperature axial hydrothermal systems. Our data and field data from the JDR flank indicate that uptake of U into basalt at moderate temperature could remove between 9.9 and 15 × 10 6 mol U yr −1 from the oceans. This is higher than a recent estimate based on measurements of U in altered ocean crust (5.7 ± 3.3 × 10 6 mol yr −1), which concords with arguments that the Δ element/heat ratios of JDR flank fluids are too large to be representative of average global flank fluids. K, Ca, Sr, Ba, Li, Si, and B are leached from terrigenous sediments between 51 and 350°C, and U is taken up. Cs and Rb are removed from the fluid below 100°C and leached from the sediment at higher temperature. Sr isotope data show that Sr is preferentially mobilised from volcanic components within terrigenous sediments, which may lead to an overestimation of the ridge-flank Sr isotope flux at the JDR if there is exchange of sediment pore fluids with basement.

Uranium-series Disequilibria in Mid-ocean Ridge Basalts: Observations and Models of Basalt Genesis

Research Article| January 01, 2003 Uranium-series Disequilibria in Mid-ocean Ridge Basalts: Observations and Models of Basalt Genesis Craig C. Lundstrom Craig C. Lundstrom Department of Geology, University of Illinois at Urbana Champaign, 245 NHB, 1301 W. Green St., Urbana, Illinois, 61801, U.S.A. Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2003) 52 (1): 175–214. https://doi.org/10.2113/0520175 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Craig C. Lundstrom; Uranium-series Disequilibria in Mid-ocean Ridge Basalts: Observations and Models of Basalt Genesis. Reviews in Mineralogy and Geochemistry 2003;; 52 (1): 175–214. doi: https://doi.org/10.2113/0520175 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Mid-ocean ridges account for more than 75% of the annual magmatic output of planet earth and are a critical piece of the plate tectonic model. Understanding the mantle melting process beneath ridges is essential for discerning basic physical and chemical processes of the earth. For instance, ridges play an intrinsic role in the geochemical flux balance for many elements on the earth and provide a tectonic framework for investigating the physical process of solid mantle flow. Ridges are also the simplest tectonic regime to study the mantle melting process because interactions with the lithosphere are thought to be minimal. Yet... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Trace metal fluxes to the ocean: The importance of high‐standing oceanic islands

Trace metal analyses of river channel sediments from Taiwan and New Zealand provide new estimates for global budgets of trace metal inputs to the world's ocean. Mean trace metal values from high standing island river sediments are less chemically weathered than observed in sediments collected from large, continental, rivers and more similar to upper continental crust (UCC) values. Our mean values for Cu, Mn, Co and Pb are similar to the UCC; Zn, Ni and Cr are more highly enriched, compared to UCC values. Our new estimates of global trace metal budgets, combining previous estimates with these new data, show that geochemical fluxes of metals into the oceans are lower than previous estimates, ranging from 9% lower for Co to 24% lower for Cu.

Hydrothermal plume-particle fluxes at 13°N on the East Pacific Rise

We have investigated the geochemical flux to sediment traps deployed close to the Totem vent site, 13°N EPR. An important emphasis has been to investigate what proportion of this settling flux derives from direct co-precipitation of vent-fluid material as polymetallic sulphides and what proportion is in the form of Fe oxyhydroxide material which not only co-precipitates vent-fluid metals but can also scavenge dissolved material from seawater. Mass fluxes and major element compositions (Fe, S, Al, Mn, CaCO 3 and C org) for our near vent samples compare well with results from previously reported Pacific hydrothermal sediment trap studies, both at this site and on the Endeavour Ridge. Our samples record large fluxes of Cu, Zn and Pb, as well as V and P, all of which are in excess over typical open-ocean trap values. If P and V are transported to the traps as sinking Fe-oxyhydroxide material from the neutrally buoyant plume, we calculate that 10–20% of the Fe entering the near vent traps occurs as oxidised material with the remaining 80–90% being supplied by polymetallic sulphides. Shale-normalised REE distribution patterns for near-vent trap samples are similar to those for local vent fluids and sulphidic sediments. Detailed mass balance calculations, however, reveal evidence for additional input from hydrothermal Fe-oxyhydroxide material with a scavenged REE composition that is less “evolved” than that reported for local neutrally buoyant plume particles. U fluxes into the near vent traps are high and consistent with uptake by sulphides. 210Pb fluxes are also high and appear dominated by co-precipitation direct from vent-fluids as Pb-sulphides. In contrast, Fe-oxyhydroxide scavenging from seawater can account for the entire 230Th and 232Th fluxes reported. If the scavenging processes identified here were similarly active in neutrally buoyant plumes, we would predict hydrothermal scavenging to impact ocean biogeochemical cycles significantly, e.g. causing removal of ca. 10% of the dissolved 230Th production from the deep water column, out to a distance of ca. 10–100 km off-axis, along the entire 60,000 km global ridge-crest.

Comparison of bacterial production in sediments, epiphyton and the pelagic zone of a lowland river

1. The microbial metabolism of organic matter in rivers has received little study compared with that of small streams. Therefore, we investigated the rate and location of bacterial production in a sixth‐order lowland river (Spree, Germany). To estimate the contribution of various habitats (sediments, epiphyton, and the pelagic zone) to total bacterial production, we quantified the contribution of these habitats to areal production by bacteria.2. Large areas of the river bottom were characterized by loose and shifting sands of relatively homogenous particle size distribution. Aquatic macrophytes grew on 40% of the river bottom. Leaf areas of 2.8 m2 m−2 river bottom were found in a 6.6 km river stretch.3. The epiphyton supported a bacterial production of 5–58 ng C cm−2 h−1. Bacterial production in the pelagic zone was 0.9–3.9 μg C L−1 h−1, and abundance was 4.0–7.8 × 109 cells L−1. Bacterial production in the uppermost 2 cm of sediments ranged from 1 to 8 μg C cm−3 h−1, and abundance from 0.84 to 6.7 × 109 cells cm−3. Bacteria were larger and more active in sediments than in the pelagic zone.4. In spite of relatively low macrophyte abundance, areal production by bacteria in the pelagic zone was only slightly higher than in the epiphyton. Bacterial biomass in the uppermost 2 cm of sediments exceeded pelagic biomass by factors of 6–22, and sedimentary bacterial production was 17–35 times higher than in the overlying water column.5. On a square meter basis, total bacterial production in the Spree was clearly higher than primary productivity. Thus, the lowland river Spree is a heterotrophic system with benthic processes dominating. Therefore, sedimentary and epiphytic bacterial productivity form important components of ecosystem carbon metabolism in rivers and shallow lakes.6. The sediments are focal sites of microbial degradation of organic carbon in a sand‐bottomed lowland river. The presence of a lowland river section within a river continuum probably greatly changes the geochemical fluxes within the river network. This implies that current concepts of longitudinal biogeochemical relationships within river systems have to be revised.

Alteration Associated with Gold Deposition at the Getchell Carlin-Type Gold Deposit, North-Central Nevada

Wall-rock alteration at the Getchell underground deposit was examined to determine the effects of Au-bear-ing fluids on host lithologies and the relationship between K-bearing alteration minerals and Au deposition. The major, minor, and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the Getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike. Each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high-grade ore to moderately altered rock or waste rock. Analyses were obtained for 39 elements, 10 oxides, and loss on ignition, using multiple techniques. Petrographic studies were integrated with geochemistry and X-ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes. Geochemical analyses indicate that Ti, Al, Zr, and Th behave as immobile elements. Immobility isocon diagrams show that significant Hg, Sb, Se, Te, Tl, and Cs are typically added to the wall rocks with Au. Fe is im-mobile in some transects, added to wall rocks in others, and removed from rhyodacite host rocks. Minor S and W are usually added to the rock, whereas minor Cu and Mo are variably added or removed. SiO 2 appears to be either added during alteration or remain constant. Elements that are consistently removed during alteration include Ca, Mn, Sr, and Sc and minor Mg, Ba, and K. Na was significantly removed from the rhyodacite. Au correlates positively with Ag, Hg, Sb, Se, Si, Te, and Tl and negatively with Ca, Mn, and Sc. Element fluxes correspond with observations of mineral abundances and show that sulfidation and decar-bonitization are spatially associated with gold mineralization. Au, Hg, Sb, Tl, Cu, and possibly Te and W were incorporated in trace element-rich pyrite as host-rock Fe was sulfidized. Fe was added to mineralized rocks in some transects, indicating that pyritization occurred in addition to sulfidation. Cs is a component of galkhaite, a trace mineral that is relatively common in some parts of the Getchell system. Decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of Ca, Mn, Sr, and Sc. Although SiO 2 , As, and S are significantly added to Carlin-type systems, they do not appear as major added components in the isocon diagrams. This is because the halos for these components are much larger than the areas examined by the sample transects. X-ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals, including illite and montmorillonite. Kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks. Montmorillonite does not correlate with gold and textures are consistent with pore-filling deposition of montmorillonite following Au deposition. Illite abundance exhibits a positive correlation with Au ( r 2 = 0.80). Intergrowths of illite with ore-stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the Au-related hydrothermal event. Mineral abundances and textures suggest that illite formed by alteration of K feldspar during Au deposition. Results show that dating of K-bearing phases in sedimentary rocks, other than illite, would provide ages unrelated to gold deposition.

Composition of pore and spring waters from Baby Bare: global implications of geochemical fluxes from a ridge flank hydrothermal system

Warm hydrothermal springs were discovered on Baby Bare, which is an isolated basement outcrop on 3.5 Ma-old crust on the eastern flank of the Juan de Fuca Ridge. We have sampled these spring waters from a manned submersible, along with associated sediment pore waters from 48 gravity and piston cores. Systematic variations in the chemical composition of these waters indicate that hydrothermal reactions in basement at moderate temperatures (63°C in uppermost basement at this site) remove Na, K, Li, Rb, Mg, TCO 2, alkalinity, and phosphate from the circulating seawater and leach Ca, Sr, Si, B, and Mn from the oceanic crust; and that reactions with the turbidite sediment surrounding Baby Bare remove Na, Li, Mg, Ca, Sr, and sulfate from the pore water while producing ammonium and Si and both producing and consuming phosphate, nitrate, alkalinity, Mn, and Fe. K, Rb, and B are relatively unreactive in the sediment column. These data confirm the earlier inference that sediment pore waters from areas of upwelling can be used to estimate the composition of altered seawater in the underlying basement, even for those elements that are reactive in the sediment column or are affected by sampling artifacts. The composition of altered seawater in basement at Baby Bare is similar to the inferred composition of 58°C formation water from crust nearly twice as old (5.9 Ma) on the southern flank of the Costa Rica Rift. The Baby Bare fluids also exhibit the same directions of net elemental transfer between basalt and seawater as solutions produced in laboratory experiments at a similar temperature, and complement compositional changes from seawater observed in seafloor basalts altered at cool to moderate temperatures. The common parameter among the two ridge flanks and experiments is temperature, suggesting that the residence time of seawater in basement at the two ridge-flank sites is sufficiently long for the solutions to equilibrate with altered basalt. This conclusion is supported by compilations of data from other ridge-flank sites, which show a systematic relationship between inferred basement water composition and temperature. We use the Baby Bare spring waters along with constraints from the riverine flux of Mg to estimate upper limits on the global fluxes of 14 elements at warm ridge-flank sites such as Baby Bare. Maximum calculated fluxes of Mg, Ca, sulfate, B, and K may equal or exceed 25% of the riverine flux, and such sites may represent the missing, high K/Rb sink required for the K budget. Additional fluxes from/to the crust on ridge flanks are expected from cool (<20–25°C) hydrothermal sites.