Heat flow measurements in the southern portion of the Gulf of California

Twenty-five new heat flow measurements made in the Gulf of California are presented. All the values except two at the mouth of the Gulf and two in the Sal si Puedes basin are high. The values ranged from 2.0 to greater than 10 μcal/cm² sec (82 to > 420 mW/m²) with eight values greater than 5.2 (210 mW/m²). Due to high rates of sedimentation throughout the Gulf, the actual heat flow, in many cases, may be up to 25% greater than that recorded.Most of the heat flow stations are concentrated in the Farallon and Guaymas basins and show a marked increase towards the central deeps, where new crust is believed to be forming. The heat flow values in the Farallon basin show a sharp peak 10–15 km southeast of the central depression while those in the Guaymas basin peak in the depression.The heat flow profiles across the Guaymas and Farallon basins are remarkably similar to those observed on other well sedimented spreading centers such as the northern portion of the Explorer trough. Thus they may provide evidence that the crust is being created by an axially symmetric intrusion process with a major loss of heat due to hydrothermal circulation. The absence of magnetic anomalies in the Gulf has been attributed to the supposed presence of large grains in the intruded basalt. Large grains form by the slow cooling of the basalt under a layer of sediment. Prominent magnetic anomalies have been observed on the northern portion of the Explorer trough. Observational data suggest that the thermal processes at this ridge axis and the center of the Farallon basin are identical. We suggest that further careful study is needed in the Gulf before the slow cooling model is accepted as an explanation for the attenuation of the magnetic anomalies.     

Seasonal and interannual thermohaline variability in the Guaymas Basin of the Gulf of California

Temperature and salinity data for the years 1939–1983 are used to investigate seasonal and interannual scales of the hydrographic variability across the Guaymas Basin, which is located between 27° and 28°N in the Central Gulf of California. Winter conditions extend from December to April and summer conditions from June to October, with transition periods in May and November. Sea surface temperature increases from about 16°C in February–March to 31°C in August. No clear seasonal cycle in surface salinity was found. Typical values are above 35.1‰ even in winter, and up to 35.5‰ in November.Relatively cold and low salinity near-surface waters observed in June 1957 and in June 1982, suggest advection of California Current Water to the Guaymas Basin. Subtropical Subsurface Water may occur around the year, but is obscured by vertical mixing with Gulf Water mainly during winter, when vertical stratification is weaker. The Intermediate and Deep Pacific Water masses successively fill the Guaymas Basin to the bottom (2000 m), showing very stable T-S characteristics.Positive sea level anomalies at Guaymas increases during El Nin˜o years, and anomalous low salinity and high temperature at the surface indicate the presence in the Guaymas Basin of water from the south. Observed differences reached 0.4‰ in surface salinity and 3°–5°C in surface temperature. There is evidence that the observed low salinities could not be due to abundant precipitation. An additional effect is a deepening of the winter pycnocline down about 200 m, compared to the usual depth of <100 m. In summer, this effect is not as clear as in winter, due to the strong stratification. The effects of the very strong 1982–1983 and 1957–1958 ENSO episodes may have lasted for one and two years, respectively. It is argued that during an ENSO event the Transition Water of the California Current meet and mix near the Gulf entrance with the Tropical Surface Water of the Costa Rica Coastal Current. This mixed water could have been carried north into the Gulf by local surface circulation. In 1983 it was found at the surface in the Guaymas Basin above the southeastward flow of the colder and saltier Gulf Water.     

Manganese geochemistry near high-temperature vents in the Mid-Atlantic Ridge rift valley

In July 1985 the first high-temperature, hydrothermal vents ever discovered on the MAR were found at 26°N (TAG). A transponder-navigated survey of a 4 × 8 km area was begun when on-board measurements of manganese revealed concentrated hydrothermal plumes in the water column comparable in size and intensity to those found over geothermal fields on the EPR. These plumes also possess anomalously high light scattering properties and anomalously warm potential temperatures. Using relay-transponder navigation with a rosette-CTD system, it was possible to sample bottom waters with temperature anomalies of several decidegrees. Samples collected near vent orifices possessed silica anomalies of up to 17 μM and dissolved manganese anomalies as large as 360 nmol/kg. These manganese and silica anomalies are linearly related and predict aMn/Si ratio of 20 mmol/mol for these MAR vent fluids, a value essentially identical to that measured in 350°C fluid from the EPR at 21°N. The distribution of temperature anomalies in the water column over this area requires the presence of more than one active vent site. A rough estimate of the convective heat loss from this section of the MAR based onθ-z profiles is 1.2 × 10⁸ cal/s which is similar to the heat loss estimated for the vent field on the EPR at 21°N.     

Spatio-temporal dynamics of suspended matter properties and bacterial communities in the back-barrier tidal flat system of Spiekeroog Island

Back-barrier tidal flat systems are characterized by basins and inlets through which water is exchanged with the coastal sea by tidal water movements. The hydrographic and morphometric properties at the inlets and in the basins vary considerably, but there is little information available how biogeochemical properties in the water column at these different sites respond to these differences. Therefore, we investigated tidal dynamics of suspended particulate matter (SPM), particulate and dissolved organic carbon (DOC), chlorophyll a, phaeopigments, numbers of particle-associated (PA) and free-living bacteria (FL), bacterial biomass production, and concentrations of dissolved manganese (Mn). Samples were taken at the surface, a mid-depth and 1 m above the bottom at a fixed station at the inlet and in the basin of the Spiekeroog back-barrier tidal flat system in the German Wadden Sea. Five tidal cycles representative for typical seasonal situations, January (winter), April and May (late spring bloom), July (summer), and November (late fall) were studied in 2005 and 2006. In July, processes related to phytoplankton dynamics and bacterial decomposition were much more enhanced in the basin, whereas in April, these processes were enhanced at the inlet but were particularly low within the basin itself. The low values within the basin were a result of the settled phytoplankton spring bloom and represent a rather short period at the decline of this bloom. In November and January, differences were much less pronounced than during the growing season and restricted mainly to SPM and PA bacteria, exhibiting higher values in the basin. FL bacteria, DOC, and dissolved Mn exhibited different patterns and much less differences between the two stations, indicating that biogeochemical processes in the dissolved phase were controlled by different factors than PA biogeochemical processes. These differences reflect the retentive properties of the basin for particles and PA biogeochemical processes, particularly during the growing season, and in general emphasize the high productivity of back-barrier tidal flat systems.     

Source and distribution of mercury in natural gas of major petroliferous basins in China

Based on the analysis of the natural gas samples of 146 wells from four major petroliferous basins of China, the source and distribution of mercury in natural gas in major petroliferous basins of China were identified. Studies have shown that the mercury concentration of natural gas in petroliferous basins of China varies widely, ranging from 0.01 to 4050 μg/m~3. The gas well with the highest mercury concentration is in the Xushen gas field in the Songliao Basin. The mercury concentration in the craton basin is relatively low, and the secondary gas reservoir in the Tarim Basin has the lowest mercury concentration. In the rift basin, due to the presence of deep faults which cut the basement and mixing effects of deep fluids, the mercury concentration in natural gas may be abnormally high, e.g. the Xushen gas field in the Songliao Basin. In relatively stable basins where deep and large faults do not develop, mercury is primarily of organic origin. In rift basins where deep fluids are mixed with gas reservoirs,mercury in natural gas may mainly come from the deep fluids and is characterized by abnormally high mercury concentration and R/Ra values.     

Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

As part of the 1984–1985 NOAA VENTS program on the Mid-Atlantic Ridge, nephelometry was used to provide real-time detection and tracking of dispersed hydrothermal plumes. At all nine 1984 study sites, hydrothermal activity was detected by in-situ, real-time nephelometer measurements and later confirmed by dissolved Mn and particulate Fe measurements. These same techniques were employed in a site-specific survey of the Trans-Atlantic Geotraverse (TAG) area in 1985 where large water-column anomalies in turbidity and in dissolved Mn helped lead to the discovery of high-temperature black smokers.The optical response of the nephelometer was to hydrothermally-derived particulate matter. Thus strong correlations existed between the nephelometer readings and total suspended matter (r = 0.98, n = 34), and particulate Fe (r = 0.88, n = 32). In addition, digital nephelometer data correlated well with dissolved Mn (r = 0.88; n = 78) throughout a large concentration range (0.2–31.0 nmol/kg).These data provide good evidence for the utility of in-situ nephelometer measurements for locating and surveying plumes from hydrothermal vents. It also appears possible, within limits, to predict concentrations of in-situ total suspended matter, of particulate Fe and of dissolved Mn.     

Palaeoceanography of Mesozoic ribbon radiolarites

Mesozoic ribbon radiolarites have no parallel in rocks cored from extant ocean basins; their mineralogy is comparable but their repetitively bedded aspect is unique. The peculiarities of the chert-argillite couplet we relate primarily to variations in surface-water productivity on a tens-of-thousands-of-years scale, to a lesser extent to redeposition from turbidity and particularly bottom currents, and diagenesis, the latter being accentuated by the pressure-temperature gradients imposed during major tectonic events: ribbon cherts are unique to orogenic belts.

We suggest that these peculiar siliceous rocks were formed, as deep-water deposits (traces of sulphate minerals notwithstanding), in small basins of various types; arc-related regions (e.g. the Great Valley Sequence and, possibly, the Franciscan of California) and embryonic oceans dominated by transform faulting such as those of the western Tethys. Given that diatoms have replaced radiolarians in processing most of the World Ocean's dissolved silica and that some Tertiary-Recent diatomites were deposited at comparable rates (expressed as g/cm² 10³ years) to Mesozoic ribbon radiolarites it is instructive to discover that certain present-day sites of diatomaceous sedimentation (e.g. Sea of Okhotsk, Bering Sea, Japan Sea and Gulf of California) provide tectonic analogues for Mesozoic siliceous basins.

In these settings, namely arc-related or transform-dominated, the first significant depositional interface probably lay at a depth of 3–3.5 km which was below the Late Jurassic (pre-Tithonian) calcite compensation depth, favouring sedimentation of an entirely siliceous record: thus most Jurassic ophiolites show a basalt-chert contact. The Cretaceous radiolarites of Cyprus and the Middle East which also rest on basalt probably result, however, from local carbonate dissolution in lava-hollows by debouching low-pH hydrothermal fluids; they are not necessarily comparable with their Jurassic counterparts.

The prevalence of mid to Late Jurassic radiolarites in the Californian and Tethyan areas is attributed to heightened fertility and elevated CCD in small ocean basins rather than any factor favouring global radiolarian productivity at this time. Depth and sea-floor topography also controlled facies developments on the continental margins of the western Tethys, shallower-water regions being characterized by pelagic-carbonate sediments.

Mesozoic ribbon cherts thus provide a tantalizing record of local palaeoceanographic parameters, palaeotectonic regimes and sedimentary environments but they tell us little about the behaviour of the World Ocean during Jurassic and Cretaceous time.     

Fluid seepage along the San Clemente Fault scarp: basin-wide impact on barium cycling

Material fluxes associated with fluid expulsion at cold seeps and their contribution to oceanographic budgets have not been accurately constrained. Here we present evidence that the barium released at cold seeps along the San Clemente Fault zone may significantly impact the geochemical budget of barium within the basin. Barium fluxes at seep localities on the fault scarp, measured with benthic chambers, reach values as high as 5 mmol m⁻² day⁻¹. This is the largest dissolved barium flux measured to date at a cold seep. The discharge of barium-rich fluids results in formation of massive barite deposits along the escarpment wall. The deposits are young (approximately 8 yr) and appear to grow at a minimum rate of 0.2 cm yr⁻¹. This rapid growth rate requires a barium efflux rate that is about two orders of magnitude higher than the measured dissolved flux. We believe that the discrepancy reflects a highly localized seepage system and that chambers positioned as close as possible to the growing chimneys did not sample the foci of fluid discharge. Transport of fine barite particles from the seeps may be responsible for excess rates of barium accumulation throughout the San Clemente Basin, relative to other basins in the California Margin. Based on a preliminary budget, we estimate that cold-seep barite is accumulating at the basin floor in San Clemente at a rate of 2 μmol m⁻² day⁻¹, a value that is comparable to the total barium accumulation rates driven by detrital and biogenic components in neighboring basins. Remobilization of cold-seep barite on the basin floor adds to that driven by the biogenic barium flux and results in benthic barium recycling rates (effluxes) within the San Clemente Basin that are as much as seven times higher than the effluxes from surrounding borderland basins. Our estimates imply that processes associated with fluid seepage along the San Clemente Fault significantly contribute to the basin’s barium cycle. The strontium isotopic composition of the seep barite is significantly different from marine ‘biogenic’ barite, which is known to accurately record seawater composition. In addition, the seep deposits are depleted in ²²⁶Ra relative to their modern biogenic counterparts, and are likely to be a source of radium-depleted particulate barium to the basin. Thus the impact of barite transport from seeps on the San Clemente escarpment to the basin floor might also have implications for the geochemistry of elements other than barium.     

Geology, ore deposits and hydrothermal venting in Bahía Concepción, Baja California Sur, Mexico

Abstract Bahía Concepción is located in the eastern coast of the Baja California peninsula and it is shaped by northwestern–southeastern normal faults. These are associated with a 12–6 Ma rifting episode, although some have been reactivated since the Pliocene. The most abundant rocks correspond to the arc related Comondú Group, Oligocene to Miocene, which forms a mainly calc‐alkaline volcanic and volcaniclastic sequence. There are less extensive outcrops of sedimentary rocks, lava flows, domes and pyroclastic rocks of Pliocene to Quaternary ages. The Neogene volcanism in the area indicates a shift from a subduction regime to an intraplate volcanism related to continental extension and the opening of an oceanic basin. The Bahía Concepción area contains numerous Mn ore deposits, being the biggest at El Gavilán and Guadalupe. The Mn deposits occur as veins, breccias and stockworks, and are composed by Mn oxides (pyrolusite, coronadite, romanechite), dolomite, quartz and barite. The deposits are hosted in volcanic rocks of the Comondú Group and, locally, in Pliocene sedimentary rocks. Thus, the Mn deposits formed between the Middle Miocene and the Pliocene. The mineralized structures are associated with Miocene northwestern–southeastern fault systems, which are analogous to those associated with the Cu‐Co‐Zn‐Mn deposits of El Boleo. The Bahía Concepción area also bears subaerial and submarine hot springs, which are associated with the same fault systems and host rocks. The submarine and subaerial geothermal manifestations south of the bay are possibly related with recent volcanism. The geothermal manifestations within the bay are intertidal hot springs and shallow submarine diffuse venting areas. Around the submarine vents (5–15 m deep, 87°C), Fe‐oxyhydroxide crusts with pyrite and cinnabar precipitate. In the intertidal vents (62°C), aggregates of opal, calcite, barite and Ba‐rich Mn oxides occur covered by silica‐carbonate stromatolitic sinters. Some 10–30 cm thick crustiform veins formed by chalcedony, calcite and barite are also found close to the vents. The hydrothermal fluids exhibit mixed isotopic compositions between δ¹⁸O‐enriched meteoric and local marine water. The precipitation of Ba‐rich Mn oxides around the vent sites could be an active analog for the processes that produced Miocene to Pliocene hydrothermal Mn‐deposits.     

Mercury distribution in Douro estuary (Portugal)

Determinations of dissolved reactive and total dissolved mercury, particulate and sedimentary mercury, dissolved organic carbon (DOC), particulate organic carbon (POC) and suspended particulate matter (SPM) have been made in the estuary of river Douro, in northern Portugal. The estuary was stratified by salinity along most of its length, it had low concentrations of SPM, typically <20 mg dm(-3), and concentrations of DOC in the range <1.0-1.8 mg dm(-3). The surface waters had a maximum dissolved concentration of reactive mercury of about 10 ng dm(-3), whereas for the more saline bottom waters it was about 65 ng dm(-3). The surface waters had maximum concentrations of total suspended particulate mercury of approximately 7 microg g(-1) and the bottom waters were always <1 microg g(-1). Concentrations of mercury in sediments was low and in the range from 0.06 to 0.18 microg g(-1). The transport of mercury in surface waters was mainly associated with organic-rich particulate matter, while in bottom waters the dissolved phase transport of mercury is more important. Lower particulate organic matter, formation of chlorocomplexes in more saline waters and eventually the presence of colloids appear to explain the difference of mercury partitioning in Douro estuarine waters.     

The composition of nutrient fluxes from contrasting UK river basins

Accurate estimates of N and P loads were obtained for four contrasting UK river basins over a complete annual cycle. The fractionation of these loads into dissolved and particulate, and inorganic and organic components allowed a detailed examination of the nutrient load composition and of the factors influencing both the relative and absolute magnitude of these components. The particulate phosphorus (TPP) loads account for 26–75% of the annual total phosphorus (TP) transport and are predominantly inorganic. The inorganic (PIP) and organic (POP) fractions of the TPP loads represent 20–47% and 6–28% of the annual TP transport, respectively. In contrast, the particulate nitrogen loads (TPN) represent 8% or less of the annual total nitrogen (TN) loads and are predominately organic. For dissolved P transport, the dissolved inorganic fraction (DIP) is more important, representing 15–70% of the TP loads, whereas the dissolved organic fraction (DOP) represents only 3–9% of the TP loads. The TN loads are dominated by the dissolved component and more particularly the total oxidized fraction (TON), which is composed of nitrate and nitrite and represents 76–82% of the annual TN transport. The remaining dissolved N species, ammonium (NH₄-N) and organic N (DON) account for 0·3–1·2% and 13–16% of the annual TN transport, respectively. The TPN and TPP fluxes closely reflect the suspended sediment dynamics of the study basins, which are in turn controlled by basin size and morphology. The dissolved inorganic nutrient fluxes are influenced by point source inputs to the study basins, especially for P, although the TON flux is primarily influenced by diffuse source contributions and the hydrological connectivity between the river and its catchment area. The dissolved organic fractions are closely related to the dissolved organic carbon (DOC) dynamics, which are in turn influenced by land use and basin size. The magnitude of the NH₄-N fraction was dependent on the proximity of the monitoring station to point source discharges, because of rapid nitrification within the water column. However, during storm events, desorption from suspended sediment may be temporarily important. Both the magnitude and relative contribution of the different nutrient fractions exhibit significant seasonal variability in response to the hydrological regime, sediment mobilization, the degree of dilution of point source inputs and biological processes. © 1998 John Wiley & Sons, Ltd.     

A high-temperature hydrothermal deposit on the seabed at a gulf of California spreading center

A submersible dive on a turbidite-covered spreading axis in Guaymas Basin photographed and sampled extensive terraces and ledges of talc. The rock contains siliceous microfossils, smectite, and euhedral pyrrhotite as well as rather pure iron-rich talc. Sulfur and oxygen isotopes indicate precipitation around a hydrothermal vent, at about 280°C.     

The Earth’s crust of the deep platform basins in the Northern Eurasia and their origin

Several large basins with a depth up to 15–20 km are revealed in the platform regions of Northern Eurasia—the Pre-Caspian, Vilyui, Pur-Gydan, and Kara–Barents platforms. All these basins have two structural features in common: they all have a rounded shape with steep walls and are marked with the reduced thickness of the Earth’s crust. With the basins' depth of 15–20 km, the basement top is flat and has an almost horizontal surface. The basins differ by the average seismic velocities in the crystalline crust which vary from 6.2–64 to 6.8–7.0 km/s. Another distinction is the type of the crystalline crust ranging from continental with a thick granite-gneiss layer to suboceanic, represented by the basite layer. For explaining the formation of these basins, we suggest a combined petrophysical model which includes several geodynamical processes of different intensity: rifting, basification, and eclogitization of the Earth’s crust.The model also takes into account the process of material outflow from beneath a basin through the midcrustal layer of increased porosity and fluid saturation. This accounts for the strong reduction of the granite-gneiss layer with the preservation of the basement’s flat surface and for the formation of significant source areas of clastic material around the basin. The formation of these basins requires an extensive and sufficiently laterally uniform and longoperating energy source. The intrusion of the mantle material saturated with fluids into the bottom portions of the crust or, simply, the long-lasting inflow of the deep fluids are the most probable sources of this kind.     

3D AND 2½ D MODELLING OF GRAVITY ANOMALIES WITH VARIABLE DENSITY CONTRAST1

The decrease of density contrast in sedimentary basins may be approximated by a quadratic function. A sedimentary basin may be viewed as a number of prisms placed in juxtaposition. Equations in closed form for the gravity anomalies of 3D and 2½ D prismatic models are derived. Approximate equations for these models are also derived for rapid calculations. Efficient methods are developed for anomaly calculation by an appropriate use of the exact and approximate equations, and hence, for 3D and 2½ D modelling. The depths to the basement are adjusted iteratively by comparing the calculated anomalies with the observed anomalies. These methods are applied for analysis of the residual anomaly map of the Los Angeles Basin, California.     

Zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys

Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ⁶⁶Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ⁶⁶Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ⁶⁶Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ⁶⁶Zn of chimney minerals was lighter or similar to the fluid δ⁶⁶Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ⁶⁶Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ⁶⁶Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ⁶⁶Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes.     

Benthic foraminifera show some resilience to ocean acidification in the northern Gulf of California,Mexico

Extensive CO₂ vents have been discovered in the Wagner Basin, northern Gulf of California, where they create large areas with lowered seawater pH. Such areas are suitable for investigations of long-term biological effects of ocean acidification and effects of CO₂ leakage from subsea carbon capture storage. Here, we show responses of benthic foraminifera to seawater pH gradients at 74–207 m water depth. Living (rose Bengal stained) benthic foraminifera included Nonionella basispinata, Epistominella bradyana and Bulimina marginata. Studies on foraminifera at CO₂ vents in the Mediterranean and off Papua New Guinea have shown dramatic long-term effects of acidified seawater. We found living calcareous benthic foraminifera in low pH conditions in the northern Gulf of California, although there was an impoverished species assemblage and evidence of post-mortem test dissolution.     

The scientific connotation of oil and gas formations under deep fluids and organic-inorganic interaction

As a relatively stable craton block in the earth system, the petroliferous basin is influenced by the evolution of the earth system from the early development environment of source rocks, hydrocarbon formation, and reservoir dissolution to hydrocarbon accumulation or destruction. As a link between the internal and external factors of the basin, deep fluids run through the whole process of hydrocarbon formation and accumulation through organic-inorganic interaction. The nutrients carried by deep fluids promote the bloom of hydrocarbon-generating organisms and extra addition of carbon and hydrogen source, which are beneficial to the development of high-quality source rock and enhancement of the hydrocarbon generation potential. The energy carried by the deep fluid promotes the early maturation of the source rock and facilitates the hydrocarbon generation by activation and hydrogenation in high-mature hydrocarbon sources. The dissolution alteration of carbonate rocks and clastic reservoirs by CO_2-rich deep fluids improves the deep reservoir space, thus extending the oil and gas reservoir space into greater depth. The extraction of deeply retained crude oil by deep supercritical CO_2 and the displacement of CH_4 in shale have both improved the hydrocarbon fluidity in deep and tight reservoirs. Simultaneously, the energy and material carried by deep fluids(C, H, and catalytic substances) not only induce inorganic CH_4 formation by Fischer-Tropsch(F-T) synthesis and "hydrothermal petroleum" generation from organic matter by thermal activity but also cause the hydrothermal alteration of crude oil from organic sources. Therefore, from the perspective of the interaction of the earth's sphere, deep fluids not only input a significant amount of exogenous C and H into sedimentary basins but also improve the reservoir space for oil and gas, as well as their enrichment and accumulation efficiencies.     

Volcanic lake systematics II. Chemical constraints

A database of 373 lake water analyses from the published literature was compiled and used to explore the geochemical systematics of volcanic lakes. Binary correlations and principal component analysis indicate strong internal coherence among most chemical parameters. Compositional variations are influenced by the flux of magmatic volatiles and/or deep hydrothermal fluids. The chemistry of the fluid entering a lake may be dominated by a high-temperature volcanic gas component or by a lower-temperature fluid that has interacted extensively with volcanic rocks. Precipitation of minerals like gypsum and silica can strongly affect the concentrations of Ca and Si in some lakes. A much less concentrated geothermal input fluid provides the mineralized components of some more dilute lakes. Temporal variations in dilution and evaporation rates ultimately control absolute concentrations of dissolved constituents, but not conservative element ratios.Most volcanic lake waters, and presumably their deep hydrothermal fluid inputs, classify as immature acid fluids that have not equilibrated with common secondary silicates such as clays or zeolites. Many such fluids may have equilibrated with secondary minerals earlier in their history but were re-acidified by mixing with fresh volcanic fluids. We use the concept of ‘degree of neutralization’ as a new parameter to characterize these acid fluids. This leads to a classification of gas-dominated versus rock-dominated lake waters. A further classification is based on a cluster analysis and a hydrothermal speedometer concept which uses the degree of silica equilibration of a fluid during cooling and dilution to evaluate the rate of fluid equilibration in volcano-hydrothermal systems.     

226Ra and222Rn contents of Galapagos Rift hydrothermal waters —the importance of low-temperature interactions with crustal rocks

Hydrothermal waters collected by “Alvin” from the Galapagos Spreading Center are enriched in²²²Rn by factors of 50–200 over bottom waters. The²²⁶Ra in the same samples, however, is enriched by less than a factor of four over bottom waters. Enrichments of²²²Rn result primarily from α-recoil from rock surfaces while²²⁶Ra enrichments are dominantly produced by high-temperature alteration of cooling ridge volcanics. The abundances of both nuclides exhibit positive correlations with temperature. The data extrapolate to bottom water temperatures and compositions, demonstrating the importance of seawater mixing. Different vents, however, have different mixing lines, and vents with high²²²Rn have low²²⁶Ra. We propose these patterns result from variations in the extent of low-temperature crustal interaction with the hydrothermal fluids. Low-temperature crustal waters can maintain high steady state²²²Rn contents due to the α-recoil additions to the fluids. The²²⁶Ra, however, is strongly adsorbed at low-temperatures resulting in low concentrations of this nuclide in low-temperature crustal waters. Thus, physical mixing of a crustal water component with hydrothermal waters or variable crustal path lengths of the hydrothermal fluids can account for the variable mixing lines and²²²Rn/²²⁶Ra values of the hot springs.The²²²Rn/²²⁶Ra value appears to be a sensitive indicator of low-temperature crustal interaction. Values > 100 have experienced extensive crustal interaction and are indicative of diffuse hydrothermal flow. Values between 1 and 10 are indicative of primary hydrothermal fluids which have not experienced significant interaction with the crust. Values of²²²Rn/²²⁶Ra between 10³ and 10⁴ are indicative of interaction of the hydrothermal fluids with sediments. Such values are observed in water samples from the Galapagos hydrothermal mounds.     

Near bottom temperature anomalies in the Dead Sea

A bottom photographic and temperature study was carried out in the Dead Sea using a miniature version of the unmanned camera system ANGUS (mini-ANGUS). Due to the low transparency of the Dead Sea water, the bottom photographs provide very poor results. Only in a very few locations was the floor visible and in those cases it was found to be a white undulating sedimentary surface.The bottom temperature measurements, which were made continuously along the ship track, indicate the presence of a large zone of temperature anomalies. This zone is located in the deep part of the north basin at a water depth of over 330 m. The anomalies occur above a portion of an east-west fault which cuts through the Dead Sea suggesting the presence of hydrothermal activity.