Reiner Botz

Carbon isotope fractionation during bacterial methanogenesis by CO2 reduction

The carbon isotope fractionation between CO2 and CH4 was studied during open system (related to gas flow) culture experiments with CO2-reducing methanogenic Archaea. To study the temperature dependence of isotope fractionation during biological methanogenesis, three representatives of the order Methanococcales were cultivated in the temperature range 35-85°C. In the stationary growth phase, the carbon isotope fractionation factor between CO2 and CH4 was found to range between 1.048 and 1.079, depending on the growth temperature and on the type of fermentor. In contrast to published data derived from culture experiments, our results fall in the range of naturally occurring carbon isotope fractionations of coexisting CO2---CH4 pairs in marine sediments. Moreover, the fractionation closely approached the thermodynamic equilibrium between both gases, although thermal isotope exchange processes are unlikely to occur below 200°C. Our findings suggest that flow-through culture experiments are useful when studying biological methanogenesis and associated (carbon-) isotope fractionation as a means of deciphering complex methanogenic processes in sediments.

Oxygen isotopes of marine diatoms and relations to opal-A maturation

In order to develop the potential tool of diatom oxygen isotopes for paleoenvironmental studies we compared oxygen isotopes of natural marine diatoms sampled from ocean surface water, sediment traps and surface sediments with oxygen isotopic fractionations determined for laboratory diatom cultures. Freshly grown natural diatoms (phytoplankton samples and sediment trap material) and cultured diatoms reveal similar oxygen isotope fractionation factors. The fresh diatoms have 3 to 10 parts per thousand lower isotope fractionation factors than fossil (sedimentary) diatoms. A temperature-related oxygen isotope fractionation could not be established for the laboratory cultures (and the natural phytoplankton samples), and there is evidence that diatom growth rate until reaching the stationary growth state also controls the measured silica-water oxygen isotope fractionation factor. It is possible, however, that slow diatom growth in sea surface water may well lead to a temperature-dependent silica-water oxygen isotope fractionation which is the prerequisite for a use of diatom oxygen isotopes in palco-surface water studies. FTIR-spectroscopic analyses of various diatomaceous materials revealed that the ratio of integrated peak intensities for Si-O-Si/Si-OH correlates with the 3 to 10 parts per thousand delta O-18(silica) increase from fresh to fossil diatoms. Open-system (flow-through) silica dissolution experiments suggest that the diatom frustules are isotopically homogenous and that the increase in O-18 is therefore not due to dissolution of isotopically light surficial Si-OH groups. It is concluded that slow internal condensation reactions during silica maturation in surface sediments cause both an increase in the intensity ratio of Si-O-Si/Si-OH and the O-18 content of framework oxygen. These findings also indicate that the oxygen isotope compositions of marine sediment diatoms do not indicate sea surface water temperature but rather reflect variable O-18 contents of surface sediments. Copyright (C) 2001 Elsevier Science Ltd.

First observations of high-temperature submarine hydrothermal vents and massive anhydrite deposits off the north coast of Iceland

High-temperature (250°C) hydrothermal vents and massive anhydrite deposits have been found in a shallow water, sediment-filled graben near 66°36′N in the Tjornes Fracture Zone north of Iceland. The site is located about 30 km offshore, near the small island of Grimsey. The main vent field occurs at a depth of 400 m and consists of about 20 large-diameter (up to 10 m) mounds and 1–3 m chimneys and spires of anhydrite and talc. A north–south alignment of the mounds over a 1-km strike length of the valley floor suggests that their distribution is controlled by a buried fault. Widespread shimmering water and extensive white patches of anhydrite in the sediment between the mounds indicates that the entire 1-km2 area occupied by the vents is thermally active. A 2-man research submersible JAGO was used to map the area and to sample vent waters, gases, and chimneys. Actively boiling hydrothermal vents occur on most of the mounds, and extensive two-phase venting indicates that the field is underlain by a large boiling zone (200×300 m). The presence of boiling fluids in shallow aquifers beneath the deposits was confirmed by sediment coring. The highest-temperature pore fluids were encountered in talc- and anhydrite-rich sedimentary layers that occur up to 7 m below the mounds. Baked muds underlie the talc and anhydrite layers, and pyrite is common in stockwork-like fractures and veins in the hydrothermally altered sediments. However, massive sulfides (pyrite–marcasite crusts) were found in only one relict mound. Subseafloor boiling has likely affected the metal-carrying capacity of the hydrothermal fluids, and deposition of sulfides may be occurring at greater depth. Although the mounds and chimneys at Grimsey resemble other deposits at sedimented ridges (e.g. Middle Valley, Escanaba Trough, Guaymas Basin), the shallow water setting and extensive boiling of the hydrothermal fluids represent a distinctive new type of seafloor hydrothermal system.

OXYGEN ISOTOPES IN MARINE DIATOMS : A COMPARATIVE STUDY OF ANALYTICAL TECHNIQUES AND NEW RESULTS ON THE ISOTOPE COMPOSITION OF RECENT MARINE DIATOMS

Oxygen isotope analyses of marine diatoms were performed in two independent ways. Stepwise fluorination of hydrous opal-A results in plateau δ180 values representing the isotopic composition of the silica frame oxygen. A method of controlled isotope exchange before silica dehydration also produces reliable results, although the exchangeability of the silica was variable. Consequently, a calibration of the isotope exchange method using the results from stepwise fluorination experiments is very useful (and sometimes essential) in order to select a water vapor of an appropriate isotopic composition to be used for equilibration. Sediment diatom samples Ethmodiscus rex and Thalassiothrix longissima from the Antarctic and the North Atlantic Ocean, respectively, show strong 180 enrichments of 46.8 and 44.1‰, which are caused by large isotope fractionation occurring at the low temperature prevailing during silica-water isotope exchange reactions. However, phytoplankton samples from surface waters of the Norwegian-Greenland Sea and the Bellingshausen Sea (Antarctica) have δ180 values between 30.4 and 35.0‰. Thus, the true silica-water isotopic fractionation is approximately 3 to 10‰ lower than the temperature-dependent silica-water equilibrium published in the literature for sedimentary diatoms. Our results indicate that successive isotope exchange reactions of diatomaceous silica with ambient seawater and/or pore water determine the isotope values of diatoms in sediments.

HYDROTHERMAL GASES OFFSHORE MILOS ISLAND, GREECE

Hydrothermal fluids emerge from the seafloor of Paleohori Bay on Milos. The gases in these fluids contain mostly CO2 but CH4 concentrations up to 2% are present. The stable carbon isotopic composition of the CO2 (near 0%) indicates an inorganic carbon source (dissociation of underlying marine carbonates). The carbon and hydrogen isotopes of most CH4 samples are enriched in the heavy species ([delta]13C = -9.4 to -17.8[per mille sign]; [delta]D = -102 to -189[per mille sign]) which is believed to be characteristic for an abiogenic production of CH4 by CO2-reduction (Fischer-Tropsch reactions). Depletions in the deuterium content of three CH4 samples (to -377%) are probably caused by unknown subsurface rock alteration processes. Secondary hydrogen isotope exchange processes between methane, hydrogen and water are most likely responsible for calculated unrealistic methane formation temperatures. We show that excess helium, slightly enriched in 3He, is present in the hydrothermal fluids emerging the seafloor of Paleohori Bay. When the isotopic ratio of the excess component is calculated a 3He/4Heexcess of 3.6 · 10-6 is obtained: This indicates that the excess component consists of about one third of mantle helium and two thirds of radiogenic helium. We infer that the mantle-derived component has been strongly diluted by radiogenic helium during the ascent of the fluids to the surface.

Constraints on origin and evolution of Red Sea brines from helium and argon isotopes

Brines from three depressions along the axis of the Red Sea, the Atlantis II, the Discovery and the Kebrit Deep, were sampled and analyzed for helium and argon isotopes. We identified two principally different geochemical fingerprints that reflect the geological setting of the deeps. The Atlantis II and the Discovery brines originating from locations in the central Red Sea show 4 He concentrations up to 1.2U10 35 cm 3 STP g 31 and a 3 He/ 4 He ratio of 1.27U10 35 . The MORB-like 3 He/ 4 He ratio is typical of an active hydrothermal vent system and clearly indicates a mantle origin of the helium component within the brines. 40 Ar/ 36 Ar ratios are as high as 305 implying that mantle-derived 40 Ar excesses of up to 3% of the total argon concentration are present in the brines and transported along with the mantle helium signal. The mean ( 4 He/ 40 Ar)excess ratio of 2.1 is in agreement with the theoretical mantle production ratio. In the Kebrit Deep, located in the northern Red Sea, we found a helium excess of 5.7U10 37 cm 3 STP g 31 . The low 3 He/ 4 He ratio of 1U10 36 points to a predominantly radiogenic source of the helium excess with only a minor mantle contribution of approximately 9%. We propose a new scenario assuming that the Kebrit brine accumulates a diffusive helium flux that migrates from deeper sedimentary or crustal horizons. In contrast to the Atlantis II and Discovery Deep, the Kebrit brine shows no sign of an active hydrothermal input. fl 2001 Elsevier Science B.V. All rights reserved.

Methane in Red Sea brines

Concentrations of hydrocarbon gases and stable carbon isotope ratios of methane from the water column of Shaban, Kebrit, Atlantis II and Discovery deeps, Red Sea, have been determined. Methane concentrations (yield C1) range from < 50 nL/L (Red Sea deep water) to ca. 22 × 10−3 L/L (Kebrit brine). Stable carbon isotopes of methane are between −30 and 43‰. Hydrocarbon gases in the brines are originally of thermogenetic origin (Kebrit: C1/C2∼ 57; δ13C1 ∼ −30‰). Methane concentrations in the transition zones between brines and Red Sea deep water decreased, especially in the Atlantis II/Discovery deeps, associated with a strong shift of δ13C1 to positive values. This shift is related to bacterial oxidation of methane in the transition zone between brine layers and overlying Red Sea deep water. Oxidized methane mixes with Red Sea deep water methane. A connection between the Atlantis II and the Discovery brine is postulated on the basis of the geochemical data.

Origin of trace gases in submarine hydrothermal vents of the Kolbeinsey Ridge, north Iceland

Two hydrothermal fields of the Kolbeinsey Ridge area, north of Iceland, show vent gas characteristics which can be related to the subsurface conditions. Helium isotopes (R/R-air = 9.8, 10.9) indicate a mantle-derived origin and can be considered as a mixture of MORE helium and a deep-mantle plume helium component. The carbon isotope composition of CO2 ranges between -2.4 and -7.8 parts per thousand. The less negative delta(13)C-CO2 values were-found at Grimsey. The data from Grimsey are very similar to those previously published and regarded as being characteristic for the Icelandic magmatic source. However, small amounts of biogenic CO2 and/or subsurface calcite precipitation are responsible for the lighter isotope values of CO2 from Kolbeinsey. CH4/He-3 ratios which are higher than in MORB indicate an additional (sedimentary) methane source for Kolbeinsey and Grimsey hydrothermal gases. The presence of higher hydrocarbons up to butane, together with the carbon isotope values of methane (delta(13)C = -26.1 to -39.8 parts per thousand) suggest a probably high-mature organic source within thick sediments of the Tjornes Fracture Zone and smaller depressions on the west side of the Kolbeinsey Ridge crest. Geochemical characteristics of hydrocarbons present in KR hydrothermal fluids are, however, typical for a mixed (thermogenic and high-temperature hydrothermal, e.g. EPR-type) origin. Moreover, it is likely that secondary processes such as bacterial oxidation and thermal cracking determined the geochemical characteristics of the gases

Sub sea floor boiling of Red Sea Brines: New indication from noble gas data

Hydrothermal brines from the Atlantis II Deep, Red Sea, have been sampled in situ and analyzed for noble gases. The atmospheric noble gas concentrations (Ne, Aratm, Kr, Xe) in the deepest layer (LCL) are depleted by 20 to 30% relative to the initial concentrations in ambient Red Sea Deep Water without a systematic mass fractionation between the different noble gases. Sub surface boiling during the hydrothermal circulation and subsequent phase separation is shown to be a consistent explanation for the observed depletion pattern. Using a conceptual model of phase separation under sub-critical conditions, in which gases are partitioned according to Henrys Law, we reconstruct the fluid history before injection into the Atlantis II Deep: after having circulated through evaporites and young oceanic crust, where it becomes enriched in HeMORB and ArMORB, the ascending fluid boils, and the residual liquid becomes depleted in noble gas concentrations. The depleted fluid rises to the sediment surface and feeds the Atlantis II basin. The relatively low boiling degree of about 3% (i.e., the percentage of fluid removed as vapor) derived from the model indicates that the Atlantis II system represents an early stage of boiling with relatively small gas loss, in contrast to hydrothermal systems at sediment-free mid-ocean ridges.

Geology of Macdonald Seamount region, Austral Islands: Recent hotspot volcanism in the south Pacific

The southeastern extension of the Austral Islands volcanic chain terminates near 29°S, 140°W at the active Macdonald Seamount. The ‘hotspot’ region near Macdonald consists of at least five other volcanic edifices each more than 500 m high, included in an area about 50–100 km in diameter. On the basis of the sea-floor topography, the southeastern limit of the hotspot area is located about 20 km east of the base of Macdonald, where it is defined by the 3950 m isobath. At the edge of the hotspot area, there is a marked deepening of the seafloor from c.3900 m down to 4000–4300 m. The deeper sea-floor is faulted and heavily sedimented. The Macdonald volcano itself stands 3760 m above the surrounding seafloor, and has a basal diameter of 45 km. Its summit in January 1987 was 39 m below sea level, and it seems likely that Macdonald will emerge at the surface in the near future.Recent (March and November 1986) phreatic explosions on Macdonald Seamount erupted fragments of ultramafic and mafic plutonic blocks together with basic lapilli (volcaniclastic sand). The plutonic blocks have been variably altered and metamorphosed, and in some cases show signs of mineralisation (disseminated sulphides). The blocks presumably come from deeper levels in the volcanic system. The volcanics so far dredged from Macdonald consist of olivine and clinopyroxene cumulus-enriched basalts, evolved basalts, and mugearite. On the basis of incompatible element variations, simple crystal fractionation seems to be controlling the chemical evolution of Macdonald magmas.