Sulfur mineralogy and geochemistry of serpentinites and gabbros of the Atlantis Massif (IODP Site U1309)

In-situ uplifted portions of oceanic crust at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were drilled during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) and a 1.4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks recovered from Hole 1309D. These data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the Atlantis Massif, which hosts the Lost City Hydrothermal Field, 5 km to the south. The oceanic crust at the central dome is characterized by Ni-rich sulfides reflecting reducing conditions and limited seawater circulation. During uplift and exhumation, seawater interaction in gabbroic-dominated domains was limited, as indicated by homogeneous mantle-like sulfur contents and isotope compositions of gabbroic rocks and olivine-rich troctolites. Local variations from mantle compositions are related to magmatic variability or to interaction with seawater-derived fluids channeled along fault zones. The concomitant occurrence of mackinawite in olivine-rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150-200 °C for late circulating fluids along local brittle faults below 700 m depth. In contrast, the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry. The serpentinites in the upper part of the hole are characterized by high total sulfide contents, high δ³⁴S_sulfide values and low δ³⁴S_sulfate values, which reflect a multistage history primarily controlled by seawater-gabbro interaction and subsequent serpentinization. The basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the Lost City hydrothermal system. Our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time. Heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity.     

Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30°N): Insights from B and Sr isotope data

The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30°N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, O, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150-250 °C and at high integrated W/R ratios that led to a marked boron enrichment (34-91 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in δ¹¹B (11-16‰) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature-pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270-350 °C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater-gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of δ¹¹B values (9-10‰), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings.     

Carbon, oxygen, and strontium isotope geochemistry of carbonate rocks of the upper Miocene Kudankulam Formation, southern India: Implications for paleoenvironment and diagenesis

The carbon, oxygen, and strontium isotope compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaeoenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in ¹⁸O and ¹³C compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on δ¹³C and δ¹⁸O values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (δ¹³C from −7.8‰ to −6.0‰ and −9.0‰ to −7.0‰; δ¹⁸O from −9.2‰ to −6.5‰ and −9.4‰ to −2.6‰, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative δ¹⁸O values of various carbonate components (about −9.4‰ to −4.1‰ for whole rocks; about −8.4‰ to −2.6‰ for ooids) suggest that during the late Miocene the paleoclimate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (−7.9‰ to −3.6‰ for whole rocks; −4.9‰ to −1.5‰ for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C₄ type plants, supporting a scenario of expansion of C₄ plants during the late Miocene in the Indian subcontinent as far south as the southern tip of India. The ⁸⁷Sr/⁸⁶Sr ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic ⁸⁷Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene.     

Fluid–rock interaction during seprentinization of oceanic ultramafic rocks hosting the Lost City hydrothermal field, 30° N,MAR

The estimation of the fluid/rock (W/R) ratio during serpentinization on the basis of oxygen isotope characteristics is peculiar, because this process is accompanied by not only changes in the stoichiometric proportions of oxygen in fluid and rock, but also by the formation of associated minerals. These factors should be taken into account for environments when the volume of aqueous fluid is limited, for instance, for serpentinization of the deep-seated rocks of oceanic lithosphere under low spreading rates. We studied isotope characteristics of samples collected in dives of submersible MIR during Cruise 50 of the R/V Akademik Mstislav Keldysh along vertical profile on the southern slope of the Atlantis Massif, which hosts the Lost City hydrothermal field. Almost all studied serpentinites have homogenous strontium isotope composition corresponding to the composition of the modern seawater. Oxygen isotope composition of these serpentinites shows systematic variations from 2. 6 to 6.1‰ with sampling depth, which indicates the preservation of stratigraphic position of samples in the sequence of the Atlantis Massif and the global serpentinization of the entire plutonic sequence. The value of the fluid–rock ratio during serpentinization in a system closed to fluid was estimated using the dissolution–crystallization model. This model takes into account the variable stoichiometry of oxygen and the effect of the simultaneous crystallization of brucite on the oxygen isotope composition of newly formed serpentine. The results show that at moderately elevated temperatures (≈300°C) and 0.1 < W/R < 5, fluid, crystallizing serpentine, and brucite are characterized by sharp variations in oxygen isotope composition: 1.3–7.8, 2.5–8.9, and 4.5–1.9‰, respectively. The model explains the observed range of δ¹⁸O in the serpentinized harzburgites of the Atlantis Massif. According to our estimates, the rocks of the studied sequence of the Atlantis Massif were serpentinized at 270–350°C and W/R = 0.7–3. For lower temperature serpentinization, for instance, at T = 250°C, the W/R ratio can be as high as 6. The present-day serpentinization of the deepseated zones of the Atlantis Massif with the Lost City fluid participance proceeds at T > 270°C and W/R ratio <1. These conditions are similar to those of serpentinization of harzburgites from the lower parts of the studied sequence of the Atlantis Massif.     

Insights into C and H storage in the altered oceanic crust: Results from ODP/IODP Hole 1256D

Carbon and hydrogen concentrations and isotopic compositions were measured in 19 samples from altered oceanic crust cored in ODP/IODP Hole 1256D through lavas, dikes down to the gabbroic rocks. Bulk water content varies from 0.32 to 2.14 wt% with δD values from −64‰ to −25‰. All samples are enriched in water relative to fresh basalts. The δD values are interpreted in terms of mixing between magmatic water and another source that can be either secondary hydrous minerals and/or H contained in organic compounds such as hydrocarbons. Total CO₂, extracted by step-heating technique, ranges between 564 and 2823 ppm with δ¹³C values from −14.9‰ to −26.6‰. As for water, these altered samples are enriched in carbon relative to fresh basalts. The carbon isotope compositions are interpreted in terms of a mixing between two components: (1) a carbonate with δ¹³C = −4.5‰ and (2) an organic compound with δ¹³C = −26.6‰. A mixing model calculation indicates that, for most samples (17 of 19), more than 75% of the total C occurs as organic compounds while carbonates represent less than 25%. This result is also supported by independent estimates of carbonate content from CO₂ yield after H₃PO₄ attack. A comparison between the carbon concentration in our samples, seawater DIC (Dissolved Inorganic Carbon) and DOC (Dissolved Organic Carbon), and hydrothermal fluids suggests that CO₂ degassed from magmatic reservoirs is the main source of organic C addition to the crust during the alteration process. A reduction step of dissolved CO₂ is thus required, and can be either biologically mediated or not. Abiotic processes are necessary for the deeper part of the crust (>1000 mbsf) because alteration temperatures are greater than any hyperthermophilic living organism (i.e. T > 110 °C). Even if not required, we cannot rule out the contribution of microbial activity in the low-temperature alteration zones. We propose a two-step model for carbon cycling during crustal alteration: (1) when “fresh” oceanic crust forms at or close to ridge axis, alteration starts with hot hydrothermal fluids enriched in magmatic CO₂, leading to the formation of organic compounds during Fischer-Tropsch-type reactions; (2) when the crust moves away from the ridge axis, these interactions with hot hydrothermal fluids decrease and are replaced by seawater interactions with carbonate precipitation in fractures. Taking into account this organic carbon, we estimate C isotope composition of mean altered oceanic crust at ∼ −4.7‰, similar to the δ¹³C of the C degassed from the mantle at ridge axis, and discuss the global carbon budget. The total flux of C stored in the altered oceanic crust, as carbonate and organic compound, is 2.9 ± 0.4 × 10¹² molC/yr.     

Variscan and post-Variscan lead–zinc mineralization, Rhenish Massif, Germany: evidence for sulfide precipitation via thermochemical sulfate reduction

A mechanical decrepitation device coupled with a gas chromatograph has been used to characterize the molecular composition of gaseous and liquid hydrocarbons contained in minerals. Application of this technique allows the identification of low-molecular-weight n-alkanes and some aromatic hydrocarbons in sulfides and gangue minerals from epigenetic Variscan and post-Variscan lead–zinc deposits in the Rhenish Massif, Germany. Based on the analysis of 200 samples, Variscan and post-Variscan mineralization can be distinguished by the composition of associated hydrocarbons. Variscan sulfides and gangue minerals contain high abundancies of methane. In contrast, n-alkanes in the C₂–C₉ range and aromatic hydrocarbons (benzene, toluene) are dominant in post-Variscan mineralization. The absence of high-molecular-weight hydrocarbons in ore minerals suggests highly mature gas associated with hydrothermal activity, during which hydrothermal fluids caused an increase in thermal maturation of organic matter and the generation of low-molecular-weight hydrocarbons in the adjacent organic-rich rocks. The hydrocarbon compositions contained in fluid inclusions of Variscan and post-Variscan minerals are probably governed by the maturation level of the potential source rocks. In Variscan time tectonic brines (T > 175 °C) generated predominantly methane, whereas basement brines (T < 175 °C) expelled higher-molecular-weight hydrocarbons (wet gases, condensates, aromatic hydrocarbons) from adjacent rocks during the Mesozoic event. The specific role of hydrocarbons in sulfide precipitation via thermochemical sulfate reduction is indicated by geochemical characteristics of organic matter associated with the Plombières Pb–Zn deposit, in eastern Belgium. Intense alteration phenomena were observed in near-ore kerogens, compared with unaltered kerogens far from the ore body, as well as by a very high maturity (5.40% R_o), a systematic depletion in ¹²C towards the vein-type mineralization, high atomic S/C ratios (0.49), and by low atomic H/C ratios (0.29). The data suggest that hydrothermal solutions caused a drastic increase in the thermal maturation of organic matter within the adjacent wall rock. Increased thermal maturation resulted in increased δ¹³C-values of organic carbon due to the preferential release of ¹²C. The change in the organic matter to a H-depleted and S-enriched bulk composition in association with sulfide ores strongly suggests that thermochemical sulfate reduction was responsible for organic degradation. Thus, thermochemical sulfate reduction probably triggered base metal sulfide precipitation in Variscan and post-Variscan ore deposits of the Rhenish Massif. Finally, based on data from this study and previous investigations, new genetic models are presented for both Variscan and post-Variscan mineralization in the Rhenish Massif. Received: 15 September 1999 / Accepted: 2 December 1999     

Sulfur in peridotites and gabbros at Lost City (30°N, MAR): Implications for hydrothermal alteration and microbial activity during serpentinization

Variations in sulfur mineralogy and chemistry of serpentinized peridotites and gabbros beneath the Lost City Hydrothermal Field at the southern face of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were examined to better understand serpentinization and alteration processes and to study fluid fluxes, redox conditions, and the influence of microbial activity in this active, peridotite-hosted hydrothermal system. The serpentinized peridotites are characterized by low total sulfur contents and high bulk δ³⁴S values close to seawater composition. Low concentrations of ³⁴S-enriched sulfide phases and the predominance of sulfate with seawater-like δ³⁴S values indicate oxidation, loss of sulfide minerals and incorporation of seawater sulfate into the serpentinites. The predominance of pyrite in both serpentinites and gabbros indicates relatively high fO₂ conditions during progressive serpentinization and alteration, which likely result from high fluid fluxes during hydrothermal circulation and evolution of the Lost City system from temperatures of ∼250 to 150 °C. Sulfate and sulfide minerals in samples from near the base of hydrothermal carbonate towers at Lost City show δ³⁴S values that reflect the influence of microbial activity. Our study highlights the variations in sulfur chemistry of serpentinized peridotites in different marine environments and the influence of long-lived, moderate temperature peridotite-hosted hydrothermal system and high seawater fluxes on the global sulfur cycle.     

U-Th systematics and Th ages of carbonate chimneys at the Lost City Hydrothermal Field

The Lost City Hydrothermal Field (LCHF) is a serpentinite-hosted vent field located 15 km west of the spreading axis of the Mid-Atlantic Ridge. In this study, uranium-thorium (U-Th) geochronological techniques have been used to examine the U-Th systematics of hydrothermal fluids and the ²³⁰Th ages of hydrothermally-precipitated carbonate chimneys at the LCHF. Fluid sample analyses indicate that endmember fluids likely contain only 0.0073 ng/g U or less compared to 3.28 ± 0.03 ng/g of U in ambient seawater. For fluid samples containing only 2-21% ambient seawater (1.1-11 mmol/kg Mg), Th concentration is 0.11-0.13 pg/g and surrounding seawater concentrations average 0.133 ± 0.016 pg/g. The ²³⁰Th/²³²Th atomic ratios of the vent fluids range from 1 (±10) × 10⁻⁶ to 11 (±5) × 10⁻⁶, are less than those of seawater, and indicate that the vent fluids may contribute a minor amount of non-radiogenic ²³⁰Th to the LCHF carbonate chimney deposits. Chimney ²³⁸U concentrations range from 1 to 10 μg/g and the average chimney corrected initial δ²³⁴U is 147.2 ± 0.8, which is not significantly different from the ambient seawater value of 146.5 ± 0.6. Carbonate ²³²Th concentrations range broadly from 0.0038 ± 0.0003 to 125 ± 16 ng/g and ²³⁰Th/²³²Th atomic ratios vary from near seawater values of 43 (±8) × 10⁻⁶ up to 530 (±25) × 10⁻³. Chimney ages, corrected for initial ²³⁰Th, range from 17 ± 6 yrs to 120 ± 13 kyrs. The youngest chimneys are at the intersection of two active, steeply-dipping normal faults that cut the Atlantis Massif; the oldest chimneys are located in the southwest portion of the field. Vent deposits on a steep, fault-bounded wall on the east side of the field are all <4 kyrs old, indicating that mass wasting in this region is relatively recent. Comparison of results to prior age-dating investigations of submarine hydrothermal systems shows that the LCHF is the most long-lived hydrothermal system known to date. It is likely that seismic activity and active faulting within the Atlantis Massif and the Atlantis Fracture Zone, coupled with volumetric expansion of the underlying serpentinized host rocks play major roles in sustaining hydrothermal activity at this site. The longevity of venting at the LCHF may have implications for ecological succession of microorganisms within serpentinite-hosted vent environments.     

Paleomagnetic and rock magnetic results from lower crustal rocks of IODP Site U1309: Implication for thermal and accretion history of the Atlantis Massif

Integrated Ocean Drilling Program (IODP) Expeditions 304/305 recovered a total of 1.4 km sequence of lower crustal gabbroic and minor ultramafic rocks from the Atlantis Massif oceanic core complex on the western flank of the Mid Atlantic Ridge (MAR) at 30°N. We conducted an integrated paleomagnetic and rock magnetic study on this sequence to help address the interplay between magmatism and detachment faulting. Detailed thermal and alternating-field demagnetization results demonstrate that stable components of magnetization of mainly reversed polarity with unblocking temperatures below the Curie temperature of magnetite are retained in gabbroic rocks at IODP Site U1309. Several samples also contain multicomponent remanences of both normal and reversed polarities that were acquired over sharply defined blocking temperature intervals, providing evidence for localized reheating of some intervals during both normal and reversed polarity periods. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier rocks recovered at Site U1309D. The overall magnetic inclination of Hole U1309D is -35°, implying significant (up to ~ 50° counterclockwise, viewed to the north) rotation of the footwall around a horizontal axis parallel to the rift axis (010°) may have occurred. The tectonic rotations inferred by the paleomagnetic data suggest that the original fault orientation dipped relative steeply toward the spreading axis and subsequently rotated to a shallower angle. Coupled with the newly published U–Pb zircon ages for Hole U1309D rocks [Grimes, C.B., John, B.E., Wooden, J.L., 2008. Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from ²⁰⁶Pb/²³⁸U zircon ages from Atlantis Massif and IODP Hole 1309D, (30°N, MAR). Geochem. Geophys. Geosyst. 9, Q08012. doi:1029/2008GC002063], the new paleomagnetic data provide temporal and thermal constraints on the accretion history of the Atlantis Massif.     

Microbial carbon cycling in oligotrophic regional aquifers near the Tono Uranium Mine, Japan as inferred from δC and ΔC values of in situ phospholipid fatty acids and carbon sources

Microorganisms are ubiquitous in deep subsurface environments, but their role in the global carbon cycle is not well-understood. The natural abundance δ¹³C and Δ¹⁴C values of microbial membrane phospholipid fatty acids (PLFAs) were measured and used to assess the carbon sources of bacteria in sedimentary and granitic groundwaters sampled from three boreholes in the vicinity of the Tono Uranium Mine, Gifu, Japan. Sample storage experiments were performed and drill waters analyzed to characterize potential sources of microbial contamination. The most abundant PLFA structures in all waters sampled were 16:0, 16:1ω7c, cy17:0, and 18:1ω7c. A PLFA biomarker for type II methanotrophs, 18:1ω8c, comprised 3% and 18% of total PLFAs in anoxic sedimentary and granitic waters, respectively, sampled from the KNA-6 borehole. The presence of this biomarker was unexpected given that type II methanotrophs are considered obligate aerobes. However, a bacterium that grows aerobically with CH₄ as the sole energy source and which also produces 56% of its total PLFAs as 18:1ω8c was isolated from both waters, providing additional evidence for the presence of type II methanotrophs. The Δ¹⁴C values determined for type II methanotroph PLFAs in the sedimentary (−861‰) and granite (−867‰) waters were very similar to the Δ¹⁴C values of dissolved inorganic carbon (DIC) in each water (∼−850‰). This suggests that type II methanotrophs ultimately derive all their carbon from inorganic sources, whether directly from DIC and/or from CH₄ produced by the reduction of DIC. In contrast, δ¹³C values of type II PLFAs in the sedimentary (−93‰) and granite (−60‰) waters indicate that these organisms use different carbon assimilation schemes in each environment despite very similar δ¹³C_CH4 values (∼−95‰) for each water. The δ¹³C_PLFA values (−28‰ to −45‰) of non-methanotrophic bacteria in the KNA-6 LTL water do not clearly distinguish between heterotrophic and autotrophic metabolisms, but Δ¹⁴C_PLFA values indicate that >65% of total bacteria filtered from the KNA-6 LTL water are heterotrophs. Ancient Δ¹⁴C values (∼−1000‰) of some PLFAs suggest that many heterotrophs utilize ancient organic matter, perhaps from lignite seams within the sedimentary rocks. The more negative range of δ¹³C_PLFA values determined for the KNA-6 granitic water (−42‰ to −66‰) are likely the result of a microbial ecosystem dominated by chemolithoautotrophy, perhaps fuelled by abiogenic H₂. Results of sample storage experiments showed substantial shifts in microbial community composition and δ¹³C_PLFA values (as much as 5‰) during 2-4 days of dark, refrigerated, aseptic storage. However, water samples collected and immediately filtered back in the lab from freshly drilled MSB-2 borehole appeared to maintain the same relative relationships between δ¹³C_PLFA values for sedimentary and granitic host rocks as observed for samples directly filtered under artesian flow from the KNA-6 borehole of the Tono Uranium Mine.     

A chlorine isotope study of DSDP/ODP serpentinized ultramafic rocks: Insights into the serpentinization process

Eight DSDP/ODP cores were analyzed for major ion concentrations and δ³⁷Cl values of water-soluble chloride (δ³⁷Cl_WSC) and structurally bound chloride (δ³⁷Cl_SBC) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition.The average total Cl content of all 86 samples is 0.26 ± 0.16 wt.% (0.19 ± 0.10 wt.% as water-soluble Cl (X_WSC) and 0.09 ± 0.09 wt.% as structurally bound Cl (X_SBC)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl⁻ site and the water-soluble Cl⁻ site varies from − 1.08‰ to + 1.16‰, averaging to + 0.21‰. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk δ³⁷Cl values (+ 0.05‰ to + 0.36‰); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk δ³⁷Cl values (− 1.26‰ and − 0.54‰). The cores with negative δ³⁷Cl values also have variable Cl⁻ / SO₄² ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ∼1‰ with depth for both the water-soluble and structurally bound Cl fractions.Non-zero bulk δ³⁷Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive δ³⁷Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of ³⁷Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low δ³⁷Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative δ³⁷Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.     

Br/Cl ratios and O, H, C, and B isotopic constraints on the origin of saline waters from eastern Canada

Saline groundwaters were recovered from undisturbed (Restigouche deposit) and active (Brunswick #12 mine) Zn-Pb volcanogenic massive sulfide deposits in the Bathurst Mining Camp (BMC), northern New Brunswick, Canada. These groundwaters, along with fresh to brackish meteoric ground and surface waters from the BMC, have been analyzed to determine their major, trace element and stable isotopic (O, H, C, and B) compositions. Saline groundwaters (total dissolved solids = 22-45 g/L) are characterized by relatively high Na/Ca ratios compared to brines from the Canadian Shield and low Na/Cl_molar and δ¹¹B isotopic compositions (−2.5‰ to 11.1‰) compared to seawater. Although saline waters from the Canadian Shield commonly have oxygen and hydrogen isotopic compositions that plot to the left of the global meteoric water line, those from the BMC fall close to the water line. Fracture and vein carbonate minerals at the Restigouche deposit have restricted carbon isotopic compositions of around −5‰ to −6‰. The carbon isotopic compositions of the saline waters at the Restigouche deposit (+12‰ δ¹³C_DIC) are the result of fractionation of dissolved inorganic carbon by methanogenesis. We suggest that, unlike previous models for shield brines, the composition of saline waters in the BMC is best explained by prolonged water-rock reaction, with no requirement of precursor seawater. We suggest that elevated Br/Cl ratios of saline waters compared to seawater may be explained by differential uptake of Br and Cl during groundwater evolution through water-rock reaction.     

Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C₁/C₂₊ > 100), high C₂/C₃ and i-C₄/n-C₄ ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ¹³C_CH4 from −79.5‰ to −45.0‰, δ¹³C_C2H6 from −53.7‰ to −32.2‰, δ¹³C_C3H8 from −36.5‰ to −20.1‰, δ¹³C_nC4H10 from −32.7‰ to −24.5‰, and δ¹³C_CO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with ¹³C enriched methane and ¹³C depleted CO₂, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, ¹³C enriched methane and variable δ¹³C of CO₂, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with ¹³C depleted methane and ¹³C enriched CO₂, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain ¹³C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.     

Serpentinization of abyssal peridotites from the MARK area, Mid-Atlantic Ridge: sulfur geochemistry and reaction modeling

The opaque mineralogy and the contents and isotope compositions of sulfur in serpentinized peridotites from the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) area were examined to understand the conditions of serpentinization and evaluate this process as a sink for seawater sulfur. The serpentinites contain a sulfur-rich secondary mineral assemblage and have high sulfur contents (up to 1 wt.%) and elevated δ³⁴S_sulfide (3.7 to 12.7‰). Geochemical reaction modeling indicates that seawater-peridotite interaction at 300 to 400°C alone cannot account for both the high sulfur contents and high δ³⁴S_sulfide. These require a multistage reaction with leaching of sulfide from subjacent gabbro during higher temperature (∼400°C) reactions with seawater and subsequent deposition of sulfide during serpentinization of peridotite at ∼300°C. Serpentinization produces highly reducing conditions and significant amounts of H₂ and results in the partial reduction of seawater carbonate to methane. The latter is documented by formation of carbonate veins enriched in ¹³C (up to 4.5‰) at temperatures above 250°C. Although different processes produce variable sulfur isotope effects in other oceanic serpentinites, sulfur is consistently added to abyssal peridotites during serpentinization. Data for serpentinites drilled and dredged from oceanic crust and from ophiolites indicate that oceanic peridotites are a sink for up to 0.4 to 6.0 × 10¹² g seawater S yr⁻¹. This is comparable to sulfur exchange that occurs in hydrothermal systems in mafic oceanic crust at midocean ridges and on ridge flanks and amounts to 2 to 30% of the riverine sulfate source and sedimentary sulfide sink in the oceans. The high concentrations and modified isotope compositions of sulfur in serpentinites could be important for mantle metasomatism during subduction of crust generated at slow spreading rates.     

Fractionation of carbon isotopes in biosynthesis of fatty acids by a piezophilic bacterium Moritella japonica strain DSK1

We examined stable carbon isotope fractionation in biosynthesis of fatty acids of a piezophilic bacterium Moritella japonica strain DSK1. The bacterium was grown to stationary phase at pressures of 0.1, 10, 20, and 50 MPa in media prepared using sterile-filtered natural seawater supplied with glucose as the sole carbon source. Strain DSK1 synthesized typical bacterial fatty acids (C_14-19 saturated, monounsaturated, and cyclopropane fatty acids) as well as long-chain polyunsaturated fatty acids (PUFA) (20:6ω3). Bacterial cell biomass and individual fatty acids exhibited consistent pressure-dependent carbon isotope fractionations relative to glucose. The observed Δδ_FA-glucose (−1.0‰ to −11.9‰) at 0.1 MPa was comparable to or slightly higher than fractionations reported in surface bacteria. However, bulk biomass and fatty acids became more depleted in ¹³C with pressure. Average carbon isotope fractionation (Δδ_FA-glucose) at high pressures was much higher than that for surface bacteria: −15.7‰, −15.3‰, and −18.3‰ at 10, 20, and 50 MPa, respectively. PUFA were more ¹³C depleted than saturated and monounsaturated fatty acids at all pressures. The observed isotope effects may be ascribed to the kinetics of enzymatic reactions that are affected by hydrostatic pressure and to biosynthetic pathways that are different for short-chain and long-chain fatty acids. A simple quantitative calculation suggests that in situ piezophilic bacterial contribution of polyunsaturated fatty acids to marine sediments is nearly two orders of magnitude higher than that of marine phytoplankton and that the carbon isotope imprint of piezophilic bacteria can override that of surface phytoplankton. Our results have important implications for marine biogeochemistry. Depleted fatty acids reported in marine sediments and the water column may be derived simply from piezophilic bacteria resynthesis of organic matter, not from bacterial utilization of a ¹³C-depleted carbon source (i.e., methane). The interpretation of carbon isotope signatures of marine lipids must be based on principles derived from piezophilic bacteria.     

The isotopic composition of particulate organic carbon in mountain rivers of Taiwan

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ¹³C_org) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ¹³C_org that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ¹³C_org from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ¹³C_org and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ¹³C_org, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ¹³C_org is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC.In order to quantify the fraction of suspended load POC derived from non-fossil sources (F_nf) as well as the isotopic composition of fossil POC (δ¹³C_fossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ¹³C_org that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, F_nf reproduces the fraction modern (F_mod) in our samples, determined from ¹⁴C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean F_nf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ¹³C_fossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ¹³C_org found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ¹³C_org observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ¹³C_org of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.     

Diet and habitat of toxodont megaherbivores (Mammalia, Notoungulata) from the late Quaternary of South and Central America

The toxodont megaherbivores Toxodon and Mixotoxodon were endemic to South and Central America during the late Quaternary. Isotopic signatures of 47 toxodont teeth were analyzed to reconstruct diet and ancient habitat. Tooth enamel carbon isotope data from six regions of South and Central America indicate significant differences in toxodont diet and local vegetation during the late Quaternary. Toxodonts ranged ecologically from C₃ forest browsers in the Amazon (mean δ¹³C = −13.4‰), to mixed C₃ grazers and/or browsers living either in C₃ grasslands, or mixed C₃ forested and grassland habitats in Honduras (mean δ¹³C = −9.3‰), Buenos Aires province, Argentina (δ¹³C = −8.7‰), and Bahia, Brazil (mean δ¹³C = −8.6‰), to predominantly C₄ grazers in northern Argentina (δ¹³C = −4.4‰), to specialized C₄ grazers in the Chaco of Bolivia (δ¹³C = −0.1‰). Although these toxodonts had very high-crowned teeth classically interpreted for grazing, the isotopic data indicate that these megaherbivores had the evolutionary capacity to feed on a variety of dominant local vegetation. In the ancient Amazon region, carbon isotope data for the toxodonts indicate a C₃-based tropical rainforest habitat with no evidence for grasslands as would be predicted from the Neotropical forest refugia hypothesis.     

Carbon, sulfur, and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the Ediacaran Hankalchough glaciation

In order to understand spatial variations of stable isotope geochemistry in the Quruqtagh basin (northwestern China) in the aftermath of an Ediacaran glaciation, we analyzed carbonate carbon isotopes (δ¹³C_carb), carbonate oxygen isotopes (δ¹⁸O_carb), carbonate associated sulfate sulfur (δ³⁴S_CAS) and oxygen isotopes (δ¹⁸O_CAS), and pyrite sulfur isotopes (δ³⁴S_py) of a cap dolostone atop the Ediacaran Hankalchough glacial diamictite at four sections. The four studied sections (YKG, MK, H and ZBS) represent an onshore-offshore transect in the Quruqtagh basin. Our data show a strong paleobathymetry-dependent isotopic gradient. From the onshore to offshore sections, δ¹³C_carb values decrease from −2‰ to −16‰ (VPDB), whereas δ¹⁸O_carb values increase from −4‰ to −1‰ (VPDB). Both δ³⁴S_CAS and δ³⁴S_py show stratigraphic variations in the two onshore sections (MK and YKG), but are more stable in the two offshore sections (H and ZBS). δ¹⁸O_CAS values of onshore samples are consistent with terrestrial oxidative weathering of pyrite. We propose that following the Hankalchough glaciation seawater in the Quruqtagh basin was characterized by a strong isotopic gradient. The isotopic data may be interpreted using a three-component mixing model that involves three reservoirs: deep-basin water, surface water, and terrestrial weathering input. In this model, the negative δ¹³C_carb values in the offshore sections are related to the upwelling of deep-basin water (where anaerobic oxidation of dissolved organic carbon resulted in ¹³C-depleted DIC), whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite. The new data provide evidence for the oceanic oxidation following the Hankalchough glaciation.     

A compound-specific n-alkane δC and δD approach for assessing source and delivery processes of terrestrial organic matter within a forested watershed in northern Japan

We measured molecular distributions and compound-specific hydrogen (δD) and stable carbon isotopic ratios (δ¹³C) of mid- and long-chain n-alkanes in forest soils, wetland peats and lake sediments within the Dorokawa watershed, Hokkaido, Japan, to better understand sources and processes associate with delivery of terrestrial organic matter into the lake sediments. δ¹³C values of odd carbon numbered C₂₃-C₃₃n-alkanes ranged from −37.2‰ to −31.5‰, while δD values of these alkanes showed a large degree of variability that ranged from −244‰ to −180‰. Molecular distributions in combination with stable carbon isotopic compositions indicate a large contribution of C3 trees as the main source of n-alkanes in forested soils whereas n-alkanes in wetland soil are exclusively derived from marsh grass and/or moss. We found that the n-alkane δD values are much higher in forest soils than wetland peat. The higher δD values in forest samples could be explained by the enrichment of deuterium in leaf and soil waters due to increased evapotranspiration in the forest or differences in physiology of source plants between wetland and forest. A δ¹³C vs. δD diagram of n-alkanes among forest, wetland and lake samples showed that C₂₅-C₃₁n-alkanes deposited in lake sediments are mainly derived from tree leaves due to the preferential transport of the forest soil organic matter over the wetland or an increased contribution of atmospheric input of tree leaf wax in the offshore sites. This study demonstrates that compound-specific δD analysis provides a useful approach for better understanding source and transport of terrestrial biomarkers in a C3 plant-dominated catchment.     

Latitudinal distribution of terrestrial lipid biomarkers and n-alkane compound-specific stable carbon isotope ratios in the atmosphere over the western Pacific and Southern Ocean

We investigated the latitudinal changes in atmospheric transport of organic matter to the western Pacific and Southern Ocean (27.58°N-64.70°S). Molecular distributions of lipid compound classes (homologous series of C₁₅ to C₃₅n-alkanes, C₈ to C₃₄n-alkanoic acids, C₁₂ to C₃₀n-alkanols) and compound-specific stable isotopes (δ¹³C of C₂₉ and C₃₁n-alkanes) were measured in marine aerosol filter samples collected during a cruise by the R/V Hakuho Maru. The geographical source areas for each sample were estimated from air-mass back-trajectory computations. Concentrations of TC and lipid compound classes were several orders of magnitude lower than observations from urban sites in Asia. A stronger signature of terrestrial higher plant inputs was apparent in three samples collected under conditions of strong terrestrial winds. Unresolved complex mixtures (UCM) showed increasing values in the North Pacific, highlighting the influence of the plume of polluted air exported from East Asia. n-Alkane average chain length (ACL) distribution had two clusters, with samples showing a relation to latitude between 28°N and 47°S (highest ACL values in the tropics), whilst a subset of southern samples had anomalously high ACL values. Compound-specific carbon isotopic analysis of the C₂₉ (−25.6‰ to −34.5‰) and C₃₁n-alkanes (−28.3‰ to −37‰) revealed heavier δ¹³C values in the northern latitudes with a transition to lighter values in the Southern Ocean. By comparing the isotopic measurements with back-trajectory analysis it was generally possible to discriminate between different source areas. The terrestrial vegetation source for a subset of the southernmost Southern Ocean is enigmatic; the back-trajectories indicate eastern Antarctica as the only intercepted terrestrial source area. These samples may represent a southern hemisphere background of well mixed and very long range transported higher plant organic material.