Geological and Environmental Sciences

Authigenic carbonates from five continental margin locations, the Eel River Basin, Monterey Bay, Santa Barbara Basin, the Sea of Okhotsk, and the North Sea, exhibit a wide range of mineralogical and stable isotopic compositions. These precipitates include aragonite, low-and high-Mg ...

Authigenic carbonates from five continental margin locations, the Eel River Basin, Monterey Bay, Santa Barbara Basin, the Sea of Okhotsk, and the North Sea, exhibit a wide range of mineralogical and stable isotopic compositions. These precipitates include aragonite, low-and high-Mg calcite, and dolomite. The carbon isotopic composition of carbonates varies widely, ranging from À60% to +26%, indicating complex carbon sources that include 13 C-depleted microbial and thermogenic methane and residual, 13 C-enriched, bicarbonate. A similarly large variability of d 18 O values (À5.5% to +8.9%) demonstrates the geochemical complexity of these sites, with some samples pointing toward an 18 O-enriched oxygen source possibly related to advection of 18 O-enriched formation water or to the decomposition of gas hydrate. Samples depleted in 18 O are consistent with formation deeper in the sediment or mixing of pore fluids with meteoric water during carbonate precipitation. A wide range of isotopic and mineralogical variation in authigenic carbonate composition within individual study areas but common trends across multiple geographic areas suggest that these parameters alone are not indicative for certain tectonic or geochemical settings. Rather, the observed variations probably reflect local controls on the flux of carbon and other reduced ions, such as faults, fluid conduits, the presence or absence of gas hydrate in the sediment, and the temporal evolution of the local carbon reservoir. Areas with seafloor carbonates that indicate formation at greater depth below the sediment-water interface must have undergone uplift and erosion in the past or are still being uplifted. Consequently, the occurrence of carbonate slabs on the seafloor in areas of active hydrocarbon seepage is commonly an indicator of exhumation following carbonate precipitation in the shallow subsurface. Therefore, careful petrographic and geochemical analyses are critical components necessary for the correct interpretation of processes related to hydrocarbon seepage in continental margin environments and elsewhere.

Oxygen isotope ratios were obtained from authigenic clinoptilolites from Barbados Accretionary Complex, Yamato Basin, and Exmouth Plateau sediments (ODP Sites 672, 797, and 762) in order to investigate the isotopic fractionation between clinoptilolite and pore water at early diagenetic stages and low temperatures. Dehydrated clinoptilolites display isotopic ratios for the zeolite framework (Ž 18 O f) that extend from C18.7‰ to C32.8‰ (vs. SMOW). In combination with associated pore water isotope data, the oxygen isotopic fractionation between clinoptilolite and pore fluids could be assessed in the temperature range from 25ºC to 40ºC. The resulting fractionation factors of 1.032 at 25ºC and 1.027 at 40ºC are in good agreement with the theoretically determined oxygen isotope fractionation between clinoptilolite and water. Calculations of isotopic temperatures illustrate that clinoptilolite formation occurred at relatively low temperatures of 17ºC to 29ºC in Barbados Ridge sediments and at 33ºC to 62ºC in the Yamato Basin. These data support a low-temperature origin of clinoptilolite and contradict the assumption that elevated temperatures are the main controlling factor for authigenic clinoptilolite formation. Increasing clinoptilolite Ž 18 O f values with depth indicate that clinoptilolites which are now in the deeper parts of the zeolite-bearing intervals had either formed at lower temperatures (17-20ºC) or under closed system conditions.

Evidence of active and dormant fluid seepage in the Santa Barbara Basin is observed as active venting of gas and oil, bacterial mats, precipitates of authigenic carbonate, and mud and tar volcanoes. Fluid seepage occurs preferentially in the proximity to faults and faulted anticlines, and to slump scarps. Seepage next to faults and anticlines indicates that hydrocarbon migration and pore fluid expulsion is controlled structurally, with faults acting as preferred conduits for fluid flow across units of low matrix permeability.

Chemosynthetic biological communities and authigenic mineral formation are common along continental margins worldwide where they indicate the availability of reduced compounds on or immediately below the seafloor. Some of these sites, historically termed "cold seeps", are associated with obvious hydrocarbon venting (e.g., Gulf of Mexico, Santa Barbara Basin, and Eel River Basin), whereas fluid flow at other sites has been inferred by the presence of chemosynthetic communities and authigenic mineral deposits on the seafloor (e.g., Monterey Bay). Authigenic mineral deposits include carbonate-group minerals, most commonly aragonite, Mg-calcite, and dolomite, which form via microbially mediated anaerobic oxidation of methane. However, depending on the chemistry of the expelled fluids, minerals other than carbonate (i.e., barite) may form deposits on the seafloor. To develop a more comprehensive understanding of the complex geochemical, physical, and biological interactions at ...

Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.

A high-resolution elemental, isotopic, and molecular record of organic matter was established for the uppermost sediments of Hole 1229E (0-700 cm below seafloor), drilled during Ocean Drilling Program (ODP) Leg 201 on the Peruvian margin. Diagnostic parameters of organic matter (OM) such as the atomic C org over total nitrogen ratio (C/N), the organic carbon isotopic signatures (δ 13 C org), and ligninderived biomarker concentrations (Σ8) all suggest that terrigenous OM influx was more important during glacial than interglacial periods on the Peruvian shelf. In contrast to other shelf environments, however, fluctuations in lignin ratios ([Ad/Al]v, S/V, and C/V) across the Pleistocene-Holocene transition point to shifting composition in terrigenous OM sources from coarse and undegraded woody materials during the Last Glacial Maximum (LGM) to much more altered and finer soft tissue materials during the Holocene. In addition, the total OM content preserved in the Peruvian shelf sediments was lower during the LGM. This observation is in agreement with prior work in the region and may be explained by a migration of the upwelling cells toward deeper sections of the shelf during glacial lower sea level stances. The potential decrease in upwelling strength at ODP Site 1229 and other similar shallow sites of the region may thus have been responsible for lower water

Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sediments that overlay the Blake Ridge Diapir on the Carolina Rise (Ocean Drilling Program [ODP] Site 996). Active chemosynthetic communities at this site are apparently fed by fluid conduits extending beneath a bottom-simulating reflector (BSR). Gas hydrates occur at several depth intervals in these near-surface sediments. The carbonate nodules are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Authigenic aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The δ 13 C values of the authigenic aragonite vary between-48.4‰ and-30.5‰ (Peedee belemnite [PDB]), indicating that carbon derived from 13 C-depleted methane is incorporated into these carbonates. The δ 13 C of pore water ΣCO 2 values are most negative in the upper 10 mbsf, near the sediment/water interface (-38‰ ± 5‰), but noticeably more positive below 25 mbsf (+5‰ ± 6‰). Because carbonates derive their carbon from HCO 3-, dissimilarities between the δ 13 C values of carbonate precipitates recovered from greater than 10 mbsf and δ 13 C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1‰ in the oxygen isotopic composition of carbonate precipitates from different depths are possibly related to changes in bottom-water conditions during glacial and interglacial time periods. Measurements of the strontium isotopic composition on 13 carbonate samples show 87 Sr/ 86 Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87 Sr/ 86 Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87 Sr/ 86 Sr = 0.709175), and to 87 Sr/ 86 Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The 87 Sr/ 86 Sr values of the authigenic carbonates at Site 996 are not consistent with the Sr isotopic values predicted for carbonates precipitated from fluids transported upward along fault conduits extending through the base of the gas hydrate-stability zone. Based on our data, we see no evidence of continuing carbonate diagenesis with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Because these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 yr.

† In the publication of this article, first authorship is shared by D.H.C. and O.U.M. D.H.C. was the coordinating and principle author of the manuscript, performed iTAG preparation and analyses, and calculated and interpreted beta diversity analyses.

A high-resolution elemental, isotopic, and molecular record of or-ganic matter was established for the uppermost sediments of Hole 1229E (0–700 cm below seafloor), drilled during Ocean Drilling Pro-gram (ODP) Leg 201 on the Peruvian margin. Diagnostic parameters of organic matter (OM) such as the atomic C org over total nitrogen ratio (C/N), the organic carbon isotopic signatures (δ 13 C org), and lignin-derived biomarker concentrations (Σ8) all suggest that terrigenous OM influx was more important during glacial than interglacial periods on the Peruvian shelf. In contrast to other shelf environments, however, fluctuations in lignin ratios ([Ad/Al]v, S/V, and C/V) across the Pleis-tocene–Holocene transition point to shifting composition in terrige-nous OM sources from coarse and undegraded woody materials during the Last Glacial Maximum (LGM) to much more altered and finer soft tissue materials during the Holocene. In addition, the total OM content preserved in the Peruvian shelf se...

Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sediments that overlay the Blake Ridge Diapir on the Carolina Rise (Ocean Drilling Program [ODP] Site 996). Active chemosynthetic communities at this site are apparently fed by fluid conduits extending beneath a bottom-simulating reflector (BSR). Gas hydrates occur at several depth intervals in these near-surface sediments. The carbonate nodules are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Authigenic aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The δ 13 C values of the authigenic aragonite vary between-48.4‰ and-30.5‰ (Peedee belemnite [PDB]), indicating that carbon derived from 13 C-depleted methane is incorporated into these carbonates. The δ 13 C of pore water ΣCO 2 values are most negative in the upper 10 mbsf, near the sediment/water interface (-38‰ ± 5‰), but noticeably more positive below 25 mbsf (+5‰ ± 6‰). Because carbonates derive their carbon from HCO 3-, dissimilarities between the δ 13 C values of carbonate precipitates recovered from greater than 10 mbsf and δ 13 C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1‰ in the oxygen isotopic composition of carbonate precipitates from different depths are possibly related to changes in bottom-water conditions during glacial and interglacial time periods. Measurements of the strontium isotopic composition on 13 carbonate samples show 87 Sr/ 86 Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87 Sr/ 86 Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87 Sr/ 86 Sr = 0.709175), and to 87 Sr/ 86 Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The 87 Sr/ 86 Sr values of the authigenic carbonates at Site 996 are not consistent with the Sr isotopic values predicted for carbonates precipitated from fluids transported upward along fault conduits extending through the base of the gas hydrate-stability zone. Based on our data, we see no evidence of continuing carbonate diagenesis with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Because these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 yr.

Authigenic carbonates from five continental margin locations, the Eel River Basin, Monterey Bay, Santa Barbara Basin, the Sea of Okhotsk, and the North Sea, exhibit a wide range of mineralogical and stable isotopic compositions. These precipitates include aragonite, low-and high-Mg calcite, and dolomite. The carbon isotopic composition of carbonates varies widely, ranging from À60% to +26%, indicating complex carbon sources that include 13 C-depleted microbial and thermogenic methane and residual, 13 C-enriched, bicarbonate. A similarly large variability of d 18 O values (À5.5% to +8.9%) demonstrates the geochemical complexity of these sites, with some samples pointing toward an 18 O-enriched oxygen source possibly related to advection of 18 O-enriched formation water or to the decomposition of gas hydrate. Samples depleted in 18 O are consistent with formation deeper in the sediment or mixing of pore fluids with meteoric water during carbonate precipitation. A wide range of isotopic and mineralogical variation in authigenic carbonate composition within individual study areas but common trends across multiple geographic areas suggest that these parameters alone are not indicative for certain tectonic or geochemical settings. Rather, the observed variations probably reflect local controls on the flux of carbon and other reduced ions, such as faults, fluid conduits, the presence or absence of gas hydrate in the sediment, and the temporal evolution of the local carbon reservoir. Areas with seafloor carbonates that indicate formation at greater depth below the sediment-water interface must have undergone uplift and erosion in the past or are still being uplifted. Consequently, the occurrence of carbonate slabs on the seafloor in areas of active hydrocarbon seepage is commonly an indicator of exhumation following carbonate precipitation in the shallow subsurface. Therefore, careful petrographic and geochemical analyses are critical components necessary for the correct interpretation of processes related to hydrocarbon seepage in continental margin environments and elsewhere.

Offshore hydrocarbon accumulations in the Gulf of Mexico (GoM) are often accompanied by natural seepage of oil and gas from subsurface reservoirs into shallow sediments and the water column. This study investigated the temporal patterns and carbon-sulfur (C-S) coupling associated with authigenic carbonate samples recovered from surface sediments of a crude oil seepage site in southern GoM (Chapopote asphalt volcano, Bay of Campeche) using radioactive U-Th dates, and stable C, O, and S isotopes. The results were compared with data from multiple seep sites in the northern GoM where methane seepage is dominant along with non-methane hydrocarbons (ethane , propane, crude oil, etc.). U-Th age-dating of Chapopote seep carbonate samples yielded ages of 13.5 ka to 4.6 ka before present (BP), suggesting that Chapopote asphalt seepage has been ongoing for thousands of years. The results are also consistent with previous studies from the northern GoM that hypothesize that seeps along the GoM c...

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