Tectonic geomorphology and active strain of the Northern Apennines mountain front

The Northern Apennines (NA) chain is the expression of the active plate margin between Europe and Adria. Given the low convergence rates and the moderate seismic activity, ambiguities still occur in defining a seismotectonic framework and many different scenarios have been proposed for the mountain front evolution. Differently from older models that indicate the mountain front as an active thrust at the surface, a recently proposed scenario describes the latter as the frontal limb of a long-wavelength fold (> 150 km) formed by a thrust fault tipped around 17 km at depth, and considered as the active subduction boundary. East of Bologna, this frontal limb is remarkably very straight and its surface is riddled with small, but pervasive high- angle normal faults. However, west of Bologna, some recesses are visible along strike of the mountain front: these perturbations seem due to the presence of shorter wavelength (15 to 25 km along strike) structures showing both NE and NW-vergence. The Pleistocene activity of these structures was already suggested, but not quantitative reconstructions are available in literature. This research investigates the tectonic geomorphology of the NA mountain front with the specific aim to quantify active deformations and infer possible deep causes of both short- and long-wavelength structures. This study documents the presence of a network of active extensional faults, in the foothills south and east of Bologna. For these structures, the strain rate has been measured to find a constant throw-to-length relationship and the slip rates have been compared with measured rates of erosion. Fluvial geomorphology and quantitative analysis of the topography document in detail the active tectonics of two growing domal structures (Castelvetro - Vignola foothills and the Ghiardo plateau) embedded in the mountain front west of Bologna. Here, tilting and river incision rates (interpreted as that long-term uplift rates) have been measured respectively at the mountain front and in the Enza and Panaro valleys, using a well defined stratigraphy of Pleistocene to Holocene river terraces and alluvial fan deposits as growth strata, and seismic reflection profiles relationships. The geometry and uplift rates of the anticlines constrain a simple trishear fault propagation folding model that inverts for blind thrust ramp depth, dip, and slip. Topographic swath profiles and the steepness index of river longitudinal profiles that traverse the anti- clines are consistent with stratigraphy, structures, aquifer geometry, and seismic reflection profiles. Available focal mechanisms of earthquakes with magnitude between Mw 4.1 to 5.4, obtained from a dataset of the instrumental seismicity for the last 30 years, evidence a clear vertical separation at around 15 km between shallow extensional and deeper compressional hypocenters along the mountain front and adjacent foothills. In summary, the studied anticlines appear to grow at rates slower than the growing rate of the longer- wavelength structure that defines the mountain front of the NA. The domal structures show evidences of NW-verging deformation and reactivations of older (late Neogene) thrusts. The reconstructed river incision rates together with rates coming from several other rivers along a 250 km wide stretch of the NA mountain front and recently available in the literature, all indicate a general increase from Middle to Late Pleistocene. This suggests focusing of deformation along a deep structure, as confirmed by the deep compressional seismicity. The maximum rate is however not constant along the mountain front, but varies from 0.2 mm/yr in the west to more than 2.2 mm/yr in the eastern sector, suggesting a similar (eastward-increasing) trend of the apenninic subduction.

Active regulation of cerebral venous tone: simultaneous arterial and venous transcranial Doppler sonography during a Valsalva manoeuvre

The aim of this study was to analyse the cerebral venous outflow in relation to the arterial inflow during a Valsalva manoeuvre (VM). In 19 healthy volunteers (mean age 24.1 +/- 2.6 years), the middle cerebral artery (MCA) and the straight sinus (SRS) were insonated by transcranial Doppler sonography. Simultaneously the arterial blood pressure was recorded using a photoplethysmographic method. Two VM of 10 s length were performed per participant. Tracings of the variables were then transformed to equidistantly re-sampled data. Phases of the VM were analysed regarding the increase of the flow velocities and the latency to the peak. The typical four phases of the VM were also found in the SRS signal. The relative flow velocity (FV) increase was significantly higher in the SRS than in the MCA for all phases, particularly that of phase IV (p < 0.01). Comparison of the time latency of the VM phases of the MCA and SRS only showed a significant difference for phase I (p < 0.01). In particular, there was no significant difference for phase IV (15.8 +/- 0.29 vs. 16.0 +/- 0.28 s). Alterations in venous outflow in phase I are best explained by a cross-sectional change of the lumen of the SRS, while phases II and III are compatible with a Starling resistor. However, the significantly lager venous than the arterial overshoot in phase IV may be explained by the active regulation of the venous tone.

ANKLE IMPEDANCE AND ANKLE ANGLES DURING STEP TURN AND STRAIGHT WALK: IMPLICATIONS FOR THE DESIGN OF A STEERABLE ANKLE-FOOT PROSTHETIC ROBOT

During locomotion, turning is a common and recurring event which is largely neglected in the current state-of-the-art ankle-foot prostheses, forcing amputees to use different steering mechanisms for turning, compared to non-amputees. A better understanding of the complexities surrounding lower limb prostheses will lead to increased health and well-being of amputees. The aim of this research is to develop a steerable ankle-foot prosthesis that mimics the human ankle mechanical properties. Experiments were developed to estimate the mechanical impedance of the ankle and the ankles angles during straight walk and step turn. Next, this information was used in the design of a prototype, powered steerable ankle-foot prosthesis with two controllable degrees of freedom. One of the possible approaches in design of the prosthetic robots is to use the human joints’ parameters, especially their impedance. A series of experiments were conducted to estimate the stochastic mechanical impedance of the human ankle when muscles were fully relaxed and co-contracting antagonistically. A rehabilitation robot for the ankle, Anklebot, was employed to provide torque perturbations to the ankle. The experiments were performed in two different configurations, one with relaxed muscles, and one with 10% of maximum voluntary contraction (MVC). Surface electromyography (sEMG) was used to monitor muscle activation levels and these sEMG signals were displayed to subjects who attempted to maintain them constant. Time histories of ankle torques and angles in the lateral/medial (LM) directions, inversion-eversion (IE), and dorsiflexionplantarflexion (DP) were recorded. Linear time-invariant transfer functions between the measured torques and angles were estimated providing an estimate of ankle mechanical impedance. High coherence was observed over a frequency range up to 30 Hz. The main effect of muscle activation was to increase the magnitude of ankle mechanical impedance in all degrees of freedom of the ankle. Another experiment compared the three-dimensional angles of the ankle during step turn and straight walking. These angles were measured to be used for developing the control strategy of the ankle-foot prosthesis. An infrared camera system was used to track the trajectories and angles of the foot and leg. The combined phases of heel strike and loading response, mid stance, and terminal stance and pre-swing were determined and used to measure the average angles at each combined phase. The Range of motion (ROM) in IE increased during turning while ML rotation decreased and DP changed the least. During the turning step, ankle displacement in DP started with similar angles to straight walk and progressively showed less plantarflexion. In IE, the ankle showed increased inversion leaning the body toward the inside of the turn. ML rotation initiated with an increased medial rotation during the step turn relative to the straight walk transitioning to increased lateral rotation at the toe off. A prototype ankle-foot prosthesis capable of controlling both DP and IE using a cable driven mechanism was developed and assessed as part of a feasibility study. The design is capable of reproducing the angles required for straight walk and step turn; generates 712N of lifting force in plantarflexion, and shows passive stiffness comparable to a nonload bearing ankle impedance. To evaluate the performance of the ankle-foot prosthesis, a circular treadmill was developed to mimic human gait during steering. Preliminary results show that the device can appropriately simulate human gait with loading and unloading the ankle joint during the gait in circular paths.

Quantification of Popliteal Artery Deformation During Leg Flexion in Subjects With Peripheral Artery Disease: A Pilot Study

Purpose: To quantify the in vivo deformations of the popliteal artery during leg flexion in subjects with clinically relevant peripheral artery disease (PAD). Methods: Five patients (4 men; mean age 69 years, range 56–79) with varying calcification levels of the popliteal artery undergoing endovascular revascularization underwent 3-dimensional (3D) rotational angiography. Image acquisition was performed with the leg straight and with a flexion of 70°/20° in the knee/hip joints. The arterial centerline and the corresponding branches in both positions were segmented to create 3D reconstructions of the arterial trees. Axial deformation, twisting, and curvatures were quantified. Furthermore, the relationships between the calcification levels and the deformations were investigated. Results: An average shortening of 5.9%±2.5% and twist rate of 3.8±2.2°/cm in the popliteal artery were observed. Maximal curvatures in the straight and flexed positions were 0.12±0.04 cm−1 and 0.24±0.09 cm−1, respectively. As the severity of calcification increased, the maximal curvature in the straight position increased from 0.08 to 0.17 cm−1, while an increase from 0.17 to 0.39 cm−1 was observed for the flexed position. Axial elongations and arterial twisting were not affected by the calcification levels. Conclusion: The popliteal artery of patients with symptomatic PAD is exposed to significant deformations during flexion of the knee joint. The severity of calcification directly affects curvature, but not arterial length or twisting angles. This pilot study also showed the ability of rotational angiography to quantify the 3D deformations of the popliteal artery in patients with various levels of calcification.

(Table 1) Sr, Nd, and Pb isotopic composition of ODP Leg 129 basalts

Basaltic rocks recovered from three drill sites in the western Pacific during Ocean Drilling Program Leg 129 have fairly distinct Sr, Nd, and Pb isotopic compositions. The Cretaceous alkali olivine dolerites from Site 800 in the northern part of Pigafetta Basin have fairly low 87Sr/86Sri (0.70292-0.70320) and 143Nd/144Ndi (0.51277-0.51281) and high present-day Pb isotopic ratios (206Pb/204Pb = 20.53-21.45; 207Pb/204Pb = 15.70-15.77; 208Pb/204Pb = 40.02-40.68). The Middle Jurassic tholeiites from Site 801 in the southern part of the basin have low 87Sr/86Sri (0.70237-0.70248), high 143Nd/144Ndi (0.51298-0.51322), and moderate present-day Pb isotopic ratios (206Pb/204Pb = 18.20-19.12; 207Pb/204Pb = 15.47-15.60; 208Pb/204Pb = 37.56-38.18); isotopic compositions of the alkali olivine basalts overlying the tholeiites fall between those of the tholeiites and Site 800 dolerites. The Cretaceous tholeiites from Site 802 in the East Mariana Basin have high 87Sr/86Sri (0.70360-0.70372), fairly low 143Nd/144Ndi (0.51277-0.51280), and fairly low and homogeneous present-day Pb isotopic ratios (206Pb/204Pb = 18.37-18.39; 207Pb/204Pb = 15.49-15.51; 208Pb/204Pb = 38.34-38.39). Isotopic compositions of Site 801 tholeiites are indistinguishable from those of modern mid-ocean ridge basalts, consistent with the proposal that these tholeiites are a part of the oldest Pacific crust. The diverse isotopic compositions of the younger basalts appear to be the result of Jurassic Pacific plate migration over the geologically anomalous south-central Pacific region, wherein they acquired their distinct isotopic compositions. The anomalous region was volcanically more active during the Cretaceous than at present.

Chlorine stable isotopic composition of basement fluids from ODP Leg 168 sites

This paper presents chlorine stable isotope compositions (delta37Cl) of sediment pore waters collected by squeezing sediment cores from the sediment-basement interface along an East-West transect through the eastern flank of the Juan de Fuca Ridge (ODP Leg 168). These "near basement fluids" (NBF) are generally thought to be representative of low-temperature fluids circulating in the off-axis basaltic crust. The delta37Cl value of the fluid directly sampled from a flow at the base of Site 1026 (WSTP1026) is also reported. NBF display delta37Cl values between -2.09? and -0.12? relative to the Standard Mean Ocean Chloride (SMOC defined as 0?) and small variations in chlorinity (~4%). These data contrast with the homogeneity of delta37Cl values associated with highly variable chlorinities observed in high-temperature on-axis fluids [Bonifacie et al., 2005, doi:10.1016/j.chemgeo.2005.06.008]. The NBF delta37Cl values show a general decreasing trend with distance from the ridge-axis except for two fluids. When plotted against delta18O values, the delta37Cl of the NBF show two different trends. This paper discusses the possible contributions on NBF delta37Cl values of fluid-mixing, water-rock interactions and transport processes (diffusion, ion membrane filtration) that can occur in the igneous basement. However, as none of these processes can fully explain the observed delta37Cl variations, the potential effect of the sediment cover is also investigated. At site 1026, the interstitial pore fluid displays a delta37Cl signature significantly lower than that of the fluid discharge sample (-1.90? and -0.28?, respectively). This difference, demonstrated here cannot be an artifact of the sampling method, rather indicates the influence of the sediment cover on NBF delta37Cl values. The potential contributions of physical processes associated with transport/compaction (e.g., diffusion, ion membrane filtration, adsorption, ion exchange) on NBF delta37Cl values are qualitatively discussed here but require additional studies for further insights. However, this study indicates that "near basement fluids" (NBF) are not, at least for Cl isotopic compositions, necessarily as representative of fluids circulating in the basaltic crust as initially thought. These results add new constraints on Cl geodynamics and show that Cl-isotopes fractionate during low-temperature circulation of fluids in off-axis and off-margin flow contexts, but not to the extent observed for active margins. Fluids circulating at low-temperature in the magmatic and/or the sedimentary part of the oceanic crust might have played a major role on the delta37Cl evolution of seawater over geologic time.

Isotope ratios and trace element concentrations of basalts from DSDP Leg 15 sites (Appendix 1)

Formation of the Cretaceous Caribbean plateau, including the komatiites of Gorgona, has been linked to the currently active Galápagos hotspot. We use Hf-Nd isotopes and trace element data to characterise both the Caribbean plateau and the Galápagos hotspot, and to investigate the relationship between them. Four geochemical components are identified in the Galápagos mantle plume: two 'enriched' components with epsilon-Hf and epsilon-Nd similar to enriched components observed in other mantle plumes, one moderately enriched component with high Nb/Y, and a fourth component which most likely represents depleted MORB source mantle. The Caribbean plateau basalt data form a linear array in Hf-Nd isotope space, consistent with mixing between two mantle components. Combined Hf-Nd-Pb-Sr-He isotope and trace element data from this study and the literature suggest that the more enriched Caribbean end member corresponds to one or both of the enriched components identified on Galápagos. Likewise, the depleted end member of the array is geochemically indistinguishable from MORB and corresponds to the depleted component of the Galápagos system. Enriched basalts from Gorgona partially overlap with the Caribbean plateau array in epsilon-Hf vs. epsilon-Nd, whereas depleted basalts, picrites and komatiites from Gorgona have a high epsilon-Hf for a given epsilon-Nd, defining a high-epsilon-Hf depleted end member that is not observed elsewhere within the Caribbean plateau sequences. This component is similar, however, in terms of Hf-Nd-Pb-He isotopes and trace elements to the depleted plume component recognised in basalts from Iceland and along the Reykjanes Ridge. We suggest that the Caribbean plateau represents the initial outpourings of the ancestral Galápagos plume. Absence of a moderately enriched, high Nb/Y component in the older Caribbean plateau (but found today on the island of Floreana) is either due to changing source compositions of the plume over its 90 Ma history, or is an artifact of limited sampling. The high-epsilon-Hf depleted component sampled by the Gorgona komatiites and depleted basalts is unique to Gorgona and is not found in the Caribbean plateau. This may be an indication of the scale of heterogeneity of the Caribbean plateau system; alternatively Gorgona may represent a separate oceanic plateau derived from a completely different Pacific plume, such as the Sala y Gomez.

Mineralogy of serpentine muds of ODP Leg 125 holes (Table 1)

Large serpentinite seamounts are common in the forearc regions between the trench axis and the active volcanic fronts of the Mariana and Izu-Bonin intraoceanic arcs. The seamounts apparently form both as mud volcanoes, composed of unconsolidated serpentine mud flows that have entrained metamorphosed ultramafic and mafic rocks, and as horst blocks, possibly diapirically emplaced, of serpentinized ultramafics partially draped with unconsolidated serpentine slump deposits and mud flows. The clayand silt-sized serpentine recovered from three sites on Conical Seamount on the Mariana forearc region and from two sites on Torishima Forearc Seamount on the Izu-Bonin forearc region is composed predominantly of chrysotile, brucite, chlorite, and clays. A variety of accessory minerals attest to the presence of unusual pore fluids in some of the samples. Aragonite, unstable at the depths at which the serpentine deposits were drilled, is present in many of the surficial cores from Conical Seamount. Sjogrenite minerals, commonly found as weathering products of serpentine resulting from interaction with groundwater, are found in most of the samples. The presence of aragonite and carbonate-hydroxide hydrate minerals argues for interaction of the serpentine deposits with fluids other than seawater. There are numerous examples of sedimentary serpentinite deposits exposed on land that are very similar to the deposits recovered from the serpentine seamounts drilled on ODP Leg 125. We suggest that Conical Seamount may be a type locality for the study of in situ formation of many of these sedimentary serpentinite bodies. Further, we suggest that both the deposits drilled on Conical Seamount and on Torishima Forearc Seamount demonstrate that serpentinization can continue in situ within the seamounts through interaction of the serpentine deposits with both seawater and subduction-related fluids.

Major and minor elements in hydrothermal and pelagic sediments at DSDP Leg 70 Holes

Interaction between young basaltic crust and seawater near the oceanic speading centers is one of the important processes affecting the chemical composition of the oceanic layer. The formation of metalliferous hydrothermal sediments results from this interaction. The importance of the interaction between seawater and basalt in determining the chemical composition of pore waters from sediments is well known. The influence of mineral solutions derived from this interaction on ocean water composition and the significant flux of some elements (e.g., Mn) are reported by Lyle (1976), Bogdanov et al. (1979), and others. Metal-rich sediments found in active zones of the ocean basins illustrate the influence of seawater-basalt interaction and its effect on the sedimentary cover in such areas. The role of hydrothermal activity and seawater circulation in basalts with regard to global geochemistry cycles has recently been demonstrated by Edmond, Measures, McDuff, McDuff et al. (1979), and Edmond, Measures, Mangum (1979). In the area of the Galapagos Spreading Center the interaction of sediments and solutions derived from interaction of seawater and basalt has resulted in the formation of hydrothermal mounds. The mounds are composed of manganese crusts and green clay interbedded and mixed with pelagic nannofossil ooze. These mounds are observed only in areas characterized by high heat flow (Honnorez, et al., 1981) and high hydrothermal activity.

Geochemistry of pore water and sediments from ODP Leg 136 sites

During Leg 136 drilling was conducted at two sites in pelagic sediments of the north central Pacific Ocean. In this report, pore-water analyses for major seawater constituents, alkalinity, ammonia, nitrate, phosphate, silica, Ba, Fe, Li, Mn, and Sr are presented. Although concentration gradients are generally weak, resulting from slow sedimentation and concomitant diffusive communication with overlying water, there is evidence of sediment/pore-water interactions, associated sediment diagenesis, and formation of authigenic minerals. Bulk major and trace element compositions of the sediments are consistent with reactions inferred to occur within the sediments and with the lithology and mineralogy. Elemental compositions of the sediments are not strongly affected by diagenesis and are primarily related to the dominant mineralogy. Sediments are typical of deep ocean pelagic settings with a significant contribution from the alteration of volcanic ash and the formation of zeolites. Sedimentary rare earth element patterns also provide evidence of active scavenging processes by Mn and Fe oxide phases in the deeper sediments at Site 842.

Radiochemistry at DSDP Leg 54 and 70 Holes

Use of the hydraulic piston corer during DSDP Leg 70 in the Galapagos mounds area allowed recovery of an undisturbed sedimentary sequence down to the basement. It thus became possible to establish the chronology of different events. Several holes on and off the mounds were studied, using uranium series disequilibrium methods of age determination and oxygen isotope stratigraphy. The following sequence was thereby established: 1) From 600,000 to 300,000 years ago there was normal pelagic sedimentation, with an injection of uranium-rich solution, probably of hydrothermal origin, between 400,000 and 300,000 years ago. 2) From 300,000 to 90,000 years ago, nontronitic clay formed, replacing a pre-existing sediment. 3) From 60,000 to 20,000 years ago, manganese oxide deposits formed, probably also replacing pre-existing sediments. 4) About 19,000 years ago there occurred a uranium injection from seawater, attributed to the end of the hydrothermal circulation. In some holes, especially Hole 424, Leg 54, younger manganese oxides have been found, indicating that some mounds may be presently active.

Ultimate and failure strengths of normal muds and serpentinite muds of ODP Leg 125 samples

Serpentinite seamounts in the Mariana forearc have been explained as diapirs rising from the Benioff zone. This hypothesis predicts that the serpentinites should have low strengths as well as low densities relative to the surrounding rocks. Drilling during Leg 125 showed that the materials forming Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc are water-charged serpentinite muds of density <2 g/cm**3. Wykeham-Farrance torsion-vane tests showed that they are plastic solids with a rheology that bears many similarities to the idealized Cam clay soil model and is well described by critical-state soil mechanics. The serpentinite muds have ultimate strengths of 1.3 to 273.7 kPa and yield strengths of approximately 1.0 to 50 kPa. These muds thus are orders of magnitude weaker than salt and are, in fact, comparable in density and strength to common deep-sea clay muds. Such weak and low-density materials easily become diapiric. Serpentinite muds from the summit of Conical Seamount are weaker and more ductile than those on its flanks or on Torishima Forearc Seamount. Moreover, the summit muds do not contain the pronounced pure- and simple-shear fabrics that characterize those on the seamount flanks. The seamounts are morphologically similar to shield volcanoes, and anastomosing serpentinite debris flows descending from their summits are similar in map view to pahoehoe flows. These morphologic features, together with the physical properties of the muds and their similarities to other oceanic muds and the geochemistry of the entrained waters, suggest that many forearc serpentinite seamounts are gigantic (10-20 km wide, 1.5-2.0 km high) mud volcanoes that formed by the eruption of highly liquid serpentinite muds. Torishima Forearc Seamount, which is blanketed by more ìnormalî pelagic/volcaniclastic sediment, has probably been inactive since the Miocene. Conical Seamount, which seems to consist entirely of serpentinite mud and is venting fresh water of unusual chemistry from its summit, is presently active. Muds from the flanks of Conical Seamount are stronger and more brittle than those from the summit site, and muds from Torishima Forearc Seamount are stronger yet; this suggests that the serpentinite debris flows are compacted and dewatered as they mature. The shear fabrics probably result from downslope creep and flow, but may also be inherited.

Igneous geochemistry of OP Leg 126 samples

Major element, trace element, and radiogenic isotope compositions of samples collected from Ocean Drilling Program Leg 126 in the Izu-Bonin forearc basin are presented. Lavas from the center of the basin (Site 793) are high-MgO, low-Ti, two-pyroxene basaltic andesites, and represent the products of synrift volcanism in the forearc region. These synrift lavas share many of the geochemical and petrographic characteristics of boninites. In terms of their element abundances, ratios, and isotope systematics they are intermediate between low-Ti arc tholeiites from the active arc and boninites of the outer-arc high. These features suggest a systematic geochemical gradation between volcanics related to trench distance and a variably depleted source. A basement high drilled on the western flank of the basin (Site 792) comprises a series of plagioclase-rich two-pyroxene andesites with calc-alkaline affinities. These lavas are similar to calc-alkaline volcanics from Japan, but have lower contents of Ti, Zr, and low-field-strength elements (LFSE). Lavas from Site 793 show inter-element variations between Zr, Ti, Sr, Ni, and Cr that are consistent with those predicted during crystallization and melting processes. In comparison, concentrations of P, Y, LFSE, and the rare-earth elements (REE) are anomalous. These elements have been redistributed within the lava pile, concentrating particularly in sections of massive and pillowed flows. Relative movement of these two-element groupings can be related to the alteration of interstitial basaltic andesite glass to a clay mineral assemblage by a post-eruptive process. Fluid-rock interaction has produced similar effects in the basement lavas of Site 792. In this sequence, andesites and dacites have undergone a volume change related to silica mobility. As a result of this process, some lithologies have the major element characteristics of basaltic andesite and rhyolite, but can be related to andesitic or dacitic precursors by silica removal or addition.

(Table T2) Sulfur isotopes of pore water from ODP Leg 201 sites

Fifty-seven interstitial water samples from six sites (Ocean Drilling Program Sites 1225-1229 and 1231) in the eastern equatorial Pacific Ocean and the Peru margin were analyzed for the stable sulfur isotopic composition (34S/32S) of dissolved sulfate along with major and minor ions. With the exception of Site 1231, sulfate from the interstitial fluids (d34S values as much as 89 per mil vs. the SF6-based Vienna-Canyon Diablo troilite standard) is found at depth to be enriched in 34S with respect to modern seawater sulfate (d34S = ~21 per mil), indicating that microbial sulfate reduction (MSR) took place to different extents at all investigated sites. Deeper sediments at Sites 1228 and 1229 are additionally influenced by diffusion of a sulfate-rich brine that has already undergone sulfate reduction. The intensity of MSR depends on the availability of substrate (organic matter), sedimentation conditions, and the active bacterial community structure. Formation of isotopically heavy diagenetic barite at the sulfate-methane transition zone is expected at Sites 1227 (one front), 1229 (two fronts), and probably Site 1228. At Site 1231, the constant sulfur isotopic composition of sulfate and concentrations of minor pore water ions indicate that suboxic (essentially iron and manganese oxide based) diagenesis dominates and no net MSR occurs.

Geochemical and isotopic composition of sediments and pore waters from ODP Leg 164 sites

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 d13C values of the authigenic aragonite vary between -48.4 per mil and -30.5 per mil (Peedee belemnite [PDB]), indicating that carbon derived from 13C-depleted methane is incorporated into these carbonates. The d13C of pore water sum CO2 values are most negative in the upper 10 mbsf, near the sediment/water interface (-38 per mil ± 5 per mil), but noticeably more positive below 25 mbsf (+5 per mil ± 6 per mil). Because carbonates derive their carbon from HCO3-, dissimilarities between the d13C values of carbonate precipitates recovered from greater than 10 mbsf and d13C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1 per mil 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 87Sr/86Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87Sr/86Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87Sr/86Sr = 0.709175), and to 87Sr/86Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The 87Sr/86Sr 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.