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.

Characterization of diagenetic fluids at ODP Site 103-639

Site 639, drilled during Leg 103 of the Ocean Drilling Program, penetrated an Upper Jurassic to Lower Cretaceous carbonate platform on a tilted fault block along the Galicia margin off the northwest Iberian Peninsula. The carbonate platform is composed primarily of a sequence of dolomite overlying limestone. Samples were analyzed for mineral chemistry, stable isotope geochemistry, fluid inclusion microthermometry, and volatile contents and by dolomite pyrolysis mass spectrometry for trace sulfate minerals. The dolomite recovered from the Galicia margin at Site 639 formed during shallow burial from sulfate-bearing, hypersaline brines at slightly elevated temperatures. The light oxygen isotopic signatures of the dolomite are interpreted as the result of the evaporative loop and slightly elevated temperatures during dolomite formation or from reequilibration at higher temperatures during deeper burial. The hypersalinity is interpreted to be associated with a nearby, shallow restricted basin that formed during rifting of the Iberian margin from Newfoundland. The dolomitization of the platform is therefore a by-product of the rifting.

Temperature and salinity reconstruction for the Last Interglacial Period in the North Atlantic Ocean

Eight deep-sea sediment cores from the North Atlantic Ocean ranging from 31° to 72°N are studied to reconstruct the meridional gradients in surface hydrographic conditions during the interval of minimum ice volume within the last interglacial period. Using benthic foraminiferal ?18O measurements and estimates of Sea Surface Temperature (SST) and Sea Surface Salinity (SSS), we show that summer SSTs and SSSs decreased gradually during the interval of minimum ice volume at high-latitude sites (52°-72°N) whereas they were stable or increased during the same time period at low-latitude sites (31°-41°N). This increase in meridional gradients of SSTs and SSSs may have been due to changes in the latitudinal distribution of summer and annual-average insolation and associated oceanic and atmospheric feedbacks. These trends documented for the Eemian ice volume minimum period are similar to corresponding changes observed during the Holocene and may have had a similar origin.

d18O, lithologies, mineralogy and determination of temperature and depth of formation of DSDP Site 69-504 siliceous rocks

Seven opal-CT-rich and five quartz-rich porcellanites and cherts from Site 504 have a range in oxygen-isotope values of 24.4 and 29.4 per mil. In opal-CT rocks, d18O becomes larger with sub-bottom depth and with age. Quartz-rich rocks do not show these trends. Boron, in general, increases with decreasing d18O for porcellanites and cherts considered together, supporting the conclusion that boron is incorporated within the quartz crystal structure during precipitation of the SiO2. Silicification of the chalks at Site 504 began 1 m.y. ago - that is, 5 m.y. after sedimentation commenced on the oceanic crust. Temperatures of chert formation determined from oxygen-isotope compositions reflect diagenetic temperatures rather than bottom-water temperatures, and are comparable to temperatures of formation determined by down-hole measurements. Opal-A in the chalks began conversion to opal-CT when a temperature of 50°C was reached in the sediment column. Conversion of opal-CT to quartz started at 55 °C. Silicification occurred over a stratigraphic thickness of about 10 meters when the temperature at the top of the 10 meters reached about 50°C. It took about 250,000 years to complete the silica transformation within each 10-meter interval of sediment at Site 504. Quartz formed over a stratigraphic range of at least 30 meters, at temperatures of about 54 to 60°C. The time and temperatures of silicification of Site 504 rocks are more like those at continental margins than those in deep-sea, open-ocean deposits.

Stable isotope composition of carbonates in the Tyrrhenian Sea

The Messinian evaporitic succession recovered at ODP Sites 652, 653, and 654 in the Tyrrhenian Sea was generated under various environmental conditions which ranged from brackish to hypersaline, as deduced from the sedimentary facies and stable isotope compositions of the carbonate and sulfate deposits. Water in the basins had to be shallow to undergo such rapid and large geochemical variations. The marine influence was omnipresent in the basin at least during the deposition of sulfate evaporites; seawater or marine brines might have been supplied either by direct input into evaporitic lagoons as at Sites 653 and 654, or by subterraneous infiltration in marginal areas as at Site 652. Episodes of severe dilution by continental waters occurred frequently throughout Messinian times in the more basinal areas at Sites 653 and 654, while a fresh water body was standing permanently at Site 652. The high heat flow present at Site 652 was responsible for a major late authigenesis of iron-rich dolomites, which was initiated during the subsidence of the basin and ended before Pliocene.