Organic geochemistry, visual composition, atomic ratios, and reflectance at DSDP Site 87-582, 87-583 and 87-584

Geochemical analyses of organic matter were carried out on Quaternary sediments from Sites 582 and 583 (Nankai Trough) and on Pliocene to Miocene sediments from Site 584 (Japan Trench), DSDP Leg 87, to evaluate petroleum-generating potential and to characterize the organic matter. The vitrinite-huminite reflectances of indigenous materials for these sites are less than 0.3% indicating the immature nature of the sediments. The sediments, however, contain remarkable amounts of recycled organic materials. The Quaternary sediments from Sites 582 and 583 contain small amounts of amorphous organic matter (less than 0.75 wt.% organic carbon and 66-90% amorphous debris), which is composed of predominantly recycled, oxidized, and over-matured (or matured) Type III material. The amount of hydrocarbon yield indicates that those sediments have lean-source potential for commercial hydrocarbon generation. The Pliocene to Miocene sediments from Site 584 contain organic matter (0.3-1.09 wt.% organic carbon) of predominantly amorphous debris (68-96%) that originated in two sources, an indigenous Type II material and a recycled, over-matured material. Pyrolysis shows an upward increase in the section of hydrocarbon yield and the same trend is also observed in organic-carbon content. The amount of the yield indicates that the Miocene sediments have lean-to-fair source potential and the Pliocene sediments have fair-to-good source potential.

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.

Geochemistry of phosphatic crusts from the shelf off Peru

Authigenic phosphorite crusts from the shelf off Peru (9°40°S to 13°30°S) consist of a facies with phosphatic coated grains covered by younger phosphatic laminite. The crusts are composed of carbonate fluorapatite, which probably formed via an amorphous precursor close to the sediment water interface as indicated by low F/P2O5 ratios, Sr and Ca isotopes, as well as rare earth element patterns agreeing with seawater-dominated fluids. Small negative Ce anomalies and U enrichment in the laminite suggest suboxic conditions close to the sediment-water interface during its formation. Increased contents of chalcophilic elements and abundant sulfide minerals in the facies with phosphatic coated grains as well as in the laminite denote sulfate reduction and, consequently, point to episodical development of anoxic conditions during phosphogenesis. The Peruvian phosphorites formed episodically over an extended period of time lasting from Middle Miocene to Pleistocene. Individual phosphatic coated grains show a succession of phosphatic layers with varying contents of organic matter and sulfide-rich phosphatic layers. Coated grains supposedly formed as a result of episodic suspension caused by high turbulence and shifting redox conditions. Episodic anoxia in the pore water induced pyritization in the outermost carbonate fluorapatite layer. Phosphatic coated grains were later transported to the place of crust formation, where subsequent laminite formation was favored under lower energy conditions. A similar succession of phosphatic layers with varying contents of organic matter and sulfide-rich layers in the laminite suggests a formation mechanism analogous to that of individual coated grains.

Mineralogy and geochemistry of sediments of the Cretaceous/Tertiary boundary at DSDP Holes 62-465 and 62-465A

The complete Paleocene section begins with the basal Tertiary Globigerina eugubina Zone. This zone occurs at 465A-3-3, 4 cm to 465A-3-3, 144 cm and belongs to Lithologic Unit I (Site 465 report, this volume), a homogeneous, white, moderately to highly disturbed nannofossil ooze.

Petrology and geochemistry at DSDP Site 59-448

This chemical and petrologic study of rocks from Site 448 on the Palau-Kyushu Ridge is designed to answer some fundamental questions concerning the volcanic origin of remnant island arcs. According to the reconstruction of the Western Pacific prior to about 45 m.y. ago (Hilde et al., 1977), the site of the Palau-Kyushu Ridge was a major transform fault. From a synthesis of existing geological and geophysical data (R. Scott et al., this volume), it appears that the ridge originated by subduction of the Pacific plate under the West Philippine Basin. Thus the Palau-Kyushu Ridge should be a prime example of both initial volcanism of an incipient arc formed by interaction of oceanic lithospheric plates and remnant-arc volcanic evolution. The Palau-Kyushu Ridge was an active island arc from about 42 to 30 m.y. ago, after which initiation of back-arc spreading formed the Parece Vela Basin (R. Scott et al., this volume; Karig, 1975a). This spreading left the western portion of the ridge as a remnant arc that separates the West Philippine Basin from the Parece Vela Basin. In spite of numerous oceanographic expeditions to the Philippine Sea, including the two previous DSDP Legs 6 and 31 (Fischer, Heezen et al., 1971; Karig, Ingle et al., 1975), and even though the origins of inter-arc basins have been linked by various hypotheses to that of remnant island arcs (Karig, 1971, 1972, 1975a, and 1975b; Gill, 1976; Uyeda and Ben-Avraham, 1972; Hilde et al., 1977), very little hard data are available on inactive remnant arcs.

Geochemistry of black shales from Breitbach Creek

Lower Cretaceous sediments are frequently characterized by a well expressed cyclicity. While the processes influencing environments above the carbonate compensation depth (CCD) are reasonably well understood, almost nothing is known about the deep ocean. Cretaceous sub-CCD sediments from the Tethys and Atlantic Oceans typically show rhythmic black/green shale successions. To gain insight into the nature of these black/green shale cycles, we performed detailed geochemical analyses (X-ray fluorescence, Rock-Eval and reactive iron analysis) on a 3 m long section of latest Aptian age. The major-element distribution of the analyzed shale sequence indicates a periodic change from a high-productivity and well-oxygenated green shale mode to a low-productivity oxygen-deficient black shale mode. It is proposed here that the preservation of organic matter was dependent on the strength of salinity-driven deepwater generation. Furthermore, the data show that the Corg content covaries with changes in the detrital composition. Therefore we hypothesize that Tethyan deepwater circulation was sensitive to changes in the monsoonal system. Time series analysis suggests that these changes are periodic in nature, although we are currently unable to prove that the dominant periodicity is related to the precession component of the Milankovitch frequencies.

Geochemistry of DSDP Hole 83-504B minerals

Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m below the seafloor (BSF). The hole previously extended through 274.5 m of sediment and 561.5 m of pillow basalts altered at low temperature (< 100°C), to 836 m BSF. Leg 83 drilling penetrated an additional 10 m of pillows, a 209-m transition zone, and 295 m into a sheeted dike complex. Leg 83 basalts (836-1350 m BSF) generally contain superimposed greenschist and zeolite-facies mineral parageneses. Alteration of pillows and dikes from 836 to 898 m BSF occurred under reducing conditions at low water/rock ratios, and at temperatures probably greater than 100°C. Evolution of fluid composition resulted in the formation of (1) clay minerals, followed by (2) zeolites, anhydrite, and calcite. Alteration of basalts in the transition zone and dike sections (898-1350 m BSF) occurred in three basic stages, defined by the opening of fractures and the formation of characteristic secondary minerals. (1) Chlorite, actinolite, pyrite, albite, sphene, and minor quartz formed in veins and host basalts from partially reacted seawater (Mg-bearing, locally metal-and Si-enriched) at temperatures of at least 200-250°C. (2) Quartz, epidote, and sulfides formed in veins at temperatures of up to 380°C, from more evolved (Mg-depleted, metal-, Si-, and 18O-enriched) fluids. (3) The last stage is characterized by zeolite formation: (a) analcite and stilbite formed locally, possibly at temperatures less than 200°C followed by (b) formation of laumontite, heulàndite, scolecite, calcite, and prehnite from solutions depleted in Mg and enriched in Ca and 18O, at temperatures of up to 250°C. The presence of small amounts of anhydrite locally may be due to ingress of relatively unaltered seawater into the system during Stage 3. Alteration was controlled by the permeability of the crust and is characterized by generally incomplete recrystallization and replacement reactions among secondary minerals. Secondary mineralogy in the host basalts is strongly controlled by primary mineralogy. The alteration of Leg 83 basalts can be interpreted in terms of an evolving hydrothermal system, with (a) changes in solution composition because of reaction of seawater fluids with basalts at high temperatures; (b) variations in permeability caused by several stages of sealing and reopening of cracks; and (c) a general cooling of the system, caused either by the cooling of a magma chamber beneath the spreading center and/or the movement of the crust away from the heat source. The relationship of the high-temperature alteration in the transition zone and dike sections to the low-temperature alteration in the overlying pillow section remains uncertain.

Geochemistry of Late Paleocene thermal maximum sediments

The late Paleocene thermal maximum (LPTM) was a dramatic, short-term global warming event that occurred ~55 Ma. Warming of high-latitude surface waters and global deep waters during the LPTM has been well documented; however, current data suggest that subtropical and tropical sea surface temperatures (SSTs) did not change during the event. Conventional paradigms of global climate change, such as CO2-induced greenhouse warming, predict greater warming in the high latitudes than in the tropics or subtropics but, nonetheless, cannot account for the stable tropical/subtropical SSTs. We measured the stable isotope values of well-preserved late Paleocene to early Eocene planktonic foraminifera from South Atlantic Deep Sea Drilling Project (DSDP) Site 527 to evaluate the subtropical response to the climatic and environmental changes of the LPTM. Planktonic foraminiferal d18O values at Site 527 decrease by ~0.94 per mil from pre-LPTM to excursion values, providing the first evidence for subtropical warming during the LPTM. We estimate that subtropical South Atlantic SSTs warmed by at least ~1°-4°C, on the basis of possible changes in evaporation and precipitation. The new evidence for subtropical SST warming supports a greenhouse mechanism for global warming involving elevated atmospheric CO2 levels.

Geochemistry at DSDP Hole 76-534A

At Site 534 in the Blake-Bahama Basin, western North Atlantic, an interval of 68 m of Maestrichtian (Upper Cretaceous) and upper middle to upper Eocene sediments consists of terrigenous siltstones, mudstones, and varicolored zeolitic claystones; minor recovery of micritic limestones, porcellanites, and quartzitic chert was made at this site as well. Comparisons with other Deep Sea Drilling Project (DSDP) sites in the western North Atlantic suggest that the following formations are present in this interval: Hatteras (Maestrichtian), Plantagenet (Maestrichtian and upper Eocene), Bermuda Rise (upper middle to upper Eocene), and the basal Blake Ridge Formation (upper middle to upper Eocene). Recognition of a Tertiary interval of the Plantagenet allows that formation to be divided into lower and upper informal units. Condensation makes this formal lithostratigraphic subdivision difficult. Together the formations record marked net condensed sedimentation (average rate ca. 2.5 m/m.y.) in strongly oxidizing bottom waters. From sedimentary structures and petrography, it is inferred that the terrigenous siltstones and micritic limestones were redeposited from the continental margin by turbidity currents. Chemical data plus petrography confirm relatively high plankton productivity during the upper Eocene. Much of the nonrecovered Eocene interval may represent chert and porcellanite. Fragments recovered were formed by replacement of relatively porous calciturbidites by opal-CT and quartz. Radiolarians in interbedded claystones rich in clinoptilolite show extensive dissolution. Relative to typical hemipelagic sediments, the claystones are enriched in many metals (Cu, Ni, Zn, Pb), particularly within manganese micronodules. The metal accumulation is related to a 30-m.y. period of slow net sediment accumulation, rather than to hydrothermal enrichment or to upward mobilization of metals from the underlying reduced Hatteras black shale facies. Elsewhere in the Blake-Bahama Basin, at Site 391, 22 km to the northwest, upper Eocene facies are missing, reportedly due to deep seafloor erosion of up to 800 m of the sedimentary succession. By contrast, the discovery that this interval is preserved at nearby Site 534 points to much less extensive seafloor erosion, possibly mostly in the Oligocene, which is missing at both DSDP Sites.

Geochemistry of oceanic arc basement rocks of Sumisu Rift and Ohmachi Seamount

Prehnite-pumpellyite facies metamorphism is described in the oceanic-arc basement rocks of Ocean Drilling Program Leg 126, Site 791 in the Sumisu Rift, western Pacific. Chemical variations of pumpellyite, epidote, chlorite, and prehnite are examined and paragenetic relations discussed. The metamorphism took place during the pre-rifting stage of an intraoceanic arc. During the backarc rifting stage, the geothermal gradient of the area was not as high as that of a spreading mid-oceanic ridge.

Interstitial-water geochemistry of Kerguelen Plateau sediments

This report synthesizes all of the interstitial-water chemistry studies associated with the Kerguelen Plateau phase of ODP Leg 119. Sediments were cored at six sites (49°24'S to 59°36'S) in water depths ranging from 564 to 4082 m. A total of 77 interstitial-water samples was recovered as part of the routine sampling protocol. In addition, a novel, highresolution pore-water sampling program was tested during Leg 119 that enabled us to pinpoint reaction zones and extend our data base to deeper, drier levels that were heretofore inaccessible. Data collected include interstitial-water sodium, potassium, calcium, magnesium, pH, alkalinity, sulfate, ammonia, phosphate, aqueous silica, salinity, chloride, oxidation-reduction potentials, and sediment chemistry. The northern sector (Sites 736 and 737) is characterized by the highest sedimentation rates (up to 140 m/m.y.) and thermal gradients (70°-98°C/km) encountered on the Kerguelen Plateau during Leg 119. Site 737 represents the most reactive sediment column cored on the Kerguelen Plateau. Major cation fluxes at Site 737 are the strongest measured during Leg 119. High dissolved calcium concentrations (141.5 mM) were encountered near the bottom of Hole 737B. Elevated temperatures promote silica diagenesis and the alteration of volcanic material below 300 mbsf, and a diagenetic front was discovered near 370 mbsf at Site 737. The southern portion of the Kerguelen Plateau (Sites 738 and 744) records the lowest sedimentation rates (less than 5 m/m.y.) and thermal gradients (43°C/km) of the three study areas. Major cation fluxes at the southern sites are the lowest that we measured on the Kerguelen Plateau. High-resolution sampling provided evidence for significant silica release to the pore waters during the weathering of basement basalt. The relatively low thermal gradient does not appear to be sufficient for the formation of the opal-CT and quartz chert beds and nodules that were encountered below 120 mbsf at Site 738. Sediment-accumulation rates on the Eastern Kerguelen Sediment Ridge (Sites 745 and 746) are intermediate to those of the northern and southern sites. Deposition below the regional CCD accounts for the nearly carbonate-free, siliceous sediments. Despite their low organic carbon contents (mean = 0.15%), sediments on the Eastern Kerguelen Sediment Ridge exhibit the highest pore-water alkalinity (6.77 mM), ammonium (0.50 mM), and phosphate (23 µM) concentrations measured on the Kerguelen Plateau. Major cation fluxes are intermediate to those calculated for the northern and southern sites. The Eastern Kerguelen Sediment Ridge interstitial waters are unusual, however, in that the downward flux of magnesium is greater than the upward flux of calcium.