Geochemistry at DSDP Legs 56-57 Holes

About 200 volcanic ash layers were recovered during DSDP Leg 57. The volcanic glass in some of these layers was investigated petrographically and chemically in this study. Volcanic glass is mainly rhyolitic and/or rhyodacitic in chemical composition, and its refractive index ranges from 1.496 to 1.529. Some volcanic ash layers consist of multiple grains of different chemical compositions. All the volcanic glass belongs to the tholeiitic and the calc-alkalic volcanic rock series, in SiO2-(Na2O + K2O) diagram and FeO*/MgO-SiO2 diagram. We correlated successfully three volcanic ash layers from the standpoint of chemical composition and biostratigraphy. Hydration of volcanic glass from Leg 57 is less intense than in other DSDP cores.

Neogene and Paleocene-Maestrichtian calcareous nannofossils in sediments of the upper continental rise off New Jersey

Maestrichtian to Holocene calcareous nannofossils from two closely spaced sites on the upper continental rise some 100 miles (161 km) southeast of Atlantic City, New Jersey, were zoned in order to help date a major canyon-cutting event in the late Miocene and to delineate and correlate other hiatuses with seismic stratigraphy. Mid-middle Eocene through middle Miocene sediments (Zones CP14 to CN6) were not recovered in these holes, but nearly all other zones are accounted for. The Eocene section is described in a companion chapter (Applegate and Wise, 1987, doi:10.2973/dsdp.proc.93.118.1987). Nannofossils are generally sparse and moderately preserved in the clastic sediments of Site 604. Sedimentation rates are extremely high for the upper Pleistocene (201 m/m.y. minimum) above a hiatus calculated to span 0.44 to 1.1 Ma. The associated disconformity is correlated with local seismic reflection Horizon Pr . Sedimentation rates continue to be high (93 m/m.y.) down to a second hiatus in the upper Pliocene dated from about 2.4 to 2.9 (or possibly 3.3) Ma. The disconformity associated with this hiatus is correlated with local seismic reflection Horizon P2 and regional Reflector Blue, which can be interpreted to mark either the onset of Northern Hemisphere continental glaciation or circulation changes associated with the closure of the Central American Seaway. Sedimentation rates in the pre-glacial lower Pliocene are only about a third those in the glacial upper Pliocene. A prominent disconformity in the upper Miocene marks a major lithologic boundary that separates Messinian(?) glauconitic claystones above from lower Tortonian conglomeratic debris flows and turbidites below. The debris flows recovered are assigned to nannofossil Zones CN8a and CN7, but drilling difficulties prevented penetration of the bottom of this sequence some 100 m below the terminal depth of the hole. Correlation of the lower bounding seismic reflector (M2/Merlin?) to a drift sequence drilled on the lower rise at DSDP Site 603, however, predicts that the debris flows began close to the beginning of the late Miocene (upper Zone CN6 time) at about 10.5 Ma. The debris flows represent a major canyon-cutting event that we correlate with the beginning of the particularly severe late Miocene glaciations believed to be associated with the formation of the West Antarctic Ice Sheet. The existence of these spectacular debris flows strongly suggest that the late Miocene glacio-eustatic low stand occurred during Vail Cycle TM3.1 (lower Tortonian) rather than during Vail Cycle TM3.2 (Messinian) as originally published. Beneath a set of coalesced regional disconformities centered upon seismic reflection Horizon Au, coccoliths are abundant and in general are moderately preserved at Site 605 in a 619-m carbonate section extending from the middle Eocene Zone CP13b to the upper Maestrichtian Lithraphidites quadratus Zone. Sedimentation rates are 37 m/m.y. in the Eocene down to a condensed interval near the base (Zone CP9). A disconformity is suspected near the Eocene/Paleocene boundary. Sedimentation rates for the upper Paleocene Zone CP8 are similar to those of the Eocene, but Zones CP7 and CP6 lie within another condensed interval. The highest Paleocene rates are 67 m/m.y. down through Zones CP5 and CP4 to a major disconformity that separates the upper Paleocene from the Danian. This hiatus spans about 2.6 m.y. (upper Zone CP3 to lower Zone CP2) and corresponds to the major sea-level drop at the base of Vail Cycle TE2.1. As the most prominent break in this Paleogene section, it may correspond to seismic reflection Horizon A* of the North American Basin. Sedimentation rates from this point to the Cretaceous/Tertiary boundary drop to 11 m/m.y., still high for a Paleocene DSDP section. No major break in deposition could be detected at the Cretaceous/Tertiary boundary.

Velocity-density systematics for basalts at DSPD Leg 54 Holes

Precise velocity and density measurements at atmospheric and elevated pressures have been obtained on basalts drilled in 13 holes during Leg 54. The vp-sigma data show systematics which are controlled by the degree of crystallinity (or conversely, glassiness), microstructure, and original mineralogy and chemistry. Extensively fractionated basalts with marked iron enrichment produce anomalously low measured velocities at densities above 2.90 g/cm**3. Also, the effective in situ pressure acting on Leg 54 basalts is less than hydrostatic, and perhaps close to zero. At zero effective pressure, the measured velocities average 2.5 km/s higher than East Pacific Rise upper crustal velocities determined by seismic refraction. This implies that the in situ velocities are undoubtedly a result of the highly fragmented nature of East Pacific Rise crust.

Elemental and stable isotopic composition of some metalliferous and pelagic sediments at DSDP Leg 70 Holes

Nontronite, the main metalliferous phase of the Galapagos mounds, occurs at subsurface depths of about 2 to 20 meters; Mn-oxide material is limited to the upper 2 meters of the mounds. The nontronite forms intervals of up to a few meters' thickness, consisting essentially of 100% nontronite granules, which alternate with intervals of normal pelagic sediment. Electron microprobe analyses of nontronite granules from different core samples indicate that: (1) there is little difference in major element composition between nontronites from varying locations within the mounds, with adjacent granules from a given sample having very similar compositions; (2) individual granules show little internal variation in composition. This indicates that the granules are composed of a single mineral of essentially constant composition, consistent with relatively uniform conditions of Eh and composition during nontronite formation. Mn-oxide crusts have very low Fe contents, a feature characteristic of rapidly deposited Mn-oxide crusts formed under hydrothermal influences. The rare-earth element (REE) abundances of the nontronites are generally extremely low, totalling less than several ppm. Two samples have the negatively Ce anomaly typical of authigenic precipitates formed relatively rapidly from seawater. A Mn-oxide crust sample has low REE contents, typical of Mn-oxide crusts formed under hydrothermal influences, but no negative Ce anomaly. A sample of unusual Mn-Fe-oxide mud has relatively high REE concentrations and a seawater-type pattern; both of these features are also found for metalliferous sediments from the East Pacific Rise. The oxygen and hydrogen isotopic composition of the nontronites define a restricted field within a d18O-dD plot. In manganiferous sediments, d18O and dD appear to decrease with increase in the Mn-oxide content of the sediment. From the d18O values of the nontronites, formation temperatures in the range of about 20-30°C have been estimated. By comparison, temperatures of up to 11.5 °C at a 9-meter depth have been directly measured within the mounds (Corliss et al., 1979), and heat-flow data suggest present basement/sediment interface temperatures of 15-25°C. In a plot of Fe + Mn vs. d18O, the Mn-oxide crust and Mn-Fe-ooze plot near the tie-lines for authigenic Mn nodules and silicate phases, implying that they have formed in isotopic equilibrium with seawater at or close to bottom-water temperatures.

Quartz content and mass accumulation rates at DSDP Hole 86-576A

The amount and the accumulation rate of quartz were measured in 33 samples from Hole 576A. The amount and source of mineral aerosol being deposited in the northwest Pacific during the Cenozoic are evaluated using these data. When Hole 576A is compared to a Cenozoic record in the central North Pacific, a strong uniformity in the composition of the mineral aerosol across the North Pacific is seen. The data suggest that Hole 576A entered the influence of the westerlies about 15 m.y. ago and that since that time the rates of sediment deposition have increased. Only the dramatic change in quartz accumulation 2.5 m.y. ago can be clearly related to a climatic event, but a gradual increase in quartz accumulation through the Miocene and early Pliocene is probably a result of increasing Northern Hemisphere aridity and intensified atmospheric activity associated with global cooling during the interval.

Mass-accumulation rates for the non-authigenic, inorganic, crystalline, eolian component of sediments recovered at DSDP Sites 62-464 to 62-466

The mass-accumulation rate (MAR) of the non-authigenic, inorganic, crystalline component of deep-sea sediments from the Pacific aseismic rises apparently reflects influx of eolian sediment. The eolian sediment usually is dominated by volcanic material, except during glacial times. Sediments from Hess Rise provide a discontinuous record of eolian MARs. During Albian to Cenomanian time, the influx of volcanic material was fairly high (0.35-0.6 g/cm**2/10**3 yr), recording the latest stages of the Albian volcanism that formed Hess Rise. From the Campanian through the Paleocene, influx of eolian sediment was low, averaging 0.03 g/cm**2/10**3 yr. None of the four Hess Rise drill sites show evidence of the Late Cretaceous volcanic episode recorded at many sites now in the equatorial to subtropical Pacific. Pliocene to Pleistocene samples record a peak in volcanic influx about 4 to 5 m.y. ago, which has been well documented elsewhere. The several-fold increase in eolian accumulation rates elsewhere which are correlated with the onset of severe northernhemisphere glaciation 2.5 m.y. ago is not obvious in the Hess Rise data.

Zeolite and silica diagenesis and sandstone petrography at DSDP Sites 57-438 and 57-439

We detected authigenic clinoptilolites in two core samples of tuffaceous, siliceous mudstone in the lower Miocene section of Hole 439. They occur as prismatic and tabular crystals as long as 0.03 mm in various voids of dissolved glass shards, radiolarian shells, calcareous foraminifers, and calcareous algae. They are high in alkalies, especially Na, and in silica varieties. There is a slight difference in composition among them. The Si : (Al+ Fe3+) ratio is highest (4.65) in radiolarian voids, intermediate (4.34) in dissolved glass voids, and lowest (4.26) in voids of calcareous organisms. This difference corresponds to the association of authigenic silica minerals revealed by the scanning electron microscope: There are abundant opal-CT lepispheres in radiolarian voids, low cristobalite and some lepispheres in dissolved glass voids, and a lack of silica minerals in the voids of calcareous organisms. Although it contains some silica from biogenic opal and alkalies from trapped sea water, clinoptilolite derives principally from dissolved glass. Although they are scattered in core samples of Quaternary through lower Miocene diatomaceous and siliceous deposits, acidic glass fragments react with interstitial water to form clinoptilolite only at a sub-bottom depth of 935 meters at approximately 25°C. Analcimes occur in sand-sized clasts of altered acidic vitric tuff in the uppermost Oligocene sandstones. The analcimic tuff clasts were probably reworked from the Upper Cretaceous terrain adjacent to Site 439. Low cristobalite and opal-CT are found in tuffaceous, siliceous mudstone of the middle and lower Miocene sections at Sites 438 and 439. Low cristobalite derives from acidic volcanic glass and opal-CT from biogenic silica. Both siliceous organic remains and acidic glass fragments occur in sediments from the Quaternary through lower Miocene sections. However, the shallowest occurrence is at 700 meters subbottom in Hole 438A, where temperature is estimated to be 21°C. The d(101) spacing of opal-CT varies from 4.09 to 4.11 Å and that of low cristobalite from 4.04 to 4.06 Å. Some opal-CT lepispheres are precipitated onto clinoptilolites in the voids of radiolarian shells at a sub-bottom depth of 950 meters in Hole 439. Sandstone interlaminated with Upper Cretaceous shale is chlorite- calcite cemented and feldspathic. Sandstones in the uppermost Oligocene section are lithic graywacke and consist of large amounts of lithic clasts grouped into older sedimentary and weakly metamorphosed rocks, younger sedimentary rocks, and acidic volcanic rocks. The acidic volcanic clasts probably originated from the volcanic high, which supplied the basal conglomerate with dacite gravels. The older sedimentary and weakly metamorphosed rocks and green rock correspond to the lithologies of the lower Mesozoic to upper Paleozoic Sorachi Group, including the chert, limestone, and slate in south-central Hokkaido. However, the angular shape and coarseness of the clasts and the abundance of carbonate rock fragments indicate a nearby provenance, which is probably the southern offshore extension of the Sorachi Group. The younger sedimentary rocks, including mudstone, carbonaceous shale, and analcime-bearing tuff, correspond to the lithologies of the Upper Cretaceous strata in south-central Hokkaido. Their clasts were reworked from the southern offshore extension of the strata. Because of the discontinuity of the zeolite zoning due to burial diagenesis, an overburden several kilometers thick must have been denuded before the deposition of sediments in the early Oligocene.