Edward Schreiber

Mass Mortality and Its Environmental and Evolutionary Consequences

The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 102 or 103 years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the oceans planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.

Elastic Moduli of Single‐Crystal Spinel at 25°C and to 2 kbar

The elastic properties of a synthetic single‐crystal spinel with a composition of MgO·2.6Al2O3 have been determined at 25°C and up to 2 kbar by measuring propagational velocities of ultrasonic waves using the method of pulse superposition. The isotropic moduli and Poissons ratio were computed from the single‐crystal values using the Voigt‐Reuss‐Hill approximation.Values of the adiabatic elastic moduli at 1 bar and 2 kbar and at 25°C are c11c12c441 bar2985.71537.21575.8(kbar)2 kbar2995.51545.01577.5(kbar).The adiabatic bulk modulus was computed to be 2019.9 kbar, and the pressure derivative of the adiabatic bulk modulus was found to be 4.18.

The geology of the Caribbean crust: Tertiary sediments, granitic and basic rocks from the Aves ridge☆

Abstract In 1967 aboard Duke Universitys R.V. “Eastward”, escarpments and pedestals near the southern end of the Aves ridge were sampled. From four pedestals, thirteen dredges, ranging in depth from 400 to 1,400 m, sampled limestones, marls and cherts. The age of the fossiliferous samples ranges from Middle Eocene to Recent. The mid-Eocene to Lower Miocene limestones were deposited in shallow, carbonate-shelf environments. The mid-Miocene to Pleistocene samples were deposited in an open-ocean planktonic environment. Three dredges were located on the north wall of a precipitous east-west trending escarpment. Two of the dredges recoverd 3,500 kg of granodiorite. The third dredge located about 40 km away from the two granitic hauls contained diabase, porphyritic basalt and metamorphosed basalt. Potassium-argon dates of three granitic samples and one diabase sample indicate that the rocks are no older than middle Cretaceous and no younger than Paleocene. The compressional wave velocity at 1–2 kbar confining pressure of the granitic samples ranged from 6.0 to 6.4 km/sec and these values correlate with the 6.0–6.3 km/sec crustal layer recorded on the Aves ridge by seismic refraction methods. These results suggest that the Aves ridge is underlain by granitic rock Late Mesozoic in age. The dredged carbonate samples indicate that during the Eocene to Early Miocene, portions of the Aves ridge were characterized by a shallow water carbonate shelf environment. The deep water pelagic samples of Mid-Miocene to Recent age indicate a subsidence of 400–1,400 m.

Critical thermal gradients in the mantle

Critical thermal gradients based upon ultrasonic laboratory data for a great variety of minerals confirm that it is possible to have a low-velocity layer for shear waves without requiring one for compressional waves. The discovery that the shear wave velocity decreases with increasing pressure for trevorite (NiFe2O4), which crystallizes in the spinel structure, together with the previously reported low-pressure derivative of the shear wave velocity in magnesium-aluminate spinel supports the conclusion that the shear velocity may decrease with depth in the transition region of the mantle. The low values of the critical thermal gradient for shear waves are the consequence of low values of the pressure derivative of the isotropic shear modulus (μ). This derivative, (∂μ/∂P)T, is determined by Poissons ratio and the coordination of the ions in the crystallographic lattice.

Density and P-wave velocity of rocks from the FAMOUS region and their implication to the structure of the oceanic crust

In the laboratory the bulk density and the compressional wave velocity as a function of varying confining pressure (0.001 to 6.0 kb) was measured for 25 samples. Nineteen of these samples were fresh, unaltered basalts and, on the basis of their mineralogy, could be separated into olivine basalt (7), plagioclase basalt (9), and pyroxene basalt (3). The basalts had textures typical of extrusive and shallow intrusive volcanics. Of particular interest during the study was the effect of varying basalt mineralogy at confining pressures representative of the estimated lithostatic confining pressure of layer 2A. At 0.5 kb confining pressure, the average compressional wave velocity is olivine basalt (7) 5.62 km/sec (σ = 0.31), plagioclase basalt (5) 5.49 km/sec (σ = 0.19), pyroxene basalt 5.45 km/sec (σ = 0.32). There is no significant difference between the velocities of the three basalt groups, thus suggesting that at 0.5-kb confining pressure the control on velocity is principally textural. The other six samples (serpentinite (3), metagabbro (1), greenstone (1), and metabasalt breccia (1)) were recovered from the escarpments of fracture zone B. These rock types and their measured compressional wave velocities are typical of rocks recovered from transform faults.

Spring Peas from New York State: Nucleation and Growth of Fresh Water Hollow Ooliths and Pisoliths

ABSTRACT Delicate hollow spheroids (1 to 2 mm) composed of radially fibrous calcite occur in, and on the surface of, calcareous travertine being deposited from water of the Orenda Spring, Saratoga Springs, New York State. Field observations suggest that spheroids form by the precipitation of calcite on the surface of water droplets that contain newly formed bubbles of CO2. Although many of the spheroids subsequently disintegrate, some grow by the addition of laminae consisting of radial bundles of fibrous calcite, commonly separated by thin (? seasonal) laminae of fine-grained calcite and detritus. These spheroids are either incorporated into laminar travertine sheets or grow by successive increments into fresh-water ooliths and pisoliths with both a concentric and a radial structur .

DSDP Leg 73: Contributions to Paleogene stratigraphy in nomenclature, chronology and sedimentation rates

Abstract DSDP Leg 73 was successful in determining magnetostratigraphic—biostratigraphic correlations throughout much of the Paleogene. This paper treats three aspects of the data analysis. The first section treats the development of a chron nomenclature which facilitates the precise correlation of arbitrary events with respect to the geomagnetic polarity history. The second section analyzes the accuracy of radiometric dates for the Paleogene. The conclusion is that despite the recent advances in radiochronology ‘South Atlantic Standard’ remains the most convenient and probably the most reliable chronological standard. The final section studies the correlation in sedimentation rates between the Umbrian and South Atlantic sites. The conclusion is that sedimentation rate changes determined from magnetostratigraphy provide a high-resolution source of paleoenvironmental information. Strong correlations are noted between sites and with respect to other paleo-environmental studies involving oxygen isotope ratios, biogeography and CCD fluctuations within the Paleogene marine sediments.

Sound Velocity and Compressibility for Lunar Rocks 17 and 46 and for Glass Spheres from the Lunar Soil

Four experiments on lunar materials are reported: (i) resonance on glass spheres from the soil; (ii) compressibility of rock 10017; (iii) sound velocities of rocks 10046 and 10017; (iv) sound velocity of the lunar fines. The data overlap and are mutually consistent. The glass beads and rock 10017 have mechanical properties which correspond to terrestrial materials. Results of (iv) are consistent with low seismic travel times in the lunar maria. Results of analysis of the microbreccia (10046) agreed with the soil during the first pressure cycle, but after overpressure the rock changed, and it then resembled rock 10017. Three models of the lunar surface were constructed giving density and velocity profiles.

Numerical ages of Cenozoic biostratigraphic datum levels: Results of South Atlantic Leg 73 drilling

Six sites were drilled in the South Atlantic during the Leg 73 cruise of the Deep Sea Drilling Project (DSDP). Hydraulic piston coring at five of the six sites obtained a nearly complete sequence of undisturbed Cenozoic samples. The magnetostratigraphy at those sites was investigated by close sampling representing time intervals of about 10 4 yr. Most of the Cenozoic nannofossil and many of the foraminiferal zonal boundaries were accurately determined and magnetostratigraphically calibrated at those five Leg 73 boreholes. Their numerical ages have been computed assuming a linear spreading rate and a radiometric age of 66.5 m.y. for the Cretaceous-Tertiary boundary. Alternative magnetostratigraphic ages were obtained with the adoption of a 63.5-m.y. age for the Cenozoic. Our data confirm previous determinations of the Pleistocene-Pliocene boundary at 1.8 (1.7) m.y. B.P. and of the Pliocene-Miocene boundary at 5.1 (5.0) m.y. B.P. The Miocene-Oligocene boundary is placed within chron C-6 C, with a magnetostratigraphic age of 23.8–24.0 (22.7–22.9) m.y. The Oligocene-Eocene age is also very precisely located within chron C-13 R, with a magnetostratigraphic age of 37.1–37.2 (35.5–35.6) m.y. The Eocene-Paleocene boundary should be located within an uncored interval of chron C-24, with a magnetostratigraphic age of 59.0 (55.4) ± 0.2 m.y. A general accord of the magnetostratigraphic ages from Leg 73 sites and the radiometric ages published in the literature for the various zonal boundaries validates the assumption of a linear sea-floor–spreading rate during the Cenozoic.

Properties and composition of lunar materials: Earth analogies.

The sound velocity data for the lunar rocks were compared to numerous terrestrial rock types and were found to deviate widely from them. A group of terrestrial materials were found which have velocities comparable to those of the lunar rocks, but they do obey velocity-density relations proposed for earth rocks.