Billy P. Glass

Paleomagnetic Study of Antarctic Deep-Sea Cores

The magnetic inclinations and inten sities of about 650 samples from seven deepsea cores taken in the Antarctic were measured on a spinner magnetometer. This series of measurements provided a magnetic stratigraphy, based on zones of normally or reversally polar ized specimens for each core, which was then correlated with the magnetic stra tigraphy of Cox et al. (1). One core (V16-134) gave a continuous record of the paleomagnetic field back to about 3.5 million years. When selected samples were subject ed to alternatingfield demagnetization, most were found to have an unstable component that was removed by fields of 150 oersteds; all samples from two cores were partially demagnetized in a field of 150 oersteds. The average inclination in these two cores was then in good agreement with the average inclination of the ambient field for the latitude of the core site. It was also found that the intensities of the samples decreased at the points of reversal; this finding is to be expected if, as has been postulated by the dynamo theory, the intensity of the dipole field decreases to zero and builds again with opposite polarity. We believe that the magnetiza tion of the cores results from the pres ence of detrital magnetite, although other magnetic minerals also may be present. Four faunal zones (, X, , and ) have been recognized in these Antarctic cores on the basis of upward sequential disappearance of Radiolaria. The faunal boundaries and reversals consistently have the same relations to one another, indicating that they are both timedependent phenomena. Using previously determined times of reversal, one may date the following events in the cores: 1) Radiolarian faunal boundaries:-X, 2 million years; X-, 0.7 million years; -, 0.4 to 0.5 million years. These dates are in good agreement with ages previously extrapolated from radio metric dates. 2) Initiation of Antarctic diatom ooze deposition, approximately 2.0 mil-lion years ago. 3) First occurrence of ice- rafted detritus, approximately 2.5 million years ago. One can also calculate rates of sedi mentation, which vary in the cores studied from 1.1 to about 8.0 millimeters per 1000 years. Sedimentation rates for the Indian Ocean cores are higher than for the Bellingshausen Sea cores. The near coincidence of faunal changes and reversals in the cores suggests but does not prove a causal relation. We conclude from this study that paleomagnetic stratigraphy is a unique method for correlating and dating deep sea cores, and that future work with such cores may provide a complete or nearly complete record of the history of the earths magnetic field beyond 4 million years.

Geochemistry and age of Ivory Coast tektites and microtektites

Abstract Ivory Coast tektites were first reported in 1934 from a geographically restricted area at Ivory Coast, West Africa. Although some additional specimens have been found later, the total number remains small (a few hundred). The Bosumtwi impact crater in Ghana is most likely the source crater for the Ivory Coast tektites, based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions. In addition to tektites on land, microtektites were found in (so far) eleven deep-sea cores off the West African coast, between about 9°N and 11°S and 0° and 23°W, defining the extent of the Ivory Coast tektite strewn field. In this study we analyzed eleven Ivory Coast tektites for their major and trace element composition, studied their petrographical characteristics, provided major element data for 111 microtektites, and major and trace element data for four microtektites. We determined the 40Ar 39Ar step-heating age of five Ivory Coast tektites and four microtektites and obtained fission-track dates for ten tektites and one Bosumtwi impact glass. The tektites have very small intersample and intrasample variations of their major and trace element composition. 111 Ivory Coast microtektites from eleven cores were analyzed for their major element compositional range. Their compositional range is significantly wider than that of the Ivory Coast tektites, but the majority of all microtektites have compositions very similar to those of the tektites (within a factor of 1.2). Trace element compositions of the tektites also show little variation between samples. The samples do not show any distinct Eu anomaly in the REE patterns. This characteristic, as well as the high absolute REE abundances and La NYbN ratios of about 8, indicate that Archean rocks are plausible source rocks. The major and trace element contents of four individually analyzed Ivory Coast microtektites show compositions that are very similar to those of the Ivory Coast tektites. However, the microtektites contain >20 rel% higher abundances of some of the lithophile and siderophile trace elements, such as Sc, Cr, Co, Ni, Sr, Zr, Ba, Hf, Ta, Th, and the REEs. These differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite-forming melt. The Ivory Coast microtektites also have a uniform internal composition. Duplicate 40Ar 39Ar step-heating age analyses were performed on five tektites. The best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs, resulting in an age of 1.1 ± 0.05 Ma. We also tried to date four microtektites by 40Ar 39Ar age analyses, but their young age and small sample size makes it impossible to assign a reliable age to the microtektites. One run yielded satisfactory results that were similar to the tektite age. In addition, we determined the fission-track ages for ten individual Ivory Coast tektite samples and for one impact glass sample from the Bosumtwi crater. The track-size corrected ages for the Ivory Coast tektites ranged from 0.91 to 1.18 Ma, resulting in an average fission-track age of 1.05 ± 0.11 Ma. This age is, within errors, identical to that of the Bosumtwi impact glass at 1.03 ± 0.11 Ma, and to the 40Ar 39Ar age of 1.1 ± 0.05 Ma. The preferred age of the Ivory Coast tektite event is 1.07 Ma.

Tektites and microtektites: key facts and inferences

Abstract Tektites are generally small, black, rounded, silicate glass bodies found scattered over several widely separated areas of the Earths surface called strewn fields. Four major tektite strewn fields are known: (1) Australasian, (2) Ivory Coast, (3) Czechoslovakian and (4) North American. Tektites from these four strewn fields have been dated at 0.7, 1, 15, and 34 Ma respectively. Most tektites are splash forms (i.e., spheres, teardrops, dumbbells, etc.), but some have undergone a second period of melting to produce ablated forms. Tektites with a blocky, layered appearance are called Muong Nong-types. Microtektites (microscopic tektites) belonging to the Australasian, Ivory Coast and North American strewn fields have been found in deep-sea sediments. The geographic occurrence and abundance of microtektites at each site have been used to determine the size and mass of each strewn field. The Australasian, Ivory Coast and North American strewn fields contain 100 million, 20 million and 1 billion tonnes of glass respectively. Although tektites resemble obsidian, they can be distinguished from terrestrial volcanic glasses by their composition and petrography. Tektites, for example, are drier than volcanic glasses and have higher FeO/Fe 2 O 3 ratios. In addition, tektites, unlike volcanic glasses, contain lechatelierite particles (SiO 2 glass) and are devoid of primary crystallites. Although some authors favor a lunar volcanic origin, most of the compositional and petrographic data indicate a terrestrial impact origin. The major, trace, and isotopic compositions of tektites indicate that the parent material was a terrestrial sedimentary deposit, and the presence of lechatelierite, coesite and shocked relict inclusions favors an impact origin. Furthermore, two tektite strewn fields have been associated with known impact craters, and tektites from one strewn field have been found associated with impact ejecta. Unlike the Cretaceous-Tertiary boundary layer, microtektite layers are not associated with mass extinctions or iridium anomalies.

Coesite and shocked quartz discovered in the, Australasian and North American, microtektite layers

Samples from the North American, Australasian, and Ivory Coast microtektite layers were examined for unmelted impact ejecta (>125 οm size). Coesite and shocked quartz, identified by X-ray diffraction, were found at all the sites where a well-defined North American microtektite layer had previously been documented. Coesite and shocked quartz were also foundin seven out of 33 cores from the Australasian strewn field; however, no coesite or shocked quartz was found associated with the Ivory Coast microtektite layer. Stishovite was also found at three of the Australasian sites. All of the Australasian cores containing coesite and shocked quartz were taken from within 2000 km of Indochina, which is believed by most to be the source area for the Australasian tektites and microtektites. Six of the seven cores have the highest concentrations of microtektites ever determined for Australasian microtektite- bearing cores. The seventh core did not contain any microtektites, which may be consistent with the hypothesis that the microtektites were deposited in a raylike pattern. The discovery of coesite and impact ejecta associated with both the North American and Australasian microtektite layers provides further support for the hypothesis of terrestrial impact origin of tektites.

Geomagnetic Reversals and Pleistocene Chronology

The palaeontology and palaeomagnetic stratigraphy of several Atlantic and Pacific cores has been determined to establish the age relationship of various palaeontological boundaries which have been used to define a Pliocene–Pleistocene boundary in deep sea sediments.

Reworking of deep-sea sediments as indicated by the vertical dispersion of the Australasian and ivory coast microtektite horizons

Abstract Microtektites (small glassy objects) have been found in deep-sea pelagic sediments associated with both the Australasian and Ivory Coast tektite strewn fields. Because of their wide geographical distribution, age of deposition (0.7 to 1.0 m.y. ago), size (∼1 mm down to at least 20 microns in diameter) and ease of recognition, an investigation of the vertical dispersion of the microtektite horizons can tell us much about reworking of pelagic sediments. In the eight cores investigated the microtektites are dispersed through a vertical section of from 35 to 90 cm. This represents an average of 120,000 yr of deposition. The distribution of the microtektites within the dispersed zone seems to be related in a general way to the amount of burrowing evident in the core.

North American microtektites from the Caribbean Sea and their fission track age

Over 6000 microscopic glass spherules between 125 μm and 1 mm in diameter were found in a sediment core (RC9-58) from the Caribbean Sea. These glassy objects are mostly confined to a zone ∼ 40 cm thick at a depth of ∼ 250 cm. We believe that the microscopic glass objects are microtektites belonging to the North American strewnfield, based on their geographical location, appearance, physical properties, stratigraphic age (middle Upper Eocene), fission track age (∼34.6 my) and major element compositions. The occurrence of North American microtektites in the Caribbean Sea indicates that the North American strewnfield is two to three times larger than previously indicated. An estimate on the abundance of microtektites in core RC9-58 indicates that the North American strewnfield may contain greater than 1017 g of tektite material.

Chemical composition of Ivory Coast microtektites

Abstract The chemical composition of twenty-eight microscopic glassy objects, found in a deep-sea sediment core taken near the Ivory Coast, has been determined by electron microprobe analysis. Like the Australasian microtektites, these glassy objects can be divided into two main groups. One group is composed of individuals with chemical compositions similar to tektites. These are referred to as “normal” microtektites. The other group is composed of bottle-green glassy objects with low silica contents ( MgO CaO ratio (>3) and an average Na 2 O K 2 O ratio greater than unity. Although the bottlegreen objects differ from the “normal” microtektites in their appearance, physical properties and chemical compositions, they are nevertheless found in association with them and are probably therefore related to them. Both the Ivory Coast and Australasian microtektites have higher MgO and lower Na2O contents than igneous glasses with comparable SiO2 contents. The chemical data presented in this paper are interpreted as confirming a previous suggestion that the glassy objects found in deep-sea sediments near the Ivory Coast are microtektites and that they represent a portion of the Ivory Coast tektite strewn field.

North American microtektites in Deep Sea Drilling Project cores from the Caribbean Sea and Gulf of Mexico

Several thousand microtektites (125 to 1,000 µ m in diameter) have been found in cores from two Deep Sea Drilling Project sites: site 94 in the Gulf of Mexico and site 149 in the Caribbean Sea. The microtektites occur in upper Eocene sediments associated with the last occurrence of at least five species of Radiolaria. X-ray diffraction data, energy dispersive X-ray analysis, and petrographic studies indicate the presence of inclusions of quartz, cristobalite, and lechatelierite. Major-element compositions were determined for 57 of the microtektites using energy-dispersive X-ray analysis. The geographical location, age, petrography, and chemistry of the microtektites indicate that they belong to the North American tektite strewn field (∼34 m.y. old). Calculations indicate that there may be as much as 10 9 t (metric tons) of microtektites in this strewn field.

Tests for size and shape dependency in deep-sea mixing

Abstract Bioturbation is generally not a strongly size-dependent or shape-dependent process in deep-sea sediments. Different sizes of ash (11–250 μm) deposited during a single eruption are usually mixed about equally, although in a few cores we found minor tendencies toward greater mixing of finer ash. Platey volcanic ash and spherical microtektites initially deposited within a few centimeters of each other in the sediment column are mixed with roughly comparable intensity. Radiocarbon dating of three sizes of CaCO3 near the top of one box core produced ages within the analytical error; this also suggests no size dependency in the mixing process. Another core showed age differences opposite in sense to those that would be predicted if the finer fractions were more intensely mixed than the coarser fractions. The sizes examined (11–250 μm) encompass most of the sedimentary components used in paleoclimatic analyses of deep-sea cores. From these findings, we infer that significant artificial lead/lag offsets in paleoclimatic signals will not be created by size-selective or shape-selective mixing. Mixing models calibrated on one sediment size fraction should be generally applicable to other sizes and kinds of material.