Bradford M. Clement

Miocene isotope reference section, Deep Sea Drilling Project Site 608: An evaluation of isotope and biostratigraphic resolution

We developed an isotope (87Sr/86Sr, δ18O) reference section for the uppermost Oligocene to lower upper Miocene (ca. 25–8 Ma) at Site 608 in the northeastern North Atlantic. This site contains the least ambiguous magnetostratigraphic record of Miocene polarity changes available, providing direct correlations to the Geomagnetic Polarity Time Scale (GPTS). We integrate biostratigraphic, magnetostratigraphic, Sr isotope, and stable isotope data to provide a reference section for Miocene isotope fluctuations. The direct correlation of isotopes and biostratigraphy to the Geomagnetic Polarity Time Scale (GPTS) provides relatively precise age estimates. We use these age estimates to evaluate the timing of first and last occurrences of planktonic foraminifera, and conclude that many of these are synchronous within a 0.5 m.y. resolution between subtropical Site 563 (33°N) and high-latitude Site 608 (43°N). In addition, we use this chronology to estimate the ages of previously established Miocene oxygen isotope Zones Mi 1 through Mi 7 and to compare the Sr isotope record at Site 608 with previously published 87Sr/86Sr records. We approximate latest Oligocene to early late Miocene (25–8 Ma) Sr isotope changes with two linear regressions. The rate of increase of 87Sr/86Sr was high from the latest Oligocene (∼25 Ma) to earliest middle Miocene (∼15 Ma), with an estimated rate of 0.000059/m.y. Our ability to reproduce Sr isotope measurements is ±0.000030 or better, yielding a stratigraphic resolution of as good as ±0.5 m.y. for this interval. The rate of change was much lower from about 15 to 8 Ma (on average, 0.000013/m.y.), yielding Sr isotope stratigraphic resolution of worse than ±2.3 m.y. The causes of the late Eocene to Miocene 87Sr/86Sr increases are not known. We speculate that a moderate 87Sr/86Sr increase (0.000030/m.y) which occurred during the late Eocene-latest Oligocene can be explained by intermittent glaciations and deglaciations of the Antarctic continent. These pulse-like changes in the input of glacial weathering products yield what appears to be a monotonic, linear increase. The increase in the frequency of glaciations during the latest Oligocene-early Miocene can explain the higher rate of change of 87Sr/86Sr at this time. We speculate that by the middle Miocene, the development of a permanent east Antarctica ice sheet resulted in decreased input of glacial weathering products and a lower rate of 87Sr/86Sr change. Appendix 1 is available with entire article on microfiche.Order from American Geophysical Union, 2000 FloridaAvenue N.W., Washington, D.C. 20009. Document P90-001;

Geographical distribution of transitional VGPs: Evidence for non-zonal equatorial symmetry during the Matuyama-Brunhes geomagnetic reversal

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Dependence of the duration of geomagnetic polarity reversals on site latitude

Abstract Paleomagnetic records of the Matuyama-Brunhes polarity reversal obtained recently from the northern, equatorial and southern latitudes of the Atlantic sector display a distinct symmetry with respect to the Equator. The virtual geomagnetic pole (VGP) paths from the mid-latitude sites in the northern and southern hemispheres (DSDP Site 609 and Core V16-58) are nearly coincident, tracking northward through the Americas. The VGP path from the equatorial site (ODP Site 664) tracks northward through eastern Asia, nearly antipodal to the other two paths. Modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records. These results are consistent with the bulk of the data available from other records of this polarity transition, suggesting that the transitional VGPs during this reversal tend to fall along two distinct longitudinal bands. The grouping of transitional VGPs suggests that there is a strong geographical control over the reversal process. An apparent correlation between the longitudinal bands of transitional VGPs and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core-mantle boundary [1] suggests that the processes which cause these flux concentrations may have influenced the geometry of the Matuyama-Brunhes transitional field.

A Southern Hemisphere record of the Matuyama‐Brunhes polarity reversal

An important constraint on the processes governing the geodynamo—the flow in the outer core responsible for generating Earths magnetic field—is the duration of geomagnetic polarity reversals; that is, how long it takes for Earths magnetic field to reverse. It is generally accepted that Earths magnetic field strength drops to low levels during polarity reversals, and the field direction progresses through a 180° change while the field is weak. The time it takes for this process to happen, however, remains uncertain, with estimates ranging from a few thousand up to 28,000 years. Here I present an analysis of the available sediment records of the four most recent polarity reversals. These records yield an average estimate of about 7,000 years for the time it takes for the directional change to occur. The variation about this mean duration is not random, but instead varies with site latitude, with shorter durations observed at low-latitude sites, and longer durations observed at mid- to high-latitude sites. Such variation of duration with site latitude is predicted by simple geometrical reversal models, in which non-dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction, and provides a constraint on numerical dynamo models.

Equatorial and mid-latitude records of the last geomagnetic reversal from the Atlantic Ocean

The authors present a record of the Matuyama-Brunhes (0.73 Ma) polarity transition from a southern hemisphere deep-sea sediment core (V16-58; 46{degree}S, 30{degree}E). The transition is recorded across at least 30 sm of section and is defined by a nearly 180{degree} change in directions which occurs during a low in the relative intensities. An increase in the {sup 10}Be/{sup 9}Be ratios associated with the intensity low suggests that the relative intensities may document a decrease in the strength of the geomagnetic field. The virtual geomagnetic pole (VGP) path is neither clearly near- or far-sided, but tracks approximately 120{degree} west of the site longitude. Considered with other Matuyama-Brunhes transition records, this southern hemisphere record supports previous interpretations that the Matuyama-Brunhes transitional field was not dipolar. Instead, the V16-58 transition together with other records of this reversal recently obtained from this longitudinal sector suggests that nonzonal terms which are symmetric about the equator were important during this transitional field.

Continuous monitoring of high‐temperature fumaroles on an active lava dome, Volcán Colima, Mexico: Evidence of mass flow variation in response to atmospheric forcing

Abstract Three records of the Matuyama-Brunhes reversal have been obtained from ODP deep-sea cores distributed along the West African margin in the Atlantic Ocean. These studies in addition to the record from DSDP Site 609B provide a latitudinal transect extending from the equator to 50°N. Simulations of natural smoothing by post-depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records, especially at the equator. The results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition.

A quantitative comparison of two paleomagnetic records of the Cobb Mountain Subchron from North Atlantic deep‐sea sediments

Rapid, multichannel monitoring of fumaroles on Volcan Colima, Mexico, provides new insight into the time-scales and magnitudes of fumarole temperature variation. Temperatures in five fumaroles, all located along a single fracture cutting the summit lava dome of the volcano, were monitored at 20-min intervals between May 1991 and May 1992. Measurements were made using a programmable data logger deployed near the fumarole field, and data were radiotelemetered to a nearby volcano observatory at regular intervals. Mean fumarole temperatures varied between 350°C and 550°C. Statistical analysis of these time series shows that significant diurnal variation occurs in each fumarole. Magnitudes of these daily fluctuations are generally between 25°C and 50°C, although larger-amplitude variations occur, especially in cooler fumaroles. Simultaneous monitoring of atmospheric pressure at the fumaroles indicates that these variations in temperature are inversely correlated with barometric pressure. These observations indicate that fumarole temperatures respond to atmospheric forcing. A numerical model developed to explore the dependence of fumarole temperature on mass flow demonstrates that many aspects of observed temperature variation are accounted for by mass flow variation, resulting from small changes in barometric pressure. The relationship between mass flow and fumarole temperature is nonlinear: the response of fumarole temperatures to a given change in mass flow is greatest in fumaroles with low mass flow (and cool temperatures). The nature of this dependence is little affected by fumarole geometry for the cases considered. Continuous measurement of fumarole temperatures may be an effective means of monitoring local mass flow on volcanoes. At Volcan Colima, average temperatures changed by less than 100°C during the 1-year sampling period. During and immediately following effusive activity, changes in degassing were abrupt and inconsistent along the length of the fracture. Following this period, temperatures decreased gradually, and there was a higher degree of correlation between fumaroles. The method described here represents a substantial improvement over traditional fumarole-monitoring techniques because subtle variation can be quickly identified using standard statistical techniques, and the method provides regular information about thermal activity on a volcano, minimizing the hazards normally associated with the collection of these data on a regular basis.

Paleomagnetic estimate of the emplacement temperature of the long-runout Nevado de Colima volcanic debris avalanche deposit, Mexico

We present a new paleomagnetic record of the Cobb Mountain Subchron obtained from deep-sea sediments cored at Ocean Drilling Program site 647 in the southern Labrador Sea. The details of the transitional field behavior documented by this record appear to be very similar to those recorded in a previously published record of this subchron obtained at Deep-Sea Drilling Project (DSDP) site 609 in the North Atlantic (Clement and Kent, 1987). We used a quantitative correlation technique (Martinson et al., 1982) to establish statistically the degree of similarity between these two records and thereby constrain the spatial variability in these transitional fields. The error in the alignments is reduced significantly by aligning records of virtual geomagnetic pole positions rather than directions, indicating that these records document such large scale changes in the fields that we can not distinguish them from dipolar changes, given the proximity of these two sites. These replicate records of the Cobb Mountain Subchron provide evidence that deep-sea sediments are capable of providing high resolution records of geomagnetic field behavior. A reexamination of the sequence of polarity transitions recorded at DSDP site 609 in light of these results suggests the presence of two preferred transitional field configurations. The field appears to change from one configuration to the other for several reversals and then back to the original configuration, suggesting that a geographical influence on the reversal process persists through this sequence. The variability in these reversal records provides insights into the response of the geodynamo to this geographical influence.

Records of the Cobb Mountain Subchron from the Bermuda Rise (ODP LEG 172)

Stoopes and Sheridan have mapped a volcanic debris avalanche of Nevado de Colima which has an exceptionally long runout (120 km) and low fall-height to length ratio (H/L = 0.04). We present paleomagnetic results from this volcanic debris avalanche deposit which provide evidence that this avalanche was emplaced at elevated temperatures. The majority of samples, collected from lithic clasts in the volcanic debris avalanche deposit, exhibit two-component remanent magnetizations with a low-temperature component (25–350°C) which is well grouped about the geomagnetic field direction at Colima and a high-temperature component (350–580°C) which is randomly oriented. Although the temperature of the deposit most likely varied with distance from the volcanic source and the thickness of the deposit, our results suggest an emplacement temperature of approximately 350°C at intermediate distances (18–26 km) from the source. In order for the rock clasts (20–40 cm diameter) to be heated to these temperatures, the avalanche was most likely the results of a magmatic, Bezymianny-type eruption. The mixing of hot, juvenile gases with the clasts provides an explanation for the high degree of fluidization of this material, as evidenced by the long runout of this avalanche deposit.

INNER-CORE ANISOTROPY, ANOMALIES IN THE TIME-AVERAGED PALEOMAGNETIC FIELD, AND POLARITY TRANSITION PATHS

Abstract Duplicate records of the Cobb Mountain Subchron were obtained from two holes (Holes 1063B at 33°41.204′N, 57°36′W and 1063C at 33°41.181′N, 57°36′W) drilled in a sediment drift at Ocean Drilling Program (ODP) Site 1063 on the northeast Bermuda Rise. We sub-sampled the cores from both holes using U-channels (up to 1.5 m long samples with 2×2 cm cross-section) focusing on continuous sampling through the late Matuyama interval containing the Cobb Mountain Subchron. Results of standard progressive alternating field demagnetization experiments reveal that these sediments generally have a stable characteristic component after removal of a low-coercivity drilling overprint. Additional discrete samples (∼7 cm 3 oriented cubes) used in progressive thermal demagnetization studies display similar behavior. By taking duplicate sets of U-channels, we were able to identify and remove several measurement artifacts, such as edge effects that affect measurements made near the ends of the U-channel samples. The upper normal-to-reverse polarity transition was not obtained because of a coring gap or an interval of coring deformation. After removing measurement artifacts, the records from the two holes were stacked to obtain a composite record. Virtual geomagnetic poles (VGPs) computed from the composite record follow a path through the central Pacific during the lower polarity transition, very similar to VGP paths from Cobb Mountain Subchron records from other sites around the world. Furthermore, the VGPs cluster near southern Africa early in the reversal process and then group in a nearly antipodal patch in the north Pacific late in the reversal. Taken together, these records indicate that the transitional field remained nearly dipolar through the polarity reversal, or at least during the beginning and end of the reversal when the field was dominated by an axial dipole tilted about 20–40° to the spin axis. If a tilted dipole is an inherent part of the initiation and ending of a reversal, it may help explain why many sediment recorders, which are thought to smooth the field, record antipodal VGP paths through many reversals.