Kenneth A. Hoffman

Dating transitionally magnetized lavas of the late Matuyama chron : Toward a new 40Ar/39Ar timescale of reversals and events

The K-Ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed, but rarely dated, the transitions between polarity intervals. Subsequent 40Ar/39Ar dating indicated that the ages of some polarity transitions had been underestimated by about 6%. Although the accepted ages of the polarity chron boundaries have increased, their precise temporal definition remained uncertain. We have taken a different approach and used incremental-heating techniques to obtain 18 new 40Ar/39Ar ages from basaltic lavas within flow sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Hawaii. These lavas record transitional paleomagnetic directions corresponding to four mid-Pleistocene polarity reversals or events. Three lavas from Punaruu Valley previously thought to record the Cobb Mountain Normal Polarity Subchron (CMNS) gave a mean age of 1.105 ± 0.005 Ma, indicating that they were erupted about 76 kyr after the CMNS; this period of transitional field behavior is designated the Punaruu event. In addition, seven new 40Ar/39Ar ages from the Punaruu Valley indicate that the Jaramillo Normal Polarity Subchron (JNS) lasted about 67 kyr, starting at 1.053 ± 0.006 Ma and ending 0.986 ± 0.005 Ma. This agrees with astronomical estimates but conflicts with JNS ages proposed by Spell and McDougall [1992] and Izett and Obradovich [1994] on the basis of 40Ar/39Ar dating of rhyolite domes in the Valles Caldera. Indistinguishable 40Ar/39Ar ages of seven lavas, including one from Punaruu Valley and six from Haleakala that record broadly similar intermediate paleodirections, suggest that the Kamikatsura event occurred at 0.886 ± 0.003 Ma. Moreover, these data indicate that the Kamikatsura event occurred 20–40 kyr after another geomagnetic event, most probably taking place at 0.92 Ma. We designate this earlier field behavior the Santa Rosa event, adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the JNS in the 1968 study of Doell et al. [1968]. The discovery of these new short-lived polarity events during the Matuyama reversed chron suggests that the 400 kyr period between 1.18 and 0.78 Ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse. This newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks.

Earliest presence of humans in northeast Asia

The timing of the earliest habitation and oldest stone technologies in different regions of the world remains a contentious topic in the study of human evolution. Here we contribute to this debate with detailed magnetostratigraphic results on two exposed parallel sections of lacustrine sediments at Xiaochangliang in the Nihewan Basin, north China; these results place stringent controls on the age of Palaeolithic stone artifacts that were originally reported over two decades ago. Our palaeomagnetic findings indicate that the artifact layer resides in a reverse polarity magnetozone bounded by the Olduvai and Jaramillo subchrons. Coupled with an estimated rate of sedimentation, these findings constrain the layers age to roughly 1.36 million years ago. This result represents the age of the oldest known stone assemblage comprising recognizable types of Palaeolithic tool in east Asia, and the earliest definite occupation in this region as far north as 40° N.

Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale

[1] A detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of Barranco de los Tilos on the island of La Palma, Canary Islands, reveals a complex, temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events. The Matuyama-Brunhes (M-B) reversal is recorded in five transitionally magnetized lava flows from the north (TN) section. The isochrons obtained from three of the lower four M-B lavas are defined by 14 incremental heating experiments that, together with a previous age determination, yielded a weighted mean of 798.4 ± 6.2 ka (all uncertainties ±2s). In addition, a 780.3 ± 10.3 ka isochron was determined for the overlying transitionally magnetized flow, indicating that it was erupted during a distinctly younger portion of the transition. Near the base of the south (TS) section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole (VGP) positions in the southwest Indian Ocean between latitudes 56� S and 65� S, suggesting instability of the geomagnetic field beyond that of typical secular variation. 40 Ar/ 39 Ar isochrons from three of these flows, defined by 11 separate incremental heating experiments, gave a weighted mean of 822.2 ± 8.7 ka. This anomalous field behavior recorded 24 ± 11 kyr prior to the M-B reversal may coincide with an event featured in several marine sediment records. Directly above two normal polarity flows ( 40 Ar/ 39 Ar isochrons of 751.9 ± 18.1 ka and 675.0 ± 15.7 ka) are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude VGPs spanning 23� –60� N. These flows are then capped by a single flow possessing normal polarity. Based on 12 incremental heating experiments, 40 Ar/ 39 Ar isochrons of five of these nine lavas, along with the uppermost flow, gave a weighted mean age of 580.2 ± 7.8 ka for this period of transitional to normal field behavior. From these same transitional lavas, Quidelleur et al. [1999] reported three unspiked K-Ar ages with a weighted mean of 602 ± 24 ka and proposed a new event called the ‘‘La Palma’’ excursion. However, the 40 Ar/ 39 Ar age presented here is three times more precise than the K-Ar age and is indistinguishable at the 95% confidence level from the 40 Ar/ 39 Ar age of a lava from the Snake River Plain, Idaho, that originally defined the Big Lost event. Transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the Big Lost event recorded at La Palma was indeed global in extent. Rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process. INDEX TERMS: 1560 Geomagnetism and Paleomagnetism: Time variations—secular and long term; 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 1035 Geochemistry: Geochronology; 1513 Geomagnetism and Paleomagnetism: Geomagnetic excursions; KEYWORDS: argonargon, dating, lavas, excursion, paleomagnetism, reversal

Magnetostratigraphic dating of early humans in China

Abstract China is a key area for research into human occupation in the Old World after the initial expansion of early humans out of Africa. Reliable age determinations are pivotal for assessing the patterns of human evolution and dispersal in this region. This paper reviews magnetostratigraphic studies of some early Pleistocene strata bearing hominin remains and/or artifact stone tools from northern to southern China. The quality and reliability of the paleomagnetic dates are also evaluated. New magnetostratigraphic results for the Xihoudu Paleolithic site in north-central China are also presented. Among the few hominin or Paleolithic sites in China from the early Pleistocene, five have been well investigated. Three of these are in northern China at Xiaochangliang, Donggutuo and Gongwangling; the remaining two in southern China at Longgupo and Yuanmou. Considerable progress has been made during the past three decades towards paleomagnetically dating these sites. Indeed, the age estimates at Xiaochangliang and Gongwangling have been widely accepted. Magnetostratigraphic consensus about the age of the Donggutuo site has also been reached. However, the age determination for the two sites in southern China is still contested. The integration of rock-magnetic stratigraphy with magnetic polarity stratigraphy has resulted in a determined age of 1.36 Ma for a stone tool-containing layer of lacustrine sediments at the Xiaochangliang site in the Nihewan Basin. Lithostratigraphic constraints have significantly contributed to determining the age of Lantian Homo erectus at Gongwangling, which occurred in a loess–paleosol sequence of the southern Loess Plateau. Its age, paleomagnetically derived, is about 1.15 Ma. These two paleomagnetic ages suggest an expansion and flourishing of human groups from northern to north-central China during the early Pleistocene. This suggestion has been reinforced by our new magnetostratigraphic age estimate of about 1.27 Ma for the Xihoudu site as will be presented in this paper. However, more work is needed for the age determinations of the hominin-bearing strata at the Longgupo and Yuanmou sites in southern China. Finally, we stress that it is necessary to combine biostratigraphy, lithostratigraphy and rock-magnetic stratigraphy with the traditional magnetic polarity stratigraphy in order to obtain reliable age determinations for strata bearing hominin remains or stone artifacts.

Structural and temporal requirements for geomagnetic field reversal deduced from lava flows

Reversals of the Earths magnetic field reflect changes in the geodynamo—flow within the outer core—that generates the field. Constraining core processes or mantle properties that induce or modulate reversals requires knowing the timing and morphology of field changes that precede and accompany these reversals. But the short duration of transitional field states and fragmentary nature of even the best palaeomagnetic records make it difficult to provide a timeline for the reversal process. 40Ar/39Ar dating of lavas on Tahiti, long thought to record the primary part of the most recent ‘Matuyama–Brunhes’ reversal, gives an age of 795 ± 7 kyr, indistinguishable from that of lavas in Chile and La Palma that record a transition in the Earths magnetic field, but older than the accepted age for the reversal. Only the ‘transitional’ lavas on Maui and one from La Palma (dated at 776 ± 2 kyr), agree with the astronomical age for the reversal. Here we propose that the older lavas record the onset of a geodynamo process, which only on occasion would result in polarity change. This initial instability, associated with the first of two decreases in field intensity, began ∼18 kyr before the actual polarity switch. These data support the claim that complete reversals require a significant period for magnetic flux to escape from the solid inner core and sufficiently weaken its stabilizing effect.

Polarity Transition Records and the Geomagnetic Dynamo

The Parker-Levy approach to reversals of the geomagnetic field predicts meridional transitional paths of the virtual geomagnetic pole (VGP) which pass either through the site of observation or through its antipode, depending upon the site location and the sense of the polarity transition. Comparison with the most detailed transitional VGP path records presently available gives some indication of the above behavior as predicted by the Parker-Levy model. Discrepancies may be due to complexities in the distribution of cyclonic convection cells in the core not considered in the formal mathematical treatment. The predicted variation in transitional field intensity experienced at any given site also is compatible with several reported transition records.

Separation of multi-component NRM: A general method

A method is presented which can successfully isolate components of remanent magnetization having intermediate relative stability in a single rock sample which contains any number of remanence components with overlapping coercivity or blocking temperature spectra. The approach consists of analysis of the path swept out by the vector destroyed during a detailed alternating field or thermal demagnetization run. The point of intersection determined for any two neighboring great circle segments identified in such a difference vector path defines the direction of such a component. Samples cored from a fragment of a Jurassic pillow basalt, shown to contain several components of magnetization, serve to illustrate the utility of the method for the case when the Zijderveld approach is unsuccessful.

Testing the accuracy of the geomagnetic polarity time-scale (GPTS) at 2–5 Ma, utilizing40Ar/39Ar incremental heating data on whole-rock basalts

Abstract Recent40Ar/39Ar dating and astrochronologic calculations indicate that the K-Ar derived GPTS underestimates the ages of geomagnetic field reversals in the interval ∼ 2-0 Ma. We report tests to check the accuracy of K-Ar data utilized to date the Reunion Event (∼ 2.1 Ma) and part of the early Pliocene portion (∼ 4.6 Ma) of the GPTS.40Ar/39Ar incremental heating studies on whole-rock basalts from the island of Reunion yield an age of2.14 ± 0.03 Ma for the Reunion Event, somewhat older than the earlier K-Ar value of2.07 ± 0.02 Ma on the same set of lava flows. Study of a single specimen from the Newer Volcanics, Australia, indicates that the earlier K-Ar date was accurate; but the new40Ar/39Ar results yield a plateau age (4.56 Ma) with a standard error of ∼ 0.6%, as compared to the K-Ar value ∼ 3%. The improved precision and accuracy resulting from such40Ar/39Ar incremental heating studies will play a key role in calibrating more precisely the GPTS for the interval 10-0 Ma.

Behavior of the geodynamo during reversal: A phenomenological model

A model is presented which interprets observed transitional behavior of the geomagnetic field to be reflective of a reversal process which begins in a localized region of the core. Azimuthal as well as latitudinal extension (or “flooding”) of reversed flux is then assumed to continue until the entire magnetic source region has been affected. Comparison with the paleomagnetic records corresponding to the Matuyama-Brunhes transition presently available from five site localities scattered about the northern hemisphere indicates that the model can successfully simulate global time-dependent variation in the reversing geomagnetic vector field. For low- and mid-latitude sites, predicted behavior during transitions is found to be compatible with that outlined earlier for the case of axisymmetric flooding. The model also successfully accounts for the high degree of dispersion among paths of the virtual geomagnetic pole (VGP) associated with Icelandic data. Comparison of VGP paths corresponding to four Cenozoic transitions recorded in the western United States suggests that there may exist a small number of discrete sites in the core at which reversals originate. It is not as yet clear whether these findings lend support to any particular kinematic or hydromagnetic model of the geodynamo.

Self‐reversal of thermoremanent magnetization in the ilmenite‐hematite System: Order‐disorder, symmetry, and spin alignment

A model of the process by which certain members of the ilmenite-hematite solid solution series acquire reverse thermoremanent magnetization (reverse TRM) is proposed. It is based on the observation that the cation order-disorder transformation at work in this mineral system produces cation-ordered regions, or domains, antiphase with respect to cation positions. Given the fundamental assumption that the so-called “x phase”, the weakly magnetic phase known to control self-reversal, is in fact Fe-rich cation-disordered boundary material between the two types of cation-ordered domains, symmetry considerations help eliminate the possibility that the direction of magnetic remanence associated with the boundaries lies parallel to the line of Fe spin. Rather, the net magnetization of the boundary material need be essentially at right angles to the orientation of spin, as is the case for pure hematite. Results of thermomagnetic experiments further suggest that superexchange with the Fe-rich boundary material produces a femmagnetic structure within each cation-ordered domain such that its net moment lies along the direction of spin, that is, perpendicular to the net boundary moment. At this stage the moments of each pair of cation-ordered domains (antiphase in composition) cancel. We propose that reverse TRM arises during cooling in an applied field through rotation of particular Fe spins within each cation-ordered domain, causing the spin arrangement to become progressively noncollinear. Competing superexchange interactions within the cation-ordered regions may be responsible for such a spin rotation. According to this model, each cation domain provides a component of magnetization opposite to the net magnetization associated with the boundary material. Provided that there exists in a given grain equal fractions of each type of cation domain, the grain will fully self-reverse. Synthesized polycrystalline samples as well as rocks containing ferrian ilmenites having compositions in the self-reversing range will acquire reverse TRM.