Joseph C. Liddicoat

Age of the Mono Lake excursion and associated tephra

Abstract The Mono Lake excursion (MLE) is an important time marker that has been found in lake and marine sediments across much of the Northern Hemisphere. Dating of this event at its type locality, the Mono Basin of California, has yielded controversial results with the most recent effort concluding that the MLE may actually be the Laschamp excursion (Earth Planet. Sci. Lett. 197 (2002) 151). We show that a volcanic tephra (Ash ♯15) that occurs near the midpoint of the MLE has a date (not corrected for reservoir effect) of 28,620±300 14C yr BP (∼32,400 GISP2 yr BP) in the Pyramid Lake Basin of Nevada. Given the location of Ash ♯15 and the duration of the MLE in the Mono Basin, the event occurred between 31,500 and 33,300 GISP2 yr BP, an age range consistent with the position and age of the uppermost of two paleointensity minima in the NAPIS-75 stack that has been associated with the MLE (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009). The lower paleointensity minimum in the NAPIS-75 stack is considered to be the Laschamp excursion (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009).

A detailed record of paleomagnetic field change from Searles Lake, California: 2. The Gauss/Matuyama polarity reversal

This new study of the Gauss/Matuyama transition from Searles Lake, California conjoined with other records from the western United States, provides interesting insights into the structure of the reversing magnetic field. The present study employs improved measurement and data reduction techniques, multiple parallel strings of samples, and a finer sampling interval than was used in the original study by Liddicoat[1982]. Particularly crucial to this investigation was the use of overprint directions to reconstruct declinations, required because the core was rotary drilled. The results of this technique were corroborated by employing an independent method that uses anisotropy of magnetic susceptibility to resolve a sediment fabric: the fabric facilitated the alignment of core segments. The new record reveals that the main swing of the transition occurs over a significantly shorter time span than was found in the original study. In addition, it brings out several small scale variations that were absent in the old record, some of which take the form of relatively rapid jumps in direction that punctuate more steadily varying changes. This alternating steady and rapid field change is similar to behavior observed in volcanic records, which argues that such behavior is not merely an artifact of episodic volcanism. The Searles Lake record is strongly nonzonal and is defined in the Northern Hemisphere by a swath of virtual geomagnetic poles (VGPs) stretching from northern Eurasia to west Africa and to the northwest Pacific. Glen et al. [1994] showed that a collection (spanning >15 Myr) of western North American transition and excursion records displays this same pattern, indicating that the VGP swath is a persistent feature of the transitional field. In addition, the compilation reveals that the swath extends into the Southern Hemisphere, outlining a region marked by an absence of poles that is centered on the Indian Ocean. The fact that this pattern is offset from a similar one seen in global compilations suggests that the persistent fields have a significant nondipolar component. Seven additional records are now available, making the western North America data set perhaps the finest regional set of high-resolution records consisting of both igneous and sedimentary records. The new records, which provide an important test of the existence of the VGP pattern, strongly support the findings that reveal the presence of persistent, long-term (>15 Myr) nondipolar transitional fields.

Persistent features of polarity transition records from western North America

We report on a high resolution transition record from Searles Valley, California that appears not to be hampered by problems of smoothing or recording breakdown, and, unlike many other sedimentary records, defines a path unconfined in longitude. A prominent feature of the record is the steady oscillation of the pole along a swath stretching from west Africa across north Eurasia to the northwest Pacific. The endpoints of the swath are associated with a change between steady and rapid field variation, consistent with results for an older transition from a nearby volcanic record. The transition culminates with poles migrating from west Africa to South America, midway between the two earlier-proposed longitudinal bands. This path indicates the reversing field has either a wider spectrum of behavior than recently suggested or is described by a more complicated field geometry than one dominated by an equatorial dipole or strong low-order nondipole component. From a comparison with existing records from the same general geographic area, we address the significance of the swath, the lack of longitudinal confinement, patterns of steady and rapid field changes, and the temporal persistence of these features in relation to the geometry of the reversing field.

Short Reversal of the Palaeomagnetic Field about 280 000 Years Ago at Long Valley, California

A reversal of the palaeomagnetic field is recorded in exposed lake sediments at Long Valley and Mono Basin in east-central California. The reversal is estimated to be several thousand years long and 280 000 years old. The chronology is based on correlation of volcanic ash beds at Long Valley and Mono Basin with ones at Summer Lake, Oregon, and correlation of ash beds at Summer Lake with ash beds in a core at Tulelake, California, where age control is provided by tephrochronology and magnetostratigraphy.

Nongeocentric axial dipole field behavior during the Mono Lake excursion

A new record of the Mono Lake excursion (MLE) is reported from the Summer Lake Basin of Oregon, USA. Sediment magnetic properties indicate magnetite as the magnetization carrier and imply suitability of the sediments as accurate recorders of the magnetic field including relative paleointensity (RPI) variations. The magnitudes and phases of the declination, inclination, and RPI components of the new record correlate well with other coeval but lower resolution records from western North America including records from the Wilson Creek Formation exposed around Mono Lake. The virtual geomagnetic pole (VGP) path of the new record is similar to that from another high-resolution record of the MLE from Ocean Drilling Program (ODP) Site 919 in the Irminger Basin between Iceland and Greenland but different from the VGP path for the Laschamp excursion (LE), including that found lower in the ODP-919 core. Thus, the prominent excursion recorded at Mono Lake, California, is not the LE but rather one that is several thousands of years younger. The MLE VGP path contains clusters, the locations of which coincide with nonaxial dipole features found in the Holocene geomagnetic field. The clusters are occupied in the same time progression by VGPs from Summer Lake and the Irminger Basin, but the phase of occupation is offset, a behavior that suggests time-transgressive decay and return of the principal field components at the beginning and end of the MLE, respectively, leaving the nonaxial dipole features associated with the clusters dominant during the excursion.

A detailed record of paleomagnetic field change from Searles Lake, California. 1. Long-term secular variation bounding the Gauss/Matuyama polarity reversal

More than 33 m of 2.5 Ma sediment from Searles Lake, California was studied in order to construct a record of secular variation (SV) across the Gauss/Matuyama (G/M) normal-to-reverse polarity transition. The behavior of the field preceding and following the reversal is considered here, while in a companion paper [Glen et al., this issue] the details of the transition are discussed. The record encompasses an interval of roughly 183,000 years beginning 50 kyr (9 m) before and extending more than 128 kyr (23 m) beyond the transition, while the main phase of the transition lasts for nearly 5 kyr (1 m). Because the core was rotary drilled, and declinations lost, SV was characterized by the inclination and its angular dispersion. Inclination-only statistics reveal that (1) the record displays overall higher than expected values of angular dispersion (normal S∼20°; reverse S∼19°; expected S∼15.5°), suggesting that the field proximal to transitions may be more noisy than the distal field. In addition, normal data from immediately before the transition display higher S than reverse data immediately following it, implying that the postransitional field is more stable than the pretransitional field. One of the most prominent features of this record is an excursion of the field occurring roughly 4 kyr prior to the onset of the reversal. A record of the G/M transition from Chinese loess (R. Zhu et al., submitted manuscript, 1999) displays a similar event (also occurring roughly 4 kyr before the transition). This and the fact that the event is associated with anomalously low intensities suggest that the disturbance may be global in nature. The fact that comparable features are associated with other transitions [Hartl and Tauxe, 1996; Clement, 1992] intimates that the field may commonly show signs of early instability. This precursory event is actually one of a sequence of oscillations (in inclination and intensity) preceding the transition. That these fluctuations occur at roughly 4 kyr intervals leading up to the reversal (which also appears at this same interval) strongly suggests that an oscillatory disturbance in the core, active over at least 15 kyr prior to the transition, had eventually triggered the reversal. In addition, that these waveforms are absent from the postransitional record suggests the reversal process actively rejuvenates and stabilizes the field.

Late Neogene–Quaternary tephrochronology, stratigraphy, and paleoclimate of Death Valley, California, USA

Sedimentary deposits in midlatitude continental basins often preserve a paleoclimate record complementary to marine-based records. However, deriving that paleoclimate record depends on having well-exposed deposits and establishing a sufficiently robust geochronology. After decades of research, we have been able to correlate 77 tephra beds exposed in multiple stratigraphic sections in the Death Valley area, California, United States. These correlations identify 25 different tephra beds that erupted from at least five different volcanic centers from older than 3.58 Ma to ca. 32 ka. We have informally named and determined the ages for seven previously unrecognized beds: ca. 3.54 Ma tuff of Curry canyon, ca. 3.45 Ma tuff of Furnace Creek, ca. 3.1 Ma tuff of Kit Fox Hills, ca. 3.1 Ma tuff of Mesquite Flat, ca. 3.15 Ma tuff of Texas Spring, 3.117 ± 0.011 Ma tuff of Echo Canyon, and the ca. 1.3 Ma Amargosa ash bed. Several of these tephra beds are found as far northeast as central Utah and could be important marker beds in western North America. Our tephrochronologic data, combined with magnetic polarity data and ^(40)Ar/^(39)Ar age determinations, redefine Neogene sedimentary deposits exposed across 175 km^2 of the Death Valley area. The alluvial/lacustrine Furnace Creek Formation is a time-transgressive sedimentary sequence ranging from ca. 6.0 to 2.5 Ma in age. The ca. 2.5−1.7 Ma Funeral Formation is typically exposed as a proximal alluvial-fan facies overlying the Furnace Creek Formation. We have correlated deposits in the Kit Fox Hills, Salt Creek, Nova Basin, and southern Death Valley with the informally named ca. 1.3−0.5 Ma Mormon Point formation. In addition, our correlation of the late Pleistocene Wilson Creek ash bed 15 in the Lake Rogers deposits represents the first unambiguous sequences deposited during the Last Glacial Maximum (marine isotope stage [MIS] 2) in Death Valley. Based on this new stratigraphic framework, we show that the Pliocene and Pleistocene climate in Death Valley is consistent with the well-established marine tropical/subtropical record. Pluvial lakes in Death Valley and Searles Valley began to form ca. 3.5−3.4 Ma in the late Pliocene during MIS MG5. Initiation of lakes in these two hydrologically separated valleys at the same time at the beginning of a cooling trend in the marine climate record suggests a link to a cooler, wetter (glacial) regional climate in North America. The Death Valley lake persisted until ca. 3.30 Ma, at the peak of the M2 glaciation, after which there is no evidence of Pliocene lacustrine deposition, even at the peak of the Northern Hemisphere Glaciation (ca. 2.75 Ma). If pluvial lakes in the Pliocene are an indirect record of glacial climate conditions, as they are for the Pleistocene, then a glacial climate was present in western North America for ∼200,000 yr during the Pliocene, encompassing MIS MG5−M2. Pleistocene pluvial lakes in Death Valley that formed ca. 1.98−1.78 Ma, 1.3−1.0 Ma, and ca. 0.6 Ma (MIS 16) are consistent with other regional climate records that indicate a regional glacial climate; however, Death Valley was relatively dry at ca. 0.77 Ma (MIS 19), when large lakes existed in other basins. The limited extent of the MIS 2 marsh/shallow lake in the Lake Rogers basin of northern Death Valley reflects the well-known regional glacial climate at that time; however, Death Valley received relatively lower inflow and rainfall in comparison.