Paleointensity Research Articles

New 40Ar/39Ar age of the full vector Upper Mammoth geomagnetic polarity transition recorded in the Pu’u Kualakauila volcanic sequence, Hawaii

Paleomagnetic measurements, coupled with 40Ar/39Ar dating, are improving our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the Earth's magnetic field. As part of an investigation of the Wai'anae Volcano, Oahu, and the short-term behavior of the geomagnetic field, we sampled a long volcanic section located on the volcano's collapsed flank at a locality known as Pu’u Kaulakauila. Prior paleomagnetic investigations of the Kamaile'unu Volcanic Series (i.e. Herrero-Bervera and Coe, 1999, Herrero-Bervera and Valet, 1999, 2005) revealed transitional directions. The fresh lava flows, easy access, and close geographical proximity to KAr dated flows made this ~215-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 47 successive flow sites, were stepwise demagnetized by both alternating field and thermal methods. Magnetostratigraphic results indicate the existence of four excursions or aborted reversals occurring at approximately ~36, ~75, ~130, and ~ 151 m above flow#1. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼10 m of the section characterized by truly excursional/Cryptochron directions (~7 lava flows). Paleointensity (P.I.) determinations indicate a steady decreasing of the PI values from 75 μ-T to ~12.2 μ-T reaching a minimum of 7.1 μ-T. These very low PI values are within the transitional/excursional part of the record. The corresponding VGPs are located on the western part of Australia. 40Ar/39Ar incremental heating experiments on groundmass from transitional flow sites at different stratigraphic levels yield a weighted mean plateau age of 3.201 ± 0.041 Ma, which, combined with the overall reversed polarity and two older polarity reversals, strongly suggests that the transitional lavas correspond to the Upper Mammoth polarity transition.

Study of Declination, Inclination and Absolute Paleointensity of the Short-Term Geomagnetic Behavior (i.e. Cryptochron C2r.2r-1, ca. 2.46 ± 0.13 Ma) Recorded at the Type Section of Halawa Valley, Koo’lau Volcano, Oahu, Hawaii, USA

A novel determination of the absolute paleointensity (P.I) of 28 flows that recorded Cryptochron C2r.2r-1 (ca. 2.46 ± 0.13 Ma) using the modified Thellier-Coe method was undertaken to understand the geomagnetic evolution of the paleofield during an excursion with the existence of less than 10 Kyrs. The successful PI determinations along the 120-meter sequential erupted flows indicate that within the truly transitional/excursional portion of the record there is a conspicuous decrease of the PI values ranging from 20 μT to low values of about 5 μT. These values are comparable to lows similar to polarity transitions of the geomagnetic field. At the base of the sequence, the record shows oscillations of the paleofied ranging from 120 μT and as low as ~10 μT with very variable changes of the non-transitional/excursional paleofield.

A comparison of Thellier-type and multispecimen paleointensity determinations on Pleistocene and historical lava flows from Lanzarote (Canary Islands, Spain)

project CGL2012-32149 (Ministerio de Econom ia y Competitividad, Spain), project 320/2011 (Ministerio de Medio Ambiente y Medio Rural y Marino, Spain) and the European Regional Development Fund (ERDF).

Is there a link between geomagnetic reversal frequency and paleointensity? A Bayesian approach

AbstractOver the last 25 years, several studies have tested for a link between geomagnetic field intensity and reversal frequency. However, despite a large increase in the number of absolute paleointensity determinations, and improved methods for obtaining such data, two competing models have arisen. Here we employ a new tool for objectively analyzing paleomagnetic time series to investigate the possibility of a link between reversal frequency and paleointensity. Transdimensional Markov chain Monte Carlo techniques are applied to a quality‐filtered version of the global paleointensity (PINT) database for the last 202 Myr to model long‐term paleointensity behavior. A large ensemble of models is sampled, from which a final representative mean model is extracted. The resulting paleointensity model confirms published conclusions that the single‐silicate crystal method gives significantly different results from more conventional whole rock paleointensity methods; this makes it difficult to jointly model the two data types in the same analysis. When the much larger whole rock data set is considered, a stable paleointensity of 5.46±0.28×1022 A/m2 for the last 202 Myr is consistent with the 95% confidence interval of the paleointensity model. Statistical tests indicate no significant correlation between reversal frequency and field intensity at the 0.05 level. However, this result is likely due to the characteristics of the PINT database rather than being a genuine, physically representative conclusion. Given the paucity of data and general state of the global paleointensity database, concerted efforts to increase the number of high‐quality, well‐dated paleointensity data are required before conclusions about a link between geomagnetic field intensity and reversal frequency can be confidently drawn.

Paleointensity Database Updated and Upgraded

The global absolute paleointensity (PINT) database, sponsored by the International Association of Geomagnetism and Aeronomy, recently was updated with new data and published alongside a new queryable interface at http://earth.liv.ac.uk/pint/.Absolute paleointensity determinations are spot readings of the strength of the geomagnetic field at the time and place at which a rock cooled down from elevated temperatures. The time generally can be estimated using an independent dating technique applied to the rock itself, and the place (or at least the paleolatitude) of the rock often can be estimated from the associated paleomagnetic direction.

Comparison between the microwave and multispecimen parallel difference pTRM paleointensity methods

SUMMARY The San Quintin volcanic field (SQVF) in Baja California (Mexico) consists of at least 13 volcanic complexes with about 30 lava flows. 40Ar/39Ar step-heating and 3He surface exposure dating experiments suggest eruption ages between 22 and 165 ka. Paleodirections of 14 flows reported here are consistent with normal secular variation. Because of low Curie temperatures and thermal alteration starting at ∼350 °C, classical Thellier–Thellier paleointensity (PI) experiments were deemed unsuccessful. Microwave PI experiments were carried out on 139 samples with 97 being interpreted to be successful, yielding values smaller than 20 μT for all sites and averaging 16.5 μT. Another sample set was studied with the multispecimen parallel differential pTRM method; temperatures for acquisition of pTRM were chosen between 200—and 225 °C. In two cases of higher unblocking temperatures, it was set at 480 °C. The experiments were successful for 104 out of 120 specimens with PI values ranging between 14 and 21 μT and clustering around 18 μT. Three noticeably higher values between 28 and 35.7 μT were obtained. Both methods yield in general well comparable PI data with the microwave PI showing a tendency to be slightly smaller than those from the multispecimen PI. The reason for it is presently not known with certainty. Possible causes include 1) incipient thermal alteration, anticipated to be slightly more prominent in the multispecimen method. 2) Cooling time effects: the microwave method involves heating during only 10–30 s compared to 2400 or more seconds in the multispecimen method. This may account for a relative underestimate of the field intensity by about 4 per cent. 3) A general PI underestimate (by ∼4 per cent) inherent to the microwave perpendicular field method applied to multidomain particles in comparison to parallel field methods. 4) A PI overestimate for small multidomain particles in the multispecimen method. However, both PI methods produced very similar results even for difficult materials as the SQVF lavas giving credit to the veracity of the obtained PI values. The multispecimen parallel differential pTRM paleointensity method thus presents a valuable alternative for traditional thermal PI determinations, as it can be applied using standard laboratory equipment. The microwave paleointensity method may yield better results but requires specialized equipment not yet available commercially.

Cooling rate corrected paleointensities from the Xitle lava flow: Evaluation of within-site scatter for single spot-reading cooling units

Abstract The historic Xitle lava, which covers a great part of southern Mexico City, is one of the most studied (magnetically) volcanic unit worldwide. Studies include detailed paleomagnetic and paleointensity investigations, which have documented an enigmatic within-flow variation of absolute paleointensity first recognized in the decade of 1960s. However, attempts to find possible explanations in terms of physical/magnetic parameters or geomagnetic effects have been unsatisfactory. As an effort to understand the over- and underestimating of geomagnetic paleointensity (PI) within the Xitle lava flow, we investigated the relation of the cooling rate upon the acquisition of thermoremanent magnetization (TRM). Contrary to archaeomagnetic investigations, most paleointensity experiments on volcanic rocks do not consider cooling rate effects in a systematic manner. Our results show that the scatter and overestimation of PI values obtained on the single Xitle lava flow are drastically reduced when using the cooling rate correction to raw selected data. The Thellier method combined with cooling rate experiments provide a higher precision compared to conventional Thellier and microwave techniques. The cooling rate effects upon acquisition of TRM in volcanic rocks seem to be as critical as in archaeomagnetic investigations.

Variation of Rock Magnetic Parameters and Paleointensities over a Single Holocene Lava Flow.

A recent (2000 BP) lava flow from central Mexico has been sampled along a vertical profile with 55 cores covering a total flow thickness of 6.6 m. A wide range of physical and magnetic parameters have been studied to characterise the samples: Curie temperature and saturation magnetisation as intrinsic properties; density, magnetic susceptibility and remanence intensity as bulk properties; Konigsberger Q-factor and hysteresis parameters as coercivity parameters. All parameters vary smoothly over the profile, most probably due to grain size variation of the magnetic minerals present in the samples. Optical observations indicate that the main opaque minerals are deuterically oxidised titanomagnetites (C3-C5) and ilmenites (R2-R3), which increase in size away from the edges of the flow. Paleointensity (PI) was determined using the double heating Thellier-Thellier method with pTRM checks. According to reliability parameters (f-, g-, and q-factor) the obtained PIs are of reasonable to good quality. PI shows marked variation with vertical position in the flow, across a range of about 25 to 125 μT, with most samples having a PI between 50 and 100 μT. The flow-mean PI of 72 μT is higher than the present day field, consistent with global data for this time-period. No obvious correlation could be found between PI and any other measured parameter. The variation of PI with vertical position in the flow may show some systematic behaviour. It is important, therefore, to sample a flow both horizontally and vertically in order to obtain a reliable paleointensity.

Preliminary paleointensity measurements and detailed magnetic analyses of basalts from the Skalamaelifell excursion, southwest Iceland

Among the many excursions reported in the late Brunhes, the Skalamaelifell excursion, recorded in 22 flows in three hills of assumed late Wisconsin basalts in the active rift zone of the Reykjanes peninsula, is one of the most reliable. It is characterized by a single, well‐defined transitional direction (D = 260, I = −16). Paleointensity (PI) measurements, using the Thellier method as modified by Coe in a vacuum with partial thermoremanent magnetization checks, were made on six samples from the excursion flows. For comparison, 10 samples from the adjacent and probably nearly contemporaneous normally magnetized flows were also measured. Because of sample scarcity, four of the excursion samples and seven of the normal samples had been already af demagnetized to 20 mT in Iceland. The samples, however, were chosen because of their high coercivity and lack of viscous remanent magnetization. They retained 70–95% of their natural remanent magnetization (NRM) after 20 mT demagnetization, and their remanence directions were stable during the 20 mT demagnetization and during 2–6 years of storage following it. Saturation magnetization (Js) measurements suggest that the reason for the low NRM of the excursion samples was a paleofield one fourth to one tenth that of the normal basalts. The mean PI from four excursion samples is 4.3±0.6 μT, whereas the mean PI of the five normal samples considered reliable is 30±9 μT. The quality of the NRM‐thermoremanent magnetization (TRM) data correlated well with the reversibility of the Js‐T curves. The PI differences agree, in general, with those predicted by the NRM/Js data, especially when comparing the means of the two groups, which average the effects of different grain sizes. A prior, partial alternating field (af) demagnetization of one sample in each of two pairs from the same cores caused the PI to be underestimated by only 10%. Continuous af demagnetization during the NRM‐TRM measurements of one sample in another pair that had nonlinear TRM‐TRM plots, even in the undemagnetized sample, caused the PI to be overestimated by 40%. These results, combined with other studies, suggest that the effect of partial af demagnetization on high‐coercivity samples depends on when the demagnetization is done, i.e., only prior to or during the NRM‐TRM measurements, and, probably more importantly, the thermochemical stability of the sample. If correct, this increases the usefulness for PI determinations of old, hard to recollect, partially demagnetized collections. The very low PI of 4 μT is similar to that found for excursions and transitions in Iceland and other places. However, o its virtual geomagnetic pole lies 90° from a north–south vertical plane through the sites, which rules out a purely axisymmetric model for the excursion. The low normal intensities are similar to those found in the Chaine des Puys (France) and elsewhere for the period 10,0000–50,000 years B.P. Given its very low PI and a direction only 70° from complete reversal, the Skalamaelifell excursion is most likely the result of nondipole fields and an unusually weak dipole field, such as that observed in France for the 10,000‐year interval, 35,000–45,000 years B.P., surrounding the Laschamp excursion.

Paleomagnetic and sedimentological studies at Lake Tahoe, California-Nevada

Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis. Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of 14C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P. X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction. Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined. Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.