Source: https://sp.lyellcollection.org/content/373/1/261?ijkey=ae853093bcdbd2cad3b96f742b66ead4e25ff5d0&keytype2=tf_ipsecsha
Timestamp: 2019-04-22 00:51:10+00:00

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We studied the detailed characteristics of the Pringle Falls excursion from samples at the original site recovered from four profiles drilled along the Deschutes River, Oregon. We drilled 827 samples spaced along 5 km for their detailed directional study. The profiles registered a high-resolution (>10 cm/ka) palaeomagnetic record of the excursion (c. 211±13 ka) recorded by diatomaceous lacustrine sediments. We conducted palaeomagnetic and rock magnetic studies to investigate the reproducibility of the signal throughout the profiles. We performed low-field susceptibility v. temperature analysis that indicated that the main magnetic carrier is pure magnetite (Curie point 575 °C). The magnetic grain size also indicated single domain–multi domain (SD–MD) magnetite. The demagnetization was performed by alternating field experiments and the mean directions were determined by principal component analyses. The detailed behaviour of the palaeosignal is highly consistent since they are rapidly deposited sediments providing a detailed representation of the palaeofield. The dissected virtual geomagnetic pole paths in three different phases are highly internally consistent and are defined by clockwise and anticlockwise loops travelling from high northern latitudes over eastern North America and the North Atlantic to South America and then to high southern latitudes; then they return to high northern latitudes through the Pacific and over to Kamchatka.
An improved understanding of the origin of the geomagnetic field, and the process by which it reverses its polarity, is a longstanding goal in the Earth Sciences. One of the means by which this goal can be achieved is to examine, in detail, the temporal variations in the geomagnetic field direction and intensity associated with polarity reversals and short polarity excursions or events. One of the more fruitful recent efforts in this field is the utilization of geodynamo models (e.g. Glatzmaier & Roberts 1995; Camps & Prevot 1996; Glatzmaier et al. 1999; Coe et al. 2000; Hoffman & Singer 2008; Valet et al. 2008a, b) to test and refine models for geomagnetic field origin and reversal mechanisms, with comparisons made between spatially and temporally robust sets of full-vector (direction and palaeointensity) geomagnetic field data. One consequence of these efforts has been the recognition that there is a need for many high-quality records of geomagnetic field behaviour from a well-distributed set of site locations for a given time interval of geomagnetic field evolution. In the case of short polarity events or excursions, a number of high-quality records of palaeomagnetic directions and absolute palaeointensities from a variety of locations are needed to gain a full understanding of the origin of these geomagnetic field phenomena and how their behaviour compares with the process by which full reversals of the geomagnetic field occur. Such data could ultimately be used to determine if polarity excursions and reversals are similar geomagnetic field phenomena. In particular, two questions are still pending: are excursions ‘aborted reversals’ (e.g. Merrill & McFadden 1999; Hoffman & Singer 2004), or are they possibly manifestations of a different process (e.g. Gubbins 1999; Valet et al. 2008a, b)? Finding answers to these questions certainly would improve the knowledge and understanding of geodynamo processes in general.
While there are many excellent studies of geomagnetic field polarity reversals (Valet et al. 1986, 1988a, b, 1989, 1992; Clement 1991; Clement & Kent 1991; Laj et al. 1991, 1992a, b; Tric et al. 1991a, b; Clement & Martinson 1992; Herrero-Bervera & Khan 1992; van Hoof & Langereis 1992a, b; McFadden et al. 1993; Zhu et al. 1994; Cisowski 1995; Channell & Lehman 1997; Herrero-Bervera & Coe 1999; Clement 2004; Herrero-Bervera & Valet 2005; Herrero-Bervera et al. 2007; Valet, J. P. & Herrero-Bervera, E. 2010; and others), there are very few full-vector studies of short-lived polarity events or excursions. The majority of such data for excursions are from sedimentary rocks, for which only relative palaeointensity values can be inferred (Levi & Banerjee 1976; Meynadier et al. 1992). Studies of relative palaeointensity derived from sedimentary rocks typically find correlations between relative palaeointensity lows and geomagnetic field reversals or excursions (e.g. Meynadier et al. 1992, 1994; Valet & Meynadier 1993; Verosub et al. 1996; Channell 1999; Guyodo & Valet 1999; Lund et al. 2001; Channell et al. 2002; Laj et al. 2006; Channell 2006; Fuller 2006). While these types of studies are of great value too, today the interpretation of relative palaeointensity data recovered from sediments and sedimentary rocks characterized by high sedimentation rates (e.g. high-resolution records) is relatively simple. Certainly, if the directional data associated with these records is available (i.e. data are obtained from azimuthally oriented marine and lacustrine sediment cores), the information thus obtained augments its value. Unfortunately, owing to the relatively short duration of the geomagnetic field reversal/event process, volcanic and sediments/sedimentary records of geomagnetic field behaviour during reversals or excursions are relatively rare. There are only a handful of directional and palaeointensity records from polarity excursions (Coe et al. 1984; Roperch et al. 1988; Chauvin et al. 1989, 1991; Carlut et al. 1999; Tanaka & Kobayashi 2003; Riisager et al. 2004; Herrero-Bervera & Valet 2005; Cassata et al. 2008). Most of these studies document separate excursions (ranging in age from the Eocene to the Pliestocene), so there is clearly a need for additional studies of particular excursions recorded in several widely-spaced geographic areas (Merrill & McFadden 2005).
Of the short polarity excursions, the Blake and Pringle Falls polarity episodes are among the better recognized and studied, but they remain somewhat enigmatic. The age of the Blake event is generally reported as 110 ka (Tric et al. 1991a, b) based on studies of sediment cores from many parts of the world. In igneous rocks, partial records of the Blake event (usually a single cooling unit) have been recorded in China (Zhu et al. 2000) and in tephras from Japan (Takai et al. 2002). Of these studies, K–Ar ages provide age estimates of 123±7 ka (Zhu et al. 2000), and an older age of 141 ka for the Aso-2 tephra (Takai et al. 2002). The nature of the geomagnetic field during the Blake event is complex and somewhat controversial. Some studies have documented two excursion ‘pulses’ (Herrero-Bervera et al. 1989; Tric et al. 1991a, b) or three (Zhu et al. 1994), spanning 10–50 ky, while in some sedimentary sequences the Blake event is absent in the directional records, and is at times only manifest by a low in relative palaeointensity values (see e.g. Channell 2006; Stoner & St-Onge 2007). Thus, excursions are a very important part of the geodynamo behaviour over geological time scales that, in addition to reversals of the main dipole field, need to be fully studied and definitely considered when evaluating and assessing whether theoretical and numerical simulation of the dynamo produces Earth-like results (e.g. Herrero-Bervera & Helsley 1993; Herrero-Bervera & Runcorn 1997; Coe et al. 2000).
In this paper we present the directional behaviour of the geomagnetic signature of four successful records and their associated palaeomagnetic and rock magnetic characteristics of a short-duration geomagnetic event recorded at Pringle Falls, Oregon. This event took place at c. 211±13 ka, based on a variety of age determinations from either tephras in sedimentary rocks, or from a limited number of locations where volcanic rocks record this event. This event can be considered to represent a very well-documented aborted reversal, particularly in terms of its age, directions and anisotropy of magnetic susceptibility (AMS) correlations (Herrero-Bervera et al. 1994; Singer et al. 2008; Canon-Tapia & Herrero-Bervera 2009). One of the greatest advantages of the record presented here is that the sediment cores have been azimuthally oriented, therefore allowing us to characterize the directional behaviour of the palaeofield with a high degree of confidence. Since our Pringle Falls record is characterized by an extremely high temporal resolution, this study will improve the knowledge of short excursions, which is extremely important now that excursions are regarded as two successive reversals bracketing an aborted polarity interval (Valet et al. 2008a, b).
As described by MacLeod et al. (1982), the sedimentary lake sequence sampled originally for palaeomagnetic studies by Herrero-Bervera et al. (1989, 1994) as well as the subsequent re-visited sites sampled for this study, including the two additional profiles, are part of an extensive pre-historic fluvial and lacustrine system formed during the last million years located east of the Cascade mountains. The most consistent interpretation of the local geology suggests that the lake in question is the result of a late Pliocene/Pleistocene increase of the base level east of the sampling area close to the western margin of the Basin and Range structural province, intimately related to the development of the extensive volcanism associated with the Newberry volcano.
At the sampling sites, the lacustrine sequence is overlain by glacial sand and gravel. The unconformity has c. 30 m of local relief. It is unlikely that we have sampled the youngest portion of the sequence, as glacial erosion has removed at least 20 m of section at this particular site. The age of the lacustrine sequence is constrained by three events. These are (from the oldest to the youngest): (a) according to MacLeod et al. (1982) the onset of volcanism for the Newberry volcano, which is presumed to be younger than 700 ka, as all portions of the volcano are <500 ka; (b) a single diatom age on a sample taken at 195 m in a deep well that penetrates into 410 m of lake sediment which was dated as 600–200 ka based upon the similarity to the diatoms at Clear Lake; and (c) an upper-boundary date of 18–15 ka, based on the age of the latest glaciation in the area.
If one assumes that the basin has an age of no more than 1 Ma, and that the 410 m of section drilled near Bend, Oregon represents a typical maximum thickness, then the youngest sediment can be no younger than 68 ka (20 m times 2500 year m−1, average sedimentation rate plus age of unconformity of 18 ka). Thus, we have sampled the Pringle Falls excursion even though the samples that contain the Pringle Falls geomagnetic polarity episode record come from the very top of our exposed section (see Herrero-Bervera et al. 1989, 1994). We have expanded the study of the original directional study (Herrero-Bervera et al. 1989, 1994) and radiometric dating (Singer et al. 2008) of the original Pringle Falls site. We have, thus far, drilled five separate sites (see Figs 1 & 2) that are currently under investigation and are presented and discussed in this paper.
Map of the southeast portion of Pringle Falls. Outcrops of the five sampling sites of our study. Profile 4 depicted in the figure is a site that was sampled but the sediments did not show a record of the Pringle Falls excursion.
Map of the sampling sites of our current study of the palaeosignal of Pringle Falls. The distance between Profile 2 and Profile 4 is 5 km. Stratigraphic correlation of the expanded study of the Pringle Falls episode along the Deschutes River in Oregon. Notice the Pumice Layer used as a marker horizon to correlate the sites. Profile 4 depicted in the figure is a site that was sampled but the sediments did not show a record of the Pringle Falls excursion.
We drilled a total of 827 samples, and we studied for our palaeomagnetic analyses 1397 specimens (i.e. by means of a gasoline-powered portable drill and orientation using a magnetic compass). The lengths of the recovered cores ranged from 5 to 15 cm in length and were drilled with inclination angles from 3° to c. 60° with respect to the horizontal plane, allowing us to have a continuous sampling of the sediments in question. In some instances, this sampling technique provided an independent observation of the same stratigraphic level, as was the case for the three profiles (i.e. Profiles named 1989 La Pine and 1, 2 and 3) published by Herrero-Bervera et al. (1989, 1994) and Canon-Tapia & Herrero-Bervera 2009 (see Figs 1 and 2). The two additional profiles 1 and 3 were also drilled from the uppermost portions of the diatomaceous sediments in the lacustrine sedimentary sequences adjacent to Pringle Falls (43.7°N, 238°E) along the Deschutes River in Oregon, as was the case for the profiles published already. Profile 4 shown in Figures 1 and 2 is a very short segment of sediments that did not record the Pringle Falls excursion. Therefore such segment of sediments has been omitted from the description and interpretation of the excursion.
The four widely separated localities are excellent exposures owing to recent erosion near the falls, and also by the fact that the sections were drilled from the continuous exposures of flat-lying sediments. All the collected samples were cut into c. 2.5 cm-long cylinders and were measured on JR-5 spinner, ScT and 2G 755 SRM cryogenic magnetometers at the SOEST-HIGP Paleomagnetics and Petrofabrics Laboratory.
Magnetic properties were analysed to identify the magnetic carriers of the natural remanent magnetization (NRM) and to investigate the origin of the NRM. Studies of magnetic mineralogy were performed first using at least one sample from each of the five profiles under study. Low-field susceptibility v. temperature (k–T) experiments were conducted in air using a KYL2 instrument in order to determine the Curie temperature of the samples. Thirteen specimens were progressively heated from room temperature up to 700 °C and subsequently cooled down using a KLY2-CS3 apparatus (Hrouda 1994; Hrouda et al. 1997) located at the SOEST-HIGP Petrofabrics and Paleomagnetics Laboratory. Several typical diagrams of susceptibility v. temperature (k–T) are shown in Figure 3. The curves have very similar heating and cooling patterns. Both show the presence of the Curie temperature of pure magnetite.
Example thermomagnetic curves for lacustrine sediments. Data obtained from the HIGP KLY-2 low-field v. temperature (k–T) susceptibility instrument. These samples, as well as other samples analysed, have simple thermomagnetic behaviour, with Curie temperatures ranging from 550 to 565 °C. The samples also have very good reversibility upon cooling, indicating little alteration of the magnetic mineralogy during heating and cooling experiments. Blue curve is heating and red indicates cooling.
We found that all of the specimens studied had reversible heating and cooling results, with single inflection points, indicating Curie temperatures between 550 and 565 °C. We interpreted these data to indicate the presence of low-Ti magnetite to pure magnetite as the primary magnetic minerals in these samples. The most probable sources of the magnetic carriers are therefore wind-blown Ti-low magnetites from the Newberry and Three Sisters volcanoes located close by the sampling sites.
Magnetic hysteresis measurements were performed on c. 200 mg of powder from the diatomaceous lacustrine silt sediments using a variable field translation balance up to 1.2 T. Saturation remanent magnetization (Mr), saturation magnetization (Ms), and coercive force (Hc) were calculated after removing the paramagnetic contribution. We determined the hysteresis loops and the back-field demagnetization curve of the saturation isothermal remanent magnetization. The variable field translation balance instrument has a measuring range of 10−8–10−2 A m2. The coercivity of remanence (Hcr) suggests that the isothermal remanent magnetization is carried by low-coercivity grains. The ratios of hysteresis parameters were plotted in Figure 4 as a Day diagram (Day et al. 1977) following recent modifications by Dunlop (2002) for type curves and regions that have been defined for pure magnetite. Most grain sizes are tightly clustered within the pseudo-single domain range for the Pringle Falls profiles. It is striking that the distribution of coercivities is related to the unblocking temperatures. If we do not consider the influence of grain sizes, it is interesting to associate this distribution with the magnetic phases that are present in the sample. The hysteresis parameters have been initially defined from synthetic crystals of magnetite (Dunlop & Özdemir 1997, 2000). This would probably explain why most studies deal with samples that are actually found within the pseudo-single domain range (e.g. Herrero-Bervera & Valet 2003, 2009).
Day plot showing magnetic grain size variations of Pringle Falls diatomaceous lacustrine silt sediments. Diagram represent the Hysteresis parameters, Mr/Ms v. Hcr/Hc plotted in the manner of Day et al. (1977) corrected according to Dunlop (2002). As is depicted in this figure we have found that the great majority of the specimens studied cover a range of magnetic sizes located in the pseudo single domain (PSD) area of the Day plot.
The remanent magnetization was measured with a 2G 755 Superconducting Rock Magnetometer (SRM) housed in the shielded room of the SOEST-HIGP Petrofabrics and Paleomagnetics Laboratory of the University of Hawaii. The samples under question were stepwise demagnetized by alternating fields (a.f.) from 5 to 60 mT. Typical demagnetization diagrams obtained are shown in Figure 5. The stable direction of the characteristic remanent magnetization (ChRM) was determined with no ambiguity. The ChRM was calculated using principal component analysis for the demagnetization diagrams with a well-defined component trending to the origin.
Alternating field demagnetization results from an ‘excursional’ Pringle Falls sample. The top diagram shows vector end points of palaeomagnetic directions on orthogonal demagnetization diagrams or modified Zijderveld (1967) plots (squares are inclinations and circles are declinations depicting the z-axis v. the horizontal plane). The second orthogonal diagram on the right is an enlargement of the plot on the left side of the figure. The best-fit lines from principal component analysis (PCA) are shown for the Free PCA option (solid black line) and Anchored PCA option (dashed green line) for the vertical component only. The lower diagram shows the normalized intensity variation with progressive demagnetization as well as the stereographic projections of the declinations and inclinations.
The samples either showed consistent directions of magnetization to within 5° of the initial value for a.f. demagnetization up to 60 mT or showed a removal of a low-coercivity component and then univectorial behaviour (see Figs 5 & 6). The vectorial diagrams show that a characteristic component of magnetization was isolated by c. 15 mT and that a univectorial component (Zijderveld 1967) was isolated for the typical samples shown in Figure 6. No bias or systematic departure from the origin was accepted and in all cases the ChRM relies on a minimum of seven successive and up to 16 directions isolated during a.f. stepwise demagnetization. Complete demagnetization of the samples was obtained with a.f. demagnetization techniques. The demagnetization curves confirm that magnetite carries the NRM with a soft resistance to alternating fields.
Alternating field demagnetization results from six Pringle Falls samples. For each sample, the top diagram shows vector end points of palaeomagnetic directions on orthogonal demagnetization diagrams or modified Zijderveld plots (Zijderveld 1967) (squares are inclinations and circles are declinations). The best-fit lines from principal component analysis (PCA) are shown for the FREE PCA option (solid black line) and ANCHORED PCA option (dashed green line) for the vertical component only. For each sample, the lower diagram shows the normalized intensity variation with progressive demagnetization.
The magnetostratigraphic results obtained previously and published by Herrero-Bervera et al. (1989) of the so-called La Pine profile are shown in Figure 7a. The magnetostratigraphic results of Profile 2 are also shown in Figure 7b and have already been published by Herrero-Bervera et al. (1994). The separation between the two sites is approximately 2 km, as can be seen in Figure 1, and both were drilled along the Deschutes River as depicted in Figures 1 and 2. The demagnetization results of both profiles show that the directional characteristics in terms of the declination, inclination and intensity of magnetization of the drilled samples are repeated arguing for an excellent intrabasinal correlation of the two sites since the geomagnetic inclinations features A, B and C are conspicuous in both records. The second important characteristic of the La Pine record is the well-defined normal polarity from the base of the section (20 m from the top) to c. 7 m from the top.
(a) Magnetostratigraphic plot of declination, inclination and intensity of magnetization of the Pringle Falls Profile, originally named ‘La Pine, Oregon’, showing the characteristic excursional features that represent the magnetic signature of the Pringle Falls polarity episode (from Herrero-Bervera et al. 1989). (b) Magnetostratigraphic plot of declination, inclination and intensity (D, I, J) record of Pringle Falls Profile 2 depicting the characteristic features A, B and C as indicated by the red arrows. The distance between the two records is approximately 1.5 km (from Herrero-Bervera et al. 1994).
The inclination from 7 to 20 m shows a close similarity to that of the present ambient field (62°) at the site from which the samples were obtained. The observed inclination values (61±2°; α95 = 2.6°) were not significantly different from each other or from the inclination of the axial dipole field at the site, which was 62° as reported by Herrero-Bervera et al. (1989). This agreement between the observed inclination and that given by the geocentric axial dipole formula can be taken as evidence for the lack of an inclination error, especially as the data are averaged over a temporal and spatial extent that is satisfactory. More details of the demagnetized results can be found in the original paper published by Herrero-Bervera et al. (1989), particularly those related to the 20 m intensity record of the La Pine profile.
In order to correlate the Pringle Falls palaeosignal and verify the reproducibility of the directional magnetic signature of the excursion, we have studied two additional profiles. Figure 8 shows the directional results, that is, inclination, declination and intensity of magnetization of Profile 1 (Fig. 8a) located adjacent (about 5 m apart) to the so-called La Pine profile, and also an additional fourth profile named Profile 3 (see Fig. 8b). Profile 1 is approximately 8.5 m thick whereas Profile 3 is approximately c. 18.4 m thick, despite the fact that such profile (i.e. Profile 3) is located about c. 1.7 km east from Profiles 1 and the original La Pine 1989 site (see Figs 1 & 2). The red arrows of Figure 8a, b indicate the three characteristic geomagnetic features of the Pringle Falls excursion, namely A, B and C. Figures 8b and 9 do not show the place where feature C should be recorded, and we argue that it is a gap where we do not have samples owing to a possible lack of suitable material for the study of such geomagnetic features.
(a) Magnetostatigraphic plot of declination, inclination and intensity of magnetization (D, I, J) Pringle Falls Profile 1, showing the characteristic excursional features, A, B and C as depicted by the red arrows that represent the magnetic signature of the Pringle Falls polarity episode. (b) Magnetostratigraphic plot of declination, inclination and intensity record of Pringle Falls Profile 3, depicting the characteristic features A and B, as indicated by the red arrows; geomagnetic feature C was not recorded at Profile 3. The distance between the two records is approximately 3.0 km.
It is also very important to mention that Figure 2 shows Profile 4 which is higher in the section than the other profiles (i.e. Profiles 1989, 1, 2 and 3) but we are not showing the magnetostratigraphic results owing to the lack of a record of the Pringle Falls excursional features, namely A, B and C, recorded in the other profiles under question.
The reproducibility of the palaeosignal can be easily shown by putting together four inclination records where the geomagnetic characteristic features A, B and C are conspicuous and where a correlation can be attained amongst the four sites in question. Figure 9 shows such intra-basinal correlation.
Intra-basinal correlation of the inclination results of four profiles from Pringle Falls, showing features A, B and C (except for Profile 3). Notice the reproducibility of the three geomagnetic characteristic features of the excursion as indicated by the red arrows and above the recently already radiometrically dated pumice layer ‘D’ by 40Ar/39Ar methods, yielding an age of 211±11 ka.
The declination and inclination data presented in Figures 7 and 8 can be converted to virtual geomagnetic pole (VGP) plots representing the apparent motion of the pole during this event (Fig. 10). The directional data obtained at Pringle Falls (from four sites 1.5 and 3.0 km apart) are very detailed with a great number of transitional directions, from which we have calculated successive VGP positions. Figures 7 and 8 depict the directional records (e.g. D, I and J) from the four localities and Figure 10 shows the totality of the VGPs obtained from the four profiles in question. It can be seen that the transitional VGPs and directions that make up the polarity episode are represented from the bottom to the top of the inclination records (see Figs 7, 8, 9) in both cases.
Virtual geomagnetic pole paths of four profiles recovered along the Deschutes River Oregon. Profile labelled as 1989 and Profile 2 have already been published by Herrero-Bervera et al. (1994) and the distance between them is c. 1.5 km. Profiles 1 and 3 were recently obtained and studied for this study. The distance between Profile 3 and Profile 2 is c. 3.0 km. The lower VGP path named Pringle Falls is the overlapping of the four paths in question, slightly centred to the east, and therefore showing more VGPs on the eastern part of Brazil. All of these paths show the extraordinary intra-basinal correlation between and amongst the four drilled profiles.
The descriptive characteristics of the Pringle Falls VGPs of the three profiles published so far, that is, La Pine (Herrero-Bervera et al. 1989), Profiles 1 and 2 (Herrero-Bervera et al. 1989, 1994; Herrero-Bervera and Helsley 1993; Valet et al. 2008a, b), have been analysed from the view point of a geometrical configuration including the totality of the individual VGPs plotted up together. The two additional profiles that have not been published so far, that is, Profiles 1 and 3, also show the same general geometric characteristics as La Pine and Profile 2. The unique geomagnetic characteristics of the poles depicted in Figure 10 include rapid directional changes and slower periods or stand-still intervals. Also shown is the preferred behaviour of the VGP path(s) over the Americas and over Asia (Laj et al. 1991, 1992a, b). In addition to the travel of the VGPs through the Pacific Ocean, there is the antipodal behaviour of the path travelled between the eastern part of South America, the central Pacific and the final lingering of the poles to the Kamchatka Peninsula.
The composite VGP loop in Figure 10 suggests that this excursion may represent an aborted reversal (e.g. Hoffman 1981) because the data include directions that are virtually antipodal to the present field. The overlapped VGP paths of the four studied profiles show a high degree of reproducibility of the palaeosignal, as was demonstrated when we analysed the directional records depicted in Figures 7, 8, 9. These overlapping VGP paths shown at the bottom of Figure 10 also attest to the intra-basinal reproducibility of the Pringle Falls palaeosignal of the excursion or ‘aborted reversal’ at the original Pringle Falls site sampled along the Deschutes River in Oregon.
In order to have a clear visualization of the different stages of the evolution of the Pringle Falls excursional palaeofield we have broken down the VGP paths into three separate phases that encompass the three geomagnetic features that define the signature of the aborted reversal, namely A, B and C. Figure 11 shows the behaviour of the transitional field that we have called Phase A corresponding to the early part of the excursion (i.e. feature A, see Figs 7 & 8), followed by Phase B corresponding to the middle of the excursion path (i.e. feature B) and then Phase C correlated to the late part of the excursion and corresponding to feature C of the excursion. It is easy to observe that, once the entire excursion has been dissected, the complex transitional evolution of the behaviour of the palaeofield emerges and is identifiable.
Virtual geomagnetic pole paths of the dissected portions of the Pringle Falls geomagnetic signature. The data (i.e. the VGPS) used for the plot correspond to only one of the sampled profiles and not an integration of the entire data set of the four profiles in question. The upper left corner path represents the Early part of the excursional path corresponding to Phase A (i.e. geomagnetic feature A); the upper right corner path corresponds to the Middle part of the excursional travel and correlates to the Phase B (geomagnetic feature B). The lower left corner of the diagram represents the Late portion of the path corresponding to Phase C (i.e. geomagnetic feature C). The lower right corner of the diagram represents the overlapping or composite diagram of all the discrete VGPs of the entire excursion under question. The black arrows represent the direction of motion of the path from younger to older; notice the characteristic clockwise loop through the Pacific that identifies one of the signatures of the Pringle Falls excursion. Notice that the program used to plot the VGPs of this figure is different from the program used to plot the VGPs of Figure 10 and there is slight shift of the data to the east.
The dissected Pringle Falls paths show that the youngest feature A is characterized by an initial cluster of excursional VGPs, then a clockwise travel to South America followed by a departure of the field from the Patagonia area through the middle of the Pacific Ocean with a tendency towards the eastern part of the Pacific (i.e. Phase A, early part of the VGP path, see Fig. 11).
Subsequently, there is a lingering of the VGPs from the northeast part of the American continent and Greenland, travelling south to the eastern part of the continent through the Gulf of Mexico on to the southern part of Baja California and then relatively fast travel to East Asia in the vicinity of Kamchatka, Japan and Korea with a sudden displacement of the VGPs to the eastern part of South America between Colombia–Venezuela to the northeast part of Brazil corresponding to feature B (i.e. Phase B, middle part of the VGP path, see Fig. 11).
The last part of the evolution of the excursional field is shown on the third dissected VGP path of Figure 11 and corresponds to feature C (i.e. Phase C, late part of the path). Such a path is characterized by the clustering of VGPs lingering between Western Europe and the entire North American continent with an initial departure from South America.
We compared the excursional VGP paths that have been published recently belonging to the Brunhes Chron with those of the recently obtained Pringle Falls recorded at the original site of Pringle Falls. Figure 12 shows that, if one makes a visual comparison of such VGP paths, one finds that all of them display remarkably different VGP paths (Valet et al. 2008a, b). The detailed description of the characteristics of the ages and VGP behaviour has recently been published by Valet et al. (2008a, b). It is important here to point out that there are two excursions that have been reported to be the same in terms of their age at the 195–200 ka level, but very recently such excursions have been identified with a certain degree of certainty with respect to the ages and they have been identified as two separate excursions. These two excursions are the Iceland basin excursion reported by Channell (2006), and Lund et al. (2006) with a recognized age date of 180–188 ka and the Pringle Falls aborted reversal dated twice at the Pringle Falls site yielding an age of 218±10 ka (Herrero-Bervera et al. 1994) and 211±12.8 ka (Singer et al. 2008). These two excursions are different with their respective VGP paths and their unique radiometric age determinations since the radiometric difference is of the order of 23 000 years (e.g. Valet et al. 2008a, b), as shown in Figure 12.
Virtual geomagnetic pole paths of the most detailed records of excursions during the Brunhes Chron. After Valet et al. (2008a, b).
It is known by now that geomagnetic excursions represent short episodes of a few thousand years at most during which the field considerably exceeds its normal range of variability during a polarity state. Palaeomagnetic records such as the Pringle Falls described here have now been obtained with extremely high temporal resolutions that have improved our knowledge of these short events.
Up until today, excursions have been defined as a single process linked to a large diminution of the dipole field and a large departure of the field from its initial polarity followed by a recovery to the initial state. According to this interpretation, excursions could be interpreted as manifestations of large-amplitude secular variations caused by the ‘failure’ and/or weakening of the dipole.
The presence of a period of a reversed polarity with a partially restored dipole even for a short time suggests a different scenario. We already know that by definition, a reversal is restricted to the period over which the pole flips from one polarity to the other where pole positions are usually considered as transitional or excursional only when they are 60° apart in terms of latitude. Therefore, the onset and termination phases of excursions are actual field reversals, as they show the flip of the dipole between the two polarities, and the very short episode of the partial restoration of the dipole in the opposite polarity that can be seen as an ‘aborted polarity state’.
Excursions as published by Valet et al. (2008a, b) then should be described ‘as two successive reversals separated by an aborted polarity interval’. Note that this scheme is inconsistent with the hypothesis that excursions would represent a single clockwise (or anticlockwise rotation) of the dipole field. Owing to its large decrease the dipole becomes too weak to be dominant and the transitions are necessarily controlled by the secular variation of the non-dipole field (see Valet et al. 2008a, b). As can be seen from Figures 7, 8, 10 and 11, the quadruplicate directional profiles as well as the VGP paths show that the directions also reach opposite polarity, in addition, southern pole positions are also observed. Other Pringle Falls records, such as the one published by Channell (2006) from ODP Site 919 in the northern Atlantic, show reversed directions at all sites, which is an indication that such directions are associated with a dipolar field geometry.
In order to study the detailed excursional characteristics of the Pringle Falls polarity episode recorded at the original sampling site, we have studied four records of such aborted reversal. The Pringle Falls excursion is one of the five to six globally identified polarity episodes between 579 and 30 ka that have been well documented in several marine sediment cores or dated using 40Ar/39Ar methods on terrestrial rocks. Together with at least five other well-dated excursions between 730 and 520 ka, there are 10 excursions that define the Geomagnetic Instability Time Scale (e.g. Singer et al. 2008) for the Brunhes Chron. The Pringle Falls excursion recorded at the original site satisfies the minimum conditions to characterize it as a global excursion, namely, the Pringle Falls excursion has been dated by radiometric methods such as 40Ar/39Ar from two different lithologies such as igneous rocks (Singer et al. 2008) and lake and deep-sea sediments (e.g. Herrero-Bervera et al. 1989; 1994; Channell 2006) as well as from far removed localities from the Northern and Southern Hemispheres (e.g. McWilliams 2001).
The study of the detailed excursional characteristics of the Pringle Falls polarity episode recorded at the original sampling site was undertaken by drilling a total of 827 samples recovered from four widely spaced profiles sampled along the Deschutes River Oregon. The four profiles sampled documented a high-resolution palaeomagnetic excursion of the Pringle Falls magnetic polarity episode (c. 211±13 ka, Singer et al. 2008), recorded by diatomaceous lacustrine sediments.
This sedimentary sequence was sampled as part of an extensive prehistoric fluvial and lacustrine complex that formed east of the Cascade Mountains during the last 1.0 Ma. The lake appears to have resulted from a late Pliocene/Pleistocene rise in the base level to the east of the sampling area near the western margin of the Basin and Range structural province and is related to the development of the extensive volcanism associated with the Newberry volcano.
We conducted palaeomagnetic and rock magnetic studies in order to investigate the reproducibility of the palaeomagnetic signal throughout the 5 km of the sampling of the four profiles that documented the Pringle Falls excursion. We performed low-field susceptibility v. temperature analysis to determine the magnetic carriers of the sediments and we found that the main magnetic carriers are pure and Ti-poor magnetite with Curie temperature ranging from 550 to 565 °C. Most grain sizes are tightly clustered within the pseudo-single domain range for the Pringle Falls profiles. The demagnetization of the sediments was done by means of alternating field methods, and the determination of the mean directions of Profiles 1 and 3 was done by principal component analyses. The level of detail of the palaeosignal of these four records is highly consistent since they are characterized by rapidly deposited sediments (greater than 10 cm/ka) that provide a detailed representation of field behaviour during the excursion.
The VGP paths are highly internally consistent and are defined by a clockwise loop travelling from high northern latitudes over the eastern part of North America and the North Atlantic to South America and then to high southern latitudes, then a return to high northern latitudes through the Pacific and over Kamchatka associated with the initial phase of the excursion, which corresponds to geomagnetic feature A. The other two geomagnetic features, namely B and C, corresponding to the middle and late stages of the evolution of the excursional field, have their own looping indicating a complex non-dipolar behaviour of the field. The initial or early phase Pringle Falls clockwise loop is characteristic of other recently found excursions like the Iceland Basin excursion (188 ka). The published age of the Pringle Falls excursion (c. 218±14 ka; Herrero-Bervera et al. 1994) and the most recent radiometric ages at the Pringle Falls site (weighted mean 211±13 ka; Singer et al. 2008), indicate that the dominance of such VGP paths (i.e. clockwise looping of the Pringle Falls, the Iceland Basin excursion and other excursions of the same age) shows that the excursional palaeofield had a relatively simple geometric characteristic. A corollary of the latter option is that palaeomagnetic polarity episodes of different ages may have similar transition and excursional polar paths, a conclusion implying that a common mechanism of the generation of the palaeofield was involved (e.g. see Herrero-Bervera et al. 1994).
We are very grateful to J. Lau for his field and laboratory assistance. Also we would like to express our thanks to the two anonymous referees for their very constructive and very helpful criticisms of our research work. Financial support to E. Herrero-Bervera was provided by SOEST-HIGP and the National Science Foundation grants EAR-9909206, EAR-INT-9906221, EAR-0207787, EAR-0213441, EAR-0510061, EAR-0710571, EAR-1015328 and EAR12-1215070, and the NSF EPSCoR program. This is SOEST contribution number 8701 and HIGP contribution number 1980.
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