Emilio Herrero-Bervera

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Age and correlation of a paleomagnetic episode in the western United States by 40Ar/39Ar dating and tephrochronology: The Jamaica, Blake, or a new polarity episode?

High-resolution paleomagnetic records from two sites near Pringle Falls, Oregon, are compared with similar records from Summer Lake, Oregon, ∼170 km to the southeast: Paoha Island, in Mono Lake, ∼660 km to the southeast and Benton Crossing, in Long Valley, approximately 700 km to the southeast, in east-central California. The sequences at Pringle Falls contain a distinctive coarse pumice-lapilli tephra layer which we have dated as 218±10 ka by 40Ar/39Ar step-heating of plagioclase feldspar. Stratigraphically, this tephra is closely associated with a suite of several other tephra layers that bracket the interval studied paleomagnetically. Each tephra layer is distinguished by the unique chemical composition of its volcanic glass shards. The pumice layer dated at Pringle Falls is correlated with layers at three of the other localities. Using all the tephra layers, we can correlate the lake stratigraphic sequences and associated paleomagnetic records among the four distant localities. Additional age control is obtained from a fifth locality at Tulelake in northern California, where the stratigraphic interval of interest is bracketed between ∼171±43 and approximately 140 ka. Characteristics of the paleomagnetic records indicate virtually identical paleofield variation, particularly the geometry of a normal to normal (N-N) geomagnetic polarity episode. The observed paleofield behavior resembles the Blake geomagnetic polarity episode, but is significantly older than the generally accepted age of the Blake episode. Either the age of the Blake episode is significantly underestimated, or the polarity episode documented here is older, perhaps the Jamaica episode, or is an as yet unreported episode. A corollary of the latter option is that paleomagnetic polarity episodes of different ages may have similar transition polar paths, a conclusion implying that a common mechanism is involved.

Stratigraphic constraints on the timing and emplacement of the Alika 2 giant Hawaiian submarine landslide

Abstract Previous work has found evidence for giant tsunami waves that impacted the coasts of Lanai, Molokai and other southern Hawaiian Islands, tentatively dated at 100+ and 200+ ka by U-series methods on uplifted coral clasts. Seafloor imaging and related work off Hawaii Island has suggested the Alika phase 2 debris avalanche as the source of the ∼100 ka “giant wave deposits”, although its precise age has been elusive. More recently, a basaltic sand bed in ODP site 842 (∼300 km west of Hawaii) estimated at 100±20 ka has been suggested to correlate with this or another large Hawaiian landslide. Our approach to the timing and linkage of giant submarine landslides and paleo-tsunami deposits is a detailed stratigraphic survey of pelagic deposits proximal to the landslide feature, beginning with a suite of seven piston, gravity and box cores collected in the vicinity of the Alika 2 slide. We used U-series dating techniques, including excess 230 Th and 210 Pb profiling, high-resolution paleomagnetic stratigraphy, including continuous, U-channel analysis, δ 18 O stratigraphy, visual and X-ray sediment lithology, and the petrology and geochemistry of the included turbidites and ash layers. Minimum ages for the Alika phase 2a slide from detailed investigation of two of the cores are 112±15 ka and 125±24 ka (2σ) based on excess 230 Th dating. A less precise age for the Alika phase 1 and/or South Kona slide is 242±80 ka (2σ), consistent with previous geological estimates. Oxygen isotope analyses of entrained planktonic foraminifera better constrain the Alika phase 2a maximum age at 127±5 ka, which corresponds to the beginning of the stage 5e interglacial period. It is proposed that triggering of these giant landslides may be related to climate change when wetter periods increase the possibility of groundwater intrusion and consequent phreatomagmatic eruptions of shallow magma chambers. Our study indicates the contemporaneity of the Alika giant submarine landslides and distal deposits from enormous turbidity currents as well as coral clasts reported to be tsunami deposits on Lanai and Molokai through direct dating and compositional analysis of the landslide deposits.

A possible lacustrine paleomagnetic record of the Blake episode from Pringle Falls, Oregon, U.S.A.

Abstract We have studied a high-resolution paleomagnetic record of a magnetic polarity episode recovered from a 20-m section of diatomaceous lacustrine sediments from Pringle Falls near La Pine, Oregon (43.7° N, 238.6° E). A total of 474 samples (982 specimens) were collected at about 5-cm intervals from the 20-m section. The age of this record is at present constrained between 0.6 MA and 18 000 yr based upon a pollen date from the older part of the lakebed sequence and the age of the younger glacial deposits that overlie the lakebeds. The characteristic magnetization of the samples was determined by alternating field (AF) demagnetization to 15 mT. The average normal polarity stable declination and inclination of the non-magnetic polarity episode sampled section (61°; α 95 = 2.6°) is statistically equivalent to that expected at this site for a geocentric axial dipole (62°). The record provides a Virtual Geomagnetic Pole (VGP) path that identifies a well-defined clockwise NE-SW loop with a radius of 60°, followed by a second younger feature that traces out a counter-clockwise loop 15° in radius, residing in the NE quadrant, and characterized by low latitudes. Interpretation of the natural remanent magnetization/anhysteretic remanent magnetization (NRM/ARM) ratios for the samples indicates that the field intensity may have fallen and then increased above the average dipole field intensity. The low-intensity interval ends before the onset of the polarity episode as defined by directions and may suggest that the episode is a result of an unstable regeneration of the field after a significant lowering of overall field intensity. These two characteristic features are similar to the two features present in the Blake geomagnetic polarity episode at 114 000 and 105 000 yr BP observed in different parts of the world.

Magnetic fabric and inferred flow direction of dikes, conesheets and sill swarms, Isle of Skye, Scotland

Abstract We have measured the AMS of 16 dikes, 35 conesheets and three sills associated with the Cuillin Hills magmatic center of the Isle of Skye in the Inner Hebrides of Scotland and of nine dikes located in the regional dike swarm of Skye. Sixty-three intrusives, totalling 734 samples, were studied to determine the plumbing of the Skye volcanic system. Low-field susceptibility versus temperature ( k – T ) identified three different mineral phases in the area, namely Ti-rich magnetite, pyrrhotite and titanomaghemite. The petrofabrics of the 63 intrusives yielded coherent flow azimuths regardless of their time of emplacement. Three main types of magnetic fabric, (A–C) were found. Fabric Type A (plane K max – K int parallel to the dike plane) represents the magma-flow direction within the intrusives and is the dominant fabric (55% of all the intrusives) within the Cuillin Hills magmatic center and its regional dike swarm. The K max inclinations show that 55% of the intrusives were fed by horizontal to sub-horizontal (AMS inclination of the K max axis is equal or less than 30°) magma fluxes and the rest of them were fed by inclined to vertical fluxes. Horizontal magma flow means lateral magma injection inside fractures and becomes more probable as the source is located further away.

Dynamical similarity of geomagnetic field reversals

No consensus has been reached so far on the properties of the geomagnetic field during reversals or on the main features that might reveal its dynamics. A main characteristic of the reversing field is a large decrease in the axial dipole and the dominant role of non-dipole components. Other features strongly depend on whether they are derived from sedimentary or volcanic records. Only thermal remanent magnetization of lava flows can capture faithful records of a rapidly varying non-dipole field, but, because of episodic volcanic activity, sequences of overlying flows yield incomplete records. Here we show that the ten most detailed volcanic records of reversals can be matched in a very satisfactory way, under the assumption of a common duration, revealing common dynamical characteristics. We infer that the reversal process has remained unchanged, with the same time constants and durations, at least since 180 million years ago. We propose that the reversing field is characterized by three successive phases: a precursory event, a 180° polarity switch and a rebound. The first and third phases reflect the emergence of the non-dipole field with large-amplitude secular variation. They are rarely both recorded at the same site owing to the rapidly changing field geometry and last for less than 2,500 years. The actual transit between the two polarities does not last longer than 1,000 years and might therefore result from mechanisms other than those governing normal secular variation. Such changes are too brief to be accurately recorded by most sediments.

Absolute paleointensity from Hawaiian lavas younger than 35 ka

Abstract Paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3.3 and 28.2 ka from the Mauna Loa volcano in the big island of Hawaii. Determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in Hawaii, although the overall average field intensity appears to be lower. Since the present results were determined at higher temperatures than in the previous studies, this discrepancy raises questions regarding the selection of low versus high-temperature segments that are usually made for absolute paleointensity. The virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows. When averaged within finite time intervals, the worldwide values match nicely the variations of the Sint-200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period. The convergence between the existing records at Hawaii and the rest of the world does not favour the presence of persistent strong non-dipole components beneath Hawaii for this period.

Secular variation of the geomagnetic dipole during the past 2000 years

We have constructed a very simple model of a time-varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka. The predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe. They were also compared with the Hongre et al. (1998) time-varying spherical harmonic model and with the CALS7K-2 model by Korte et al. (2005). We find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the European sites and not strikingly larger for the rest of the world. Many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records, which, along with the small number and poor geographical distribution of sites, leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2. It appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years. In a second step, we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment. The present decrease of the dipole did not begin prior to 1000 years ago, and the dipole was actually increasing from 0 until A.D. 500. The dipole moment of CALS7K is lower than the present estimate, probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data. The tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at Paris during the past 2 ka. The results show that the “archeomagnetic jerks” are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field.

Statistical assessment of the preferred longitudinal bands for recent geomagnetic reversal records

Records of recent polarity reversals recorded in sediments show a preponderance of transitional VGPs situated in the longitudinal sector over the Americas and, to a lesser extent, over its antipode. A series of tests based on the methods of circular statistics show that this distribution is not consistent with the statistical fluctuations of a random probability distribution. The longitudinal bias in the distribution, therefore, requires a physical explanation.

Rapid regional perturbations to the recent global geomagnetic decay revealed by a new Hawaiian record

The dominant dipolar component of the Earth’s magnetic field has been steadily weakening for at least the last 170 years. Prior to these direct measurements, archaeomagnetic records show short periods of significantly elevated geomagnetic intensity. These striking phenomena are not captured by current field models and their relationship to the recent dipole decay is highly unclear. Here we apply a novel multi-method archaeomagnetic approach to produce a new high-quality record of geomagnetic intensity variations for Hawaii, a crucial locality in the central Pacific. It reveals a short period of high intensity occurring ~1,000 years ago, qualitatively similar to behaviour observed 200 years earlier in Europe and 500 years later in Mesoamerica. We combine these records with one from Japan to produce a coherent picture that includes the dipole decaying steadily over the last millennium. Strong, regional, short-term intensity perturbations are superimposed on this global trend; their asynchronicity necessitates a highly non-dipolar nature.