Shaul Levi

Late Pleistocene geomagnetic excursion in Icelandic lavas : Confirmation of the Laschamp excursion

In 1980 Kristjansson and Gudmundsson [1] reported a late glacial geomagnetic excursion in three hills in the Reykjanes peninsula, Iceland, with shallow negative inclinations and westerly declinations. They named it the Skalamaelifell excursion. More extensive field work has identified the same excursional paleomagnetic direction (declination= 258°,inclination= −15°) at four additional outcrops in a 10 × 10 km area in the Reykjanes peninsula. The excursion lavas are olivine tholeiites with similar petrography and chemical compositions. Paleointensity determinations by the Thellier method average 4.2 ± 0.2 μT for 8 samples, more than an order of magnitude weaker than the present geomagnetic field in Iceland. Together, these results suggest extrusion of the excursion lavas in a very brief span of time, probably less than a few hundred years. KAr dating of the excursion lavas gives a mean age for 19 determinations of 42.9 ± 7.8ka(2σ). Compilation of thirty KAr ages of the Laschamp and Olby flows by three laboratories yield a new age for the Laschamp excursion in France of 46.6 ± 2.4ka(2σ). The age of the excursion in southwestern Iceland is statistically indistinguishable from the Laschamp excursion at the 95% confidence level, and both have very low paleointensities. Therefore, we suggest that the Laschamp and Olby flows in France and the Skalamaelifell units of Iceland recorded essentially the same geomagnetic excursion. Differences in the virtual paleomagnetic poles (VGPs) of these excursions may be due to (1) the probable non-dipole character of the geomagnetic field during the excursion, (2) rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in France and Iceland, and/or (3) crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations.

Paleointensity of the earth's magnetic field during the Laschamp excursion and its geomagnetic implications

The reversed paleomagnetic direction of the Laschamp and Olby flows represents a specific feature of the geomagnetic field. This is supported by paleomagnetic evidence, showing that the same anomalous direction was recorded at several distinct sites, including scoria of the Laschamp volcano. To examine this anomalous geomagnetic fluctuation, we studied the paleointensity of the Laschamp and Olby flows, using the Thellier method. Twenty-five samples were selected for the paleointensity experiments, and from seven we obtained reliable results. Because the paleointensity results of the Olby and Laschamp flows as well as Laschamp scoria are very similar, they can be represented by a single mean paleointensity,F = 7.7 μT. Considering that this low paleointensity is less than 1/6 of the present geomagnetic field and is more characteristic of transitional behavior, our results suggest that the paleomagnetic directions of the Laschamp and Olby flows were not acquired during a stable reversed polarity interval. A more likely explanation is that the Laschamp excursion represents an unsuccessful or aborted reversal.

Hydrothermal mounds and young ocean crust of the Galapagos: Preliminary Deep Sea Drilling results, Leg 70

A total of 32 holes at 5 sites near 1°N, 86°W drilled on Deep Sea Drilling Project (DSDP) Leg 70 (November–December 1979) provide unique data on the origin of the hydrothermal mounds on the south flank of the Galapagos spreading center. Hydrothermal sediments, primarily Mn-oxide and nontronite, are restricted to the immediate vicinity of the mounds (⩽100 m) and are probably formed by the interaction of upward-percolating hydrothermal solutions with sea water and pelagic sediments above locally permeable zones of ocean crust. Mounds as much as 25 m in height form in less than a few × 10 5 yrs, and geothermal and geochemical gradients indicate that they are actively forming today. The lack of alteration of upper basement rocks directly below the mounds and throughout the Galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust.

A sixty thousand year paleomagnetic record from Gulf of California sediments: secular variation, late Quaternary excursions and geomagnetic implications

A unique inclination record of geomagnetic secular variation for the past 60, 000 years (60 ka) has been obtained from continuously deposited sediments in the Gulf of California, recovered during Deep Sea Drilling Project (DSDP) Leg 64 by hydraulic piston coring at Site 480. The chronology of Hole 480 was determined by δ18O stratigraphy and varve counts, indicating sedimentation rates approaching 1 m/ka. The paleomagnetic results of the upper 49 m show: (1) The average inclination over this interval is identical to the geocentric axial dipole value at the sampling site. (2) Excursion directions occur between about 53 and 22 ka before present (BP). During this time, the geomagnetic field was generally “noisier” than in the overlying and underlying sections, with greater dispersion of the inclination. (3) The Laschamp excursion was apparently recorded at Hole 480 between about 51 and 49 ka BP and the Mono Lake excursion between about 29 and 26 ka BP. In addition, a narrow 0.3–0.4 m zone near 23 ka BP has a very similar paleomagnetic signature as the excursion observed at Summer Lake, Oregon [1], and we suggest that the Summer Lake is distinct from and younger than the Mono Lake excursion by 3–5 ka and of considerably shorter duration, lasting no more than a few hundred years. (4) Recurring inclination fluctuations were identified at Site 480, characterized by end points with steep inclinations and shallow intermediate value(s), as compared with the geocentric axial dipole. The inclination cycles are particularly apparent from 54 to 24 ka BP with a characteristic period of about 4.4 ka. (5) The “noisier” inclination record between 54 and 24 ka BP might be related to the generally reduced dipole moment between about 20 and 50 ka, and particularly low paleointensities for the Laschamp and Mono Lake excursions.

Why are marine magnetic anomalies suppressed over sedimented spreading centers

The absence of lineated marine magnetic anomalies over sedimented, recent spreading centers has been observed in several areas. We propose that magnetic anomalies can be suppressed as a result of pervasive hydrothermal reactions underneath thick blankets of sediment. The relatively impermeable sediment cover produces comparatively closed hydrothermal systems, which increase the residence time of the hot fluids in basaltic layer 2, causing more thorough leaching of the remanence-carrying iron-titanium oxides and diminishing the marine magnetic anomalies. This contrasts with lesser alteration at sparsely or unsedimented zones of extension with more open circulation. Conversely, spreading ridges and oceanic crust characterized by absent or subdued lineated anomalies may signal sites of extensive hydrothermal mineralization. This process might also provide an explanation for some magnetic “quiet zones,” such as in the Gulf of California, the Gulf of Aden, and the northern Red Sea.

Geomagnetic record in Minnesota lake sediments—Absence of the Gothenburg and Erieau excursions

Several excursions of the geomagnetic field during the Brunhes epoch have been postulated on the basis of paleomagnetic data from sediments. It has further been suggested that these excursions may represent sudden fluctuations of the geomagnetic field, manifested especially in fluctuations of the local geomagnetic inclination. In this study we present high-resolution data of inclination variations recorded in the sediments of two Minnesota postglacial lakes. To the best of our knowledge, our data cover two overlapping time windows, 0 to 9600 B.P. and about 9000 to 16,000 B.P. The results strongly suggest that no excursions have occurred in Minnesota over the past 16,000 yr. Thus, it appears that the Gothenburg (Sweden) excursion at about 12,500 B.P. is not worldwide and that the regional Erieau (Lake Erie) excursion does not extend to Minnesota. We suggest that these two previously claimed excursions might not reflect geomagnetic field behavior but merely poor paleomagnetic “recording” within the sediments used for these studies.

When did hominids first leave Africa?: New high-resolution magnetostratigraphy from the Erk-el-Ahmar Formation, Israel

New paleomagnetic results from the Erk-el-Ahmar Formation, Israel, resolve age ambiguities of one of the oldest hominid sites outside Africa, where ancient Oldowan tools and artifacts were excavated. We identified in the section the upper and lower boundaries of the Olduvai subchron, and we conclude that these sediments were deposited between ca. 1.7 and 2.0 Ma. This result is consistent with the hypothesis that earliest hominid migrations from Africa to Eurasia during the early Paleolithic traversed the Levantine corridor.

Magnetic property zonation in a thick lava flow

In this study, grain size and composition-dependent magnetic properties of titanomagnetite minerals are used as indicators of intraflow structures and magmatic evolution in an extensive and thick (30–60 m) basaltic lava flow. Similar zonation occurs in this flow at three localities separated by tens of kilometers. The magnetic properties subdivide the flow to three zones. The upper layer, representing the top 1/3 of the lava (≤ 20 m), has higher magnetic stability due to smaller and more deuterically oxidized titanomagnetite grains, approaching pure magnetite. The central layer in the underlying 2/3 of the flow (≤ 35 m) has larger, magnetically less stable, and less oxidized grains with relatively uniform magnetic properties. The basal layer, the bottom 1/10 of the flow (≤ 5 m), has near primary, least oxidized titanomagnetites (Ulv68Mag32). The upper intraflow boundary of the magnetic properties appears to coincide with the transition from entablature (above) to colonnade (below), distinguishing between regions of faster and slower cooling. Microprobe data indicate that the intraflow oxidation state (Fe3+/Fe2+) of the initially precipitated primary titanomagnetites increases with falling equilibrium temperature from the flow margins to a maximum near the center, the position of lowest equilibrium temperature. In contrast, Curie temperature measurements indicate that titanomagnetite oxidation increases with height in the flow. Modification of the initially symmetric equilibrium titanomagnetite compositions was caused by subsolidus high-temperature oxidation possibly due to hydrogen loss produced by dissociation of magmatic water, as well as unknown contributions of circulating air and percolating water from above. The titanomagnetites of the basal layer of the flow remain essentially unaltered.

Paleomagnetic constraints on the initiation of uplift on the Santa Susana Fault, Western Transverse Ranges, California

The Plio-Pleistocene nonmarine Saugus Formation is widely exposed in the east Ventura basin and the northern San Fernando Valley, north and south of the Santa Susana fault. In the east Ventura basin, the Saugus is overlain by the Pacoima Formation. Magnetostratigraphy of the fine-grained Saugus and Pacoima strata and the presence of the 0.76 Ma Bishop ash are used to calculate their average sedimentation rates: 0.9 km/m.y. in the east Ventura basin and 1.1 km/m.y. in the northern San Fernando Valley. Extrapolation of the Saugus and Pacoima sedimentation rates shows that they were deposited from about 2.3 to 0.5 Ma both north and south of the Santa Susana fault. The shift from remote to locally derived clasts marks the initiation of uplift of the Santa Susana Mountains. The extrapolated age of this boundary is about 0.7–0.6 Ma.

Geomagnetic fluctuations during a polarity transition

The extensive Roza Member of the Columbia River Basalt Group (Washington State) has intermediate paleomagnetic directions, bracketed by underlying normal and overlying reverse polarity flows. A consistent paleomagnetic direction was measured at 11 widely distributed outcrops; the average direction has a declination of 189° and an inclination of −5°, with greater variation in the inclination [Rietman, 1966]. In this study the Roza Member was sampled in two Pasco Basin drillcores, where it is a single cooling unit and its thickness exceeds 50 m. Excellent core recovery allowed uniform and dense sampling of the drillcores. During its protracted cooling, the Roza flow in the drillcores recorded part of a 15.5 Ma geomagnetic polarity transition. The inclination has symmetric, quasicyclic intraflow variation, while the declination is nearly constant, consistent with the results from the outcrops. Thermal models of the cooling flow provide the timing for remanence acquisition. The inclination is inferred to have progressed from 0° to −15° and back to −3°over a period of 15 to 60 years, at rates of 1.6° to 0.5°/yr. Because the geomagnetic intensity was probably weak during the transition, these apparently high rates of change are not significantly different from present-day secular variation. These results agree with the hypothesis that normal secular variation persists through geomagnetic transitions. The Iow-amplitude quasicyclical fluctuations of the field over tens of years, recorded by Roza, suggest that the geomagnetic field reverses in discrete steps, and that more than 15–60 years were required to complete this reversal.