Paleomagnetic Laboratory

Overview of the Paleomagnetic Laboratory

Background
The paleomagnetic laboratory was initiated by the Geophysical Institute of UAF in 1965, and is currently housed in the recently built Natural Sciences building. The initial aims of the laboratory were to investigate paleosecular variation recorded in volcanic rocks, and to study the possibility of oroclinal bending in Alaska. The latter study eventually led to the first paleomagnetic evidence for terranes and terrane motions. Since then many other projects have come and gone, and new viable projects have appeared.

Ongoing Research
Tectonics of Northeast Russia. The main set of projects over the last decade has been focused on the tectonic history of the terranes that make up most of Northeast Russia. We started paleomagnetic studies there in 1990, part of a cooperative program with Michigan State University, the Yakutsk Science Center and the Northeast Interdisciplinary Scientific Research Institute (NEISRI) in Magadan. Through this program we have collected oriented samples for paleomagnetism and samples for geochronology from many sites throughout Sakha (Yakutia), the Magadan district and in Chukotka. All the sampling programs were designed to attempt to unravel the very complicated tectonic history of the area. So far the paleomagnetic part of the project has been bedeviled by the discovery that a large percentage of our samples have been remagnetized, making the original magnetization (and hence original paleolatitude) difficult or impossible to determine. This now appears to have been a blessing in disguise inasmuch as the overprint is so consistent from area to area that we are now able to interpret it as a record of the accretion of the various terranes, and incorporate this record with recent results from sites where the application of detailed demagnetization techniques has enabled us to see the original magnetization.

The existence of a pervasive overprint through all of our study areas in Northeast Russia is mirrored in Alaska. Arctic Alaska has the best evidence for a major overprint, and it has been suggested that this is related to the time, and perhaps mechanisms of formation of the Brooks Range fold and thrust belt. Since Arctic Alaska has large potential and known oil reserves, there is considerable industry interest in the details of the tectonic history. The same arguments apply to the fold and thrust belts of Northeast Russia, particularly in relation to the Verkhoyansk Mountains and the other collisional zones.

Lake Cores. We are involved in lake core studies from Skilak Lake on the Kenai Peninsula that started off as a USGS funded project to look for paleomagnetic evidence of the Great Alaska Earthquake of 1964. In the process of doing this, we discovered that the USGS estimates of the time versus sediment thickness were in error by a factor between about 5 and 10. This meant that the 1964 event was in sediment too soft to measure. We continued the work by looking for a record of the previous major earthquake in the same area, but failed to get more funding. We did, however, obtain an excellent magnetic susceptibility record showing most (all?) of the significant volcanic eruptions in the Cook Inlet area.

Elgygytgyn Impact Crater. Lake Elgygytgyn is located in a 3.4 Ma impact crater in Chukotka. The crater was created in cretaceous extrusives, formed a substantial rim which now forms the whole catchment for the lake. This implies that the lake sediments contain a paleoclimate record from close to the time of impact to the present day. A major drilling program has been proposed (under the leadership of Prof. Julie Brigham-Grette, U. Mass.) and will open many opportunities for other sampling programs, including opportunities to devise techniques to study the impact itself.

Field 5Ma. This is part of an NSF funded multi-university project to obtain high quality paleomagnetic data from lava flows erupted within the last 5 million years, and located within a restricted band of longitudes (roughly the western edge of the Americas). The GI-UAF part of this involved collections as far north as we could find suitable lava flows. The best collections were from the Aleutian Islands. The results from the Alaskan volcanic rocks reinforce the measurements and conclusions we made in 1972, but the quality (and thus believability) of the data have been much improved.

Chulitna. One of the first of the “micro-terranes” recognized in Alaska. The whole package of rocks representing the terrane is less than a few tens of kilometers in any direction. Our project is an attempt to determine the magnetic latitude at which the rocks, principally Triassic volcanics, were formed. This is a joint project with the Alaska Division of Geophysical and Geologic Surveys, who supplied all the field logistics and student support. Since the ADGGS project has now finished, it is unlikely that we can find the logistic support needed to do a more thorough job, but the existing data re providing interesting insights into the history of the terrane.

Service Projects. The lab performs a number of services to both the teaching and research programs. Tours are given to show beginning science students what paleomagnetism and cryogenic magnetometers are all about, and selected groups from other classes are given more complete explanations.

A number of undergraduate students have worked in the lab, and have been given enough explanation that they (hopefully) end up knowing the basics of the process and the operation of the equipment. Of these, a few have taken an individual study credit class where they have to complete a paleomagnetism project. One has gone on to get a PhD based on a major paleomagnetic study.

The lab is called upon from time to time to make measurements related to other research. The most recent example is a set of measurements requested to test whether magnetite was being created in high temperature-pressure experiments. Another is a series of test measurements to see if variations in the magnetic properties can be used as a correlation tool in a field study of carbonate successions. The lab equipment is frequently used by the Limnology group at UAF for correlating between sediment cores.

Possible Future Studies
In addition to the research areas mentioned, there is a strong national and international movement to promote geo-studies in the Arctic Ocean. These will include deep and shallow drilling and coring, both of which will require paleomagnetic measurements for correlating between cores and for dating based on geomagnetic reversals. We are already involved in proposals to drill parts of the ‘Bering Land Bridge’, and hope to become involved in plans to drill the Lomonosov Ridge. The Lomonosov Ridge is a major Arctic Ocean crossing feature that is thought to be a fragment of continental shelf that was split off when today’s active spreading center migrated from NE Greenland to the Laptev Sea.

Equipment
The paleomagnetic laboratory has dedicated space alongside the Geology and Geophysics department, which includes a magnetically shielded room. Existing equipment consists of a 2G 2-axis cryogenic magnetometer with PC controlled stepper motor drives for inserting and rotating the samples. Thermal demagnetization is done with a Schonstedt oven and AF demagnetization with a Schonstedt AF unit. A Bartington susceptibility meter is used for basic measurements and has a furnace attachment for making rather poor susceptibility versus temperature measurements . Ancillary equipment includes a Varian electromagnet, various fluxgate and Hall effect probes for the lab, and an array of drills and field equipment. Field equipment includes several Zodiac boats (one 20+ feet long which makes an excellent platform for lake coring) and metal riverboats. We have a basic Mackereth mini-corer and a 3 meter conventional Mackereth corer. The 3-meter corer is currently being converted to hydraulic operation to allow deeper water lakes to be sampled.

The University operates a number of laboratories of interest, including Geochronology (see Paul Layer, under this section), and a Microprobe and XRF facility (see Ken Severin, under Geology and Geophysics/Economic Geology and Geochemistry).

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