Rapid Field Change or Remagnetization Artifact? An Ongoing Rock Magnetic Investigation
Twenty years ago streaked directions were reported in a basalt flow within the Steens Mountain reversal transition zone that might have resulted from very rapidly changing field direction while the lava was cooling (Coe and Prevot, EPSL, 1989). This unusual flow is located at a point in the transition where the direction jumps almost 90 degrees between two bracketing groups of stably magnetized flows. After cleaning to 500C, sample directions from the more slowly cooled interior of this flow were very different from those from more quickly cooled parts closer to both the lower and upper margins. Recently we resampled this flow to test whether the streaked directions might instead be an artifact of remagnetization. So far this rock magnetic investigation has raised more questions than it has answered for how to interpret the natural remanent magnetization. Samples spanning most of the thickness of the flow display two magnetic phases with Curie temperatures TC1=100-150C and TC2=500-565C in both strong and weak field thermomagnetic curves, except for samples very near the top and bottom that exhibit just TC2. Only modest change in these curves occurs after heating to 580C in air as well as in argon. Room temperature hysteresis parameters also change little after progressive heating cycles up to 500C; then, after the next heating cycle to 580C, both Mrs/Ms and Hcr/Hc increase by up to 30 percent. In view of its thermal stability to 500C, which is above the Curie temperature of fully oxidized titanomagnetite with x-value corresponding to TC1, it was a surprise when examination of mildly etched polished sections from this flow revealed shrinkage cracks in the host titanomagnetite grains indicative of maghemitization by low-temperature oxidation. The above observations and additional rock magnetic data in this ongoing study will be presented in relation to the crucial question for testing the rapid field change hypothesis: when and how did the natural remanence with unblocking temperatures over 500C form?
New Wine in an old Bottle? A Model for Magnetism-Climate (tele)Connection
Inversion of modern topsoil or paleosol magnetism to environmental change parameters require that we understand correctly the physical, chemical and biogeochemical controls on soil formation per se, and loessic soil formation in particular. Accumulating field evidence from loess deposits worldwide point decisively to the formation of both magnetically enhanced and depleted soils compared to parent loess. Only in some cases magnetic enhancement in parent loess may be explained by detrital windblown magnetite. In general however, neoformed ultrafine (less than a few micrometers in size) 'iron oxide' ("sensu lato"), particles include strongly magnetic ferrimagnets and weakly magnetic antiferromagnets. A knowledge of the balance between the two may lead to a more correct and comprehensive understanding of multiple environmental parameters of the past, rather than simply the past rainfall, as has been done in the past. An approach based on the fundamental magnetochemistry of alteration is not only desirable but feasible. Thus, the formation and stability of ferrous ions on nanoparticles of ferric oxides and hydroxides may be the most fundamental step for neoformation of magnetite and maghemite. The ferrous ion may, however, be unstable in the Earth's surface environment and invert back to ferric, generating weakly magnetic hematite and goethite. In both cases, information about past environmental parameters (temperature, precipitation minus evaporation, soil acidity, microbial presence) may be embedded in soil magnetic (and other) parameters. To test the validity of the above heuristic model, we review recent experimental evidence (low temperature magnetism, Mössbauer spectra, surface chemical reactivity, and conventional and synchrotron X-ray diffraction and absorption, etc). We then advance model environments which would lead to the magnetic observables. In addition, we inspect computational models of a reactive 'iron oxide' surface to investigate their compatibility with magnetic and non-magnetic evidence of alteration from the laboratory and the field situations.
Is a Universal Model for Loess Magnetism / Climate Connection Utopian?
Pleistocene loess deposition punctuated by periods of soil formation is observed, predominantly at mid- latitudes, over the Asian, European and North American continents. Geoscientists exercising in different disciplines have seized the opportunity handed by loess and paleosol deposits to study the climate of the past from a continental perspective. Paleomagnetists and mineral magnetists have already contributed significantly, most notably from Chinese Loess Plateau sequences. The former provided chronological constraints through the recovery of geomagnetic polarity changes and the latter discovered that roughly 30 nm ferrimagnetic particles were the source of magnetic susceptibility peak values in paleosol. Semi-quantitative models linking magnetic susceptibility to annual precipitation have been proposed but in all cases these are geographically restricted to local or regional models. How can we move forward, beyond the dominantly qualitative and regional models, towards a quantitative and a global model capable of inverting data from loess to paleoclimatic parameters? Is this utopian? The objective of this presentation is two-fold. First, we will take a wide angle look at the question/task at hand. Potential variables to be included are parent material (composition, grain size), post-depositional inputs (organic material, organisms), climate (temperature, moisture, etc.), physical parameters (slope, vegetation, pH, etc.), alteration (neoformation, dissolution, remobilization, recrystallisation). How do current magnetism based models address these different variables? What can we learn from the data, models and approaches of other disciplines such as elemental and isotope geochemistry, sedimentology and pedology? Secondly, we will explore, from the point of view of working with natural sample, the merits of different approaches such as physical and chemical separations. A comprehensive investigation, as outlined above, complemented by a similar systematic investigation using synthetic minerals and controlled laboratory settings will undoubtedly lead to the identification of the dominant variables and to the construction of a widely, perhaps globally, applicable model connecting loess magnetism to climate.
Diffuse Oceanic Plate Boundaries, Plate Non-Rigidity, True Polar Wander, and Motion Between Hotspots: Results From Investigations of Marine Magnetic Anomalies
Marine magnetic anomalies due to seafloor spreading record reversals of Earth's magnetic field and the orientation of the paleomagnetic field. They can be used to make precise estimates of relative plate motion and of the apparent polar wander of oceanic plates. In this talk I will present the results of several studies that include analyses of marine magnetic anomalies. A new set of geologically current relative plate angular velocities, termed MORVEL, has been determined in part from 1696 rates of seafloor spreading estimated from marine magnetic anomalies (DeMets, Gordon, & Argus 2009). The MORVEL set of angular velocities supersede those of NUVEL-1A (DeMets et al. 1994). A new feature of MORVEL is the assumed existence of many diffuse oceanic plate boundaries, such as that between the Indian and Capricorn plates. An important result from MORVEL is that several plate circuits fail closure, that is, the relative plate angular velocities summed around the circuit differ significantly from zero as would be expected if all the plates are rigid. Thus, it appears that at least some plates are not rigid. The most dramatic example of plate circuit non-closure is for the Pacific-Nazca-Cocos plate circuit, which encloses the Galapagos triple junction and fails to close by a stunning 14 ± 5 mm/yr (95% confidence limits). Part of the observed non-rigidity is likely due to predictable horizontal thermal contraction as oceanic lithosphere cools and subsides (Kumar & Gordon 2009). I will present simple illustrations of the velocity field within a plate expected from horizontal thermal contraction and speculate on how it may relate to observed plate circuit non-closures. The shapes of magnetic anomalies due to seafloor spreading contain valuable information about the location of the paleomagnetic pole, especially for the Pacific plate for which oriented rock samples are scarce. Particularly useful are Pacific-Farallon magnetic anomaly crossings near the paleo-equator. I use results from anomaly 12r (32 Ma, Horner-Johnson & Gordon 2009) to illustrate the value of these data. The results show that the hotspots in the Pacific basin have moved in unison with those in the Indian and Atlantic basins relative to the spin axis, a process most simply interpreted as true polar wander. Plate reconstructions based on fits of magnetic anomalies are used to place limits of 0 to 10 mm/yr on the rate that hotspots in the Pacific basin move relative to hotspots in other basins over the past 50 Ma (Koivisto, Andrews, & Gordon, 2009).