ROCK MAGNETISM OF CARBONATES


Modern paleomagnetic methods using superconducting magnetometers (paired with ultra-clean acid-washed sample holders) have high enough sensitivity to obtain very high-quality paleomagnetic records from carbonate rocks (limestones, dolostones). Such high-sensitivity is necessary since the concentration of magnetic minerals is generally low in carbonate rocks resulting in magnetizations that can be quite weak. These rocks are a potentially quite powerful paleomagnetic archive where in an ideal situation their magnetization would be dominated by a primary signal that could be used to determine past latitude and paleogeography. Not only are carbonate rocks abundant throughout the geological record, but furthermore as chemically precipitated sediments they can provide geoscientists with invaluable information about the physical, chemical and biological conditions in ancient environments. Geochemical data from carbonate rocks inform much of our understanding of how the surface of the Earth has changed through time as they can reveal changes to the carbon cycle as well as the cycles of other major and trace elements. Thus, the potential to pair information about paleogeographic data with these other types of data is quite compelling.

However, there have been multiple well-documented examples of remagnetization of carbonate rocks. A central example of such remagnetization is in Paleozoic limestones of eastern North America where the primary remanence is completely obscured by subsequent remagnetization (itself of an ancient origin). However, there certainly are are primary magnetic minerals in limestones and convincing examples where primary magnetization has been preserved on geological timescales. Are there ways we can gain insight into whether a carbonate rock records a primary signal or if its magnetization is dominantly a secondary signal.

SEDIMENTARY RECORDS OF FUEGIAN TECTONICS


Adjacent to mountains the lithosphere bends in response to the heavy load creating a foreland basin. As mountains grow, continual weathering and erosion shed sediment into this depression. The changing composition of this sediment records the history of the mountain belt giving us insight into its development.

One of the strongest proposed connections between tectonics and climate in Cenozoic Earth History is the hypothesis that the opening of the oceanic pathway between the southernmost Andes and the Antarctic peninsula thermally isolated Antarctica 35 million years ago. Once this passage was opened the strong Antarctic Circumpolar Current developed around the continent hindering the passage of warm-water gyres and in the process facilitating the glaciation of the continent. The role the opening of the Drake Passage played in the initial glaciation of Antarctica is currently a subject of hot debate. A better understanding of the timing of the passage’s opening is needed in order to  further evaluate the connection between the tectonic processes that formed the passage and the climatic transition.
Working with the illustrious Team Barbeau (the tectonics and sedimentation lab at the University of South Carolina led by Prof David Barbeau), I have been part of a project designed to further understand the history of the southernmost Andes. Unlike the majority of the Andes that run north-south the southernmost Andes curve into the Patagonian Andes and run east-west forming the northern margin of the Drake Passage. U/Pb age data from detrital zircons collected from the Magallanes foreland basin show a distinct shift in the sources that were contributing zircon grains to the basin at the end of the middle Eocene.

We interpret this shift in the zircon composition of the sediment as resulting from a peak in tectonic activity that caused out-of-sequence thrusting and led to the development of a structural blockage that prevented zircons of the Patagonian batholith from reaching the basin. The timing of this shift falls right between the first geochemical evidence for Pacific waters entering the Atlantic and the major glaciation event suggesting a connection between increased tectonism in the sourthernmost Andes and the deepening of the Drake Passage. A paper describing these results was published in 2009 in Earth and Planetary Science Letters (doi:10.1016/j.epsl.2009.05.014).

Over the past three years members of Team Barbeau have ventured across the Drake Passage to extend the fieldwork and to develop constraints on the composition of age-equivalent strata in Antarctica. The group has also been developing thermochronology data that constrains the timing of uplift in both the southernmost Andes and the Antarctic Peninsula. Information about this ongoing work and the expeditions themselves can be found at the SCOTIA project website. I joined an expedition to the Antarctic Peninsula abroad the Laurence M. Gould research vessel from mid-November to late-December of 2009 as we continued to work to unravel the connections between tectonic motions and climatic change. Photos from this recent expedition can be found here.

Lonar Crater, India: Bolide impact in basalt as an analog for planetary surface processes and magnetization.

Rappelling into a well while mapping the Lonar Crater ejecta blanket (photo and illustration from Maloof et al., 2009)