Dr. Joshua Moser Feinberg

Institute for Rock Magnetism
University of Minnesota

Shining a Light in the Dark: How magnetic minerals in cave deposits illuminate the history of Earth’s magnetic field and past environmental change

This lecture will review the latest paleomagnetic and environmental magnetic lessons gathered from speleothems, including stalagmites and flowstones. No matter how quiet and pristine a cave setting may appear, all speleothems contain assemblages of magnetic minerals. In most settings these iron oxide minerals are derived largely from overlying soils, though minor fractions may also come from the residuum of dissolved bedrock, reworked sediment carried by episodic floods, geomicrobiological activity, and even windblown dust. Regardless of their origin, these minerals become aligned with Earth’s ambient magnetic field before they are fixed within a speleothem’s growing carbonate matrix. Modern paleomagnetic studies are using ever-smaller speleothem samples, which has allowed researchers to study the finer details of important forms of short-term geomagnetic behavior, ranging from excursions to temporal oscillations of the South Atlantic magnetic anomaly. It has also enabled some researchers to document systematic remanence anisotropy along the steeply dipping flanks of stalagmites, a phenomenon that appears to be controlled by speleothems’ carbonate fabrics. While collecting samples along the growth axis of a stalagmite is a good strategy to minimize such remanence bias, there are more robust, multi-sample methods to correct for this effect, which will become increasingly important as speleothem-based paleomagnetic studies are incorporated into global and regional geomagnetic field models. Environmental magnetic studies from speleothem continue to provide critical information about the hydrogeology and soil development immediately above cavern systems. By integrating information about the composition, concentration, and grain size distribution of magnetic minerals with trace element and isotopic geochemistry data from speleothems, rock magnetists can play pivotal roles in studies of paleoclimate, anthropological activities, wildfires, and much, much more.

About Joshua M. Feinberg

Josh Feinberg is a geologist and geophysicist best known for helping run the Institute for Rock Magnetism, an international center at the University of Minnesota whose core mission is to serve the greater geomagnetic community by providing free-of-charge access to state-of-the-art facilities and technical expertise, and by encouraging visiting researchers in their studies of important new topics in rock magnetism and related interdisciplinary fields. Feinberg’s active research interests include geophysics, geology, environmental science, archaeology, and climatology. His research group and collaborators have led a renaissance in the use of speleothems (stalagmites and flowstone) as high-resolution recorders of geomagnetic behavior and environmental change. Feinberg’s main research tools are magnetic methods combined with mineral characterization and detailed field work. Feinberg is also committed to advancing undergraduate education through the development of new undergraduate programs to enable students to be competitive within a variety of geoscience career paths.