Sarah Slotznick - “Deep-time” Environmental Magnetism
Channel: Institute for Rock Magnetism
Date: 2021-06-03

12th Institute for Rock Magnetism Conference

Session 2: Environmental Magnetism and Proxies
Conveners: Andrew Roberts (ANU) and Anna Lindquist (Macalaster College)

Title: “Deep-time” Environmental Magnetism - Untangling Redox Conditions, Diagenesis, and Metamorphism

Authors: Sarah Slotznick (1), Nicholas Swanson-Hysell (2), Joseph Kirschvink (3), Woodward Fischer (3), Erik Sperling (4), Luke Fairchild (2), Yiming Zhang (2), Samuel Webb (5), Don Winston (6)



Institutions:
(1) Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire, USA
(2) Earth & Planetary Science, University of California, Berkeley, California, USA
(3) California Institute of Technology, Pasadena, California, USA
(4) Department of Geological Sciences, Stanford University, California, USA
(5) Stanford Synchrotron Radiation Lightsource, Stanford University, California, USA
(6) Geosciences, University of Montana, Missoula, Montana, USA



Abstract:
Environmental magnetism is a broad sub-discipline utilizing magnetic minerals to understand litho- and bio- geochemical processes occurring on Earth today and in the past. Early applications focused on sediment records especially cores, both marine and terrestrial, to interpret for environmental change. In the past 10 years, a few workers have begun applying environmental magnetic techniques to the sedimentary rock record, notably several studies on Precambrian sedimentary units.

A main focus for the Precambrian Earth History community been paleoenvironmental reconstructions, specifically redox state and oxygen levels due to their importance for biogeochemical processes. Major first-order questions still remain about atmospheric and aqueous environments surrounding the Great Oxidation Event at ~2.3 Ga and in the subsequent billion and a half years during the evolution of eukaryotes and multicellularity. Bulk rock magnetic techniques are perfectly poised for addressing these questions as they can identify and quantify redox-sensitive iron mineralogy non-destructively and at lower abundances than traditional techniques (optical petrography, X-ray diffraction, geochemical extractions, etc.) Initial applications confirm the utility of magnetic methods in providing new insight into ambiguous and contradictory geochemical signals.


However, despite the promise, there are challenges. All Precambrian-age sedimentary rocks contain multiple generations of mineralization so it is vital to classify iron-bearing minerals into primary phases, preserving information about the ancient environment, and secondary phases, telling the story of later alteration during diagenesis/metamorphism. Paleomagnetism can offer insight into the absolute timing of mineral formation by comparing the magnetic direction(s) recorded by these minerals to the apparent polar wander path of the terrain/continent. This technique has been applied for decades by tectonicists and economic geologists, but recently has been applied to Precambrian sedimentary rocks to understand their petrogenesis. Beyond this new direction, additional studies understanding surficial weathering and diagenesis in modern and recent sediments are vital for the expansion of “deep-time” environmental magnetism; there are many open questions spanning from the timing of diagenetic transformations to the oxygen levels needed for (mag)hemitization to the ubiquitous presence of goethite.