Downward remagnetization of a ∼74-m-thick zone in lake sediments from palaeo-Lake Idaho (NW United States)—Locating the Gauss/Matuyama geomagnetic boundary within a dual-polarity zone. Issue 2 (29th April 2020)

• Record Type: Journal Article
• Authors: Allstädt, Frederik J; Appel, Erwin; Rösler, Wolfgang; Prokopenko, Alexander A; Neumann, Udo; Wenzel, Thomas; Pross, Jörg
• Is Part Of: Geophysical journal international. Volume 222:Issue 2(2020)
• Page Start: 754
• Page End: 768
• Publication Date: 2020-04-29
• Subjects: Magnetic mineralogy and petrology -- Magnetostratigraphy -- Palaeomagnetism -- Remagnetization; Geophysics -- Periodicals; 550
• DOI: 10.1093/gji/ggaa165
• Languages: English
• ISSNs: 0956-540X

Abstract

SUMMARY: Remagnetization is an important issue in palaeomagnetism. Here, we discuss an extraordinarily thick (∼74 m) dual-polarity transition zone between the Gauss and Matuyama Chrons. The studied succession is from a drill core through lacustrine sediments of palaeo-Lake Idaho (Snake River Plain, NW United States of America) that are intercalated with basalt units. We identified detrital Ti-rich titanomagnetite and magnetite in lamellar exsolutions as the main carriers of a primary remanence, likely derived from the basalts that erupted in the Snake River Plain. Stepwise thermal demagnetization revealed a single-component remanent magnetization with reversed and normal polarities above and below the transition zone, respectively. Based on rock-magnetic results, microscopic observations, and previously known events in the evolution of palaeo-Lake Idaho, the reversed-polarity component in the transition zone represents a secondary chemical remanent magnetization caused by magnetic mineral alteration or partial neo-formation of magnetite, in association with strong depletion of the primary detrital magnetic minerals that affected a wide depth range below the level where the remagnetization event occurred. This remagnetization event was most likely related to lake-level lowering and partial desiccation of palaeo-Lake Idaho. Understanding the nature and origin of the remagnetization allows to identify the polarity boundary in the unusual case of a secondary magnetization with reversed polarity produced downward in a sequence to an extraordinary large depth. Based on available age information, the observed reversal represents the Gauss/Matuyama boundary, which provides an important age constraint for palaeoclimatic interpretation of the succession.