Thermal instability from rock magnetic measurements confirms the underestimates of absolute paleointensity records during the Santa Rosa geomagnetic excursion

Abstract

Geomagnetic excursion events have been widely studied in recent years as a key process for understanding the evolution of the Earth's magnetic field. The Santa Rosa geomagnetic excursion (SRE) event during the Matuyama chron has been globally recorded in sediment sequences and lava flows. Galapagos lavas distributed in near-equatorial with an ⁴⁰Ar/³⁹Ar age of 925.7 ± 4.6 ka display absolute paleointensity values of about 14% of the modern magnetic field, which is a valuable record of the Earth's magnetic field strength during the SRE event. However, the above extremely low estimates of paleointensities during the SRE were fitting from higher temperature segments (400 °C–575 °C) from previous paleointensity experiments, which is biased by the thermal instability of Galapagos lava samples during high-temperature heating treatments. From our comprehensive rock magnetic experiments in this study, Galapagos lava samples exhibit thermal instability after heating treatments higher than 400 °C. The severe thermal alteration occurred after the heating temperature reached 500 °C, mainly manifested as an increase in remanence-carrying capacities, such as the enhanced ability of paleointensity specimens to record partial thermoremanent magnetization, resulting in underestimated paleointensities during the SRE. In-depth experiments on rock magnetism and hysteresis parameters analysis provide a powerful method to detect the thermal instability of lava samples, which can help us confirm the biased geomagnetic field strength during this short-lived excursion period and prevent misinterpretations of the Earth's magnetic field evolution through erroneous low paleointensity records.