[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-06] Exploring quasi-continuous time variations in geomagnetic field intensity during the Gauss-Gilbert chrons through Icelandic lava sequences

*Yuhji Yamamoto¹, Maxwell Christopher Brown^2,3, Hiroyuki Hoshi⁴, Vivian Sinnen^3,10, Hiroto Ishikawa¹, Brian Jicha⁵, Masaru Kono⁶, Hidefumi Tanaka¹, Takehiro Koyaguchi⁷, Kohei Masaoka^1,8, Justin Tonti-Filippini^3,9 (1.Kochi University, 2.University of Minnesota, 3.University of Iceland, 4.Aichi University of Education, 5.University of Wisconsin-Madison, 6.Tokyo Institute of Technology, 7.University of Tokyo, 8.Kyushu University, 9.University of Oslo, 10.Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research)

Iceland is a volcanic island with thousands of plateau lava flows emplaced sequentially in piles over the past ~16 Ma. These lava sequences are a unique data source for investigating quasi-continuous time variations in the past geomagnetic field. The comprehensive compilation of published Icelandic paleomagnetic data, ICEPMAG (Tonti-Filippini and Brown, 2019), contains entries for over 9400 lavas, but only ~6% are associated with absolute paleointensity (API) data. These API data come from 24 papers published between 1970 and 2016 on a variety of lava sequences in different regions, but mostly in southern and eastern Iceland, and none from western Iceland.

To investigate quasi-continuous time variations in API during the Gauss Normal polarity chron and the upper Gilbert Reversed polarity chron, we have targeted the lava sequences of Lundarreykjadalur valley in the Borgarfjörður region of western Iceland. We collected oriented paleomagnetic core samples from ~390 lavas spanning 21 sections along an 18 km transect, during field campaigns in 1994 and 2016-2023. Paleodirections, K-Ar dates (in the 1990s) and new ⁴⁰Ar/³⁹Ar dates from these lavas indicate that the targeted lavas recorded quasi-continuous time variations in the geomagnetic field from ~2.6 Ma to ~4.3 Ma.

API determinations from a selection of these lavas have been obtained using the Tsunakawa-Shaw method. To assess the quality of the API data, we have conducted detailed rock magnetic measurements, including hysteresis, FORC, and thermomagnetic measurements. We present preliminary API results and discuss intensity variations during the Gauss Normal polarity chron and the upper Gilbert Reversed polarity chron. The API data from this study is expected to provide one of the most detailed records of time-series changes in API for this period.