A Full Vector Study of a Terrestrial Geomagnetic Record of the Porcupine Excursion (ca. 2737 ka) Recovered From a Long Volcanic Sequence at Makapuu Point, Koolau Volcano, Hawaii, USA.

New paleomagnetic measurements, coupled with 40Ar/39Ar dating are revolutionizing our understanding of the geodynamo by providing terrestrial lava records of the short-term behavior of the paleofield. As part of an investigation of the Koolau volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled the exposed flows of a long volcanic section (i.e. 191-m) located on the volcano's southwest collapsed flank at a locality known as Makapuu Point. Paleomagnetic and K-Ar investigations of the Koolau Volcanic Series have revealed excursional directions for lavas ranging from 2-3 Ma. The easy access and close geographical proximity to the K-Ar dated lava flows made this newly studied 191-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least 10 samples, collected from each of the successive sites, were stepwise demagnetized by both a.f. (5-100 mT) and thermal (28 to 700 °C) methods. Mean directions were obtained by p.c. analysis. All samples yielded a strong and stable ChRM vector demagnetization diagrams based on 7 or more demagnetization steps, with thermal and a.f. results differing insignificantly. k-T analysis conducted on individual lava flows indicated 50% with reversible curves. Curie points from these analyses revealed a temperature close to or equal to 150-250oC, 575°C and 620oC, indicative of Ti-poor and Ti-rich magnetite as well as titanomaghemite ranging from single domain to pseudosingle domain grain sizes. The mean directions of the base of the section sampled up to ∼14m of the section are excursional ( 10 flows). We have also conducted absolute paleointensity (PI) determinations of the excursional flows using the Thellier-Coe protocol yielding PI values as low as 19 mT and up to 88 mT within the excursional zone of the record. 40Ar/39Ar incremental heating experiments on the groundmass from at least one flow site at 9-m from sea level that yields a plateau with an age of 2.60±0.13 Ma, suggesting that the excursion corresponds to the Porcupine excursion (ca. 2737 ka) reported by Channell et al., (2016). This is the first terrestrial record of the Porcupine excursion and the age is 2-3% younger than in the reported timescale. The obtained VGPs are located over the northeastern part of Asia, close to the artic Circle and as far south as the East China Sea.