Study of Declination, Inclination and Absolute Paleointensity of the Short-Term Geomagnetic Behavior (i.e. Cryptochron C2r.2r-1, ca. 2.46 ± 0.13 Ma) Recorded at the Type Section of Halawa Valley, Koo’lau Volcano, Oahu, Hawaii, USA

A novel determination of the absolute paleointensity (P.I) of 28 flows that recorded Cryptochron C2r.2r-1 (ca. 2.46 ± 0.13 Ma) using the modified Thellier-Coe method was undertaken to understand the geomagnetic evolution of the paleofield during an excursion with the existence of less than 10 Kyrs. The successful PI determinations along the 120-meter sequential erupted flows indicate that within the truly transitional/excursional portion of the record there is a conspicuous decrease of the PI values ranging from 20 μT to low values of about 5 μT. These values are comparable to lows similar to polarity transitions of the geomagnetic field. At the base of the sequence, the record shows oscillations of the paleofied ranging from 120 μT and as low as ~10 μT with very variable changes of the non-transitional/excursional paleofield.


Introduction
The study of the short-term behavior of the geomagnetic field is still one of the most relevant and important topics in geophysics today. The mechanisms by which the geomagnetic field is generated in the liquid outer core of the planet is crucial to understand the composite spectrum for the geomagnetic diploe moment and the amplitude spectrum of geomagnetic variation as a function of fre- The discovery of a terrestrial record of a Cryptochron at Halawa Valley of the Koo'lau volcano, Oahu Hawaii triggered the study of the directional as well as the 40 Ar/ 39 Ar radiometric determination of the age of C2r.2r-1 (Cande & Kent, 1995) of 28 successive lava flow sites of a 120-m volcanic sequence (Herrero-Bervera et al., 2007).
The motivation of this study is because there is a lack of short-term geomagnetic behavior with durations less than 10 2 to 10 4 extruded by lavas. Therefore, the coupling of the latest 40 Ar/ 39 Ar determinations and the directional as well as absolute paleointensity results makes this study a very new and unique opportunity to gain an insight into how the generation of the paleofield is documented during very fast fluctuations of the Earth's magnetic field (i.e. few hundreds of years at best). Therefore, the aim of this study is to study the third missing component of the geomagnetic vector, i.e. the absolute paleointensity determination of those 28 flows at the 120-m long Halawa type section to investigate the full vector behavior of the Cryptochron.

The Cryptochron Record at the Halawa Type Section
The directional (i.e. declination and inclination) record of the Cryptochron registered at the type section of Halawa valley of the Koo'lau volcano (21.3743701˚N, 157.9079793˚W) has been already published (Herrero-Bervera et al., 2007). A brief summary of the results indicates that the Halawa Valley is one of the very few sites characterized by a normal polarity. Twenty eight flows were sampled and between 7 -17 magnetically and sun oriented 2.5 cm diameter cores were drilled from a 120-m thick volcanic sequence for paleomagnetic and rock magnetic properties studies (see Figure 1).
The alternating field (a.f.) and thermal demagnetization cleaning experiments were performed on 252 specimens in order to determine the mean declination and inclination of the 28 flows in question ( Figure 2).
The characteristic remanent magnetization was isolated applying a.f. field between 10 to 100 mT and 300˚C -450˚C pointing towards magnetite as the main mineral magnetic carrier of the 28 flows. The mean flow directions were successfully attained by averaging both the a.f. and thermal demagnetization data into a single data set per flow, (see Table 1 of (Herrero-Bervera et al., 2007)) see Figure 3.
Rock magnetic tests were performed to characterize the magnetic mineralogy of the 28 flows studied. The low-field vs temperature experiments (k-T) were      Induced magnetization experiments were applied to all 28 flows mainly to determine grain sizes to test the magnetic behavior of the samples upon demagnetization and eventually check the stability of magnetization of the entire set of flows.
Saturation remanent magnetization (Mr), saturation magnetization (Ms), and coercive force (Hc) were calculated after removal of the paramagnetic contribution. The coercivity of remanence (Hcr) suggests that the NRM is carried by low-coercivity grains. The ratios of the hysteresis parameters plotted as a Day et al. (1977), diagram and modified according to Dunlop (2002) in Figure Day et al. (1977) and corrected according to Dunlop (2002) Grain size approximations using hysteresis parameters are consistent with single domain (SD) and pseudo-single domain (PSD) grains. However they could also represent a mixing of SD and MD particles. This methods are very useful to find out the magnetic grain sizes of carriers of magnetization of the materials under question. Taken from Herrero-Bervera et al. (2007). Figure 6. Shows the corresponding 40 Ar/ 39 Ar radiometric determinations of 9 individual flows. The isochrons of the nine lava flow sites are between 2.64 ± 0.23 and 2.37 ± 0.17 Ma, but these ages are indistinguishable from one another at the 95% confidence level indicating that the entire sequence of flows was most probably erupted over a period of time shorter than our best analytical precision.. The best inverse-variance weighted mean age of the nine flows studied is 2.514 ± 0.039 Ma (MSWD = 062). This age has been recalculated and published by (Singer, 2014) as 2.46 ± 0.08 Ma. This novel 40 Ar/ 39 Ar methodology is used to find out the age of the flows that recorded the fast oscillations of the paleofield.  results dated this period of low inclination between 2.52 ± 0.10 and 2.40 ± 0.17 Ma. The combination of their two most reliable ages allowed them to propose an age of 2.46 ± 0.13 Ma for the magnetic anomaly that they attribute to Cryptochon C2r.2r-1. The original date obtained by the published study of (Herrero-Bervera et al., 2007), was 2.514 ± 0.039 and later on corrected and published by (Singer, 2014) as 2.46 ± 0.08 Ma (Figure 8).

Absolute Paleointensity Determinations of the Halawa Cryptochron
The determination of the absolute paleointensity of the geomagnetic field from the 28 flows recorded at the Halawa valley volcanic sequence was achieved by using the modified Thellier-Coe protocol, (Coe, 1967) of the Thellier and Thellier-experiments but with a different protocol. Instead of measuring the NRM first, we preferred to apply a TRM before heating the sample in zero field (Aitken et al., 1988;Aitken, 1990;Valet et al., 1998;Herrero-Bervera & Valet, 2005 (Singer, 2014)). Ages in bold along the GITS are relative to FCs at 28.187 Ma, ages in italics are from astrochronology. From (Guillou et al, 2018).

Conclusion
The conclusions of the study of the Halawa Crytochron recorded at the type sec-    (Guillou et al., 2018). These results from the Koo'lau volcano have an excellent age correlation with studies from Cape Verde, in the Atlantic.
The successful absolute paleoitntensity (P.I.) results obtained from 27 flows indicate very low values ranging from about 5 -10 micro-Teslas within the truly part of the Cryptochron as well as very high oscillations of the PI values paleosignal outside the truly excursional/transitional zones of the 28 flow record.
The combined rock magnetic and radiometric methods yielded the best results with regards to the age of the flows that recorded the relatively fast changes of the excursion of the so called Cryptochron recorded by the Koo'lau volcano, Oahu, Hawaii.