Paleomagnetic secular variation at the Azores during the last 3 ka

Abstract

We report on 33 new paleomagnetic directions obtained from 16 lava flows emplaced in the last 3 ka on São Miguel, the largest island of the Azores. The data provide 27 well‐dated directions from historical or 14C dated flows which, together with 6 directions previously gathered from the same flows by Johnson et al. (1998), yield the first paleomagnetic directional record of the last 3 ka from the Atlantic Ocean. Within‐flow directions are consistent, suggesting that inclination swings from 60° to 25° and declination changes between −10° to 20° reflect variations in the geomagnetic field over the last 3 ka. To a first approximation, the declination record is consistent with predictions from CALS3k.4 and gufm1 global field models. Conversely, inclination values are lower than model predictions at two different ages: 1) four sites from the 1652 AD flow yield I = 48° instead of I = 63° predicted by gufm1; 2) data from several flows nicely mimic the inclination minimum of 800–1400 AD, but inclination values are lower by ∼10° than CALS3k.4 model predictions. By interpolating a cubic spline fit on declination / inclination versus age data, we tentatively infer the directional evolution of the geomagnetic field at the Azores from 1000 BC to 1600 AD. The obtained curve shows three tracks in virtual overlap during the 1000–800 BC, 800–500 BC, and 400–700 AD time spans.

Chapter 10 Distribution and significance of basaltic eruptive centres: São Miguel, Azores

“…All of the samples obtained from basaltic lava flows from Fogo 1 were considered to represent the 1652 eruption products and yielded lower inclinations than the ones predicted by the global field models for the Atlantic Ocean for the seventeenth century. In fact this age fits well with the age that would be provided by the palaeosecular variation curve for the Azores for this time period, in view of the fact that the inclination and declination results given by Fogo 1 lavas (Johnson et al 1998;Chiara et al 2012) are similar to those obtained for the Pico do Carvão and Pico das Mós eruptions (1.28 + 0.15 and 1.25 + 0.15 ka BP; Moore & Rubin 1991). The presence of trachytic pumice lapilli between the upper levels of Pico do Fogo deposits, which may correspond to the deposit P17 (Queiroz 1997;Queiroz et al 2015), suggests that the basaltic eruption could be prehistoric and have occurred just before the settlement, almost at the same time as the most recent explosive eruption of Sete Cidades (Ferreira 2000).…”

“…This older age has serious implications for the palaeosecular variation curve for São Miguel established by Chiara et al (2012), who considered that the Pico do Fogo lavas belonged to the 1652 eruption. This older age has serious implications for the palaeosecular variation curve for São Miguel established by Chiara et al (2012), who considered that the Pico do Fogo lavas belonged to the 1652 eruption.…”

The multimethod palaeointensity approach applied to volcanics from Terceira: full-vector geomagnetic data for the past 50 kyr

“…7a) and inclinations ( Fig. 7b) were compared to the data produced by Di Chiara et al (2012) and the newest generation of geomagnetic field model predictions (Nilsson et al 2014;Pavón-Carrasco et al 2014). Except for site 26 (7673 BC), our data concur well with both data from the nearby island of São Miguel (Di Chiara et al 2012) and the field models.…”

“…Di Chiara . The declination and inclination data from this study are depicted as grey circles, with the vertical error bar being the α 95 interval; data of (Di Chiara et al 2012) are open diamonds. Intensities (c) obtained using the IZZI-Thellier protocol are green squares; the MSP-DSC result is a red circle; blue triangles are the pseudo-Thellier results; data from ) are yellow squares.…”

“…Six flows are from the Holocene, the other four units are considerably older: 25.7, 46.3, 116.3 and 401 ka. The Holocene flows will add to the current fullvector curves of secular variation in the Azores region (Di Chiara et al 2012. The youngest three of the older units are (nearly) coeval to known excursions of the geomagnetic field: the Mono Lake, Laschamps and Blake events, respectively, and may therefore shed light on the expression of these events in this remote region of the Mid-Atlantic.…”