Late-Quaternary secular variation data from Mexican volcanoes

Abstract

We present 32 new paleomagnetic directions and 21 absolute paleomagnetic intensities (PI) from 33 volcanoes, sampled at 66 sites and covering the last 46 ka. Of these, 29 were radiocarbon-dated, 3 by thermo-luminescence, and one is of historical age (AD 1793). Rock magnetic experiments show that the dominant minerals are magnetite and titanomagnetite of low to intermediate titanium content, and of pseudo single domain size. Paleodirections were determined using stepwise demagnetization protocols, and PI by the IZZI or Thellier-Coe protocols, and strict selection criteria were applied to ensure the reliability of the data. Previously published data were evaluated to fulfill similar quality criteria as our data, resulting in: 65 directional data point; 18 paleointensity entries from lavas; 59 archeointensities are found to be acceptable. All accepted data come from a region within an 860 km radius around Mexico City. Paleosecular variation (PSV) curves of the past geomagnetic full-vector were established by using the bootstrap-resampling algorithm combined with cubic P-Splines for smoothing and interpolation. Because of the uneven time distribution of the data, this was done for two successive periods, between AD 2000 – 1600 BC and 1600 BC - 44000 BC. The full-vector PSV curves are unique for the American continents because of their temporal extension end because they include both, directions and absolute PI. The recent PSV curves, the last 3600 years, are of high resolutions which certainly have implication for both regional and global geomagnetic field models. Fast PSV change centered around AD 700 and AD 900, separated by a period of much slower PSV were observed, and rapid PI changes were found around 400 BC and AD 1000. In the period between 1600 BC and 44000 BC, PSV curves are well defined only within some periods and otherwise only define general PSV trends. Several intensity peaks with values up to 65 μT (VADM 15⋅1022 Am2) were observed around 250 BC, 12000 BC, 15000 BC and 27000 BC, and intensity lows around AD 700, 1200 BC, 24000-17000 BC, and 34000 BC. Abnormal directions were found around 26000 BC, with negative inclinations down to about -30°, which may be related to the Mono Lake or Rockall event. A comparison with other PSV curves is complicated by the large distance to Europe and the Hawaiian islands, with longitude differences of 99° and 57°, respectively. Lake sediment data from southern Texas show more differences than agreements, suggesting that either of the data may be affected by systematic errors.