Holocene paleointensities: Thellier Experiments on submarine basaltic glass from the East Pacific Rise

Abstract

A complete description of the geomagnetic field requires both paleodirectional and paleointensity data. Although the paleointensity data base has grown steadily over the last three decades, it remains limited in time and space (the majority of data are of Holocene age and come from Europe). Furthermore, it has been difficult to assess the reliability of the paleointensity determinations. Here we present paleointensity determinations on precisely dated Holocene (0 to 3500 years old) submarine basaltic glass from the East Pacific Rise (15°S to 22°S). Although hysteresis measurements and low‐temperature isothermal remanent magnetization (IRM) acquisition experiments document a significant contribution of superparamagnetic grains, high blocking temperatures (above 400°C) and Curie temperatures between 490°C and 550°C indicate a single‐domain low‐Ti magnetite as the carrier of the remanent magnetization. This notion is further supported by the fact that saturation of remanence is achieved in moderate fields of about 200–300 mT. Submarine basaltic glass proves to be nearly ideal for paleointensity determinations in that it produces a high success rate for Thellier experiments. Twenty‐six out of 30 samples resulted in acceptable paleointenisty determinations. Multiple experiments on splits from the same sample show good reproducibility. The paleointensities for zero age glasses correspond precisely with the present field intensity at the site of recovery. The results of the remaining samples range from 16.7 to 53.9 μT with corresponding virtual axial dipole moments (VADM) of 3.61 ×1022 to 11.9× 1022 A m2. The intensities vary rapidly with time excluding a westward drifting nondipole component as the source for these fluctuations. Basaltic glass is frequently recovered in both dredged and drilled material from the ocean floor. The availability of submarine basaltic glass throughout the world oceans therefore holds great potential for a better distribution of paleointensity data through time and space.