Paper No. 57-2
Presentation Time: 1:55 PM
REFINING THE AGE OF HOMINID SITES USING MAGNETOSTRATIGRAPHY
Magnetostratigraphy makes use of the off-kilter metronome of Earth’s polarity changes. This geodynamo-generated barcode can be seen on each side of ocean ridges, and it is also imprinted on all sedimentary deposits both on the ocean floor and in continental basins. Research on Quaternary lava flows in the early 1960’s determined both the pattern and approximate timing of these polarity switches. This immediately led to a clearer understanding of global tectonics, as well as a new type of global geochronometer that could utilize diverse rock types and be applied in many different stratigraphic settings. An early application of magnetostratigraphy was to refine and confirm the initial radiometric dating of hominid-bearing deposits in the East African Rift Valleys. In sequences where volcanic material does not exist, magnetostratigraphy may be the primary dating tool available, especially in the Early Pleistocene and Pliocene. An example is in Spain’s Baza Basin where lithic tool-bearing strata (with Early Pleistocene biostratigraphy) were found to be of Reverse polarity, thus broadly delimiting their age to 1.8 - 0.8 Ma. Expanding the magnetostratigraphy revealed that these strata were only a small part of a long sequence of Reverse polarity, so that an interpolated age for the tools is now 1.3 to 1.2 Ma. Even in sequences where volcanic strata are found, such as the Middle Pliocene in the Afar Rift Valley of Ethiopia, there can be stratigraphic intervals or fossil locations where polarity zonation provides the most detailed age control. Magnetostratigraphy can also be applied to isolated deposits, such as cave breccia and flowstones that lack a basin-wide context, but can have concentrations of vertebrate bones and/or lithic tool material. When Reverse polarity strata are found, then an age of >0.8 Ma is required.
The boundary between polarity zones is also useful as a precise global event horizon. The duration for a polarity change has a relative age precision of just a few thousand years. Wherever identified, a specific polarity boundary can be used to synchronize geological comparisons between sections, both regionally and globally. Polarity boundaries can be combined with other event horizons (e.g. ash beds, and lithologies that reflect astronomical forcing) to provide snapshots of the ancient Earth’s surface.