Direct dating of fossils
Last month, many around the world read about the unveiling of the remarkably intact remains of two Australopithecus sediba individuals from the Malapa cave site in South Africa.
A team of researchers from the Natural History Museum in Vienna, from the University of Vienna in Austria and from the Washington University, USA recently conducted the first successful direct dating of the material.
Several previous attempts to radiocarbon date the Mladeč specimens directly have failed, but in the present attempt by using teeth as dating material reliable results were obtained.
Every so often, a curious thing happens to the Earth’s magnetic field.
We don’t really know why it happens, or even when it is likely to happen next, but every several hundred thousand years or so, the Earth’s magnetic field reverses. We know this, because when rocks are formed, they are indelibly marked with the normal or reverse polarity of their birth time, or chron.
Nearly all fossilized bone is recrystallized after burial; however, the durations of recrystallization are typically poorly constrained. (2011) present Lu-Hf geochronology data for 72 bones that showed prolonged episodes of trace-element uptake and open system behavior during recrystallization.
Modeled durations range from hundreds to millions of years (Herwartz et al., 2011; Koenig et al., 2009, and references therein). Both studies demonstrate that the chemical complexity of fossilized bones led to a wide range of ages.A Cretaceous dinosaur bone collected from just below the Cretaceous-Paleogene interface yielded a U-Pb date of 73.6 ± 0.9 Ma, in excellent agreement with a previously determined 40Ar/39Ar date of 73.04 ± 0.25 Ma for an ash bed near this site.The second dinosaur bone sample from Paleocene strata just above the Cretaceous-Paleogene interface yielded a Paleocene U-Pb date of 64.8 ± 0.9 Ma, consistent with palynologic, paleomagnetic, and fossil-mammal biochronologic data.Fassett et al.’s failure to present geochemical data that provide insight into the process and duration of recrystallization undermines their conclusion that their age estimates are closely associated with time of death and early fossilization of the dinosaurs in question.Without greater knowledge of the recrystallization process and its duration, it is difficult to interpret what useful information, if any, can be derived from U-Pb or Lu-Hf dating efforts on fossil bone. present the first use of laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) U-Pb dating of fossil bone, but their reference to U-Pb dating studies of igneous zircon and titanite as being similar to the methods for recrystallized apatite is unsupported. (2011) present the detailed methodology required to ensure valid LA-MC-ICP-MS U-Pb geochronology of apatite.Apart from being a geological curiosity, the constant toing-and-froing of the Earth’s polarity has proven extraordinarily useful in pin-pointing the geological ages of rocks.