84 ANALYTICAL METHODS/Geochronological Techniques Relative Geochronological Techniques Biostratigraphy Methodology Biostratigraphy refers to correlation and age determination of rocks through use of fossils Determining the environment in which the fossil species lived is inherent in this type of analysis Theoretically, any fossil can be used to make physical correlations between stratigraphic horizons, but fossils that are best suited for making precise age correlations (time-stratigraphic correlations) represent organisms that (1) had wide geographic dispersal, (2) were shortlived, and/or (3) had distinct and rapidly developed evolutionary features by which they can now be identified Fossils fulfilling these criteria are termed ‘index’ fossils Both evolution and changes in local environment can cause the appearance or disappearance of a species, thus the time-significance of a particular index fossil must be demonstrated regionally through distinctions made between local environmental effects and time-significant events Environmental effects may bring about the appearance/disappearance of a species because of local conditions, whereas time-significant effects may bring about the appearance/disappearance of a species because of evolution, extinction, or regional migration Local environmental effects are not necessarily time significant and cannot be used in time correlations between different sedimentary units with magnetic reversal frequencies typically between and My Some rock minerals (such as hematite or magnetite) may become magnetized in the same direction as Earth’s magnetic field (normal or reversed), either when a magmatic rock cools or when sedimentary rocks are deposited As geochronological tools, palaeomagnetism and magnetostratigraphy rely on determining the magnetic polarity, including magnetic declination and inclination, of the sample’s remanent magnetic component Palaeomagnetism uses these parameters to calculate a palaeomagnetic pole for the sampling site An age for the pole is determined by matching the pole to a part of the apparent polar wander path (APWP) for that continent (Figure 4) Instead of using poles, magnetostratigraphy, as outlined previously, identifies a sequence of magnetic reversals in a sedimentary or volcanic section (Figure 2) The magnetostratigraphic profile is compared and matched to similar patterns in the GPTS and a chronology for the sampled interval is established The absolute chronology of the GPTS is tied by radiogenic isotope methods, by calibration against the ATS, and/or by calibration with a well-defined biostratigraphic zone (see Magnetostratigraphy, Palaeomagnetism) Application Fossils from the marine sedimentary record indicate existence of primitive life perhaps as early as 2.1 By ago, although the explosion of abundant life in the seas is usually tied to the start of the Palaeozoic era 544 million years ago (Ma) The continental sedimentary record indicates existence of plants and animals by Early Palaeozoic times, with recent indications of animals making forays from the seas onto land perhaps 530 Ma Palaeozoic biostratigraphy, especially for the marine sedimentary record, is tied to precise, absolute ages for most period and stage boundaries, but gaps in the fossil record and/or the lack of isotopically datable rocks at key boundaries leave some discrepancies yet to be resolved Biostratigraphy and fossil zone correlation are most precisely defined for the Mesozoic and Cenozoic eras; this is largely due to the ability to calibrate biostratigraphy not only with radiogenic isotope ages, but also with the GPTS and the ATS for these time periods Application Palaeomagnetism and magnetostratigraphy are most successfully applied to fine-grained volcanic and sedimentary rocks; the latter include red beds, siltstones, mudstones, and limestones Matching of palaeomagnetic poles to established APWPs yields imprecise ages for rocks, but is useful for reasonable, first-order age estimates, probably within about Æ10 My for Phanerozoic through Late Proterozoic rocks The GPTS is most accurately refined through about 175 Ma because of the availability of marine magnetic anomaly profiles to which onshore data can be referenced; nonetheless, magnetic stratigraphy and the GPTS extend through the Palaeozoic to the earliest datable Cambrian sedimentary rocks (Figure 2) Well-constrained magnetostratigraphy yields very precise ages for the following reasons: (1) geomagnetic polarity reversals are rapid, globally synchronous events, and lend themselves well to global, time-significant correlations; (2) polarity reversals are not predictable and yield unique reversal patterns; (3) significant parts of the GPTS have been astronomically tuned, intercalibrated with detailed biostratigraphy, and/or constrained with absolute radiometric ages Palaeomagnetism and Magnetostratigraphy Chemostratigraphy Methodology Earth’s magnetic field, generated in the liquid outer core, undergoes periodic reversals, Methodology Non-radiogenic chemical geochronological tools for sedimentary rocks fall into one of