Palaeoclimatic changes from rock magnetic proxies
Lithostratigraphic, magnetostratigraphic and rock-magnetic cyclostratigraphic data were combined to create a high-resolution age model for 342 m of Late Pliocene–Middle Pleistocene marine deposits exposed in the Stirone River, northern Italy. Magnetostratigraphic analysis of 74 oriented samples at 21 stratigraphic horizons recognized five polarity zones between c. 3.0 and 1.0 Ma. Unoriented samples were collected every metre between 0 and 311 m and low-field magnetic susceptibility (χ) was measured for cyclostratigraphic analysis. The χ data series was tied to absolute time using the magnetostratigraphy and subjected to multi-taper method spectral analysis. The resultant power spectra revealed significant frequency peaks that are aligned with eccentricity, obliquity and precession Milankovitch orbital cycles. The χ data, correlated to the 41 ka obliquity and the 23 ka/19 ka precession cycles and anchored to a well-established biostratigraphic horizon, were used to create a high-resolution age model for the Stirone section between 2.99 and 1.81 Ma, where stratigraphic positions of magnetic reversals were previously poorly defined. This cyclostratigraphic age model reveals that the length of an important depositional hiatus at the base of the C2An.1n subchron is 200 ka shorter than previously determined. We link the precession-aligned variability in χ to global mid-latitude, insolation-induced variability in runoff and ocean circulation.
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Magnetic Methods and the Timing of Geological Processes
Magnetostratigraphy is best known as a technique that employs correlation among different stratigraphic sections using the magnetic directions defining geomagnetic polarity reversals as marker horizons. The ages of the polarity reversals provide common tie points among the sections, allowing accurate time correlation. Recently, studies of magnetic methods and the timing of geological processes have acquired a broader meaning, now referring to many types of magnetic measurements within a stratigraphic sequence. Many of these measurements provide correlation and age control not only for the older and younger boundaries of a polarity interval, but also within intervals. Thus, magnetostratigraphy no longer represents a dating tool based only on geomagnetic polarity reversals, but comprises a set of techniques that includes measurements of geomagnetic field parameters, environmental magnetism, rock-magnetic properties, radiometric dating and astronomically forced palaeoclimatic change recorded in sedimentary rocks, and key corrections to magnetic directions related to geodynamics, palaeocurrents, tectonics and diagenetic processes.