Magnetic signature of environmental changes in the last 1.2 Myr at ODP Site 1146, South China Sea

A detailed magnetic analysis has been done on the upper 145 m of the splice of ODP Site 1146 drilled during Leg 184 in the South China Sea. First, a magnetostratigraphic record was obtained. The Bruhnes–Matuyama reversal, preceded by a precursor, and the two Jaramillo reversals were identified at 115, 133 and 137.5 mcd (meter composite depth), respectively. Several known excursions during the Brunhes period, as well as the Kamikatsura event during the Matuyama chron, were also identified. Coupled with the oxygen isotope record obtained from benthic foraminifera, this allows us to establish a precise age model, showing that the studied interval extends to 1.18 Myr. Long-term and short-term variations have been documented in the magnetic parameters, which appear at least in part related to changes in monsoon activity. On the long-term trend, three main intervals with different magnetic trends were identified: 1.2–0.7, 0.7–0.2 and the most recent 0.2 Myr. During the oldest period, the magnetic content decreases, while the grain size of the magnetic mineral increases with large fluctuations. The coercivity of the magnetic minerals also increases consistently with data obtained by other authors from Pacific cores. During the following 500 kyr all the magnetic properties are rather uniform, with rather low abundance and relatively large grain sizes and low S-ratio. This probably illustrates a period of enhanced winter monsoon. Then, in the most recent period, the magnetic grain size decreases again with pronounced fluctuations. On the short-term scale, a 23 kyr component is identified by power spectrum analysis in all magnetic parameters in the bottom part of the studied interval. This component is correlated with the oxygen isotope record, as shown by coherence analysis. Except for the most recent 200 kyr, where the pattern changes, cold/warm periods coincide with low/high magnetic content, and magnetic grains are coarser during cold stages and finer during warm periods. This is consistent with the clay mineral analysis and illustrates changes in the balance between arid/humid periods corresponding to dominant winter/summer monsoon and giving rise to physical erosion and wind blowing/chemical weathering in the main fluvial plains and transport by oceanic current.