USING FIRST ORDER REVERSAL CURVES (FORC) TO BETTER CHARACTERIZE THE ROCK-MAGNETIC PALEOENVIRONMENTAL RECORD FROM LAKE BOSUMTWI, WEST AFRICA

Located in West Africa, Lake Bosumtwi occupies a 1Ma old impact crater. This hydrologically closed lake is sensitive to changes in moisture balance and the strength of the West African Monsoon. The complete 1 million year lacustrine sediment record was recovered in 291 m of sediment drill core from the crater's annular moat. Prior studies of Lake Bosumtwi sediment have shown rock-magnetic parameters of magnetic mineralogy, concentration, and grain size document climate variability on both abrupt and glacial-interglacial timescales.

This undergraduate research project studies magnetic hysteresis First Order Reversal Curves (FORC) to better characterize the sediment and the use of rock-magnetics as a paleoclimate proxy. The procedure for measuring a FORC involves measuring 123 partial hysteresis loops in order to better characterize the coercivity distribution within a sample. FORC diagrams are used to interpret both the grain size and mineralogy of the magnetic components of the sediment. Some samples that plot in the pseudo-single domain (PSD) region of the Day Plot are shown to be bimodal; consisting of both multi-domain (MD) and single domain (SD) sized particles when analyzed by the FORC method. Thus the FORC diagram is better for analysis of magnetic grain size than the Day plot alone. The rock-magnetic ratio of saturation remanent magnetization to low-field susceptibility suggests the presence of greigite in Lake Bosumtwi sediment with a SD, high-coercivity component. FORC diagrams of this sediment show, SD, high coercivity closed contours from 65-70mT with strong magnetic interactions. These Bosumtwi FORC diagrams are similar to published FORC diagrams of greigite samples.