Determination of Relative Contributions from Marine and Terrestrial Sediment Sources
in the Cariaco Basin using a Magnetic Mixing Model

Verosub, K L, Acton, G - Geology Department, University of California, Davis, CA 95616 United States
Shumway, R H - Statistics Department, University of California, Davis, CA 95616 United States

The Cariaco Basin, an anoxic basin off the coast of Venezuela, acts as a depocenter for sediments of both marine and terrestrial origin. Sedimentation rates for the upper 170 m of sediment recovered during ODP Leg 165 at Site 1002 average 35 cm/k.y. and are fairly constant over the entire 600-k.y.-long interval cored. We have used a magnetic mixing model to identify and quantify the contributions to the sediment at this site from marine and terrestrial sources during the past 27,000 years. Our approach is based on a broad array of environmental magnetic parameters. Using the appropriate statistical methods, we first determine the extent to which different pairs of parameters are interrelated. In the case of the Cariaco Basin, many of the parameters can be shown to vary coherently. We then use the extremal values of each particular magnetic parameter to determine the magnetic signature of two hypothetical source materials. A magnetic mixing model is used to calculate the contribution at any point in the core from each of the sources. The magnetic signatures of the two hypothetical sources can be compared to actual source materials that have contributed sediment to the basin. For the Cariaco core, recent geochemical studies have shown that variations in Ti and Fe content correlate with precipitation and that wetter intervals are associated with more riverine (terrigenous) input and higher Ti and Fe content while dryer intervals are associated with more marine (biogenic) input and lower Ti and Fe content. The relative contributions from our two hypothetical sources, as determined by the magnetic mixing model, show a good correlation with variations between higher and lower Ti and Fe content. This suggests that we have successfully determined the magnetic signatures of the marine and terrestrial components in the sediment.