| Abstract | Variations in magnetic mineralogy, geochemistry, and physical properties of sediments from Grass Lake and Tule Lake were used to interpret the environmental factors which influenced sedimentation over the past 130 ky. Grass Lake and Tule Lake are located in northern California in the southern Cascade Range. Volcanic rocks are the predominant lithology in the catchment of both lakes, but the lakes differ in elevation, lake size, and catchment area. Petrographic and Curie temperature analysis shows the magnetic minerals to be detrital. Grass Lake is a small, subalpine lake in which characteristic relations among magnetic mineral abundance, contents of Ti and Zr, and grain size reflect sedimentation during periods of volcanic activity in the catchment or periods of volcanic quiescence. In the absence of volcanic activity, oxidative weathering, erosion, sediment transport, and deposition produced lacustrine beds in which the concentrations of magnetic iron oxide minerals show a close correspondence between immobile elements (Ti and Zr) in certain heavy minerals. Periods of volcanic activity, at approximately 80 Ka, 70 Ka, and from 32 to 10 ka, are interpreted from coarse grain size, a fluctuating Zr/Ti ratio, and in some cases, high hematite content. Throughout the whole core, reduction reactions in the sediments may also have affected the magnetic properties through the dissolution of magnetite. Tule Lake is a larger lake in which comparison of sediment magnetic properties to diatom populations shows a correspondence between lake level changes (caused by glacial-interglacial climate changes), and magnetic mineral concentration. During mostly warm periods, from 160 to 140 ka, and 9 ka to the present, high lake level led to relatively low magnetite content at the core site. Such a relation may arise from low depositional energies at the core site. Moreover, high contents of organic carbon suggest that during warm periods lithogenic sediment was diluted by high concentrations of organic matter. During periods of low lake level, associated with glacial periods, magnetic mineral contents increased along with Ti and Zr contents. During these periods variations in sediment transport and depositional processes regulated the concentration of heavy minerals in the sediments. |