Sedimentary Changes and Authigenic Mineralogy Determined From Site 307 Cores, Academician Ridge, Lake Baikal, Siberia

Depth-related variations in mineral and sedimentary components have been studied in bottom sediments of Academician Ridge, Lake Baikal, southeastern Siberia. Gravity cores from Sites 307R and 307A-3 were divided into 62 sedimentary units that differed in structure, color, and/or grain size. Samples from the units were analyzed on smear slides using a petrographic (light) microscope. Bulk and ultrafine particulate mineralogy of representative samples were determined with X-ray diffraction. Authigenic Fe and Mn oxide precipitates were studied with scanning electron microscopy. X-ray diffraction analysis of bulk samples and petrography of detrital grains indicate a granitic provenance for all samples including those older than Holocene. The sediment is composed of fresh quartz, alkali feldspar, minor sodic plagioclase, muscovite, biotite, hornblende, and chlorite in a fine-grained clay-mineral matrix of smectite, illite, chlorite, and kaolinite. Intervals containing thin layers and lenses of coarse sediment enriched in perthite (a potassium feldspar) are probably the result of Pleistocene events. The sediment is composed of more than 50 wt.% clay minerals, and the ultrafine (< 0.1 μm) clay fraction is dominated by well-crystallized dioctahedral aluminous smectite resembling montmorillonite in composition. In oxidized layers of Core 307R, ultrafine sediment chemistry indicates that the smectite is associated with excess ferric iron, amorphous silica, and manganese. A thin crust of sediment cemented with three components is present a few centimeters below the sediment-water interface in both cores. The crust cement includes some form of SiO2, an FeIII-oxide, and a mineral phase composed of Ca, Fe phosphate. Vivianite nodules have formed on the lower surface of the crust indicating precipitation of ferrous iron under reducing conditions. The crust forms the base of a surficial oxidized zone containing abundant Fe- and Mn-oxide mineral phases. Barium is associated with Mn-rich units. Some of the muds have alternately been subjected to oxidizing and then reducing conditions, or the reverse. Framboidal pyrite in oxidized surficial sediments indicates reducing conditions in the past, and residual oxidized FeIII phases in deeply buried reduced sediments indicates previously oxidizing conditions. In one core, an abundant green phase of unknown composition resembling a secondary mineral is present in Pleistocene sediments.