ALBERT

Description: Speleothems from a well-ventilated dolomitic cave in the Pokhara Valley, central Nepal, preserve a mineralogic record of Indian summer monsoon variability over the past 2300 yr. Annually deposited aragonite layers formed between 2300 and 1500 yr B.P., indicating reduced monsoon precipitation and increased cave aridity, whereas alternating calcite/aragonite laminae deposited after 1500 yr B.P. record elevated summer monsoon precipitation and increased cave humidity. Dense, optically clear calcite layers deposited from 450 ± 5 to 360 ± 20 yr B.P. (1550 to 1640 A.D.) indicate a less-evaporative cave environment and suggest moister and/or cooler conditions, possibly related to climatic change associated with the onset of the Little Ice Age.

Description: Speleothem carbon and oxygen isotopic records from Onondaga Cave, south-central Missouri, and Beckham Creek Cave, north-central Arkansas, are compared with the Cupola Pond and Oldfield Swamp pollen series from southeastern Missouri and the Rodgers Shelter and Modoc Shelter vertebrate biostratigraphic sequences from central Missouri and southwestern Illinois. Similar, and roughly contemporaneous, shifts between deciduous forest and steppe indicators throughout the Holocene are revealed in each database. These independent proxies record steppe conditions between approximately 9000 and 1500 cal yr B.P. A shift toward lighter speleothem carbon may reflect a change from warm and dry to cool and dry conditions between 4500 and 3000 yr B.P. The sensitive response of speleothem δ13C to changes in vegetation emphasizes their importance as paleoclimate records in an area containing few other millenial-scale climate proxies.

Description: Turrialba volcano, the southeasternmost volcano in the Central American arc, is constructed of medium to high-K calcalkaline basalts, andesites, and dacites, plus rare basalts with unusually high Nb concentrations. The compositions of these high-Nb basalts are more similar to those of intraplate basalts than they are to typical calcalkaline or arc-tholeiitic basalts. The association of calcalkaline and high-Nb basalts is rare in arc front volcanoes, seemingly being restricted to volcanoes that overlie Oligocene or younger subducting crust or that overlie the edges of subducting plates. The calcalkaline and high-Nb basalts at Turrialba have generally similar major element, trace element, and isotopic compositions but differ significantly in their Ba/La and La/Nb ratios. The geochemical similarities imply that they were derived from similar ocean island basalt sources. Their geochemical differences suggest that residual rutile stabilized by a large ion lithophile element bearing slab-derived fluid was present during calcalkaline basalt genesis but not during high-Nb basalt genesis. To explain the stability of rutile in a calcalkaline melt with a relatively low TiO2 concentration, we use a model that involves two stages of melting for both basalt types. Silica saturated high degree melts with mid-ocean ridge basalt like incompatible element concentrations generated by upwelling mantle are used as mixing end-members for both the calcalkaline and the high-Nb basalts. The calcalkaline basalts represent mixtures of the high-degree melts and oxidized small-degree melts generated by amphibole breakdown in mantle overlying the subducting slab. This small-degree melt has high incompatible element concentrations and is saturated in rutile. Arc-related lamprophyric rocks have compositions that are appropriate for these small-degree melts. High-Nb basalts are mixtures of the high-degree melts and more reduced small-degree melts that are undersaturated in rutile. These reduced melts may migrate around or through the subducting slab into the wedge to become involved in arc magma genesis.