(Table 2) Analyses of prominent ash layers recovered at DSDP Site 18-178, Gulf of Alaska

A sequence of ash layers recovered from site 178 of the Deep Sea Drilling Project in the Gulf of Alaska was studied to determine the nature of highly explosive volcanic eruptions associated with the Aleutian Arc and Alaskan Peninsula during the last 8 m.y. The major-element chemistry of 25 distinct ash layers was determined. When the analyses are plotted on conventional major-element variation diagrams, the unusual, highly evolved, calc-alkalic characteristics of the ashes are apparent. Perhaps more significantly, there is a good correlation of certain indices of the degree of chemical evolution of each ash (SiO2 content and Larsen index) with sample age. Both parameters vary cyclically, with maximum values of both indices occurring at present, 2.5, and about 5.0 m.y. ago. The cause of the cyclic activity, as well as discontinuous volcanic activity reported for other areas by other investigators, is not precisely known. However, we suggest that variable rates of subduction provide a viable hypothesis for discontinuous volcanic activity associated with convergent plate boundaries.

Results of the sensitivity experiments LOVECLIM as NetCDF files

Several abrupt climatic events during the present interglacial have been associated with catastrophic freshwater forcing, such as the events at 9.2and 8.2 ka BP (Alley et al., 1997; Barber et al., 1999; Marshall et al. 2007; Fleitmann et al. 2008). Proxy evidence suggests that similar events may have occurred during the last interglacial (e.g., Beets & Beets 2003; Beets et al., 2006), suggesting that freshwater-induced perturbations are an important mechanism for abrupt climate change in interglacial climates. In addition solar variability (Neff et al., 2001; Wang et al., 2005) and explosive volcanic eruptions (Crowley, 2000; Shindell et al., 2003; Jansen et al., 2007) can trigger centennial-scale climate events during interglacials and may thus have been responsible for a part of interglacial climate variability. We investigate the sensitivity of the present and last interglacial climates to realistic perturbations resulting from freshwater, solar or volcanic forcings. We will compare the differences between the two interglacial periods, between different climate models and evaluate the resulting using proxy archives.

Composition of basalts from the north part of the Bougault anomaly, Mid-Atlantic Ridge 15°N

A quantitative model of development of magmatic and ore-magmatic systems under crests of mid-ocean ridges is constructed. Correct physical models of melting zone formation in approximation to active spreading, non-stationary dynamics of magma intrusion from a center of generation, filling of magma chambers of various shapes, feeding of fissure-type volcanoes, and retrograde boiling of melts during solidification of intrusive bodies beneath axial zones of spreading in crests of ridges are proposed. Physicochemical and mathematical theories of disintegration of multi-component solutions, growth of liquational drops of ore melts, and sublimation of components from magmatic gases are elaborated. Methods for constructing physically correct models of heat and mass transfer in heterophase media are devised. Modeling of development of magmatic and ore-magmatic systems on the basis of the Usov-Kuznetsov facies method and the Pospelov system approach are advanced. For quantitative models numerical circuits are developed and numerical experiments are carried out.

Ash layers, hyaloclastite, and alteration of basaltic glass at DSDP Leg 65 Holes

Three types of tephra deposits were recovered on Leg 65 of the Deep Sea Drilling Project (DSDP) from three drill sites at the mouth of the Gulf of California: (1) a series of white ash layers at Sites 483, 484, and 485; (2) a layer of plagioclase- phyric sideromelane shards at Site 483; and (3) an indurated, cross-bedded hyaloclastite in Hole 483B. The ash layers in (1) are composed of colorless, fresh rhyolitic glass shards with minor dacitic and rare basaltic shards. These are thought to be derived from explosive volcanoes on the Mexican mainland. Most of the shards in (2) are fresh, but some show marginal to complete alteration to palagonite. The composition of the glass is that of a MORB-type tholeiite, low in Fe and moderately high in Ti, and possibly erupted from off-axis seamounts. Basaltic glass shards occurring in silt about 45 meters above the basement at Site 484 A in the Tamayo Fracture Zone show a distinctly alkalic composition similar to that of the single basement basalt specimen drilled at this site. The hyaloclastite in (3) is made up chiefly of angular sideromelane shards altered to smectite and zeolites (mainly phillipsite) and minor admixtures of terrigenous silt. A very high K and Ba content indicates significant uptake of at least these elements from seawater. Nevertheless, the unusual chemical composition of the underlying massive basalt flow is believed to be reflected in that of the hyaloclastite. This is a powerful argument for interpreting the massive basalt as a surface flow rather than an intrusion. Glass alteration is different in the glassy margins of flows than in thicker glassy pillow rinds. Also, it appears to proceed faster in coarse- than fine-grained sediments.

Petrology and geochemistry of sideromelane glass shards from Pleistocene as layers north and south of Gran Canaria

Major elements, S, F, Cl concentrations and relative proportions of S6+ to total S were analyzed with electron microprobe in sideromelane glass shards from Pleistocene volcaniclastic sediments drilled during ODP Leg 157. Glasses are moderately to strongly evolved and represent a spectrum from alkali basalt, basanite and nephelinite through hawaiite, mugearite and tephrite to phonolitic tephrite. Measured S6+/SumS (0.03±0.98) and calculated Fe2+/Fe3+ (2.5±5.8) ratios in the melt yield preeruptive redox conditions ranging from NNO-1.4 to NNO+2.1. The morphology of the glass shards, variations of S and Cl concentrations (0.010±0.127 wt% S, 0.018±0.129 wt% Cl), calculated preeruptive temperatures (1030±1200 °C) and oxygen fugacities suggest that glasses deposited even within the same ash layers have diverse origin and may have resulted from both submarine and subaerial eruptions. Most vesicle-free glasses are characterized by high concentrations of S and represent undegassed or slightly degassed submarine lavas, whereas vesiculated glasses with low concentrations of S and Cl are strongly degassed and can be ascribed to the eruptions in shallow water or on land. Sideromelane glass shards at Sites 953 are thought to have resulted from submarine eruptions northeast of Gran Canaria, glasses at Site 954 represent mostly volcaniclastic material of shallow water submarine and subaerial eruptions on Gran Canaria and Tenerife, and glasses deposited at Site 956 resulted from submarine or explosive eruptions on Tenerife.

(Table1) Sedimentation rates and age-control points form ODP Hole 152-919A

Planktic d18O and d13C records and point count records of biogenic, volcanic, and nonvolcanic terrigenous [ice-rafted debris (IRD)] sediment components from Hole 919A in the Irminger basin, northern North Atlantic provide a comprehensive dataset from which a paleoceanographic reconstruction for the last 630 kyr has been developed. The paleoceanographic evolution of the Irminger basin during this time contains both long-term patterns and significant developmental steps. One long-term pattern observed is the persistent deposition of hematite-stained ice-rafted debris. This record suggests that the modern and late Pleistocene discharges of icebergs from northern redbed regions to the Irminger Sea lie in the low end of the range observed over the last 630 kyr. In addition, Arctic front fluctuations appear to have been the main controlling factor on the long-term accumulation patterns of IRD and planktic biogenic groups. The Hole 919A sediment record also contains a long-term association between felsic volcanic ash abundances and light d18O excursions in both interglacial and glacial stages, which suggests a causal link between deglaciations and explosive Icelandic eruptions. A significant developmental step in the paleoceanographic reconstruction based on benthic evidence was for diminished supply of Denmark Strait Overflow Water (DSOW) beginning at ~380 ka, possibly initiated by the influx of meltwater from broad-scale iceberg discharges along the east Greenland coast. There is also planktic evidence of a two-step cooling of sea surface conditions in the Irminger basin, first at ~338-309 ka and later at ~211-190 ka, after which both glacials and interglacials were colder as the Arctic front migrated southeast of Site 919. In addition to offering these findings, this reconstruction provides a longer-term geologic context for the interpretation of more recent paleoceanographic events and patterns of deposition from this region.