Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998^99) and four from Legs 119 and 120 (1987^88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at V110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatilerich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet^biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet^biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate. ß

Seawater 187 Os= 188 Os ratios for the Middle Miocene were reconstructed by measuring the 187 Os= 188 Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187 Os= 188 Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187 Os= 188 Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01=Myr increase in marine 187 Os= 188 Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87 Sr= 86 Sr ratios at 16 Ma. A large increase of 1‰ in benthic foraminiferal Ž 18 O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate Ž 13 C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187 Os= 188 Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187 Os= 188 Os ratios and also nearly constant 87 Sr= 86 Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO 2 levels after 15 Ma. The temporal resolution (three samples=Myr) of the 187 Os= 188 Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187 Os= 188 Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187 Os= 188 Os variations may correlate inversely with short-term benthic foraminiferal Ž 18 O and bulk carbonate Ž 13 C variations that reflect glacioeustatic events.

During the latest Eocene, as Earth's climate transitioned from a greenhouse to an icehouse state, likely forced by declining atmospheric carbon dioxide pressure ( pCO 2 ), a large tract of basic and ultrabasic seafl oor breached sea level in the New Caledonian region of the southwestern Pacifi c Ocean. A plausible mechanism for CO 2 drawdown at this precise time, 35-34 Ma, invokes weathering of the seafl oor rocks, composed of highly soluble Ca-and Mg-rich silicates, and related organic carbon burial. Carbon burial fl uxes based on estimates of paleo-area, paleo-erosion rate, and paleo-sedimentation rate suggest a peak perturbation of 0.3-0.5 Emol (10 18 mol) m.y. -1 This perturbation may have been suffi cient to lower atmospheric pCO 2 ~100 ppmv, thus triggering growth of the East Antarctic ice sheet and a host of related environmental changes.

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The Dog Bay Line, a Silurian suture key to deciphering Appalachian accretionary history, was first recognized in Newfoundland. It marks where the Ordovician Tetagouch–Exploits ensimatic back-arc basin (TEB), which had opened within the leading peri-Gondwanan Gander terrane, finally closed. Here, we extrapolate this suture into New England, placing it between the Liberty–Orrington–Miramichi inliers (LOM) and the Merrimack–Fredericton trough (MFT). Southeastward, marine strata of the MFT overlie the TEB passive margin, exposed in the Ganderian St. Croix block, and display southeast-vergent structures transected by Acadian cleavage. They structurally underlie southeast-vergent thrusts at the base of the LOM. Northwestward, the LOM, Central Maine – Matapedia trough (CMMT), and Lower Silurian igneous rocks record elements of the upper plate trench–arc system, respectively, a subduction complex, forearc basin, and arc. The CMMT forearc received detritus both from the northwesterly arc region, and also from the Early Silurian-exhumed subduction complex. Minimal contrast in Silurian turbidites near the line may be due to sediment bypassing the subduction complex, and (or) a common provenance when the complex emerged above sea level. Salinic unconformities in the upper plate (arc–trench) reflect episodes of shortening, within an overall extensional setting that resulted in thinned, weakened lithosphere, and also final uplift accompanying latest Silurian slab breakoff. Silurian strata of the Coastal Volcanic Belt document a separate arc system built on Ganderia’s trailing edge, where northwest-directed subduction of a narrow seaway led to latest Silurian collision with buoyant, strong lithosphere of Avalonia’s passive margin, and the onset of Acadian typically dextral-oblique, northwest-vergent deformation.