We have constructed high-resolution (104-105 years) benthic foraminiferal/513C and b180 records for the upper Eocene through lower Oligocene of two pelagic sequences, Deep Sea Drilling Project (DSDP) Site 522 in the Angola Basin, South Atlantic Ocean, and Ocean Drilling Program (ODP) Site 744 in the southern Indian Ocean. These records provide improved constraints on both the timing and magnitude of marine oxygen and carbon isotope events from 30 to 35 Ma. The oxygen isotope records indicate that the ubiquitous b180 increase (Oi-1), which marks the rapid expansion of continental ice sheets and a minimum of 3 ø to 4øC of cooling of bottom waters in the earliest Oligocene (33.6 Ma), occurred in <350 kyr. More than half the transition occurred over the final 40-50 kyr. This period of lower temperatures and widespread continental glaciation persisted for roughly 400 kyr (i.e., the duration of magnetochron C 13n). These records also indicate that this interval was characterized by at least two -100-kyr waxing and waning cycles (Oi-la and Oi-lb) and possibly several higher-frequency events. The benthic foraminifera115•3C records show a positive 0.8%0 excursion that is nearly isochronous with the Oil oxygen isotope increase. Similar magnitude 15•3C increases at other sites indicate this was a global phenomenon suggestive of an unusually large perturbation to the carbon cycle. This excursion was followed by smaller amplitude 15•3C oscillations with periods of roughly -400 kyr. We suspect that the ubiquitous Oi-1 15•3C excursion resulted from a brief but substantial increase in export production and carbon burial. Introduction Antarctic ice sheets have a significant influence on the climate and ecology of the oceans on both regional and global scales. They enhance the vigor of atmospheric circulation by acting as an enormous heat sink that steepens the latitudinal thermal gradient of the southern hemisphere [e.g., Flohn, 1984]. Low temperatures and strong, persistent winds generated over the ice sheets help sustain the rapid production of Antarctic Bottom Water (AABW) by accelerating sea-ice formation and cooling of surface waters. These same winds also heighten the fertility of the Southern Oceans by intensifying surface circulation and creating upwelling along frontal zones and by delivering fine iron rich dust from Antarctica [e.g., Martin, 1990]. On geologic timescales, continental ice sheets also influence other critical components of the Earth system Copyright 1996 by the American Geophysical Union. Paper number 96PA00571. 0883-8305/96/96PA-00571512.00 including sea level, land/sea surface coverage, planetary albedo, continental weathering rates, and ocean chemistry, each of which has the potential to trigger large-scale physical and geochemical feedbacks [e.g., Matthews and Poore, 1980; Barron, 1985; Gibbs and Kurnp, 1994]. Hence efforts to understand the nature of past changes in global climate, particularly in ocean and atmospheric circulation, require a fundamental understanding of Antarctic ice sheet evolution....

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The fie ld of ocean geochemistry has recently been expanded to include in situ laser Raman spectroscopic measurements in the deep ocean. While this technique has proved to be successful for transparent targets, such as fluids and gases, d iff iculty ex ists in u sin g deep sub mergence vehicle man ipulators to position and control the very small laser spot with respect to opaque samples of interest, such as many rocks, minerals, bacterial mats, and seafloor gas hydrates.We have developed, tested, and successfully deployed by remotely operated vehicle (RO V) a precision underwater positioner (P UP ) which provides the stability and precision movement required to perform spectroscopic measurements usin g the Deep Ocean In Situ Spectrometer (DORISS) instru ment on opaque targets in the deep ocean for geochemical research. The positioner is also adaptable to other sensors, such as electrodes, which require precise control and positionin g on the seafloor. P UP is capable of translating the DORI SS optical head with a precision of 0.1 mm in three dimension s over a range of at least 15 cm, at depths up to 4000 m, and under the nor mal range of oceanic conditions (T, P , current velocity). The positioner is controlled, and spectra are obtained, in real time via Ethernet by scientists aboard the surface vessel. Th is capability has allowed us to acquire high quality Raman spectra of targets such as rocks, shells, and gas hydrates on the seafloor, includ in g the ability to scan the laser spot across a rock surface in sub-millimeter increments to identify the constituent mineral gra ins. These developments have greatly enhanced the ability to obtain in situ Raman spectra on the seafloor from an enormous range of specimens.

[1] Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino, California. In spite of high sedimentation rates and high biological productivity, direct or indirect indicators of gas hydrate presence had not previously been recognized in this region, or along transform margins in general. Gas is indicated by a bottom simulating reflection (BSR) observed near the Gorda Escarpment, by ''bright spots'' and ''gas curtains'' scattered throughout the sedimentary basin to the south, and by d C and d18 O isotopes of carbonates, which are similar to those recovered from other hydrate-bearing regions. The BSR reflection coefficient of À0.13 ± 0.04 and interval velocities as low as 1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR toward the north facing Gorda Escarpment and beneath a channel near the crest suggests fluid flow toward the seafloor. Integrating these various observations, we suggest a scenario in which methane is formed in thick Miocene and Pliocene deposits of organicrich sediments that fill the marginal basin south of the transform fault. Dissolved and free gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate formation and in channels, slumps and chemosynthetic communities on the face of the escarpment. We conclude that the BSR appears where hydrate-bearing sediments are uplifted because of current triple junction tectonics.