ALBERT

Description: The Middle America Trench between the Cocos Ridge and a well-studied corridor off the Nicoya Peninsula has a more varied morphology and structure than previously reported. The morphological positive features on the lower plate significantly affect the upper plate structure. The Cocos Ridge has uplifted the margin opposite the Osa Peninsula. Northwest of Cocos Ridge, numerous seamounts on the oceanic crust sculptured the margin as they subducted. A seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted. The greater portion of the trench sediment is subducted beneath a lower slope accretionary mass. Beneath the middle and upper slope is a margin wedge consisting of a high-velocity rock with few internal reflections. Its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area. Overlying this surface is a low-velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred Eocene section beneath the shelf. The shelf basement is composed of Nicoya complex (ophiolite) with the same acoustic velocity, similar structure, and no apparent dividing geologic boundary with the margin wedge. We favor a seaward continuation of the Nicoya complex to the middle slope and emphasize the evidence for a non-steady state Tertiary tectonic history.

Description: The flow of the low‐salinity Antarctic Intermediate Water (AAIW) at 700–1150 m depth across the Rio Grande Rise and the lower Santos Plateau is studied under the auspices of the World Ocean Circulation Experiment (WOCE) in the context of the Deep Basin Experiment. Our data set consists of several hydrographic sections, a collection of 15 RAFOS float trajectories, and records from 14 moored current meters. The data were gathered during different intervals between 1990 and 1994. The inferred flow field strongly supports a basinwide anticyclonic recirculation cell in the subtropical South Atlantic underneath the wind‐driven gyre. Its center, which appears to be southeast of the Rio Grande Rise, separates the eastward advection of AAIW below the South Atlantic Current from the westward flowing, recirculating AAIW. The two near‐shelf limbs closing the circumference of AAIW flow are formed in the east by the deep Benguela Current, potentially modulated by salty inflow of Indian Ocean Intermediate Water, and in the west by the Brazil Current system. Further important circulation elements are the Brazil‐Falkland (Malvinas) Confluence Zone at 40°S and an unnamed divergence at 28°S close to the 1000 m isobath. The resulting broad southward flow of AAIW augments the share of modified, i.e., saltier, intermediate water in the source region of the South Atlantic Current, while the smaller northward flow marks the source of a narrow equatorward Western Intermediate Boundary Current, ultimately leaving the South Atlantic. This shelf‐trapped jet is clearly documented in hydrographic data from 19°S and in nearby current meter records. The jet contrasts a sluggish flow across this latitude east of 35°W. A continuous flow of AAIW from its subpolar region in the southwestern Argentine Basin all along the western slope toward the equator appears unlikely between 35°S and 25°S.

Description: Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long-term history remains a matter of speculation. To determine if Pacific-type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr-Nd-Pb isotope data, combined with trace element and 40Ar-39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87Sr/86Sr (0.7025–0.7029) and 206Pb/204Pb (18.80–19.48) ratios typical of Pacific-type MORB indicating that Pacific-type upper mantle existed east of the Australian-Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030–0.7035), low 206Pb/204Pb (17.99–18.10) and trace element characteristics typical of present day Indian-type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with an Indian-type MORB source (Site 280A) and, in the second case, transitional between Pacific-type and Indian-type mantle sources. The occurrence of seafloor basalts with transitional or Indian-type isotopic characteristics well to the east of the present Indian-Pacific MORB isotopic boundary within the AAD strongly implies that Pacific-type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma.

Description: Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic.

Description: The El Niño/Southern Oscillation (ENSO) phenomenon is the strongest natural interannual climate fluctuation1. ENSO originates in the tropical Pacific Ocean and has large effects on the ecology of the region, but it also influences the entire global climate system and affects the societies and economies of manycountries2. ENSO can be understood as an irregular low-frequency oscillation between a warm (El Niño) and a cold (La Niña) state. The strong El Niños of 1982/1983 and 1997/1998, along with the more frequent occurrences of El Niños during the past few decades, raise the question of whether human-induced 'greenhouse' warming affects, or will affect, ENSO3. Several global climate models have been applied to transient greenhouse-gas-induced warming simulations to address this question4, 6, but the results have been debated owing to the inability of the models to fully simulate ENSO (because of their coarse equatorial resolution)7. Here we present results from a global climate model with sufficient resolution in the tropics to adequately represent the narrow equatorial upwelling and low-frequency waves. When the model is forced by a realistic future scenario of increasing greenhouse-gas concentrations, more frequent El-Niño-like conditions and stronger cold events in the tropical Pacific Ocean result

Description: Recent captures of two female giant squid ( Architeuthis ) off southern Australia have provided the first record of a mated female specimen of these almost mythical deepsea creatures. We found sperm packages (spermatophores) embedded within the skin of both ventral arms of the larger of the two specimens. It seems that male giant squids may use their muscular elongate penis to ‘inject’ sperm packages under pressure directly into the arms of females.

Description: Three lakes of concentrated brine were discovered in the seabed of the Mediterranean Ridge, southwest of Crete in late 1993 and early 1994. These lakes have a distinctive appearance on side-scan sonar images with no backscattering from the lake surface and enhanced backscattering at their edges (Figure 1). Measurements with conductivity-temperature-depth (CTD) and heat flow probe show in one case a near-isothermal temperature for the lake water and in another thermal layering. The chemistry of the brines varies in each lake and shows that the brines are derived from selective solution of different parts of the Messinian evaporites that lie beneath the seafloor. Flow of seawater through the sediments containing the Messinian evaporites appears to be required to form the lakes.