ABSTRACT Deep-sea sediments from the eastern equatorial Atlantic indicate that dissolution of carbonate was greater during the late Pleistocene glacial stages than at present or during the last interglacial stage. Surface (Holocene) carbonate distribution shows a correlation with the 1.9°C potential temperature isotherm which represents the top of Antarctic Bottom Water. A comparison of the surface versus glacial Pleistocene (18,000 YBP) carbonate distribution indicates that the effects of Antarctic Bottom Water were felt 200 to 700 meters shallower during glacial then interglacial conditions. Records of carbonate composition for the past 200,000 years show increases in both carbonate dissolution and terrigenous noncarbonate dilution during glacial stages. Likewise, microscopic examination of the planktonic foraminiferal faunas indicates much greater test fragmentation and destruction and higher percentages of benthonic forams per total foram populations during glacial conditions. The available evidence suggests that Antarctic Bottom Water was the mechanism that produced the increased carbonate dissolution. Therefore, probably either an increase in the production and circulation of Antarctic Bottom Water during glacial stages or the production of a glacial North Atlantic Bottom Water occurred. Regardless of the model used to explain the observations, all of these data suggest that the eastern equatorial Atlantic experienced greater carbonate dissolution during late Pleistocene glacial stages.

Abstract The effects of strong circulation for the generation of erosional surfaces in the deep sea are observed in the region of the Rio Grande Gap, Western South Atlantic. Conspicuous unconformities are observed on multichannel seismic lines shot by the University of Texas Institute of Geophysics (UTIG) research ship R/V Fred Moore in July 1979, while surveying the Rio Grande Gap and the Brazil basin for future locations of DSDP sites. The Rio Grande Gap, a low basement area with an average width of 150 km (93 mi), is located between the Rio Grande Rise and the basement high to the west (Figure 1). The Rio Grande Gap is the major connection between the Argentine basin to the south and the Brazil basin to the north. The 600-km (372-mi) long Vema Channel stretches along the western limit of the Gap (Figure 1). This channel allows significant quantities of northward-flowing Antarctic Bottom Water (AABW) to enter the Brazil basin. Abroad terrace extends from the Vema Channel to the base of the Rio Grande Rise. Regional seismic studies of this part of the South Atlantic (for example, Le Pichon et al, 1971; Gamboa, 1981) have revealed a complex depositional history in the area, mainly related to the onset and fluctuations of the Antarctic Bottom Water circulation. The purpose of this paper is to present some examples of these erosional and depositional events identified on seismic lines across the Rio Grande Gap and the southern portion of the Brazil basin. Analyses of UTIG multichannel seismic data in the Rio Grande Gap allow us to distinguish four major seismic sequences within the sedimentary cover of the region. The sediments in the Rio Grande Gap are about 1.2 km (.7 mi) thick and the sequences are designated by letters A to D from the base to the top (Figures 2 and 3). Sequence A lies on a strong reflector inferred to be top of oceanic crust. In general, the basement reflector is fairly smooth, but in places considerable relief is observed, which appears to indicate offset by faulting. Sequence A is characterized by weak (relatively low amplitude) but continuous subparallel internal reflectors. The lower part of this sequence onlaps and fills the relief on the basement. The upper limit of Sequence A is defined by a prominent regional unconformity (unconformity A) which truncates this sequence at several places. This unconformity is a fairly smooth and level surface and probably marks a major change in the bottom-water circulation through this area. Sequence B is acoustically transparent, showing only few discontinuous reflectors. Sequence B thins and pinches out locally beneath the axis of the Vema Channel. The upper boundary of Sequence B is a prominent regional reflector, unconformity B. Sequence B probably represents a regime of restricted sedimentation controlled by deep sea currents, which began to affect the Rio Grande Gap area. Initiation of these currents probably eroded sequence A to produce unconformity A. Sequence C forms the major part of the terrace to the east of the Vema Channel. This sequence is characterized by dipping (prograding) and contorted internal reflectors (Figures 2 and 3). In cross section Sequence C is somewhat similar in geometry to an alluvial terrace (Figures 1, 2 and 3). This sequence represents a striking change in the sedimentation pattern in the Rio Grande Gap area. Its dipping layers indicate a progradation of sediments transported along the bottom, which filled the gap and formed the terrace to the east of the Vema Channel.

Frequency distributions were determined for more than 250 species of benthic foraminifera from 121 core-top samples taken on the Nazca plate, in the Peru-Chile Trench, and on the Peru shelf and slope. According to their species content, the benthic faunas may be regarded as continental margin, plate-bathyal, or trench and plate-abyssal faunas. About two-thirds of the species from the Peru Margin live in depths shallower than middle bathyal, for which there is no comparable environment on the plate. The displacement of these shallow-dwelling species downslope serves to differentiate the lower-bathyal continental-slope assemblages from those of the plate. Five margin assemblages were recognized on the basis of the shallowest occurrences of species and modes of their percentage distributions. The shallowest assemblages—outer shelf and upper bathyal—are characterized by species of Bolivina living under low-O 2 conditions. Many of the species of these assemblages extend northward only as far as Central America. In contrast, assemblages in upper and lower middle-bathyal and lower-bathyal depths are more widespread along the west coast of the Americas; therefore, wider comparison of their distribution and wider potential use of them as paleobathymetric indices are permitted. One species, Bolivina costata, was found in all samples from the margin and in some samples from the eastern extremity of the plate; this suggests its potential use as a paleoecologic indicator of plate-boundary conditions. The predominantly calcareous plate-bathyal populations occupy the sea floor between 2,600- and 4,100-m water depth. About one-fourth of the species of this group occur also in each of the other two groups. Q-mode factor analysis indicates the presence of three principal and two subordinate assemblages. As in other oceans, the deepest calcareous assemblage is dominated by Nuttallides umbonifera, which occupies a zone associated with the Antarctic Bottom Water Mass in which there is intensive solution of calcium carbonate. This zone of dissolution occurs between about 3,700 and 4,100 m in the study area. The Epistominella exigua Assemblage dominates the shallower parts of the plate between 2,600 and 3,700 m, except for the sea floor beneath the equatorial region of high productivity, where the Gyroidina turgida Assemblage predominates. The trench and plate-abyssal faunas consist entirely of agglutinated species that occur deeper than 4,100-m water depth, the regional CCD (carbonate compensation depth). Half of the species are restricted to the abyss. Q-mode factor analysis indicates one principal and two subordinate assemblages. The Reophax dentaliniformis Assemblage occupies the Peru-Chile Trench, which is the principal path for the movement of Antarctic Bottom Water to the north. The distribution of the assemblage is also coincident with a high-productivity belt associated with the Peru Current. Two distinctive agglutinated species, Ammomarginulina sp. and Saccammina tubulata, are restricted to the trench. Water-mass and substrate preferences of the benthic species may be used to interpret the paleoenvironments of sedimentary sequences in deep-sea cores as well as the origins of sedimentary rocks forming the continental margin. Because this is the first documentation of modern benthic foraminifera in the area, all pertinent species are illustrated.

This paper reports the results of a detailed analysis of 25 Miocene Pacific Deep Sea Drilling Project (DSDP), sites at which 110 benthic foraminiferal species in 255 DSDP samples were quantified. The sites range in depth from 1.5 to 4.5 km between 45 degrees north and south latitudes. The faunal distributions were studied for many Pacific Ocean sites at different water depths through time; diagrams depicting both areal and water depth distributions are presented. Miocene deep-sea benthic foraminifera form clear distribution patterns. Some species appear to have reacted to changes in water mass quality; others appear to reflect changes in food availability and sample preservation resulting from changes in surface productivity related to paleoceanographic change. Many changes in faunal distribution, as well as evolutionary originations and extinctions, occurred between 13 and 16 Ma, concomitant with paleoclimatic and oceanographic changes presumably related to Antarctic glacier expansion and cooling of the deep Pacific Ocean. Late Miocene faunal changes between 8 and 10 Ma were primarily changes in species abundances and depth distribution which appear to be related to the effects of further Antarctic glaciation. The oxygen isotopic record for most of the faunal samples is shown for different water depths through time. The data show clear isotopic changes 14.5 Ma and 8-10 Ma, concurrent with many changes in the foraminiferal distribution patterns. A new species, Cibicidoides cenop n. sp. is named (Appendix I). It evolved in the early Miocene and is apparently an indicator of cold-water conditions in the Miocene Pacific Ocean. The benthic foraminiferal data are compatible with the following paleoceanographic interpretations: (1) a warm early Miocene deep ocean with a less well-developed surface-to-bottom thermal-gradient that in mid-late Miocene time; 2) an increase in ocean margin surface productivity after 16 Ma related to intensification of atmospheric-oceanic circulation and upwelling; 3) a change in the deep ocean water mass after 15 Ma, including a cooling and a shallowing of deep-ocean conditions related to permanent establishment of the Antarctic ice sheet; 4) an increase in surface productivity along the equator after 15 Ma, related to intensification of the latitudinal thermal gradient, more vigorous deep-ocean circulation and perhaps establishment of the Equatorial Counter-current and Undercurrent systems; 5) an increase in sediment organic carbon and intensification of the low oxygen zone after 8-10 Ma as well as an increase in deep-ocean dissolution and Antarctic bottom water influence related to further Antarctic ice sheet expansion in latest Miocene time; and 6) a deep water corridor across the Indo-Pacific until at least 21 Ma.

The bottom potential temperature distribution for the Pacific Ocean north of 11° S. is presented, based upon 495 data points derived from hydrographic stations, the Lamont thermograd, and an in-situ salinity-temperature-depth recorder. Although the entire range of potential temperature is only 1.2° C, an oceanwide pattern is apparent. The coldest bottom water (< 0.8° C potential temperature) occurs in the equatorial region between 160° and 180° W. This feature can be traced into the area north of Hawaii by paths both east and west of these islands with a warming of only 0.5° C. In order to separate the effect of the decrease of potential temperature with depth (which occurs in the upper 4000 to 5000 m) from the bottom variation due to influx of cold water, the anomaly of bottom potential temperature is plotted. This parameter is defined as the difference between the observed bottom potential temperature for a particular station and the average bottom potential temperature for the North Pacific at the depth of the observation. The water between 160° and 180° W. has the largest negative anomaly, 0.3° C colder than the average t p found at corresponding depths. The water north of Hawaii has a positive anomaly, indicating that in this region the deeper water is relatively warm compared to that in the equatorial region between 160° and 180° W., even after the depth effect is removed. The bottom water over and east of the crest of the mid-ocean ridge at 120° W. has large positive anomalies, suggesting the possibility that this water is warmed by geothermal heating. From the bottom potential temperature distribution and the anomalies of these values, together with recently obtained Lamont bottom current measurements, it is possible to trace the basic pattern of bottom circulation (which should also reflect the circulation for the water below 2000 to 2500 m). The sole influx of water is the Antarctic Bottom Water flowing northward along 175° W. from 20° S. to 16° N. This current is no doubt a continuation of the deep western boundary current along the Tonga Trench described recently for the South Pacific. From this point, the flow divides into two zonal branches. The eastern branch passes between the Johnston Island and Christmas Island Ridges and the western branch passes between the Marshall Islands and the Marcus-Necker Ridge. After trans-versing these respective passages, both flows turn northward, eventually converging in the North Pacific Basin between the Hawaiian and the Aleutian Islands, where deep upward transport is expected. It is estimated that the time necessary for the water to flow from 175° W., 10° S. to the center of the convergence area is 750 years.

We present an overview of the Eocene-Oligocene transition from a marine perspective and posit that growth of a continent-scale Antarctic ice sheet (25 × 10 6 km 3 ) was a primary cause of a dramatic reorganization of ocean circulation and chemistry. The Eocene-Oligocene transition (EOT) was the culmination of long-term (10 7 yr drawdown and related cooling that triggered a 0.5‰–0.9‰ transient pre-scale) CO 2 cursor benthic foraminiferal δ 18 O increase at 33.80 Ma (EOT-1), a 0.8‰ δ 18 O increase at 33.63 Ma (EOT-2), and a 1.0‰ δ 18 O increase at 33.55 Ma (oxygen isotope event Oi-1). We show that a small (~25 m) sea-level lowering was associated with the precursor EOT-1 increase, suggesting that the δ 18 O increase primarily reflected 1–2 °C of cooling. Global sea level dropped by 80 ± 25 m at Oi-1 time, implying that the deep-sea foraminiferal δ 18 O increase was due to the growth of a continent-sized Antarctic ice sheet and 1–4 °C of cooling. The Antarctic ice sheet reached the coastline for the first time at ca. 33.6 Ma and became a driver of Antarctic circulation, which in turn affected global climate, causing increased latitudinal thermal gradients and a “spinning up” of the oceans that resulted in: (1) increased thermohaline circulation and erosional pulses of Northern Component Water and Antarctic Bottom Water; (2) increased deep-basin ventilation, which caused a decrease in oceanic residence time, a decrease in deep-ocean acidity, and a deepening of the calcite compensation depth (CCD); and (3) increased diatom diversity due to intensified upwelling.