(Table 1) Age model for ODP Site 175-1077 based on oxygen isotopes of Globigerinoides ruber pink

Palynological records from the Congo fan reveal environmental change in equatorial Africa occurring 1.05 Ma ago, 100 k.y. before the mid-Pleistocene climatic shift at 0.9 Ma. Prior to 1.05 Ma, a glacial-interglacial rhythm is not obvious in the African vegetation variation. Afterwards, Podocarpus spread in the mountains of central Africa mainly during glacials and Congo River discharge decreased. The sequence of vegetation variation associated with the mid-Pleistocene glacials and interglacials differed from that observed during the late Pleistocene. Between 0.9 and 0.6 Ma, interglacials were characterized by warm dry conditions and glacials were characterized by cool humid conditions, while during the past 0.2 Ma glacials were cold and dry and interglacials warm and humid. Our data indicate that before the Northern Hemisphere ice caps dramatically increased in size (0.9-0.6 Ma), low-latitude climate forcing and response in the tropics played an important role in the initiation of 100 k.y. ice-age cycles. During the mid to late Pleistocene, however, the climate conditions in the tropics were increasingly influenced by the glacial-interglacial variations of continental ice sheets.

Age model for ODP Leg 175 sites

Upwelling along the western coast of Africa south of the equator may be partitioned into three major areas, each having its own dynamics and history: (1) the eastern equatorial region, comprising the Congo Fan and the area of Mid-Angola; (2) the Namibia upwelling system, extending from the Walvis Ridge to Lüderitz; and (3) the Cape Province region, where upwelling is subdued. The highest nutrient contents in thermocline waters are in the northern region, the lowest in the southern one. Wind effects are at a maximum near the southern end of the Namibia upwelling system, and maximum productivity occurs near Walvis Bay, where the product between upwelling rate and nutrient content of upwelled waters is at a maximum. In the Congo/Angola region, opal tends to follow organic carbon quite closely in the Quaternary record. However, organic carbon has a strong precessional component, while opal does not. Despite relatively low opal content, sediments off Angola show the same patterns as those off the Congo; thus, they are part of the same regime. The spectrum shows nonlinear interference patterns between high- and low-latitude forcing, presumably tied to thermocline fertility and wind. On Walvis Ridge, as in the Congo-Angola region, the organic matter record behaves normally; that is, supply is high during glacial periods. In contrast, interglacial periods are favorable for opal deposition. The pattern suggests reduction in silicate content of the thermocline during glacial periods. The reversed phase (opal abundant during interglacials) persists during the entire Pleistocene and can be demonstrated deep into the Pliocene, not just on Walvis Ridge but all the way to the Oranje River and off the Cape Province. From comparison with other regions, it appears that silicate is diminished in the global thermocline, on average, whenever winds become strong enough to substantially shorten the residence time of silicate in upper waters (Walvis Hypothesis, solving the Walvis Paradox of reversed phase in opal deposition). The central discovery during Leg 175 was the documentation of a late Pliocene opal maximum for the entire Namibia upwelling system (early Matuyama Diatom Maximum [MDM]). The maximum is centered on the period between the end of the Gauss Chron and the beginning of the Olduvai Chron. A rather sharp increase in both organic matter deposition and opal deposition occurs near 3 Ma in the middle of the Gauss Chron, in association with a series of major cooling steps. As concerns organic matter, high production persists at least to 1 Ma, when there are large changes in variability, heralding subsequent pulsed production periods. From 3 to 2 Ma, organic matter and opal deposition run more or less parallel, but after 2 Ma opal goes out of phase with organic matter. Apparently, this is the point when silicate becomes limiting to opal production. Thus, the MDM conundrum is solved by linking planetary cooling to increased mixing and upwelling (ramping up to the MDM) and a general removal of silicate from the upper ocean through excess precipitation over global supply (ramping down from the MDM). The hypothesis concerning the origin of the Namibia opal acme or MDM is fundamentally the same as the Walvis Hypothesis, stating that glacial conditions result in removal of silicate from the thermocline (and quite likely from the ocean as a whole, given enough time). The Namibia opal acme, and other opal maxima in the latest Neogene in other regions of the ocean, marks the interval when a cooling ocean selectively removes the abundant silicate inherited from a warm ocean. When the excess silicate is removed, the process ceases. According to the data gathered during Leg 175, major upwelling started in the late part of the late Miocene. Presumably, this process contributed to the drawing down of carbon dioxide from the atmosphere, helping to prepare the way for Northern Hemisphere glaciation.

Age model and stable isotope record of DSDP holes 48-401 and 86-577

Early Paleogene warm climates may have been linked to different modes and sources of deepwater formation. Warm polar temperatures of the Paleocene and Eocene may have resulted from either increased atmospheric trace gases or increased heat transport through deep and intermediate waters. The possibility of increasing ocean heat transport through the production of warm saline deep waters (WSDW) in the Tethyan region has generated considerable interest. In addition, General Circulation Model results indicate that deepwater source regions may be highly sensitive to changing basin configurations. To decipher deepwater changes, we examined detailed benthic foraminiferal faunal and isotopic records of the late Paleocene through the early Eocene (~60 to 50 Ma) from two critical regions: the North Atlantic (Bay of Biscay Site 401) and the Pacific (Shatsky Rise Site 577). These records are compared with published data from the Southern Ocean (Maud Rise Site 690, Islas Orcadas Rise Site 702). During the late Paleocene, similar benthic foraminiferal delta18O values were recorded at all four sites. This indicates uniform deepwater temperatures, consistent with a single source of deep water. The highest delta13C values were recorded in the Southern Ocean and were 0.5 per mil more positive than those of the Pacific. We infer that the Southern Ocean was proximal to a source of nutrient-depleted deep water during the late Paleocene. Upper Paleocene Reflector Ab was cut on the western Bermuda Rise by cyclonically circulating bottom water, also suggesting a vigorous source of bottom water in the Southern Ocean. A dramatic negative excursion in both carbon and oxygen isotopes occurred in the latest Paleocene in the Southern Ocean. This is a short-term (<100 kyr), globally synchronous event which also is apparent in both the Atlantic and Pacific records as a carbon isotopic excursion of approximately 1 per mil. Faunal analyses from the North Atlantic and Pacific sites indicate that the largest benthic foraminiferal faunal turnover of the Cenozoic was synchronous with the isotopic excursion, lending support to the hypothesis that the extinctions were caused by a change in deepwater circulation. We speculate that the Southern Ocean deepwater source was reduced or eliminated at the time of the excursion. During the early Eocene, Southern Ocean delta13C values remained enriched relative to the North Atlantic and Pacific. However, the Southern Ocean was also enriched in delta18O relative to these basins. We interpret that these patterns indicate that although the Southern Ocean was proximal to a source of cool, nutrient-depleted water, the intermediate to upper deep water sites of the North Atlantic and Pacific were ventilated by a different source that probably originated in low latitudes, i.e., WSDW.