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[1] Middle/late Miocene to early Pliocene sedimentary sequences along the continental margin of southwest Africa have changes that correspond to the carbonate crash (12-9 Ma) and biogenic bloom events ($7-4 Ma) described in the equatorial Pacific by Farrell et al. [1995]. To explore the origins of these changes, we analyzed the carbon and coarse fraction contents of sediments from ODP Sites 1085, 1086, and 1087 at a time resolution of 5 to 30 kyr. Several major drops in CaCO 3 concentration between 12 and 9 Ma are caused by dilution from major increases in clastic input from the Oranje River during global sea level regressions. Abundant pyrite crystals and good preservation of fish debris reflect low oxygenation of bottom/pore waters. Regional productivity was enhanced during the time equivalent to the carbonate crash period. Higher benthic/planktic foraminiferal ratios indicate that CaCO 3 dissolution at Site 1085 peaked between 9 to 7 Ma, which was after the global carbonate crash. This period of enhanced dissolution suggests that Site 1085 was located within a lowoxygen water mass that dissolved CaCO 3 more easily than North Atlantic Deep Water, which began to bathe this site at 7 Ma. At 7 to 6 Ma, the onset of the biogenic bloom, increases and variations in total organic carbon and benthic foraminiferal accumulation rates show that paleoproductivity increased significantly above values observed during the carbonate crash period and fluctuated widely. We attribute the late Miocene paleoproductivity increase off southwest Africa to ocean-wide increases in nutrient supply and delivery 1 .

The ~100 k.y. cyclicity of the late Pleistocene ice ages started during the mid-Pleistocene transition (MPT), as ice sheets became larger and persisted for longer. The climate system feedbacks responsible for introducing this nonlinear ice sheet response to orbital variations in insolation remain uncertain. Here we present benthic foraminiferal stable isotope (d 18 O, d 13 C) and trace metal records (Cd/Ca, B/Ca, U/Ca) from Deep Sea Drilling Project Site 607 in the North Atlantic. During the onset of the MPT, glacial-interglacial changes in d 13 C values are associated with changes in nutrient content and carbonate saturation state, consistent with a change in water mass at our site from a nutrient-poor northern source during interglacial intervals to a nutrient-rich, corrosive southern source during glacial intervals. The respired carbon content of glacial Atlantic deep water increased across the MPT. Increased dominance of corrosive bottom waters during glacial intervals would have raised mean ocean alkalinity and lowered atmospheric pCO 2 . The amplitude of glacial-interglacial changes in d 13 C increased across the MPT, but this was not mirrored by changes in nutrient content.We interpret this in terms of air-sea CO 2 exchange effects, which changed the d 13 C signature of dissolved inorganic carbon in the deep water mass source regions. Increased sea ice cover or ocean stratification during glacial times may have reduced CO 2 outgassing in the Southern Ocean, providing an additional mechanism for reducing glacial atmospheric pCO 2 . Conversely, following the establishment of the ~100 k.y. glacial cycles, d 13 C of interglacial northern-sourced waters increased, perhaps reflecting reduced invasion of CO 2 into the North Atlantic following the MPT.

[1] We reconstruct paleoproductivity at three sites in the Atlantic Ocean (Ocean Drilling Program Sites 982, 925, and 1088) to investigate the presence and extent of the late Miocene to early Pliocene ''biogenic bloom'' from 9 to 3 Ma. Our approach involves construction of multiple records including benthic foraminiferal and CaCO 3 accumulation rates, Uvigerina counts, dissolution proxies, and geochemical tracers for biogenic and detrital fluxes. This time interval also contains the so-called late Miocene carbon isotope shift, a well-known decrease in benthic foraminiferal d13 C values. We find that the timing of paleoproductivity maxima differs among the three sites. At Site 982 (North Atlantic), benthic foraminifera and CaCO 3 accumulation were both at a maximum at $5 Ma, with smaller peaks at $6 Ma. The paleoproductivity maximum was centered earlier ($6.6-6.0 Ma) in the tropical Atlantic (Site 925). In the South Atlantic (Site 1088), paleoproductivity increased even earlier, between 8.2 Ma and 6.2 Ma, and remained relatively high until $5.4 Ma. We note that there is some overlap between the interval of maximum productivity between Sites 925 and 1088, as well as the minor productivity increase at Site 982. We conclude that the paleoproductivity results support hypotheses aiming to place the biogenic bloom into a global context of enhanced productivity. In addition, we find that at all three sites the d 13 C shift is accompanied by carbonate dissolution. This observation is consistent with published studies that have sought a relationship between the late Miocene carbon isotope shift and carbonate preservation.Citation: Diester-Haass, L., K. Billups, and K. C. Emeis (2005), In search of the late Miocene -early Pliocene ''biogenic