Neogene ocean history is dominated by the theme of stepwise global cooling (with occasional reversals); the main trends of carbonate sedimentation on the Ontong Java Plateau show the regional response of productivity, dissolution, winnowing, and redeposition to this overall climatic change. The relative importance of these processes in controlling accumulation rates and carbonate content is difficult to assess for any given place and time. Thus, the outstanding feature of the carbonate record, the Tortonian-Messinian accumulation rate peak centered in the latest Miocene (maximum sedimentation rate >60 m/m.y.), is the product of a complex interplay of a general late Miocene to early Pliocene productivity maximum combined with increased mechanical and chemical erosion before and after the peak. The mix of erosional factors depends on the depth level considered and changes with time. Increased productivity apparently derives from high nutrient content in Pacific deep waters, caused by increased production of North Atlantic Deep Water (NADW) in the latest Miocene and basin-basin fractionation. Enhancement of the thermocline strength is indicated at that time from an increase in planktonic foraminifers living at intermediate depths. A fundamental change in the mode of productivity (toward pulsed productivity?) is indicated by changes in the coccolith flora.The main focus of this study is the definition of major patterns of sedimentation and associated open questions, as follows:1. Carbonate records are parallel over a wide depth range. Does this mean that dissolution is also important on the upper plateau? Or is there a "conspiracy" of separate factors acting in concert?2. Dissolution of carbonate cannot explain both carbonate and sedimentation rate patterns. The "loss paradox*' arises from the fact that carbonate percentages at different depths are so similar that the differences in carbonate are insufficient to account for differences in sedimentation rate, assuming that dissolution produces these differences.3. Equatorial crossings have little or no effect on carbonate content or sedimentation rate. "Equatorial insensitivity" indicates that equatorial upwelling is of subordinate importance in biogenic sedimentation on the plateau in the late Neogene (as is the case today).4. There is evidence for a general insensitivity of both carbonate and sedimentation rate records with regard to global changes in conditions, as seen in commonly used proxies. Changes in δ 18 θ of benthic foraminifers, for example, and sea-level changes (as mapped by sequence stratigraphy) are not clearly correlated with the main parameters of Neogene carbonate sedimentation on the plateau. Correspondence to the δ 3 C record is somewhat better, however. Proxies may be ill defined, or the regional overprint may obscure global relationships.The issues listed above are of a very general nature. Without a successful attack on these questions, the major patterns of carbonate sedimentation on the plateau will remain enigmatic, as will many phe...

Ocean Drilling Program Site 882 (50°22'N, 167°36'E) in the northwest Pacific is an excellent site to monitor changes in the surface-and deep-water circulation. Site 882 provides the first high-resolution GRAPE density, magnetic susceptibility, carbonate, opal, total organic carbon, and foraminifers (planktonic and benthic) stable isotope records between 3.2 and 2.4 Ma in the North Pacific Ocean. We observe a dramatic increase in ice-rafting debris at Site 882 at 2.75 Ma, which is coeval with that found in the Norwegian Sea, suggesting that the Arctic and northeast Asia were significantly glaciated from 2.75 Ma onward. At Site 882 this major change was accompanied by a dramatic drop in sea-surface temperatures (>7.5°C) and opal mass accumulation rates (fivefold decrease), and is overlain by a more gradual long-term decrease in both total organic carbon and CaCO 3 MARs. We suggest that the onset of Eurasian Arctic and northeast Asia glaciation is about 100 k.y. before the glaciation of Alaska and 200 k.y. before the glaciation of Greenland and the northeast American continent. We suggest that the initiation and intensification of Northern Hemisphere glaciation was forced by changes in the orbital parameters: in particular, the gradual increase in the amplitude of obliquity from 3.5 to 2.5 Ma and a sharp rise in the amplitude of precession and thus insolation between 2.8 and 2.55 Ma.

A high-resolution carbonate record from the northwest Pacific covering roughly the last 6 m.y. generally shows "Atlantic-type" interglacial spikes of carbonate preservation. These spikes parallel maxima in the biogenic opal and total organic carbon records, indicating increased productivity during interglacials. The variations can be explained best by changes in the advection of nutrientrich deep water originating in the North Atlantic and ultimately upwelling in the northwestern Pacific, the terminus of the global salinity conveyor belt. During interglacial times, the conveyor was enhanced, which led to increased upwelling of nutrient-rich deep water and, therefore, to a regionally enhanced surface productivity. As a result, carbonate production and accumulation at Site 882 exceeded the local carbonate dissolution during early/peak interglacial times, in contrast to the central Pacific where CaCO 3 corrosion is characteristic of warm stages as a response to an increase in the circulation of the salinity conveyor belt, leading to the "Pacific type" of interglacial minima of carbonate accumulation.

Shipboard paleomagnetic studies on Ocean Drilling Program Leg 130 were plagued by a severe reduction in the remanence intensity, which affected sediments below a sharply defined onset at sub-bottom depths of 50 mbsf or less. This loss of intensity was accompanied by a reduction in magnetic stability and consequent loss of polarity and paleolatitude information. Viscous remanent magnetization and drilling-induced remanences contributed high-coercivity overprints, further obscuring the greatly diminished primary magnetization. The depth at which the reduction in intensity occurred correlates with the organic carbon content of the sediments, and a further relationship is present between intensity downhole and the level of sulfate reduction. This implies that loss of intensity is related to the microbial reduction of magnetite. Reduction of sulfate results in the generation of a magnetic iron sulfide, which appears to be the carrier of the high-coercivity overprints. Oxidation and drying of the sediment after sampling leads to partial destruction of this sulfide, and complete decomposition occurs at temperatures below 300°C. These features identify the magnetic sulfide as the metastable phase, greigite. Varying inputs of organic material in past times, presumably reflecting both latitude and climatic changes, resulted in varying degrees of reduction, which are now reflected in differences in magnetic intensity, degree of overprinting, and survival of primary magnetization. The highest reduction levels result in the almost total dissolution of magnetite, and the weak surviving remanence is dominated by greigite. At lower reduction states, multidomain magnetite persists together with greigite. Still-less-reducing conditions in turn cause the survival of fine-grained, single-domain magnetite, and then of hematite.An earlier paleomagnetic study of Ontong Java Plateau sediments from Deep Sea Drilling Project Leg 30 produced a paleolatitude curve that differed significantly from other records of Pacific paleolatitude. It is likely that the samples in this earlier study also carried high-coercivity overprints, which biased the results. Those samples from Leg 130 that appear to preserve a primary magnetization yielded paleolatitudes consistent with expected Pacific Plate motion.

The quantity and quality of organic carbon of Eocene to Holocene sediments from ODP Sites 645, 646, and 647 were investigated to reconstruct depositional environments. Results were based on organic-carbon and nitrogen deter minations, Rock-Eval pyrolysis, and kerogen microscopy.The sediments at Site 645 in Baffin Bay are characterized by relatively high organic-carbon values, most of which range from 0.5% to almost 3%, with maximum values in the middle Miocene. Distinct maxima of organic-carbon ac cumulation rates occur between 18 and 12.5 Ma and between 3.4 and 0 Ma. At Sites 646 and 647 in the Labrador Sea, organic-carbon contents vary between 0.1% and 0.75%. Cyclic "Milankovitch-type" changes in organic-carbon depo sition imply climate-controlled mechanisms that cause these fluctuations.The composition of organic matter at Site 645 is dominated by terrigenous components throughout the entire sedi ment sequence. An increased content of marine organic carbon was recorded only in the late-middle Miocene. At Sites 646 and 647, the origin of the organic matter most probably is marine.Oceanic paleoproductivity values were estimated, based on the amount of marine organic carbon. During most of the Neogene time interval at Site 645, productivity was low, i.e., similar or less than that measured in Baffin Bay today. Higher values of up to 150 (200) gC • m -2 • y -1 may have occurred only in the Miocene. At Sites 646 and 647, mean paleoproductivity values vary between 90 and 170 gC ■ m -2 • y -1 ; i.e., these are also similar to those measured in the Labrador Sea today. Lower values of 40 to 70 gC • m -2 • y -1 were estimated for the early Eocene and (middle) Mio cene.