(Table 1) U and Th isotope analyses of pyrite and anhydrite of ODP Site 158-957

Hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to ~400 °C before venting at spectacular 'black smokers'. These hydrothermal systems exert a strong influence on ocean chemistry (Edmond et al., 1979, doi:10.1016/0012-821X(79)90061-X), yet their structure, longevity and magnitude remain largely unresolved (Elderfield and Schultz., 1996, doi:10.1146/annurev.earth.24.1.191). The active Transatlantic Geotraverse (TAG) deposit, at 26° N on the Mid-Atlantic Ridge, is one of the largest, oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black-smoker fields. Here we report ages of sulphides and anhydrites from the recently drilled (Humphris et al., 1995, doi:10.1038/377713a0) TAG substrate structures -determined from 234U-230Th systematics analysed by thermal ionization mass spectrometry. The new precise ages combined with existing data (Lalou et al., 1993, doi:10.1029/92JB01898; 1998, doi:10.2973/odp.proc.sr.158.214.1998) show that the oldest material (11,000 to 37,000 years old) forms a layer across the centre of the deposit with younger material (2,300-7,800 years old) both above and below. This stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid (James and Elderfield, 1996, doi:10.1130/0091-7613(1996)024<1147:COOFFA>2.3.CO;2). The age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2,000 years.

Geochemistry of the oldest Atlantic oceanic crust

Controversy has surrounded the issue of whether mantle plume activity was responsible for Pangaean continental rifting and massive flood volcanism (resulting in the Central Atlantic Magmatic Province or CAMP, emplaced around 200 Ma) preceding the opening of the central Atlantic Ocean in the Early Mesozoic. Our new Sr-Nd-Pb isotopic and trace element data for the oldest basalts sampled from central Atlantic oceanic crust by deep-sea drilling show that oceanic crust generated from about 160 to 120 Ma displays clear isotopic and chemical signals of plume contamination (e.g., 87Sr/86Sr(i) = 0.7032-0.7036, epsilonNd(t) =+6.2 to +8.2, incompatible element patterns with positive Nb anomalies), but these signals are muted or absent in crust generated between 120 and 80 Ma, which resembles young Atlantic normal mid-ocean ridge basalt. The plume-affected pre-120 Ma Atlantic crustal basalts are isotopically similar to lavas from the Ontong Java Plateau, and may represent one isotopic end-member for CAMP basalts. The strongest plume signature is displayed near the center of CAMP magmatism but the hotspots presently located nearest this location in the mantle reference frame do not appear to be older than latest Cretaceous and are isotopically distinct from the oldest Atlantic crust. The evidence for widespread plume contamination of the nascent Atlantic upper mantle, combined with a lack of evidence for a long-lived volcanic chain associated with this plume, leads us to propose that the enriched signature of early Atlantic crust and possibly the eruption of the CAMP were caused by a relatively short-lived, but large volume plume feature that was not rooted at a mantle boundary layer. Such a phenomenon has been predicted by recent numerical models of mantle circulation.

(Supplement Table S2) Results of 230Th/U-dating

We compared the suitability of two skeletal materials of the Atlantic brain coral Diploria strigosa for 230Th/U-dating: the commonly used bulk material comprising all skeletal elements and the denser theca wall material. Eight fossil corals of presumably Last Interglacial age from Bonaire, southern Caribbean Sea, were investigated, and several sub-samples were dated from each coral. For four corals, both the ages and the activity ratios of the bulk material and theca wall agree within uncertainty. Three corals show significantly older ages for their bulk material than for their theca wall material as well as substantially elevated 232Th content and (230Th/238U) ratios. The bulk material samples of another coral show younger ages and lower (230Th/238U) ratios than the corresponding theca wall samples. This coral also contains a considerable amount of 232Th. The application of the available open-system models developed to account for post-depositional diagenetic effects in corals shows that none of the models can successfully be applied to the Bonaire corals. The most likely explanation for this observation is that the assumptions of the models are not fulfilled by our data set. Comparison of the theca wall and bulk material data enables us to obtain information about the open-system processes that affected the corals. The corals showing apparently older ages for their bulk material were probably affected by contamination with a secondary (detrital) phase. The most likely source of the detrital material is carbonate sand. The higher (230Th/232Th) ratio of this material implies that detrital contamination would have a much stronger impact on the ages than a contaminant with a bulk Earth (230Th/232Th) ratio and that the threshold for the commonly applied 232Th reliability criterion would be much lower than the generally used value of 1 ng g^-1. The coral showing apparently younger ages for its bulk material was probably influenced by more than one diagenetic process. A potential scenario is a combination of detrital contamination and U addition by secondary pore infillings. Our results show that the dense theca wall material of D. strigosa is generally less affected by post-depositional open-system behaviour and better suited for 230Th/U-dating than the bulk material. This is also obvious from the fact that all ages of theca wall material reflect a Last Interglacial origin (~125 ka), whereas the bulk material samples are either substantially older or younger. However, for some corals, the 230Th/U-ages and activity ratios of the bulk material and the theca wall samples are similar. This shows that strictly reliable 230Th/U-ages can also be obtained from bulk material samples of exceptionally well-preserved corals. However, the bulk material samples more frequently show elevated activity ratios and ages than the corresponding theca wall samples. Our findings should be generally applicable to brain corals (Mussidae) that are found in tropical oceans worldwide and may enable reliable 230Th/U-dating of fossil corals with similar skeletal architecture, even if their bulk skeleton is altered by diagenesis. The 230Th/U-ages we consider reliable (120-130 ka), along with a recently published age of 118 ka, provide the first comprehensive dating of the elevated lower reef terrace at Bonaire (118-130 ka), which is in agreement in timing and duration with other Last Interglacial records.

Fallout and concentration of aerosol-borne radioactivity over the Atlantic (Appendix)

Measurements of atmospheric radioactivity attached to aerosols are described. Fallout was collected in a vessel of large area. Emphasis was on separation of "wet" and "dry" samples. For strontium 90 a ratio of "wet" to "dry" fallout of 5:1 has been found independent of latitude. The total fallout was smaller than comparable values from continents because of very small amounts of rainfall in the equatorial zone. In order to achieve consistency in the global balance a better knowledge not only of radioactivity but also of precipitation over the ocean is required. Fallout of Ra-D clearly shows the ITC as a barrier for the latitudinal movement of near sea-surface air masses. The concentration of short-lived emanation daughters shows large variations according to varying geographic conditions. A variation with time could not be explained. The specific activity of long-lived radioactive substances shows the expected effect of the ITC as well as a seasonal diminuation of average concentration, similar to that measured at Heidelberg.

Stable isotope record and lithic fragments distribution in sediments of MIS9-14 of the North Atlantic

Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9-14 (300-540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north - south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.

Age and chemical composition of apatites from five DSDP and ODP Sites in the Pacific, Atlantic and Indian Oceans

Since studies on deep-sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing. Due to sluggish annealing kinetics, this effect cannot be quantified by laboratory annealing experiments. The unknown amount of low-temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data, particularly for samples which experienced slow cooling in shallow crustal levels. To further elucidate these uncertainties, we studied volcanogenic sediments from five deep-sea drill cores, that were exposed to maximum temperatures between ~10° and 70°C over geological time scales of ~15-120 Ma. Mean track lengths (MTL) and etch pit diameters (Dpar) of all samples were measured, and the chemical composition of each grain analyzed for age and track length measurements was determined by electron microprobe analysis. Thermal histories of the sampled sites were independently reconstructed, based on vitrinite reflectance measurements and/or 1D numerical modelling. These reconstructions were used to test the most widely used annealing models for their ability to predict low-temperature annealing. Our results show that long-term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11%. Both chlorine content and Dpar values explain the downhole annealing patterns equally well. Low chlorine apatite from one drill core revealed a systematic relation between Si-content and Dpar value. The question whether Si-substitution in apatite has direct and systematic effects on annealing properties however, cannot be addressed by our data. For samples, which remained at temperatures <30°C, and which are low in chlorine, the Laslett et al. [Laslett G., Green P., Duddy I. and Gleadow A. (1987) Thermal annealing of fission tracks in apatite. Chem. Geol. 65, 1-13] annealing model predicts MTL up to 0.6 µm longer than those actually measured, whereas for apatites with intermediate to high chlorine content, which experienced temperatures >30°C, the predictions of the Laslett et al. (1987) model agree with the measured MTL data within error levels. With few exceptions, predictions by the Ketcham et al. [Ketcham R., Donelick R. and Carlson W. (1999) Variability of apatite fission-track annealing kinetics. III: Extrapolation to geological time scales. Am. Mineral. 84/9, 1235-1255] annealing model are consistent with the measured data for samples which remained at temperatures below ~30°C. For samples which experienced maximum temperatures between ~30 and 70°C, and which are rich in chlorine, the Ketcham et al. (1999) model overestimates track annealing.

Stable oxygen isotope ratios and grain size analyses of sediment cores from the Propeller Mound, Northeast Atlantic

The first detailed stratigraphic record from a deep-water carbonate mound in the Northeast Atlantic based on absolute datings (U/Th and AMS 14C) and stable oxygen isotope records reveals that its top sediment sequences are condensed by numerous hiatuses. According to stable isotope data, mainly sediments with an intermediate signal are preserved on the mound, while almost all fully glacial and interglacial sediments have either not been deposited or have been eroded later. The resulting hiatuses reduce the Late Pleistocene sediment accumulation at Propeller Mound to amounts smaller than the background sedimentation. The hiatuses most likely result due to the sweeping of the mound in turn with the re-establishment of vigour interglacial circulation patterns after sluggish current regimes during glacials. Thus, within the discussion if internal, fluid-driven or external environmentally driven processes control the evolution of such carbonate mounds, our findings for Propeller Mound clearly point to environmental forcing as the dominant mechanism shaping deep-water carbonate mounds in the NE Atlantic during the Late Pleistocene and Holocene.

Cenozoic radiolaria in the western North Atlantic

Eight Cenozoic radiolarian zones were recognized in samples from two holes at Site 603, drilled on the lower continental rise off North America during Leg 93 of the Deep Sea Drilling Project. Paleocene to early Eocene radiolarian zones (Bekoma bidartensis, Buryella clinata, and Phormocyrtis striata striata zones) and early to late Miocene radiolarian zones (Calocycletta costata, Dorcadospyris alata, Diartus petterssoni, and Didymocyrtis antepenultima zones) were recognized in sediments from Holes 603 and 603B. In addition, a new Paleocene Bekoma campechensis radiolarian Zone is defined by the interval between the first morphotypic appearance of B. campechensis and the B. campechensis-B. bidartensis evolutionary transition. This zone is immediately below the B. bidartensis Zone of Foreman (1973), and has previously been discussed as a Paleocene "unnamed zone" by other investigators. A hiatus between Neogene and Paleogene sequences was also recognized in the radiolarian faunas.

Deep thermohaline Circulation in the low-latitude Atlantic during the Last Glacial

Present-day low-latitude eastern and western Atlantic basins are geochemically distinct below the sill depth of the Mid-Atlantic Ridge. While Antarctic Bottom Water (AABW) circulates freely in the western Atlantic, flow into the eastern Atlantic is restricted below 4 km which results in filling the abyssal depths of this basin with water of geochemical similarity to nutrient depleted North Atlantic Deep Water. Using carbon isotopes and Cd/Ca ratios in benthic foraminifera we reconstruct the geochemistry of these basins during the last glacial maximum. Results indicate that deep eastern and western Atlantic basins became geochemically identical during the last glacial. This was achieved by shoaling of the upper surface of AABW above the sill depth of the Mid-Atlantic Ridge, which allowed bottom waters in both basins to be filled with the same water mass. Although AABW became the dominant water mass in the deep eastern Atlantic basin during the glacial, Holocene-glacial delta13C-PO4 shifts in this basin are in Redfield proportions, unlike the disproportionate Holocene-glacial delta13C-PO4 shifts observed in the Southern Ocean. By examining the composition of deep and intermediate waters throughout the Atlantic, we show that this effect was induced by a change in gradient of the delta13C-PO4 deepwater mixing line during glacial times. Evidence from high-latitude planktonic data suggests that the change in gradient of the deepwater mixing line was brought about through a significant reduction in the thermodynamic effect on Southern Ocean surface waters. By using coupled delta13C-PO4 data to constrain the composition of end member water masses in the glacial Atlantic, we conclude that deep waters in the low-latitude glacial Atlantic were composed of a mixture of northern and southern source waters in a ratio of 1:3.

Thorium and radiocarbon measurements on sediment cores from the Northseast Atlantic

Radiocarbon and 230Thexcess data from six NE Atlantic box cores are considered. The cores form a transect from the Porcupine Abyssal Plain over the East Thulean Rise to the southern end of Feni Drift. The chronology for the cores is established from bulk sediment carbonate radiocarbon data and reveals that sections exhibiting constant accumulation rates can be identified in all the cores, with rates of 3.0-3.5 cm/kyr on the plain through the Holocene and late Holocene rates of 4.3-6.6 cm/kyr elsewhere. Five out of the six cores show accumulations of more 230Thexcess than is produced in the overlying water column, with the greatest inventories (up to 225% of production) in the cores from the rise and drift. A size fraction comparison between two cores from the plain and rise reveals that the higher overall accumulation rates and 230Thexcess inventories in the off-plain cores are due to an increased fine (<5 µm) component fraction, whereas the flux of coarser material is similar to that received on the plain. This suggests that the higher fluxes of materials observed are physically (rather than biogeochemically) driven and also that drift formation has been continuously active in the late Holocene. Sections of all the cores where regular accumulation is defined by the radiocarbon data are modeled first by a linear radiocarbon age/depth model and second by a constant rain (230Thexcess)0 model prorated for the observed core inventories. These modeling approaches yield historical mass accumulation rate estimates which are generally in reasonable agreement (±30%), but the differences observed appear to be well organized in time rather than random.