Chemical composition of basalts from the Atlantic Ocean

Basalt recovered beneath Jurassic sediments in the western Atlantic at Deep Sea Drilling Project sites 100 and 105 of leg 11 has petrographic features characteristic of water-quenched basalt extruded along modern ocean ridges. Site 100 basalt appears to represent two or three massive cooling units, and an extrusive emplacement is probable. Site 105 basalt is less altered and appears to be a compositionally homogeneous pillow lava sequence related to a single eruptive episode. Although the leg 11 basalts are much more closely related in time to the Triassic lavas and intrusives of eastern continental North America, their geochemical features are closely comparable to those of modern Mid-Atlantic Ridge basalts unrelated to postulated "mantle plume" activity. Projection of leg 11 sites back along accepted spreading "flow lines" to their presumed points of origin shows that these origins are also outside the influence of modern" plume" activity. Thus, these oldest Atlantic seafloor basalts provide no information on the time of initiation of these "plumes". The Triassic continental diabases show north to south compositional variations in Rb, Ba, La, and Sr which lie within the range of " plume "-related basalt on the Mid-Atlantic Ridge (20° - 40° N) This suggests that these diabases had mantle sources similar in composition to those beneath the present Mid-Atlantic Ridge. "Plumes" related to deep mantle sources may have contributed to the LIL-element enrichment in the Triassic diabase and may also have been instrumental in initiating the rifting of the North Atlantic. Systematically high values for K and Sr87/Sr86 in the Triassic diabases may reflect superimposed effects of crustal contamination in the Triassic magmas.

(Table 2) Uranium concentrations and isotopic ratios in surface waters of the Atlantic Ocean in April 1977

Uranium concentrations and isotope ratios were measured in thirteen surface-water samples collected across the entire Atlantic Ocean. The mean isotope ratio was 1.15+/-0.01, and the mean concentration 3.1+/-0.2 µg/l.

Feldspar Pb-isotope ratios for earliest Pleistocene sediments of the sub-polar North Atlantic Ocean

Relatively little is known in detail about the locations of the early Pleistocene ice-sheets responsible for ice-rafted debris (IRD) inputs to the sub-polar North Atlantic Ocean during intensification of northern hemisphere glaciation (iNHG). To shed new light on this problem, we present the first combined in-depth analysis of IRD flux and geochemical provenance of individual sand-sized IRD deposited in the sub-polar North Atlantic Ocean during the earliest large amplitude Pleistocene glacial, marine isotope stage (MIS) 100 (~2.52 Ma), arguably the key glacial during iNHG. IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single feldspar grains. We find that the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) of individual ice-rafted (>150 µm) feldspars deposited at DSDP Site 611A, ODP Site 981 and IODP Site U1308 during MIS 100 records a shift from predominantly Archaean-aged circum-North Atlantic Ocean continental sources during early glacial ice-rafting events to dominantly Palaeozoic and Proterozoic-aged sources during full glacial conditions. The distribution of feldspars in Pb-Pb space for full glacial MIS 100 more closely resembles that documented for feldspars deposited at the centre of the last glacial IRD belt (at IODP/DSDP Site U1308/609) during ambient (non-Heinrich-event) ice-rafting episodes of MIS 2 (~23.8 ka) than that documented for MIS 5d (~106 ka). Comparison of our early Pleistocene and last glacial cycle datasets suggests that MIS 100 was characterised by abundant iceberg calving from large ice-sheets on multiple continents in the high northern latitudes (not just on Greenland).

Cadmium/Calcium and stable isotope ratios of benthic foraminifera from the northeast Atlantic Ocean

The delta13C and Cd measurements from benthic foraminifera from Biogeochemical Ocean Flux Study (BOFS) northeast Atlantic Ocean sediment cores are presented. The delta13C values in glacial foraminifera are consistent with those from elsewhere in the North Atlantic Ocean. For intermediate water (1000 - 2000 m water depth), delta13C values were higher at the last glacial maximum than in present North Atlantic Deep Water (NADW), whereas for deep water (>2000 m) they were lower during the glacial maximum. The Cd concentrations of glacial northeast Atlantic intermediate water were lower than those of present NADW. However, deepwater Cd concentrations increased to values between NADW and present Pacific Deep Water (PDW). The delta13C and Cd data are consistent and show that the northeast Atlantic Ocean was strongly stratified with 13C enriched, low Cd intermediate water overlying 13C depleted, high Cd deep water. The glacial water column comprised two different water masses: deep water, similar in character to present Antarctic Bottom Water (AABW), and intermediate water, different in character from both AABW and NADW, and any present intermediate-depth North Atlantic water. The characteristics of glacial intermediate water were, however, similar to present near-surface waters in the North Atlantic, which suggests rapid ventilation of the glacial ocean to depths of up to 2000 m by cold, nutrient-depleted young surface waters.

Stable oxygen and carbon isotope ratios of Pliocene benthic foraminifera in the Atlantic Ocean

Large changes in benthic foraminiferal delta180 and delta13C occurred during the Pliocene (between 3.0 and 2.0 Ma) at Hole 665A. Oxygen isotopic compositions increased to maximum values at 2.4 Ma, correlating with an 18O enrichment observed at Hole 552A and other locations (Shackleton et al., 1984). As at Hole 606 (Keigwin, 1986), however, maximum delta180 values at 2.4 Ma were not as great as at Hole 552A, and enrichments in delta180 also occurred before 2.4 Ma. We believe that the section representing sediments from 2.5 to 2.7 or 2.8 Ma is missing at Hole 552A because of incomplete core recovery. Consequently, the older delta180 increases are not found at Hole 552A. Benthic foraminiferal delta13C values are much lower at Hole 665A than at Hole 552A, approaching the low values observed in the Pliocene Pacific Ocean. This geographic distribution of delta13C suggests that, like late Quaternary glaciations, the equatorial Atlantic Ocean was dominated during the Pliocene by deep water that originated in the Southern Ocean and had chemical characteristics very similar to the Pacific Ocean. Reduced O2 values were probably associated with low delta13C values and contributed to increased preservation of organic carbon during enriched 180 intervals of the Pliocene equatorial Atlantic.