Chemistry of peridotites from ODP 109-670A

During Leg 109 of the Ocean Drilling Program, about 100 m of serpentinized peridotites were drilled on the western wall of the M.A.R. axial rift valley, 45 km south of the Kane Fracture Zone. The present study reports petrological and mineralogical data obtained from 29 small pieces of these ultramafic rocks, including about 60% serpentinized harzburgites, 26% serpentinized lherzolites, 14% serpentinized dunites, and one sample of olivine websterite. Modal analyses show that all these rocks are plagioclase-free four-phase peridotites equilibrated in the spinel lherzolite facies. The estimated average modal composition of the sample set is about 80% olivine, 14% opx, 5% cpx, and 1% spinel, that is, a cpx-poor lherzolite. The well developed porphyroclastic structures and mineralogical characteristics of these rocks indicate their affinity with the group of residual mantle tectonites, among the abyssal peridotites. Features typical of magmatic cumulates are lacking. The high contents in Al2O3 of the cpx (average 5.4%) and of the opx (average 4.3%) porphyroclasts, the low Cr# of the spinels (average 22.9%), and the rather high content in modal cpx (about 5%), indicate a moderate percentage of melting, of the order of 10%-15%. Site 670 peridotites plot close to the least depleted mantle rocks collected in the oceans in most diagrams used to define the average trend of the ocean-floor peridotites. Microprobe traverses across the cores of the exsolved opx and cpx porphyroclasts permitted the recalculation of the magmatic compositions of these pyroxenes: the 'primitive' opx were equilibrated at about 1300°C, probably at the end of the main melting episodes, whereas the 'primitive' cpx show lower equilibration temperatures, at about 1200°C, reflecting a more complex thermal history. The subsolidus evolution is well recorded, from 1200°C to about 950CC, by the exsolved pyroxenes and the olivine and spinel phases. Unusually high blocking temperatures, close to 1000°C, indicate that the peridotite body was cooled very rapidly between 1000°C and the beginning of serpentinization. Oxygen fugacities, calculated for 10 kb and at the blocking temperatures indicated by the olivine/spinel geothermometer, are close to the usual fugacities calculated in oceanic peridotites and basalts (of the order of 10**-10 to 10**-11, on the QFM buffer). Site 670 peridotites have compositions close to those of the peridotites collected in the Kane Fracture Zone area, and obviously belong to the moderately depleted mantle peridotites which characterize abyssal peridotites collected away from mantle plumes and oceanic islands. In particular, they differ from the highly residual harzburgites collected along the M.A.R. over the Azores bulge.

Stable isotope ratios on 3 sediment cores from Tagus prodelta off Lisbon

A high-resolution sedimentary sequence recovered from the Tagus prodelta has been studied with the objective to reconstruct multi-decadal to centennial-scale climate variability on the western Iberian Margin and to discuss the observations in a wider oceanographic and climatic context. Between ca. 100 BC and AD 400 the foraminiferal fauna and high abundance of Globorotalia inflata indicate advection of subtropical waters via the Azores Current and the winter-time warm Portugal Coastal Current. Between ca. AD 400 and 1350, encompassing the Medieval Climate Anomaly (MCA), enhanced upwelling is indicated by the planktonic foraminiferal fauna, in particular by the high abundance of upwelling indicator species Globigerina bulloides. Relatively light d18O values and high sea surface temperature (SST) (reconstructed from foraminiferal assemblages) point to upwelling of subtropical Eastern North Atlantic Central Water. Between ca. AD 1350 and 1750, i.e. most of the Little Ice Age, relatively heavy d18O values and low reconstructed SST, as well as high abundances of Neogloboquadrina incompta, indicate the advection of cold subpolar waters to the area and a southward deflection of the subpolar front in the North Atlantic, as well as changes in the mode of the North Atlantic Oscillation. In addition, the assemblage composition together with the other proxy data reveals less upwelling and stronger river input than during the MCA. Stronger Azores Current influence on the Iberian Margin and strong anthropogenic effect on the climate after AD 1750 is indicated by the foraminiferal fauna. The foraminiferal assemblage shows a significant change in surface water conditions at ca. AD 1900, including enhanced river runoff, a rapid increase in temperature and increased influence of the Azores Current. The Tagus record displays a high degree of similarity to other North Atlantic records, indicating that the site is influenced by atmospheric-oceanic processes operating throughout the North Atlantic, as well as by local changes.

(Appendix A) Relative abundance of Emiliania huxleyi in sediment cores of the North Atlantic

The coccolithophore species Emiliania huxleyi is characterized by a wide range of sizes, which can be easily distinguished in the light microscope. In this study we have quantified the abundance of large (coccoliths > 4 µm in maximum length) E. huxleyi specimens during the last 25 kyr in sedimentary records from eleven cores and drill sites in the NE Atlantic and W Mediterranean Sea, to prove its usefulness in the reconstruction of water mass dynamics and biostratigraphic potential. During the Last Glacial Maximum this large form, a cold-water indicator, was common in the NE Atlantic and Mediterranean, and its regional variation in abundance indicates a displacement of the climatic zones southwards in agreement with the development of ice sheets and sea ice in the Northern Hemisphere during this period. On the other hand, the gradient between northern and southern surface water masses in the Subtropical Gyre appears to have been more pronounced than at present, while the Portugal and Canary Currents were more intense. In the western Mediterranean basin temperatures were cooler than in the adjacent Atlantic, provoking a quasi-endemism of these specimens until the end of Heinrich Event 1. This may have been due to a restriction in the communication between the Atlantic and Mediterranean through the Strait of Gibraltar, the arrival of cold surface water and the amplification of cooling after the development of ice sheets in the Northern Hemisphere. During the deglaciation, large E. huxleyi specimens decreased in abundance at medium and low latitudes, but were still numerous close to the Subarctic region during the Holocene. In transitional waters this decrease to present day abundances occurred after Termination Ib. The abrupt change in abundance of this large E. huxleyi form is proposed as a new biostratigraphic event to characterize the Holocene in mid- to low-latitude water masses in the North Atlantic, although this horizon seems to be diachronous by 5 kyr from tropical to subarctic regions, in agreement with the gradual onset of warm conditions.