ALBERT

Description: Nd and Pb isotopes were measured on the fine fraction of one sediment core drilled off southern Greenland. This work aims to reconstruct the evolution of deep circulation patterns in the North Atlantic during the Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan-African and Variscan crusts, and the Mid-Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2-6.5 ka) is composed of material derived from the North American shield and from a "young" crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the Mid-Atlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene.

Description: Data and observation from Drifting Program Leg 121 and plate-tectonic reconstructions indicate that the Ninetyeast Ridge (Indian Ocean) was derived from the interaction of a deep-seated Dupal hotspot and a nearby spreading-ridge axis. The 5000-km-long ridge, from lat 34°S to lat 10°N, was drilled at three sites during Leg 121. About 178 m of basalt, 〉38 to 〉80 Ma, were recovered from a total penetration of ~310 m. Shipboard petrographic and geochemical studies showed that each site has distinctive characteristics. Most of the cored lavas have a tholeiitic basalt composition. Incompatible-element abundanes and ratios show systematic trends, consistent with an origin for the Ninetyeast Ridge lavas by mixing between a depleted component-Indian Ocean mid-ocean ridge basalt-and an enriched component-oceanic-island basalt similar to that observed in the youngest alkalic basalts from the Kerguelen archipelago. Preliminary shore-based trace element abundance and isotopic data are compatible with this hypothesis, although Pb isotopes indicate the involvement of another component. The long-lasting and more or less continuous activity of the Kerguelen-Heard plume (ca. 115 Ma), now located under Heard Island, south of the Southeast Indian Ridge, provides evidence that the source of the Dupal anomaly is deep seated.

Description: The Alpha–Mendeleev ridge complex is a prominent physiographic and geological feature of the Arctic Amerasia Basin. The Alpha and Mendeleev ridges are, respectively, the eastern and western components of a continuous seafloor high that is approximately 2000 km long and 200–400 km wide. A surge of interest in the tectonic evolution of Arctic submarine features has led to a wealth of new geophysical data collected from the Alpha Ridge. Current interpretations of its origin vary but there is compelling evidence that the Alpha Ridge may have formed as an oceanic plateau during the Late Cretaceous. Geological samples are rare but most samples recovered indicate a genetic link with the High Arctic Large Igneous Province (HALIP). In August 2016, Canada’s Extended Continental Margin-United Nations Convention on the Law of the Sea Program dredged approximately 100 kg of volcanic rocks from the Alpha Ridge. The large size and pristine state of the samples enabled the first comprehensive study of a single eruptive event in the volcanic record of the Alpha Ridge. The dredge sample is a lapilli tuff containing vitric and basaltic clasts. Textural evidence and the coexistence of juvenile and cognate clasts suggest a phreatomagmatic eruption. The vitric fragments consist of sideromelane glass with abundant plagioclase microlites. Texturally, these basaltic glass lapilli display a fresh glassy core surrounded by Fe- and Ti-rich zones and a palagonite rim. Major and trace element analyses of glassy cores indicate remarkably uniform, mildly alkaline basaltic compositions. The plagioclase-bearing glass yielded a 40Ar/39Ar plateau age of 90.40±0.26 Ma (2σ error) which included 89% of 39 Ar released. We interpret this result to represent the eruption age of the plagioclase microlites and consequently, of the host basaltic glass lapilli in the tuff. Volatile species analyses by infrared spectroscopy on the fresh basaltic glass suggests that the melt was effectively degassed to shallow level. Assuming equilibrium degassing, the homogeneous resulting values of H2O total in the range 0.1 to 0.19 wt.% (1σ error) indicate subaerial or shallow eruption (surface to 80 m). The new 40Ar/39Ar age for the sample is consistent with a 40 Ar/39Ar age of 89±1 Ma obtained for a sample of tholeiitic basalt dredged from the central part of the Alpha Ridge, and with the range of ages reported for HALIP igneous rocks exposed onshore in the Canadian Arctic Archipelago (130-80 Ma). Our new data provide evidence for local emergence of the Alpha Ridge in the Late Cretaceous. A comparison the Alpha Ridge and Kerguelen Plateau–Broken Ridge Large Igneous Province (LIP) provides new insights on the episodic nature of LIP magmatism and variations in eruptive style through time.