Exp. 395 Reykjanes Mantle Convection and Climate

Expedition 395 は IODP プロポーザル 892-Full2 (Mantle Dynamics, Paleoceanography and Climate Evolution in North Atlantic Ocean) に基づいており、北大西洋のアイスランド南部のレイキャネス海嶺の掘削航海です。約30Maまでの海洋底拡大に伴うアイスランドプルーム活動、V字型地形の形成、マントル溶融の進化過程、堆積物に記録された千年スケールの古気候、海洋循環の時間変化などの解明を目指す分野横断型の掘削航海です。すでにExp. 384とExp. 395Cにて5つのサイトを掘削しています。

The intersection between the Mid-Atlantic Ridge and Iceland hotspot provides us with a natural laboratory where the composition and dynamics of Earth’s upper mantle can be observed. Plume-ridge interaction drives variations in the melting regime, resulting in a range of crustal types including a series of V-shaped ridges and troughs south of Iceland. Time-dependent mantle upwelling beneath Iceland dynamically supports regional bathymetry, leading to changes in the height of oceanic gateways which control the strength of deep-water flow over geologic timescales. We propose a drilling program that contains three objectives: (1) to test contrasting hypotheses for the formation of V-shaped ridges; (2) to understand temporal changes in ocean circulation, and explore connections with plume activity; (3) to reconstruct the evolving chemistry of hydrothermal fluids with increasing crustal age, varying sediment thickness and crustal architecture. This drilling program will recover basaltic samples from crust that is blanketed by thick sediments and is thus inaccessible with dredging. Major, trace and isotope geochemistry of basalts will allow us to observe spatial and temporal variations in mantle melting processes. We will test the hypothesis that the Iceland plume thermally pulses on two timescales (5-10 Ma, and ~30 Ma), leading to fundamental changes in crustal architecture. This idea will be tested against alternative hypotheses involving propagating rifts and buoyant mantle upwelling. Millennial-scale paleoclimate records are contained within rapidly accumulated sediments of contourite drifts in this region. The accumulation rate of these sediments is a proxy for current strength, which is moderated by dynamic support of oceanic gateways such as the Greenland-Scotland Ridge. These sediments also provide constraints for climatic events including Pliocene warmth, the onset of Northern Hemisphere Glaciation and abrupt Late Pleistocene climate change. Our combined approach will explore relationships between deep Earth processes, ocean circulation and climate. Our objectives can only be addressed by recovering sedimentary and basaltic cores, and we plan to penetrate 200 m into igneous basement at five sites east of Reykjanes Ridge. Four sites intersect V-shaped ridges/troughs pairs, one of which coincides with Bjorn Drift. A fifth site is located over 32.4 Ma oceanic crust devoid of V-shaped features, chosen to intersect Oligo-Miocene sediments of Gardar Drift. Sediments and basalts recovered during this program will provide a major advance in our understanding of mantle dynamics, and of the coupled nature of Earth’s deep and surfical domains.