ALBERT

Description: Earthquake locations along the southern Mid-Atlantic Ridge have large uncertainties due to the sparse distribution of permanent seismological stations in and around the South Atlantic Ocean. Most of the earthquakes are associated with plate tectonic processes related to the formation of new oceanic lithosphere, as they are located close to the ridge axis or in the immediate vicinity of transform faults. A local seismological network of ocean-bottom seismometers and land stations on and around the archipelago of Tristan da Cunha, allowed for the first time a local earthquake survey for one year. We relate intra-plate seismicity within the African oceanic plate segment north of the island partly to extensional stresses induced by a bordering large transform fault and to the existence of the Tristan mantle plume. The temporal propagation of earthquakes within the segment reflects the prevailing stress field. The strong extensional stresses in addition with the plume weaken the lithosphere and might hint at an incipient ridge jump. An apparently aseismic zone coincides with the proposed location of the Tristan conduit in the upper mantle southwest of the islands. The margins of this zone describe the transition between the ductile and the surrounding brittle regime. Moreover, we observe seismicity close to the islands of Tristan da Cunha and nearby seamounts, which we relate to ongoing tectono-magmatic activity.

Description: Ultraslow spreading ridges are poorly understood plate boundaries consisting of magmatic and amagmatic segments that expose mostly mantle peridotite and only traces of basalt and gabbro. The slowest part of the global spreading system is represented by the eastern Gakkel Ridge in the Central Arctic Ocean, where crustal accretion is characterized by extreme focusing of melt to discrete magmatic centers. Close to its eastern tip lies the unusual 5,310 m deep Gakkel Rift Deep (GRD) with limited sediment infill, which is in strong contrast to the broader sediment‐filled rift valleys to the east and west. Here, we report an 40Ar/39Ar age of 3.65±0.01 Ma for a pillow basalt from a seamount located on the rim the GRD confirming ultraslow spreading rates of ~7 mm/yr close to the Laptev Sea as suggested from aeromagnetic data. Its geochemistry points to an alkaline lava, attributed to partial melting of a source that underwent prior geochemical enrichment. We note that the GRD extracts compositionally similar melts as the sparsely magmatic zone further west but at much slower spreading velocities of only ~6‐7 mm/yr, indicating the widespread occurrence of similarly fertile mantle in the High Arctic. This enriched source differs from sub‐continental lithospheric mantle that influences magmatism along the Western Volcanic Zone (Goldstein et al. 2008) and is similar to metasomatized mantle ‐ shown to influence melt genesis along the Eastern Volcanic Zone.

Description: Understanding the enigmatic intraplate volcanism in the Tristan da Cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it. We measured phase-velocity curves of Rayleigh waves using cross-correlation of teleseismic seismograms from an array of ocean-bottom seismometers around Tristan, constrained a region-average, shear-velocity structure, and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot. The ocean-bottom data set presented some challenges, which required data-processing and measurement approaches different from those tuned for land-based arrays of stations. Having derived a robust, phase-velocity curve for the Tristan area, we inverted it for a shear wave velocity profile using a probabilistic (Markov chain Monte Carlo) approach. The model shows a pronounced low-velocity anomaly from 70 to at least 120 km depth. VS in the low velocity zone is 4.1-4.2 km/s, not as low as reported for Hawaii (∼4.0 km/s), which probably indicates a less pronounced thermal anomaly and, possibly, less partial melting. Petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle (mantle potential temperature of 1,410-1,430°C, an excess of about 50-120°C compared to the global average) and a melt fraction smaller than 1%. Both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65-70 km, consistent with recent estimates from receiver functions. The presence of warmer-than-average asthenosphere beneath Tristan is consistent with a hot upwelling (plume) from the deep mantle. However, the excess temperature we determine is smaller than that reported for some other major hotspots, in particular Hawaii.

Description: Since about 15 years a growing number of evidence is found in water depth up to more than 1000 m of the Arctic Ocean that grounding of ice has occurred in various places including the "Beringian" continental margin north of the present Chukchi and East-Siberian seas and the Lomonosov Ridge. These landforms include moraines, drumlinized features, glacigenic debris flows, till wedges, mega-scale glacial lineations (MSGL), and iceberg plough marks (Polyak et al. 2001, Niessen et al. 2013, Dove et al. 2014, Jakobsson et al. 2014). They suggest that thick ice has occurred not only on nearly all margins of the Arctic Ocean but also covered pelagic areas. In a recent paper, Jakobsson et al. (2016) present more evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, thereby revitalising an old modelling concept of a kilometre-thick ice shelf extending over the entire central Arctic Ocean (Hughes et al. 1977) now dated to Marine Isotope Stage (MIS) 6. Other (including our) studies, however, suggest that the pattern, and, in particular, the timing of these glaciations is more complex. Most recent discoveries on the Lomonosov Ridge have not only gained different information on Pleistocene glaciations but also allowed for the first time to reconstruct upper Miocene Arctic Ocean sea-ice and SST conditions. This became possible since submarine sliding (likely associated with ice grounding) led to removal of younger sediments from steep headwalls and thus exhumation of Miocene to early Quaternary sediments close to the seafloor, allowing the retrieval and analysis of such old sediments by gravity coring (Stein et al. 2016). Submarine glacial landforms from the western and central Arctic Ocean were discovered and investigated during the cruises of RV "Polarstern" in 2008 and 2014, and RV "Araon" in 2012 and 2015. Orientations of some of these landforms suggest that thick ice has flown north into the deep Arctic Ocean from the continental margin of the East Siberian Sea repeatedly (Niessen et al. 2013), thereby grounded on plateaus and seamounts of the Medeleev Ridge. In addition, hydro-acoustic data is presented from the Lomonosov Ridge (Siberian side to close to the North Pole), which support the hypothesis of widespread grounding of ice in the Arctic Ocean, of which the sources are still difficult to determine. The data suggest that thick ice-shelves could have developed from continental ice sheets on a nearly circum-arctic scale, which disintegrated into large icebergs during glacial terminations. On the slopes of the East Siberian Sea and/or on the Arlis Plateau, three northerly-directed ice advances occurred, which are dated by sediment cores using the chronology of brown layers (B1 to B7) as suggested by Stein et al. (2010). According to our age model, the latest advance is slightly older than B2 (MIS-3/4), which has been interpreted as MIS-6 by Jakobsson et al. (2016). A larger well-constrained glaciation has occurred during MIS-4, of which an ice shelf grounded to 900 m on the Arlis Plateau. In the western Arctic Ocean, the oldest datable ice advance has an intra-MIS-5 age. In our data, the chronology of older ice advances along the East Siberian margin are not well constrained but may extend back as far as MIS-16. In contrast, cores from the southern and central Lomonosov Ridge indicate that the youngest ice grounding there has occurred during MIS-6. This grounding was less intense than previous ice-shelf groundings in the area, of which the chronology remains speculative until longer cores become available. Along the Lomonosov Ridge, detailed bathymetric mapping between 81° and 84°N exhibit numerous amphitheatre-like slide scars, under which large amounts of Cenozoic sediments were remobilized into mass-wasting features on both the Makarov and Amundsen sides of the ridge. In areas shallower than 1000 metres, slide scars appear to be associated with streamlined glacial lineations, whereby some of the bedforms have been removed by sliding. It appears that at least some of the mass-wasting events have been triggered by moving and/or loading of grounded ice. Sub-bottom seismic profiling discovered at least three generations of debris-flow deposits near the ridge, which were generated by the slides. In places, the nearly randomly distributed slide scars and debris-flow deposits make it hard to interpret past ice-flow directions from landforms and re-deposited sediments. The pattern allows interpretation of both directions off East Siberia (e.g. Jakobsson et al. 2016) and off Eurasia (e.g. Polyak et al. 2001) towards the central Arctic Ocean. Underneath the slide scars escarpments of up to 400 m in height were formed. Near the southern end of the Lomonosov Ridge the last exhumation of old sediments has occurred during MIS-6. Some of the old sediments recovered in 2014 were studied in more detail (Stein et al., 2016). We can show for the first time that the mid/late Miocene central Arctic Ocean was relatively warm (4-7°C) and ice-free during summer, but sea ice occurred during spring and autumn/winter. A comparison of our biomarker proxy data with Miocene climate simulations seems to favour relatively high late Miocene atmospheric CO2 concentrations. References Dove, D., Polyak, L. & Coakley, B., 2014. Widespread, multi-source glacial erosion on the Chukchi margin, Arctic Ocean. Quat. Sci. Rev. 92, 112–122 Hughes, T. J., Denton, G. H. & Grosswald, M. G., 1977. Was there a late-Würm Arctic ice sheet? Nature, 266, 596–602 Jakobsson, M. et al., 2014. Arctic Ocean glacial history. Quat. Sci. Rev. 92, 40-67 Jakobsson, M., et al., 2016. Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation. Nat. Comm., 7, 10365, DOI: 10.1038/ncomms10365, 1-10 Niessen, F. et al., 2013. Repeated Pleistocene glaciation of the East Siberian continental margin. Nat. Geosci. 6, 842–846 Polyak, L., Edwards, M. H., Coakley, B. J. & Jakobsson, M., 2001. Ice shelves in the Pleistocene Arctic Ocean inferred from glaciogenic deep-sea bedforms. Nature 410, 453–459 Stein, R., Matthiessen, J., Niessen, F., Krylov, A., Nam, S., Bazhenova, E., 2010. Towards a better (litho-) stratigraphy and reconstruction of Quaternary paleoenvironment in the Amerasian Basin (Arctic Ocean), Polarforschung, 79 (2), 97-121 Stein, R., K. Fahl, Schreck, M., Knorr, G., Niessen, F., Forwick, M., Gebhardt, C., Jensen, L., Kaminski, M., Kopf, A., Matthiessen, J., Jokat, W., and Lohmann, G., 2016. Evidence for ice-free summers in the late Miocene central Arctic Ocean. Nature Communications 7:11148, doi:10.1038/ncomms11148.

Description: The modern polar cryosphere reflects an extreme climate state with profound temperature gradients towards high-latitudes. It developed in association with stepwise Cenozoic cooling, beginning with ephemeral glaciations and the appearance of sea ice in the late middle Eocene. The polar ocean gateways played a pivotal role in changing the polar and global climate, along with declining greenhouse gas levels. The opening of the Drake Passage finalized the oceanographic isolation of Antarctica, some 40 Ma ago. The Arctic Ocean was an isolated basin until the early Miocene when rifting and subsequent sea-floor spreading started between Greenland and Svalbard, initiating the opening of the Fram Strait / Arctic-Atlantic Gateway (AAG). Although this gateway is known to be important in Earth’s past and modern climate, little is known about its Cenozoic development. However, the opening history and AAG’s consecutive widening and deepening must have had a strong impact on circulation and water mass exchange between the Arctic Ocean and the North Atlantic. To study the AAG’s complete history, ocean drilling at two primary sites and one alternate site located between 73°N and 78°N in the Boreas Basin and along the East Greenland continental margin are proposed. These sites will provide unprecedented sedimentary records that will unveil (1) the history of shallow-water exchange between the Arctic Ocean and the North Atlantic, and (2) the development of the AAG to a deep-water connection and its influence on the global climate system. The specific overarching goals of our proposal are to study: (1) the influence of distinct tectonic events in the development of the AAG and the formation of deep water passage on the North Atlantic and Arctic paleoceanography, and (2) the role of the AAG in the climate transition from the Paleogene greenhouse to the Neogene icehouse for the long-term (~50 Ma) climate history of the northern North Atlantic. Getting a continuous record of the Cenozoic sedimentary succession that recorded the evolution of the Arctic-North Atlantic horizontal and vertical motions, and land and water connections will also help better understanding the post-breakup evolution of the NE Atlantic conjugate margins and associated sedimentary basins.