RV SONNE Fahrtbericht / Cruise Report SO233 WALVIS II,14.05-21.06.2014,Cape Town, South Africa - Walvis Bay, Namibia

Abstract

R/V SONNE cruise SO-233 WALVIS II conducted geological, morphological, and biological studies in the area of the aseismic Walvis Ridge and the adjacent ocean floor (South Atlantic). The Walvis Ridge is a textbook example of a hotspot track connected to a continental flood basalt province and represents the Atlantic “type locality” for the enriched mantle one (EM-I) geochemical endmember in intraplate volcanic rocks. Despite its importance in the global hotspot reference frame, endmember geochemical composition, and uncertainties in its formation and evolution, basement sampling of the Walvis Ridge remained poor to date, in particular along its easternmost 1500 km. The geological studies carried out during SO-233 therefore aimed for extensive multi-beam mapping using a SIMRAD EM 120 echo-sounding system, sediment echo-sounding using a ATLAS PARASOUND sub-bottom profiling system, and hard rock sampling by dredging and TV-grab of the Walvis Ridge and associated features. The major targets of the WALVIS II project are (1) to test for age progressive volcanism along the ridge, (2) to differentiate between classical hotspot and plate fracturing models for its formation, and (3) to constrain the origin, temporal and spatial evolution of melting conditions and source compositions (in particular regarding the EM-I endmember and proposed zonation models of mantle plumes). The biology program conducted on SO-233 comprised sampling of benthic organisms and meiofauna using a TV-multi-corer, a TV-grab, sediment traps installed in the geological dredges, and by collecting marine invertebrates from the hard rocks yielded by dredging. The biological investigations of the WALVIS II project intend to describe the benthic diversity of deep-sea invertebrates of the Walvis Ridge and will help to identify proxies of species connectivity and dispersal between the Walvis Ridge and neighboring ridge like structures (e.g. Agulhas Ridge). Another objective is to test whether connectivity of benthic communities in the Angola and Cape Basins is interrupted by the Walvis Ridge. SO-233 multi-beam mapping revealed that the southern bifurcated section of the Walvis Ridge appears to have formed through the coalescence of former volcanic islands. The new bathymetric data also yielded several evidence for large-scale extensional tectonic movements which are most likely related to the separation of the Walvis Ridge and Rio Grande Rise that were rifted apart by the mid Atlantic Ridge. Seventy-one dredge hauls have been conducted during SO-233. Of these, 28 delivered massive lavas, 24 volcaniclastic rocks including breccias containing lava fragments, 22 sedimentary rocks, and 11 Mn-Fe-oxide crusts and nodules. The volcanic rocks comprise a broad variety of lavas as well as epiclastic, hydroclastic, and pyroclastic rocks. Carbonates dominate among the non-volcanic rocks, many of them represent relicts of fossil coral reefs. Despite technical problems with the EM 120 system and difficult weather and seafloor conditions occasionally constraining rock sampling, SO-233 achieved its major goals, i.e. bathymetric mapping and representative hard rock sampling of all major geomorphological units of the Walvis Ridge and of associated features. The set of rock samples recovered during SO-233 represents the by far most detailed sampling of the Walvis Ridge to date. Out of 91 collecting stations, 80 stations yielded the total amount of 80 kg of sediment from sediment traps in the geological dredges, TV-multi-corer tubes and TV-grab. At 44 stations we could collect macrofaunal organisms, partly in large quantities. Ninety specimens of living brachiopods representing 6 genera were found at all depths and will mainly be used for molecular diversity studies. The remaining living macrofauna was largely composed of sponges, octocorals, some deep water hexacorals, molluscs, polychaetes, bryozoans, cirriped crustaceans and a few isopods and amphipods, mainly occurring in small numbers and medium diversity. The most spectacular finding was a fossil cold water reef mound community, which shows similarities in species composition to North Atlantic cold water reefs and proofs the influence of Antarctic benthos communities on the Walvis Ridge fauna mediated by northbound cold water currents. The samples represent the most diverse collection of benthos organisms ever retrieved from the Walvis Ridge region.