RV SONNE Fahrtbericht / Cruise Report SO233 WALVIS II, 14.05-21.06.2014, Cape Town, South Africa - Walvis Bay, Namibia
Hoernle, Kaj, Werner, Reinhard and Lüter, Carsten, eds. (2014) RV SONNE Fahrtbericht / Cruise Report SO233 WALVIS II, 14.05-21.06.2014, Cape Town, South Africa - Walvis Bay, Namibia . GEOMAR Report, N.Ser. 022 . GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, 153 pp. DOI 10.3289/GEOMAR_REP_NS_22_2014.
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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.
|Document Type:||Report (Cruise Report)|
|Keywords:||RV SONNE, SO233, cruise report, Southeast Atlantic, Walvis Ridge, Bathymetry, Volcanology, Geochronology, Magmatic Geochemistry, Mantelplumes, Seamounts, Biology, Marine Benthos, Meiofauna, Biodiversity, Biogeography|
|Research affiliation:||OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems|
|Date Deposited:||14 Nov 2014 09:48|
|Last Modified:||11 Feb 2015 12:45|
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