Werner, Reinhard, Nürnberg, Dirk and Hauff, Folkmar, eds. and SO225 Shipboard Scientific Party (2013) RV SONNE Fahrtbericht/Cruise Report SO225, Manihiki II Leg 2, The Manihiki Plateau - Origin, structure and effects of oceanic plateaus and Pleistocene dynamic of the West Pacific warm water pool ; 19.11.2012 - 06.01.2013, Suva/Fiji - Auckland/New Zealand . . GEOMAR Report, N. Ser. 006 . GEOMAR Helmholtz-Zentrum für Ozeanforschung, Kiel, Germany, 176 pp. DOI 10.3289/GEOMAR_REP_NS_6_2013.

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Abstract

R/V SONNE cruises SO-224 and SO-225 are part of the cooperative project MANIHIKI II
between GEOMAR Helmholtz Centre for Ocean Research Kiel and the Alfred Wegener
Institute Helmholtz Centre for Polar and Marine Research (AWI), funded by the German
Ministry of Education and Research (BMBF). This multidisciplinary project continues previous
research at the Manihiki Plateau conducted since 2007 (SO-193) on morphological,
volcanological, geochemical, and geochronological studies and is now broadened by
geophysical and paleoceanographic research foci.
SO-225 focused on stratigraphically controlled sampling of the igneous successions of the
Manihiki Plateau. This has been accomplished by using the remotely operated vehicle ROV
Kiel 6000 and chain bag dredges. Coring of deep sea sediments and sampling of the overlying
water column has been added to the program. SO-225 and subsequent shore-based research
in the home institutes mainly address (1) the temporal, spatial, and compositional evolution of
the igneous basement of Manihiki Plateau, (2) the environmental impact of the large volcanic
eruptions, which formed the Manihiki Plateau, (3) the Plio-Pleistocene dynamics and evolution
of the West Pacific Warm Pool during the last ~3 million years, and (4) the potential
oceanographic interaction between the equatorial Pacific and the Southern Ocean (“ocean
tunnel hypothesis”) and its climatic responses. The integration of scientific results from SO-224
and SO-225 with existing data from the West Pacific large igneous provinces Manihiki,
Hikurangi, and Ontong Java will contribute towards a better understanding of the origin and
effects of volcanic mega events, the formation of large igneous provinces, and the
paleoceanography and paleoclimate of the equatorial West Pacific.
R/V SONNE cruise SO-225 started in Suva/Fiji on November 21st, 2012, and ended in
Auckland/New Zealand on Januar 5th, 2013. Complementing 2,940 nm multi-beam mapping
and 2,250 nm sediment echo-sounding, a total of 62 deployments of various devices have
been carried out during SO-225. Ten of 11 multi corers yielded sediment samples, 16 piston
corer and 3 gravity corer deployments recovered altogether 131.6 m sediment cores. The
sampling of the water column by CTD and multi net was successful. Foraminiferal sand and
ooze dominate among the sediment samples, some cores also contained sandy clayey silt rich
in foraminifers and nanno ooze. The sediment cores cover a more than 1,100 km core transect
extending from the ocean floor to the north of the Manihiki Plateau to the southern foothills of
the High Plateau. Preliminary studies on board showed that the SO-225 sediment sampling
yielded excellent paleoceanographic archives which can be correlated along the entire core
transect and dated back to Pliocene. Further preliminary results include that past climate
changes significantly affected the West Pacific Warm Pool. The sediment samples also will
allow to reconstruct the Plio/Pleistocene variability of equatorial currents and the Antarctic
intermediate water. Four ROV dives yielded 32 rock samples from two profiles across the
slopes in the northern and central part of the Manihiki Plateau (North Plateau and Danger
Island Troughs). Stratigraphically controlled sampling along c. 3 km long profile reaching from
4,600 m up to 3,260 m water depth across the flank of the south-eastern foothills of the North
Plateau was particularly successful. Due to a series of unfortunate circumstances beyond our
control, further ROV sampling on SO-225 had to be cancelled. Instead we decided to run
dredges to considerably broaden the range of samples from the Manihiki Plateau basement by
dredging. Twenty-three dredge hauls have been conducted in an average water depth of
4,380 m. Of these, 20 delivered magmatic rocks, 12 volcaniclastics, 8 sedimentary rocks, and
13 Mn-Fe-Oxide crusts. Notably, some of the dredged rocks show spinifex textures indicating
unusual high eruption temperatures and several dredges contained fresh volcanic glass. The
recovery of fresh glass from a presumably c. 120 million years old flood basalt province is a
great achievement which will enable detailed petrological and geochemical studies of the
plateau forming melts. Finally, mapping of submarine volcano Monowai en route on the transit
to Auckland SO-225 should contribute to a time series of maps which continuously document
the evolution of the volcano. During profiling, however, a sudden and significant increase in
volcanic activity hindered us in mapping the top area of Monowai.

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