SO225

MANIHIKI II

Wochenbericht Nr. 4

(10.12. – 16.12.2012

Werner, Reinhard

English

OceanRep

  SO225 MANIHIKI II Weekly Report No. 4 (10.12. – 16.12.2012)  R/V SONNE  06°40,9´S / 162°44,6´W  This week, RV SONNE cruise SO225 did not remain on „the sunny side of the street“ as before. After an unexpected stop-over at Pago Pago on American Samoa at the beginning of the week, we experienced the effects of the tropical cyclone „Evan“, which caused large damage in Apia, the capitol of the Republic of Samoa. Luckily, we were not close enough that „Evan“ could threaten us heavily, but station work was hampered and delayed. During the second half of the week we concentrated on mapping the ocean floor in the central and northern parts of Manihiki Plateau using multi-beam and sediment echosounding techniques. Unfortunately, we remained unlucky during the course of the week. After having left the influence of „Evan“, we realized that the Posidonia antenna mounted below the ship was severely damaged, possibly by floating refuse. Posidonia is necessary for operating and navigating our diving robot ROV Kiel 6000. As we are not able to repair the antenna aboard RV SONNE, we will no longer operate the ROV, but instead will run dredges to recover hard rock from the seafloor. Thanks to the crew of RV SONNE, we managed to change equipment during calm weather conditions on Sunday, December 16. We are very positive that the combined use of both ROV and dredges will allow us to fulfill the petrologicial part of MANIHIKI II.    The remotely operated vehicle ROV KIEL 6000 in the photic layer of the SW Pacific.  Scientists study rock samples yielded by dredging. Late Sunday afternoon, the first dredge trail was accomplished at the northern margin of Manihiki Plateau. Based on results from RV SONNE cruise SO193 in 2007, we already know that the basement of Manihiki Plateau is tholeiitic of origin and approximately 123-126 million years old. Since SO225 focuses on the temporal development of the basement at Manihiki Plateau, we intend to considerably broaden the range of samples. The first dredge yielded lava, lithified sediments, and manganese. During the 4th week of the expedition, the paleoceanographic working group concentrated entirely on laboratory work. The 1 m-core segments meanwhile stored in the cooling room were cut into two halves, one to be archived, the other to be worked on. After the smoothing of the sediment surfaces, the sediment lithology was documented by core photography and visual core description, which allows first insight into the past depositional environment and climatic conditions. These approaches were supported by rapid and continuous core-logging techniques. The magnetic susceptibility measures the amount of magnetizable minerals to quantify the terrigenous portion of the sediment. Color and lightness of the sediment was assessed from other scanning techniques. Both logging approaches help to characterize and classify the sedimentary deposits. Most important result of this week is the correlation of recovered sediment records over wide distances and the first stratigraphical classification. To yield a first idea of the age of the sediment, the characteristic pattern of our logging data was compared and correlated to dated and already published reference records. From this approach we suggest to conclude that our sedimentary records cover the past ca. 1 million years, allowing us to describe the pronounced Pleistocene climatic variability in the equatorial West Pacific Warmpool area. The preliminary age model will be verified at home by oxygen isotope stratigraphy. The oxygen isotope signal of calcitic microfossils preserved in the sediments varies down-core and will be related to an astronomic timescale, the approach of which is even more accurate than absolute radiometric dating techniques.   Cutting sediment cores during SO225. The light  sediments consisting primarily of calcitic microfossils, are cut into cm-slices. Thousands of samples will be brought to the home labs for further analyses.   Photographs of a c. 15.5 m long sediment core recovered out of 3,250 m water depth at the "North Plateau". Foraminiferal sand and ooze dominate the lithology, and carbonate preservation is excellent. In order to allow for the highest temporal resolution of our paleoceanographic reconstructions, sampling of sediment records was performed at 1 cm-resolution, which is indeed time-consuming and not yet finished considering that c. 110 m of sediment are piled up in our cooling facilities. After arrival in the home labs, various, mostly (isotope)geochemical analyses will be applied to decipher ocean temperature and salinity conditions, the variability of the upper ocean, and its changes in ventilation and stratification on glacial to interglacial timescales.  For the ongoing cruise, we expect further interesting core locations on the "High Plateau" expanding our core transect to the south. Here, sedimentation appears to be much higher than on the "Western Plateaus" and the "North Plateau", as deduced from available sediment-echosounding data. In particular, the geophysical studies performed during SO224 were valuable for the selection of coring sites. During the night from Thursday, December 12, to Friday, December 13, we celebrated a little party in the GeoLab, as half of the cruise passed already by. All participants send greetings from the Southwest Pacific to everyone at home.  Reinhard Werner  

4. Wochenbericht SO225

SO225

MANIHIKI II

Wochenbericht Nr. 3

(03.12. – 09.12.2012

  SO225 MANIHIKI II Weekly Report No. 3 (03.12. – 09.12.2012)  R/V SONNE  03°00,26´S / 165°07,1´W  In the 3rd week R/V SONNE expedition SO225 focused on the northernmost part of the Manihiki Plateau, the so-called "North Plateau", and the ocean floor between Manihiki and the equator. On Monday and Tuesday, two dives with the ROV Kiel 6000 have been conducted in order to sample a c. 3 km long profile reaching from 4,605 m up to 3,260 m water depth across the flank of the south-eastern foothills of the North Plateau. The major objective of ROV sampling on SO225 is to sample stratigraphically controlled the real plateau basement. Previous investigations, however, revealed that large areas of the plateau are overprinted by younger volcanism. We selected the location at the North Plateau for sampling because rock samples dredged in the top region of this site on the former cruise SO193 yielded ages of 125 Ma and therefore fall into the age range assumed for the plateau basement.   Crinoidae on chaotic, pillow-like lava in 3.376 m water depth at a steep slope in the south-eastern part of the North Plateau.   Some ROV samples are too big and must be crushed with a sledge-hammer prior to further processing (photo N. Furchheim).  The pictures from these slope provided by the ROV show a rough, chaotic landscape dominated by steep slopes, small sediment-covered terraces, canyons, and ridge-like structures. Large parts of the slope are covered with rock debris including up to several meter-sized blocs. In between the debris bizarre rock formations are frequently exposed and consist of pillow lava, breccias, and massive rocks. As for the Danger Island Troughs, the abundance of debris, faults, and overall morphology of the slope also indicate for this area intense tectonic movements after formation of the plateau basement. Based on our observations we assume, that relocation of rocks by slope failures continues up to date. Altogether 28 rock samples have been taken by the ROV during the two dives. They mainly consist of fine-grained, relatively fresh volcanics, which are interpreted as dikes in the lower part of the profile. In the same region, sampling yielded coarse crystalline gabbros. Gabbros form when magma resides in the crust and slowly cools until complete crystallization. These rocks are considered to be important witnesses of former magma reservoirs. By laboratory studies at home, we hope to get detailed information on the compositional and age relation between the plutonic rocks and the lavas of the Manihiki Plateau. Interestingly, the ROV collected lava with fresh pyroxene phenocrysts from the upper part of the profile which indicate a higher degree of differentiation for the eruption products of this magma complex. Some rocks show a weak metamorphic overprinting, which becomes apparent by a slightly greenish color (chloritization) and numerous veins along cracks. Thus the rocks show clear evidence for fluid transport within the plateau crust accompanied by tectonic overprinting as already indicated by the field observations with the ROV. The remaining week was under the lead of the Paleoceanography working group. A coring station on the southeastern part of the North Plateau was the beginning of a series of 4 coring sites along a N-S-trending transect, which ended ca. 330 km south of the equator. Our accompanying CTD-measurements indicated that we gradually came under the influence of the equatorial currents, namely the South Equatorial Current and Countercurrent. Accordingly, the dominant sediment type changed from pure foraminiferal sands, which were difficult to recover, to more clayey and silty foraminiferal oozes. The larger portions of calcareous phytoplankton in the sediments point to a clearly higher primary productivity in the equatorial region, also reflected in a relatively lively sea fauna with calamari, fish, and birds. The recovery of the unique sediment material will definitely allow us to reconstruct the variability of the upper ocean over geological timescales, mainly due to the fact that the preservation of calcitic microfossil shells, which serve as biotic carriers for our geochemical proxies, is excellent down to even great water depths. Surprisingly, our TV-multi corer provided images of sediment ripples at c. 2 km water depth, pointing to highly dynamic deep water masses.   First investigations of a sediment core using a color scanner (photo GEOMAR). Deployment of the multi net at night (photo D. Nürnberg). In the night from Saturday to Sunday we deployed the multi net for the first time on this expedition which can sample plankton from varying water depths and should serve for proxy calibration studies. The multi net has been lowered 7 times up to 500 m water depth and sampled, besides larger organisms like Copepoda, large amounts of plankton from the water column of the nutritious, equatorial water masses. In the early afternoon of Sunday, the studies in the north of the Manihiki Plateaus were completed with a final piston corer deployment and R/V Sonne headed back to the south where we plan to conduct further coring and ROV sampling during next week.  As during the past week the temperature has been very high, but the heavy rain showers occurred more rarely in the north. In the meantime a table tennis tournament has been initiated in which 18 scientists and crew members take part and hope to reach at least the finals. All participants send greetings from the Southwest Pacific to everyone at home.  Reinhard Werner 

4. Wochenbericht SO225

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