SO225

MANIHIKI II

Wochenbericht Nr. 3

(03.12. – 09.12.2012

Werner, Reinhard

English

SO225

MANIHIKI II

Wochenbericht Nr. 5

(17.12. – 23.12.2012

OceanRep

  SO225 MANIHIKI II Weekly Report No. 5 (17.12. – 23.12.2012)  R/V SONNE  11°03,1´S / 161°31,9´W  Sampling the flanks of the Danger Island Troughs by dredging was the focus of our work during the 5th week of R/V SONNE expedition SO225 to the Manihiki Plateau. The Danger Islands Troughs are elongated, deep basins, which represent large fault systems bifurcating the plateau into the Western Plateaus, to the west, and the High Plateau, to the east. Their flanks as well as the northern margin of the High Plateau expose the interior of the Manihiki Plateau which is not accessible elsewhere. First, however, we finished sampling along the northern margin of the High Plateau. Two dredge hauls at a ridge-like feature at the foot of the northern flank and at its base recovered partly olivine bearing sheet and pillow lavas as well as a large fragment of a pillow breccia. Afterwards R/V Sonne sailed to the northern end of the Danger Island Troughs. On Tuesday, December 18th, the deepest dredge haul of this cruise has been carried out at a small ridge located in the center of the northernmost basin in more than 5.800 m water depth. The dredge returned altered lava fragments and lithified sediments. Two dredges conducted in an adjacent trough yielded similar results, confirming our earlier impression from cruise SO193 in 2007, that lithified sediments dominate the upper rock layers in the entire northern part of the Danger Island Troughs. Therefore we decided to continue further south, where a SO193 dredge haul yielded particularly interesting, c. 120 m.y. old rocks of the plateau basement and where a seismic profile conducted on the previous cruise SO224 showed exposed plateau basement.   The geological lab full of rocks....   Cut sample of an only slightly altered lava.  Here, we conducted 11 dredge hauls in 4.800 to 3.000 m water depth between the early morning of Wednesday an Friday evening. Here, a structure of most likely tectonic origin, characterized by several terraces along its southwestern slope, has been studied particularly intense, through systematic sampling of the cliffs below each terrace. The dredges returned various lavas, a broad variety of volcaniclastic rocks and also some intrusives. Some lava fragments appeared oxidizied, which may indicate eruptions under subaerial conditions. A special highlight, however, was the recovery of considerable amounts of fresh volcanic glass from the base of this feature. Volcanic glass forms by rapid cooling of lava for example when hot lava comes in contact with water. Such unaltered glasses is of particular interest to study the original composition of melts. Volcanic glass, however, is not stable and alters over the course of time; initially to palagonite and finally to clay. This process can significantly accelerate when glass is exposed to sea water. The recovery of fresh glass from a presumably c. 120 million years old flood basalt province is therefore something special and a great achievement which will enable detailed petrological and geochemical studies of the plateau forming melts. Some 15 nm further north, two dredge tracks not only recovered lava and sediments, but also metamorphically overprinted rocks (greenshists). Most notably is the spinifex-texture of the lava, named after a sharp Australian grass species that describes the needle-like growth of pyroxene and olvine crystals. The very beautiful spinifex-texture forms by rapid crystallization of very hot, mostly ultramafic melts, pointing to unusual high magma temperatures during the formation of Manihiki Plateau. Finally, we dredged samples from the western and eastern flanks of Danger Island Troughs, at the intersection with detailed seismic profiles of SO224. On the western slope, sheet and pillow lava dominate, partly also with spinifex-texture. The eastern slope mainly exhibited lithified sediments and manganese crusts. Complementing mapping of the ocean floor and sediment echo sounding, a total of 17 dredges have been carried out during the first five days of this week. Of these, 12 dredges recovered magmatic rocks, 7 volcaniclastic rocks, and 6 lithified sediments. At the very end of the week, the paleoceanography group started again, carrying out 2 coring stations on the High Plateau. Site selection was based on seismic profiles from SO224, which showed sediment deposits characterized by hundreds of meters in thickness. Unfortunately, sediment recovery was not as easy as thought, with many coring failures and loss of equipment probably due to the hard and lithified sediment even at very shallow core depths. After many attempts at extremely hot conditions on the working deck, the paleoceanographers were finally paid off with a high quality sediment record.    Merry Christmas from the SO225 scientists. The picture was taken in the geology laboratory of R/V SONNE, as it started raining just prior to the shooting. On December 21, we celebrated that earth did not tumble down as predicted by the Mayan calendar, together with the birthday of a colleague in the seismic laboratory. On Christmas Eve, the scientific program of SO225 will be continued with dredging and mapping at Suvorov Trough in the southern part of Manihiki Plateau. In the evening, we will celebrate Christmas jointly in the mess, and with a ceremonial lunch on the first Christmas day. All SO225 participants send cordial greeting from the SW Pacific: Merry Christmas to all of you.  Reinhard Werner 

5. Wochenbericht SO225

SO225

MANIHIKI II

Wochenbericht Nr. 2

(26.11. – 02.12.2012

  SO225 MANIHIKI II Weekly Report No. 2 (26.11. – 02.12.2012)  R/V SONNE  08°18,3´S / 164°29,6´W  In the night from November 25th to 26th, R/V Sonne arrived at the so-called Western Plateaus, which represent the western part of the Manihiki Plateau. We headed for a seamount, which has already been investigated on cruise SO193 in 2007. The data compiled on that cruise show that the top region of this seamount is a large plateau covered by thick sediment layers. Here, we conducted the first coring station of our cruise in 1,500 m water depth. Such relatively shallow stations are of great interest for the paleoceanographers as at shallow water depths carbonate shells of planktonic micro fossils (foraminifera) are well-preserved and thus, allow to reconstruct environmental conditions during the past. At each coring station, we commonly deploy one to two piston corers to recover sediment cores as long as possible. Additionally, the multi-corer is run to sample bottom water and the sediment surface, which is usually destroyed in the cores recovered by piston corer. At the first station, unfortunately, the foraminiferal sand turned out to be difficult to sample as it was easily washed out during heaving of the sampling tools. Finally, we managed to obtain a core as well as samples form the sediment surface. Furthermore, we sampled and analyzed the water column in various depths from the ocean floor up to the water surface using a CTD (Conductivity, Temperature, Density) and a rosette water sampler.     Samples of the sediment surface yielded with a multi-corer (photo: Torsten Bierstedt).  Scientists investigate sediment cores and rocks in the geology lab on board R/V Sonne and prepare them for subsequent shore based research in the home institutes (photo: David Poggemann). During the following days, we gave priority to further coring stations located on a N-S-profile across the western plateaus, since a dive with the remotely operated vehicle ROV Kiel 6000 had to be cancelled due to unforeseeable technical problems. Sediment sampling at these stations was very successful and yielded up to 16 m long sediment cores. First core logging methods conducted on board show that even in the equatorial West Pacific Warm Pool area the lithological changes are quite prominent, pointing to pronounced variations in the ecosystem in response to oceanographic and climatic changes during the Pleistocene. Furthermore, we discovered millimeter-sized lithogenic particles of most likely volcanic origin in only a few centimeter sediment depth. Because of their size and density, such particles were neither drifted by currents nor transported by wind over long distances. We presume that they originate from volcanic eruptions nearby the location where they were deposited. Volcanism during the Late Pleistocene, however, has not yet been described for the area of the Manihiki Plateau. On Friday, November 30th, R/V Sonne sailed back to the Danger Island Troughs, where several ROV dives were scheduled. The Danger Islands Troughs are a large fault-bounded series of en echelon bathymetric depressions, named after the atolls at its southern end. The basins strike north-south, bifurcate the Manihiki Plateau into the High  Plateau to the east and the Western Plateaus to the west, and consist of deep, elongated basins with water depths as deep as 6,000 m. From SO193 we know that thick lava successions are exposed along the steep slopes of the basins. These lava successions are the main target for stratigraphically controlled ROV sampling in order to reconstruct the temporal and compositional evolution of the Manihiki Plateau.    Rock sampling in 3,500 m water depth using ROV Kiel 6000. A conventional chisel hold by the manipulator of the ROV turned out to be the optimal tool for rock sampling.  The manipulator of the ROV recovers a rock sample form the eastern flank of the Danger Island Troughs. In the morning of December 1st, we were able to conduct the first ROV dive of the cruise thanks to the tireless commitment of the GEOMAR ROV team. The dive covered a depth profile extending from 3,500 to 3,000 m water depth across the steep upper slope of the central basin of the Danger Island Troughs. The images? taken by the ROV from the sea floor reveal a spectacular rough landscape, which is dominated by sheet und pillow lavas, volcanic breccias, dikes and debris consisting of lava blocks. The in situ rock successions are just right for the planned systematic sampling, but turned out to be extremely solid and robust. Nevertheless, we were able to recover rock samples thanks to the skillful handling of the manipulator by the ROV pilots. The samples comprise dense lavas and also, somewhat surprisingly, highly vesicular volcanics. We are optimistic that we will complete our sample set during the upcoming days by further dives. The last week also let us recognize that the sun does not always shine in the equatorial Pacific. Quite often short, but very intense rain showers surprised us. This and the burning heat makes work on the aft deck somewhat unpleasant. Overall, we enjoy the warm tropical evenings with now and then spectacular sun sets. All participants are well and send greetings to everyone at home. For all cruise participants Reinhard Werner   

2. Wochenbericht SO225

  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 

3. Wochenbericht SO225

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