The shear strength was determined by a rotation viscosimeter RV3 (KASSEN S 1990; BLANZ 1992).Th e rotation
viscosimeter measures th e undrained shea r strength , tha t can be equated to apparent cohesio n in fine-grained sediment s when the angle of internal frictio n decreases to zero (BISHO P &
BJERRUM 1 9 61).
For the purpose of measuring th e shear strength a
hydrometric curren t meter FL-1000 (1 0 mm x 8.8 mm) was lowere d 1 cm parallel to the bedding int o the sediment an d was rotate d at a constant spee d of 4 rotations per minute. Depending on the sediment core' s litholog y 2 to 3 measurements were carrie d out in a vertical depth interva l of 5 to 10 cm to determine the natural variability at each depth.
The resistance serve s as a measure of the shear strength , i.e., th e shear stress that is opposed by the sediment sampl e with the rotation motions of the rotation wing. This shea r
stress i s measured by means of a twist potentiometer in the probe and the measured analo g values are recorde d on a Rikadenski recorder . The shear strength T is calculated fro m the measured values (S ) by means of a specific probe shea r factor. The results are shown in Fig. 3.5.3.-1.
T = (A x S) (kPa ) T = shear strengt h
A = shear facto r S = scale parts
Cruise Report S082 5 3
Natural water content and humid unit weight
Parallel to the shear strengt h measurements, 10 ccm of sediment was extracted by constant volume sample r to determine the water content and the humid unit weight. These defined sediment sample s (1 0 ccm) were weighed in wet and dry form onboard (dryin g at 105°C in the drying cabinet) . The balance onboard is a wave-compensated la b balance tha t according to its operating instruction s weights precisely + - 1%.
The water content W (see Fig. 3.5.3.-2) is calculated as follows:
W = W w / W s x l 0 0 (%)
W = natural water conten t (%) Ww = water mass (g)
Ws = mass of dried sediment sampl e (g)
Values over 100 % can occur because the natural water conten t is correlated with the dry weight.
The humid unit weight is defined as follows (RICHARD S 1962 ) : Wbd = M / V (g/ccm)
The shear strength increase s with depth (RICHARD S 1961;
ALMAGOR 1979 ) in normally consolidate d sediment s (SKEMPTO N 197 0). Amon g other things, the shear strength depends on the grain size distribution, the sediment composition , th e
sedimentation rate and the grade of consolidation.
The box cores (S0821KAL , S0822KAL, S0823KAL , S082 -4KAL, S082-5KAL and S082-7KAL) contain mainly gre y to olive-grey, silty to clayey sediment s with different content s of sand. I n all core sections the occurrence of sponge needles is striking. In some horizons (se e core descriptions) spong e needles are strongly accumulated . These horizons (se e Fig.
3.5.3.-1) are characterized by distinct shea r strength peaks with strengths of up to 22 kPa. The general increas e in shear
strength with depth (RICHARD S 1961) that is known fro m other core sites and from other sea areas (KASSEN S 1990; BLANZ 1992 !
SO
82-1S O
82-2S O
82-3S O
82-5S O
82-4S O
82-7Shear Strengt h (kPa) 0 10 20
1 I
Shear Strengt h (kPa) 0 10 20
I
Shear Strengt h (kPa) 0 1 0 2 0
I < I i I i I
Shear Strengt h (kPa) 0 10 20
I i I i I i
Shear Strengt h (kPa) 0 10 20
I • I . I . I
Shear Strengt h (kPa) 0 1 0 2 0
I . I i I i I
Fig. 3.5.3.-1 Shear strength of the giant gravity cores
could not exactly be determined i n any of the sediment cores.
The relatively "high " shear strength (fro m near to the bottom 10 kPa) i s surprising an d remains constan t t o the core basis with fluctuation s o f +- 5 kPa (RICHARD S 1962) .
B) Water content
The natural water content i s a criterion to assess the
sediment's degree of strength (ALMAGO R 1979) . Moreover, thi s degree depends on the degree of compaction and the sediment' s composition (RICHARD S 1962) .
On average, the water conten t profiles (se e Fig. 3.5.3.-2) range fro m 150% to 1 2 0% without being dependent o n depth, i.e., th e water content does not decrease with depth and is thus correlated with the shear strength that neither depend s on depth. Neither there are distinct water content profile minima nor maxima. The horizons with a high shear strengt h have yet a low water content (approx . 100%) . However, they do not occur as distinct peaks i n the water content profiles.
For a better interpretation of the curves it is necessary
Cruise Report S082 55
SO 82-1
Water Content (% dry weight)
100 20 0
SO 82-2
Water Content (% dry weight) 100 200
SO 82-3
Water Content (% dry weight) 100 200
l . i . l
SO 82-5
Water Content (% dry weight)
100 20 0 I i I i I
SO 82-4
Water Content (% dry weight)
100 20 0 I i I i I
SO 82-7
Water Content {% dry weight)
100 20 0
Fig.3.5.3.-2 Natural water content of the giant gravity cores
to carry out som e additional investigations . Only th e analyse of radiographs, grain size distributions, sedimen t
compositions an d sedimentation rate s will contribut e t o the interpretation of the exact sedimentatio n history an d it s climatic background with concerns of the soil mechanics.
3 . 5 . 4 . Benthic s t u d i e s
( J . S v a v a r s o n )
The benthic faun a of the deeper, oceanic region s of the Reykjanes Ridge i s still poorly known, not only i n terms of its structure (specie s composition, biomass), but also in terms of physical and biological patterns tha t determine the composition of the fauna. The area i s quite complex i n its topography an d presumably als o i n its physical characteristic s
(currents, sedimentation, etc.) , althoug h th e watermasses may be homogeneous. This complexity o f the topography may be
reflected i n heterogeneous habitats within this seamoun t
environment, resulting i n patchiness o f the benthic fauna . The aim of the present stud y i s to:
A) characterize th e benthic faun a (macrofaun a and meiofauna) i n terms of species composition, biomass and abundance and relate these patterns t o geophysical an d geological characteristic s o f the recent sediment s and to biological parameters . Also of interest is:
B) the interaction between benthic animals , some of which (foraminifers) are important considerin g th e geological tim e scale. Recent studie s have show n that benthic isopod s
(Crustacea) actually fee d on (an d crunch) benthic
foraminifers, and may thu s have influenc e on the species composition of the foraminifers and possibly also on the fragmentation of the foraminifers . The impac t of this has, however, not yet been quantified.
Samples fo r studies of the benthos were taken with 0.2 5 cm2 boxcorer, supplie d with photographic equipment . Altogether, samples were taken at seven stations at depths between 110 2 and 186 7 meters. The part o f the boxcorer planned fo r
biological studie s was isolated fro m the rest of the sample with 20 * 40 cm frame , while still having surfac e water overlying th e sediments. This was, however, not alway s possible. From each boxcorer subsample s were taken for the study of chloroplastic pigment s ( 2 syringes with 1.25 c m diameter), tota l carbon (1 0 cm2, upper 2 cm layer) , meiofaun a
(three subcorers each 1 0 cm2), and the rest was preserved t o study th e macrofauna. Some of the meiofauna sample s were subdivided int o 1 cm thick layers to allow study of the vertical distributio n of the meiofauna.
A priori one would expec t coars e sediment s occurring nea r the central part of the ridge, that are possibly erode d by the seamounts o f the area. All sample s except fo r two were,
however, taken fro m bottoms with very fine , soft mud. These were at depths of 140 9 to 1777 m. Here the surface sediment s are characterized by an abundance of globigerinoid an d
pteropod mollusc shell s together with numerous calcareou s sponge spicules . This bottom typ e could indicat e rather slo w currents and rapid sedimentatio n of both planktonic an d
benthic, resuspended material. Some of the samples seem to
Cruise Report S082 5 7
have rich benthic fauna .
One of the boxcorer containe d "har d bottom", which was actually fin e mud, foraminifer a shell and calcareous spong e spicules, packed int o hard substratum . This hard bottom had a rich epifauna, with ophiuroids, sponges, hydroids and
polychaete tubes . It is not evident whether thi s type of bottom sediment s has recently been incorporated int o the
surface, or whether it is older sedimen t being eroded . Exac t quantities of the benthic animal s and the species compositio n can not be seen until th e material has been sorte d in the laboratory.
4. Summar y
For the firs t tim e after the re-unification of Germany a research cruise with RV'Sonne" had has been planned, organize d and performed under the leadership of an institute fro m the former GDR, th e Institute of Baltic Se a Research Warnemunde (formerly Institute of Marine Research - Warnemunde). Thi s task was solve d with good results i n very clos e cooperatio n and with the help of the GEOMAR Marine Research Center.
The cruise was divided int o two parts. During th e first part a short geophysical program was carried out. Two advanced
seismoacoustic profiling systems , the Parasound-Paradigma narrow beam echosounder an d the SEL9 0 sediment penetratin g echosounder, were installe d an d tested with good results.
Furthermore, a single Hydrosweep-profile over the
Reykjanes Ridge Crest was run. The recorded bathymetric strip e allows a classification into two main segments . The souther n segment between the Charles Gibbs Fracture Zon e and the Bight Discontinuity i s a typical part o f the northern Mid Atlantic Ridge with rift mountains bounding a median valley. The Bight Discontinuity seem s to be a transition zon e between the
influenced an d non - influenced region s of the Icelan d
hotspot. From this transition zon e the second segmen t trend s 3 6° northeastward towar d Iceland . Oblique spreadin g and en echelon ridges, striking perpendicularly t o the spreading
direction, are typical. The overall structur e of the Reykjanes Ridge support s a mainly amagmatic-tectoni c evolutio n as
typical of episodic spreadin g models.
During th e second part sedimentatio n processes, includin g their spatial and temporal variability were investigate d i n the central part of the Reykjanes Ridg e at 59° 30'N. Along 1 0 lines seismoacousti c profilin g an d Hydrosweep - mapping were performed. First topographi c maps of the working area were
created on board. At seve n selected station s sedimen t sample s were take n using severa l samplin g devices. The firs t result s
suggest tha t th e sedimentation condition s ar e very complex.
They can be traced back t o five essential sedimentatio n processes. Apart fro m pelagic sedimentatio n of biogenic
carbonate, sedimentation o f sponge spicules plays an importan t role. Volcanic particles are attributed t o active submarin e volcanism i n the central Reykjanes Ridge . Additionally, inpu t of ice-rafted detritus i s recorded. However, it s percentage plays only a minor role i n the sediment. Moreover , the
sediments were partly superimpose d by strong bottom currents.
Physical properties o f the sediments were estimated at 6 sampling stations . They sho w high water contents, ranging fro m 120% to 150% with som e peaks up to 200%. The average shea r strength i s about 1 0 kPa with maximum values up to 20 kPa mainly a t horizons where sponge needles are accumulated. No clear increas e or decrease i n the data with depth was
observed, neither i n water content nor i n shear strength . This
59 suggests tha t the sediments are very weakly consolidated ,
The benthic faun a (macrofaun a and meiofauna) in terms of species composition, biomass and abundance were
investigated a t the sampling sites . Fine-grained, sof t surfac e sediments are characterized b y an abundance of globigerinoi d and pteropod mollusc shell s together with numerous calcareou s sponge spicules. This bottom typ e could indicat e rather slo w currents and rapid sedimentation of both planktic an d benthic, resuspended material. Hard bottom had rich epifauna, with
ophiuroids, sponges , hydroids and polychaete tube s
5. Acknowledgements
The S082 project was financiall y supporte d by the
"Federal Minister fo r Research and Technology". We acknowledg e the efforts of Dr.G.Ehrlich and Dr.B.Steingrobe fro m PLR
Julich during preparation and performance of the cruise.
Furthermore we thank masters and crew of RV"Sonne" fo r their assistance during cruis e S082 . Participatio n o f scientist s of the Universities o f Reykjavik, Bremen, Greifswald, Hamburg , Kiel and Rostock as well a s from Geomar Kiel and the Institut e of Baltic Se a Research Warnemiinde i s gratefully acknowledged . Special thank s are due to Dr.F.Kogler and Dr.F.Werner, Kiel University, fo r supporting th e sediment-physica l
investigations. We also thank Dr.T.Floden, Stockhol m University, fo r critically readin g the manuscript an d C.Hoffmann fo r correcting th e English text.
6. References
ALMAGOR,G. (1979) : A review: Marine geotechnical studie s at continental margins.- Geol. Surve y o f Israe l Report MG/79/3:
1-100.
BALLARD,R.D.& van ANDEL,T.H.(1977): Morphology an d tectonic s of the inner rift valley a t lat 36°50'N on the Mid-Atlantic Ridge, Bull. Geol. Soc . Am. 88 , 507 - 530, 1977
BEIERSDORF, H. et. al.(1993) : F S Sonne, Benutzerhandbuch, 2.
Auflage . Bundesanstalt fu r Geowissenschaften und Rohstoffe, 125 S.
BISHOP,A.W.& BJERRUM,L.B. (1961) : Bedeutung und Anwendbarkei t des Dreiachsialversuchs fu r die Losung von Standsicherheits-aufgaben.- Norges Geotekniske Inst . Publ. 43: 61 S..
BLANZ, T. (1992) : Sedimentologische un d sedimentphysikalisch e Eigenschaften holozaner Sedimentkern e au s der Alkor- und der Littorina-Tiefe /nordliches Kattegat.- Meyniana 44: 75-95, Kiel.
CRAINE,K.et al.(1992) : Investigatin g th e Reykjanes Ridge near 59°50'N with the Russian Mir Submersibles, EOS, Vol. 73, p530 EKDALE, A., BROMLEY, R.G. & Pemberton, S.G. (1984) : ICHNOLOG Y - The Use of Trace Fossils i n Sedimentology an d Stratigraphy . - Society o f Economic Paleontologist s an d Mineralogists,
Tulsa, Oklahoma, 15, 317 p.
FLEISCHER,U.(1974): The Reykjanes Ridge: A summary of
geophysical data , in: Geodynamics of Iceland and the North Atlantic Area, ed. by L. Kristjansson, NATO Advanced Stud y
Institute, D. Reidel, Hingham, Mass, 1 7 - 33,
JACOBY,W.R.& GIRARDIN,N. (1980) : The evolution of the
lithosphere at the southeast flan k of the Reykjanes Ridge, J.
Geophys. 47 , 271 - 277,
JOHNSON,G.L.& JAKOBSSON,S.P. (1985) : Structure and petrology of the Reykjanes Ridge between 62°55' N and 63°48'N , J. Geophys.
Res. 90 , 10073 - 10083,
GENTE,P.(1987) Etude Morphostructurale Comparativ e des Dorsales Oceanique a Taux d'Expansion Varies, in These de Doctorat, Univ. de Bretagne Occidentale, Brest, France,
KAPPEL,E.S.& RYAN,W.B.F.(1986): Volcanic episodicit y an d a non steady stat e rift valley alon g northeast pacific spreadin g centers: Evidence fro m Sea MARK I , J. Geophys. Res. 91, 13 92 5 - 13940 ,
KASSENS, H. (1990) : Verfestigte Sedimentlage n und seismisch e Reflektoren: Fruhdiagenese und Palaoozeanographie i n der Norwegischen See. - Berichte SF B 313, Uni. Kiel 24: 117 S..
LACKSCHEWITZ, K.S. (1991 ) : Sedimentationsprozesse am aktiven mittelozeanischen Kolbeinsey Riicke n (nordlic h von Island) . -GEOMAR Report 9 , 1-133 .
LAUGHTON,A.S.; SEARLE,R.C.& ROBERTS,D.G.(1979): The Reykjanes Ridge Crest and it s transition between it s rifted and non-rifted regions, Tectonophysics 55 , 173 - 177,
LEWIS,B.R.T.(1979): Periodicitie s i n volcanism and
longitudinal magma flo w on the East Pacifi c Rise at 23°N, Geophys. Res. Lett. 6, 753 - 756,
MACDONALD,K.C.& FOX,P.J.(1990): The Mid-Ocean Ridge. Sci.Am.
6, 42 - 49,
NISHIMURA,C.E. et al.(1989): Investigatio n o f a possible underwater volcanic eruptio n on the Reykjanes Ridge by
61 airborne sonobuoy s and AXBT's, EOS, Vol. 70, pl301,
RICHARDS, A. F. (1961):Investigation s of deep-sea sedimen t cores, part 1 : Shear strengh bearing capacit y an d
consolidation. - U.S. Navy Hydrographic Office , Technical Report 63 , Washington D.C.: 7 0 S.
RICHARDS, A. F. (1962) : Investigation s o f deep-sea sedimen t cores, part 2 : Mass physical properties.- U.S. navy
Hydrographic Office. Technical Report 106, Washington D.C. : 145 S. .
SEMPERE,J.-C.; PURDY,G.M. & SCHOUTEN,H. (1990) : Segmentatio n of the Mid-Atlantic Ridg e between 24°N and 30°40'N . - Nature, Vol 344, 427-431
SHOR, A.W. & POORE, R.Z. (1979) : Bottom Currents an d Ic e Rafting i n the North Atlantic. - In: LUYENDYK, B.P., CANN, J.R., e t al., Init . Repts. DSDP 49, 859-872.
SKEMPTON, A. W. (1970) : The consolidation o f clay by
gravitational.- Quat. Journ. Geol. Soc . London 125: 373-412.
SMITH,D.K.& CANN,J.R.(1992): The role of seamoun t volcanism in crustal constructio n at the Mid - Atlantic Ridg e (2 4 - 30°N), J. Geophys. Res. 97, 1645 - 1658,
SPIES, V.(1993):Digitale Sedimentechographie - Neue Wege zu einer hochauflosenden Akustostratigraphie. , Habil.-schrift Univ. Bremen,
TALWANI,M.;WINDISCH,C.& LANSETH,M.(1971): The Reykjanes Ridg e Crest: A detailed geophysical study , J. Geophys. Res. 76, 473 - 517,