Fig. 6.4.4-4: Physical properties of selected cores taken on Lomonosov Ridge.
nents and corne, no doubt, frorn a continental source. Basin. Overflow currents rnight have winnowed the fine-grained, rnore easily suspended clay, but this process should be restricted to the low-lying saddles and should not cause a general coarsening.
It rnay be that these sedirnents represent the "true" glacio-marine sedirnent, transported to the Lornonosov Ridge frozen in icebergs and pack ice and undiluted by the rain of rnuddy to clayey turbidites which fill the basins. Further post-cruise work will be required to corne to sorne settled conclusion as to the origin of these sedirnents.
6.4.5 Sediments in the Makarov Basin (R. Stein and K. Moran)
The wedge-shaped Makarov Basin lies between the Lornonosov Ridge and the Alpha-Mendeleev-Ridge (Fig. 6.3-1) and cornprises two abyssal plains: (i) the Wrangel Abyssal Plain which is about 2,800 rn deep and adjoins the East Siberian shelf, and (ii) the Siberian Abyssal Plain which is about 4,000 rn deep and adjoins the Lornonosov Ridge. Both abyssal plains are connected via the Arlis Gap. The two geological stations (PS21 78 and PS2180; Fig. 6.3-3) visited during ARK-VIIIl3, were both in the Siberian Abyssal Plain in water depths of 4,005 rn and 4,009 rn, respectively. At these two stations, three box cores, one Set of rnulticores, two piston cores, and one kastenlot core were recovered. These are the first sedirnent cores ever recovered frorn this area.
Sediment compositon a n d lithostratigraphy
The surface-near sedirnents from the Makarov Basin are cornposed of an upperrnost 15-20 crn thick sequence of dark yellowish brown silty clay, underlain by a more grayish brown to olive brown clay. In the upperrnost sequence, the coarse fraction is characterized by the dorninance of planktic forarninifers (Neogloboqua- drina pachyderma), rare occurrence of benthic forarninifers, sponge spicules, and ostracodes, and rare to abundant occurrence of quartz and rare occurrence of rnica
and opaque rninerals. Increased arnounts of dropstones (quartz grains, terrigenous carbonate, and siltstones >1rnrn) were observed in the rniddle part of this sequence. In the underlain clay, only very small arnounts of planktonic forarninifers occur and the coarse fraction is clearly dominated by fine-grained quartz.
The sedirnents from the long cores are mainly composed of olive, olive brown to brown clay. Occasionally, rnore sandy intervals and fining-upwards sequences occur. Between about 1.8 and 5 rn sub-bottom depth, sequences of sandy rnud with frequent rnud clasts of 2-10 mrn in diarneter ("cottage cheese texture") were recorded. In the kastenlot Core PS2178-5 (2.1-2.4 m sub-bottorn depth) several large dropstones of up to 7 cm in diameter were found. Based on smear-slide estimates, clay rninerals and quartz are the dominant rnineral phases; heavy rninerals, opaques, rnicro-nodules, and volcanic glass occur in rninor arnounts (Fig.
6.4.5-1). Biogenic particles were not observed.
Fig. 6.4.5-1: Sediment texture of kastenlot Core PS2178-5 frorn srnear slide analyses.
In the uppermost 15-20 crn of the sedirnents from the Makarov Basin rare abundances of Emiliania huxleyi, Gephyrocapsa spp., and Coccolithus pelagicus were found. Below this depth, the sedirnent is barren of nannofossils and other rnicrofossils.
In the Makarov Basin Cores 2178-5 and 2180-2, rnagnetic susceptibility ranges from 150-350 X 10 E-6 SI (Fig. 6.4.5-2). Although the level is not very high the logs of both cores allow a reliable correlation which is confirmed by sediment colors.
The large peak in the lowerrnost section of Core PS2180-2 is caused by the core Cutter lamella which was torn off the core catcher during penetration.
2178-5 K A L 2180-2 G P C
Fig. 6.4.5-2: Magnetic susceptibility of cores taken in the Makarov Basin.
Physical Properties
Physical properties of sediment frorn the Makarov Basin can be characterized in two types of depositional environments: (i) rapidly deposited sediment which show no increase in strength with depth and (ii) normally consolidated sediment with increasing strength with depth. In both of these sediment types, physical properties also vary with input of coarse-grained sediment and mud clasts. These variations show up in the physical property record as an 'overprint' on the general trend. For exarnple, Core PS2180-2 shows the upper 4 rn of sediment with no increase in shear strength with depth, indicating that the sediment is likely rapidly deposited (Fig. 6.4.5-3). Within this unit, peaks of shear strength occur and are associated with sand and mud clasts. From 4-1 1 m below seafloor (mbsf) in Core PS2180-2, the sediment is generally normally consolidating with increasing shear strength with depth. However, large peaks in shear strength occur within this general trend at intervals where sand, silty sand and mud clasts are also deposited. From 11 m to
the bottorn of the core, shear strength decreases with depth. The acoustic velocity of Core PS21 80-2 is predorninantly low (1,490-1,520 rnlsec) which is characteristic of clay dorninated sedirnent. As with the shear strength profile, peaks in velocity are associated with grain size changes, specifically sand layers and intervals of rnud clasts.
Shcar Strength (kPa) Acoustic Velocity (m/s) 4 8 12 16 20 1400 1500 I600 1700 1800
Fig. 6.4.5-3: Physical properties of sedirnent Core 2180-2 in the Makarov Basin.
Depositional Environment
The sedirnents in the Makarov Basin are clearly dorninated by siliciclastic cornpo- nents such as quartz, clay rninerals, and rnica, indicating the continental origin of the sedirnent. Based On grain size distribution and sedirnent composition and texture, two rnajor sedirnent types with different transport rnechanisrns can be distinguished:
-
The occurrence of large dropstones as well as irregular-shaped mud clasts indicate an ice-rafted origin of these sedirnents. Several of the coarse-sand- sized quartz grains found in the coarse fraction of the surface-near sedirnents are well-rounded and highly-polished. This may suggest fluvial, current, or wave transport rnechanisms in coastal and shallow shelf areas before the sedirnent was incorporated into grounded sea ice or icebergs and transported into the Central Arctic Ocean.- The fining-upwards sequences with gradational changes frorn sandy rnud to clay are interpreted as distal turbidites. The source area of the sedirnent rnight be the surrounding ridges or continental shelf areas.
The abundant occurrence of the planktonic foraminifer N. pachyderma in the coarse fraction of the upperrnost sedirnents may indicate increased carbonate productivity
carbonate dissolution in the Makarov Basin.
More detailed sedirnentological and geochernical investigations as well as a rnuch better stratigraphic frarnework are absolutely necessary for a rnore detailed reconstruction of the origin of the different sedirnent types, their changes through time, and their paleoenvironrnental significance.
6.4.6 Sediments on the Morris Jesup Rise (R. Stein)
The Morris Jesup Rise is a rnore than 200 km broad aseismic plateau that trends northeastward frorn the northern coast of Greenland into the Amundsen Basin (Fig.
6.3-1). It has probably been generated in connection with the Yerrnak Plateau as a single oceanic plateau in the period between magnetic anomalies 18 and 13 (40- 34 Ma).
During the ARK-VIIIl3 cruise, coring operations were concentrated On two transects, one transect at the northeastern slope (Figs. 6.3-1, 6.3-4 and 6.4.6-1, sites PS21 98, PS2199, PS2200, and PS2201) and the other at the eastern slope of the Morris Jesup Rise (Figs. 6.3-1 and 6.3-4; sites PS2202, PS2203, and PS2204). A total of five box cores, four Sets of rnulticores, two piston cores, one kastenlot core, and four gravity cores were recovered in water depths between 1,070 and 3,900 m (Appendix 8.1). The long piston and kastenlot cores are all taken at shallow water depth.
Sediment Composition a n d Lithostratigraphy
The near surface sedirnents from the Morris Jesup Rise (at 1,080 rn water depth) are composed of an upperrnost 10 cm thick unit of dark brown silty clay, underlain by a more grayish brown to olive brown silty clay. In the uppermost interval, the coarse fraction is dorninated by planktonic foraminifers (Neogloboquadrina pachyderma). Benthic forarninifers, sponge spicules, bivalves, echinoderrns, serpels, pteropodes, and ostracodes are rare to comrnon. In one box corer (PS2201-I), the sedirnent surface was cornpletely covered by siliceous sponge spicules and single sponges of up to 20 crn in diameter. The siliciclastic cornponents in the coarse fraction of the upperrnost unit, cornrnon to abundant in occurrence, are rnainly quartz and very coarse-grained rock fragrnents (dropstones). The underlain silty clay is dorninated by siliciclastic cornponents, i.e.
quartz, rock fragments, and opaque rninerals. The rock fragrnents are very coarse- grained (dropstones) and cornposed of siltstones, terrigenous carbonates, and rnetarnorphites. Biogenic cornponents are restricted to rare to comrnon occurrence of planktic forarninifers; benthic foraminifers are of very rninor importance.
Furtherrnore, coal fragments were observed in this unit.
The sedirnents recovered in the long cores rnainly consist of (rhythmic?) alternations between light olive brown to brown sandy to silty mud and grayish brown to brown, often rnottled, sornetirnes silty, clay. Mud clasts and large dropstones frequently occur in the sandy rnud intervals. Based on srnear-slide
l
oozz
Fig. 6.4.6-1: Sketch of bathyrnetric section from Arnundsen Basin to Morris Jesup Rise. Sarnple locations are indicated by arrows.
reaching up to 80 % of the bulk sedirnent. Sand- and silt-sized material, however, rnay become a rnajor proportion of the sedirnent in several intervals (Fig. 6.4.6-2).
The silt and sand fractions are cornposed of quartz as the rnost irnportant cornponent, followed by terrigenous carbonates, feldspars, and opaques as less irnportant cornponents. Planktic forarninifers also occur
-
at least in rninor arnounts-
in the deeper part of the sedirnent sequences of Morris Jesup Rise (Fig. 6.4.6-2).
Magnetic Susceptibility
A rnean susceptibility of about 260 X 10 E-6 SI was detected for cores at the Morris Jesup Rise. Three cores taken On top of the rise (PS2200-1, PS2200-5, and PS2202-5) yielded alrnost identical Patterns, but there is no obvious correlation to Core PS2198-2 frorn the lower slope of the rise (Fig. 6.4.6-3).
Sedirnentary En vironrnent
The sediments at the Morris Jesup Rise are cornposed dorninantly by siliciclastic cornponents, i.e., rnainly clay rninerals and quartz as well as rock fragments. The cornmon occurrence of irregular-shaped rnud clasts and coarse-sand-sized rock fragrnents suggests a glacio-marine origin of these sedirnents. The large drop- stones occasionallv found in the seauence rnay indicate transport by icebergs. The dark gray terrigenous carbonates, rnetarnor- cornposition of t h e ' d r ~ ~ s t o n e s , rnainiy
0 20 40 60 80 100