Open Session - SWISS GEOSCIENCE MEETINGs

8
Symposium 2: Mineralogy-Petrology-Geochemistry
2.
Geochemistry of the rift valley sediments at the Arctic Mid-Ocean Ridge:
Preliminary results of the H2DEEP expedition to the southern Knipovich
Ridge at 3°N
Baumberger Tamara*, Früh-Green Gretchen L.*, Pedersen Rolf B.**, Thorseth Ingunn H.**, Bernasconi Stefano M.*** & Plötze
Michael****
*Institute for Mineralogy and Petrology, ETH Zurich, Clausiusstrasse 25, CH-8092 Zurich (tamara.baumberger@erdw.ethz.ch)
**Centre for Geobiology, University of Bergen, Allegaten 41, N-5007 Bergen
***Geological Institute, ETH Zurich, Universitätsstrasse 16, CH-8092 Zurich
****Laboratory of Clay Mineralogy, IGT, ETH Zurich, CH-8092 Zurich
The southern Knipovich Ridge (SKR, 73°N, 8°E) in the Norwegian-Greenland Sea is a permanently ice-free arctic ridge segment
with an effective spreading rate of only 6 mm/year and is thus one of the most slow-spreading segments of the global
ridge system. This ultra-slow spreading ridge is characterised by magmatic and amagmatic accretionary ridge segments and
a partly sediment-covered rift valley, reaching water depths from 2000-2500 metres to 3500 metres in the deepest area. These
sediments likely represent an important hydrological and thermal boundary to heat and fluid flow and provide a broad
archive of the tectonic and alteration history. H2DEEP is an interdisciplinary, international project aimed at studying geodynamic
and hydrothermal processes and their links to the deep hydrogen-based biosphere along the SKR. Our individual
project of H2DEEP focuses on understanding water-rock interactions and the links between chemical and microbiological
processes associated with alteration of a sediment-covered oceanic ridge and with serpentinisation in ultra-slow spreading
environments.
Here we present preliminary results of mineralogical and geochemical investigations of the rift valley sediments obtained
by gravity cores of the upper 4 metres of the sedimentary cover in the summer of 2007. The rift valley sediments are primarily
characterised by hemipelagic sedimentation interrupted by turbiditic events. Visual observations identified glass- and
iron-rich layers representing previous volcanic and probably hydrothermal activity. Radiographic analyses show a range of
manganese-rich layers likely indicating hydrothermal events in the past. Powder x-ray diffraction investigations of bulk samples
indicate that most of the minerals are detrital in origin. The most common mineral in the clay fraction is smectite,
presumably derived from an increased volcanic activity in the investigated area. In one core, clay minerals of possible authigenic
origin were identified in a layer at ≈2 metre depth. Carbon geochemistry of the sediments shows elevated total organic
carbon contents, and carbon isotope compositions of total carbon reflect mixing of organic carbon with marine carbonates.
Increased inorganic carbon contents in the upper sediment layers are associated with the presence of foraminifera. A significant
increase in alkalinity and dissolved inorganic carbon content with sediment depth was detected in geochemical pore
water profiles in the sediments from the deeper area of the rift valley. The corresponding δ 13 CDIC values decline remarkably
with increasing alkalinity and dissolved inorganic carbon contents. Our preliminary mineralogical and geochemical studies
of the rift valley sediments point to an active history with various volcanic and hydrothermal events affecting the investigated
area over the past twelve to twenty thousand years.
REFERENCES
Centre for Geobiology of the University of Bergen, Norway
Institute for Mineralogy and Petrology of the ETH Zurich, Switzerland
Marine Geology and Geochemistry Group