The Southeast Greenland Transect (Leg 152) was drilled from September to November 1993 in order to gain information about (I) the nature and subsidence history of
the Seaward Dipping Reflector Sequence (SDRS), (II) the paleoceanographic and glaciation history of the southeast Greenland shelf and the Irminger Basin, and (III) the evolution of the breakup volcanism of the North Atlantic (Larsen et al., 1994). The sediments of the Irminger Basin probably record several important paleoceanographic events as e.g., the initial overflow of the North Atlantic Deep Water (NADW) and the onset of the northern hemisphere glaciation.
Site 918 is located on the upper continental rise, approximately 130 km off the east Greenland coast, in 1868.5 m of water1. The volcanic basement of the SDRS is overlain by about 1200 m of sediment, which span in age from Paleocene to recent. As already shown by shipboard analyses, total organic carbon contents of the sediments recovered at Site 918 are relatively low, ranging between 0.1 and 0.5 wt%. In this study, the major aim is to investigate the quantity and quality of organic matter in terms of a correlation of changes in the composition with changes in lithology, and thus, in paleoclimatic and paleoceanographic conditions. Several different methods were applied to study the sediments of Site 918: elemental analyses (total organic carbon and total nitrogen content), Rock Eval pyrolysis parameters, maceral microscopy, and specific biomarker extraction
(n-alkanes).
The recorded amount and composition of organic matter within the sediments of Site 918 very well mirror the long term changes in climatic and paleoenvironmental development of the southeast Greenland region. Paleoceanographic conditions are strongly dependant on the intensity of the cold southward directed East Greenland Current (Larsen et al., 1994), and thus, influence the accumulation and preservation of organic carbon. The discussed section (1108 to 0 mbsf) of Site 918 comprises three major lithological units:
During Eocene and Oligocene times (Lith. Unit III, ca. 1108 - 800 mbsf) relatively warm climatic conditions and shallow water depth (<1500 m) lead to the deposition of mainly terrigenous organic material derived from the surrounding coastal areas. Dense vegetation results in enhanced deposition of plant material confirmed by vitrinites and long chain alkanes. Finely dispersed wood pieces also indicate transport by rivers or occasionally driftwood. Marine production was probably very low during this time. No evidence for increased productivity could be found in the sediment parameters. Low sedimentation rates (Larsen et al., 1994) and oxic water column conditions may have caused an intense degradation of the labile marine organic matter (Stein, 1991).
In the early to late Miocene sediments of Lith. Unit II
(ca. 800-600 mbsf) increased organic carbon contents and amounts of planktonic organisms point to an enhanced surface water productivity. This is supported by microscopical findings (strong background fluorescence, lack of terrigenous macerals). The alkane distribution, hydrogen index values, and C/N-ratios, however, indicate a mixture of marine and terrigenous organic material. During this time interval a highly oxidizing water column (bioturbation)
and extremely low sedimentation rates support the diagenesis and remineralisation of the organic material. The depositional conditions are comparable to a hemipelagic oxic environment, marine dominated with a minor terrigenous organic input.
At about 5 Ma (ca. 500 mbsf) a dramatic change in
depositional conditions occurred at Site 918 (Larsen et al., 1994). The sedimentation rate of siliciclastic material increased to values up to 19.5 cm/ky and biogenic carbonate production almost terminated. Flux rates of organic carbon increased (mean values of about 0.7 g cm-2 ky-1) and the preservation of organic particles is moderate. Well preserved alginites sparsely occur. Climatic conditions turned to colder temperatures (increase in dropstone occurrence), the dense vegetation cover disappeared, and thus, terrigenous organic material is only a minor component of the organic sediment fraction. This is supported by alkane pattern, hydrogen index values and microscopical observations. A rapid burial of the organic matter may have led to a better preservation and higher flux rates during this time interval.
Larsen, H.C., Saunders, A.D., Clift, P.D. et al., Proc. ODP, Init. Repts., 152, 977 pp. (1994).
Stein, R., Lecture Notes in Earth Sciences 34, 217 pp. (Springer, 1991).