Forschung

PALAEOENVIRONMENTAL IMPLICATIONS FROM TERTIARY SEDIMENTS OF KAINAN MARU SEAMOUNT AND NORTHERN GUNNERUS RIDGE

Prof. Dr. Werner Ehrmann

Dr. Claus-Dieter Hillenbrand
Alfred Wegener Institute for Polar and Marine Research
P.O. Box 12 01 61, D-27515 Bremerhaven, Germany

Scientific Background:

This project aimed in the reconstruction of the Antarctic history during the Tertiary by analyzing sediments from Kainan Maru Seamount and northern Gunnerus Ridge, southernmost Indian Ocean (Fig. 1). We investigated the composition of the sediments in order to reconstruct the palaeoclimate, the glacial history of Antarctica, the palaeo-oceanography, the transport processes and the depositional history. The working area is close enough to Antarctica to document all processes in the sedimentary record. However, it is distant enough to enable dating of the sediments by biostratigraphic and palaeomagnetic means.

Investigations:

For this study, we investigated Neogene and Oligocene sedimentary sections that were sampled by gravity and piston coring during cruise ANT-VIII/6 of the RV "Polarstern". The sediments crop out along erosional features at Kainan Maru Seamount and northern Gunnerus Ridge. The investigated sediments cover the time intervals 30.1-29.0 Ma (early Oligocene), 14.1-12.8 Ma (middle Miocene), 8.8-6.5 Ma (late Miocene) and 5.1-2.7 Ma (Pliocene). Main emphasis was given to the generation of grain size and clay mineral data.

Fig. 1: a The study area in the broader context of Antarctica. Ice shelves are shown in white, prominent submarine topographic features in gray. Schematic circulation patterns of surface waters (black arrows) and of deep and bottom waters (white arrows) after Orsi et al. (1995, 1999), Whitworth et al. (1998) and Schröder & Fahrbach (1999). AIS/LG: Amery Ice Shelf and Lambert Glacier, AR: Astrid Ridge, GR: Gunnerus Ridge, KMS: Kainan Maru Seamount, KN: Kapp Norvegia, KP: Kerguelen Plateau, MR: Maud Rise, PB: Prydz Bay, RIP: Riiser-Larsen Peninsula.
b Detail map of Gunnerus Ridge and Kainan Maru Seamount with the locations of the investigated sediment cores. Bathymetric contours are taken from Gersonde et al. (1998).

First Results:

(1) Our study showed that the sediments of Kainan Maru Seamount and Gunnerus Ridge are suitable for reconstructing the Cenozoic palaeoenvironment, palaeoclimate and glacial history of Antarctica. The sediments cropping out at erosional features are therefore promising targets for further sediment sampling without the need for a drilling ship.

(2) The sediments are dominated by siliceous oozes and siliceous muds implying deposition south of the Polar Front.

(3) Icebergs reached the area throughout the time represented by the investigated cores. However, the intensity of ice-rafting was very variable.

(4) The clay mineral assemblages are dominated by illite and smectite, whereas chlorite and kaolinite occur only in traces. Illite and chlorite are derived from physical weathering of an East Antarctic source. Smectite and kaolinite probably are derived from the erosion of Cenozoic or Mesozoic shelf sediments. Kaolinite also might be derived from weathering of Permo-Triassic rocks in the hinterland of Prydz Bay.

(5) The early Oligocene sediments (30.1-29.0 Ma) indicate that surface water temperatures were high enough to allow biosiliceous production and to make most icebergs melt south of the area. Palaeoenvironmental conditions on Kainan Maru Seamount obviously were quite different from those on Maud Rise and Kerguelen Plateau, which are in a similar position. This leads to the assumption that the Southern Ocean was subdivided into different zones.

(6) The sediments of the middle Miocene time interval 14.1-13.5 Ma point to similar palaeoenvironmental conditions as those of the early Oligocene. The diatom oozes deposited 13.5-13.1 Ma indicate relative warm conditions with perennial siliceous phytoplankton production and less glacial reworking of smectie-enriched shelf sediments. The 13.1-12.8 Ma old sediments document an intensification of East Antarctic glaciation resulting in high concentrations of IRD.

(7) Compared to the older sediments, the late Miocene sediments (8.7-6.5 Ma) have higher contents of poorly crystalline and biotite-like illites and of IRD and silt. This change indicates a continued intensification of the glaciation. The strong fluctuations in the sedimentological parameters point to a much more dynamic ice sheet.

(8) The Pliocene sediments (5.1-2.7 Ma) show an even more pronounced cyclic sedimentation mirroring glacial and interglacial conditions. The duration of one cycle is about 400 ka.

Publication:

Hillenbrand, C.-D. & Ehrmann, W. (2003): Palaeoenvironmental Implications of Tertiary Sediments from Kainan Maru Seamount and Northern Gunnerus Ridge. - Antarctic Science, 15 (4): 522-536.