Bellingshausen Sea

NEOGENE SEDIMENTATION AND CLIMATE HISTORY, NORTHERN BELLINGSHAUSEN SEA, ANTARCTIC PENINSULA
(Ocean Drilling Program, Leg 178, Sites 1095 and 1096)

Prof. Dr. Werner Ehrmann

Dr. Claus-Dieter Hillenbrand
Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven

Scientific Background:

The recent Antarctic ice sheet plays an important role in the global climate system by influencing the atmospheric and oceanic circulation of our planet. Very little, however, is known about the development of the Neogene Antarctic ice sheet and its impact on the environment. Until now, the Neogene glacial and climatic history of Antarctica has been deduced mainly from proxy data obtained in low latitudes, such as oxygen isotopes, which were measured on benthic foraminifers recovered from well dated and complete marine sedimentary sequences in the pelagic sea. However, the isotope data are ambiguous, because they record both the volume of the ice sheets and the temperature of seawater.

The behavior of the Cenozoic Antarctic ice sheet can best be studied from sediments drilled in proximal glaciomarine settings, because there direct evidence exists for ice advances and retreats through time. Such settings have been drilled in Prydz Bay and in the Ross Sea. However, core recovery on the Prydz Bay shelf was very poor, dating was problematic for all cores from the shelf areas, and all proximal sequences show evidence for long and repeated hiatuses.

Fig. 1: Location map of ODP Sites 1095 and 1096 and Core PS1565 west of the Antarctic Peninsula and clay mineral assemblages in surface sediments west of the Antarctic Peninsula based on data by Petschick et al. (1996) and Diekmann et al. (2000). For Site 1096 and gravity core PS1565 the clay mineral assemblages of the uppermost core sediments (0-1 cm below seafloor) are also shown. For Site 1097 the mean clay mineral composition of all investigated samples is given. The clay mineral assemblage on the shelf in the northeast is characterized by high smectite contents (smectite province), whereas shelf sediments farther in the southwest contain more chlorite and illite (chlorite-illite province). Surface sediments at the continental rise offshore from the chlorite-illite province are characterized by enhanced smectite contents in respect to the deposits on the adjacent shelf.

Scientific Objectives:

The main objectives of Leg 178 of the Ocean Drilling Program (ODP) were to recover and to investigate complete high-resolution Neogene to Quaternary sedimentary sequences at the Antarctic continental margin and, based on these studies, to reconstruct the evolution of the Antarctic ice sheet and its influence on the global climate system over the past ~10 m.y. The target area was the Bellingshausen Sea, west of the Antarctic Peninsula (Fig. 1).

Our studies contribute to the understanding of the glacial and climatic history by focusing on the clay mineral assemblages in sediments recovered at ODP Sites 1095 and 1096 (Fig. 1). We used the clay minerals to reconstruct the provenance and transport paths of the sediments in response to the glacial dynamics in the Antarctic Peninsula region during both the present and late Neogene to Quaternary times.

Fig. 2: Ternary plot of the clay mineral composition of the 192 investigated samples from Site 1095 and the 119 investigated samples from Site 1096. Kaolinite occurs only in trace amounts, and was added to the chlorite content. Clay mineral fluctuations at Sites 1095 and 1096 alternate between two endmember assemblages. One assemblage is characterized by <20% smectite and >40% chlorite. The second assemblage has >20% smectite and <40% chlorite.

Results:

1. Clay mineral assemblages in surficial continental margin sediments west of the Antarctic Peninsula consist of smectite, chlorite, and illite, with kaolinite occurring in trace amounts only. High smectite concentrations typify the shelf sediments off northern Graham Land, whereas chlorite and illite dominate in shelf deposits offshore from the southern Antarctic Peninsula. At the continental rise a bottom current transports smectite supplied from the northern Antarctic Peninsula to the southwest (Fig. 1).

2. An upper Quaternary sediment sequence from site PS1565 comprises an interglacial clay mineral assemblage with high smectite amounts and a glacial assemblage characterized by enhanced chlorite concentrations. During glacial periods, chlorite-enriched detritus was supplied to the continental rise by gravitational downslope processes triggered by the advance of grounded ice streams to the shelf break.

3. Upper Miocene to Quaternary sediments recovered at ODP Leg 178 continental rise Sites 1095 and 1096 exhibit clay mineral fluctuations alternating between two endmember assemblages. One assemblage is characterized by <20% smectite and >40% chlorite. The other assemblage has >20% smectite and <40% chlorite. The clay mineral fluctuations are similar to those observed at site PS1565. The short-term changes in clay mineral composition at Sites 1095 and 1096 we ascribe to repeated ice advances and retreats across the shelf west of the Antarctic Peninsula, reflecting glacial-interglacial cyclicity (Fig. 2). We conclude that oscillations in Antarctic ice volume may have influenced the global climate already during the late Miocene.

4. Only slight long-term changes are observed in the clay mineral assemblages deposited at ODP Sites 1095 and 1096. A slight enhancement of smectite contents between ~7.3 Ma and ~5.3 Ma at Site 1095 may be a consequence of a higher smectite supply caused by an intensification of volcanism on the South Shetland Islands and by a strengthening of glacial erosion in northern Graham Land in response to local uplift associated with ridge crest?trench collision. An increase of illite at Site 1096 between ~1.5 Ma and ~0.2 Ma is probably caused by changes in the supply of glacial debris from different source areas on the southern Antarctic Peninsula.

5. The clay mineral assemblages deposited at Sites 1095 and 1096 point to an onset of vast glaciation in the Antarctic Peninsula region before 9 m.y., but give no evidence for major deglaciation events since then.

The results have been summarized in two papers:

Hillenbrand, C.-D. und Ehrmann, W. (2001): Distribution of Clay Minerals in Drift Sediments on the Continental Rise West of the Antarctic Peninsula, ODP Leg 178, Sites 1095 and 1096. - In: Barker, P.F., Camerlenghi, A., Acton, G.D. & Ramsay, A.T.S. (Eds.): Proc. ODP, Sci. Results, 178: 1-29 (PDF-Format)

Hillenbrand, C.-D. & Ehrmann, W. (2005): Late Neogene to Quaternary environmental changes in the Antarctic Peninsula region: Evidence from drift sediments. - Global and Planetary Change, 45: 165-191.

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