Middle Eocene to early Miocene environmental changes in the sub-Antarctic Southern Ocean: evidence from biogenic and terrigenous depositional patterns at ODP Site 1090

Abstract

During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved middle Eocene to lower Miocene sediment record was recovered at Site 1090 on the Agulhas Ridge in the Atlantic sector of the Southern Ocean. This new sediment record shows evidence of a hitherto unknown late Eocene opal pulse. Lithological variations, compositional data, mass-accumulation rates of biogenic and lithogenic sediment constituents, grain-size distributions, geochemistry, and clay mineralogy are used to gain insights into mid-Cenozoic environmental changes and to explore the circumstances of the late Eocene opal pulse in terms of reorganizations in ocean circulation.

The base of the section is composed of middle Eocene nannofossil oozes mixed with red clays enriched in authigenic clinoptilolite and smectite, deposited at low sedimentation rates (≤2 cm ka⁻¹). It indicates reduced terrigenous sediment input and moderate biological productivity during this preglacial warm climatic stage. The basal strata are overlain by an extended succession (100 m, 4 cm ka⁻¹) of biosiliceous oozes and muds, comprising the upper middle Eocene, the entire late Eocene, and the lowermost early Oligocene. The opal pulse occurred between 37.5 and 33.5 Ma and documents the development of upwelling cells along topographic highs, and the utilization of a marine nutrient- and silica reservoir established during the pre-late Eocene through enhanced submarine hydrothermal activity and the introduction of terrigenous solutions from chemical weathering on adjacent continents. This palaeoceanographic overturn probably was initiated through the onset of increased meridional ocean circulation, caused by the diversion of the Indian equatorial current to the south. The opal pulse was accompanied by increased influxes of terrigenous detritus from southern African sources (illite), mediated by enhanced ocean particle advection in response to modified ocean circulation.

The opal pulse ended because of frontal shifts to the south around the Eocene/Oligocene boundary, possibly in response to the opening of the Drake Passage and the incipient establishment of the Antarctic Circumpolar Current. Condensed sediments and a hiatus within the early Oligocene part of the section possibly point to an invigoration of the deep-reaching Antarctic Circumpolar Current. The mid-Oligocene to lower Miocene section on long time scale exhibits less pronounced lithological variations than the older section and points to relatively stable palaeoceanographic conditions after the dramatic changes in the late Eocene to early Oligocene.

Introduction

In terms of Cenozoic climate and environmental history, the Eocene–Oligocene period has attracted palaeoclimatic research, as this time went along with climate deterioration, initial Antarctic glaciation, and the development of the circum-Antarctic Southern Ocean. From the present knowledge, inferred from terrestrial archives and sediment records on the Antarctic shelves, ephemeral ice masses possibly established during the late Eocene Abreu and Anderson, 1998, Barker et al., 1999, Barrett, 1999. The Eocene/Oligocene boundary marks an abrupt cooling step that was associated with the first significant establishment of permanent Antarctic ice sheets and enhanced formation of cool deep-water masses along Antarctica Lear et al., 2000, Zachos et al., 2001.

Changes in palaeocenography were moreover mediated through reorganizations in southern hemispheric ocean circulation. Since the late Eocene the diversion of the Indian equatorial current through the progressive northward motion of the Indian plate and the restriction of Mediterranean basins gave rise to meridional current patterns, which affected the current systems of the southern Indian Ocean and the southeastern South Atlantic (Lawver and Gahagan, 1998). At the earliest since the early Oligocene, continental drift led to the opening of deep sea conduits between Antarctica and South America (Drake Passage) and between Antarctica and Australia (Tasmanian Gateway), promoting the thermal isolation of Antarctica Lawver and Gahagan, 1998, Barker, 2001, Exon et al., 2001. The exact timing of the latter tectonic processes, however, still is a matter of controversial debate.

Important archives of Eocene–Oligocene environmental change occur in the form of marine deposits encountered around Antarctica in the Southern Ocean that bear environmental signals recorded by the biogenic and terrigenous sediment components Kennett and Barker, 1990, Barron et al., 1991, Ehrmann and Mackensen, 1992, Diester-Haass et al., 1993, Lazarus and Caulet, 1993, Thomas and Gooday, 1996. During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved and almost complete and undisturbed succession of middle Eocene to lower Miocene marine deposits was recovered at Site 1090 in the southeastern Atlantic sector of the sub-Antarctic Southern Ocean (Gersonde et al., 1999) (Fig. 1). As a unique feature, which so far was not recognized in its dimension in the Southern Ocean, it includes a ∼100-m-thick succession of upper middle Eocene to lower Oligocene diatomaceous oozes, which apparently is linked with late Eocene climate cooling prior to the onset of extended Antarctic glaciation. This observation is of particular interest, because biosiliceous blooms also punctuated global cooling events and changes in ocean circulation during former periods in earth history, as during the late Devonian and at the Cretaceous/Tertiary boundary (Racki, 1999). Another example is a late Pliocene opal pulse in the Benguela upwelling area off southwestern Africa, which preceded the late Cenozoic cooling step Lange et al., 1999, Berger et al., 2002.

The goals of this paper are to gain new insights into environmental changes during the middle Eocene to early Miocene and to explore the circumstances of the late Eocene opal pulse in the sub-Antarctic South Atlantic in terms of climate change and reorganizations in ocean circulation. Our interpretations are based on the survey of the depositional environment and compositional variations of the biogenic and lithogenic sediment fractions of the Site 1090 record.