Deep-sea drilling in the Antarctic region (Deep-Sea Drilling Project legs 28, 29, 35, and 36) has provided many new data about the development of circum-Antarctic circulation and the closely related glacial evolution of Antarctica. The Antarctic continent has been in a high-latitude position since the middle to late Mesozoic. Glaciation commenced much later, in the middle Tertiary, demonstrating that near-polar position is not sufficient for glacial development. Instead, continental glaciation developed as the present-day Southern Ocean circulation system became established when obstructing land masses moved aside. During the Paleocene (t = ∼65 to 55 m.y. ago), Australia and Antarctica were joined. In the early Eocene (t = ∼55 m.y. ago), Australia began to drift northward from Antarctica, forming an ocean, although circum-Antarctic flow was blocked by the continental South Tasman Rise and Tasmania. During the Eocene (t = 55 to 38 m.y. ago) the Southern Ocean was relatively warm and the continent largely nonglaciated. Cool temperate vegetation existed in some regions. By the late Eocene (t = ∼39 m.y. ago) a shallow water connection had developed between the southern Indian and Pacific oceans over the South Tasman Rise. The first major climatic-glacial threshold was crossed 38 m.y. ago near the Eocene-Oligocene boundary, when substantial Antarctic sea ice began to form. This resulted in a rapid temperature drop in bottom waters of about 5°C and a major crisis in deep-sea faunas. Thermohaline oceanic circulation was initiated at this time much like that of the present day. The resulting change in climatic regime increased bottom water activity over wide areas of the deep ocean basins, creating much sediment erosion, especially in western parts of oceans. A major (∼2000 m) and apparently rapid deepening also occurred in the calcium carbonate compensation depth (CCD). This climatic threshold was crossed as a result of the gradual isolation of Antarctica from Australia and perhaps the opening of the Drake Passage. During the Oligocene (t = 38 to 22 m.y. ago), widespread glaciation probably occurred throughout Antarctica, although no ice cap existed. By the middle to late Oligocene (t = ∼30 to 25 m.y. ago), deep-seated circum-Antarctic flow had developed south of the South Tasman Rise, as this had separated sufficiently from Victoria Land, Antarctica. Major reorganization resulted in southern hemisphere deep-sea sediment distribution patterns. The next principal climatic threshold was crossed during the middle Miocene (t = 14 to 11 m.y. ago) when the Antarctic ice cap formed. This occurred at about the time of closure of the Australian-Indonesian deep-sea passage. During the early Miocene, calcareous biogenic sediments began to be displaced northward by siliceous biogenic sediments with higher rates of sedimentation reflecting the beginning of circulation related to the development of the Antarctic Convergence. Since the middle Miocene the East Antarctic ice cap has remained a semipermanent feature exhibiting some changes in volume. The most important of these occurred during the latest Miocene (t = ∼5 m.y. ago) when ice volumes increased beyond those of the present day. This event was related to global climatic cooling, a rapid northward movement of about 300 km of the Antarctic Convergence, and a eustatic sea level drop that may have been partly responsible for the isolation of the Mediterranean basin. Northern hemisphere ice sheet development began about 2.5–3 m.y. ago, representing the next major global climatic threshold, and was followed by the well-known major oscillations in northern ice sheets. In the Southern Ocean the Quaternary marks a peak in activity of oceanic circulation as reflected by widespread deep-sea erosion, very high biogenic productivity at the Antarctic Convergence and resulting high rates of biogenic sedimentation, and maximum northward distribution of ice-rafted debris.

Cenozoic evolution of Antarctic glaciation the Circum-Antarctic Ocean and their impact on global paleoceanography

Oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on Antarctica since the earliest Oligocene (circa 35 Ma). The growth and decay of ice sheets during the Oligocene to modern “ice house world” caused glacioeustatic sea level changes. The early Eocene was an ice-free “greenhouse world,” but it is not clear if ice sheets existed during the middle to late Eocene “doubt house world.” Benthic foraminiferal δ18O records place limits on the history of glaciation, suggesting the presence of ice sheets at least intermittently since the earliest Oligocene. The best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic δ18O records. Although planktonic isotope records from the western equatorial regions are limited, subtropical planktonic foraminifera may also record such ice volume changes. It is difficult to apply these established principles to the Cenozoic δ18O record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events. We improved Oligocene to Miocene correlations of δ18O records and erected eight oxygen isotope zones (Oi1-Oi2, Mi1-Mi6). Benthic foraminiferal δ18O increases which are associated with the bases of Zones Oil (circa 35.8 Ma), Oi2 (circa 32.5 Ma), and Mil (circa 23.5 Ma) can be linked with δ18O increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near Antarctica. Our new correlations of middle Miocene benthic and western equatorial planktonic δ18O records show remarkable agreement in timing and amplitude. We interpret benthic-planktonic covariance to reflect substantial ice volume increases near the bases of Zones Mi2 (circa 16.1 Ma), Mi3 (circa 13.6 Ma), and possibly Mi5 (circa 11.3 Ma). Possible glacioeustatic lowerings are associated with the δ18O increases which culminated with the bases of Zone Mi4 (circa 12.6 Ma) and Mi6 (circa 9.6 Ma), although low-latitude planktonic δ18O records are required to test this. These inferred glacioeustatic lowerings can be linked to seismic and rock disconformities. For example, we link 12 Oligocene-early late Miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries (= inferred eustatic lowerings) of Haq et al. (1987).

Unlocking the Ice House: Oligocene‐Miocene oxygen isotopes, eustasy, and margin erosion

Summary

All available ship and aeroplane tracks across the Indian Ocean were searched for identifiable magnetic anomalies and transform faults, and hence the age and direction of motion at the time of formation of about two-thirds of the floor of the ocean established. The magnetic lineations show that India moved away from Antarctica at about 18 cm/y for 20 My in the Early Tertiary. This rapid motion ceased in the Eocene and was followed by a period in which little or no spreading took place west of the Ninety East Ridge. Australia separated from Antarctica during this period. The present spreading episode began about 36 My ago. This detailed study has permitted instantaneous poles of rotation to be obtained, and has established that Africa is now moving northward at 2cm/y relative to Antarctica in the South West Indian Ocean. The evolution of the triple junction between the South East, South West and Central Indian Ridges is clearly reflected in the topography and magnetic lineations. The depth of parts of the ocean formed since the Late Cretaceous increases with age in the manner expected from the temperature structure of a cooling plate, and supports the evolution determined from the magnetic lineations in a most remarkable way. Heat flow observations are more scattered but also consistent with the same thermal model. The proposed evolution agrees with the distribution of known continental fragments and with the Late Cretaceous palaeomagnetic poles from surrounding continents and one obtained from the shape of the magnetic lineations south of India. It is, however, not yet clear how to reconstruct Gondwanaland from the Late Cretaceous reconstructions.

The Evolution of the Indian Ocean since the Late Cretaceous

The equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals: late Paleocene, early Eocene, early middle Eocene, late Eocene, and early Oligocene. Paleotemperatures are calculated using standard oxygen isotope/temperature equations with adjustments to account for (1) variations in sea water δ18O related to changes in global ice volume over time and (2) latitudinal gradients in surface water δ18O. These reconstructions indicate that sea-surface temperatures (SST) of the Southern Oceans in the early Eocene were as high as 15°C, whereas temperatures during the late Paleocene and early middle Eocene reached maximum levels of 10°–12°C. By the late Eocene and early Oligocene high latitude SST had declined to 6 and 4°C, respectively. For most of the early Paleogene, low latitude sub-tropical temperatures remained constant and well within the range of Holocene temperatures (24°ndash;25°C) but by the late Eocene and early Oligocene declined to values in the range of 18° to 22°C. The late Paleogene apparent decline in tropical temperatures, however, might be artificial because of dissolution of near-surface foraminifera tests which biased sediment assemblages toward deeper-dwelling foraminifera. Moreover, according to recent plate reconstructions, it appears that the majority of sites upon which the late Eocene and early Oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling. Global deepwater temperature on average paralleled southern ocean SST for most of the Paleogene. We speculate based on the overall timing and character of marine sea surface temperature variation during the Paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high-latitude warmth of the early Eocene.

Evolution of Early Cenozoic marine temperatures

The History of the Earth's Surface Temperature During the Past 100 Million Years

Cenozoic deep-sea sedimentation in the southwest Pacific area was controlled by large changes in the patterns of bottom-water circulation and erosion. The circulation patterns were largely controlled by the development of the Circum-Antarctic Current south of Australia. Development of the Circum-Antarctic Current did not occur until the middle to late Oligocene when final separation occurred south of the South Tasman Rise, although initial sea-floor spreading between Australia and Antarctica commenced in the late early Eocene. Before the late Oligocene an erosive western boundary current flowed northwards through the Tasman and Coral Sea areas creating a regional unconformity centered near the EoceneOligocene boundary (Leg 21). When circum-Antarctic flow was established in the late Oligocene, a regional Neogene unconformity formed south of Australia and New Zealand, and sedimentation recommenced in the northern Tasman-Coral Sea area. This was due to the western boundary flow which earlier passed through the region and was largely diverted to the area east of New Zealand and into the Tonga Trench. A world-wide Oligocene unconformity was created by a major change in bottom-water circulation, in turn caused by increased bottom-water production related to the onset of substantial Antarctic glaciation near the Eocene-Oligocene boundary. The separation of Australia from Antarctica led to a fundamental change in the world's oceanic circulation and its climate that marks the onset of the modern climatic regime.

http://www.deepseadrilling.org/29/volume/dsdp29_44.pdf

44. cenozoic paleoceanography in the southwest pacific ocean, antarctic glaciation, and the development of the circum-antarctic current

Planktonic foraminifera species diversity and ice-rafted sand grains, indication of glaciation during most of Cenozoic, middle Miocene warming trend

Cenozoic paleoglacial history of Antarctica recorded in Subantarctic deep-sea cores

Study of 18 Cenozoic South Pacific deep-sea cores indicates an association of glacially derived ice-rafted sands and relatively low planktonic foraminiferal diversity with cooling of the Southern Ocean during the Lower Eocene, upper Middle Eocene, and Oligocene. Increased species diversity and reduction or absence of ice-rafted sands in Lower and Middle Miocene cores indicate a warming trend that ended in the Upper Miocene. Antarctic continental glaciation appears to have prevailed throughout much of the Cenozoic.

Antarctic Glaciation during the Tertiary Recorded in Sub-Antarctic Deep-Sea Cores.

Examination of Holocene Bahamian ooelites by scanning electron and light microscopy has revealed the morphology and orientation of aragonite crystals in the lamellar ooelitic envelope, and their modification by the boring activities of endolithic algae. The voids produced by these algae are found in progressive stages of being lined and filled with precipitated microcrystalline aragonite, which is similar to the process of micrite envelope formation in molluscan and other skeletal carbonate grains.

Endolithic Algae and Micrite Envelope Formation in Bahamian Oölites as Revealed by Scanning Electron Microscopy

A suite of orange-peel grab sediment samples collected during Deep Freeze II from the Antarctic continental shelf of the Weddell Sea contained several well-sorted sand samples. These were collected in 1957 from Berkner Bank in water depths of 282 to 300 m. Samples from greater and lesser depths show poorer sorting as well as differences in other size parameters (Fig. 1). The well-sorted sands include both striated and fractured quartz grains with a considerable degree of polish. Photographs and electron micrographs of grain surfaces and replicas suggest that entire grain surfaces of both the fractured surface morphology and the polished surface have undergone quartz dissolution suggesting an extensive period of exposure to seawater. Detailed studies show features characteristic of both beach and dune sand abrasion overlying glacial features. A variable amount of rock flour is present in all samples, but is least abundant in the 290 ± 10m depth samples. The samples from depths both greater and less than 290 ± 10 m appear to be typical of periglacial marine sediments with a poorly sorted heterogeneous assortment of minerals and rock fragments. The 290 ± 10 m samples appear strikingly different and strongly resemble quartz dune or beach sands. The very good sorting, size, and surface features suggest that these are dune sands. These data indicate that Berkner Bank may have been exposed at the surface during an interglacial period when the Antarctic land surface stood approximately 300 m higher with respect to sea level than it does today.

Stanley V. Margolis

Papers

44. cenozoic paleoceanography in the southwest pacific ocean, antarctic glaciation, and the development of the circum-antarctic current

Cenozoic paleoglacial history of Antarctica recorded in Subantarctic deep-sea cores

Antarctic Glaciation during the Tertiary Recorded in Sub-Antarctic Deep-Sea Cores.

Endolithic Algae and Micrite Envelope Formation in Bahamian Oölites as Revealed by Scanning Electron Microscopy

Possible Interglacial Dune Sands from 300 Meters Water Depth in the Weddell Sea, Antarctica