To groundtruth these acoustic profiles with compositional data on the sedimentary deposits, sophisticated coring technologies from the Deep Sea Drilling Program (DSDP) and its Ocean Drilling Program successor have been required. For example, cores from DSDP Leg 28, which were collected from the Ross Sea by the Glomar Challenger in 1972-73, recovered sediments from the early Cenozoic when Antarctica was separating from Australia (Table 6.1). These Ross Sea sediment cores contained deposits of planktonic algae (diatoms) and single-celled animals (foraminifera) that were distributed by currents and water masses in the ocean (Chapter 7: Flowing Planet). In particular, the diatom oozes reflect high concentrations of nutrients upwelling from the deep sea that enhanced their productivity (Chapter 9: Living Planet). In addition, sediment cores from DSDP Leg 28 contained ethane and methane which subsequently roused international interest in potential Antarctic mineral resources (Chapter 11: Environmental Protection).

Together, the combination of sediment acoustic and core profiles (Fig. 6.4) indicate that there was extensive ice-sheet scouring in the Ross Sea region by 24 million years ago. During the next 15 million years, there were episodes of glacial erosion followed by prolonged periods of marine deposition when the ice sheet was not grounded on the sea floor. Since the beginning of the Pleistocene epoch, nearly 2 million years ago, the frequency of ice-sheet grounding events has increased along with the pronounced occurrence of glacial-interglacial cycles in the Earth system (Table 6.1).

Sediments from other deep-sea cores around Antarctica indicate that diatom deposition has shifted toward the lower latitudes along with the northern boundary of the Southern Ocean since the early Oligocene. Throughout this period, there also was a northward shift in the deposition of ice-rafted debris (stones dropped onto the seafloor) from melting icebergs around Antarctica. Similarly, there was a northward displacement of planktonic foraminifera, which have calcium carbonate shells that increasingly dissolve as seawater temperatures become colder. Together, these sedimentary deposits paint a rough picture of intensifying Antarctic glaciation and climate cooling during the last 38 million years.

Within individual foraminifera, however, there are higher resolution geochemi-cal signatures of the ambient environmental conditions that existed when their calcareous shells were precipitated. Specifically, shells of these microscopic animals contain both heavy and light isotopes of oxygen (18O and 16O, respectively) that can be related directly to global temperatures and ice volumes. During periods of seawater cooling, marine carbonates increasingly incorporate 18O relative to 16O. In addition, because 18O evaporates more slowly than 16O, seawater concentrations of the heavy isotope increase as the light isotope is transported by water vapor from the ocean onto the ice caps. Therefore, during glacial periods when the seawater is relatively cold and enriched in the heavy oxygen isotope, calcareous marine species precipitate more 18O than 16O.

The ratios (R) of 18O/16O in samples and standards, after being multiplied by

1000 to magnify the small differences, conventionally are expressed by delta notation (5) in units of parts per thousand (per mil, %o):

Given the fractionation of oxygen isotopes, <518O values become more positive in marine carbonates and more negative in glacial ice during climate cooling conditions. Conversely, under climate warming conditions, <518O values become more negative in marine carbonates and more positive in glacial ice. For marine carbonates, a 1%o increase in <518O equates with a temperature decrease of nearly 4°C in the ambient seawater.

Cenozoic sedimentary deposits from around Antarctica contain calcareous for-aminifera that had been living in the water (plankton) near the sea surface as well as on the sediment (benthos) in the deep sea. Even though these planktonic and benthic foraminifera lived in widely separated habitats, the <518O values in their calcium carbonate shells had increased overall from around -1% to 4% during the past 55 million years. This 5% change in the oxygen isotope composition of the foraminifera equates with a temperature drop of more than 15°C—reflect-ing the long-term cooling trend of the Earth system throughout the Cenozoic

Within the Cenozoic epochs (Table 6.1), seawater temperatures dropped suddenly at the Eocene-Oligocene boundary, to approximately 5°C. During the Oligocene it appears that Antarctic glaciation became widespread and that there was extensive sea-ice production. As a consequence of Antarctic cooling, surface waters became denser and began sinking into the deep sea, where they subsequently form the bottom waters of the ocean.

The next major climate cooling threshold was achieved during the middle Miocene, around 14 million years ago, when foraminifera <518O values from the Southern Ocean dramatically increased by more than 2%c as seawater temperatures decreased to nearly 0°C (Fig. 6.5). This temperature decrease was associated with enlargement of the East Antarctic Ice Sheet and a circumpolar temperature drop in the coastal zone around the continent. As a consequence of this middle Miocene cooling event around Antarctica, the slope of the temperature gradient between high and low latitudes markedly steepened in the ocean and atmosphere.

The most recent climatic cooling transition was reached during the Pliocene, around 3 million years ago, when seawater temperatures dipped below 0°C around Antarctica and ice sheets began developing in the Northern Hemisphere. Decreasing temperatures associated with expanding ice sheets in West Antarctica and circumpolar sea-ice extent during the Pliocene intensified the circulation, upwelling, and ensuing productivity of the Southern Ocean. During this period, coastal marine species' extinctions occurred in the Arctic and Antarctic along with the emergence of deep-sea faunas into shallow polar oceans. At lower latitudes, in areas such as New Zealand, there also were the first appearances of cold-water marine fauna during the late Pliocene.

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