PP54B-01 INVITED 16:05h
Interactions between Antarctic Sea Ice, Ice Sheets, and Climate Through the Cenozoic
The seasonal distribution and thickness of Antarctic sea ice has important climatic effects on radiation balance, energy transfer between the atmosphere and ocean, moisture availability, and thermohaline circulation. Here, we explore the role of sea ice and related feedbacks in the Cenozoic evolution of Antarctic climate and ice sheets. We use a numerical climate model with explicit, dynamical representations of sea ice and grounded continental ice sheets to test: 1) the sensitivity of Southern Hemisphere sea ice to early Cenozoic climate forcing (paleogeography, CO2, orbital cycles, and ice sheet configuration); and 2) the importance of sea ice-atmosphere feedbacks on glacial mass balance in the Antarctic interior. In our model, little or no sea ice forms around the Antarctic margin with 3xCO2, regardless of orbital forcing or ice sheet configuration. At 2xCO2 seasonal sea ice distribution is shown to be highly sensitive to ice sheet size and configuration, via the ice sheet's control on Southern Ocean surface temperature and the low-level wind field. As in prior modeling studies, the growth of sea ice produces significant local-regional changes in net radiation and surface heat flux heat, with statistically significant effects on temperature, precipitation, and surface pressure over the sea ice zone and coastal areas. Only limited effects are seen in the continental interior, however, and changes in net annual snow budgets are too small to affect the pace of a growing East Antarctic ice sheet. These results suggest the Cenozoic appearance of Antarctic sea ice was a response to grounded ice volume and was not a critical factor in Paleogene and Neogene episodes of glaciation. The East Antarctic Ice Sheet's control of equatorward sea ice extent has important implications for Southern Ocean deepwater production and implies proxy reconstructions of ancient sea ice may be indicative of conditions in the continental interior. According to our model, the most persistent and thickest sea ice, prior to the development of the West Antarctic Ice Sheet, would have been located along the western margin of the shallow seaway separating east and west Antarctica, a critical area for the formation of ice shelves that could have influenced the early development of the West Antarctic Ice Sheet.
PP54B-02 16:20h
Timing and Nature of the Deepening of the Tasmanian Gateway
In the late Paleogene, Australia separated from Antarctica and continued to drift northwards allowing the eventual development of the Antarctic Circumpolar Current (ACC). The exact timing and nature of the opening of this gateway during the Eocene-Oligocene (E/O) transition is of interest in respect to its apparent synchroneity with climate deterioration on continental Antarctica. The recovery of a continuous marine sedimentary record across the E/O transition at 4 sites within the Tasmanian Gateway (TG) during ODP 189, allows detailed paleoenvironmental changes to be documented at high resolution in this region. The critical sedimentary units are barren of calcareous microfossils yet siliceous and organic-walled microfossils (notably diatoms and dinocysts) are abundant and excellently preserved, allowing an integrated and detailed paleoeonvironmental analysis of the E/O transition. We present results from Site 1172 and report on the timing and nature of several step-wise deepening events and paleooceanographic changes across the E/O transition in the TG. We use integrated diatom, dinocyst, geochemical, lithological and physical property data to show that the TG deepened at 35.5 Ma, preceding the E/O Antarctic glaciation event by 2 Ma. Importantly our microfossil data indicate a pre-deepening shallow-water pro-deltaic setting characterized by highly endemic biota influenced by a cool clockwise rotating `proto-Ross Sea gyre', to a post-deepening pelagic setting in the earliest Oligocene characterized by cosmopolitan biota. Interestingly these findings indicate a warming in the TG at the exact time when, according to previous hypotheses, a cool-ACC should be influencing the region. These conclusions are corroborated independently by the modeling results of Huber et al., (in press, Palaeoceanography).
PP54B-03 INVITED 16:35h
ACEX: A First Look at Arctic Ocean Cenozoic History
The first Integrated Ocean Drilling Program mission specificplatform expedition (ACEX - Arctic Coring Expedition) drilled and recovered core from five holes at four sites through Cenozoic sediments draping the crest of the Lomonosov Ridge in the central Arctic Ocean. Coring continued into the underlying Cretaceous sedimentary bedrock. Sites are located only a few nautical miles apart along a single seismic line (AWI-91090), showing an identical and coherent Cenozoic seismostratigraphy. Preliminary results from shipboard investigations of core-catcher-based bio- and lithostratigraphy, pore water analyses and core logger data describe a thick (~160 m) middle Miocene through Pleistocene sequence that shows large amplitude, cyclic variability in the density, magnetic susceptibility and acoustic velocity of the sediments. Sediments are largely carbonate free. Pleistocene sedimentation rates are close to 3 cm/ka, whereas Pliocene sediments are by-and-large missing. A sharp change in physical properties at ~200 m defines the transition into a 200+ m thick Paleogene sequence that is initially dominated by large numbers of dinoflagellate cysts. The early Miocene, Oligocene and late Eocene appear to be largely missing in a hiatus. However, a 32 m thick interval separates the overlying middle Miocene from the underlying middle Eocene and presumably preserves some of the early Neogene and late Paleogene sections. Dinoflagellate cysts, diatoms, ebridians and silicoflagellates are common to abundant in the middle Eocene section, which bottoms in a spectacular layer showing massive occurrences of glochidia and massulae (megaspores) of the freshwater hydropterid fern Azolla (duckweed) at the early/middle Eocene boundary (~306 m), suggesting strongly reduced surface water salinity or perhaps even a brief episode of fresh water conditions at the surface. Biosilica is not present prior to the late early Eocene (~320 m). The (sub-) tropical dinoflagellate species Apectodinium augustum occurs abundantly at around 380m in pyrite-rich mudstones, indicating that the Paleocene/Eocene boundary and the associated Carbon Isotope Excursion (CIE) interval were recovered, and that the Arctic Ocean experienced surface temperatures on the order of 20°C during the Paleocene Eocene Thermal Maximum (PETM). Benthic foraminifers indicate that the early Eocene through latest Paleocene sediments were deposited in shallow-marine, neritic to inner neritic, environments. The mudstone of late Paleocene age rests unconformably on Campanian marine sands, sandstone and mudstone.
PP54B-04 16:50h
From Greenhouse to Icehouse: Evidence for Late Early Eocene Concomitant Cooling of Southern Ocean Surface Waters and Global Deep Waters From Dinoflagellate Endemism
ODP Leg 189 drilling around Tasmania retrieved continuous Eocene records from the Southern Ocean - Antarctic Margin. The shallow marine, pro-deltaic successions of Sites 1170, 1171 and 1172 include the interval representing the onset (55 Ma) and termination (50 Ma) of the Early Eocene Climatic Optimum (EECO). The end of the EECO is globally reflected in the oceans by the onset of increasingly cooler deep-water temperatures, and marks the onset of the trend towards the Icehouse world. Here we show that a strong increase of endemic Antarctic dinoflagellates precisely matches the termination of the EECO in the Southern (Pacific) Ocean. The record of these surface-dwelling organisms thus indicates that changes of surface water parameters, notably temperature, occurred near simultaneously with global deep-water temperature changes. Moreover, the signal coincides with the return to heavier d13C-values, and atmospheric CO2 decline. Comparison of the field data with predictions from fully coupled climate model simulations, and a new basic understanding of Eocene Southern Ocean circulation, suggests that changes in carbon burial was driving changes in atmospheric greenhouse gasses, and the apparently coupled surface- and deep-water temperature signals.
PP54B-05 17:05h
From Greenhouse to Icehouse: Marine and Terrestrial Palynological Evidence for Climatic and Oceanic Change Through the Cenozoic of the Arctic
The marine and terrestrial biotas of northern Alaska and the Canadian Beaufort Mackenzie Basin (BMB) are intimately linked to changes in the climate and oceanography of the region. These changes can be reconstructed using palynological data from surface sections and numerous exploration wells drilled in the region over the past 30 years. During the Late Triassic to Early Eocene, marine dinoflagellate cyst (dinocyst) and terrestrial miospore (pollen and spore) palynomorphs were diverse and abundant across the region, reflecting the presence of a relatively warm and productive polar ocean that was fringed by extensive forests. The region was heated by northward-flowing Pacific currents, but lay north of the Arctic Circle and had seasonal 24 hour winter darkness and summer daylight. No modern analogue exists for this environment. A dramatic change occurred at the end of the Early Eocene as global climate shifted from the greenhouse towards the modern icehouse world. This had a particularly strong effect in high latitudes. A succession of major extinction events reflected falling sea and air temperatures in the Arctic and progressively eliminated marine and terrestrial species from the region. These events can be correlated with Eocene cooling steps known from the North Atlantic, where they had a milder effect, and provide a chronostratigraphic link between the regions. By Oligocene time the Arctic populations were strongly impoverished, but Miocene warming permitted the immigration of cold-temperate species including marine dinoflagellates and terrestrial angiosperms. Following this warm phase, the marine and terrestrial populations became increasingly restricted as air and water temperatures fell during the Plio-Pleistocene, leading to the modern highly endemic Arctic biotas.
PP54B-06 17:20h
Arctic Dinoflagellate Migration Marks the Oligocene Glacial Maximum: Implications for the Rupelian-Chattian Boundary
Various geochemical and biotic climate proxies, and notably deep-sea benthic foraminiferal $\delta$$^{18}$O records indicate that the Eocene 'greenhouse' state of the Earth gradually evolved towards an earliest Oligocene 'icehouse' state, eventually triggering the abrupt appearance of large continental ice-sheets on Antarctic at $\sim$33.3 Ma (Oi-1 event). This, however, was only the first of two major glacial events in the Oligocene. Benthic foraminiferal $\delta$$^{18}$O records show a second positive excursion in the mid Oligocene, consistent with a significant ice-sheet expansion and/or cooling at 27.1 Ma (Oi-2b) coincident with magnetosubchron C9n. Here, we report on a mid Oligocene, globally synchronous, Arctic dinoflagellate migration event, calibrated against the upper half of C9n. A sudden appearance, and abundance increases of the Arctic taxon {\it Svalbardella} at lower-middle latitudes coincides with the so-called Oi-2b benthic $\delta$$^{18}$O event, dated at $\sim$27.1 Ma. This phenomenon is taken to indicate significant high-latitude surface water cooling, concomitant Antarctic ice-sheet growth, and sea level lowering. The duration of the {\it Svalbardella} migrations, and the episode of profound cooling is estimated as $\sim$500 ka, and is here termed the Oligocene Glacial Maximum (OGM). Our records suggest a close link between the OGM, sea-level fall, and the classic Rupelian-Chattian boundary, magnetostratigraphically dating this boundary as $\sim$27.1 Ma.
PP54B-07 17:35h
Tectonically Induced Changes of the Global Thermohaline Circulation
The climate changes that took place on Earth during the Tertiary are thought to be partly due to changes in the global thermohaline circulation. At present, there is a conveyor type of circulation with sinking mainly in the North Atlantic Ocean and no sinking in the North Pacific. Climate simulations of the Cretaceous climate have indicated a state of the thermohaline circulation with deep convection occurring at high latitudes on both hemispheres and upwelling at the equator. Within a fully-coupled climate model with continental geometries corresponding to the late Oligocene and early Miocene, we find a northern sinking state of the global thermohaline circulation, with a (shallow and weak) overturning circulation in both the Pacific and the Atlantic Oceans. In this presentation, we study the evolution of the thermohaline circulation during the Tertiary with the use of coupled climate simulations and idealized ocean models. We focus on the circulation changes which are induced by tectonic changes, as, e.g., the opening of the Southern Ocean gateways and the gradual closing of the equatorial seaways.