PP51D-1351 0800h
Radiolarian and Sedimentologic Evidence for Late Eocene Origin of Southern Ocean Environments
Benthic foram $\delta$$^{18}$O and Mg/Ca records indicate that the deep ocean cooled by 5-$10\deg$C between the early Eocene and earliest Oligocene, although the timing, magnitude and dominant style of change (continuous vs. steps) is still debated. The largest change occurs at the Eocene/Oligocene boundary (33.5 Ma), associated with the first major Antarctic glaciation. Current debate concerns whether this glaciation was a threshold event related to long-term decline in CO$_{2}$ or was caused by opening of the Tasmanian Gateway, with thermal isolation of Antarctica and development of circumpolar circulation. The Southern Ocean, characterised by circumpolar current flows and bounded by the Polar Front, has contained since the mid Paleogene a distinct Antarctic radiolarian biota consisting of endemic forms and bipolar high-latitude taxa adapted to seasonal high productivity environments. It is also characterised by common biogenic silica in pelagic sediments. Past changes in the abundance and geographic distribution of these faunas and sediments provide qualitative proxies for the origin and development of this environment. We examine trends in radiolarian evolution, biogeography and biosiliceous facies development from the mid-Eocene to early Oligocene. We calculate diversity, speciation and extinction rates; track the origin and dispersal patterns of Antarctic radiolarian biota (percent Antarctic species at each site); and map sedimentary facies distribution changes through a series of time slices from the late Middle Eocene to the middle Early Oligocene. Data comes from numerous DSDP and ODP sites drawn from the Pacific, Indian and Atlantic sectors of the Southern Ocean. Results show maxima in rates of evolutionary change, increases in Antarctic species, and appearance of biosiliceous components in sediments in the late Middle to middle Late Eocene interval. There is little evidence for major change in, or expansion of the Antarctic radiolarian fauna; or increase of biosiliceous sediment at the time of the opening of the Tasmanian Gateway and Eocene-Oligocene glaciation. We conclude that the Antarctic environment largely developed prior to the Eocene-Oligocene boundary.
PP51D-1352 0800h
Evolution of North Atlantic Thermohaline Circulation: From the Greenhouse to the Icehouse
A growing body of data indicates that the pattern of oceanic thermohaline circulation has varied through the geologic past. At any given time, tectonic and climatic boundary conditions will tend to dictate the operating pattern of thermohaline circulation. For example, in the Early Cenozoic prior to the opening of the northern North Atlantic basins, the Southern Ocean was the dominant source region for deep water formation. As global climate cooled and tectonic gateways changed through the Cenozoic, this mode gradually evolved into the Late Cenozoic mode, characterized by Southern Ocean and North Atlantic sources of deep water formation. To better understand how the evolution of Cenozoic thermohaline circulation related to changes in global climate and ocean basin configuration, we generated Nd isotope records from a depth transect of Ocean Drilling Program sites in the southeastern Atlantic to track deep water mass composition through time. We used fossil fish debris from ODP sites 1262-1264 (Leg 208), which span present-day water depths of 2500 m to 4750 m, to reconstruct the isotopic signature of deep waters over the past 55 Ma. The data indicate an initial transition from relatively non-radiogenic values ($\sim$-10 $\epsilon$$_{Nd}$ units) at 55 Ma to more radiogenic values ($\sim$-8.5) at 32 Ma. From 32 Ma to 3.85 Ma, the Nd signal becomes more non-radiogenic, approximately -12.3 at the top of our record. Comparison of our data with Nd isotopic records derived from a North Atlantic Fe-Mn crust show similar non-radiogenic values ($\sim$-10.5) in the 50 - 32 Ma interval and a trend toward more non-radiogenic values beginning at approximately 20 Ma. The data likely reflect an overall shift from a Southern Ocean deep water source to the ultimate incursion of deep waters from the North Atlantic. The non-radiogenic values at the base of the record reflect a Southern Ocean source of deep water. The shift toward more radiogenic values indicates an increased contribution of Pacific waters to the Southern Ocean source as the Drake Passage began to open to deep flow at $\sim$33 Ma. Though the subsequent trend toward more non-radiogenic Nd isotope values does not directly correlate to any major tectonic event, it is approximately coincident with the increase of benthic foraminiferal $\delta$$^{18}$O values, based on comparison with the Zachos et al (2001) global compilation. Thus the build-up of continental ice on Antarctica and global cooling may have altered the character of Southern Ocean deep waters during the early Oligocene.
PP51D-1353 0800h
The early Eocene in the Southern Ocean; an integrated dinocyst and geochemical analysis of ODP Leg 189 sites 1171 and 1172, Tasman Sea.
The early Eocene is known as the warmest and most extreme long-term greenhouse interval of the Cenozoic. Superimposed on these conditions, short-lived 'hyperthermal' events, apparently caused by the fast input of carbon in the ocean-atmosphere system, are associated with dramatic changes in ocean chemistry, the global hydrological cycle and bio-provincialism and evolution. The `Paleocene/Eocene Thermal Maximum' or PETM event (55 Ma) is the most extreme hyperthermal event, but recently similar events within this time period have been identified. Many relatively complete late Paleocene through early Eocene sequences have by now been reported from around the world, but most are from ancient low- to mid-latitude sites. ODP Leg 189 in the Tasman Sea recovered the southernmost successions ever encountered from this critical phase in Earth's history at the marginal marine Sites 1171 and 1172 (at 70\deg to 65\deg S paleolatitude). On the basis of more detailed investigations of downhole and core logging data, in combination with bio- (dinocysts) magnetostratigraphy and organic stable isotope geochemistry, we have identified the PETM, and possibly also the younger Elmo event (see Sluijs et al., AGU fall meeting abstract volume 2004) and provide correlation and analysis of the late Paleocene through early Eocene in terms of completeness and paleoenvironment.
PP51D-1354 0800h
Orbital forced sea level fluctuations during the Middle Eocene (ODP site 1172, East Tasman Plateau)
Ocean Drilling Program leg 189 was undertaken to test and refine the hypothesis (by Kennett et al., 1975), that the reconfiguration of continents around Antarctica (e.g.: the opening of the Tasmanian Gateway and Drake passage) led to the onset of the Antarctic Circumpolar Current that, in turn, would cause thermal isolation and hence cooling of Antarctica. This would possibly even cause global cooling, as suggested by the 33.3 Ma Oi1 event. The cores of leg 189, site 1172 on the eastern side of the Tasmanian Gateway provided a nearly complete succession of Eocene and Oligocene sediments. Cyclostratigraphic analysis based on XRF derived Ca and Fe records indicates distinct Milankovitch cyclicity between 40 and 36 Ma. (Röhl et al, in press). In the core-section representing magnetochron 18n-1n, the Ca record shows precession cycles in combination with obliquity, suggested to reflect sea level fluctuations (Röhl et al, in press). New datasets include microfossil data (organic-walled dinoflagellate cysts, pollen/spores and diatoms), loss-on-ignition measurements, magnetic data (environmental magnetics - ARM). Here, we aim to further investigate the proposed relationship between astronomical forcing and sea-level fluctuations. Additionally, we aim to obtain insight in the palaeoecology of the distinct endemic circum-Antarctic late Middle to Late Eocene dinoflagellate cyst assemblages. Results corroborate the concept that the cyclicity recorded by Ca and Fe measurements is the result of sea-level fluctuations. This implies that during late Middle Eocene times, astronomical forcing has modulated sea level - most likely through Antarctic ice buildup and meltdown. In turn, this would indicate the presence of significant, though probably modest, ice masses already ~40 Ma ago, well before the onset of the Antarctic Circumpolar Current. Kennett, J. P., R. E. Houtz, et al. (1975). Development of the circum-Antarctic current. Science 186: 144-147. Röhl, U.; H. Brinkhuis, C.E. Stickley, M. Fuller, S.A. Schellenberg, G. Wefer, G. Williams, Cyclostratigraphy of Middle and Late Eocene sediments from the East Tasman Plateau (site 1172), in press.
PP51D-1355 0800h
Eocene-Oligocene Southern Ocean Paleo-bathymetry maps generated with geophysical, sedimentological and microfossil data.
The Eocene-Oligocene transition is characterized by the cooling of the Southern Ocean region and growth of the Antarctic ice-sheet. Through this transition, changes in spatial distribution and topography of land masses and ocean basins have played a role. Our objective is to produce a set of enhanced paleo-bathymetry maps based on both plate reconstruction models for the Southern Ocean and sedimentological and microfossil data. We used paleo-age grids to create a series of paleoceanic basement-depth grids, based on a thermal boundary layer depth-age model. We further constrained these paleo-bathymetry grids, especially for continental margins, with sedimentological and microfossil data (diatoms, foraminifera, nannofossils, organic walled dinoflagellates, pollen & spores and radiolarians); derived from new and published data from land based sections and DSDP/ODP (and other) cores. In particular, we aim to map the opening of oceanic gateways (Tasmanian Gateway and Drake Passage) and changing ocean topography through Eocene-Oligocene times. It is known that the evolution of circum Antarctic current systems were strongly dependent on bottom topography, particularly the deepening of oceanic gateways and the mid-ocean ridge system. It is hoped that this work will offer a more realistic/resolved starting input for ocean basin configuration (paleo-geographic model) and topography (paleo-bathymetry model). It would be a valuable tool to assess boundary conditions for oceanographic circulation models and next generation CCM's; in particular it would help assess deep-water circulation and related climate change further back in geological time.
PP51D-1356 0800h
From Greenhouse to Icehouse: Evidence of Climatic Changes Across the Marine Eocene-Oligocene Transition From the Massignano GSSP Section (Central Italy)
The transition from global "greenhouse" conditions of the early and middle Eocene to global "icehouse" conditions of the early Oligocene marks a turning point in Cenozoic Earth history which was marked by reorganization of global ocean circulation patterns and significant turnovers in the marine and terrestrial biota (Prothero et al., 2003) and led to the development of the first East Antarctic ice-sheet, close to the Eocene/Oligocene boundary (33.7 Ma). The Massignano GSSP for the Eocene/Oligocene boundary (Premoli Silva & Jenkins, 1993), exposed in an abandoned quarry in the Monte Conero area, on the Adriatic coast of central Italy, was investigated at high-resolution in order to provide evidence for climatic changes across the marine Eocene-Oligocene transition. The Massignano section is 23-m thick and consists of alternating reddish/greenish-grey marls and calcareous marls with several biotite-rich levels of volcanic origin which were deposited in a lower bathyal depositional setting, at a paleodepth of 1000-2000 m (Coccioni & Galeotti, 2003). A complete geological record of 3 myr (from 36.2 to 33.2 Ma according to the time scale of Berggren et al., 1995) is preserved which spans the interval from the latest Eocene to the early Oligocene, from Chron C16n to C13n (Bice & Montanari, 1988; Lowrie & Lanci, 1994), and is provided by an accurate calibration of bio- and geochemical events. Cosmic signatures are also recorded in the Massignano section (Montanari et al., 1993) where three impactoclastic, iridium-rich layers occurs in the middle-lower part of the succession (Montanari et al., 1988, 1993; Bodeselitsch et al., 2004). They are possibly linked to the Popigai and Chesapeake Bay impacts and related to a comet shower over a duration of 2.2 myr (Farley et al., 1998). Calcareous nannofossil and foraminiferal assemblages (Coccioni et al., 2000; Spezzaferri et al., 2002), dinoflagellate cyst palynology (Brinkhuis & Biffi, 1993), ostracod faunas (Dall'Antonia et al., 2003), oxygen and carbon isotopes (Bodeselitsch et al., 2004), and environmental magnetism (Jovane et al., 2004) provide evidence of a major cooling trend with warm pulses. These pulses seems to be global in extent and may have been triggered by multiple impact events during the Late Eocene comet shower that may have played an important role related to the deterioration of the global climate at the end of the Eocene Epoch. The release of methane hydrate during and after an impact in a continental shelf (like the Chesapeake Bay impact) or seafloor, or impacts of $^{12}$C-rich comets may account for the observed negative isotope excursions. References Bice D. and Montanari A., 1988. IUGS Spec. Publ., Graf. Aniballi, 111-117; Bodeselitsch B. et al., 2004. E.P.S.L., 223, 283-302; Brinkhuis H. and Biffi U., 1993. Mar. Mic., 22, 131-183; Coccioni R. et al., 2000. Terra Nova, 12, 258-263; Coccioni R. and Galeotti S., 2003. In: Prothero D.R. et al., (eds.), 2003. Columbia Univ. Press, 438-452; Dall'Antonia B. et al., 2003. Mar. Mic., 48, 91-106; Farley K.A. et al., 1998. Science, 280, 1250-1253; Jovane L. et al., 2004. Geoph. Res. Let., 31, L15601, doi:10.1029/2004GL020554; Lowrie W. and Lanci L., 1994. E.P.S.L., 126, 247-258; Montanari A. et al., 1993. Palaios, 8, 420-437; Montanari A. et al., 1988. IUGS Spec. Publ., Graf. Aniballi, 195-208; Premoli Silva I. and Jenkins D.G., 1993. Episodes, 16, 379-382; Prothero D.R. et al., (eds.), 2003. Columbia Univ. Press, 541 pp.; Spezzaferri S. et al., 2002. J. Foram. Res., 32, 188-199.
PP51D-1357 0800h
The Massignano Eocene-Oligocene golden spike section revisited
In common practice, the Eocene/Oligocene (E/O) boundary is linked to the Oi-1 $\delta$ $^{18}$ O benthic isotope event, reflecting the oldest phase of major Antarctic glaciation, calibrated against magnetosubchron C13n. Yet, the IUGS-ratified, current E/O GSSP at the pelagic Massignano Quarry section, central Italy, occurs within the older magnetosubchron C13r, at metre 19 of the 23 m section. To promote further high-resolution stratigraphic and paleoecological studies at Massignano, and to extend the lower Oligocene record, the so-called Massicore was drilled about 110 m south of the stratotype section. By means of high-resolution organic-walled dinoflagellate cyst (dinocyst) analysis, in combination with biotite-rich horizons an almost perfect linear correlation between the core and the quarry was obtained, resulting in the establishment of the Massignano GSSP composite section, spanning from magnetosubchron16-2n to 12r (van Mourik and Brinkhuis, in press). The revised paleomagnetic ages of this interval (P$\ddot{a}$like et al., in prep) were used for a preliminary age model of the composite section. The paleoecological dinocysts proxies were plotted along this agemodel. A straightforward correlation of the (relative) Sea Surface Temperature (SST) curve to the 400 ky eccentricity curve of Laskar et al.'s (2004) was possible. In two intervals (5.60 to 10.50 m and 17 to 35 m) the correlation could be made to the 100 ky eccentricity curve (Laskar et al., 2004). The first astronomical timescale for the Massignano GSSP composite section could be composed, placing the GSSP (19 m) at age of 33.96 $\pm$ 0.05 Ma. The age of the onset of the Oi-1 event appears around 33.55 $\pm$ 0.01 Ma, and towards the top of the section the cold peaks in the SST get more and more pronounced. The cooler conditions are substantiated by the occurrence of restricted high latitude dinocyst species ({\it G. inflata} and {\it Sv. cooksoniae}) from 33.30 $\pm$ 0.01 Ma until the top of the section (van Mourik et al., in prep). Relying also on earlier studies, we demonstrate that the current E/O GSSP criterion, the local extinction of the tropical genus {\it Hantkenina} (planktonic foraminifera) represents an isolated event, not suitable for global correlation. Instead, we argue that selection of the Oi-1 event as E/O criterion, which is also related to the TA4.3/4.4 third order sequence boundary, and the last occurrence of the dinocyst {\it A. diktyoplokum}) would provide a much better basis for global correlation. Moreover, selection of this slightly younger Oi-1 event criterion would place the top of the classic Priabonian Stage back in the latest Eocene, and the base of the Rupelian Stage in the earliest Oligocene. However, since the correlative level is not represented by the surface (quarry) section, but only resides in the cored, not exposed, section at Massignano, a new GSSP site should be selected, for example in one of the other central Italian sections that does span C13n (van Mourik and Brinkhuis, in press).