P. Ferretti

University of Cambridge, Cambridge, ENG, United Kingdom

Are you P. Ferretti?

Claim your profile

Publications (28)138.02 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Paleoclimate and paleoenvironmental studies in the north-eastern Italian Alps have been hampered by the rarity of well-preserved high-altitude deposits and the lack of high-resolution multi-proxy records with adequate chronological control. This paper presents the first complete Late Glacial to Holocene peat succession from the Dolomites (Danta di Cadore, Belluno, Italian Alps). A 7 m core was used to evaluate the potential of the ombrotrophic Val di Ciampo peat deposit (1400 m a.s.l.) as an archive of environmental and climate change. The depth age scale is based upon independent C-14 and Pb-210 dates and combined with peat stratigraphy demonstrates that the peat core covers more than 13,200 cal BP, extending to the end of the last part of the Late Glacial. Bulk density, inorganic matter content, pore water pH, conductivity, Ca/Mg ratios, and Ca, Sr and Ti trends were used to identify changes in trophic conditions through the bog. The boundary between ombrotrophic and minerotrophic conditions occurs at approximately 400 cm below the surface and demonstrates that this core is the longest Eastern Alpine ombrotrophic record yet obtained, corresponding to 7000 cal BP. The high-resolution chemical data of this peat archive improves our understanding of European Alpine Holocene climate variability and the relationship between natural climate fluctuations and anthropogenic climate change during the present interglacial in the Dolomites.
    Quaternary International 09/2013; 306:71-79. DOI:10.1016/j.quaint.2013.05.001 · 2.13 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.
    Science 08/2012; 337(6095):704-9. DOI:10.1126/science.1221294 · 31.48 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ombrotrophic peatlands are a valuable repository of high-quality climatic signals because their only source of water is precipitation. For this reason, they constitute an authentic and detailed archive of information about past and present patterns in climate change and allow us to infer the impact of natural and human activities causing trace element contamination. In the province of Belluno (Northeastern Alps, Italy), bogs of major interest are located in Danta di Cadore, 46°34'16'' N 12°29'58'' E, and Coltrondo, 46°39'28'' N 12°26'59'' E. This study aims to provide a detailed reconstruction of past climate and environmental conditions by a novel multi-proxy method characterized by high-resolution geochemical (trace elements, rare earth elements, Pb isotopes, organic carbon, humification), and pollen analysis using the Danta di Cadore and Coltrondo peat bogs. Major and trace elements in the peat profiles were measured using the non-destructive X-Ray Fluorescence (XRF) core scanner. Moreover, lightness and colour parameters were measured on the images recorded by a digital colour line scan camera connected to the XRF core scanner. This method provides high-resolution geochemical data (0.25 cm) and sediment reflectance spectra (0.1 cm) that document the down-core distribution of major and trace elements, the changes in sediment colour, and help to infer the natural geochemical processes which affect the peat profiles. Conversion of element intensities measured by XRF core scanner to element concentrations is essential for quantitative applications involving mass-balance and flux calculations. We calibrate the of measured intensities of our XRF results with control specimens taken from the same core at strata corresponding exactly to core-scanner measurements. The control specimen compositions are quantified using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) which is a well-established method of geochemical analysis. Our measurements of specific element concentrations describe the trophic status of the peat profile and, together with a reliable age model, allow the reconstruction of the changing rates and the predominant sources of a wide variety of atmospheric trace elements. To our knowledge, this is the first attempt to provide quantitative geochemical interpretation of XRF core scanner data for this type of deposit. The results of this type of analysis provide environmental information which is currently lacking in the Northeastern Italian Alps.
    European Geosciences Union General Assembly; 04/2012
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Hudson Strait (HS) Heinrich Events, ice-rafting events in the North Atlantic originating from the Laurentide ice sheet (LIS), are among the most dramatic examples of millennial-scale climate variability and have a large influence on global climate. However, it is debated as to whether the occurrence of HS Heinrich Events in the (eastern) North Atlantic in the geological record depends on greater ice discharge, or simply from the longer survival of icebergs in cold waters. Using sediments from Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic spanning the period between 960 and 320 ka, we show that sea surface temperatures (SSTs) did not control the first occurrence of HS Heinrich(-like) Events in the sedimentary record. Using mineralogy and organic geochemistry to determine the characteristics of ice-rafting debris (IRD), we detect the first HS Heinrich(-like) Event in our record around 643 ka (Marine Isotope Stage (MIS) 16), which is similar as previously reported for Site U1308. However, the accompanying high-resolution alkenone-based SST record demonstrates that the first HS Heinrich(-like) Event did not coincide with low SSTs. Thus, the HS Heinrich(-like) Events do indicate enhanced ice discharge from the LIS at the end of the Mid-Pleistocene Transition, not simply the survivability of icebergs due to cold conditions in the North Atlantic.
    Paleoceanography 12/2011; 26:PA4201. DOI:10.1029/2011PA002135 · 3.92 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Records of benthic calcite delta18O and Mg/Ca will be presented for a site in Lower Circumpolar Deep Water in the Pacific sector of the Southern Ocean extending from the MIS 37/38 boundary at the start of the mid Pleistocene Transition. Almost half of the flux of cold bottom water entering the major basins of the world ocean does so through the Southwest Pacific Ocean as the Pacific Deep Western Boundary Current which mainly transports Circumpolar Deep Water. Thus, the site occupies a critical region for comparison with Southern Ocean atmospheric records. It should be more representative of the global deep ocean than sites that may have been influenced by hydrographic variations between ocean basins. There is a strong correlation between the climate from the ice core records and that derived from the deep sea, including warmer then Holocene MIS 5, 7, 9 and 11 (but cooler than Antarctic temperatures by 1sigma in normalized data). The lengths and intensities of interglacials will be discussed. Estimation of polar amplification suggests glacial temperature thresholds close to freezing point, not observed at a North Atlantic site in NADW with which the LDCW site will be compared. The role of carbonate ion saturation will be discussed. Seawater delta18O implies ~ 1.5 ○C cooling preceded ice-sheet build up with rapid change at MIS 22 associated with 100 kyr cyclicity. The temperature records from the South Pacific and North Atlantic Ocean show an apparent phase lead of oceanic temperature with respect to eccentricity that will be discussed.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Mid-Pleistocene transition (MPT) was the time when quasi-periodic (∼ 100 kyr), high-amplitude glacial variability developed in the absence of any significant change in the character of orbital forcing, leading to the establishment of the characteristic pattern of late Pleistocene climate variability. It has long been known that the interval around 900 ka stands out as a critical point of the MPT, when major glaciations started occurring most notably in the northern hemisphere. Here we examine the record of climatic conditions during this significant interval, using high-resolution stable isotope records from benthic and planktonic foraminifera from a sediment core in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313). We have considered the time interval from late in Marine Isotope Stage (MIS) 23 to MIS 20 (910 to 790 ka). Our data indicate that interglacial MIS 21 was a climatically unstable period and was broken into four interstadial periods, which have been identified and correlated across the North Atlantic region. These extra peaks tend to contradict previous studies that interpreted the MIS 21 variability as consisting essentially of a linear response to cyclical changes in orbital parameters. Cooling events in the surface record during MIS 21 were associated with low benthic carbon isotope excursions, suggesting a coupling between surface temperature changes and the strength of the Atlantic meridional overturning circulation. Time series analysis performed on the whole interval indicates that benthic and planktonic oxygen isotopes have significant concentrations of spectral power centered on periods of 10.7 kyr and 6 kyr, which is in agreement with the second and forth harmonic of precession. The excellent correspondence between the foraminifera δ18O records and insolation variations at the Equator in March and September suggests that a mechanism related to low-latitude precession variations, advected to the high latitudes by tropical convective processes, might have generated such a response. This scenario accounts for the presence of oscillations at frequencies equal to precession harmonics at Site U1313, as well as the occurrence of higher amplitude oscillations between the MIS22/21 transition and most of MIS 21, times of enhanced insolation variability.
    Earth and Planetary Science Letters 04/2010; 293(s 1–2):28–41. DOI:10.1016/j.epsl.2010.02.016 · 4.72 Impact Factor
  • Quaternary Science Reviews 01/2010; 29(1):160-169. · 4.57 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The sensitivity to temperature of Mg/Ca ratios in the shallow-infaunal benthic foraminifera Uvigerina spp. has been assessed. Core-top calibrations over ∼1–20°C show a range in sensitivity of 0.065–0.084mmol/mol/°C but few data are available spanning the temperature range anticipated in deep-sea records over glacial–interglacial cycles. In contrast to epibenthic foraminiferal species, carbonate ion saturation appears not to affect Mg/Ca significantly. A method based on estimating the ratio of the temperature sensitivity of foraminiferal Mg/Ca to that of δ18Ocalcite shows that sensitivity for Mg/Ca at the high end of the observed core-top range (∼0.1mmol/mol/°C) is required for consistency with LGM–Holocene differences in each property as constrained by independent proxy data. This is supported by a Mg/Ca record for Uvigerina spp. generated for the Southern Ocean over the past 440,000 years from Ocean Drilling Program Site 1123 (Chatham Rise, New Zealand). The record shows variability that correlates with climate oscillations. The LGM deep ocean temperature derived from the Mg/Ca record is −1.1±0.3°C. Transformation to temperature allows estimates to be made of changes in bottom water temperature and seawater δ18O and comparison made with literature records. Analysis reveals a ∼2.5-kyr lead in the record of temperature over calcite δ18O and a longer lead over seawater δ18O. This is a reflection of larger phase offsets at eccentricity periods; phase offsets at tilt and precession are within error zero.
    01/2010; 29(1):160-169. DOI:10.1016/j.quascirev.2009.07.013
  • [Show abstract] [Hide abstract]
    ABSTRACT: One of the most intriguing aspects of Pleistocene climate is the development of quasi-periodic (ca.100 kyr), high-amplitude glacial variability during the middle Pleistocene. A number of causes have been suggested for this Mid-Pleistocene Transition (MPT), but there is no consensus yet on this matter despite more than two decades of research. Most hypotheses invoke either a response to a long-term cooling, possibly induced by decreasing atmospheric pCO2, or changes in internal ice-sheet dynamics. Marine records from the North Atlantic and tropical-ocean upwelling regions have already provided support for the first hypothesis, documenting decreases of sea-surface temperatures (SSTs) during the MPT; in contrast, no discernible shift in SSTs has been identified in the Western Pacific Warm Pool. Little is known about how deep-water temperatures evolved during this climate transition, how deep-water temperatures correlated with the surface water variability and how temperature change is linked with the records of greenhouse gases and atmospheric temperature from EPICA Dome C. Here we present records of Mg/Ca and stable isotopes measured in planktonic and benthic foraminifera over the time interval 0.35-1 Ma (Marine Isotope Stages 10-27) from a marine sediment core recovered in the mid-Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). We have analysed three foraminiferal species: Globigerina bulloides, Globorotalia inflata and Uvigerina spp. to represent the upper sea-surface, the thermocline and the deep ocean respectively. The Mg/Ca-based temperature estimates allow us to test the hypothesis of a global cooling associated with the MPT in the Southern Hemisphere and, paired with the foraminiferal d18O record, derive the d18O of the water in which the foraminifer calcified, the latter of which combines a global glacioeustatic signal with local hydrographic effects. Temperature estimates suggest that the long term average surface- and deep- water temperatures changed relatively little in the South-west Pacific through the interval analysed, with no systematic shift across the MPT. In addition, these results permit preliminary discussion of the magnitude of surface- and deep-water temperature changes during glacial/interglacial transitions and the interglacials themselves. The phase relationship between surface- and deep-water signals has been assessed; in particular, benthic Mg/Ca and planktonic Mg/Ca are almost in phase at eccentricity, tilt and precession, with a slight lead of the planktonics over the benthics. At the 100 kyr period, Mg/Ca leads d18O both in the planktonic and benthic records. Finally, the phasing of variations in the marine record will be considered with respect to other component of the climate system, and a comparison with greenhouse gases and atmospheric temperatures from EPICA Dome C will be attempted, after evaluation of methods for precise synchronization of these records.
  • [Show abstract] [Hide abstract]
    ABSTRACT: One of the most intriguing aspects of the Pleistocene climate is the development of 100 kyr glacial variability during the middle Pleistocene. Very little is known about how deep-water temperatures evolved during this climate transition, how it correlated to the surface water variability and how it links with the records of greenhouse gases and atmospheric temperature from EPICA Dome C. We present detailed records of Mg/Ca, Sr/Ca, B/Ca, Li/Ca and stable isotopes on the infaunal benthic (Uvigerina spp.) foraminifer over the time interval 0-1 Ma from a marine sediment core recovered in the mid Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). Associated records for planktonic (Globigerina bulloides and Globorotalia inflata) foraminifera have also been obtained. The Mg/Ca record has been assessed for secondary effects additional to temperature and comparisons made with ancillary trace element data. The Mg/Ca-based water temperature estimates allow us to examine the deep ocean response to glacial-interglacial changes in the atmosphere and surface ocean over this critical period and, using the paired foraminiferal O isotope and Mg/Ca records explore the changes in seawater O isotopic compsition.
  • P. Ferretti, H. Elderfield, N. McCave
    [Show abstract] [Hide abstract]
    ABSTRACT: One of the most intriguing aspects of the Pleistocene climate is the development of quasi-periodic (ca.100 kyr), high-amplitude glacial variability during the middle Pleistocene. A number of causes have been suggested for this Mid-Pleistocene Transition (MPT), but there is no consensus yet on this matter despite more than two decades of research. Most hypotheses for the origin of the MPT invoke either a response to a long-term cooling, possibly induced by decreasing atmospheric pCO2, or changes in internal ice sheet dynamics. Marine records from the North Atlantic and tropical-ocean upwelling regions have already provided support to the first hypothesis, documenting decreases of sea surface temperatures during the MPT. In contrast, very little is known about how deep-water temperatures evolved during this climate transition, and how it correlated to the surface water variability. Here we present records of Mg/Ca and stable isotopes on planktonic (Globigerina bulloides, Globorotalia inflata) and benthic (Uvigerina spp.) foraminifera over the time interval 0.35-1 Ma from a marine sediment core recovered in the mid Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). The Mg/Ca-based temperature estimates allow us to test the hypothesis of a global cooling associated with the MPT in the Southern Hemisphere and, paired with the foraminiferal delta18O record, derive the delta18O of the water in which the foraminifer calcified, the latter of which combines a global glacioeustatic signal with local hydrographic effects. These results permit preliminary discussion of the magnitude of the surface- and deep-water temperature changes during glacial/interglacial transitions and the interglacials themselves. The phase relationship between surface- and deep-water signals is tentatively assessed. Finally, the phasing of variations in the marine record will be considered with respect to other component of the climate system, and a comparison with greenhouse gases and atmospheric temperature from EPICA Dome C will be attempted, after evaluation of methods for precise synchronization of these records.
  • [Show abstract] [Hide abstract]
    ABSTRACT: ODP Site 1240 from the Eastern Equatorial Pacific (EEP) provides a unique archive to explore, at high resolution, the role of low latitude oceanographic systems in past global climate changes. A combination of both inorganic and organic geochemical proxies is applied to reconstruct temperature and salinity changes in surface and thermocline layers. The Sea Surface Temperature (SST) record based on G. ruber Mg/Ca ratios supports a surface warming during times of rapid cooling in the North Atlantic associated to the Heinrich Events. These results are consistent with speleothems records from Brazil and coupled climate model reconstructions, and support that diminished North Atlantic Meridional Overturning Circulation (AMOC) induced a southward displacement of the Inter-Tropical Convergence Zone (ITZC) and hence a weakening of the south-east trade winds. Nevertheless, a SST multi-proxy comparison illustrates a more complex pattern with enhanced seasonality during the last deglaciation induced by both orbital and AMOC forcings interacting at this time. Thermocline proxies support that combined low precession and high obliquity conditions lead to the establishment of "La Niña-like" conditions in the EEP that alternated with diminished upwelling conditions at annual or interannual time scale which apparently resembled intense "El Niño-like" conditions.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Past hydrographical conditions in the Eastern Equatorial Pacific (EEP) are analysed throughout the multi- proxy study of the long sediment core ODP Site 1240, located at the Panama Basin (0°01.311"N, 86°27.758"W, 2921mbsl). The combination of different geochemical proxies (delta18O, delta13C and Mg/Ca) in two different foraminifera species with diverse preferential dwelling depths in the water column (G. ruber and N. dutertrei) allows the parallel reconstruction for the last 275.000 years of temperature and salinity both at the surface and thermocline layers respectively. The records obtained cover the last 275,000 years at orbital but also millennial-centennial time scales. Results show the existence of recurrent events characterized by saltier thermocline waters which are associated with an intensification of the EEP upwelling system. These events coincide with Earth's orbital configurations characterized by low precession periods and were particularly strong when co-existed with high obliquity conditions, as consistently occurs at the onset of glacial Terminations. This orbital configuration strengthened the atmospheric Walker circulation in the equatorial Pacific which eventually favours the establishment of strong La Niña-like like conditions in the EEP. The thermocline proxies also support the advection of Southern Ocean waters into the equatorial thermocline during glacial Terminations. This 'tunnelling' effect could have been reinforced by the strong La Niña-like conditions through a southward displacement of the westerlies (as supported by modern satellite observations during La Niña events), therefore enhancing the Antarctic Circumpolar Current and the resumption of the Southern Ocean upwelling system. The Site 1240 records also show significant variability at millennial time-scales and demonstrate that sea surface temperatures (SST) evolved consistently with changes in atmospheric CO2. Apparent contradictory results coming from different SST proxy records based on foraminifera Mg/Ca and biomarker (Uk"37) are interpreted in terms of seasonality intensity changes which provide new insights on the understanding of the regional paleoceanography. Results also support a close linkage between oceanographic changes in the EEP and rapid changes in the intensity of the North Atlantic Meridional Overturning circulation.
  • Source
    L D Pena, I Cacho, P Ferretti, M A Hall
    [Show abstract] [Hide abstract]
    ABSTRACT: Interannual-decadal variability in the equatorial Pacific El Niño–Southern Oscillation (ENSO) induces climate changes at global scale, but its potential influence during past global climate change is not yet well constrained. New high-resolution eastern equatorial Pacific proxy records of thermocline conditions present new evidence of strong orbital control in ENSO-like variability over the last 275,000 years. Recurrent intervals of saltier thermocline waters are associated with the dominance of La Niña–like conditions during glacial terminations, coinciding with periods of low precession and high obliquity. The parallel dominance of δ13C-depleted waters supports the advection of Antarctic origin waters toward the tropical thermocline. This “oceanic tunneling” is proposed to have reinforced orbitally induced changes in ENSO-like variability, composing a complex high- and low-latitude feedback during glacial terminations.
    Paleoceanography 09/2008; 23:PA3101, doi:10.1029/2008pa001620. DOI:10.1029/2008PA001620 · 3.92 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: It has been recently suggested "a substantial portion of the marine 100-ky cycle that has been object of so much attention over the past quarter of a century is, in reality, a deep-water temperature signal and not an ice volume signal" (Shackleton, 2000). There are currently few records available of deep-water temperature variations during the Pleistocene and most of our understanding is inferred from the oxygen isotopic composition (delta18O) of benthic foraminifera from deep-sea sediments. However, variations in benthic delta18O reflect some combination of local to regional changes in water mass properties (largely deep- water temperature) as well as global changes in seawater delta18O (delta18Osw) resulting from the growth and decay of continental ice. Recent studies suggest that benthic foraminiferal Mg/Ca may be useful in reconstructing deep-water temperature changes, but the application of this method to benthic species has been hampered by a number of unresolved issues, such as uncertainties related to the calibration for benthic Mg at the coldest temperatures. Here we present deep-sea Mg/Ca and delta18O records for the past eight glacial cycles in benthic foraminiferal ( Uvigerina spp.) calcite from a marine sediment core recovered in the mid Southern latitudes. Ocean Drilling Program Site 1123 was retrieved from Chatham Rise, east of New Zealand in the Southwest Pacific Ocean (3290 m water depth). This site lies under the Deep Western Boundary Current (DWBC) that flows into the Pacific Ocean, and is responsible for most of the deep water in that ocean; DWBC strength is directly related to processes occurring around Antarctica. Temperatures derived via pore fluid modeling of the last glacial maximum are available from Site 1123 and represent an important tool to constrain deep-water temperatures estimates using Mg/Ca. In selected time slices, we measured B/Ca ratios in Uvigerina in order to gain information on the deep-water carbonate saturation state and have data of Mg/Ca and B/Ca on planktonic species, which also provides evidence on carbonate saturation state. These results permit preliminary discussion of the magnitude of the deep-water temperature changes during glacial/interglacial transitions and the interglacials themselves. In particular, our deep-water temperature estimates confirm that interglacial stages before 430 ka were characterized by less pronounced warmth - at least in the deeper southern Pacific - than those of the past four climatic cycles, a pattern previously observed in the deuterium record from EPICA Dome C. We examine the relative contributions of deep-water temperature and ice volume to the benthic delta18O signal. The phase relationship between the two signals is tentatively assessed for the middle/late Pleistocene, when different patterns of climate variability have been inferred from marine and ice cores records.
  • Source
    Eos Transactions American Geophysical Union 03/2006; 87(13):129, 133. DOI:10.1029/2006EO130002
  • Source
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The IODP Expeditions 303 and 306 drilling sites were chosen for two reasons: (1) to capture Miocene-Quaternary millennial-scale climate variability in sensitive regions at the mouth of the Labrador Sea and in the North Atlantic ice-rafted debris (IRD) belt (Ruddiman et al., 1977), and (2) to provide the sedimentary and paleomagnetic attributes, including adequate sedimentation rates, for constructing high-resolution isotopic and magnetic stratigraphies. High accumulation rates, reaching 20 cm ky-1, permit the study of millennial-scale variations in climate and in the Earth's magnetic field over the past several million years, when the amplitude and frequency of climate variability changed substantially. Shipboard logging and scanning data (magnetic susceptibility and remanence, density, natural gamma radiation, digital images and color reflectance) and post-expedition x-ray fluorescence (XRF) scanning data have revealed that the sediment cores recovered on Expeditions 303 and 306 contain detailed histories of millennial-scale climate and geomagnetic fi eld variability throughout the late Miocene to Quaternary epochs. The climate proxies will be integrated with paleomagnetic data to place the records of millennial-scale climate change into a high resolution stratigraphy based on oxygen isotope and relative paleomagnetic intensity (RPI). The paleomagnetic record of polarity reversals, excursions and RPI in these cores is central to the construction of the stratigraphic template and will provide detailed documentation of geomagnetic field behavior.
    Scientific Drilling 01/2006; DOI:10.2204/iodp.sd.2.01.2006
  • Proc. ODP, Sci. Results, 199, Edited by Wilson, P.A. and Lyle, M. and Firth, J.V., 01/2006: pages 1-13; Ocean Drilling Program.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The transition from the extreme global warmth of the Eocene "greenhouse" to glacial conditions is one of the most prominent in Earth's climatic evolution, yet one of the most poorly understood. It is widely accepted that large ice sheets first appeared ~34 Ma, coincident with decreasing pCO2 and a deepening of the calcite compensation depth (CCD), and that glaciation in the Northern Hemisphere began much later, between 10 and 6 Ma. In a recently published study [1], we presented records of sediment and foraminiferal geochemistry covering the greenhouse-icehouse climate transition from sequences recovered during recent Ocean Drilling Program Legs 199 (Paleogene Equatorial Transect) and 208 (Walvis Ridge). The carbonate content of Leg 199 and 208 sequences contain evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from ~42 Ma, with a permanent deepening 34 Ma. The most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to ~1.5%, suggesting a lowering of global sea level by at least 100 to 125 metres during the middle Eocene through significant storage of ice in both hemispheres. Sediment cores retrieved from the Lomonosov Ridge in the Arctic Ocean during Integrated Ocean Drilling Program Leg 302 (Arctic Coring Expedition) contain ice-rafted sand into the middle Miocene and ice-rafted pebbles into the middle Eocene [2], consistent with an early glacial onset in the Northern Hemisphere. We suggest that the greenhouse-icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago. References: [1] Tripati, A., J. Backman, H. Elderfield, & P. Ferretti, 2005, Eocene bipolar glaciation associated with global carbon cycle changes. Nature 436, p. 341-346. [2] Shipboard Scientific Party, 2005, Arctic Coring Expedition (ACEX): paleoceanographic and tectonic evolution of the central Arctic Ocean. IODP Prel. Rep. 302, http://www.ecord.org/exp/acex/302PR.pdf.