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The warmest global climates of the past 65 million years occurred during the early Eocene epoch (about 55 to 48 million years ago), when the Equator-to-pole temperature gradients were much [...]

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern (1). Multi-year droughts during the instrumental period2 and decadal-length droughts of the past two millennia (1, 3) were shorter and climatically different from the future permanent, 'dust-bowl-like' megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming (4). So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies (5) to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant [C.sub.4] plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ~2°C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4-6°C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase., The hydroclimatology of the southwestern US shows significant natural variability including major historical droughts (1). Models of climate response to anthropogenic warming predict future dustbowl-like conditions that will last much [...]

Relative to the present day, meridional temperature gradients in the Early Eocene age (~56-53Myr ago) were unusually low, with slightly warmer equatorial regions (1) but with much warmer subtropical Arctic (2) and mid-latitude (3) climates. By the end of the Eocene epoch (~34Myr ago), the first major Antarctic ice sheets had appeared (4,5), suggesting that major cooling had taken place. Yet the global transition into this icehouse climate remains poorly constrained, as only a few temperature records are available portraying the Cenozoic climatic evolution of the high southern latitudes. Here we present a uniquely continuous and chronostratigraphically well-calibrated [TEX.sub.86] record of sea surface temperature (SST) from an ocean sediment core in the East Tasman Plateau (palaeolatitude 65° S). We show that southwest Pacific SSTs rose above present-day tropical values (to ~34°C) during the Early Eocene age (~53Myr ago) and had gradually decreased to about 21 °C by the early Late Eocene age (~36Myr ago). Our results imply that there was almost no latitudinal SST gradient between subequatorial and subpolar regions during the Early Eocene age (~55-50Myr ago). Thereafter, the latitudinal gradient markedly increased. In theory, if Eocene cooling was largely driven by a decrease in atmospheric greenhouse gas concentration (6), additional processes are required to explain the relative stability of tropical SSTs given that there was more significant cooling at higher latitudes., The Palaeogene temperature evolution of the Antarctic margin, particularly the Pacific sector, is still poorly resolved. One difficulty with obtaining relevant records close to the Antarctic continent is the general [...]

Author(s): Appy Sluijs (corresponding author) [1]; Henk Brinkhuis [1]; Stefan Schouten [3]; Steven M. Bohaty [4]; Cédric M. John [4, 6]; James C. Zachos [4]; Gert-Jan Reichart [2]; Jaap S. [...]

Author(s): Mark Pagani (corresponding author) [1, 7]; Nikolai Pedentchouk [1, 7]; Matthew Huber [2, 7]; Appy Sluijs [3]; Stefan Schouten [4]; Henk Brinkhuis [3]; Jaap S. Sinninghe Damsté [4, 5]; [...]

Author(s): Anna-Louise Reysenbach (corresponding author) [1]; Yitai Liu [1]; Amy B. Banta [1]; Terry J. Beveridge [2]; Julie D. Kirshtein [3]; Stefan Schouten [4]; Margaret K. Tivey [5]; Karen L. [...]

The Cenozoic Arctic Ocean Little was known about the environmental history of the Arctic Ocean before the 2004 ACEX ocean drilling expedition. Now a 430-metre sea floor sediment core has been recovered and its analysis, reported this week, provides a 56-million-year climate record spanning the transition from a warm 'greenhouse' to a colder 'icehouse' world. Several key events are identified during the Cenozoic: surface waters during the Palaeocene/Eocene thermal maximum (55 million years ago) were much warmer than previous estimates; surface-water freshening confirms an intensified hydrological cycle about 49 million years ago; and the first ice-rafted debris occurred 45 million years ago, 35 million years earlier than was thought. The revised timings for the earliest Arctic cooling events coincide with those for Antarctica, supporting suggestions that global climate changed symmetrically about the poles. A core of sediments taken from underneath the Arctic Ocean provides evidence that ocean conditions could support a free-floating fern, Azolla, during the middle Eocene epoch, roughly 50 million years ago. It has been suggested, on the basis of modern hydrology and fully coupled palaeoclimate simulations, that the warm greenhouse conditions.sup.1 that characterized the early Palaeogene period (55-45 Myr ago) probably induced an intensified hydrological cycle.sup.2 with precipitation exceeding evaporation at high latitudes.sup.3. Little field evidence, however, has been available to constrain oceanic conditions in the Arctic during this period. Here we analyse Palaeogene sediments obtained during the Arctic Coring Expedition, showing that large quantities of the free-floating fern Azolla grew and reproduced in the Arctic Ocean by the onset of the middle Eocene epoch (~50 Myr ago). The Azolla and accompanying abundant freshwater organic and siliceous microfossils indicate an episodic freshening of Arctic surface waters during an ~800,000-year interval. The abundant remains of Azolla that characterize basal middle Eocene marine deposits of all Nordic seas.sup.4,5,6,7 probably represent transported assemblages resulting from freshwater spills from the Arctic Ocean that reached as far south as the North Sea.sup.8. The termination of the Azolla phase in the Arctic coincides with a local sea surface temperature rise from ~10 °C to 13 °C, pointing to simultaneous increases in salt and heat supply owing to the influx of waters from adjacent oceans. We suggest that onset and termination of the Azolla phase depended on the degree of oceanic exchange between Arctic Ocean and adjacent seas., Author(s): Henk Brinkhuis [sup.1] , Stefan Schouten [sup.2] , Margaret E. Collinson [sup.3] , Appy Sluijs [sup.1] , Jaap S. Sinninghe Damsté [sup.2] [sup.4] , Gerald R. Dickens [sup.5] , [...]

The Cenozoic Arctic Ocean Little was known about the environmental history of the Arctic Ocean before the 2004 ACEX ocean drilling expedition. Now a 430-metre sea floor sediment core has been recovered and its analysis, reported this week, provides a 56-million-year climate record spanning the transition from a warm 'greenhouse' to a colder 'icehouse' world. Several key events are identified during the Cenozoic: surface waters during the Palaeocene/Eocene thermal maximum (55 million years ago) were much warmer than previous estimates; surface-water freshening confirms an intensified hydrological cycle about 49 million years ago; and the first ice-rafted debris occurred 45 million years ago, 35 million years earlier than was thought. The revised timings for the earliest Arctic cooling events coincide with those for Antarctica, supporting suggestions that global climate changed symmetrically about the poles. Identification of the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence shows that sea surface temperatures near the North Pole increased from roughly 18 degrees Celsius to over 23 degrees Celsius -- such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. The Palaeocene/Eocene thermal maximum, ~55 million years ago, was a brief period of widespread, extreme climatic warming.sup.1,2,3, that was associated with massive atmospheric greenhouse gas input.sup.4. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition.sup.5. We show that sea surface temperatures near the North Pole increased from ~18 °C to over 23 °C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean's bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations.sup.6, but the absolute polar temperatures that we derive before, during and after the event are more than 10 °C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms--perhaps polar stratospheric clouds.sup.7 or hurricane-induced ocean mixing.sup.8--to amplify early Palaeogene polar temperatures., Author(s): Appy Sluijs [sup.1] , Stefan Schouten [sup.2] , Mark Pagani [sup.3] , Martijn Woltering [sup.2] , Henk Brinkhuis [sup.1] , Jaap S. Sinninghe Damsté [sup.2] [sup.4] , Gerald R. [...]

Author(s): Ashna A. Raghoebarsing [1]; Arjan Pol [1]; Katinka T. van de Pas-Schoonen [1]; Alfons J. P. Smolders [2]; Katharina F. Ettwig [1]; W. Irene C. Rijpstra [3]; Stefan Schouten [...]

Author(s): Enno Schefuß (corresponding author) [1, 4]; Stefan Schouten [2]; Ralph R. Schneider [3] Past hydrological changes in Africa have been linked to various climatic processes, depending on region and [...]

Author(s): Ashna A. Raghoebarsing [1]; Alfons J. P. Smolders (corresponding author) [2]; Markus C. Schmid [1]; W. Irene C. Rijpstra [4]; Mieke Wolters-Arts [3]; Jan Derksen [3]; Mike S. M. [...]

Author(s): Hugh C. Jenkyns (corresponding author) [1]; Astrid Forster [2]; Stefan Schouten [2]; Jaap S. Sinninghe Damsté [2] To understand the climate dynamics of the warm, equable greenhouse world of [...]

Author(s): Enno Schefuß (corresponding author) [1]; Stefan Schouten; J. H. Fred Jansen; Jaap S. Sinninghe Damsté The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes [...]

The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants-adapted to warm, dry conditions and low atmospheric CO2 concentrations-have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature and CO2 concentration are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-transported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid-Pleistocene, changes in atmospheric moisture content-driven by tropical sea surface temperature changes and the strength of the African monsoon-controlled aridity on the African continent, and hence large-scale vegetation changes. [ABSTRACT FROM AUTHOR]

Past hydrological changes in Africa have been linked to various climatic processes, depending on region and timescale. Long-term precipitation changes in the regions of northern and southern Africa influenced by the monsoons are thought to have been governed by precessional variations in summer insolation. Conversely, short-term precipitation changes in the northern African tropics have been linked to North Atlantic sea surface temperature anomalies, affecting the northward extension of the Intertropical Convergence Zone and its associated rainbelt. Our knowledge of large-scale hydrological changes in equatorial Africa and their forcing factors is, however, limited. Here we analyse the isotopic composition of terrigenous plant lipids, extracted from a marine sediment core close to the Congo River mouth, in order to reconstruct past central African rainfall variations and compare this record to sea surface temperature changes in the South Atlantic Ocean. We find that central African precipitation during the past 20,000 years was mainly controlled by the difference in sea surface temperatures between the tropics and subtropics of the South Atlantic Ocean, whereas we find no evidence that changes in the position of the Intertropical Convergence Zone had a significant influence on the overall moisture availability in central Africa. We conclude that changes in ocean circulation, and hence sea surface temperature patterns, were important in modulating atmospheric moisture transport onto the central African continent.

The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants--adapted to warm, dry conditions and low atmospheric CO2 concentrations--have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature and CO2 concentration are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-transported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid-Pleistocene, changes in atmospheric moisture content--driven by tropical sea surface temperature changes and the strength of the African monsoon--controlled aridity on the African continent, and hence large-scale vegetation changes.