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Recent research has found that the subsiding Sunda Shelf (Southeast Asia) was permanently exposed prior to ca. 400 ka with initial submersion-exposure cyclicity, associated with interglacial-glacial sea-level cycles, beginning between 400 and 240 ka. We analyzed the impact submersion-exposure cycles on regional environment and climate through a 640 k.y. leaf-wax carbon isotope (d13Cwax) reconstruction at Andaman Sea Site U1448, representing relative changes in C3/C4 plant abundances. Prior to ca. 250 ka, the Sunda region was inhabited by a stable C3 (forest) biome, after which submersion-exposure cycles initiated with the deglacial sea-level rise at ca. 250 ka. During subsequent glacial-age sea-level drops, the newly exposed shelf was rapidly colonized by C4 grasses, followed by slow transitions back to C3 forests, representing a tenfold increase in the variability of C3/C4 vegetation in the Sunda region. The C3/C4 regime shift since 250 ka is coherent across the Southeast (SE) Asia peninsula and Sunda Shelf and is coincident with a shift in the east-west sea-surface temperature gradient in the equatorial Pacific Ocean. We hypothesize that the expansion of C4 grasslands promoted and sustained drier glacial-age climates over SE Asia via a feedback mechanism that contributed to weakening the ascending branch of the east-west atmospheric circulation in the equatorial Pacific region known as the Walker Circulation. Our results indicate that the Sunda Shelf region has a larger influence on Walker Circulation than is seen in current paleoclimate simulations. [ABSTRACT FROM AUTHOR]

Carbon isotope changes in paleosols from Siwalik, Pakistan, and marine sediments from the Bengal Fan indicate a major [C.sub.4] plant expansion in the Himalayan foreland during the late Miocene. However, the timing and mechanisms behind the [C.sub.4] plant expansion remain enigmatic. Here we present high-resolution (~60 k.y.) biomarker and compound-specific isotope data spanning the past 11 m.y. from Ocean Drilling Program Site 722 in the Arabian Sea. An ~5%0--6% increase in leaf wax [[delta].sup.13]C values indicates a marked rise of [C.sub.4] plants from 10 to 5.5 Ma, with accelerated expansion from 7.9 to 5.5 Ma. A concurrent ~50% rise in leaf wax [delta]D values is attributed to a combined effect of changes in precipitation amount and evaporation, indicating that source regions for the plant waxes became progressively drier from 10 to 5.5 Ma. In contrast to earlier reports, our isotope records, biomarker abundances, alkenone [U.sup.K.sub.37], and Globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval. Rather, our results suggest that large-scale hydrological changes drove the late Miocene expansion of [C.sub.4] plants in the Himalayan foreland and Arabian Peninsula. Keywords: [C.sub.4] plants, late Miocene, carbon isotope, hydrology.

The history of the Arctic Ocean during the Cenozoic era (0-65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm 'greenhouse' world, during the late Palaeocene and early Eocene epochs, to a colder 'icehouse' world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ~14 Myr, we find sedimentation rates of 1-2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (~3.2 Myr ago) and East Antarctic ice (~14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (~45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ~49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (~55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change. 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. Analysis of Arctic Ocean sediment core spanning more than 50 million years identifies several key features of Arctic climate history -- the revised timing of the earliest Arctic cooling events implied by this record coincides with those from Antarctica, supporting arguments that climate change is symmetric about the Earth's polar regions., Author(s): Kathryn Moran [sup.1] [sup.31] , Jan Backman [sup.2] , Henk Brinkhuis [sup.3] , Steven C. Clemens [sup.4] , Thomas Cronin [sup.5] , Gerald R. Dickens [sup.6] , Frédérique Eynaud [...]