Abstracts on Global Climate Change

Jun 2007

Dominant factors controlling glacial and interglacial variations in the treeline elevation in tropical Africa

Wu, HB Guiot, J Brewer, S Guo, ZT Peng, CH


The knowledge of tropical palaeoclimates is crucial for understanding global climate change, because it is a test bench for general circulation models that are ultimately used to predict future global warming. A longstanding issue concerning the last glacial maximum in the tropics is the discrepancy between the decrease in sea-surface temperatures reconstructed from marine proxies and the high-elevation decrease in land temperatures estimated from indicators of treeline elevation. In this study, an improved inverse vegetation modeling approach is used to quantitatively reconstruct palaeoclimate and to estimate the effects of different factors (temperature, precipitation, and atmospheric CO2 concentration) on changes in treeline elevation based on a set of pollen data covering an altitudinal range from 100 to 3,140 m above sea level in Africa. We show that lowering of the African treeline during the last glacial maximum was primarily triggered by regional drying, especially at upper elevations, and was amplified by decreases in atmospheric CO2 concentration and perhaps temperature. This contrasts with scenarios for the Holocene and future climates, in which the increase in treeline elevation will be dominated by temperature. Our results suggest that previous temperature changes inferred from tropical treeline shifts may have been overestimated for low-CO2 glacial periods, because the limiting factors that control changes in treeline elevation differ between glacial and interglacial periods.

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Eustasy and sea water Sr composition: application to high-resolution Sr-isotope stratigraphy of Miocene shallow-water carbonates

Kroeger, KF Reuter, M Forst, MH Breisig, S Hartmann, G Brachert, TC

SEDIMENTOLOGY 54:3 565-585

Oceanic Sr-87/Sr-86-isotope ratios are strongly influenced by rates of silicate weathering and therefore linked not only to glaciation but also to sea-level change. The present study combines analysis of sequence stratigraphy and basin architecture with Sr-isotope stratigraphy in Miocene shallow-water sediments in southern Portugal and Crete (Greece). The common method is to use smoothed global sea water Sr-isotope reference curves but here a different approach is chosen. Instead, measured Sr-isotope curves are correlated with unsmoothed reference curves by identification of similar fluctuations in the order of several 100 kyr. Transgressive intervals are characterized by increasing Sr-isotope ratios interpreted as corresponding to intensified silicate weathering as a consequence of deglaciation, while lowstand deposits have low Sr-isotope ratios. Comparison of Sr-isotope curves and sedimentary sequences in the studied basins with independent global delta O-18 data and data on global sea-level might suggest a general relationship, supporting a connection to global climate change. Because of these relationships, the method presented herein has a high potential for use in high-resolution age dating and is also applicable in shallow-water sediments.

SPotGS:paleo | /neutral/paleo | 024

Dec 2006

Interhemispheric anti-phasing of rainfall during the last glacial period

Wang, XF Auler, AS Edwards, RL Cheng, H Ito, E Solheid, M


We have obtained a high-resolution oxygen isotopic record of cave calcite from Caverna Botuvera (27 degrees 13’S, 49 degrees 09’W), southern Brazil, which covers most of the last 36 thousand years (ka), with an average resolution of a few to several decades. The chronology was determined with 46 U/Th ages from two stalagmites. Tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change. We interpret our record in terms of meteoric precipitation changes, hence the variability of South American Monsoon (SAM) intensity. The oxygen isotopic profile broadly follows local insolation changes and shows clear millennial-scale variations during the last glacial period with amplitudes as large as 3 parts per thousand but with smaller centennial-scale shifts (< 1 parts per thousand) during the Holocene. The overall record is strikingly similar to, but strongly anti-correlated with, a number of records from the Northern Hemisphere. We compared our record to other precisely dated contemporaneous records from Hulu Cave eastern China. Minima in 6180 (wet periods, intense SAM) at our site are synchronous with maxima in delta O-18 (dry periods, weak East Asian Monsoon, EAM) in eastern China (within precise dating errors) and vice versa. This anti-phased precipitation relationship between two low-latitude locations may be interhemispheric in extent, based on comparison with records from other sites. Precipitation anti-phasing may be related to north-south shifts in the mean position of the intertropical convergence zone (ITCZ) and asymmetry in Hadley circulation in two hemispheres, associated not with seasonal changes as observed today, but with millennial-scale climate shifts. The millennial-scale atmospheric see-saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor’s greenhouse properties. (c) 2006 Elsevier Ltd. All rights reserved.

melatonin:paleo | /neutral/paleo | 071

Nov 2006

Late Pliocene monsoon linkage in the tropical South China Sea

Tian, J Pak, DK Wang, PX Lea, D Cheng, XR Zhao, QH


The onset of Northern Hemisphere Glaciation (NHG) similar to 2.7 Ma ago coincided with prominent climate changes in the tropical regions such as the African and the Asian monsoons. However, the relationship between tropical and sub-tropical monsoonal variations and high northern latitude ice sheet expansion as well as processes such as late Pliocene tropical sea surface temperature (SST) change is not clear. Our late Pliocene (2.5-3.3 Ma) monsoon proxy records and Mg/Ca derived SST records at Ocean Drilling Program (ODP) Site 1143 from the southern South China Sea (SCS) reveal that while tropical SST shows a stepwise decrease of 2-3 degrees C during this period, the East Asian monsoon gradually strengthens in response to the onset of the NHG. At the 41-kyr and 23-kyr bands, ice volume change lags tropical SST by similar to 4 kyr, but leads the East Asian monsoon by similar to 12-17 kyr. Our finding highlights the significant role of the tropical Pacific region in driving global climate change in the late Pliocene, which has invariable leading phase relative to the ice volume change as in the late Pleistocene. However, the East Asian monsoon shows a linear response to the onset: of the NHG in the late Pliocene, with much bigger phase lagged at the 41-kyr and 23-kyr bands than in the Pleistocene, which suggests that at the obliquity and precession bands the phases of the Plio-Pleistocene East Asian monsoon variations relative to the global ice volume changes are not constant, but variable. Therefore, the East Asian monsoons are not only simply driven by northern summer insolation at the precession period but also modulated by global ice volume change in high latitudes. (c) 2006 Elsevier B.V. All rights reserved.

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The elevation history of the Tibetan Plateau and its implications for the Asian monsoon

Harris, N


The determination of the evolving palaeoaltitude of the Tibetan Plateau, since the India-Eurasia collision underpins our understanding of how orography in central Asia affects the intensity of the monsoon and hence global climate change. Palacoaltitudes, however, cannot be measured directly and need to be inferred from proxy observations that are usually model-dependent. Differing tectonic models for the behaviour of the lithosphere during continental collision have contrasting implications for the elevation of the plateau. However, two techniques recently employed for determining palaeo-elevation are independent of tectonic models, the first involving the variation with altitude of oxygen isotopes in precipitation and the second involving the change of leaf morphology with moist static energy of the atmosphere. Elevation studies have focused on southern Tibet, largely due to the relative ease of access to the region. There is a remarkable unanimity amongst the diverse techniques applied that the altitude of the southern plateau has not significantly changed since at least the mid Miocene (ca. 15 Ma) arguing for an onset of the monsoon system during or before the early Miocene. A range of tectonic studies suggest that the northern and eastern parts of the plateau are younger geornorphological features, but there are few quantitative constraints of the timing of elevation from these regions of Tibet. Since both the elevation and the surface area of the plateau impact on atmospheric circulation, palacoaltitude studies need to be extended to chart the increasing areas of elevated land surface through time. (c) 2006 Elsevier B.V All rights reserved.

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Atypical delta N-15 variations at the southern boundary of the East Pacific oxygen minimum zone over the last 50 ka

Martinez, P Lamy, F Robinson, RR Pichevin, L Billy, I


We report a nitrogen isotope record (ODP Site 1233) from the southern Chile margin at 41 degrees S. The site is located slightly south of the southern boundary of the Peru-Chile upwelling system and the associated oxygen minimum zone off Peru and northern Chile. We show that our nitrogen isotope record, from the time interval 0-50 calendar kiloyears before present (ka B.P.), bears an atypical pattern both in shape and timing when compared with records obtained from either the continental margin of the eastern Pacific or the Subantarctic Zone (SAZ) of the Southern Ocean. The delta N-15 values at Site 1233 are relatively high throughout the record, varying between 9 parts per thousand. and 13 parts per thousand. The iriajor features are a pronounced delta N-15 increase at the beginning of the deglaciation, a maximum from 19 to 10 ka B.P.; thereafter a large decrease during the early Holocene, and millenial scale oscillations showing an Antarctic timing. We propose that the record results from an amalgam of low-latitude and high-latitude processes. Low-latitude processes, including a stronger advection signal of heavy nitrates from the denitrifying zones off Peru and northern Chile, would explain the timing of the deglaciation rise and the heaviest values found over this interval, excluding the Antarctic Cold Reversal period. The overall differences between site 1233 and records from Peru and northwest American margins suggest however that the origin of the delta N-15 signal off Chile is largely controlled by hydrologic and climatic changes in the Southern Ocean. We propose that the interplay between nutrient demand in the SAZ and latitudinal shifts of hydrologic fronts controlled both the concentrations and the isotopic signature of the remaining nitrate delivered to the Chile margin. Then, the glacial surface waters of the southern Chile margin were likely lower in nitrate concentration and bear a higher delta N-15 than during interglacial periods. (c) 2006 Elsevier Ltd. All rights reserved.

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Oct 2006

Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

Kienast, M Kienast, SS Calvert, SE Eglinton, TI Mollenhauer, G Francois, R Mix, AC

NATURE 443:7113 846-849

Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean - atmosphere feedbacks or by changes in the Atlantic thermohaline circulation(1). North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America(2-4), as well as the Indian and East Asian monsoon systems(5,6), but not with tropical Pacific sea surface temperatures(7). Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of 1 degrees C, synchronous ( within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of 0.5 degrees C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from Th-230-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans’ thermohaline circulation, although possibly amplified by tropical ocean - atmosphere interaction as suggested before(8).

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Integration of ice-core, marine and terrestrial records for the Australian Last Glacial Maximum and Termination: a contribution from the OZ INTIMATE group

Turney, CSM Haberle, S Fink, D Kershaw, AP Barbetti, M Barrows, TT Black, M Cohen, T Correge, T Hesse, PP Hua, Q Johnston, R Morgan, V Moss, P Nanson, G Van Ommen, T Rule, S Williams, NJ Zhao, JX D’Costa, D Feng, YX Gagan, MK Mooney, S Xia, Q


The degree to which Southern Hemisphere climatic changes during the end of the last glacial period and early Holocene (30-8 ka) were influenced or initiated by events occurring in the high latitudes of the Northern Hemisphere is a complex issue. There is conflicting evidence for the degree of hemispheric ‘teleconnection’ and an unresolved debate as to the principle forcing mechanism(s). The available hypotheses are difficult to test robustly, however, because the few detailed palaeoclimatic records in the Southern Hemisphere are widely dispersed and lack duplication. Here we present climatic and environmental reconstructions from across Australia, a key region of the Southern Hemisphere because of the range of environments it covers and the potentially important role regional atmospheric and oceanic controls play in global climate change. We identify a general scheme of events for the end of the last glacial period and early Holocene but a detailed reconstruction proved problematic. Significant progress in climate quantification and geochronological control is now urgently required to robustly investigate change through this period. Copyright (c) 2006 John Wiley & Sons, Ltd.

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Oct 2005

Paleoceanographic records and sea ice extension history on the slope of the northern Bering Sea over the last 100 ka BP

Wang, RJ Li, X Xiao, WS Xia, PF Chen, RH


Quantitative analytic results of the biogenic components in Core B2-9 from the northern Bering Sea slope indicate that the coarse fraction and opal content, serving as proxies of surface productivity, have increased stepwise since the marine isotope stage(MIS)5.3, reflecting periodic enhancement in surface productivity. The surface productivity attained its highest level during the Holocene, followed by MIS 3.2 to 2 and then MIS 5.3 to 3.3 with a lowest level. High total organic carbon(TOC) contents, together with high C/N ratios, which stand mostly between 7 and 20, show that the TOC was deposited from mixing sources. Therefore,one has to be cautious to use TOC as a proxy of surface productivity. The high TOC and C/N ratio during MIS 5.1, 3.3 to 3.2 and the Holocene reflect that the terrigenous organic matter input increased during interglacial periods. Increases in the fine- and silt-grained terrigenous components from MIS 5.3 to the middle Holocene imply that with the cooling climate, sea ice on the Bering Sea slope extended continuously. Ice-rafted and charcoal detritus increased during glacial, interstadial and the last deglaciation periods and decreased during interglacial periods, suggesting that sea ice on the slope increased and melted, respectively, during glacial and interglacial periods. The extension of sea ice during glacial periods,which was linked with the climate over the North American Continent, responded to global climate change during late Quaternary glacial and interglacial cycles.

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Dec 2004

Paleoclimate and faunal evolution in the Plio-Pleistocene of Africa and South America

Quinteros, RB Behrensmeyer, AK Ormazabal, GC

AMEGHINIANA 41:4 641-649

PALEOCLIMATE AND FAUNAL EVOLUTION IN THE PLIO-PLEISTOCENE OF AFRICA AND SOUTH AMERICA. Climatic change is often hypothesized to be a major variable in forcing evolutionary change. Recent work on Pliocene and Pleistocene fossil mammals from the Turkana Basin of Kenya and Ethiopia shows that climate may play an important role in the spread of savanna environments, and thus in the increasing abundance of mammals adapted to open and seasonally and conditions. If global climate change is behind some of these trends in East Africa, we hypothesize that similar patterns of faunal change may occur elsewhere. An analysis of fossil mammals from the Pampean region of Argentina shows an increase in the number of species adapted to open and seasonally and conditions through the Plio-Pleistocene. The South American pattern is not identical to the East African pattern, but both show a significant increase in open and and adapted mammals shortly after 2 million years ago, at the very beginning of the Pleistocene. Although global climate change may be invoked as a common cause of these intercontinental trends, local and regional geography and tectonics play a critical role in modulating the global signal.

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Evolution of late glacial ice-marginal lakes on the northwestern Canadian Shield and their influence on the location of the Dubawnt Lake palaeo-ice stream

Stokes, CR Clark, CD


During deglaciation of the North American Laurentide Ice Sheet large proglacial lakes developed in positions where proglacial drainage was impeded by the ice margin. For some of these lakes, it is known that subsequent drainage had an abrupt and widespread impact on North Atlantic Ocean circulation and climate, but less is known about the impact that the lakes exerted on ice sheet dynamics. This paper reports palaeogeographic reconstructions of the evolution of proglacial lakes during deglaciation across the northwestern Canadian Shield, covering an area in excess of 1,000,000 km(2) as the ice sheet retreated some 600 km. The interactions between proglacial lakes and ice sheet flow are explored, with a particular emphasis on whether the disposition of lakes may have influenced the location of the Dubawnt Lake ice stream. This ice stream falls outside the existing paradigm for ice streams in the Laurentide Ice Sheet because it did not operate over fined-grained till or lie in a topographic trough. Ice margin positions and a digital elevation model are utilised to predict the geometry and depth of proglacial takes impounded at the margin at 30-km increments during deglaciation. Palaeogeographic reconstructions match well with previous independent estimates of lake coverage inferred from field evidence, and results suggest that the development of a deep lake in the Thelon drainage basin may have been influential in initiating the ice stream by inducing calving, drawing down ice and triggering fast ice flow. This is the only location alongside this sector of the ice sheet where large (>3000 km(2)), deep lakes (similar to120 m) are impounded for a significant length of time and exactly matches the location of the ice stream. It is speculated that the commencement of calving at the ice sheet margin may have taken the system beyond a threshold and was sufficient to trigger rapid motion but that once initiated, calving processes and losses were insignificant to the functioning of the ice stream. It is thus concluded that proglacial lakes are likely to have been an important control on ice sheet dynamics during deglaciation of the Laurentide Ice Sheet. (C) 2004 Elsevier B.V. All rights reserved.

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