Abrupt onset and termination of the African Humid Period:
Identifieur interne : 000396 ( Main/Corpus ); précédent : 000395; suivant : 000397Abrupt onset and termination of the African Humid Period:
Auteurs : Peter Demenocal ; Joseph Ortiz ; Tom Guilderson ; Jess Adkins ; Michael Sarnthein ; Linda Baker ; Martha YarusinskySource :
- Quaternary Science Reviews [ 0277-3791 ] ; 1999.
Abstract
A detailed (ca. 100yr resolution) and well-dated (18 AMS 14C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.
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DOI: 10.1016/S0277-3791(99)00081-5
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Accelerator Mass Spectrometry, Lawrence-Livermore National Laboratory, Livermore CA 94551, USA</mods:affiliation></affiliation></author><author><name sortKey="Adkins, Jess" sort="Adkins, Jess" uniqKey="Adkins J" first="Jess" last="Adkins">Jess Adkins</name><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author><author><name sortKey="Sarnthein, Michael" sort="Sarnthein, Michael" uniqKey="Sarnthein M" first="Michael" last="Sarnthein">Michael Sarnthein</name><affiliation><mods:affiliation>Institute Fuer Geowissens Chafter, Universitaet Kiel, Kiel, Germany</mods:affiliation></affiliation></author><author><name sortKey="Baker, Linda" sort="Baker, Linda" uniqKey="Baker L" first="Linda" last="Baker">Linda Baker</name><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author><author><name sortKey="Yarusinsky, Martha" sort="Yarusinsky, Martha" uniqKey="Yarusinsky M" first="Martha" last="Yarusinsky">Martha Yarusinsky</name><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">ISTEX</idno><idno type="RBID">ISTEX:C16A77669E4CDAB26534C698270243899539C50F</idno><date when="1999" year="1999">1999</date><idno type="doi">10.1016/S0277-3791(99)00081-5</idno><idno type="url">https://api.istex.fr/document/C16A77669E4CDAB26534C698270243899539C50F/fulltext/pdf</idno><idno type="wicri:Area/Main/Corpus">000396</idno></publicationStmt><sourceDesc><biblStruct><analytic><title level="a">Abrupt onset and termination of the African Humid Period:</title><author><name sortKey="Demenocal, Peter" sort="Demenocal, Peter" uniqKey="Demenocal P" first="Peter" last="Demenocal">Peter Demenocal</name><affiliation><mods:affiliation>E-mail: peter@ldeo.columbia.edu</mods:affiliation></affiliation><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author><author><name sortKey="Ortiz, Joseph" sort="Ortiz, Joseph" uniqKey="Ortiz J" first="Joseph" last="Ortiz">Joseph Ortiz</name><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author><author><name sortKey="Guilderson, Tom" sort="Guilderson, Tom" uniqKey="Guilderson T" first="Tom" last="Guilderson">Tom Guilderson</name><affiliation><mods:affiliation>Center for Accelerator Mass Spectrometry, Lawrence-Livermore National Laboratory, Livermore CA 94551, USA</mods:affiliation></affiliation></author><author><name sortKey="Adkins, Jess" sort="Adkins, Jess" uniqKey="Adkins J" first="Jess" last="Adkins">Jess Adkins</name><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author><author><name sortKey="Sarnthein, Michael" sort="Sarnthein, Michael" uniqKey="Sarnthein M" first="Michael" last="Sarnthein">Michael Sarnthein</name><affiliation><mods:affiliation>Institute Fuer Geowissens Chafter, Universitaet Kiel, Kiel, Germany</mods:affiliation></affiliation></author><author><name sortKey="Baker, Linda" sort="Baker, Linda" uniqKey="Baker L" first="Linda" last="Baker">Linda Baker</name><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author><author><name sortKey="Yarusinsky, Martha" sort="Yarusinsky, Martha" uniqKey="Yarusinsky M" first="Martha" last="Yarusinsky">Martha Yarusinsky</name><affiliation><mods:affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</mods:affiliation></affiliation></author></analytic><monogr></monogr><series><title level="j">Quaternary Science Reviews</title><title level="j" type="abbrev">JQSR</title><idno type="ISSN">0277-3791</idno><imprint><publisher>ELSEVIER</publisher><date type="published" when="1999">1999</date><biblScope unit="volume">19</biblScope><biblScope unit="issue">1–5</biblScope><biblScope unit="page" from="347">347</biblScope><biblScope unit="page" to="361">361</biblScope></imprint><idno type="ISSN">0277-3791</idno></series><idno type="istex">C16A77669E4CDAB26534C698270243899539C50F</idno><idno type="DOI">10.1016/S0277-3791(99)00081-5</idno><idno type="PII">S0277-3791(99)00081-5</idno></biblStruct></sourceDesc><seriesStmt><idno type="ISSN">0277-3791</idno></seriesStmt></fileDesc><profileDesc><textClass></textClass><langUsage><language ident="en">en</language></langUsage></profileDesc></teiHeader><front><div type="abstract" xml:lang="en">A detailed (ca. 100yr resolution) and well-dated (18 AMS 14C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.</div></front></TEI><istex><corpusName>elsevier</corpusName><author><json:item><name>Peter deMenocal</name><affiliations><json:string>E-mail: peter@ldeo.columbia.edu</json:string><json:string>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</json:string></affiliations></json:item><json:item><name>Joseph Ortiz</name><affiliations><json:string>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</json:string></affiliations></json:item><json:item><name>Tom Guilderson</name><affiliations><json:string>Center for Accelerator Mass Spectrometry, Lawrence-Livermore National Laboratory, Livermore CA 94551, USA</json:string></affiliations></json:item><json:item><name>Jess Adkins</name><affiliations><json:string>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</json:string></affiliations></json:item><json:item><name>Michael Sarnthein</name><affiliations><json:string>Institute Fuer Geowissens Chafter, Universitaet Kiel, Kiel, Germany</json:string></affiliations></json:item><json:item><name>Linda Baker</name><affiliations><json:string>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</json:string></affiliations></json:item><json:item><name>Martha Yarusinsky</name><affiliations><json:string>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</json:string></affiliations></json:item></author><language><json:string>eng</json:string></language><abstract>A detailed (ca. 100yr resolution) and well-dated (18 AMS 14C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.</abstract><qualityIndicators><score>7.568</score><pdfVersion>1.2</pdfVersion><pdfPageSize>569 x 771 pts</pdfPageSize><refBibsNative>true</refBibsNative><keywordCount>0</keywordCount><abstractCharCount>1513</abstractCharCount><pdfWordCount>9042</pdfWordCount><pdfCharCount>58087</pdfCharCount><pdfPageCount>15</pdfPageCount><abstractWordCount>214</abstractWordCount></qualityIndicators><title>Abrupt onset and termination of the African Humid Period:</title><pii><json:string>S0277-3791(99)00081-5</json:string></pii><genre><json:string>research-article</json:string></genre><host><volume>19</volume><pii><json:string>S0277-3791(00)X0046-7</json:string></pii><pages><last>361</last><first>347</first></pages><issn><json:string>0277-3791</json:string></issn><issue>1–5</issue><genre></genre><language><json:string>unknown</json:string></language><title>Quaternary Science Reviews</title><publicationDate>2000</publicationDate></host><categories><wos><json:string>GEOGRAPHY, PHYSICAL</json:string><json:string>GEOLOGY</json:string><json:string>GEOSCIENCES, MULTIDISCIPLINARY</json:string></wos></categories><publicationDate>1999</publicationDate><copyrightDate>1999</copyrightDate><doi><json:string>10.1016/S0277-3791(99)00081-5</json:string></doi><id>C16A77669E4CDAB26534C698270243899539C50F</id><fulltext><json:item><original>true</original><mimetype>application/pdf</mimetype><extension>pdf</extension><uri>https://api.istex.fr/document/C16A77669E4CDAB26534C698270243899539C50F/fulltext/pdf</uri></json:item><json:item><original>false</original><mimetype>application/zip</mimetype><extension>zip</extension><uri>https://api.istex.fr/document/C16A77669E4CDAB26534C698270243899539C50F/fulltext/zip</uri></json:item><istex:fulltextTEI uri="https://api.istex.fr/document/C16A77669E4CDAB26534C698270243899539C50F/fulltext/tei"><teiHeader><fileDesc><titleStmt><title level="a">Abrupt onset and termination of the African Humid Period:</title><title level="a" type="sub" xml:lang="">rapid climate responses to gradual insolation forcing</title></titleStmt><publicationStmt><authority>ISTEX</authority><publisher>ELSEVIER</publisher><availability><p>ELSEVIER</p></availability><date>1999</date></publicationStmt><notesStmt><note type="content">Fig. 1: Seasonal climatology of surface winds, rainfall, and atmospheric dust trajectories over subtropical West Africa. During boreal winter months the land surface cools relative to the ocean and regional atmospheric circulation is dominated by the NE trade winds which advect African dust to the eastern equatorial Atlantic. The winter African dust trajectory (stippled pattern) follows the NE–SW pattern of the transporting winter trade winds (Pye, 1987). During boreal summer increased sensible heating over central North Africa drives the cyclonic inflow of moisture-laden air from the adjacent eastern equatorial Atlantic which brings sporadic but intense monsoon rains to the sub-Saharan and Sahel regions of West Africa. The summer African dust plume (stippled pattern) results from strong surface turbulence associated with monsoonal frontal systems and entrained particles are convectively lifted to mid-tropospheric levels and then transported westward by the African Easterly Jet (Pye, 1987; Schütz et al., 1981). Interannual variations in African dust export are highly correlated to regional precipitation anomalies (Prospero, 1981; Prospero and Nees, 1977; Prospero and Nees, 1986). The location of Ocean Drilling Program Site 658 off Cap Blanc, Mauritania is shown.</note><note type="content">Fig. 2: Sediment composition data and AMS radiocarbon age control from Site 658C. Samples were taken continuously at 2cm intervals, which is roughly equivalent to 50–150yr based on the 18cm/ka average sedimention rates at Site 658C. A brief hiatus between 14.8 and 17.2cal. ka BP is indicated by two closely spaced AMS radiocarbon dates at 324 and 328cm (Table 1). Note the very abrupt changes in sediment composition which occur at 326cm (ca. 14.8cal. ka BP), 260cm (12.3cal. ka BP), and 125cm (ca. 5.5cal. ka BP).</note><note type="content">Fig. 3: Biogenic carbonate, opal and terrigenous sediment percentage and flux records (dashed lines, in g/cm2/ka) from Site 658C. Eighteen AMS radiocarbon dates were used to constrain the carbonate, opal, and terrigenous percentage time series; age control points are indicated by filled triangle symbols to the left of the figure. Radiocarbon ages were converted to calibrated calendar ages using the Calib 3.03 program (Stuiver and Reimer, 1993) after applying a 500yr reservoir correction to the raw 14C ages. Flux data were calculated using the “nominal” age model derived from a subset of 14C age control levels. Flux data are shown by dashed lines and the nominal age control points are shown by open triangles to the right of the figure. Note the abruptness of the 14.8, 12.3, and 5.5cal. ka BP sediment composition transitions, each of which were completed within several centuries.</note><note type="content">Fig. 4: Comparison of the boreal summer (JJA) average insolation computed for 20°N (Berger and Loutre, 1991) with the Site 658C benthic oxygen isotope record (from analyses of C. wuellerstorfi), and terrigenous (eolian) percentage and flux records spanning the last 25cal. ka BP. Note the onset and termination of the African Humid Period in terms of the low eolian dust flux at Site 658C between ca. 14.8ka and 5.5cal. ka BP associated with the early Holocene rise in summer insolation forcing of the African monsoon. Atmospheric methane concentrations preserved in occluded ice bubbles and the oxygen isotopic composition of glacial ice in the GISP2 Greenland ice core are also shown (Blunier et al., 1995; Dansgaard, 1993). The onset of the African Humid Period was synchronous with the end of cold, glacial conditions in Europe, which occurred by ca. 14.5cal. ka BP, corresponding to the end of Heinrich event 1 in the North Atlantic (Blunier et al., 1995; Bond et al., 1993; Broecker et al., 1993; Dansgaard, 1993). The rapid onset of humid conditions in the tropics is also recorded by the abrupt increase in atmospheric methane at ca. 14.7cal. ka BP, which documents the rapid expansion of tropical wetland methane sources (Blunier et al., 1995). The Site 658C data confirm terrestrial African paleoclimate records which document a brief (ca. 1ka) interval of more arid conditions associated with the cool Younger Dryas Chronozone (Gasse et al., 1989; Gasse et al., 1990; Roberts et al., 1993; Street-Perrott and Perrott, 1990; Williamson et al., 1993). The termination of the African Humid Period at 5.5cal. ka BP coincides with the mid-Holocene minimum in atmospheric methane concentration. The subsequent late Holocene methane rise has been attributed to the expansion of boreal wetlands which were absent during the first stages of the deglaciation (Blunier et al., 1995). The timing of the terrigenous transitions at ca. 5.5 and 14.8cal. ka BP are indicated on the JJA insolation curve (filled symbols).</note><note type="content">Fig. 5: The linear relationship between orbital insolation forcing and simulated changes in African monsoonal precipitation using an atmosphere-only climate model which does not employ coupled ocean and/or vegetation feedbacks (from Prell and Kutzbach, 1987). The monsoon is strongly linked to orbital changes in seasonal radiation forcing, but there is no indication of non-linear or threshold responses from these early atmosphere-only experiments. Strongly positive, non-linear amplifications of the primary insolation forcing of monsoonal climate are achieved in climate model simulations which explicitly include coupled vegetation, land surface processes, and surface ocean responses (see text).</note><note type="content">Fig. 6: Average Northern Hemisphere summer insolation (JJA, in W/m2) used to force the coupled ocean–atmosphere–vegetation CLIMBER2 model for the 9000yr BP to present transient simulation of Holocene changes in subtropical African climate (Claussen et al., 1999) (a). In the fully coupled model, annual Saharan precipitation (b) and fractional Saharan vegetation cover (c) decreases very abruptly at 5440±30yr BP (based on an ensemble average of 10 separate transient simulations; (Claussen et al., 1999). The abrupt African climate responses were attributed to the highly non-linear (positive) feedbacks linking progressive decreases in regional precipitation, vegetation cover loss, and increasing albedo (Claussen et al., 1999). Note the remarkable similarity between the shape and timing of the abrupt mid-Holocene termination of the African Humid Period at 5.5cal. ka BP for the climate model simulation (b), (c) and the Site 658C record of West African eolian dust supply variations (d).</note><note type="content">Table 1: AMS radiocarbon dating of Site 658C planktonic foraminifera. All values were measured at the Center for Accelerator Mass Spectrometry (CAMS) at the Lawrence Livermore National Laboratory. Samples of between 1000 and 1500 picked, monospecific planktonic foraminifera (G. bulloides) were sonicated in deionized water prior to analysis. Calibrated ages were rounded to the nearest decade (Stuiver and Reimer, 1993)b</note><note type="content">Table 2: Mean sediment component accumulation rates at Site 658C</note></notesStmt><sourceDesc><biblStruct type="inbook"><analytic><title level="a">Abrupt onset and termination of the African Humid Period:</title><title level="a" type="sub">rapid climate responses to gradual insolation forcing</title><author><persName><forename type="first">Peter</forename><surname>deMenocal</surname></persName><email>peter@ldeo.columbia.edu</email><note type="correspondence"><p>Corresponding author. Tel.: 001-914-365-8483; fax: 001-914-365-8165. Lamont–Doherty Earth Observatory, of Columbia University, Geoscience 211, Palisades, NY 10964, USA</p></note><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation></author><author><persName><forename type="first">Joseph</forename><surname>Ortiz</surname></persName><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation></author><author><persName><forename type="first">Tom</forename><surname>Guilderson</surname></persName><affiliation>Center for Accelerator Mass Spectrometry, Lawrence-Livermore National Laboratory, Livermore CA 94551, USA</affiliation></author><author><persName><forename type="first">Jess</forename><surname>Adkins</surname></persName><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation></author><author><persName><forename type="first">Michael</forename><surname>Sarnthein</surname></persName><affiliation>Institute Fuer Geowissens Chafter, Universitaet Kiel, Kiel, Germany</affiliation></author><author><persName><forename type="first">Linda</forename><surname>Baker</surname></persName><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation></author><author><persName><forename type="first">Martha</forename><surname>Yarusinsky</surname></persName><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation></author></analytic><monogr><title level="j">Quaternary Science Reviews</title><title level="j" type="abbrev">JQSR</title><idno type="pISSN">0277-3791</idno><idno type="PII">S0277-3791(00)X0046-7</idno><imprint><publisher>ELSEVIER</publisher><date type="published" when="1999"></date><biblScope unit="volume">19</biblScope><biblScope unit="issue">1–5</biblScope><biblScope unit="page" from="347">347</biblScope><biblScope unit="page" to="361">361</biblScope></imprint></monogr><idno type="istex">C16A77669E4CDAB26534C698270243899539C50F</idno><idno type="DOI">10.1016/S0277-3791(99)00081-5</idno><idno type="PII">S0277-3791(99)00081-5</idno></biblStruct></sourceDesc></fileDesc><profileDesc><creation><date>1999</date></creation><langUsage><language ident="en">en</language></langUsage><abstract xml:lang="en"><p>A detailed (ca. 100yr resolution) and well-dated (18 AMS 14C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.</p></abstract></profileDesc><revisionDesc><change when="1999">Published</change></revisionDesc></teiHeader></istex:fulltextTEI><json:item><original>false</original><mimetype>text/plain</mimetype><extension>txt</extension><uri>https://api.istex.fr/document/C16A77669E4CDAB26534C698270243899539C50F/fulltext/txt</uri></json:item></fulltext><metadata><istex:metadataXml wicri:clean="Elsevier, elements deleted: ce:floats; body; tail"><istex:xmlDeclaration>version="1.0" encoding="utf-8"</istex:xmlDeclaration><istex:docType PUBLIC="-//ES//DTD journal article DTD version 4.5.2//EN//XML" URI="art452.dtd" name="istex:docType"><istex:entity SYSTEM="gr1" NDATA="IMAGE" name="gr1"></istex:entity><istex:entity SYSTEM="gr2" NDATA="IMAGE" name="gr2"></istex:entity><istex:entity SYSTEM="gr3" NDATA="IMAGE" name="gr3"></istex:entity><istex:entity SYSTEM="gr4" NDATA="IMAGE" name="gr4"></istex:entity><istex:entity SYSTEM="gr5" NDATA="IMAGE" name="gr5"></istex:entity><istex:entity SYSTEM="gr6" NDATA="IMAGE" name="gr6"></istex:entity></istex:docType><istex:document><converted-article version="4.5.2" docsubtype="fla"><item-info><jid>JQSR</jid><aid>673</aid><ce:pii>S0277-3791(99)00081-5</ce:pii><ce:doi>10.1016/S0277-3791(99)00081-5</ce:doi><ce:copyright type="full-transfer" year="1999">Elsevier Science Ltd</ce:copyright></item-info><head><ce:title>Abrupt onset and termination of the African Humid Period:</ce:title><ce:subtitle>rapid climate responses to gradual insolation forcing</ce:subtitle><ce:author-group><ce:author><ce:given-name>Peter</ce:given-name><ce:surname>deMenocal</ce:surname><ce:cross-ref refid="ORFA"><ce:sup>a</ce:sup></ce:cross-ref><ce:cross-ref refid="CORR1">*</ce:cross-ref><ce:e-address>peter@ldeo.columbia.edu</ce:e-address></ce:author><ce:author><ce:given-name>Joseph</ce:given-name><ce:surname>Ortiz</ce:surname><ce:cross-ref refid="ORFA"><ce:sup>a</ce:sup></ce:cross-ref></ce:author><ce:author><ce:given-name>Tom</ce:given-name><ce:surname>Guilderson</ce:surname><ce:cross-ref refid="ORFB"><ce:sup>b</ce:sup></ce:cross-ref></ce:author><ce:author><ce:given-name>Jess</ce:given-name><ce:surname>Adkins</ce:surname><ce:cross-ref refid="ORFA"><ce:sup>a</ce:sup></ce:cross-ref></ce:author><ce:author><ce:given-name>Michael</ce:given-name><ce:surname>Sarnthein</ce:surname><ce:cross-ref refid="ORFC"><ce:sup>c</ce:sup></ce:cross-ref></ce:author><ce:author><ce:given-name>Linda</ce:given-name><ce:surname>Baker</ce:surname><ce:cross-ref refid="ORFA"><ce:sup>a</ce:sup></ce:cross-ref></ce:author><ce:author><ce:given-name>Martha</ce:given-name><ce:surname>Yarusinsky</ce:surname><ce:cross-ref refid="ORFA"><ce:sup>a</ce:sup></ce:cross-ref></ce:author><ce:affiliation id="ORFA"><ce:label>a</ce:label><ce:textfn>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</ce:textfn></ce:affiliation><ce:affiliation id="ORFB"><ce:label>b</ce:label><ce:textfn>Center for Accelerator Mass Spectrometry, Lawrence-Livermore National Laboratory, Livermore CA 94551, USA</ce:textfn></ce:affiliation><ce:affiliation id="ORFC"><ce:label>c</ce:label><ce:textfn>Institute Fuer Geowissens Chafter, Universitaet Kiel, Kiel, Germany</ce:textfn></ce:affiliation><ce:correspondence id="CORR1"><ce:label>*</ce:label><ce:text>Corresponding author. Tel.: 001-914-365-8483; fax: 001-914-365-8165. Lamont–Doherty Earth Observatory, of Columbia University, Geoscience 211, Palisades, NY 10964, USA</ce:text></ce:correspondence></ce:author-group><ce:abstract><ce:section-title>Abstract</ce:section-title><ce:abstract-sec><ce:simple-para>A detailed (ca. 100<ce:hsp sp="0.25"></ce:hsp>yr resolution) and well-dated (18 AMS <math altimg="si3.gif"><sup loc="pre">14</sup><rm>C</rm></math> dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.</ce:simple-para></ce:abstract-sec></ce:abstract></head></converted-article></istex:document></istex:metadataXml><mods version="3.6"><titleInfo><title>Abrupt onset and termination of the African Humid Period:</title><subTitle>rapid climate responses to gradual insolation forcing</subTitle></titleInfo><titleInfo type="alternative" contentType="CDATA"><title>Abrupt onset and termination of the African Humid Period:</title><subTitle>rapid climate responses to gradual insolation forcing</subTitle></titleInfo><name type="personal"><namePart type="given">Peter</namePart><namePart type="family">deMenocal</namePart><affiliation>E-mail: peter@ldeo.columbia.edu</affiliation><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation><description>Corresponding author. Tel.: 001-914-365-8483; fax: 001-914-365-8165. Lamont–Doherty Earth Observatory, of Columbia University, Geoscience 211, Palisades, NY 10964, USA</description><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="given">Joseph</namePart><namePart type="family">Ortiz</namePart><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="given">Tom</namePart><namePart type="family">Guilderson</namePart><affiliation>Center for Accelerator Mass Spectrometry, Lawrence-Livermore National Laboratory, Livermore CA 94551, USA</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="given">Jess</namePart><namePart type="family">Adkins</namePart><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="given">Michael</namePart><namePart type="family">Sarnthein</namePart><affiliation>Institute Fuer Geowissens Chafter, Universitaet Kiel, Kiel, Germany</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="given">Linda</namePart><namePart type="family">Baker</namePart><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="given">Martha</namePart><namePart type="family">Yarusinsky</namePart><affiliation>Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA</affiliation><role><roleTerm type="text">author</roleTerm></role></name><typeOfResource>text</typeOfResource><genre type="research-article" displayLabel="Full-length article"></genre><originInfo><publisher>ELSEVIER</publisher><dateIssued encoding="w3cdtf">1999</dateIssued><copyrightDate encoding="w3cdtf">1999</copyrightDate></originInfo><language><languageTerm type="code" authority="iso639-2b">eng</languageTerm><languageTerm type="code" authority="rfc3066">en</languageTerm></language><physicalDescription><internetMediaType>text/html</internetMediaType></physicalDescription><abstract lang="en">A detailed (ca. 100yr resolution) and well-dated (18 AMS 14C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.</abstract><note type="content">Fig. 1: Seasonal climatology of surface winds, rainfall, and atmospheric dust trajectories over subtropical West Africa. During boreal winter months the land surface cools relative to the ocean and regional atmospheric circulation is dominated by the NE trade winds which advect African dust to the eastern equatorial Atlantic. The winter African dust trajectory (stippled pattern) follows the NE–SW pattern of the transporting winter trade winds (Pye, 1987). During boreal summer increased sensible heating over central North Africa drives the cyclonic inflow of moisture-laden air from the adjacent eastern equatorial Atlantic which brings sporadic but intense monsoon rains to the sub-Saharan and Sahel regions of West Africa. The summer African dust plume (stippled pattern) results from strong surface turbulence associated with monsoonal frontal systems and entrained particles are convectively lifted to mid-tropospheric levels and then transported westward by the African Easterly Jet (Pye, 1987; Schütz et al., 1981). Interannual variations in African dust export are highly correlated to regional precipitation anomalies (Prospero, 1981; Prospero and Nees, 1977; Prospero and Nees, 1986). The location of Ocean Drilling Program Site 658 off Cap Blanc, Mauritania is shown.</note><note type="content">Fig. 2: Sediment composition data and AMS radiocarbon age control from Site 658C. Samples were taken continuously at 2cm intervals, which is roughly equivalent to 50–150yr based on the 18cm/ka average sedimention rates at Site 658C. A brief hiatus between 14.8 and 17.2cal. ka BP is indicated by two closely spaced AMS radiocarbon dates at 324 and 328cm (Table 1). Note the very abrupt changes in sediment composition which occur at 326cm (ca. 14.8cal. ka BP), 260cm (12.3cal. ka BP), and 125cm (ca. 5.5cal. ka BP).</note><note type="content">Fig. 3: Biogenic carbonate, opal and terrigenous sediment percentage and flux records (dashed lines, in g/cm2/ka) from Site 658C. Eighteen AMS radiocarbon dates were used to constrain the carbonate, opal, and terrigenous percentage time series; age control points are indicated by filled triangle symbols to the left of the figure. Radiocarbon ages were converted to calibrated calendar ages using the Calib 3.03 program (Stuiver and Reimer, 1993) after applying a 500yr reservoir correction to the raw 14C ages. Flux data were calculated using the “nominal” age model derived from a subset of 14C age control levels. Flux data are shown by dashed lines and the nominal age control points are shown by open triangles to the right of the figure. Note the abruptness of the 14.8, 12.3, and 5.5cal. ka BP sediment composition transitions, each of which were completed within several centuries.</note><note type="content">Fig. 4: Comparison of the boreal summer (JJA) average insolation computed for 20°N (Berger and Loutre, 1991) with the Site 658C benthic oxygen isotope record (from analyses of C. wuellerstorfi), and terrigenous (eolian) percentage and flux records spanning the last 25cal. ka BP. Note the onset and termination of the African Humid Period in terms of the low eolian dust flux at Site 658C between ca. 14.8ka and 5.5cal. ka BP associated with the early Holocene rise in summer insolation forcing of the African monsoon. Atmospheric methane concentrations preserved in occluded ice bubbles and the oxygen isotopic composition of glacial ice in the GISP2 Greenland ice core are also shown (Blunier et al., 1995; Dansgaard, 1993). The onset of the African Humid Period was synchronous with the end of cold, glacial conditions in Europe, which occurred by ca. 14.5cal. ka BP, corresponding to the end of Heinrich event 1 in the North Atlantic (Blunier et al., 1995; Bond et al., 1993; Broecker et al., 1993; Dansgaard, 1993). The rapid onset of humid conditions in the tropics is also recorded by the abrupt increase in atmospheric methane at ca. 14.7cal. ka BP, which documents the rapid expansion of tropical wetland methane sources (Blunier et al., 1995). The Site 658C data confirm terrestrial African paleoclimate records which document a brief (ca. 1ka) interval of more arid conditions associated with the cool Younger Dryas Chronozone (Gasse et al., 1989; Gasse et al., 1990; Roberts et al., 1993; Street-Perrott and Perrott, 1990; Williamson et al., 1993). The termination of the African Humid Period at 5.5cal. ka BP coincides with the mid-Holocene minimum in atmospheric methane concentration. The subsequent late Holocene methane rise has been attributed to the expansion of boreal wetlands which were absent during the first stages of the deglaciation (Blunier et al., 1995). The timing of the terrigenous transitions at ca. 5.5 and 14.8cal. ka BP are indicated on the JJA insolation curve (filled symbols).</note><note type="content">Fig. 5: The linear relationship between orbital insolation forcing and simulated changes in African monsoonal precipitation using an atmosphere-only climate model which does not employ coupled ocean and/or vegetation feedbacks (from Prell and Kutzbach, 1987). The monsoon is strongly linked to orbital changes in seasonal radiation forcing, but there is no indication of non-linear or threshold responses from these early atmosphere-only experiments. Strongly positive, non-linear amplifications of the primary insolation forcing of monsoonal climate are achieved in climate model simulations which explicitly include coupled vegetation, land surface processes, and surface ocean responses (see text).</note><note type="content">Fig. 6: Average Northern Hemisphere summer insolation (JJA, in W/m2) used to force the coupled ocean–atmosphere–vegetation CLIMBER2 model for the 9000yr BP to present transient simulation of Holocene changes in subtropical African climate (Claussen et al., 1999) (a). In the fully coupled model, annual Saharan precipitation (b) and fractional Saharan vegetation cover (c) decreases very abruptly at 5440±30yr BP (based on an ensemble average of 10 separate transient simulations; (Claussen et al., 1999). The abrupt African climate responses were attributed to the highly non-linear (positive) feedbacks linking progressive decreases in regional precipitation, vegetation cover loss, and increasing albedo (Claussen et al., 1999). Note the remarkable similarity between the shape and timing of the abrupt mid-Holocene termination of the African Humid Period at 5.5cal. ka BP for the climate model simulation (b), (c) and the Site 658C record of West African eolian dust supply variations (d).</note><note type="content">Table 1: AMS radiocarbon dating of Site 658C planktonic foraminifera. All values were measured at the Center for Accelerator Mass Spectrometry (CAMS) at the Lawrence Livermore National Laboratory. Samples of between 1000 and 1500 picked, monospecific planktonic foraminifera (G. bulloides) were sonicated in deionized water prior to analysis. Calibrated ages were rounded to the nearest decade (Stuiver and Reimer, 1993)b</note><note type="content">Table 2: Mean sediment component accumulation rates at Site 658C</note><relatedItem type="host"><titleInfo><title>Quaternary Science Reviews</title></titleInfo><titleInfo type="abbreviated"><title>JQSR</title></titleInfo><genre type="Journal">journal</genre><originInfo><dateIssued encoding="w3cdtf">20000101</dateIssued></originInfo><identifier type="ISSN">0277-3791</identifier><identifier type="PII">S0277-3791(00)X0046-7</identifier><part><date>20000101</date><detail type="volume"><number>19</number><caption>vol.</caption></detail><detail type="issue"><number>1–5</number><caption>no.</caption></detail><extent unit="issue pages"><start>1</start><end>480</end></extent><extent unit="pages"><start>347</start><end>361</end></extent></part></relatedItem><identifier type="istex">C16A77669E4CDAB26534C698270243899539C50F</identifier><identifier type="DOI">10.1016/S0277-3791(99)00081-5</identifier><identifier type="PII">S0277-3791(99)00081-5</identifier><accessCondition type="use and reproduction" contentType="">© 1999Elsevier Science Ltd</accessCondition><recordInfo><recordContentSource>ELSEVIER</recordContentSource><recordOrigin>Elsevier Science Ltd, ©1999</recordOrigin></recordInfo></mods></metadata><enrichments><istex:catWosTEI uri="https://api.istex.fr/document/C16A77669E4CDAB26534C698270243899539C50F/enrichments/catWos"><teiHeader><profileDesc><textClass><classCode scheme="WOS">GEOGRAPHY, PHYSICAL</classCode><classCode scheme="WOS">GEOLOGY</classCode><classCode scheme="WOS">GEOSCIENCES, MULTIDISCIPLINARY</classCode></textClass></profileDesc></teiHeader></istex:catWosTEI></enrichments><serie></serie></istex></record>Pour manipuler ce document sous Unix (Dilib)
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