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Abrupt Younger Dryas cooling in the northern tropics recorded in lake sediments from the Venezuelan Andes

Abrupt Younger Dryas cooling in the northern tropics recorded in lake sediments from the Venezuelan Andes
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:  Author's personal copy Abrupt Younger Dryas cooling in the northern tropics recorded in lake sedimentsfrom the Venezuelan Andes Nathan D. Stansell a, ⁎ , Mark B. Abbott a , Valentí Rull b , Donald T. Rodbell c ,Maximiliano Bezada d , Encarni Montoya b a Department of Geology and Planetary Science, University of Pittsburgh, 4107 O'Hara Street, Pittsburgh, PA 15260, USA b Laboratory of Palynology and Paleoecology, CSIC  – Botanic Institute of Barcelona, Pg. del Migdia s/n, Barcelona 08038, Spain c Department of Geology, Union College, Schenectady, NY 12308, USA d Departamento de Ciencias de la Tierra, Universidad Pedagógica Experimental Libertador, Av. Páez, El Paraíso, Caracas 1021, Venezuela a b s t r a c ta r t i c l e i n f o  Article history: Received 23 June 2009Received in revised form 7 February 2010Accepted 22 February 2010Available online 17 March 2010Editor: P. DeMenocal Keywords: paleoclimateglaciationpalynologylate Glacialglacier mass-balance A radiocarbon dated sediment record from Laguna de Los Anteojos, a cirque lake in the Mérida Andes of Venezuela, indicates that warmer and wetter atmospheric conditions occurred in the northern tropics at theonset of the Bølling ( ∼ 14,600 cal yr BP), and abruptly colder and drier conditions around the time of theYounger Dryas (YD). Geochemical and clastic sediment analyses from Los Anteojos show that glaciersadvanced at  ∼ 12,850 cal yr BP, reached their YD maximum extent at  ∼ 12,650 cal yr BP, and then retreateduntil complete deglaciation of the watershed at  ∼ 11,750 cal yr BP. The onset of warmer conditions thatended the coldest phase of the YD occurred several hundred years earlier at Los Anteojos than in the highlatitudes of the Northern Hemisphere. During the peak YD glacial advance, glacier equilibrium-line altitudesin the region were  ∼ 360 to 480 m lower, and temperature was  ∼ 2.2 to 2.9 °C colder than modern.Independent palynological evidence from the Los Anteojos sediment core indicates that the northern Andeswere more arid and at least 2.3 °C colder during the YD. The direction and timing of glacial  fl uctuations inVenezuela are consistent with observations of marine sediment records from the Cariaco Basin that suggestabrupt cooling occurred at  ∼ 12,850 cal yr BP, followed by a shift to higher temperature after  ∼ 12,300 cal yrBP. The timing and pattern of climatic changes in northern South America are also consistent withpaleoclimate records from the southern Tropical Andes that suggest a southward shift in the position of theIntertropical Convergence Zone occurred at the start of the cooling event, followed by a return to wetterconditions in northern South America during the late stages of the YD. The early warming of the tropicalatmosphere and invigoration of the hydrologic cycle likely contributed to the shift to increased temperaturein the higher latitudes of the Northern Hemisphere at the end of the late Glacial stage.© 2010 Elsevier B.V. All rights reserved. 1. Introduction The Younger Dryas cold reversal (YD) between  ∼ 12,850 and11,650 cal yr BP (calibrated years before A.D. 1950) in the NorthernHemisphere (Rasmussen et al., 2006) is evidence that the climate system is capable of rapid changes. Ice core records from Greenlandand terrestrial paleoclimate archives from northern Europe indicatethat major temperature shifts occurred abruptly at the beginning andendoftheYD(Alley,2000;Bakkeetal.,2009).Theserecordshighlight the persistence of this regional 1200-year cooling event and thesensitivity of the global climate system to changes in ocean andatmospheric circulation (Broecker, 2006). The exact cause of the YD cooling event is a subject of current debate, but it occurred during aperiod of increasing Northern Hemisphere temperature, rising sea-level, and melting ice sheets, which makes the YD a provocativeanalog for potential climatic surprises that may be generated bytoday's anthropogenic greenhouse warming (Alley et al., 2003). Climate models that explore the possible causes of the YD, andother abrupt global temperature shifts, typically favor either tropical(Clement et al., 2001), or high latitude (Broecker, 2006) ocean – atmospheresystemsasthedominantdrivingmechanisms.Welldatedpaleoclimate records from the low latitudes during this periodwarrant further investigation because the tropics are the center of the planet's heat budget, with energy transferred to the high latitudesthrough the oceans and atmosphere. The tropics also modulateatmospheric greenhouse gas concentrations, such as water vapor,carbon dioxide and methane (Broecker, 1997; Sowers, 2006). Therefore, regardless of the initial driver of the YD, the tropics musthaveplayedamajorroleinmodulatingtheclimatesystemduringand Earth and Planetary Science Letters 293 (2010) 154 – 163 ⁎  Corresponding author. Current address: Byrd Polar Research Center, The Ohio StateUniversity,ScottHallRoom108,1090CarmackRoad,Columbus,OH43210,USA.Tel.:+1614 292 4910; fax: +1 614 292 4697. E-mail address: (N.D. Stansell).0012-821X/$  –  see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2010.02.040 Contents lists available at ScienceDirect Earth and Planetary Science Letters  journal homepage:  Author's personal copy at the termination of the event. Comparison of terrestrial (atmo-spheric) and marine paleoclimate records from the low latitudesshould improve our scienti fi c knowledge of the mechanisms thatdrove abrupt global temperature shifts during the late Glacial stage.The pattern of low latitude atmospheric changes during the YDand the connection to high latitude temperature shifts is currentlyunclear.Thisisin-partbecausetropicalpaleoclimaterecordspresentacon fl icted pattern of past climatic conditions during the late Glacialstage(e.g.Rodbelletal.,2009),andtherearealimitednumberof welldated terrestrial records spanning this period from South America.Exceptions exist, including ice cores (e.g. Thompson et al., 1998),speleothems (Wang et al., 2004; van Breukelen et al., 2008) and lakesediments (Seltzer et al., 2000; Baker et al., 2001; Rowe et al., 2002), but most of the available records of low latitude terrestrial climaticchangearelimitedtotheSouthernHemispheresocomparativelylittleis known about how the climate system operated in the NorthernTropical Americas during the last Deglaciation.Lake sediments studies offercertain advantages over other glacial-geologic methods, because they provide continuous records, can bedated precisely with radiocarbon, and multiple proxies can beanalyzed at high-resolution to better determine the timing anddirection of tropical temperature and humidity  fl uctuations. Here,we present a new terrestrial sediment record from Laguna de LosAnteojos in the Venezuelan Andes. This continuous sedimentarysequence recovered from a proglacial lake documents the timing,direction and magnitude of local climatic change, and allows forquantitative estimates of atmospheric cooling during the YD. Thehigh-resolution analyses and  fi rm chronology of the Los Anteojossediments makes it suitable for comparison with other sites acrosstropical South America to identify the timing and pattern of lowlatitude climatic change during the YD. We also review and discussthe records of abrupt temperature, circulation and moisture-balancechanges in the northern low latitudes of the Americas during the lateGlacialstage,andsummarizetheevidenceofthetimingandpatternof aridity and temperature changes in tropical South America during theYD. 2. Study site TheLagunadeLosAnteojoslakebasin(8°32 ′ 17.88 ″ N,71°4 ′ 25.15 ″ W,3920 m a.s.l.) in the Mérida Andes of Venezuela (Figs. 1 and 2) iswell situated to record changes in the glacial status of the watershedduring the late Glacial stage. The catchment is currently ice-free, asglaciers in the area have retreated during the late Holocene to eleva-tions above  ∼ 4700 m (Schubert, 1998). The headwall of Los Anteojos( ∼ 4400 m a.s.l.) is situated in a position that is glaciated only undermoderately strongcooling events, and mostof the sediments that haveaccumulated in the lake since its inception are organic-rich. The lake isrelatively small ( ∼ 0.04 km 2 ) and deep ( ∼ 9 m) with a steep  ∼ 1 km 2 catchment set in a north-facing cirque, and the coring site at LosAnteojos is directly down-valley from a paleo-glacier headwall(Schubert,1972).Thebedrockofthewatershedconsistspredominantlyof quartzo-feldspathic meta-sediments (Schubert, 1972; Hackley et al.,2005),andisthereforemoderatelyresistanttoweatheringanderosion.The steep walls of the basin amphitheater keep the lake well protectedfrom wind mixing, and the deep water combined with seasonal strat-i fi cation minimizes the in fl uence of bioturbation of the sediments. Thebathymetry of the lake is simple, with a single bowl-shaped basinsurrounded by broad, shallow ( ∼ 1 m deep) shelves. These combinedcharacteristics allow the lake to preserve continuous,  fi nely laminatedsediments.The Mérida Andes were extensively glaciated at times during thelate Glacial stage (Mahaney et al., 2008) and the Holocene (Stansell et al., 2005; Polissar et al., 2006; Stansell et al., 2007a). Today, theclimate of the Venezuelan Andes is intermediate between the innerand outer tropics, with the majority of precipitation accumulatingduring boreal summer, and humidity is high year-round (Azocar andMonasterio, 1980). The wet and generally cloudy environment makesglacier mass-balance in this part of the Andes sensitive mostly totemperature changes (Kaser and Osmaston, 2002).The alpine vegetation in the Mérida Andes has two major biomes:the páramo above  ∼ 3000 m, and the montane cloud forest below.The highest part of the páramo, referred to as the desert páramo, orperiglacial zone, is located below ∼ 4700 m and de fi nes the altitudinalboundary between seasonal and permanent snow cover (Monasterio,1980). The elevation limit of these vegetation boundaries varies byup to a few hundred meters in this region of the Andes, and is mainlycontrolled by annual average temperature and minor differencescaused by local precipitation patterns.Precipitation in the area around Los Anteojos is relatively high(1550 mm/yr), and biomes in this region are situated slightly higherthanelsewhere inthe MéridaAndes. Forexample,theupper montanecloud forest extents up to 3200 m, and the desert páramo extends tobetween 4400 and 4700 m, which is also the uppermost limit of plantlife (Berg and Suchi, 2001). Located at 3920 m elevation, Los Anteojosis situated in an intermediate position within the páramo biome,dominated by grasses and typical columnar rosettes of   Espeletia  that Fig.1. MapidentifyingthelocationofLagunadeLosAnteojos,andthelocationsofothertropical paleoclimate records discussed in this manuscript. Fig. 2.  Glacial-geologic map of the Laguna de Los Anteojos watershed and surroundingregion(modi fi edfromSchubert,1998).Themodern(A.D.1962)icelimitis ∼ 4600 m a.s.l.(Polissar et al., 2006). Younger Dryas moraines were mapped below Pico Humboldt at ∼ 4000 m a.s.l.(Mahaneyetal.,2008),andwereusedincombinationwithresultsfromthisstudy for Δ ELA and paleo-temperature reconstructions.155 N.D. Stansell et al. / Earth and Planetary Science Letters 293 (2010) 154 – 163  Author's personal copy are re fl ected in modern pollen assemblages by the dominance of Asteraceae and Poaceae (Rull, 2006).  Podocarpus  is one of the mainarboreal components of the upper layer of Andean forests, and itspollen has been used as an indicator of changes in tree-line altitudeand climatically-driven oscillations (Salgado-Labouriau, 1979). Lowerand middle páramo levels are characterized by high to mediumpercentages of   Podocarpus , while desert páramo is completelydominated by Asteraceae and Poaceae. A special type of forestdominatedbydwarf  Polylepis treesalsooccursinsmallpatcheswithinthe páramos, and is well represented around the lake. The pollen of these trees has low wind-dispersion power and is not transportedvery far from where it grows (Rull, 2006). 3. Methods  3.1. Coring  In January, 2007, a 425-cm long continuous percussionpiston corewas collected in a single polycarbonate tube from the depocenter of Los Anteojos in ∼ 9 m of water. A surface core was also collected fromthe same location and extruded in the  fi eld at a 0.5 cm interval tocapture the sediment – water interface, and to collect the uppermost30 cm of   fl occulent material that was not recovered in the percussioncore. The total sediment thickness is 455 cm for the composite lateGlacial and Holocene sections. The core was described and cut into1.5 m-long sections in the  fi eld, and all samples were transported tothe Sediment Geochemistry Laboratory at the University of Pittsburgh.  3.2. Sediment chronology and age model The sections of sediment used for dating were wet sieved using a63 µm screen, and macrofossils  N 63 µm were picked using jewelerstweezers and a dissecting microscope. Radiocarbon samples werepretreated following established acid – base – acid pretreatment proce-dures (Abbott and Stafford, 1996). Six radiocarbon dates on aquatic macrofossil samples, spanning the early Holocene and late Glacialstagewereisolated.ThesamplesweretransportedtotheKeckCarbonCycle Accelerator Mass Spectrometer Laboratory at the University of California, Irvine where they were combusted, reduced to graphiteandmeasured.Theradiocarbonageswerecalibratedandconvertedtocalendar years before present (cal yr BP) using CALIB 6.0 with theIntCal09 dataset (Reimer et al., 2009), with present de fi ned as A.D.1950. The calibrated ages used here are the maximum likelihoodvalues exported by the CALIB program, and the 2-sigma error rangesalong with the median ages from CALIB are presented in Table 1. Anage – depth model (Fig. 3) was constructed using a 3rd orderpolynomial fi t betweenthe mediancalibrated radiocarbonage values.We note that using the radiocarbon method to date depositsduring the late Glacial stage is potentially problematic because of uncertainties in the ability to calibrate values during this time span,however, recent calibration curves have improved the precision of radiocarbonagesinthisagerange(Hughenetal.,2004a;Reimeretal.,2009). Moreover, the uncertainty of any one of our ages is minimizedby the robust and almost linear relationship between depth and themedian calibrated values of the 6 radiocarbon ages. It should also benoted that there is no limestone in the watershed to produce a hard-water effect, and soils are weakly developed, especially immediatelyfollowingdeglaciation,whichlimitstheinputof agedcarbonfromthecatchment.  3.3. Sedimentology and geochemistry Bulk density (BD) was measured on 1 cm 3 samples, taken everycentimeter down-core (Fig. 4). The samples were weighed wet, andagain after drying in a 60 °C oven for 24 h. Total carbon (C) andnitrogen (N) were measured every 2 to 5 cm on 1 cm 3 samples at theUniversity of Arizona Stable Isotope Laboratory, and are presented asatomicratios.Totalorganicmatterwasmeasuredeverycentimeterbyloss-on-ignition (LOI) at 550 °C (Dean, 1974). There is no measurablecalciumcarbonateinthesediments,basedonLOImeasurementsmadeafter burning at 1000 °C and the lack of CO 2  produced during acidi- fi cation for C and N measurements. Bulk sediment geochemistry wasmeasured every 0.1 cm using an ITRAX scanning X-ray  fl uorescence(XRF) instrument (Croudace et al., 2006) at the University of Minnesota-Duluth Large Lakes Observatory, and the values arepresented in counts per second (CPS). Magnetic susceptibility wasmeasuredata0.2 cmintervalusingaTamiscanhigh-resolutionsurfacescanning sensor connected to a Bartington susceptibility meter at theUniversity of Pittsburgh.  3.4. Biogenic silica and clastic sediments Weight percentage biogenic silica (bSiO 2 ) (Figs. 4 and 6) wasmeasured on freeze-dried samples at the University of AlbertaDepartment of Earth and Atmospheric Sciences. Samples for analysiswere collected from centimeter-thick sections, taken every 2 to 5 cmdown-core. A modi fi ed wet alkali sequential digestion method(DeMaster, 1979; DeMaster, 1981; Conley, 1998) was used onapproximately 30 mg of freeze-dried sediment (Conley and Schelske,2001), which allows for separation of bSiO 2  and the minerogenic,aluminosilicate fraction. The amount of soluble silicic acid (H 4 SiO 4 )extractwasthenanalyzedusingtheheteropolybluemethod(Clescerlet al., 1999) with a Beckman DU 520 spectrophotometer operating at815 nm. The method precision for duplicate digestions gave anaverage standard deviation of 0.52 while duplicate analysis on all theextracts gave relative differences of   b 10%.  Table 1 Radiocarbon ages of aquatic macrofossils used in study. Calibrated ages are in calendaryears before A.D. 1950 (cal yr BP). Ages presented are the median maximum likelihoodvalues (parentheses) and the 95.4% probability range.Lab # Depth(cm)Measured  14 C age Measured error(±)2 σ  calibrated age(calyr BP)UCI-37537 262 6420 20 7290-(7365)-7420UCI-37511 326 8850 20 9780-(10,010)-10,150UCI-37538 372 10,180 25 11,760-(11,890)-12,000UCI-37539 406 11,060 30 12,760-(12,960)-13,100UCI-37540 425 11,880 35 13,590-(13,740)-13,870UCI-37623 446 12,430 80 14,110-(14,520)-15,020 Fig. 3.  The age – depth model for the Laguna de Los Anteojos sediment core based on a3rd order polynomial  fi t ( r  2 =0.99) between 6 radiocarbon dated aquatic macrofossils(Table 1).156  N.D. Stansell et al. / Earth and Planetary Science Letters 293 (2010) 154 – 163  Author's personal copy BiogenicsilicacontentisalsorepresentedinSi/Tivalues,measuredusing scanning XRF (Brown et al., 2007). The Si/Ti values for the Los Anteojos core were converted to weight percent biogenic silica usinglinear regression ( r  2 =0.44,  p = b 0.001) between the XRF data andthe weight percent values that were measured independently(Stansell, 2009). The percent clastic sediment values were calculatedas the residual amount after summing the percentages of organicmatter and biogenic silica, and subtracting this from 100% since thereis no carbonate in the sediments.  3.5. Palynology Volumetric samples (2 cm 3 ) were taken every 2 to 5 cm for pollenanalyses.Amodernsurfacesamplewascollectedforcomparisonfromthe same coring location. An additional modern sample from LagunaNegra (8°47 ′ 11.61 ″ N, 70°48 ′ 21.44 ″ W, 3470 m) was also analyzed forcomparison. These samples were processed using standard palyno-logical techniques (Bennett and Willis, 2001), after spiking withLycopodium tablets (batch #124961, average 12,542 spores/tablet).Slides were mounted in silicone oil without sealing. Pollen and sporeidenti fi cations were made according to available datasets for theregion(vanderHammenandGonzalez,1960;MurilloandBless,1978;Salgado-Labouriau, 1984; Tyron and Lugardon, 1991). Counts wereconducted until a minimum of 300 pollen and spores were tabulated(excluding the superabundant  Isoëtes ) and counting continued untilthe saturation of diversity was reached (Rull, 1987). The pollen sum (Fig.5)includesallpollentypes.Thediagramwasbuiltandzonedwith  psimpoll , using the Optimal Splitting by Information Content (OSIC)and broken-stick methods (Bennett, 1996).  3.6. Paleo-temperature calculation The YD ice position in the Mérida Andes is well de fi ned in the PicoHumboldt region (Mahaney et al., 2008) that is  ∼ 5 km northeast of Los Anteojos (Fig 2). Paleo-equilibrium-line altitudes (ELA's) weredetermined by applying the terminus-headwall altitude ratio (THAR)method (Porter, 2001) with a range of values from 0.2 to 0.4 to the elevation of mapped moraines below Pico Humboldt, and assumingthe modern and YD headwall values were both 4870 m. These lowTHAR values were applied using the rationale that the majority of tropical glacier surface areas are in the accumulation zones andtongues are short due to steep ablation gradients (Kaser and Georges,1999; Benn et al., 2005; Osmaston et al., 2005). Using the same THAR method as above, the modern ELA was determined from the ice limitidenti fi ed in aerial photos taken in A.D. 1962 (Polissar et al., 2006).Paleo-temperature estimates for the YD were then calculated for the Δ ELA amounts relative to the pre-21st century values, by applying anatmospheric lapse rate calculation method of   − 6.0 °C km − 1 (Porter,2001). 4. Results 4.1. Sedimentology and geochemistry There are two clear end-member sediment facies in the LosAnteojos record (see Supplemental Table A1 in the Appendix). Firstare sections dominated by clastic (minerogenic) sediment character-ized by high values of titanium (Ti), magnetic susceptibility and drybulk density ( N 0.5 g cm − 3 ). These clastic-rich sections are low inorganic-mattercontent( b 10%)andlightgrayincolor(GLEY18/1).Incontrast, there are sections of the core with low clastic sedimentcontent that have low values of Ti, magnetic susceptibility anddry bulk density ( b 0.5 g cm − 3 ). These sections with low clastic sedi-ment concentrations are high in organic-matter content ( N 15%) anddark brown in color (7.5YR 2.5/1). The low carbon to nitrogen (C:N)atomic ratios (9 – 15) of the sediments indicate that the organicmaterial is mostly of aquatic srcin, with a limited contribution of  Fig. 4.  Stratigraphic column (see Supplemental Table A1 in the Appendix) and sediment core data from Laguna de Los Anteojos including Titanium (Ti) in counts per second (CPS),magneticsusceptibility(SIunits),clasticsediment(wt.%),bulkdensity(g cm − 3 ),biogenicsilica(wt.%),organicmatter(wt.%),andcarbonandnitrogen(atomicratio)plottedversusage.TheTiandbiogenicsilicadatawere fi lteredusingaLOWESSfunction(0.01tension),andarepresentedasraw(grayline)and fi lteredvalues(blackline).Thegrayshadingmarksmajor changes in the sedimentological and geochemical pro fi les that were used to interpret the timing of climatic changes in the Venezuelan Andes.157 N.D. Stansell et al. / Earth and Planetary Science Letters 293 (2010) 154 – 163
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