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CU-Boulder GEOL 5700 - A late Miocene subtropical-dry flora

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A late Miocene subtropical-dry £ora from the northernAltiplano, BoliviaKathryn M. Gregory-WodzickiLamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964-8000, USAReceived 18 October 2000; accepted 24 September 2001AbstractA variety of evidence suggests that the Altiplano of the Central Andes, the second highest and largest plateau onearth, underwent significant uplift in the late Miocene^Pliocene. The most important datum supporting recent uplift isa collection of the 10.66 0 0.06 Ma Jakokkota flora from west-central Bolivia, which implies a paleoelevation no morethan 1600 0 1200 m; today the site has an elevation of almost 4000 m. In order to test the reliability of this estimate,the present study analyzes a new collection of the Jakokkota flora from a lacustrine unit that is 0.2^0.5 Myr youngerthan the previously analyzed collection from a fluvial unit. Climate estimates based on leaf morphology for the twocollections are statistically indistinguishable; the combined flora has a mean annual temperature of 21.5 0 2.0‡C and amean annual precipitation of 550 0 180 mm. The similarity of the climate estimates for the two floras suggests thatthere was not a significant climate change between them, nor a significant bias in the leaf morphology due to differingtaphonomic processes. The climate estimate for the combined flora thus presents a representative picture of the lateMiocene climate of the northern Altiplano. If one assumes that the climate of the tropics has not changed significantlysince the late Miocene, as is suggested by marine isotopic data, then the paleoclimate of the Jakokkota flora implies apaleoelevation of 1160 0 600 m. Thus, the Jakokkota flora supports the hypothesis of a young age for theAltiplano. 5 2002 Elsevier Science B.V. All rights reserved.Keywords: Central Andes; Miocene; leaves; paleoclimatology; Andean Orogeny1. IntroductionThe Altiplano, which is the second highest andlargest plateau on earth and forms the heart ofthe Central Andes, is perhaps a very young fea-ture; a variety of evidence suggests that morethan half its modern elevation of 3700 m wascreated after the middle Miocene. For example,erosion rates and the deposition of evaporites sug-gest that the Central Andes began to in£uencerainfall patterns by about 15 Ma, and fossil £orasand erosion surface remnants suggest that the Al-tiplano was at elevations of no more than V1500m as recently as 10 Mya (Gregory-Wodzicki,2000a).The age of the Altiplano is of interest for sev-eral reasons. First of all, the Altiplano has a ma-jor in£uence on regional climate, and perhaps onglobal climate. It anchors the location of thesouth paci¢c subtropical anticyclone, strengthens0031-0182 / 02 / $ ^ see front matter 5 2002 Elsevier Science B.V. All rights reserved.PII: S0031-0182(01)00434-5E-mail address: [email protected](K.M. Gregory-Wodzicki).PALAEO 2793 21-5-02Palaeogeography, Palaeoclimatology, Palaeoecology 180 (2002) 331^348www.elsevier.com/locate/palaeothe austral summer anticyclone over Bolivia, andenhances convective rainfall in the Central Andes(Meehl, 1992; Lenters and Cook, 1995, 1997).Determining the uplift history of the Altiplanois thus critical to understanding the evolution ofthe South American climate and biota. Secondly,the Altiplano is tectonically a rather enigmaticfeature: a plateau formed at a non-collisionalmargin. De¢ning the timing of uplift can helpconstrain the dynamic processes responsible forits formation.As of now, the most important piece of evi-dence for the young-Altiplano hypothesis is the10.66 0 0.06 Ma Jakokkota £ora from west-cen-tral Bolivia (Fig. 1). This £ora is the most pre-cisely dated Miocene £ora from the Central An-des, and was deposited during the most recentphase of Andean orogeny. An analysis of theleaf morphology of the £ora suggests that thesite, which today has a mean annual temperature(MAT) of 8.3‡C, was signi¢cantly warmer in thepast, with a paleoMAT of 18.6^21.0 0 2.5‡C. Ifone assumes that the climate of the tropics haschanged little since the late Miocene, as suggestedby marine isotope data, the paleotemperatureimplies a paleoelevation of 590^1610 0 1200 m(Gregory-Wodzicki et al., 1998; Gregory-Wod-zicki, 2000a), which is signi¢cantly lower thanthe modern elevation of 3940 m.But does this estimate present a representativepicture of the late Miocene climate and elevationof the Altiplano? Collected from a £uvial horizonless than 25 cm thick, the Jakokkota £ora is therecord of probably no more than 10 000 yr.Though short-term climate variation before theice ages was perhaps of a smaller magnitudethan we observe during the ice ages, it probablystill was signi¢cant ; for example, marine isotopedata suggest that in the Pliocene short-term tem-perature £uctuations were on the order of 1.5^4‡C(King, 1996). Thus it is possible that this collec-tion of the Jakokkota £ora may not typify the lateMiocene paleoenvironment.This study attempts to provide a more charac-teristic estimate of late Miocene climate by ana-lyzing a sample from a new horizon at the Jakok-kota locality, a lacustrine unit found 10 m abovethe previously collected £uvial horizon. Analysisof the £ora of this younger horizon, here calledthe upper Jakokkota £ora, can improve ourunderstanding of late Miocene climate in twoways. First of all, by providing a larger sampleof the Jakokkota £ora, it reduces the error of theclimate estimate; several authors have shown thatthe accuracy of climate estimates based on leafmorphology increases with the increasing numberof species in a sample (Wolfe, 1971; Povey et al.,1994; Wilf, 1997; Burnham et al., 2001). Sec-ondly, by sampling a di¡erent time horizon anddepositional environment, the new collection willprovide some measure of short-term climate var-iation and will provide some constraints on errordue to di¡erent taphonomic processes.In this study, the climate of the upper Jakok-kota £ora is analyzed using the method of Wolfe(1993). First, the leaf morphology of the upperJakokkota £ora is scored, and then these scoresare input into models that relate leaf morphologyto MAT and mean annual precipitation. At thismoment, it is di⁄cult to determine which climateleaf morphology models provide the most accu-rate estimates of climate for Bolivian paleo£oras,so the assumption is made that the most accurateresults will be from those models that provide themost accurate climate estimates for modern vege-tation


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