Late Quaternary History of the Atacama Desert.

More documents

Recommendations

Info

latorre et al. 83extensive low cover grassland appears dominated almost exclusively by asingle species of steppe grass, Stipa frigida.We have collected and 14 C-dated 42 middens from three differentelevations at Quebrada Chaco. The middens were analysed for both thepresence of plant macrofossils (Betancourt et al. 2001) and for pollen containedwithin the midden matrix (excluding faecal pellets). The lowermost series(Fig. 6.1f ), collected within absolute desert along a small outcrop of ancienttravertines at 2600 metres, includes 15 middens that span ages from morethan 52,200 years ago to 590 cal. BP. Middens older than 15,000 cal. BPgenerally have very rich plant assemblages (especially Andean perennials)and high percentages of extralocals compared to those that date to the last1500 years, which are more impoverished. A cluster of high species richnessoccurs in middens dated to between 23,800 to 18,100 years ago, although afew older middens with greater richness appear 38,900 years ago and morethan 52,200 years ago. Pollen analyses of the midden matrix (excluding faecalpellets) reveals abundant Fabaceae and Brassicaceae pollen between 52,000to 33,000 years ago. Diverse pollen assemblages, which include Brassicaceae,Fabaceae, Asteraceae (Tubiliflora) and Chenopodiaceae pollen, characterisemiddens dated to between 25,000 and 15,000 years ago. Desert taxa pollen(Cistanthe and Caryophyllaceae) dominate late Holocene middens youngerthan 1500 cal. BP.Only six middens spanning the last 8400 years were collected fromintermediate elevations (3100–3200 metres) within the canyon. Macrofossilcontent reveals a stable and arid Holocene (Fig. 6.1g). Despite a slight increasein pollen diversity after 1500 cal. BP, older assemblages are dominated byplants found nearby at a modern spring.Closer to the lower vegetation limit, at 3450–3500 metres, we collecteda series of 20 middens spanning the last 40,500 years along severalvolcanic outcrops (Fig. 6.1h). The two middens older than 35,000 years haveimpoverished floras dominated by Andean perennials. Middens dated between19,600 and 10,100 cal. BP, however, exhibit higher numbers of taxa, withmiddens particularly rich in annuals as well as Andean perennials datedbetween 15,000 and 10,000 years ago. Only four middens are dated younger
84 part one environmental historythan 10,000 cal. BP, all of which display plant assemblages very similarto modern assemblages. The pollen assemblage from the oldest midden isdominated by Asteraceae (Tubuliflora) and Poaceae. Pollen from middensdated between 20,000 and 10,000 years ago is considerably more diverse, withPoaceae, Asteraceae (Tubuliflora), Chenopodiaceae and Chaetanthera-type allpresent. The genus Chaetanthera is found today only in the high Andes closeto the upper vegetation limit (about 4500 metres), and is strong evidence forcolder temperatures as well as increased precipitation between 20,000 and15,000 years ago. Middens between 14,000 and 10,000 cal. BP show an increasein Brassicaceae and Asteraceae (Tubuliflora) with a concomitant decreasein grass pollen. Younger middens are dominated by Chenopodiaceae andNolanaceae, implying much more arid conditions throughout the Holocene,save the last 1000 years, when an increase in Brassicaceae and Fabaceaepollen occurs.Channel deposits in Quebrada Chaco are 3–5 metres thick and containalluvial sediments (silt, sand, gravel) and wetland facies (diatomite, tufa,organic mats). We studied the age, sedimentology and stratigraphy of channeldeposits along a 20-kilometre reach of Quebrada Chaco and along four of itsmain tributaries between 2700 metres and 3300 metres. The sedimentologyof channel deposits ranges from mostly diatomite and organic mats topredominately alluvial sediments with a few beds of organic mats. Thirtyaccelerator mass spectrometer (AMS) 14 C ages, mostly on carbonised wood,provide good age control for these deposits. The oldest channel deposits areundated and lie beneath a clear unconformity. Younger channel depositsdate between 20,800 and 10,200 cal. BP with the majority of deposits datingbetween 15,400 and 10,200 years BP. Sequences of diatomite and organicmats are interpreted as marshes/wetlands that were supported by much higherlevels of ground water than are currently in the drainage (for recent discussionof wetland deposits in the Atacama, see Rech et al. 2003 and Grosjean et al.,this volume).The majority of wetland deposits in Quebrada Chaco and its tributaries,as well as midden records from upper and lower elevations dating between15,400 and 10,200 years ago, are in good agreement with late Glacial/earlyHolocene wet phases indicated in the higher middens (greater than 3450
Page 6 and 7: chapter 6Claudio Latorre, Julio L B
Page 8 and 9: latorre et al. 75associated upper-a
Page 10 and 11: latorre et al. 77fluctuations in th
Page 12 and 13: latorre et al. 79Figure 6.1Rodent m
Page 14 and 15: latorre et al. 81youngest midden in
Page 18 and 19: latorre et al. 85metres) at Quebrad
Page 20 and 21: latorre et al. 87between 18,900 and
Page 22 and 23: latorre et al. 89may have applied t

Late Quaternary History of the Atacama Desert.

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?