e question whether the mid-Holocene climate (between ca. 9 and 4 cal kyr B.P.) in the Atacama Desert and the Central Andes in general was humid or dry has wide implications with regard to the understanding of long-term climate variability in South America. Paleosols, regional groundwater tables, abiotic proxy data and pollen of aquatic plants in lake sediments show a marked and rapid shift from very humid late-glacial/early Holocene climatic conditions (between ca. 14 and 9.5 kyr B.P.) to extremely dry mid-Holocene conditions (more arid than today between ca. 9 and 4 kyr B.P.). An exception during this hyperarid period is a century-scale more humid interval around ca. 5.5–6 kyr B.P. that appears systematically in lake sediment archives. In contrast, pollen for most terrestrial plants preserved in lake sediments do not show major changes during the Holocene, whereas more humid mid-Holocene conditions (compared with late Holocene conditions) were inferred from plant macrofossils in rodent middens. Is the reason for this disagreement to be attributed to misinterpretation of the paleoenvironments or of the proxy records themselves, or to incomplete paleoclimatic interpretation of the paleoenvironments? We argue that these different paleoclimate archives record different aspects and facets of ‘climate’. While paleosols and groundwater in the Atacama Desert record low-frequency climate variability at century to millennium scales, lake sediments on the Altiplano record decade- to century-scale variability. Terrestrial vegetation responds to shorter high-frequency climate variability at seasonal to inter-annual scales and preferably to humid years. Vegetation remains in ‘hibernation’ or does not germinate during arid years. Thus information from these three types of archives is not a priori comparable and requires careful site-specific, archive-specific and time-scale-specific evaluation. What is natural in modern climatology is also true for paleoclimatology: a comprehensive assessment must account for the complex daily and seasonal cycles, for the range of climate variability and trends at different scales in space and time, for impacts of short-term extreme events, and for specific, often non-linear responses of individual bio-geo-physical archives to any of the numerous aspects of ‘climate’.
Andes; South America; Quaternary; lake sediments; pollen; climate change; paleoecology
Figures and tables from this article:
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Fig. 1. Holocene paleoenvironmental records of Laguna Miscanti and adjacent areas. Limnogeological data, aquatic plants, wetland vegetation and pollen concentrations (for details see Grosjean et al., 2001, fig. 2) show that the mid-Holocene lake levels decreased and that the lake turned into a wetland with abundant Cyperaceae, whereas the percent grasses in rodent middens (Latorre et al., 2001) and pollen of major groups (Chenopodiaceae, Gramineae; Grosjean et al., 2001) remain unchanged. The chronology on the left is based on the age–depth relation in Grosjean et al., 2001, fig. 2.
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Fig. 2. (A) Map with locations of sites mentioned in the text, shaded areas are above 4000 m. (B) Space–time diagram for paleoenvironmental archives used for the paleoclimate reconstruction in the Atacama Desert.
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Fig. 3. (A) Schematic view showing the early Holocene paleosols next to the shorelines of the paleolakes (after Grosjean and Nuñez, 1994). The paleosol is missing on the surfaces younger than 9 kyr B.P., panel B showing the missing paleosol in the creep track of wanderblocks on Co. Toco (4600 m), and panel C showing the missing paleosol formation on alluvium after 8000 14C yr B.P. in Tambillo (2400 m).
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Fig. 4. Travertine outcrops of the San Bartolo spring complex. The modern 14C reservoir effect of TDIC (total dissolved inorganic carbon) of the spring water is on the order of 5000 years. The reconstruction of groundwater tables is preliminary.
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Fig. 5. Conceptual reconstruction of early Holocene, mid-Holocene and modern precipitation on the Altiplano (ca. 24°S), and the respective responses of terrestrial vegetation and lake systems (e.g. Laguna Miscanti). A threshold of 250 mm annual (mainly summer) precipitation is assumed to trigger flowering of terrestrial plants. Absolute values (except the measured precipitation at Susques 1975–1990) in this figure are not yet established and are debatable at the current stage of knowledge.
Table 1. Short-term response of the water surfaces (km2) to precipitation events in some selected catchments in the Atacama Altiplano (based on LANDSAT MSS data, from Vuille and Baumgartner, 1993)
For locations see Fig. 2.
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The most fascinating thing I find in looking at the pictures of these sites is the fact the tops of the mountains are sheared flat. It is obvious there was an advanced level of thought and ability brought to bear in regard to this process.
The question remains...WHY?
2. The shaved mountain tops would have required a lot of work to make happen.