Mid-Holocene climate in the south-central Andes: humid or dry?

South-Central Andes: Humid or Dry? Betancourt et al. (1) recently challenged the prevailing view that the central Andes were a generally arid zone during the midHolocene (2–7). This debate has wide implications regarding monsoonal and westerly paleocirculation in this area. The archives used by Betancourt et al. (1) are organic and inorganic alluvial deposits that formed in steep valleys along the western slope of the Central Andes between 7 and 3 C kiloyears before present (ky B.P.). Consensus exists about the chronology, the calm wetland depositional environment, and the existence of higher groundwater tables at the deposit locations during that time. Disagreement exists about the interpretation of the high water tables. Whereas Betancourt et al. (1) interpret the Tulán and Puripica profiles as a result of wetter conditions and high regional groundwater tables, our studies (2, 3) have concluded that generally drier conditions prevailed. We argue that wet conditions result in higher river discharge and erosion, whereas accumulation of fine materials in a steep valley requires overall decreased river flow and a lowenergy environment. With dry conditions, finegrained but permeable sediments that accumulate in the valley bottom may form local aquifers and raise the groundwater table, despite a larger cross section in the valley and reduced discharge overall. Modern analogs show that groundwater tables in such wetlands are often self-sustained, very local, and disconnected from their surroundings, and are thus difficult to interpret in terms of climate conditions. Today, the rivers in both the Tulán and Puripica valleys are erosive and have dissected the mid-Holocene sediments down to bedrock. Typically, sediments from between 14 and 9 ky B.P., a period of well-documented, extreme humidity, are missing in these valleys because of strong erosion, whereas, as expected, sediments from this humid period are present in fossil spring complexes [Tilomonte Springs, figure 4 in (1)]. These spring deposits were indeed related to a higher regional groundwater table during the humid period 14 to 9 ky B.P. (8). Ice accumulation and levels of endorheic lakes (3–6, 8) are most sensitive to largescale climatic conditions. The Sajama ice core shows that minimal accumulation rates and lake levels on the Altiplano from 16°S to 27°S were mostly low between 7 and ,4 C ky B.P., drawing a consistent general picture of multimillennia-scale aridity. The ephemeral salt lake around 5 ky B.P. in the Salar de Atacama (9) is enigmatic. In that particular location, subsidence in the tectonically active graben must be considered; however, decadeto century-scale ephemeral saline lakes are also recorded in other Altiplano localities around 6 to 5 ky B.P. Missing paleosol formation on undisturbed surfaces after 8.5 ky B.P. speaks against generally humid midHolocene conditions, and the significant regional decline of human occupation can be interpreted as a response to extremely arid conditions (2, 10, 11). The picture of mid-Holocene aridity is more complex, however. Decadeto centuryscale humid spells found in lake sediments (2, 3, 7 ) might offer an explanation for the fossil rodent midden data (1). There does not seem to be a contradiction with the midden data, but we disagree with the interpretation drawn from wetland deposits of generally humid mid-Holocene climates in the Atacama Desert.