We welcome comments by Hartley regarding our Neogene climate reconstruction of the Atacama Desert (Rech et al., 2006) and the opportunity to further discuss the resolution and sensitivity of soils and sedimentological deposits for inferring hyperaridity in the geologic record. In regard to our Calama Basin paleosol record, Hartley agrees that the mid-Miocene transition from rooted and gleyed calcic vertisols to salic gypsisols with pedogenic nitrate likely records a change in climate. Hartley states, however, that the significance and lateral extent of this change is limited and therefore cannot be used to constrain Andean uplift, rain shadow development, or regional climate change. Moreover, Hartley argues that we do not integrate our paleosol data with the sedimentological and stratigraphic data from the region, which indicate wetter conditions during the Miocene. The sedimentary and stratigraphic evidence Hartley cites to suggest wetter conditions during the Miocene include 1) ~55 m of alluvial conglomerate and sandstone that overlie the Barros Arana gypsisol, 2) evidence of post-13 Ma alluvial activity along the west flank of the Andes, and 3) the 85-m-thick unit of palustrine carbonate (Opache Formation) deposited in the central part of the Calama Basin between 8 and 3 Ma.
Our fundamental disagreement with Hartley concerns the sensitivity and robustness of various sedimentological deposits that have been used to reconstruct long-term climate change in the Atacama Desert (Hartley and Chong, 2002; Hartley, 2003; Hartley et al., 2005). We agreee with Hartley's argument that alluvial deposits that overlie the Barros Arana gypsisol, as well as post-13 Ma alluvial deposits along the west slope of the Andes, indicate precipitation on the adjacent catchments. Occasional precipitation events occurred in the past as they do today in the Atacama, and large precipitation events are capable of eroding and depositing material downstream. This is especially true in areas of pronounced relief, such as the western flank of the Andes, or in areas influenced by tectonically induced changes in base level. We therefore find it difficult to use the presence and thickness of alluvial deposits at a mountain front to infer past climatic regimes. Hartley also suggests that the Upper Miocene–Piocene palustrine Opache Formation, deposited from ~8–3 Ma across the central part of the Calama Basin, reflects wetter conditions. Marshes and wetlands fed by extralocal precipitation in the Andes are common in the Atacama today (e.g., Rech et al., 2002) and their mere presence in the geologic record cannot be used to infer wetter conditions. Analysis of the Opache Formation by May et al. (1999) led these authors to conclude that tectonic influences on basin accommodation development and drainage patterns are the primary control on the formation of these paludal deposits.
We specifically chose to use soils and paleosols in the Atacama Desert to reconstruct climate change because there is a direct relationship between climate and soils in this region today. The hyperarid core of the Atacama Desert contains thick soils cemented by nitrate, chloride, and sulfate salts, whereas soils on the outer margins of the Atacama transition to predominately sulfate and carbonate soils with greater precipitation (Rech et al., 2003; Ewing et al., 2006). We suggest that the relationship between soil salts and precipitation is a sensitive recorder of precipitation (cf. Amit et al., 2006), as long as soil salts originate from eolian additions and not from capillary rise from groundwater. The transition from calcic vertisol soils with root traces and gleyed horizons (which do not form in the Atacama today) to the Barros Arana gypsisol with soil nitrate represents a dramatic decrease in precipitation in the Calama Basin during the mid-Miocene. We suggested in our article that this dramatic decrease in precipitation was likely caused by Andean uplift to >2 km and the development of the Andean rain shadow. We argue for the development of an Andean rain shadow by this time because the Barros Arana gyspsisol represents a period of prolonged (106 years) hyperaridity (<20 mm/yr precipitation) on the eastern margin of the Atacama. It is difficult to imagine that this hyperaridity was maintained through orbital and millennial climate variations during the mid-Miocene without the presence of an Andean rain shadow.
Hartley contends that, in the Southern Hemisphere, a mountain range is not required to create hyperarid conditions on the western margin of a continent, and notes the hyperarid coastline of the Namib Desert. However, it is unclear to us if hyperarid conditions were indeed maintained in the Namib during Quaternary climatic oscillations. If we examine the record of soil development in the Namib, thick layers of soil carbonate underlie gypsic horizons (Bao et al. 2001), which suggests to us that hyperarid conditions were not maintained through Quaternary climate oscillations.
We do agree with Hartley that the paleosols in the Calama Basin only represent one data point, and that our results need to be replicated over a broader spatial scale and with different paleoclimatic proxies to truly reconstruct the development of an Andean rain shadow. We are currently working with paleosols and stable isotopic records over a broader spatial region to this end. We can also integrate these data within the broader context of sedimentological data from the Atacama Desert, if a climatic signal can be teased apart from tectonic influences in the Andean forearc.