**5. Andean glaciers also hold the best proxy for the Amazon Holocene changes, the record left in the snow and ice chemistry**

Reliable climatic data from the Amazon are still deficient; a detailed monitoring of large areas of the Amazon basin requires a dense network of rain gauges, which in some cases is not feasible by topography and forest [44]. Therefore, any analysis in this region should be taken with caution [45, 46]. On the other hand, ice cores provide archives of the past climate record, on the climatic forcing at the time of its deposition (as changes in solar activity) and volcanic eruptions.

Several ice cores were extracted and analysed in the Andes for information on the environmental conditions of the tropics (**Figure 2**): in southern Peru (Quelccaya ice cap, 13°56′ S, 70°50′ W, 5670 m a.s.l.); in the Cordillera Branca, Peru (Huascarán mountain, 9°07′ S, 77°37′ W, 6048 m a.s.l.); in western Bolivia (Sajama ice cap, 18°06′ S, 68°53′ W, 6542 m a.s.l.) and in the central sector of the Andes (Nevado Illimani, 16°37′ S, 67°46′ W, 6350 m a.s.l.) [11, 47–51]. The Nevado Illimani record is particularly interesting for this chapter, as it is less than 500 km of the Amazon rainforest (receiving by advection the humid masses from this region), providing information on the composition and evolution of the atmospheric chemistry of the Amazon region [11, 51].

The annual climate over the tropics is dominated by two well-defined seasons (summer/wet and winter/dry) and the glaciers of the central Andes are fed during the wet season by precipitation coming from the Amazon basin. Therefore, we can consider the snow and ice layers of these glaciers as indirect indicators (proxies) of the environmental conditions of the South America.

The snowfall of the Illimani Nevado shows traces of biomass emission (e.g. ammonia, acetate, potassium) as a dominant contribution coming from the Amazon basin [52]. It is important to notice that water vapour recycles several times along its path from the Atlantic through the Amazon basin before precipitating in the Andes glaciers.

An alternative technique for the study of the climatic variables of a region can be based on the ratio in the stable isotope ratios (δD and δ18O) in rainwater and snow [53, 54]. It is known that the present proportion of these elements is controlled by meteorological parameters (temperature, precipitation volume, etc.), which allows reconstructing/estimate the climatic conditions in the past. This technique also allows the identification of the air masses that undergo precipitation [47, 51, 53, 55, 56].

The analysis of the four ice cores extracted from the Andean tropics (Huascarán, Quelccaya, Illimani and Sajama) was based (mostly) on the information deduced from the content of hydrogen and oxygen isotopes. The results showed good consistency among the records, suggesting a similar climatic history for the twentieth century [51]. Initially, Lonnie Thompson from the Ohio State University used the isotopic oxygen content record to analyse temperature changes based on similar records for high latitudes [48, 49]. Notwithstanding, when comparing the meteorological records with the isotopic records for the tropical Andes, other authors [55, 57] identified a strong correlation between changes in precipitation and changes in oxygen values. These authors come to the conclusion that changes in precipitation origin and amount are more important than temperature (as originally proposed by for high latitudes [53]) as controller of the isotopic ratios in the tropics.

A study of the Huascarán mountain ice cores identified that zonal wind variations over South America at 500 hPa are closely related to the interannual variations in the δ18O values [48]. This suggests that the sea surface temperature (SST) in the western tropical Atlantic influences the circulation at 500 hPa, the moisture isotopic fractionation process along its Amazon pathway and so, the δ18O precipitation at the ice core site.

It is not known whether the temperature effect or the amount effect predominates in the isotopic signal in tropical glaciers [58]. The authors determined the correlation of δ18O with the ENSO variability in the Illimani Nevado ice core, and identified that more negative values of this isotope ratio coincide with Pacific SST (sea surface temperature) below average. But this interpretation on stable oxygen isotopic ratios should be taken with caution, though it is assumed that the lowlands to the east of the Andes and the Atlantic Ocean are the sources of moisture for precipitation in the glaciers, the local conditions such as condensation and water recycling should not be overlooked.

The Atlantic Ocean, by the trade winds circulation, 'feeds' moisture into the Andean snowfields, so the SST variability in the Andes precipitation is first 'filtered' across the Atlantic sector. When the Pacific is hot/cold, surface temperature anomalies are redistributed to the tropical Atlantic basin, so the moisture flow responsible for the precipitation in the Andes can be remotely controlled by tropical Pacific conditions [59].

Briefly, we can say that the primary moisture source for precipitation in the tropical glaciers of the Amazon basin is the Atlantic Ocean. So, the record of an Andean ice cores is a link between the meteorological processes of the Amazon basin, Andes and the Atlantic Ocean. Although the δ18O variations in the tropical Peru are strongly controlled to ENSO variability [7, 60] on an interannual scale, the Amazon source has remained consistent since the late glacial stage (LGS) [61] and all Peruvian-Bolivian high-altitude ice core records reflect the Amazonian source.

The low nitrate record from the Huascarán and Sajama ice cores point to a less extensive Amazon Basin forest [20] during the LGS. This reduction of forest cover and expansion of savannahs are consistent with greater abundance of eolian particles at the Huascarán core site [20, 48].

For the little ice age (LIA, generally accept as a cool period from 1200 to 1800 C.E.), elevated concentrations of nitrate (NO3 − ) and relative low 18O at the Quelccaya site are observed [7], interpreting these observations as a consequence of more moist condition to the south over the Amazon Basin. Furthermore, they observed that the LIA is a marked feature in three ice core records (Huascarán, Quelccaya and Illimani). During the past 200 years, δ18O have increased, reaching the highest values for the past 6000 years [7].

Finally, the Illimani ice core records positive trends in trace species of anthropogenic origin (Cu, As, Zn, Cd, Co, Ni and Cr) from the beginning of twentieth century [1], high amounts of biomass emission tracers (ammonium, formate, acetate, oxalate, potassium) are found at the same core [62].
