Studies of Nitrate Deposits in the Atacama Desert, Chile: Insights from Triple Oxygen Isotopes and Strontium Isotopes

The Atacama Desert in northern Chile is the oldest and driest place on Earth, characterized by continuous arid conditions since mid-Tertiary times and hyperarid since the Pliocene. These extreme aridity conditions make this region an ideal environment for the accumulation of highly soluble salts, including the Nitrate Deposits which are of economic importance and scientific research. Despite almost two centuries of exploitation and research, a comprehensive generic model fitting the extremely variable settings and types of Nitrate Deposits has not been universally accepted.

Geochemical and isotopic evidence suggests predominant atmospheric deposition of sea salt and ozone oxidation products with diagnostic Δ¹⁷O anomalies in the nitrates and sulfates deposition. Current models invoke two genesis scenarios: (1)episodic rains and salts washing-down from the soil surface followed by reprecipitation due to evaporation that is assumed to concentrate nitrates and associated salts in sedimentary continental sequences, and (2)salts dissolution in low-O₂ groundwater with subsequent precipitation after capillary activity and evaporation. However, certain Nitrate Deposits, like pure nitrate veins and manto-type up to 1m thick cutting through sedimentary or volcanic rocks at depths of 25m, cannot be easily explained through these two mechanisms. They are too thick and deep to be easily related to capillarity evaporitic concentration from dilute groundwater because the capillary fringe in sediments rarely exceeds 2m. On the other hand, in some cases they are related to gypsum veins, suggesting the possibility of redissolution of primary nitrate deposits by hydrothermal, seismic activity, or precipitations, and brines emplaced into fracture systems, fault planes and stratigraphic boundaries.

This study investigates a possible origin of Nitrate Deposits by analyzing triple oxygen isotopes in nitrates and sulfates. The triple oxygen isotope values in nitrates offer constraints on the formation processes, such as bacterial denitrification and atmospheric photochemical reactions, thereby giving clues to interpret the possible origin and evolution of these deposits.

Sulfates are the dominating salt in soils of the Atacama Desert and are present in Nitrate Deposits. Thenardite and mirabilite appear in several stratigraphic sequences of saline soils between 5-65 cm below the surface. Gypsum is present in continental sequences mainly as gypsysols, but our main interest is when it occurs in groups of sulfate veins associated with nitrates cutting volcanic rocks. Triple oxygen isotope analysis of sulfate veins helps to distinguish between secondary atmospheric sulfate, deposition of sea spray, biological sulfate reduction, and reoxidation. For thenardites, appear to fall on a unique trend in the multi-isotope plots, tending towards zero in ∆¹⁷O_SO4 and very low δ¹⁸O_SO4 and δ³⁴S_SO4, extrapolating this trend to ∆¹⁷O_SO4=0‰ yields a hypothetical hydrothermal end member.

Additionally, the strontium isotopes (⁸⁷Sr/⁸⁶Sr) in Nitrate Deposits offer further insights. The strontium isotope composition reflects interactions between nitrate-bearing fluids (high and low temperature) and host rocks. Comparing these isotopic ratios with known geological formations aids in detecting potential nitrate sources.

The combination of these isotopic systems offers a comprehensive approach to understanding and provides new ideas about the origin and evolution of these still enigmatic deposits in the Atacama Desert.