⁸⁷Sr/⁸⁶Sr in recent accumulations of calcium sulfate on landscapes of hyperarid settings: A bimodal altitudinal dependence for northern Chile (19.5°S–21.5°S)

Abstract

An elevation-dependent relationship of the 87 Sr/ 86 Sr ratio of Holocene surface accumulations of sulfate salts is demonstrated for a continental margin hyperarid setting. In the Atacama Desert of northern Chile, gypsum and anhydrite of multiple origins exist widely on superficial materials that originated during the last 10,000 years. An important source of calcium sulfate is from offshore-generated stratocumulus clouds that are advected onto the continent, where they generate fog that transfers water droplets to the ground surface which, upon evaporation, leaves calcium sulfate crystals. Meteorological measurements of the cloud base and top altitudes average 400 m and 1100 m above sea level (masl), respectively. The seawater ratio of 87

87Sr/86Sr of calcium sulfate in ancient soils of hyperarid settings as a paleoaltitude proxy: Pliocene to Quaternary constraints for northern Chile (19.5–21.7°S)

“…Two reported cases of entombment in calcium sulfates of newspapers from the early twentieth century demonstrate a high rate of development of gypsum and anhydrite under modern conditions [ Searl and Rankin , ] (D. Burr and R. Jacobsen, personal communication, 2014). Based on the first‐order topographic control on the spatial extent of marine aerosols [ Cereceda et al ., ; Farías et al ., ] and the small fractionation of Sr isotopes, it has been possible to construct a modern altitude proxy based on 87 Sr/ 86 Sr of these modern accumulations of salts in the landscape (Figure b) [ Cosentino et al ., ]. This 87 Sr/ 86 Sr‐altitude distribution is bimodal, with high 87 Sr/ 86 Sr ratios inside the height band of time‐averaged (10 4 years) influence by marine aerosols, and low values outside of that elevation range.…”

“…To accomplish this, we present a new absolute paleoaltimetry proxy based on 87 Sr/ 86 Sr of ancient soils of the Atacama Desert. The paleoaltimeter is based on the observed altitude dependence of 87 Sr/ 86 Sr in modern accumulations of salts in the area [ Cosentino et al ., ]. The geological record of environmental salts are relict gypsic soils and paleosols which accumulate only under hyperarid climate conditions.…”

“…The cross represents the downdip hinge of the long‐wavelength, crustal‐scale monocline [ Jordan et al ., ]. (b) Holocene accumulations of salts' 87 Sr/ 86 Sr and altitude of formation ( n = 28) in the Atacama Desert [ Cosentino et al ., ]. The red dashed line represents the bootstrap value for the Holocene‐averaged outside‐of‐the‐fog isotopic ratio (0.70746 ± 0.00010), while the blue dashed line corresponds to the inside‐of‐the‐fog equivalent (0.70807 ± 0.00004).…”

“…The likelihood that a Gypsisol or gypsic relict soil sample formed within the paleo fog zone is calculated based on its Sr isotopic ratio and the distribution of Sr ratios of Holocene accumulations of salts in the landscape, corrected for temporal changes in the seawater Sr ratio. For a given paleosol with a 87 Sr/ 86 Sr value of x , and a given outside‐of‐the‐fog Holocene salt accumulations' sample size n , the likelihood of this paleosol having formed inside of the paleo fog zone is 100*( p / n ), where p is the number of outside‐of‐the‐fog soil samples with 87 Sr/ 86 Sr lower than x [ Cosentino et al ., ]. This way, we can infer whether a given soil has formed inside or outside of its contemporaneous paleo‐fog zone.…”

“…Sites shown by stars 1–3 illustrate the elevations at which a soil formed (open stars) and at which it is now found (solid stars). (b) Holocene accumulations of salts in the landscape and within the Holocene time‐averaged fog zone have calcium sulfate 87 Sr/ 86 Sr of 0.70807 ± 0.00004 (bootstrap 1 σ ), while for those located outside this value is 0.70746 ± 0.00010 (bootstrap 1 σ ) [ Cosentino et al ., ]. Correction factors needed to quantify elevation change are schematically indicated: the rise in seawater 87 Sr/ 86 Sr with time translates to a higher 87 Sr/ 86 Sr within the Holocene fog zone than within paleo‐fog zones; the rise in eustatic sea level results in a rise of the fog zone layer, while lower sea surface temperatures (SST) result in a lower fog zone layer.…”