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What was the Atacama Desert like before it became hyperarid? The evidence does not point to one desert-wide transformation from a uniformly wet landscape to today’s extreme dryness. Different parts of the Atacama record different histories. Paleosols near Calama show alternating wetter, chemically weathering conditions and drier, carbonate-forming intervals during the early and middle Miocene. A 2026 study from the hyperarid Coastal Cordillera core finds exceptionally old surface exposure histories and argues that intense aridity there may have started as early as the Mid to Late Eocene. Those records come from different places and measure different things, so neither provides a single onset date for the whole desert.
In other words, the Atacama was probably a landscape of shifting moisture regimes before hyperaridity became established in some sectors. Even after the core became persistently hyperarid, sediment records show modest changes in activity through glacial and interglacial cycles.
“Before hyperaridity” describes several different Atacamas
The Atacama is a large region stretching from the Coastal Cordillera through interior basins, the Precordillera and the Andean foothills. Its modern hyperarid core, especially areas below about 2,300 metres in the Coastal Cordillera, receives less than 2 millimetres of mean annual precipitation. That figure describes the present regional climate reported by Ritter and colleagues in 2026; it is not a reconstructed rainfall value for the Eocene or Miocene.
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Because the region is so extensive, a soil preserved near Calama cannot automatically describe conditions on the coastal plateau, and a stable rock surface in the Coastal Cordillera cannot automatically date the onset of aridity in the eastern Andes. Researchers therefore combine several kinds of archive:
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- Paleosols preserve the chemical and mineral signatures of the soil-forming environment.
- Cosmogenic-nuclide measurements record how long mineral grains have been exposed near the surface to cosmic rays.
- Sediment sequences show when runoff, wind or other processes moved material into a basin.
These proxies have different time resolutions and meanings. Soil chemistry can indicate whether weathering and carbonate accumulation occurred under relatively humid or dry conditions, while a cosmogenic exposure duration mainly describes landscape stability and a clast’s cumulative exposure history.
What the Calama paleosols reveal about the early and middle Miocene
Fourteen soils record alternating moisture conditions
Riquelme and co-authors examined 14 paleosols near Calama dating to the early and middle Miocene. A paleosol is an ancient soil that was buried or preserved and later exposed by erosion. Its minerals and structure can survive long after the climate that formed it has disappeared.
The Calama sequence does not show one uninterrupted wet phase. Instead, soil formation alternated between horizons shaped by strong chemical weathering and clay production and horizons dominated by pedogenic carbonate. Chemical weathering breaks down primary minerals and commonly produces clay; in this setting, that combination is interpreted as evidence for comparatively humid soil-forming conditions. Carbonate accumulating within a soil is more consistent with limited leaching under arid or semiarid conditions.
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Wetter conditions were important, but not permanent
Taken together, the 14 soils indicate repeated swings from wetter intervals to drier or semiarid intervals. The study concludes that wetter conditions prevailed in the Calama area during the early to middle Miocene, but “wetter” does not mean a continuously rainy environment comparable with a modern humid region. It means that, relative to the surrounding arid phases, enough moisture was available for stronger chemical alteration and clay formation.
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This distinction matters when interpreting the question “Was the Atacama always this dry?” At least near Calama, the answer is no: the early-to-middle Miocene record includes soil-forming conditions substantially more favorable to chemical weathering than today’s hyperarid core. But the same sequence also contains carbonate-forming intervals, showing that dryness was already part of the regional climate system.
A broader transect places extreme aridity by roughly 12–8 million years ago
A separate paleosol investigation followed an approximately 800-kilometre transect across the broader Atacama region. It reported massive gypsum-rich soils 2–5 metres thick and interpreted widespread gypsic-paleosol formation between about 12 and 8 million years ago as evidence that extreme hyperaridity was present by around 12 million years ago.
Gypsum can accumulate where evaporation is strong and water is insufficient to flush dissolved salts through the soil. Very thick gypsic profiles therefore indicate a long-lived dry surface environment, not merely a brief drought. However, the transect result is regional rather than universal. It should not be converted into a definitive date for every part of the Atacama, especially because the Calama paleosols and the Coastal Cordillera core represent different sites and archives.
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The Miocene evidence supports a landscape that was becoming, or in some sectors already was, exceptionally dry. It does not settle whether the first severe aridity began in the Miocene everywhere.
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The 2026 Coastal Cordillera study proposes a much earlier start for core aridity
What was measured
Ritter and colleagues analyzed cosmogenic radioactive isotope 21Ne in 135 quartz clasts collected from the hyperarid Coastal Cordillera. Quartz exposed near the surface is struck by cosmic rays, producing measurable isotopes. The concentration can be converted into a modelled exposure duration if production rates, erosion, burial and other parameters are estimated.
The study found that 32 of the 135 clasts—24 percent—have modelled exposure durations older than the Oligocene–Miocene boundary. The authors state: “Thirty-two clasts have modelled exposure durations of Oligocene age or older.” Some individual modelled durations reach approximately 62.5 million years.
What those ages mean
An exposure duration is not a direct measurement of rainfall, and it is not automatically the age at which a clast landed in its present position. A quartz grain may have received cosmic-ray exposure while its parent rock was being exhumed, during transport across the surface, during shallow burial and after final deposition. The calculated value integrates those stages.
Intermittent burial is especially important. If a clast spent part of its history shielded from cosmic rays, the measured isotope concentration can yield an apparent duration shorter than the total time since the landscape first became stable. The reported durations may therefore be minimum estimates of how long some surfaces remained relatively undisturbed.
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Why the authors connect the result with Eocene aridification
The long exposure histories imply very low landscape evolution in the sampled core: little erosion, transport or redeposition occurred after the clasts became exposed. Ritter and colleagues interpret that preservation as consistent with an extremely stable, intensely arid surface extending back well before the Miocene. They propose that aridification in the Coastal Cordillera may have begun during the Mid to Late Eocene, substantially earlier than the Miocene-centered account often used for the Atacama.
This is an interpretation of surface stability and landscape history, not a continuous Eocene-to-present rainfall curve. The study does not show that rain stopped on a particular date, nor does it demonstrate that the entire Atacama became hyperarid simultaneously.
How the main evidence differs
| Evidence | Location | Proxy | What it indicates | Primary limitation |
|---|---|---|---|---|
| Riquelme et al., 2016 | Near Calama | 14 early-to-middle Miocene paleosols | Alternation between chemical weathering and clay production, and carbonate accumulation; wetter conditions prevailed overall in this interval | Soil conditions at Calama do not date aridity in the Coastal Cordillera or every other sector |
| Regional paleosol transect, 2019 | Approximately 800 kilometres across the Atacama | Gypsum-rich paleosols 2–5 metres thick | Widespread gypsic-soil formation and extreme aridity by about 12–8 million years ago | A regional interval is not a single desert-wide onset date |
| Ritter et al., 2026 | Hyperarid Coastal Cordillera core | Cosmogenic 21Ne in 135 quartz clasts | Very long modelled exposure histories; 32 clasts (24 percent) older than the Oligocene–Miocene boundary | Exposure duration includes pre-depositional exposure and burial effects; it is not direct rainfall timing |
| Melles et al., 2019 | Endorheic basin in the Coastal Cordillera | Sediment delivery over 215,000 years | More transport during interglacial stages and reduced or absent transport during glacial stages | Shows variability within established hyperaridity, not the original onset of the desert |
A timeline that keeps the regions separate
- Mid to Late Eocene: The 2026 Coastal Cordillera study proposes that intense core aridification may have begun during this interval, based on exceptionally old quartz exposure histories. This remains a debated interpretation.
- Early to middle Miocene: Paleosols near Calama record alternating humid-to-dry soil-forming conditions, with wetter conditions prevailing overall in that local sequence.
- About 12–8 million years ago: The broader transect contains thick gypsic paleosols interpreted as evidence for widespread extreme hyperaridity across the sampled region.
- Last 215,000 years: Coastal Cordillera sediments show that transport varied with glacial–interglacial climate cycles even after the core was persistently hyperarid.
This sequence is not contradictory. It reflects different locations, different processes and different definitions of “aridity.” A stable coastal surface could have become extremely dry before nearby interior basins developed the thick gypsum soils found in later Miocene deposits. Calama could also have experienced wetter soil-forming intervals while the coastal core remained far drier.
Why might the Atacama have become arid so early?
Global cooling after the Early Eocene Climatic Optimum
Ritter and colleagues write, “We hypothesize that global cooling following the Early Eocene Climatic Optimum was likely a key driver of regional aridification.” The wording is important: this is a proposed mechanism, not a demonstrated single cause. Cooling can alter atmospheric circulation, moisture transport and the position of subtropical dry zones, but the clast data alone cannot test every step of that chain.
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Andean uplift and the Humboldt Current
The authors also discuss Andean uplift and the Humboldt Current as processes that may have intensified or expanded aridity later. Rising topography can change circulation and block moisture pathways, while the cold ocean current along the west coast helps maintain stable, dry atmospheric conditions. These factors may have reinforced an already dry coastal environment rather than marking one instantaneous switch for the whole region.
Why a single-cause story is inadequate
The available archives do not provide a continuous record that separates the effects of global temperature, mountain building, ocean circulation and local basin development. The most defensible conclusion is therefore regional and conditional: early aridity in the Coastal Cordillera is plausible from the exposure evidence, while later Miocene drying is clearly recorded in several soil sequences, and the timing and relative importance of each driver remain active scientific questions.
Hyperarid does not mean climate never changed again
Melles and co-authors examined sediments from an endorheic basin in the Coastal Cordillera covering the past 215,000 years. An endorheic basin has no outlet to the sea, so sediment entering it can preserve changes in runoff and transport. Their record shows greater sediment delivery during interglacial stages and diminished or absent transport during glacial stages.
The result does not imply that the basin became humid during interglacials. Instead, small precipitation or runoff changes within an overwhelmingly dry climate were enough to alter how much sediment moved. “Persistently hyperarid” therefore describes the long-term state, not a landscape frozen into identical conditions every year or every ice-age cycle.
What scientists can and cannot say about the transition
- Supported: Different Atacama sectors experienced different moisture histories; Calama had wetter early-to-middle Miocene soil-forming intervals; thick gypsum soils indicate extreme aridity in parts of the region by about 12–8 million years ago; and the Coastal Cordillera contains quartz clasts with very long modelled exposure durations.
- Not established: A single date when the entire Atacama became hyperarid, a continuous rainfall record from the Eocene to today, or proof that a particular clast marks the moment rainfall ceased.
- Still debated: Whether the oldest exposure histories require Mid to Late Eocene aridification, how synchronous drying was between subregions, and how much global cooling, Andean uplift and the Humboldt Current each contributed.
The strongest answer to “Was the Atacama always this dry?” is therefore no—but also not “it was once uniformly wet.” Before hyperaridity, the region passed through a patchwork of wetter and drier environments. Some coastal surfaces may have remained extraordinarily stable and dry since the Eocene, while interior areas such as Calama continued to form soils under alternating moisture regimes into the Miocene. Later, hyperaridity became widespread in several sectors, yet even the modern core retained modest glacial–interglacial variability.
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