What was the Atacama Desert like before it became hyperarid? The best answer is regional, not a single desert-wide story. Some areas near Calama alternated between relatively humid soil-forming conditions and arid or semiarid intervals during the early to middle Miocene. A new study of the hyperarid Coastal Cordillera core finds exceptionally old surface-exposure histories and argues that intense aridity there may have begun as early as the Middle to Late Eocene. Those records come from different places and measure different things, so neither should be treated as a precise start date for the entire Atacama.
Was the Atacama always this dry? No—not everywhere, and probably not at every time. The modern core receives less than 2 millimetres of mean annual precipitation below 2,300 metres, but past soils and sediments show both wetter episodes and continuing variability within an overall hyperarid climate. The timing of drying depends on the subregion and on whether scientists are reading soil chemistry, sediment delivery or cosmic-ray exposure.
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“Before hyperaridity” describes several Atacamas, not one uniform wet era
The Atacama is a very large, topographically varied region. The Coastal Cordillera contains the exceptionally dry core; the Calama basin lies farther inland; the Precordillera and Andean foothills have their own elevations, drainage patterns and climate histories. A soil formed near Calama records the moisture available while that soil developed. A quartz cobble on the Coastal Cordillera can preserve a history of how long its surface remained exposed to cosmic rays. A lake or closed-basin sediment sequence records episodes when runoff and sediment transport were sufficient to move material.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThese archives answer related but different questions. Paleosols reveal environmental conditions during soil formation. Sediments reveal pulses or absences of transport. Cosmogenic nuclides estimate how long mineral surfaces have been exposed during a combination of landscape processes. Treating all three as a single rainfall gauge creates a false precision that the evidence does not support.
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What the Calama paleosols reveal about the early and middle Miocene
Humid intervals left chemical-weathering signatures
Riquelme and colleagues examined 14 paleosols near Calama dating to the early and middle Miocene. Several show chemical weathering and clay production. Those processes generally require more effective moisture than the modern hyperarid core supplies, because water must infiltrate soil long enough to alter minerals and move dissolved elements.
The authors interpret these horizons as evidence for comparatively humid soil-forming conditions. Their conclusion is not that Calama was a rainforest or that the whole Atacama was wet. It is that, during parts of the early to middle Miocene, moisture was sufficient for chemical weathering and clay formation in this inland sector.
Other horizons formed under arid or semiarid conditions
The same group of paleosols also contains pedogenic carbonate: carbonate that accumulated within a developing soil rather than simply being deposited as a bed of sediment. Carbonate commonly concentrates where evaporation exceeds infiltration and where water does not remain in the soil long enough to flush dissolved material deeply downward. In the Calama record, carbonate-rich horizons alternate with the more chemically weathered, clay-producing horizons.
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That alternation is the key result. It indicates swings between wetter soil-forming conditions and drier, arid-to-semiarid conditions rather than one uninterrupted transition from a permanently wet landscape to today’s desert. The study concludes that wetter conditions prevailed in the early to middle Miocene at this site overall, while still documenting substantial dry intervals.
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What those soils cannot establish
The Calama paleosols do not provide a continuous annual-rainfall curve. Soil formation is episodic, and a soil can be modified, buried, exposed or eroded before the next soil develops. Nor can these inland profiles set the exact onset of hyperaridity in the Coastal Cordillera, which is a different part of the desert with a different archive.
A broader transect points to extreme aridity by roughly 12 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 to 5 metres thick and interpreted widespread gypsic-paleosol formation between about 12 and 8 million years ago. Gypsum can accumulate where evaporation is strong and water movement is limited, making these profiles consistent with extreme aridity.
This regional evidence supports the presence of a very dry landscape by the late Miocene in at least the sectors represented by the transect. It should not be converted into the claim that every part of the Atacama crossed the hyperarid threshold at exactly 12 million years ago. The transect combines locations with different elevations and histories, and the soils record intervals of formation rather than a single instant when rain stopped.
A 2026 study proposes that the Coastal Cordillera core became arid much earlier
What was measured
Ritter and colleagues (2026) analysed cosmogenic 21Ne in 135 quartz clasts derived locally from the hyperarid Coastal Cordillera. Cosmic rays create rare isotopes such as 21Ne in exposed minerals. The longer a stable mineral surface remains near the ground, the more cosmogenic nuclide can accumulate, allowing scientists to model an exposure duration.
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Thirty-two of the 135 clasts—24 percent—have modelled exposure durations older than the Oligocene–Miocene boundary. The study’s abstract states: “Thirty-two clasts have modelled exposure durations of Oligocene age or older.” Some individual modelled durations reach approximately 62.5 million years.
Why the authors connect this to Eocene aridity
Such long-lived exposure requires very little landscape change. If rainfall, runoff and erosion were vigorous, rock surfaces would more often be stripped, buried, transported or freshly exposed. The authors therefore interpret the old exposure histories as evidence for prolonged surface stability and exceptionally low landscape evolution in the core. They propose that intense aridification may have begun in the Middle to Late Eocene, substantially earlier than a narrative centred only on Miocene drying.
The paper explicitly presents this as an interpretation rather than a settled desert-wide date. Its proposed mechanism is also cautious: “We hypothesize that global cooling following the Early Eocene Climatic Optimum was likely a key driver of regional aridification.” The wording matters. Cooling after the Early Eocene Climatic Optimum is a suggested driver, not a demonstrated single cause.
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A quartz clast’s modelled exposure duration integrates several stages of its history. It is best understood as a surface-preservation record, not as a timestamp for the last substantial rain.
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- Exhumation: the quartz may have been exposed while being released from its original rock.
- Transport: it can receive cosmic-ray exposure while moving downslope or across a gravel surface.
- Shallow burial: partial burial reduces production, but does not necessarily erase the accumulated signal.
- Final deposition: the clast may then remain near the surface for a very long period.
- Model calculation: measured 21Ne is converted into an exposure duration using assumptions about production, shielding and the clast’s history.
Intermittent burial can make a modelled duration a lower limit to the time since the clast first began accumulating exposure. Conversely, exposure before final deposition means an old value does not prove that the clast landed at that age. The result demonstrates long preservation of a mineral surface; it does not directly measure ancient rainfall, and it does not establish that precipitation fell below a particular threshold in the Eocene.
How the main evidence compares
| Evidence | Location | Proxy | What it records | What it can and cannot date |
|---|---|---|---|---|
| Ritter et al. (2026) | Hyperarid Coastal Cordillera core | Cosmogenic 21Ne in 135 quartz clasts | Long-term exposure and surface stability; 32 clasts (24%) model older than the Oligocene–Miocene boundary | Supports an early-aridity interpretation, possibly Middle to Late Eocene; does not provide a continuous rainfall record |
| Riquelme et al. (2016) | Near Calama | 14 early-to-middle Miocene paleosols | Alternation between chemical weathering/clay production and pedogenic carbonate | Shows wetter and drier soil-forming intervals; does not set the onset of core hyperaridity |
| Regional paleosol transect (2019) | Approximately 800-kilometre transect across the Atacama | Gypsum-rich paleosols | Two- to five-metre gypsic soils formed broadly between 12 and 8 Ma | Indicates extreme aridity in represented sectors by the late Miocene; not a universal date for every subregion |
| Melles et al. (2019) | Endorheic basin in the Coastal Cordillera | Sediment delivery over 215 ka | More transport during interglacial stages and diminished or absent transport during glacial stages | Shows variability within a persistently hyperarid climate; does not reach back to the Eocene |
Hyperaridity still allowed climate swings during the last 215,000 years
The modern core should not be imagined as a landscape that never changes. Melles and colleagues (2019) analysed sediments from an endorheic basin in the Coastal Cordillera covering the past 215,000 years. They found modest precipitation-linked shifts in sediment transport: interglacial stages generally had greater transport, while glacial stages had reduced or absent transport.
In a hyperarid environment, a small change in effective moisture can determine whether hillslopes produce enough runoff to move sediment into a closed basin. The record therefore demonstrates variability superimposed on persistent aridity. “Hyperarid” describes the long-term climatic regime; it does not mean every millennium had identical rainfall or that every storm had the same geomorphic effect.
Possible causes of the early drying—and why they remain debated
Global cooling after the Early Eocene Climatic Optimum
Ritter et al. identify global cooling after the Early Eocene Climatic Optimum as a likely contributor in their hypothesis. Cooling can alter atmospheric circulation, moisture transport and the position or strength of subtropical dry zones. The study does not demonstrate a one-to-one causal chain from global temperature decline to the exact exposure ages of individual clasts.
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Andean uplift and the Humboldt Current
Topographic uplift in the Andes can increase rain-shadow effects by forcing moist air to rise on one side of the range and leaving drier air downwind. The cold Humboldt Current can stabilize the lower atmosphere and suppress rainfall along the Pacific margin. These factors are widely discussed as processes that could have intensified or expanded aridity later, even if some core surfaces were already exceptionally stable.
The most defensible interpretation is therefore additive rather than exclusive: an early regional tendency toward aridity may have been strengthened by later tectonic and oceanographic changes. The relative timing and importance of each driver remain active scientific questions.
A reconciled timeline
- Middle to Late Eocene, possibly: The Coastal Cordillera core may already have experienced intense aridity and very low landscape turnover, according to the 2026 cosmogenic-21Ne interpretation. This is a proposed early onset, not a settled date for the whole desert.
- Early to middle Miocene: Near Calama, 14 paleosols record alternating chemically weathered, clay-producing conditions and carbonate accumulation. Wetter conditions were important there, but dry intervals also occurred.
- About 12 to 8 Ma: A broader transect records thick gypsum-rich paleosols, interpreted as evidence that extreme hyperaridity was established in the represented sectors by the late Miocene.
- Past 215 ka: Coastal Cordillera sediments show changing transport between interglacial and glacial stages, demonstrating variability within a persistently hyperarid regime.
- Today: Below 2,300 metres in the modern core, mean annual precipitation is less than 2 millimetres per year. That modern measurement cannot be projected backward as an ancient rainfall value.
What scientists can—and cannot—say now
- The Atacama was not uniformly wet before hyperaridity; different places experienced different moisture regimes.
- Calama soils demonstrate early-to-middle Miocene alternation between relatively humid and arid-to-semiarid soil formation.
- Thick gypsic paleosols show that extreme aridity affected parts of the region by roughly 12–8 Ma.
- Very old cosmogenic exposure durations in the Coastal Cordillera support a possible Eocene start for intense core aridity and exceptional surface stability.
- Exposure durations are not direct precipitation measurements and may include pre-depositional exposure.
- No continuous, direct Eocene-to-present rainfall reconstruction currently covers the entire hyperarid core.
- The onset and causes of Atacama hyperaridity therefore remain geographically specific and debated, rather than reducible to one universally accepted transition date.
The clearest answer to “Was the Atacama always this dry?” is consequently “no, not as one connected landscape.” Some core surfaces may have been extraordinarily dry and stable far earlier than once assumed, while nearby or more inland regions retained wetter intervals. The desert’s history is a patchwork of local records, and its apparent transition to hyperaridity depends on which patch—and which geological clock—you examine.
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