Gypsum platforms at the Salar de Pajonales hold a sequence of biological signals that form a clear physical pattern. Terraces above Flamencos Lagoon contain relic stromatolites with fossil material in their lower zones and active microbial layers in their upper zones. This arrangement exists because gypsum traps water inside its structure, filters intense radiation, and encloses biological material as the mineral grows. The result is a set of textures and biological markers that form a consistent template. The clarity of this pattern at Pajonales provides a guide for identifying biological traces in gypsum deposits formed under similar environmental conditions elsewhere.
The region sits at high altitude in northern Chile where daytime radiation intensities are among the highest recorded on Earth. Temperatures fall below minus twenty degrees at night and rise above twenty during the day. Humidity remains low for long stretches of the year and climbs only during brief atmospheric saturation events. Even during these rare pulses, hydration at the surface increases only slightly. Most exposed materials lose moisture immediately due to strong winds and evaporative loss. Gypsum retains internal hydration because its crystal lattice binds water in a stable arrangement. This enables microscopic pockets of moisture to persist long after the surrounding environment dries.
The stromatolite structures illustrate how this mineral stability interacts with biological activity. The lower zones contain folded gypsum laminae with silicates, iron rich bands, and organic remnants. These layers preserve diatom frustules, micritic filaments, cavities left by gas bubbles, and degraded organic material. The diatoms, including forms resembling Amphora and Surirella Sella, remain enclosed inside hardened gypsum. The arrangement records earlier intervals when shallow water supported photosynthetic communities. Silica fragments, Mg Si aggregates, and spherulitic crystal clusters appear alongside these fossils. Their positions align with growth surfaces that developed during past hydration cycles. This combination forms a physical record of conditions that once allowed sustained biological activity.
The upper stromatolite layers show a different stage. Close to the surface, small translucent cavities hold modern endolithic cyanobacteria. These organisms occupy exfoliation planes where radiation intensity decreases and light still penetrates. Confocal imaging identifies active phototrophs by their red autofluorescence from chlorophyll a and phycocyanin. Filamentous forms show green autofluorescence linked to pigment breakdown. Carotenoids and phycoerythrin appear within the same microspaces. These organisms survive because the mineral maintains internal moisture during extended dry periods. When short humidity pulses occur, metabolic activity increases within these sheltered cavities.
Between the lagoon and the terraces lie halite gypsum crusts. These surfaces contain far less water and support halophilic bacterial and archaeal groups that tolerate high salinity. Their long term preservation potential differs from stromatolites because halite dissolves easily under changing moisture conditions. Unconsolidated sediments near the lagoon and adjacent ponds show greater microbial diversity due to higher water content and mixed mineral composition. These sediments contain chloroplast sequences, photoautotrophic lipids, actinobacteria, and aquatic lineages. They accumulate abundant biosignatures but remain vulnerable to oxidation and transport because they lack the encapsulation properties of gypsum.
Pajonales presents all three habitats in one location. The sediments display high biological activity and low structural preservation. The crusts display select adaptations to salinity and desiccation. The stromatolites display both fossil and active systems stored inside a mineral that protects biological traces from environmental stress. This makes the gypsum terraces the clearest long term archive in the region. Their internal textures, microfossils, pigments, and moisture gradients appear in a sequence that can be recognized and applied to other evaporitic gypsum deposits.
The significance of this pattern is that it develops through simple processes. Calcium sulfate precipitates from evaporating water. Biological material becomes trapped inside growing crystals. Subsequent changes in water availability alter which organisms can survive inside the mineral. Over time the stromatolite preserves a vertical sequence of biological stages. Earlier diatom rich layers record periods of sustained surface water. Upper layers record a shift toward endolithic communities that occupy microspaces beneath the surface. The mineral separates these stages without erasing them.
The pattern includes folded basal laminae with fossil diatoms and micritic filaments, fine bands of silica and iron aligned along growth surfaces, spherulitic gypsum clusters with embedded organic fragments, and upper translucent zones containing pigment bearing phototrophs. These features appear repeatedly in gypsum that forms under evaporitic conditions. Their consistency provides measurable clues for identifying biological microhabitats in other gypsum settings.
If gypsum in another region contains aligned silica bands, enclosed microfossils, or folded laminae with organic material, it matches the lower pattern observed at Pajonales. If it contains shallow cavities with pigment bearing cells or calcite deposits along internal planes, it matches the upper pattern. The physical indicators do not require interpretation. They appear directly in mineral textures, pigment traces, frustule casts, and microcavities.
The terraces at Pajonales demonstrate that gypsum does more than preserve life. It organizes biological traces in a chronological sequence. Ancient diatom layers occupy deeper zones. Modern cyanobacterial communities occupy the upper translucent layers. Internal hydration remains stable enough to support phototrophs even during long arid intervals. Mineral fabrics protect fragile structures from oxidation, radiation, and mechanical disturbance. This stability allows biological signatures to remain identifiable long after environmental conditions change.
The result is a mineral archive that displays clear transitions between past and present biological activity. Each layer corresponds to a different environmental regime. Each group of organisms occupies the microhabitat suited to its constraints. Each mineral texture records how water, evaporation, and crystal growth shaped the underlying biology. Pajonales provides a direct example of how life becomes preserved inside gypsum and how that preservation leaves recognizable markers.
This makes the location a reference point. It shows how biological signals become embedded in gypsum under natural conditions and how those signals remain visible after long periods of extreme dryness. It shows that fossil diatoms, micritic filaments, pigment bands, and active phototrophs occupy predictable positions inside the mineral structure. It shows that hydration trapped inside the lattice enables ongoing microbial activity while protecting earlier layers. The clarity of this arrangement provides a practical template. Gypsum elsewhere that formed through similar processes can be examined using the same physical cues that appear at Pajonales. These cues are specific, consistent, and preserved by the mineral itself.
