Xeropreservation of functionalized lipid biomarkers in hyperarid soils in the Atacama Desert

Abstract

Our understanding of long-term organic matter preservation comes mostly from studies in aquatic systems. In contrast, taphonomic processes in extremely dry environments are relatively understudied and are poorly understood. We investigated the accumulation and preservation of lipid biomarkers in hyperarid soils in the Yungay region of the Atacama Desert. Lipids from seven soil horizons in a 2.5 m vertical profile were extracted and analyzed using GC-MS and LC-MS. Diagnostic functionalized lipids and geolipids were detected and increased in abundance and diversity with depth. Deeper clay units contain fossil organic matter (radiocarbon dead) that has been protected from rainwater since the onset of hyperaridity. We show that these clay units contain lipids in an excellent state of structural preservation with functional groups and unsaturated bonds in carbon chains. This indicates that minimal degradation of lipids has occurred in these soils since the time of their deposition between >40,000 and 2 million years ago. The exceptional structural preservation of biomarkers is likely due to the long-term hyperaridity that has minimized microbial and enzymatic activity, a taphonomic process we term xeropreservation (i.e. preservation by drying). The degree of biomarker preservation allowed us to reconstruct major changes in ecology in the Yungay region that reflect a shift in hydrological regime from wet to dry since the early Quaternary. Our results suggest that hyperarid environments, which comprise 7.5% of the continental landmass, could represent a rich and relatively unexplored source of paleobiological information on Earth.

Keywords: Atacama; FAME; Mars; biomarker; desert; hyperarid; lipid; preservation.

Figure 1

Yungay Soil Pit Stratigraphy & Key Lipid Abundances: a) The soil pit contains three major units: gypsiferous soils (0–90 cm depth), clay-rich units (> 90 cm depth), and a 10 cm-thick halite unit that interrupts clay units at 150 cm depth. Gypsiferous soils are matrix-supported and contain angular lithics. A clay unit at 100 cm depth contains centimeter-sized laminations composed of course sand. A clay unit at 140 cm depth contains fine sub-cm sized laminations (Supplementary Fig. 3). The massive, well-cemented halite unit (~140–150 cm) has two major morphologies: vertical, crystalline structures and mottled halite. Beneath are alternating bands of well-sorted clay units and contain fibrous plant fragments (cm-size) that become more concentrated with depth. A more detailed description of the stratigraphic profile is provided by (Ewing et al., 2006; 2008). b, c, d) Total abundances of FAME, DCA, and Isoprenoidal GDGT were found to increase with depth with the exception of the halite unit (150 cm). Isoprenoidal GDGT and DCA were absent from upper gypsiferous soil. Isoprenoidal GDGTs are plotted on a logarithmic scale due to the presence of a high relative abundance of Archaeol in the halite unit.

Figure 2

Fatty Acid Methyl Ester (FAME) Profiles for Representative Yungay Pit Samples: Upper gypsiferous soils were dominated by n-16:0 and n-18:0 FAMEs. On the other hand, clay units were dominated by n-22:0 and greater chain length FAMES and contain a greater diversity in FAME content. The deepest clay unit had the greatest total FAME abundance. The well-cemented halite unit was dominated by n-16:0 and n-18:0 FAMEs. The vertical scale changes between plots.

Figure 3

n-Alkane Content in Yungay Pit Soils: The straight chain n-alkane content of Yungay soils reveals two distinct patterns. Clay rich units exhibit an odd-over-even chain length preference. Conversely, gypsiferous soils and the halite unit exhibit a slight even-over-odd chain length preference.