Identification of carotenoids in ancient salt from Death Valley, Saline Valley, and Searles Lake, California, using...

Carotenoids are common components of many photosynthetic organisms and are well known from the red waters of hypersaline ecosystems where they are produced by halophilic algae and prokaryotes. They are also of great interest as biomarkers in extraterrestrial samples. Few laser Raman spectroscopy studies have examined ancient field samples, where pigments and microscopic life are less defined. Here, we have identified carotenoids in ancient halite brine inclusions, 9 ka to 1.44 Ma in age, from borehole cores taken from Death Valley, Saline Valley, and Searles Lake, California, for the first time with laser Raman spectroscopy. Carotenoids occurred in fluid inclusions as colorless to red-brown amorphous and crystalline masses associated with spheroidal algal cells similar in appearance to the common halophilic alga Dunaliella. Spectra from carotenoid standards, including β-carotene, lycopene, and lutein, were compared to microscopically targeted carotenoids in fluid inclusions. Carotenoids produced characteristic bands in the Raman spectrum, 1000-1020 cm⁻¹ (v₃), 1150-1170 cm⁻¹ (v₂), and 1500-1550 cm⁻¹ (v₁), when exposed to visible laser excitation. Laser Raman analyses confirmed the presence of carotenoids with these characteristic peaks in ancient halite. A number of band sets were repeated at various depths (ages), which suggests the stability of this class of organic molecules. Carotenoids appear well preserved in ancient salt, which supports other observations, for example, preserved DNA and live cells, that fluid inclusions in buried halite deposits preserve intact halophilic microbial ecosystems. This work demonstrates the value of laser Raman spectroscopy and carotenoids in extraterrestrial exploration for remnants of microbial life.