IDENTIFICATION OF CAROTENOIDS IN ANCIENT SALT FROM DEATH VALLEY, SALINE VALLEY, AND SEARLES LAKE, CALIFORNIA USING LASER RAMAN SPECTROSCOPY

Carotenoids are common components of many photosynthetic organisms, and are well known from the red colored waters of hypersaline ecosystems where they are produced by halophilic algae and prokaryotes. They are also of great interest as biomarkers in extraterrestrial sediments. Few Raman studies have examined field samples, where pigments and microscopic life are less defined. Here we identify carotenoids in ancient halite brine inclusions containing natural microbial communities for the first time using laser Raman spectroscopy and assess their preservation.

Spectra from carotenoid standards, including β-carotene, lycopene, and lutein, were compared to microscopically targeted carotenoids in ancient salt crystals from Death Valley, Saline Valley, and Searles Lake, California. Analyses were performed at Virginia Tech with a JY Horiba LabRam High Resolution Raman microprobe and an excitation wavelength of 514 nm. Carotenoids produced characteristic peaks in three regions of the Raman spectrum when exposed to visible laser excitation. Bands appeared between 1000-1020 cm^-1 (v₃), 1150-1170 cm^-1 (v₂), and 1500-1550 cm^-1 (v₁). Analyses confirmed the presence of well-preserved carotenoids with these bands in halite ranging from 9 ka - 1.44 Ma in age. Specific carotenoids were identified such as β-carotene in Death and Saline Valleys, lycopene in Saline Valley, and lutein in Searles Lake. Carotenoid degradation was not observed on a million-year timescale. This research showed that carotenoids are well preserved in ancient salt and revealed a surprisingly diverse group of carotenoids in a small geographic location from a short period of Earth’s history.