Palaeobotanical reconstruction and assessment of polymerization extent of Miocene ambers: Insights from spectroscopy and geochemistry

Abstract

The present investigation integrates Fourier transform infrared (FTIR) spectral features with Gas Chromatography-Mass Spectrometry (GC-MS) data to explore the botanical origin of Miocene ambers from northeast India, and to assess their polymerization extent. The FTIR spectra of these ambers exhibit strong similarities to those of modern Dipterocarpaceae resins, indicating a likely origin from the Miocene Dipterocarpaceae family. The presence of a distinct triplet band at approximately 1384, 1378, and 1369 cm⁻¹ further supports this botanical linkage. This study presents a novel evidence of p-cymene (m/z 119, molecular weight 134), a volatile monoterpenoid embalmed in Miocene ambers from northeast India. Besides, these ambers comprise C₁₅ bio-sesquiterpenoids (β-bourbonene, β-elemene, β-cadinene, β-cubebene, α-humulene, and germacrene D, among others), geo-sesquiterpenoids (calamenene (cadina-1,3,5-triene), dihydro-ar-curcumene, cadina-1(10),6,8-triene, and cadalene), C₃₀ pentacyclic triterpenoids (oleana-2,13(18)-diene, oleana-2,12-diene, oleana-2,18-diene, ursa-2,12-diene, A-neo-oleana-3(5),12-diene, and A-neo-ursa-3(5),12-diene), along with β-amyrone and α-amyrone, the oxidative derivatives of β-amyrin and α-amyrin, respectively. The co-occurrence of C₁₅ sesquiterpenoids and pentacyclic triterpenoids having oleanane and ursane skeletons suggests that the ambers represent Class II (dammar-type) resins, likely secreted by the abundant Dipterocarpaceae vegetation under the warm, humid conditions of the Miocene in northeast India. Furthermore, relatively higher abundance of geoterpenoids over these C₁₅ bioterpenoids, the presence of oxidized amyrins, lower 1700/1460 values (<0.60), and a higher 1384/1243 FTIR spectral ratio (>3.00) than those of the modern Dipterocarpaceae resins, suggest mild polymerization of these Miocene resins during fossilization.