Experimental maturation of pine resin in sediment to investigate the formation of synthetic copal and amber

Abstract

Experimentally simulating fossil resin formation would improve our understanding of copal/amber and could simulate the diagenesis of resin inclusions. Resin from living Pinus underwent sediment-encased maturation under various temperature and pressure conditions. Light microscopy suggests that matured resin dries, possibly hardens, and darkens into a brittle, yellow-orange-brown translucent mass with increased luster, exhibiting flow lines, birefringence, conchoidal fracturing, and air pockets typical of copal/amber. Leached components were observed in the sediment. Infrared spectroscopy suggests that matured resins have spectra consistent with those of fossil resins and may exhibit similar differences from fresh resin-possibly decreased relative intensity of C=O stretching at ~ 1700 cm^-1. Results suggest desiccation and volatile/labile component loss, alongside potential polymerization/cross-linking of stable components into a macromolecule (although we discuss the challenge of 'Class V ambers'). 'Synthetic copal/amber' is amenable to destructive analyses and would guide studies of fossil resin inclusions in informed, predictable, and targeted manners to limit loss of rare specimens. With novel experimental methods involving fresh resin and utilizing sediment porosity, our work expands upon insights from commercial autoclaving of natural subfossil copal/fossil amber used to alter their physical properties. Considering the broad success of sediment-encased maturation to simulate carbonaceous compression fossils and ancient resins, we predict that experimental taphonomy will elucidate the fossilization potential of diverse plant biomolecules and even plant secondary metabolites related to herbivory, flavor, and pharmacology.

Fig. 1

Run A results from dry P. sylvestris resin. (A) Resin before maturation, showing pale off-white coloration and opacity. (B) Split tablet’s broken edge, showing darkening, translucence, and brittleness. (C) Close up of bottom left region of (A) showing conchoidal fracturing (arrow). (D–G) Resin we suspect likely leaked from this sample during maturation onto the surface of another, unopened tablet. Yellow, translucent, birefringent surface leakage showing prominent luster in (D) parallel polarization and (E) cross polarization settings of the microscope. Close up of the top left region of (D–E) showing circular flow lines and preference in (F) parallel polarization and (G) cross polarization.

Fig. 2

Run A results from the P. mugo resin. (A) Resin before maturation, showing light coloration and opacity. (B) Part and counterpart after maturation, showing loss of mass/volume to produce a large cavity, as well as poorer preservation than in Fig. 1.

Fig. 3

Run B results from dry P. sylvestris resin. (A–B) Resin before maturation, showing light coloration and opacity. Top (C) and side (D) view of hand-prepared tablet after maturation, showing darkened color and translucence. (E) Close up of the side view in (D) showing surface texture. (F) Front view and close up (G) showing large air pocket produced (arrow) and prominent luster.

Fig. 4

Previous experimental results from Saitta et al., using older sediment-encased maturation equipment than presented here (e.g., heat produced by placing pressurized chamber in a laboratory oven instead of by a localized, insulated heating coil around the pressurized chamber). Pine resin (scraped from living Aleppo Pine at FSA) in calcium bentonite compacted at ~ 9.1 metric tons over a 126.7 mm² surface area. Maturation was at ~ 250 °C and ~ 230–300 bar (pressurized air) for ~ 23 h. Part (A–B) and counterpart (C–D) of the tablet, showing darkly colored, translucent, hardened resin with a front of leaching labile material out into the sediment matrix (arrows) (A, C) and air pockets within an overall largely voided space (B, D). Images modified from supplemental material of Saitta et al..

Fig. 5

ATR-FTIR spectra of fresh (A), fossil (B), and sediment-encased matured (C–H) resins. (A) Fresh Aleppo Pine resin from outside of the FSA (Sierra Vista, AZ, USA). Two spectra were obtained from the same sample. (B) Natural Eocene fossil Baltic amber MAI UG 508,762 (also ATR-FTIR and baseline corrected) modified from Szwedo & Stroiński (CC BY-NC-SA 4.0). (C) Run A dry P. sylvestris resin (excluding its possible leakage). (D) Run A P. mugo resin. (E) Run B dry P. sylvestris resin. (F–H) Run C contained two samples of each resin type (e.g., two separate pieces of P. mugo resin were compacted separately into two separate clay tablets and then matured at the same time in the maturation chamber, alongside the other Run C samples). (F) Run C dry P. sylvestris resin. (G) Run C sticky P. sylvestris resin. (H) Run C P. mugo resin. Red dots indicate ~ 1700 cm^–1 C=O stretching peaks that are not the most intense peak relative to their spectrum, as expected as resins mature. Purple dots indicate ~ 1605 cm⁻¹ peaks that might derive from aromatic C=C stretching, as possibly expected during resin maturation. Negative absorbance values (grey) in (A) and especially (H) indicate difficulties in background correction (e.g., air pockets and reduced contact with resin) and/or suboptimal measurement parameters in OMNIC software; still, many characteristic peaks can be inferred in these spectra.

Fig. 6

Formulating intriguing hypotheses for candidate plant secondary metabolites to search for in the fossil record. Their diagenesis could be studied experimentally using sediment-encased maturation. In addition to resin maturation here, our method has already shown success at simulating carbonaceous compression fossils – whereby carbohydrate-based (A) plant tissues such as leaves (i.e., cellulose) and (B) arthropod cuticles (i.e., chitin) survive well through experimental maturation as compressed, darkened, organic residues (modified from Saitta et al.), as they do through natural diagenesis. (C) Candidate fossil biomolecules with predicted high diagenetic thermodynamic stability: Humulus humulone, Cinnamomum *cinnamaldehyde, Cannabis THC and CBD, Lamiaceae (e.g., Mentha) *pulegone and *menthol, Zingiberaceae gingerol, zingerone, and shogaol, and Platanaceae scopoletin. *However, structures with the highest predicted volatility may be less likely to preserve well in fossils, unless contained by or incorporated into their surrounding matrix. All chemical structures are from public domain Wikimedia Commons.