Fossilization of resin differs from that of plant parts. For example, the process is neither like the preservation of a film of carbon, as in a compression of fos-sil ized leaves, nor like mineral replacement of the original cellular structure, as in petrified wood. Not all resins fossilize, but fossilization of those that produce large deposits begins with polymerization, that is, the combining of two or more resin monomers to form a complex compound of higher molec-ular weight. Most ambers are derived from components of terpenoid resins.
For example, diterpenes with a labdanoid structure (Figure 1-4) polymerize to form some of the most abundant ambers (Figure 4-1 and Appendix 3).
Initial polymerization occurs across terminal groups located on the side chain, resulting in formation of the general polymeric structure (K. Anderson et al. 1992). Because resin is a mixture of compounds, the polymeric structure of amber may involve different monomers, cross-links with nonpolymeric compounds (e.g., succinic acid), as well as occluded compounds (e.g., mono-terpenes). Hence, amber is not based on pure or completely consistent poly-mers, but the predominant monomers in this example appear to be labda-triene acids and/or alcohols. This polymerization apparently is rapid via a free radical mechanism that is photoinitiated when the resin exudes from the plant and hardens as it is exposed to sunlight and air (Cunningham et al.
1987).
Other ambers that occur in large deposits are based on polymers of ses-quiterpene hydrocarbons such as cadinene. Unlike diterpenoid ambers, these are substantially soluble in organic solvents and may include triterpenoids.
Ambers from phenolic resins that have been analyzed are derived from poly-mers of styrene (Figure 4-1) and are less common than those with terpenoid polymers. Although most ambers result from polymerization of particular constituents in some resins, small amounts of other ambers result from preser-vation of nonpolymerizable terpenoids protected by occurrence in fossilized plant parts (Appendix 3; Otto et al. 2002 a, b).
Like other forms of organic matter, the structural characteristics of poly-merized resin undergo regular changes over geologic time in response to var-ious conditions (K. Anderson et al. 1992). These progressive changes, reflect-ing both age and burial history, are considered collectively as maturation.
With increasing age, the maturity of any given resin will increase, but the rate
FOSSILIZED RESIN AND AMBER | 145
at which it occurs depends on the prevailing geologic conditions as well as the composition of the resin. Therefore, maturity and age usually cannot be directly correlated. Changes appear to be a response primarily to geothermal stress since chemical change in the resin accelerates at higher temperatures.
Because resins are deposited under very different geologic conditions, there is a continuum of relative maturity, beginning with recently deposited resin and extending back to the oldest fossil resin in late Carboniferous sediments.
When in the maturation process does resin become fossilized? This ques-tion is one of the most persistent and controversial. The confusion increases with the addition of terms such as young amber, subfossil amber, or subfossil resin, used by some investigators (as well as amber dealers). Reactions in the maturation process, such as cross-linking, isomerization, and cyclization, account for decreased solubility (Clifford and Hatcher 1995) but assertions that maturation to fossil resin takes millions of years (e.g., Poinar 1992a) are
Figure 4-1. Skeletal structures of resin components that form polymers and, hence, fossilize. The macromolecular structures are those that characterize some of the most abundant ambers and provide the basis of a chemical classifi-cation of amber (Appendix 3). The polycadinene illustrated is the current model, but additional investigations are necessary to define the precise structural char-acteristics of these polymers (K. Anderson and Muntean 2000).
Polymer Polymer
R
Polylabdanoid
Polycadinene Polystyrene
Polymer Polymer
Polymer
Polymer R = CH3 (biformene) R = CH2OH (alcohol) R = CO2H (acid)
146 | CHAPTER 4 Amber: Resins Through Geologic Time
unsupported by data and do not contribute to understanding how resin should be distinguished as fossilized. Rice (1987) and Poinar (1992a), how-ever, have made suggestions for evaluating physical characteristics such as hardness, specific gravity, melting point, and solubility, which provide a first estimate of the maturation state of resin as well as help in the determination of imitations of amber. In fact, these are some of the criteria that have been used in describing mineralogical varieties of amber. On the other hand, there is no objective chemical analysis for reliable determination of the relative maturity of fossil resin; hence, alternative criteria for distinguishing recent from fossil resin have been proposed.
To help clarify the terminology used to distinguish the age of resins, K.
Anderson (1997) proposed a scale based on carbon-14 dating (Table 4-1), and this terminology is followed in Plant Resins. Resins older than 40,000 years are considered fossil. Dates for fossil resin older than 40,000 years would be based on stratigraphic dating of the sediment from which the resin was recov-ered because 40,000 years is the limit for carbon-14 dating. Inadequate amounts of potassium in the amber preclude using potassium–argon or other trace isotope dating methods for older materials. Gas chronology, however, involving the ratio of argon-40 to argon-39, may offer potential for dating (Landis and Snee 1991). A 40,000-year date for fossilization fits reasonably well within a paleontological framework as there is no firm age definition for the term fossil, and remains of mammoths and other extinct species consid-ered to be fossils are also of the order of tens of thousands of years old.
Between 40,000 and 5000 years the resin is considered subfossil. From 5000 to 250 years, it has the status of ancient resin, but less than 250 years, it is considered modern or recent. Although the distinctions are arbitrary and
Table 4-1
Modern versus fossil resin based on carbon-14 dating (K. Anderson 1997)
RADIOCARBON (14C) AGES (YEARS) TERMINOLOGY
0–250 Modern or recent resin
250–5000 Ancient resin
5000–40,000 Subfossil resin
>40,000 Fossil resin, amber, resinite