Origin of Baltic Amber

Alicja Pielińska

The amber nuggets extracted in the mines of Sambia or found on the Baltic beaches are the fossilised resin of coniferous trees with its species yet to be determined conclusively. In the 19 th century, this species was denoted collectively as amber-bearing pine, Pinus succinifera , which was confirmed by Kurt Schubert in 1961. These pines grew over 45 million years ago (in the Eocene) in the mixed forests of the continent called Fennoscandia.

Traces of amber-bearing trees
The fact that amber is a resin is confirmed by impressions of fragments of bark surface from the trunks of the mother trees which are visible on many amber nuggets and the remains of wood and bark which can be found inside many nuggets.

Primary natural forms of Baltic amber
In the Eocene, the resin would flow into places where tree trunks, branches and roots were damaged, close up wounded tree tissue helping it to regenerate itself, and thanks to its disinfecting properties, it protected the tree against bacteria, viruses and fungi .

Dripstone forms
The liquid resin which flowed out to the surface through the damaged parts of the pines' trunks or limbs, warmed and clarified in the rays of the sun, preserved imprints of bark, the shapes of the trees' injuries and set in the form of streams or “icicles.” Between the layers of successive swellings, small plants and especially arthropods which did not manage to break free were trapped: today, they are imbedded in amber as organic inclusions.
Amber icicles formed out of thin resin, while the massive amber drops, where no inclusion has ever been found, are the product of thicker resin.

Forms resulting from the accumulation of resin inside the trunks - the so-called internal natural casts
Liquid resin accumulated inside the tree trunks between the split layers or in various kinds of cracks in the wood or bark, sometimes under the bark, sometimes preserving even labyrinths of corridors: traces of where insect larvae used to feed. The surfaces of most nuggets which were formed inside tree trunks have characteristic mirrors of the once liquid resin. Nuggets formed of several successive outflows of resin, which are marked by visible layers of discontinuity, are often found. The borders between the neighbouring outflows are where the nuggets crack.

The amber formed inside tree trunks is usually opaque. The largest known Baltic amber nugget weighs 9,750 grams. It is an in-bark specimen which was rolled during its journey via continental glacier in the Pleistocene, over the last million years. The natural cavities in amber nuggets are the mark that remains of the branches and splinters that have long crumbled away.

In search of the amber-bearing pine
Research continues to find the answer to the primary question: what were the trees of the Eocene forests whose resin led to the formation of Baltic amber called succinite? The morphological analysis of smaller and larger amber-permeated fragments of wood and bark and the imprints of these tissues on the fossil resin's surface allowed German scientists to conclude that there were several species of these trees which received the joint name of amber-bearing pine: Pinus succinifera . The amber-bearing pines were characterised by their low wood and high cork production. The resin formed not only in the cells which surrounded the resin ducts but also in the parenchyma cells of the internal living bark of these trees.

Apart from botanical analysis, the effort to explain the origin of Baltic amber is undertaken by using various methods to compare fossil resins with the resins of contemporary trees. Infra-red spectrometry (IRS) analysis showed a similarity in the absorption curves of certain bandwidths of light by Baltic amber and the resin of contemporary cedar trees of the Cedrus atlantica species: trees from the pine family ( Pinaceae ). Some researchers see a similarity between the chemical makeup of succinite and the resin of the contemporary strongly resinous long-lived New Zealand kauri trees – Agathis australis from the family Araucariaceae . More recent research shows similarities between the properties of Baltic amber and those of the resin of Pseudolarix sp. – a tree from the pine family ( Pinaceae ).
The various results of research on the origin of amber have led to the need to revise the existing designations performed by palaeobotanists.

Why were the amber-bearing pines so resinous?
This is another question which bothers amber researchers. Resinosis, the unnaturally intense production of resin, could have been caused by the trees being injured or infected by fungi. Trees are more sensitive at the peripheries of their species range. The heavy disorder in the amber-bearing pines' physiological balance could have also been caused by sudden climate change, for instance by the transgression of the Eocene Sea . Another theory attributes special impact to the considerable contamination of the atmosphere with volcanic ash in the area of the amber-bearing forests. The clogging of the stomata in the pine needles would lead to shock, with resinosis as a reaction.
The accumulation of large amounts of amber in Baltic deposits can be explained by the dominance of amber-bearing trees in widely-dispersed forests, which lasted millions of years.

Plant inclusions in Baltic amber
Numerous traces of plants have been found in Baltic amber (succinite). These are usually small fragments of plant tissue and plant organs, which - fresh or already decomposing - fell into the aromatic resin flowing out profusely from the amber-bearing trees 45 million years ago. Attempts to identify such specimens usually prove unsuccessful. Vary rarely found inclusions of whole small plant organisms, such as liverworts and mosses, are the most rewarding for researchers of amber flora. Such plant parts as flowers, fruit, seeds, needles, leaves, branches and resin-permeated wood can also be identified down to their genus or species. Pollen and spores, just as micro-organisms in amber, have yet to garner serious attention in Polish studies. The most numerous angiosperm remains are the stellate hairs torn off young leaves or leaf buds of oak which are ubiquitous in Baltic amber.

Morphological research has allowed palaeobotanists to identify 250 species of spore-bearing, herbaceous and arborescent plants from the amber-bearing forest. These include plants from diverse habitats: mountain, lowland and swamp plants. This diversity indicates, among other things, the diverse area of the amber-bearing forests. Furthermore, the co-existence of temperate climate species alongside subtropical and tropical elements has been found. Contemporary plant species which are comparable to the fossil plants found in Baltic amber occur in Africa, America , South Eastern Asia, China , Indonesia , Japan and Oceania.

In 1997, palaeobotanist Aleksandra Kohlman-Adamska distinguished three main amber-bearing forest communities. Coniferous forests in the higher mountains had sequoias, the Koyamaki ( Sciadopitys verticillata ) or Japanese umbrella pines, firs, spruces, larches and numerous representatives of the cypress family ( Cupressaceae ): the Californian incense-cedar ( Calocedrus decurrens ), Thujopsis , Chamaecyparis , and the Thuja . In the not very thick forest steppes, which covered lower parts of mountains, there were mainly pines, palms and numerous species of oak, both evergreen and with falling leaves; other trees grew there as well: beeches, chestnuts, maples, cycads from the genus Zamia ; shrubs, such as the magnolia, holly and some Laurel family ( Lauraceae ) plants; grasses dominated the undergrowth.

 Swamp forests grew in the damp river valleys, and in them grew exotic Glyptostrobus pensilis (called water pines by the Chinese), shrubs from the Salicaceae , Myricaceae and Clethraceae families and herbaceous plants from the family Commelinaceae .

The Earth was much warmer in the Eocene. The equatorial sea currents which reached the southern part of Fennoscandia gave it a very warm subtropical climate: palm trees would grow up to 60° latitude. Further north there were conditions appropriate for warm temperate and temperate climate plants. The rivers flowing through these forests would carry smaller and larger dripstone resin forms and entire tree trunks with resin accumulated in all kinds of cracks inside and under the bark, and inside the tree. All this resinous material accumulated in the deltaic deposits of today's southern Baltic underwent gradual physical and chemical transformations, which produced the amber nuggets we find today.

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