Saturday 16 May 2015

Amber from the Late Cretaceous Otway Basin of Australia.

Amber deposits are quite rare in Australia; Miocene-Pliocene ambers are known from coal deposits in Victoria and Eocene ambers are known from similar deposits in Tasmania. A number of beach deposits in Australia also contain amber thought to have been reworked from Southeast Asian deposits (amber floats and can be transported long distances on ocean currents). To date Mesozoic amber has not been described from Australia, and is rare in the Southern Hemisphere. Late Cretaceous amber deposits are known from a range of deposits across North America and Eurasia, but in the Gondwanan continents (i.e. the modern continents that made up the Mesozoic southern supercontinent of Gondwana) amber is known only from Brazil, Argentina, South Africa, Ethiopia and New Zealand.

In a paper published in the journal PLoS One on 13 May 2015, Annie Quinney, Chris Mays and Jeffrey Stilwell of the School of Earth, Atmosphere& Environment at Monash University, Darla Zelenitsky of the Department of Geoscience at the University of Calgary and François Therrien of the Royal TyrrellMuseum of Palaeontology describe a newly discovered amber deposit from the Otway Basin off the southern coast of Victoria.

The amber samples all come from drill cores produced by offshore exploration for the hydrocarbons industry, and come from the Flaxman andWaarre formations of the Upper Cretaceous Sherbrook Group, which are judged to be Turonian in age, or about 90 million years old. During the Late Cretaceous the Otway Basin was one of a series of rift basins forming as Australia spilt away from Antarctica during the latter part of the breakup of Gondwana. These basins were flooded in their centres, but also contained a range of marginal marine environments, including numerous deltas. The Otway Basin ambers occur in thin lignite (coal) horizons or sandy siltstones and sandstones, or occasionally from within organic lenses within these sandstones, or within carbonaceous (chalky) mudstones. Importantly these deposits are thought to have originated in a high latitude austral forest, the first known record of amber from such an environment from the Late Cretaceous, the other known Southern Hemisphere ambers all having come from low-to-middle latitudes.

(Left) Turonianpalaeogeographic reconstruction of the continents from a South Polar perspective. Black box over Australia indicates study area. (Right) Study area map within the Otway Basin (grey box) off the coast of Victoria, Australia (inset). White dots indicate amber-bearing well locations including well names. Dark grey shading represents modern land; light grey represents continental crust. Quinney et al. (2015).

The majority of the amber recovered comprised small flecks, 0.5-5 mm in diameter, though a few larger nodules of 1-3 cm were also found; many of these larger nodules burst on being freed from the rock due to the release of pressure. The amber ranged from light yellow to dark red in colour, with orange ambers comprising about 65% of the sample and yellow about 15%. Most of the samples were very clear, though a few brown or cloudy brown opaque ambers were also found.

Colours of Australian amber. (A) Burst piece of yellow amber, some still embedded in matrix; (B, top): Partial nodule of orange amber embedded in matrix; (B, bottom left): Cross-section through orange amber nodule containing organic matter; (B, bottom right): Cross-section through orange amber droplet; (C) fragments from burst piece of darkorange amber; (D) fragments from burst piece of red amber; (E) fragments from burst piece of dark brown; (F) fragments from burst piece of milky brown amber. Scale bar is 4 mm.Quinney et al. (2015).

Only two identifiable fossils were found in the Otway Amber, both being plant spores, the first identified as a Lycophyte of the genus Kraeuselisporites, and the other as Cyathidites minor, a widespread Fern spore thought to have been produced by a member of the Cyatheaceae or Dicksoniaceae. Kraeuselisporites spores have previously been recorded from Late Cretaceous amber from East Africa, and from Middle to Late Cretaceous deposits from New Zealand. Cyathidites minor is the most abundant known fossil spore in Mesozoic deposits from Gondwana, and in particular is known from the Cretaceous of Antarctica, New Zealand, and southern and eastern Australia, though this is the first time it has been recorded as an inclusion in amber. Thus these spores shed little light on the precise age or palaeoenvironment of the Otway Amber, but their presence does leave the hope of more useful spores being found in the future.

The deposits also include a number of filamentous micro-organisms. Similar organisms have been interpreted as either Bacterial or Fungal filaments in other deposits, however the inclusions from the Otway Amber fall within a size range that could imply membership of either group, and in the absence of any diagnostic reproductive features Quinney et al. prefer not to make any assumptions about their origin. A large amount of fragmentary degraded organic material is also present in many samples, sometimes not completely encased within the amber.

Examples of bioinclusions from Otway amber. (A) Probable specimen of Kraeuselisporites. (B) Cyathidites minor. (C) Randomly oriented network of filamentous microorganisms. (D) Mutually aligned filamentous microorganisms pictured with ovoid and fusiform inclusions. (E) Degraded organic matter exhibiting cracks along outer margin. (F) Degraded organic matter with attached filamentous microorganisms. Scale Bars: 100 μm (A–D); 50 μm (E); 200 μm (F). Quinney et al. (2015).

As well as the identifiable inclusions there are a number of non-identifiable inclusions, which Quinney et al. describe as ‘pseudoinclusions’, an interpretation which allows for these to be preservational artefacts instead of external material encased within the amber.

 Firstly there are small spherical inclusions 5-200 μm in diameter, which range in colour from pale yellow through orange to dark orange-brown, and which are coloured independently of the amber in which they are included. Similar inclusions from other amber deposits have previously been described as testate Protists (shelled single-celled micro-organisms), however Quinney et al. can find no evidence of organelles, vacuoles or other structures within the spheres, which could be considered indicative of a biological origin, and feel that the wide range of sizes present does not reflect a true Protist ecology (which would tend to produce clusters of size values).

Secondly there are elongate fusiform and ovoid inclusions, which are again 5-200 μm in size and range in colour from pale yellow through orange to dark orange-brown, and are coloured independently of the amber in which they are included.These come in clusters of a few to several hundred, typically sharing a common orientation, and often are found alongside filamentous micro-organisms, elongate pieces of decayed organic matter and filamentous pseudoinclusions, sharing a common orientation with these also. Similar inclusions to these have again been described as Protists, but again Quinney et al. feel that the lack of internal structures and random distribution of sizes does not support such a diagnosis. In some cases these ovoid inclusions have filamentous projections which resemble germination from Fungal spores or cell plasma escaping from the broken shell of a Testate Amoeba. Quinney et al. rule out the Fungal interpretation on the basis of the size of these inclusions, 50-200 μm, which is considerably larger than any modern Fungal spore, typically 2-20 μm. A Testate Amoeba interpretation cannot be ruled out so easily, as known Testate Amoeba can range from 10 μm to 300 μm in size, though again there is no real evidence to support this either, and the filaments may simply be unassociated bodies which have come into contact with the ovoids as they were entombed in the amber.

Thirdly there are irregular filamentous inclusions of uncertain origin. These tend to be found individually rather than in clusters, but may be found alongside filamentous micro-organisms, elongate fragments of degraded plant matter or fusiform/ovoid pseudoinclusions, and where this occurs will typically share a common orientation with these. These filamentous inclusions have well defined borders and lack internal structures. They range from 5 μm to 25 μm in width and from 90 μm to 225 μm in length, and again range in colour from pale yellow through orange to dark orange-brown, and are coloured independently of the amber in which they are included. The origin of these filaments is unclear; they are too large to be of known Bacterial or Fungal origin, but show no relationship to any other group.

Finally there are amorphous inclusions, lacking any common shape and ranging in size from 10 μm to 200 μm. These can be elongate, in which case they tend to be orientated in similar directions to other elongate inclusions in the amber. The borders of these structures tend to be poorly defined, and they may include speckles or vesicles within them. Again they range in colour from pale yellow through orange to dark orange-brown, and are coloured independently of the amber in which they are included. Similar inclusions in other ambers have been interpreted as Amorphous Amoebae, but once more Quinney et al. can find no real evidence to support this.

Examples of other inclusions from Otway amber. (A) Spherical inclusions (Type A pseudoinclusions) with internal vesiculation. (B) Concentration of ovoid on the left side (dark orange) and no inclusions of the right side (lighter orange). (C) Mutually aligned fusiform inclusions. (D) Ovoid inclusion with single vesicle. (E) Ovoid inclusions (black arrows); lower ovoid inclusion is stacked on top of an irregular filamentous inclusion (grey arrow); pictured with filamentous microorganisms. (F) Vesicular ovoid inclusion with projection. (G) Irregular filamentous inclusion (grey arrow) surrounded by mutually aligned filamentous microorganisms and ovoid inclusions. (H) Amorphous inclusion. (I) Amorphous inclusions surrounded by mutually aligned ovoid inclusions. Scale bars: 50 μm (A, F); 200 μm (B); 100 μm (C–E, G–I). Quinney et al. (2015).

Orange amber is the most common type found in the Otway deposits, accounting for 65% of the samples, and including all known types of pseudoinclusions, as well as both the identifiable spore fossils, most of the filamentous micro-organisms and a large quantity of degraded plant matter. Ovoid and fusiform pseudoinclusions were the most abundant type.

Yellow amber is the second most common type present in the Otway deposits, accounting for 15% of the Amber present. Inclusions are rare in this type of amber, and restricted to decayed organic matter and spherical pseudoinclusions.

Dark orange amber comprises 8% of the total Otway Amber. This amber contained degraded plant matter and all types of pseudoinclusions, with spherical inclusions being the most common. Interestingly spherical and fusiform/ovoid inclusions appear to be concentrated in darker areas within this amber, but almost absent from the lighter areas.

Red amber comprises about 10% of the Otway material. This amber frequently includes spherical pseudoinclusions and degraded plant material, with one sample containing a possible filamentous micro-organism.

Brown amber is uncommon, comprising less than 2% of the total Otway material. This amber is largely opaque, due to the presence of large amounts of degraded plant matter, which makes it impossible to determine if other types of inclusion or pseudoinclusion are present. One single piece of brown amber had a milky colouration; this was too opaque to detect any inclusions.

Previous studies of amber have suggested that colour may be influenced by a number of factors, including material included in the amber and post-depositional chemical weathering. A previous study of amber from Charentes in France suggested that yellow ambers with few inclusions probably formed from resin that was extruded and set high in the branches of trees, where little wind-blown debris reached it, an explanation which Quinney et al. feel is also plausible for the Otway Amber. Under this interpretation orange amber would have formed lower, on the trunks of the trees, where they were more easily reached by wind-blown dust and debris. Red amber from Charentes was found to include aquatic micro-organisms, and is interpreted as having been extruded underwater and consequently set more slowly, but no such inclusions were found in red amber from Otway, making interpretation harder. Brown ambers contain abundant degraded organic material, which is likely to indicate it was either extruded underground or fell into the soil before setting. Milky brown ambers have previously been interpreted as being material extruded from roots, though only a single small piece is available from Otway, showing no other diagnostic features associated with root amber, so Quinney et al. refrain from making any diagnosis on this.

See also…

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