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United Kingdom - Russia scientific workshop ""T
«What does the Devonian of the Arctic tell us
УДК 561: 581.332: 552.52/.57: 551.734(234.83) DOI: 10.19110/2221-1381-2019-10-12-15
DEVONIAN AND EARLY CARBONIFEROUS COALS AND THE EVOLUTION OF WETLANDS
J. E. A. Marshall1, O. P. Tel'nova2, C. M. Berry3
1School of Ocean & Earth Science, University of Southampton, National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK; [email protected] 2Institute of Geology FRC Komi SC UB RAS, Syktyvkar; [email protected] 3School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, Wales, UK; [email protected]
The rise of forests in the Mid and Late Devonian has attracted much attention as potential drivers of the Earth System. In this context it is important to understand what groups contribute to the first wetlands found at the Devonian palaeoequator. A time series of Devonian coals shows that the palaeoequatorial Mid and Late Devonian humic (vitrinite rich) coals are dominated by the lycopod microspore Cymbosporites and megaspore Verrucisporites that can be confidently associated as the spores from Protolepidodendropsis. This plant has successfully colonised and probably created the wetland coal environment by the ability to shallow root and hence anchor the trees above the anaerobic layer. The progymnosperm Archaeopteris is abundant from its spores in Devonian palaeoequatorial sediments but plays no role in these coal forming wetlands. More rigorous age dating shows that there is some evidence for a Tournaisian coal gap.
Keywords: Mid and Late Devonian, Early Carboniferous, coals, evolution of wetlands.
ДЕВОНСКИЕ И РАННЕКАМЕННОУГОЛЬНЫЕ УГЛИ И ЭВОЛЮЦИЯ БОЛОТНЫХ ФАЦИЙ
Д. Э. А. Маршалл1, O. П. Тельнова2, К. М. Берри3
1 Школа изучения океана и Земли, Саутгемптонский университет, Национальный океанографический центр, Европейский путь, Саутгемптон, SO14 3ZH, UK 2 Институт геологии ФИЦ Коми НЦ УрО РАН, Сыктывкар 3 Школа изучения Земли и океана, Университет Кардиффа, Main Building, Park Place, Cardiff CF10 3AT, Wales, UK
Леса в среднем и позднем девоне привлекают большое внимание как потенциальные движущие силы биосферы. В этом контексте важно понять, какие группы растений способствовали появлению первых болотных фаций в девонском палеоэкваторе. Анализ палеоэкваториальных средне- и позднедевонских гуминовых (богатых витринитом) углей показал, что в них преобладают микроспоры Cymbosporites плауновидных растений и мегаспоры Verrucisporites, известные in situ из Protolepidodendropsis. Это растение благодаря наличию мелких корневых систем, которые укрепляли деревья над анаэробным слоем, успешно колонизировало болотные фации и, вероятно, способствовало углеобразованию. В девонских палеоэкваториальных отложениях в изобилии встречаются споры прогимносперма Archaeopteris, но это растение не является углеобразующим на заболоченных территориях. В турнейское время (ранний карбон) процесс угле-накопления имел специфические характеристики.
Ключевые слова: средний и поздний девон, ранний карбон, уголь, эволюция болотных фаций.
Introduction
The Devonian was a pivotal episode in the evolution of land vegetation as it was the time when plants transitioned from small simple forms lacking roots and leaves to tall more deeply rooted trees with extensive fronds that formed the first forests [1]. This deeper rooting changed soils into biogeochemical factories that released significant nutrients (e. g., phosphorus) which stimulated growth of both terrestrial plants and marine phytoplankton with increased sequestration of organic matter into sediments. In addition, the Ca2+ and Mg2+ released from these minerals also drewdown CO2 by forming extensive lime-
stone deposits. It has been argued [2] that these changes drove the Devonian Earth System into a series of environmental perturbations including mass extinctions and the descent into an ice house planet. One signifier of the sequestration of carbon into terrestrial sediments is the presence of Devonian and early Carboniferous coals. The best compilation of these coals [3] maps their distribution in time slices for the Mid and Late Devonian and the Tournaisian and Visean. A significant attribute of coals is that they formed from in situ vegetation. Hence, the dominant spores found within them informs us as to the likely plants that formed them. Therefore, by studying a time series of
Vestnk IG Komi SC UB RAS, October, 2019, No. 10
Devonian coals we can understand which plant groups were present in the Devonian wetland environment and how they changed through time. It is also particularly informative to study coals from the Devonian and early Carboniferous equatorial areas as it is today's equatorial areas that drive the Earth System in terms of terrestrial biomass production. Therefore, any study of these first equatorial forests is of significance.
Material and Methods
We have had access to a time series of coals from selected Devonian and Early Carboniferous localities. These are:
Akarachkino Quarry, Salair, Eifelian, lowest Momontovo Formation [4];
Mimerdalen cannel coal, Pyramiden, Spitsbergen, late Givetian with petrology [5], palynology [6, 7] plus additional samples;
Hill 251 and G217 Highway, Givetian, Hoxtolgay, Xinjiang, China [8];
A series of coals from Melville Island, Arctic Canada (Nunavut), Weatherall, Hecla Bay and Beverley Inlet Formations, Givetian to Frasnian [9];
Bj0rn0y (Bear Island), Svalbard, coals from the mid Fa-mennian [10—12];
Triungen, Spitsbergen, Visean [13];
Backlund Ridge, Geographical Society 0 (island), East Greenland, Visean [14].
The coals (0.15 g, finely crushed) were oxidised with concentrated HNO3 saturated with KClO3 followed by washing by dilution with water, sieving at 15 ^m and then a short (15 minutes) treatment with 5 % KOH which solubilised the bulk of the vitrinite material. The oxidation time (8—36 hours) was experimentally determined by a successful maceration that produced well-preserved spores that were not over-oxidised. The isolated spores were mounted in Elvacite 2044TM.
Selected coals were prepared in polished block for reflected light petrographic study. The coals were mounted in Fast-glass resin and cut down to expose the coal. They were then polished successively with 9.5 ^m, 3 ^m and 0.05 ^m alumina powder on Kemet synthetic laps. The coals were examined using a low power x40 oil immersion objective in both incident white light and UV fluorescence (filter set 9 BP 450—490, FT 510, LP 520) using a Zeiss UMSP 50 microscope.
Spores from Devonian coals
The oldest coal from Akarachino was preserved within a carbonate succession and proved to be significantly degraded. Petrographically the coal appeared fragmented but homogenous with no identifiable spores. Several oxidations were attempted but no spores were recovered. There are a number of other coals of Mid Devonian age reported particularly from Central Asia [15] and China [16] which are cuticle coals from plants such as Orestovia and Barsassia and will not be considered further.
An informative coal is that from the late Givetian in Mimer-dalen, Spitsbergen. Here it occurs in a succession of late Givetian to Frasnian age where megaspores (Contagisporites) and microspores (Geminospora) found elsewhere in situ [17] from the progymnosperm Archaeopteris are common throughout the sequence. In addition, the lycopod Protolepidodendropsis is abundant, preserved in places as upright thickets of trees. The microspore Cymbosporites magnificus and megaspore Verrucisporites submamillarius can be confidently associated [18] as the spores from this plant. There are rare Archaeopteris trees preserved as upright specimens including a putative example within a thicket of Protolepidodendropsis. The Mimerdalen coal is relatively
thick (~1 m) but somewhat tectonised and hence difficult to determine if rooted at its base. Spore analysis [6, 7] shows there to be a variety of spores present but importantly there is a significant component 15—16 % [7] from Archaeopteris as represented by Geminospora svalbardiae. In contrast Cymbosporites magnificus is rare and only reaches a maximum of 2.5 % in a single coal sample. Reflected light petrography of the coal shows a dominance of spores and hence this was a spore coal formed from the accumulation of plant debris rather plant debris anaer-obically degrading to vitrinite.
Arctic Canada (Nunavut) is particularly important for palaeoequatorial Devonian coals as these occurred within a contiguous sequence of Eifelian to Famennian age. In addition, they have been systematically reviewed as regards petrography and palaeoenvironment [9]. This shows them to be a mixture of humic and cannel coals. It was from a sub-set of these humic coals (i.e. vitrinite rich) that spores were isolated. These analyses show all the coals are dominated by Cymbosporites together with its associated megaspore Verrucisporites. Geminospora is effectively absent. This demonstrates that all these coals were deposited in wetlands with a vegetation of lycopod trees allied to Protolepidodendropsis. This is informative as there are a number of publications on the Devonian spores from Arctic Canada [19] that enable us to link together the inception of Geminospo-ra spp followed 300 m higher by Cymbosporites. The inception of Geminospora is used to pick the base of the Givetian stage [19] although with very little independent age evidence. These occurrences are substantiated by the inceptions of their me-gaspores [20] in the same sections.
The abundance of vitrinite together with the dominant Cymbosporites shows that these coals formed in an anaerobic peat deposited from a lycopod forest. This was to the exclusion of Archaeopteris that is regarded as the more advanced plant with deeper more extensive root systems, secondary growth and more extensive photosynthetic surfaces. Archaeopteris was clearly locally abundant from its presence in the dispersed spore record in the same sections but unable to compete within the wetland environment. A key to understanding this exclusion was that the lycopods would have been able to dominate [20] as they were shallowly rooted and hence above the anaerobic layer. It can now be hypothesised that it was not that lycopods were able to adapt to this wetland environment but rather that they instigated it in the Mid and Late Devonian. Lycopods would have grown in wet climates with a high P/E ratio and their remains would have accumulated to create anaerobic conditions within a water saturated peat. This would have then excluded any plant that was more deeply rooted and created a distinctive wetland ecosystem within Mid Devonian equatorial environments. This lycopod dominance in wetlands continually evolved to reach its apogee in the Late Carboniferous with the sophisticated stig-marian [21] root systems
A similar dominance of Cymbosporites also occurs in China [8] where there are palaeoequatorial coals of Givetian and Frasnian age in the isolated Altaid volcanic arc terrane of Junggar in Xinjiang. Here a number of coal seams occur and are dominated by vitrinite to the exclusion of other coal macerals particularly sporinite. However, both lycopods, Cymbosporites and Verrucisporites are present throughout the enclosing clastic sequence.
Palaeoequatorial coals of Famennian age are generally rare but occur on Bj0rn0y (Bear Island) in the Svalbard archipelago. These coals are mid Famennian in age, pre-dating the occurrence of Retispora lepidophyta that occurs higher in the succession. Palaeobotanically the coals are associated with numerous
1 & 2. Pair of incident UV fluorescence and white light images of Givetian cannel coal from Mimerdalen, Spitsbergen. The UV fluorescence image shows mostly yellow spores with very little non-fluorescent vitrinite present. Scale bar 50 ^m.
3 & 4. Pair of incident UV fluorescence and white light images of Givetian coal from Arctic Canada. Yellow-orange fluorescent spores are abundant but significant areas of made up of non-fluorescent medium grey coloured vitrinite (4). Scale bar 50 ^m.
5. Givetian humic coal from Arctic Canada composed entirely of vitrinite. Scale bar 50 ^m.
6. Verrucisporites sp, a lycopod megaspore from a Givetian coal in Arctic Canada. In Mimerdalen this genus can be confidently associated with Protolepidodendropsis. Scale bar 10 ^m.
7. Cymbosporites magnificus, a lycopod microspore from a Givetian coal in Arctic Canada. In Mimerdalen this spore can be confidently associated with Protolepidodendropsis. Scale bar 10 ^m.
8. The most abundant spore from the mid Famennian sequence with Cyclostigma coals in Bj0rn0y. This has been attributed to Hymenozonotriletes luteolus [11] and is the potential spore of Cyclostigma. Scale bar 10 ^m.
9. Cymbosporites magnificus, a lycopod microspore from the Givetian coal in Mimerdalen that can be confidently associated with Protolepidodendropsis. Scale bar 10 ^m.
10. Verrucisporites submamillarius, a lycopod megaspore from the Givetian coal in Mimerdalen that can be confidently associated with Protolepidodendropsis. Scale bar 10 ^m.
11. Geminospora lemurata, the in situ microspore of Archaeopteris from Mimerdalen, Spitsbergen. Scale bar 10 ^m.
12. Contagisporites optivus, the in situ megaspore of Archaeopteris from Mimerdalen, Spitsbergen. Scale bar 10 ^m.
1 и 2. Пара изображений живетского кеннельского угля в УФ-лучах и отраженном белом свете, Мимердален, Шпицберген. В УФ-лучах множество спор флюоресцирует в желтом цвете, незначительные включения витринита серые, не флюоресцируют. Масштабная линейка 50 мкм.
3 и 4. Пара изображений угля в УФ-лучах и отраженном белом свете из живетских обнажений Арктической Канады. В УФ-лучах множество спор флюоресцирует в желтом цвете, незначительные включения витринита серые, не флюоресцируют (4). Масштабная линейка 50 мкм.
5. Живетский гуминовый уголь из Арктической Канады полностью состоит из витринита. Масштабная линейка 50 мкм.
6. Verrucisporites sp, мегаспора плауновидного из живетского угля Арктической Канады. В обнажениях Мимердалена этот род спор может быть уверенно связан с Protolepidodendropsis. Масштабная линейка 10 мкм.
7. Cymbosporites magnificus, микроспора плауновидного из живетского угля Арктической Канады. В обнажениях Мимердалена этот род спор может быть уверенно связан с Protolepidodendropsis. Масштабная линейка 10 мкм.
8. Наиболее распространенная спора из среднефаменских углей с Cyclostigma в Медвежьем. Этот вид спор относился к Hymenozonotriletes luteolus [11], потенциальными продуцентами которых предполагались Cyclostigma. Масштабная линейка 10 мкм.
9. Cymbosporites magnificus, микроспора плауновидных из живетского угля в Мимердалене, которая может быть уверенно связана с Protolepidodendropsis. Масштабная линейка 10 мкм.
10. Verrucisporites submamillarius, мегаспора плауновидных из живетского угля в Мимердалене, которая может быть уверенно связана с Protolepidodendropsis. Масштабная линейка 10 мкм.
11. Geminospora lemurata, микроспора in situ Archaeopteris из Мимердалена, Шпицберген. Масштабная линейка 10 мкм.
12. Contagisporites optivus, мегаспора in situ Archaeopteris из Мимердалена, Шпицберген. Масштабная линейка 10 мкм.
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occurrence of the lycopod Cyclostigma kiltorkense and are accepted as being formed from this plant. The dominant spore in the succession has been referred to Hymenosporites luteolus [10,11]. However, it would appear to be more similar to Hymenozonotriletes papulosus or Spelaeotriletes microgranulatus Byvscheva var. minor Byvscheva, 1976. However, it is notable that the spore possesses an internal ring structure, a feature that is characteristic of the lepidodendroid lycopods such as Lycospora.
Tournaisian coals
Although the compilation of Boucot et al., [3] lists a number of Tournaisian coal occurrence from Spitsbergen and Bj0rn0y most of these are not supported by palynological assemblages. For example, the Spitsbergen coals in the Triungen section appear with the inception of Lycospora pusilla and are Viséan in age. Similarly, at Backlund Ridge and Rebild Bakker in East Greenland [14] the inception of coals is again coincident with Lycospora pusilla. On Bj0rn0y there are no recorded coals [10] between the last occurrence of Retispora lepidophyta and the inception of Lycospora pusilla. Although more records have to be directly tested it appears there may be either a coal gap or general paucity of the coal environment in the Tournaisian. A future challenge is to build on the compilation of Boucot and others to produce a separate Tournaisian coal distribution map.
Conclusions
A time series of Devonian palaeoequatorial coals shows that their spores were dominated by the microspore Cym-bosporites together with the megaspore Verrucisporites. Both of these spores can be confidently associated with the lycopod tree Protolepidodendropsis. The microspores and megaspores of the progymnosperm Archaeopteris are also abundant through these sequences but were absent from the coal swamp environment. A hypothesis is that the lycopods with their shallow root were preadapted for a wetland environment by being able to grow above the anaerobic layer. This prevented other, more deeply rooted plants such as progymnosperms from occupying the same environment. This explains the early (Mid Devonian) exclusivity of lycopods to the wetland coal swamps. Mid Famennian coals from Bj0rn0y were again dominated by lycopods (Cyclostig-ma). Analysis of Tournaisian coals from our collections shows that these are not Tournaisian in age but rather Viséan. This hints there may be a Tournaisian coal gap.
Acknowledgements
This contribution has originated from a Science Café in Syktyvkar supported by the Russian Academy of Sciences and the British Consulate in Ekaterinburg through the office of Her Majesty's Consul, Mr Richard Dewell. Natural Resources, Canada (Keith Dewing) are thanked for supplying the Melville Island coal samples. Ed Fleming (CASP) provided the samples from Bj0rn0y. Colleagues at NIGPAS (Nanjing) are thanked for the opportunity to work on Devonian coals in China.
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