Britain Tree-ring dating of gymnosperm wood from western Ireland

Britain lays near the equator during the Carboniferous period.

Tropical forests widely grown have been well preserved in the way of coal. These in-situ coal-bearing rock help to reconstruct the plant system before. This study combine the previous studies of plant fossil records and paleosols analysis to estimate if the humid climate influence the plant growth in Carboniferous Period. The earlier warm and wet climatic condition improve the development of the spore-producing and gymnosperm plants growth. Under the transition of the condition between humid to semi-arid climate, xerophyte such as cordaites developed well in the repetitive dry seasons period, meanwhile the gymnosperm became extinct.         1. Introduction In the beginning of the Carboniferous, Britain located in the southern margins of the Laurussian continent, which was close to the equator and covered by shallow water of the Rheic Ocean (Nance et al. 2012).

This is evidenced by a huge amount of Carboniferous coal fossils, produced from coal swamps and redbeds (e.g. Waters & Davies 2006). This paleogeographic location provided an excellent warm and wet environment, which influence biodiversity.  Well-preserved fossils wood in lower Carboniferous sediment can help to predict the paleoclimate (e.g.

Falcon-Lang 1999a, b; Waters & Davies 2006). Large in situ stumps of pteridosperm called Pitus has been found in the northern England and similar fossils, such as the stumps and ferns, were also found in the flood basin sequence of the southern Scotland (Falcon-Lang 1998). These fossils can be used to prove the existence of the prehistoric gymnospermous forests. Tree-ring dating of gymnosperm wood from western Ireland and southern Scotland suggests monsoonal climate, which is also comparable to modern tropical flora and past flora (Falcon-Lang 1999b). However, plants in Late Carboniferous showed a significant change in both species and quantity. The dominant lepidodendreae in Early Carboniferous Period decreased and tend to extinct, replaced by other xerophyte flora such as cordaites and ferns (Phillips et al. 1985). This essay will focus on the main species of plant grew in different durations of Carboniferous based mainly on published studies of fossil record in sediment strata.

In addition, this essay will try to compare different species and their indication of climate effect. 2. DescriptionFrom previous studies, the climate in Britain during the Early Carboniferous was suggested as seasonal tropical climate, with a seasonal annual rainfall connected to monsoonal circulation (Fig.1). Although there is no directly evidence for seasonal rainfall, the abundant amount of palaeosols and plant fossil record help to analyze the paleoclimate (). In the study of paleosol in Ballagan Formation in southeast Scotland (Fig.

2) during the lower Mississippian period,from the analysis of soil alkalinity range and water logging content in the aspects of geochemistry(isotope) and mineralogy, the annual precipitation were estimate around 1000 to 1500 mm in average. The variation of soil alkalinity and high annual rainfall indicate the rapid contrast seasonal climate, after combine with the deep penetrating cracks and evaporite sedimentation. Massive roots were preserved in fined-grained sandstone and siltstone beds, forming at a range of 1 to 80 cm in length. The root traces found in Northan core from Ballagan Formation have the vertical distribution with scattered sided branches preserved as carbon and brecciation indicated that these plants represent in wetland, which mainly composed of herbaceous or shrub-like plants. The diameters of the trunks found from Ballagan formation in Burnmouth are 6 to 14cm, and the height of them were calculated as less than 3.8 m.

They consists of shrubby ferns, e.g. Lyrasperma scotica. There also have tall trees such as Stamnostoma huttonense with 1.4 meters in diameters and about 25 meters in height (Timothy et al, 2016) Fig 2. Field section study in southeast Scotland (from Kearsey, T. I.

et al, 2016) In the study of Early Carboniferous climate research in northern Britain by using the anatomical features of plant fossils record, the gymnosperm fossil examples came from seven places. The subtle and discontinuous ring boundaries and ring increments in fossil collection suggested the similarity with current Araucariacean conifers, which means the ecological habitat of two plants are also similar. The growth rings of Araucariacean grow in the tropical zone nowadays illustrate the same irregular dry seasons. Furthermore, the fossil leaves also represent the xeromorphic features from outside and inside.

The most obvious feature is their hairy and rolled surface on morphology. And the extremely incrassate cell walls caused by the seasonal water stress.In upper sequence later Visean period located in Northern Birtain, the plant fossils have a significant preservation in the Asbian to Brigantian sedimentation. The fossil tree stump found in Craigleith Quarry in Edinburgh was up to 15 meters in length, which recorded by Witham in the 19th century, they mostly were gymnosperm.

In the study of the tree woods and leaves connected to the paleocliamte by Falcon-Lang (1998), the climate in northern Britain were considered to a seasonal tropical climate in later Mississippian. The main indication is the growth rings from the wood fossils, which showed a subtle and discontinuous ring boundaries with variable increments width.         The growth of tree rings record in wood fossil gave one evidence of the dry seasons interval, by comparing the fossil gymnosperm wood found in western Ireland (Fig 2) and southern Scotland and araucarian conifers in north Australia and southeast Asia. The specimens from north Australia grew in irregular tropical rainfall season have similar model of tree rings as that of in Carboniferous gymnosperm in Britain. However, macrophylla in southeast Asia have no rings, although all of these three specimen growed in similar latitude and near the equator. In addition, the anatomical features of the pteridosperm leaves indicate the xeromorphism of the plants from several aspects, such as the increasing thickness of the cell walls, pileous and rolled surface and sunken stomata (reduce the water vapor from the leaf).

These gymnosperm dominated fossils found in northern Britain highly represented the seasonality of the rainfall (1999). Figure 2. a.Western Ireland, b. Gymnosperm collected region(from Falcon-Lang,1998) However, the rainforest collapsed in late Carboniferous period, the ferns growth increased and took the place of the dominance of lycopsids. Few fossils record of large stumps from these reassembling ecosystem in middle Pennsylvanian time. These significant change of different flora domination were related to the variation of the climate (Sahney et al,2010). The immediate cause is the glacioeustatic.

The dated glacial deposits and sedimentary cyclicity suggested that the high frequent glaocio-eustasy started in the period of middle to late Mississipian (about 330 Ma) in British Isles, which began a little earlier than the cyclothems in USA at similar position in Carboniferous period, and strongly effect the high frequency of climate change. The sea level rises rapidly. (Wright and Vanstone, 2001) The main flora in glaciation were cordaites, pterdosperms and tree ferns, other plants such as lycopsids dominated the deglaciation land in earlier time were drowned by the increase sea-level at interglacial action.

Only few coniferopsid forests distribute in highland escaped from the flooding. They mainly survived in paleovalley regugia. This essay will present a synthesis of data from previous studies to estimate the connection between the major plants group and different climates in Carboniferous period.

   3. External factorsGrowth environmentThe macrofloral record from the South Wales Coalfield in Upper Carboniferous, abundant pollen and spores fossils were found the coal-bearing Westphalian strata. The fossils distributed in the eastern part of the coalfield and extended to west. This Coalfield has complete macrofloral record in Westphalian succession in UK.From the aspect of Species Richness (from fossils record), we can understand the biodiversity in order to assume the alteration of the environment. There are some main types of plant group collection from two areas (Swansea Area and Pontypridd Area), which were the lycophytes, the calamites, the pteridosperms, and the cordaites.

The lycophytes most found in the peat substrates, the ferms and pteridosperms widely distributed in direr clastic substrates (Christopher, 2007), and the cordaites intensive dispersal of driest clastic substrates (Falcon-Lang, 2003).  Sea levelThe cyclothemic sedimentation of coal-bearing strata in Late Carboniferous respond to the rapid climate change and sea level fluctuations (Falcon-Lang and Dimichele, 2009). After the begin of the glacio-eustasy in latter time of Early Carboniferous, the climate changed more frequent at low latitudes. The record of the calcret-karst strata suggested that the climate were altered between semi-arid to humid conditions. This also reflect on the fluctuation of the microfloral composition from NW Europe and North America (Wright and Vanstone, 2001).

The coal forests in Late Carboniferous formed during the glacial to interglacial transition. The fossil record during the seasonal dry glacial phases (lowstand system) grew in lowermost valley fills, which were seasonal dry vegetation consists of cordaites, pollen of conifers, pteridosperms, and only a few spores of Lycospore pusilla These kind of assemblage were rarely preserved in North America, but they help to distinguish the major tropical plants in lowstand Late Carboniferous period through Euramerican. They built a typical dry land landscape and few lepidodendrid dominant wetland in equatorial zone (2009). Species changeThe significance feature of plant species shift during Late Carboniferous period suggested a relevance between climate and plants.

From the study of coal-swamp vegetation analysis in quantity by Phillips (1985), the quantity of the floras in late Carboniferous coal-swamp indicate a general forest communities structure.The fossils collected from the coal seams from Euramerican paleotropical zone, although the alteration much more obviously recorded in North America. The numbers of lycopod showed a fluctuate variation with substantial decline in middle Pennsylvanian. Cordaites dominated the tropical forest gradually, which follow by the Tree ferns and medullosan pteridosperms. The gigantic tree fern developed prominently with large body, villous feature found  in most species possessed a period of dry seasons.        Plant diversityIn the study of macrofloral in Upper Carboniferous summerized the previous literature record from south Wales Coalfield, the  mainly flora contained sphenophytes, ferns, cordaites, etc. The proportion of each group in different substage have diffenerce.

For example, the amount of the hydrophilous plants decresed with the reduction of mudflat area. This alteration of the biomass might caused by sea-level changs and the uplift of the mountains. The coal fossils indicates that lycophyte dominate the wetlands in low altitude habitat for a long time until the fern and pteridosperm developed after the uplift of the mountain (Cleal, 2007).      3. Discussion and conclusionFrom the record of the different major wood fossils group in humid period and arid season in different period Britain, it can be held that the climate influence the type of major floras growth. The lycopods dominated the early tropical forest of Euramerica and took placed by cordaites and conifers in Late Carboniferous, only few amount of lycopods existed.Climate during the carboniferous period changed between humid to semi-arid, both helophyte and xerophyte widely spread in tropical forest in Early Carboniferous Period.

And the frequency of this transition became higher in Late Carboniferous. Diverse xerophyte developed in the dry seasons interval. Although the humid climate is not the only reason to decide the botany in different period tropical forest.In addition, the fossils record in early Carboniferous strata are more abundant and complete. Compare that, the evidence for dry phase flora in late Carboniferous period is insufficient.

The inference of the paleoclimate record can be connected with the modern climate change and further help to predict the future climate.