Fossil Trees in the Basal Purbeck Formation on Portland
- The Great Dirt Bed Forest


INTRODUCTION

In the basal Purbeck Formation of Dorset a fossil forest vegetation is preserved, representing well-developed forests that grew on the borders of the shallow, hypersaline Purbeck lagoon which covered southern England during the late Jurassic. The dominant tree was a conifer which appears from its morphology to have been adapted to growing in a semi-arid environment.


THE SEASONAL ENVIRONMENT OF THE FOSSIL FORESTS


Fossil tree in the grounds of the Portland Heights Hotel

The narrow and variable growth rings of the trees indicate that conditions were marginal for tree growth and highly irregular from year to year. Comparison with modern tree-ring data suggests that the Purbeck climate was of Mediterranean type, with warm wet winters when the trees were able to grow but with hot, arid summers suitable for the formation of evaporites. Analogous modern environments for both the forests and the seasonal lagoonal sediments can be found in the Mediterranean-type climatic regions of South Australia.

This evidence shows that within the "equable" Jurassic climate marked seasonal variations affected the whole environment. The seasonal nature of this climate supports recent palaeoclimatic models which propose that such a climate prevailed along mid-latitude continental margins during the Mesozoic.

The Mesozoic climate is considered to have been much warmer than that of today, with less extreme temperature ranges that prevented the formation of icecaps. Substantial palaeobotanical data indicate that the boundary between tropical (non-seasonal) and temperate (seasonal) floras was shifted as much as 15° poleward compared to that of today. Forest vegetation of trees with growth rings characteristic of modern warm temperate regions grew at latitudes as high as 65°- 70°S, within the Early Cretaceous polar circle. The presence of this extended subtropical zone was suggested to have been the result of the expansion of the subtropical high-pressure systems to latitudes of around 50°, with an adjacent temperate zone which stretched almost to the poles.

The three-dimensional atmospheric simulations (based on mathematical representations of physical laws) for mid Cretaceous palaeogeography implied that even with the high ocean temperatures necessary to warm polar oceans, continental interiors still experienced extreme seasonal temperatures. Large seasonal variations in rainfall were associated with mid-latitude continental margins, related to the alternation of high and low pressure centres over continents and adjacent oceans. This seasonal type of climate appears to have been a feature of mid-latitude continental margins throughout the Mesozoic.

Few palaeoclimatic indicators are able to record such short-term climatic variation. However, the growth rings of fossil trees, being a direct record of climate on land, can record the nature of the seasonality and by comparison with growth responses of modern trees to present-day climate, the nature of the palaeoclimate can be deduced.

The Purbeck Formation was deposited at a palaeolatitude of about 36°N. Comparative studies of the Lower Purbeck evaporites with similar modern deposits in the Persian Gulf imply that the climate was arid. Yet remains of petrified wood in the basal Purbeck deposits show that extensive forests existed on the margins of the large, shallow evaporitic Purbeck lagoon which covered southern England at the end of the Jurassic. This suggests that the climate was somewhat less arid, since trees were able to grow, while at the same time being suitable for evaporites. Faunal remains were considered to be indicative of a warm-temperate climate and ostracod types to be representative of a North African Mediterranean-type climatic regime. Climatic information from tree rings is particularly convincing when supported by palaeoenvironmental evidence from associated sediments, faunas and floras as illustrated in the next sections.


THE FLORA

Gymnosperm forests became established on the borders of the Purbeck lagoon in Dorset during minor regressive phases across the lagoon margin, when the development of well-drained soils became possible. The forests were subsequently inundated by rising hypersaline water and coated with algal-bound sediment which protected the tree bases and the soil from erosion. The trees are now preserved in situ as silicified stumps and branches within the palaeosols (the "Dirt Beds") and encased with algal limestone.

A summary of the basal Purbeck stratigraphy is presented below. The interpretation illustrates the alternation of hypersaline (intertidal) and fresh-water (supratidal) environments across the lagoon edge.


The structure of the wood from the petrified tree stumps indicates that the forests were dominated by one type of tree. This common conifer was a well-developed tree with a simple trunk, low branches and shallow roots spreading across the top of the limestone below the soil. The Purbeck trees, though clearly tolerating a semi-arid environment, show no special features that could be construed as adaptation to a halophytic or a swampy coastal environment On the contrary, the palaeosol indicates that the trees grew in a well-drained, calcareous soil, although the lagoon shoreline appears to have been only a short distance away. Other plant remains are much less abundant and include cycads represented by their short silicified stems found in situ between the tree stumps, and two other conifers represented only by wood fragments. Ferns and lycopods have formed the undergrowth.


Tree Bowl at Independent Quarry

One of the most important aspects of the Purbeck forests is that the trees are preserved in their original positions of growth. Many silicified tree stumps are still rooted in situ, but even where the wood is not present the original locations of the trees can be determined by the positions of circular domes of algal stromatolitic limestone which encircled the stumps at one time. These formed when the forests were drowned by rising water and algal-bound sediment collected around the tree stumps, completely covering the tops of some and collecting around the bases of others. In most of the empty central holes of the "doughnut-shaped" domes or "burrs", small fragments of silicified wood still remain and often the impression of the wood grain is present on the inside walls.

Assuming that each "open" burr formed around the base of a standing tree, an estimate of the spacing of the trees and density of the original forest can be made.



GROWTH RING ANALYSIS

Growth rings in silicified wood from the basal Purbeck Formation are fairly well-defined, particularly in specimens in which the organic matter has been retained in the cell walls. A picture of part climate can be deduced from such factors as:
(1) the presence or absence of growth rings for determining palaeolatitude (tropical or temperate climates),
(2) the absolute ring widths as indicators of growing conditions and
(3) the presence of false rings, indicative of the onset of adverse conditions during the growing season. In addition, variation in cell dimensions throughout a single ring provides some record of growing conditions.

The presence of clear growth rings within the Purbeck wood implies that the climate in which they lived was seasonal and tree growth limited to the more favourable periods. The narrowness of the rings suggests that growth was slow and the growing season relatively short. The average ring width of 1.13 mm for the Purbeck trees is comparable to growth in modern conifers from limiting environments, such as the semi-arid forests of the Western Mountains of America. At high altitudes low temperatures restrict tree growth but at the lower semi arid forest border the limiting factor is the shortage of water. Such trees under stress are particularly "sensitive" to the limiting factors. In the semi-arid Purbeck environment the most likely factor affecting tree growth to such a marked extent would have been the rainfall pattern and water availability.


Fossil tree stump in situ at Withies Croft

In order to interpret the Purbeck climate in more detail, comparisons of the growth-ring data with similar sequences from known climatic regions was made. The most similar tree-ring characteristics were found in trees from areas with a markedly seasonal climate of Mediterranean-type with warm, wet winters and hot, arid summers. Tree-rings from selected modern conifers from such areas show the same slow growth in their narrow rings and very similar variability in growth from year to year. Several rings from the modem trees also have 'false" rings within the earlywood, followed by a very narrow zone of latewood cells. Although water was available for growth in the growing season, intermittent droughts during this period caused the formation of false rings. It is not possible to determine when the growth of the Purbeck trees occurred - during the more favourable warm, wet seasons or during the summer. However, studies of conifers in Israel have shown that Cupressus sempervirens has an annual rhythm of cambial activity which coincides with the Mediterranean climatic rhythm. The first flush of extensional growth, followed by the onset of cambial activity, begins in October when temperatures drop and water becomes less limiting. This winter growth may be interrupted by short periods of inactivity in the spring caused by occasional hot, dry spells. This results in the formation of false rings. The cambium becomes dormant towards the beginning of the dry summer. A similar pattern of growth during the wet winters is envisaged for the Purbeck conifers.


SEDIMENTARY EVIDENCE OF SEASONAL CLIMATE

The lateral equivalent of the Great Dirt Bed palaeosol in the westerly Purbeck outcrop, at Portesham and Upwey, represents a marginal lagoonal clay which contains an anomalous association of both silicified wood, conifer shoots, seeds, and charophytes along with nodules of silicified evaporite pseudomorphs. This strange combination is, however, observed today in seasonal lakes in Mediterranean-type climatic regions of South Australia. In the Coorong coastal lagoon (32°S lat.) charophytes and a freshwater fauna are present in the lagoon margins whilst evaporites are forming towards the lagoon centre. The charophytes are able to grow in ephemeral waters at the lagoon margin or in adjacent fresh-water marshes during the winter season when rainfall, low evaporation and drainage water from adjacent land allows lake margins to become fresh. During the summer drought intense evaporation raises the salinity of the water to levels where evaporites are precipitated, resulting in a mixture of fresh-water and high-salinity features in the sediment. A similar seasonal variation in fresh-water flora, fauna and evaporites is envisaged for the Purbeck.


Lagoonal deposit at Tout Quarry

The Purbeck tree stumps and branches are petrified by quartzine. This is frequently considered to be a palaeoclimatic signal indicative of a semi-arid climate. The source of the silica in the Purbeck trees is not clear; there is no evidence that it came either from volcanic or organic sources. However, within the Coorong Lagoon in South Australia, comparable in some aspects to the Purbeck lagoon, chert is inorganically precipitated due to seasonal changes in lagoon water salinity. During periods of high photosynthetic activity in the lake, the pH of the water increases to values sufficiently high for the dissolution of silica (pH> 9), mainly from detrital quartz and silicate clay minerals. This silica-rich brine is then precipitated as gelatinous opal-cristobalite during the dry season in areas of lower pH such as just below the sediment surface where plant material is decaying.

A similar pattern of precipitation seems a likely mechanism for the origin of the Purbeck silica. After the Purbeck forests had been submerged by hypersaline lagoon water, the pH of the water might have been raised by photosynthesising algae and by seasonal evaporation to values high enough for dissolution of silica (probably from detrital quartz or diatoms). The silica would then be sufficiently mobile to move to areas of low pH which, in this case, would have been the decaying organic matter in the tree stumps.


Calcrete at the base of the Great Dirt Bed

The limestone at the very top of the Hard Cap contains laminar structures and nodules of secondary carbonate, called calcrete. This is present as areas of laminated or mottled patches of microcrystalline carbonate or undulating laminar coatings on nodules. Brecciated pieces of the calcrete were incorporated into the Great Dirt Bed soil as it formed, now visible as pebbles in the soil matrix, confirming that the calcrete is Jurassic in age. Calcrete is a term broadly applied to the epigenetic accumulation of secondary carbonate formed by the evaporation of carbonate-rich water, the prerequisite of which is a semi-arid, seasonal climate with periods of rainfall followed by drought. Pedogenic calcrete forms in soils in semi-arid regions and is related to the capillary action of soil waters. During the wet season, rain water percolates through the soil dissolving carbonate pebbles and shells. As the soil water then evaporates within the soil (the actual depth depending on the aridity of the climate and the intensity of evaporation) secondary carbonate is precipitated as laminar crusts on nodules, later coalescing to form indurated laminar layers.


Faunal evidence


An example of a fossil crustacean

The Lower Dirt Bed in the east of the Dorset Purbeck outcrop is not strictly a palaeosol, as previously reported, but is a finely laminated, varve-like deposit with alternating laminae of white carbonate and dark, organic-rich sediment. The finer, dark laminae have fish scales, fish vertebrae, decayed ostracods and the chitinous carapaces of crustaceans Modern crustaceans of this type live today in fresh or brackish water, often in ephemeral pools and puddles less than 30 cm deep. They are frequently found in monospecific communities in small ephemeral ponds on coastal flats associated with evaporites, such as the seasonal lagoons of South Australia. They live in these pools during the wet season but die, along with fish and other fauna, as the pool later desiccates. A similar origin is envisaged for these Purbeck Lower Dirt Bed sediments and fauna. The dark laminae represent the influx of plant debris from adjacent soils during the wet season followed by the accumulation of carbonate during the dry period.

Fossil conchostracans are useful index fossils because they occur in very restricted non-marine facies. They are also good indicators of seasonality since an assemblage on one lamina is representative of a single season. The concentric growth lines on the modern carapaces are the result of moulting every three days, and so by counting the growth lines on the fossil an estimate of its life span can be obtained. On this basis the largest conchostracan from the Lower Dirt Bed would have lived for about five months, which also suggests that the pool, and thus the wet season, lasted for a similar period.


DISCUSSION

Both the growth rings in the Purbeck wood and adjacent sedimentary and faunal associations indicate that a highly seasonal environment prevailed during the Late Jurassic in southern England. The seasons consisted of a warm wet period followed by an arid one. The modern equivalent is the Mediterranean-type climate which has a low annual temperature range but rainfall is restricted to winter months and in some regions the summer is very arid.

Whether these seasons reflect the winter and summer of one single year or in fact represent longer periods is unclear. It is conceivable that the droughts lasted considerably longer than a few months during the summer, and that growth ceased for long periods. However, the close similarity between growth ring parameters of the Purbeck tree rings and those from modern Mediterranean-type trees suggests that the seasons were annual.

Both the ephemeral aspect of the climate and the position of the forests in a rather restrictive environment on the Purbeck lagoon margin would have adversely affected the growth of the trees. The factor which would have limited growth most markedly would have been the shortage of water. Not only was rainfall concentrated in the wet winter season but even then it may have been very irregular, for one aspect of the Mediterranean climate today is not specifically the overall amount of rainfall but its erratic occurrence and great intensity for short periods. When rain does fall, it does so with the greatest intensity at the beginning and at the end of the wet season. Intermittent dry spells can interrupt the wet season for days or even months. The "false" rings within the earlywood of the fossil growth rings support the idea of adverse conditions occurring during the more favourable growing season. An indication of the length of the wet season is given by the conchostracan branchiopods which appear to have survived in pools which contained some water for at least five months.

The irregular supply of rain water probably accounts for such variable ring widths from year to year. During wet seasons water would have soaked directly into the soils or drained from higher areas in the west. Likewise plant debris from the soils would have been washed into the lagoon episodically and the water at the margins diluted by the additional fresh water, allowing fresh-water floras (charophytes) and faunas (molluscs, ostracods) to become established. Since the shallow tree roots, restricted to downward growth by the hard limestone below the soil, would not have been able to tap any groundwater supplies, the only water available would have been that in the soil. This would have been used rather quickly and, given the shallowness of the roots, the trees must have experienced the effect of drought rather rapidly. The presence of false rings in the wood and the very narrow zone of latewood is consistent with such an interpretation. A layer of indurated calcrete within the soil may have acted as an aquiclude, retaining a lens of fresh-water as a further store for the trees.

The trees themselves appear to have had xerophytic adaptations which would have helped them survive in such an arid environment. These features include small, scale-like leaves and thick cuticle with sunken papillate stomata to reduce moisture loss. It seems likely that whole shoots were shed during the dry season for the same reason. These shoots would have been suitable fuel for forest fires for which there is strong evidence in the Purbeck forests. Fossil charcoal or fusain is preserved within the palaeosols, though there is no evidence of charred tree stumps or logs to suggest that the fire was intense enough to destroy whole trees or prevent their establishment. The fire probably passed quickly through the undergrowth, charring dried twigs and shoots.

Interpretation of the Purbeck environment was mainly based on comparison of the trees and growth rings with analogous modem environments, the most comparable being the Mediterranean-type climatic regions of Australia. As indicated above, many aspects of the Coorong lagoon (32°S) can be found in the Purbeck environment, particularly the association of fresh-water and evaporitic sediments, and the seasonal influence on the fauna. However, a good analogue to the Purbeck forest is absent here since the natural modern vegetation is Eucalyptus, although another similar forest was found on Rottnest Island, near Perth where natural stands of conifers grow adjacent to ephemeral salt lakes. The habit of these conifers, being small trees with monopodial trunks, conical habit, shallow roots and low branches with thin foliage forming a closed canopy, is very similar to that envisaged for the Purbeck trees. They are also subject to frequent fires: they are fire-sensitive to the extent that the trees cannot withstand foliage scorch but regeneration is favoured by an ash layer on the soil. The growth of the trees shows the same degree of variability reflected in their growth rings. The climate here is of extreme Mediterranean-type, with very arid summers, though the mean annual rainfall is 745 mm, 82% of which falls within the five winter months from May to September. The temperatures range from a mean maximum temperature of 25.7°C in summer to 16.7°C in winter.

Analogies to the Purbeck seasonal ponds and fauna can be found on Rottnest Island too. Seasonal fresh-water swamps of limited area contain water only during the winter from the start of the first heavy winter rains in May until the last drop evaporates in late October (about five months). All crustaceans living in these ponds have eggs which can withstand drought, as the Purbeck conchostracans probably did. The salt lakes adjacent to the Rottnest Island conifer stands become saline enough during summer droughts to allow evaporites to precipitate.

The modern Mediterranean-type climate is chiefly one of coastal lowlands of mid-latitudes. Its seasonality is related to annual changes in solar radiation as the sun moves across the hemispheres, giving rise to periodic shifts in pressure centres and their related wind fields. In simple terms, low-pressure centres strengthen over continents in summer as the land surface heats up and corresponding subtropical anti-cyclones are present over the adjacent oceans. In winter this pattern is reversed and oceanic winds bring rain to coastal areas.

This type of seasonal, semi-arid climate in the Late Jurassic in southern England accounts for the paradoxical association of evaporites with well-developed forest vegetation. The relationship of these forests and sediments to this type of climate may serve as a model for other Mesozoic floras. The growth rings in fossil trees are clearly important indicators of palaeoclimate and particularly useful for testing the seasonal climate predicted for continental margins by palaeoclimatic modelling.

JANE E. FRANCIS
Earth Sciences, University of Leeds

Abridged by Bob Ford from Francis, J. E., 1984. “The seasonal environment of the Purbeck (Upper Jurassic) fossil forests” in Palaeogeography, Palaeoclimatology, Palaeoecology, 48 (1984): 285-307.
Elsevier Science Publishers B. V. , Amsterdam.

Photos by Bob Ford and Paul Crabtree