The History Of The Azolla Event

Published Date: 02 Nov 2017

65 Million Years of Evolution in our Backyard

Geology of Southeast Australia – Week 2 Trip, 2013

Jacob Nichols – 540 631

Geology of Southeast Australia

The Relationship between Climate and Floral Change in Victoria over the Last 65 Million Years

Abstract

This essay will explore the relationship between the climate and floral change in Victoria over the past 65 million years. It will chronologically disseminate these two variables from each epoch from the Cenozoic era, and display commonalities between them. It is found that the relationship does in fact go both ways, plants are altered by the climate, however there is some evidence indicating that the climate is also altered by plants.

Approximately 200 years ago a young naturalist Charles Darwin travelled to the Galapagos Archipelago and analysed the local inhabitants (Darwin, 2013). His findings on this trip led Darwin to conceiving the notions of evolution and natural selection, disputing the contemporary train of thought that species were immutable productions. Darwin claims that the species that will survive are the ones most suitably adapted to external forces that are imposed on them (Darwin, 1869). Plant species are not an exception to this notion, the climate has the greatest impact on floral propagation and evolution as evident in the correlation between palaeobotanical and palaeoclimatic records (Martin, 1982). This essay will attempt to explain the current climate of Australia, the general tendencies of the Australian climate over the past 65 million years, and expose the relationship between the climate and flora in Victoria throughout the Cenozoic era.

If the habitat of a species stays consistent then there are no selective pressures to drive evolution, however a changing environment will favour plants and animals with beneficial variations caused by mutations allowing them to survive and multiply over the others (Dobeson, 2005). It is important to keep in mind throughout this symposium that while climate may be the salient factor in terms of the evolution of flora, other factors such as; soil nutrient levels, atmospheric CO2 levels, fire frequency and intensity, glaciations and the arrival of humans also have a large impact (Hill, 2004).

Figure 1: A generalised map of the current vegetation of Australia (Martin, 2006)

Australia is the driest continent in the world; 75% of the continent receives less than 800mm of rainfall each year and even that is often unreliable (Evans, 2011). The Southeastern corner of Australia is outside of this percentage and has between 1000mm and 2000mm of rainfall spread evenly through the year, with average temperatures between about 10°C minimum and 28°C maximum (Dobeson, 2005). This corner has the richest and most abundant vegetation in the country due to the temperate conditions (Evans, 2011). Figure 1 shows the general distribution of plants throughout Australia.

65 million years of global temperatures

Figure 2: A graph of temperature fluctuations over the past 65 million years (Lappi, 2013)

‘Australia has been profoundly altered by its movement through 20 degrees of latitude since its initial rifting from Antarctica 40 Million years ago (Mya)…. causing major climactic implications both directly as Australia moved through broad climactic zones and indirectly as Australia’s movement influenced other events with a climactic outcome’ (Hill, 2004). Tectonics within Australia have probably existed much the same throughout the Cenozoic with changes being only minor and gentle (Martin, 1982). When Australia was moving northwards and breaking away from Antarctica, so was South America, the two movements allowed the flow of the circumpolar current to occur (Martin, 1982; Hill, 2004).

Palaeocene

The Palaeocene was a time of warm marine conditions, with less differentiation of climactic zones than there is today and Antarctica was essentially unglaciated (Martin, 2006). Victoria experienced high levels of precipitation and runoff and the climate was cool and temperate. The beginning of the Cenozoic supported mesothermal to microthermal (cool temperate) rainforest vegetation, with some swamps. Gymnosperms were a prominent part of the vegetation with angiosperms increasing in prominence; however Nothofagus were rare (Martin, 1982; Martin, 2006). Pteridophytes were also common inferring a year round humidity (Martin, 2006). The vegetation was richly diverse; however the coastal regions were more likely to have Casuarinaceae (Martin, 2006).

The transition between the Palaeocene and the Eocene epochs gave rise to what is now known as the Palaeocene-Eocene Thermal Maximum (PETM) 56 million years ago. This saw an increase in temperatures in tropical regions of 3-5°C compared to Late Palaeocene values (Jaramillo, 2010). These high temperatures led to an increase in tropical vegetation; however some areas were above the maximum temperature threshold for some tropical plants. The PETM only lasted about 100,000-200,000 years, thus was one of the most abrupt global warming events of the entire Cenozoic era. This event is associated with a large negative carbon isotope excursion recorded in carbonate and organic materials, reflecting a massive release of C-depleted carbon (Jaramillo, 2010).

Eocene

The Eocene was the warmest period worldwide during the Cenozoic era. Victoria continued to experience temperate conditions with considerable rainfall and runoff (approximately 1800mm per year and a mean annual temperature between 16 and 22°C). Humidity remained high and temperatures increased, the area became subtropical (Martin, 1990; Martin, 2006). The Eocene saw an upsurge of megathermal angiosperms and ferns, which were highly diverse and mangroves were present in tide-dominated environments and had their southernmost expansion to Tasmania (Hill, 2004; Martin, 2006).

The Mid-late Eocene saw the beginning of the separation of Australia from Antarctica (~45 Mya) but maintained an embayment that prevented the circumpolar current from flowing. Prior to this separation a period of global warming is evident in the vegetation where the high temperatures must have been accompanied by year round rainfall (Hill, 2004; Martin, 2006). After this brief period of warming there was a general decrease in oceanic water temperature. ‘The vegetation of coastal southeastern Australia was compared to living lowland megathermal rainforest in northeastern Queensland but an important difference is the presence of winter deciduous species, which was probably a response to high latitude winter darkness’ (Hill, 2004). High epiphyllous fungal loads on fossil leaves indicate high rainfall with no dry season during the year (Hill, 2004). This epoch saw a rise in the Brassopora type of Nothofagus, Casuarinaceae and Gymnosperms became more common than in the early Eocene (Martin, 2006). The vegetation was still predominantly rainforest but some sclerophyllous elements appeared likely due to low nutrient levels and low phosphorous levels, these species were to become adapted to a xeromorphic function when dry conditions arose (Hill, 2004; Martin, 2006).

The late Eocene saw the rift between Australia and Antarctica widened, strengthening the Circumpolar-Antarctic Current, reducing the heat transfer from the tropics to the high latitudes (Martin, 2006). This was a key interval that marked the transition from the Cretaceous-Eocene marine and atmospheric circulation to the modern climate (Martin, 2006). The climate turned cooler and wetter Angiosperm diversity declined, Nothofagus increased in salience and swamp taxa were augmented (Martin, 2006).

The Azolla Event

The relationship between the climate and flora is not as straight forward as one may think; they are quite interrelated and to an extent, symbiotic. While the climate may alter the form, and mechanisms of a plant, a plant can in turn affect the weather. The earth is currently experiencing an ‘icehouse’ effect; it is sufficiently cooler than it was ~49 Mya which may be linked to the Azolla event (Challis, 2013). Azolla is a freshwater fern like plant that can absorb and feed on large amounts of nitrogen and carbon in the air. Given the appropriate conditions, it can double its biomass in just two days (Nielson, 2010). The Azolla used these nutrients from the atmosphere to prosper, and once the Azolla died it would sink to the sea floor, but not decay, thus trapping the carbon and nitrogen with it (Nielson, 2010). It formed largely in the arctic sea during a period where it was shallow and rather dense (Nielson, 2010). From this it can be deduced that the event occurred over 800,000 years which coincides with a decrease in carbon dioxide levels from 3500ppm in the early Eocene to 650 ppm after this event (Challis, 2013).

http://files.myopera.com/nielsol/blog/azolla_production.jpg

Figure 3: A diagram of the Azolla event and the basic dynamics of its occurrence (Nielson, 2010)

Oligocene

The early Oligocene saw an abrupt cooling of ocean waters largely impacting the high latitudes (Martin, 2006). The temperatures decreased hastily due to increasing development of circumpolar currents, while precipitation remained relatively high (1500-1800mm), freshwater swamps became prominent and seasonality became apparent (Martin, 2006). The Latrobe Valley showed a high level of Nothofagus Pollen and Victoria saw an increase in the diversity of Angiosperms and Conifers, which were demonstrating trends in leaf size and stomatal distribution in response to climate change (Hill, 2004; Martin, 2006). The Oligocene is also important as xeromorphic characters first appear here, becoming increasingly more common as the continent dried out (Hill, 2004).

Miocene

The early-mid Miocene was a time of high sea levels and flooding marginal, shallow basins and warm, humid climates, which have not been experienced since the mid-Miocene (Martin, 2006). Conditions here were probably more equitable than at any time since, however it also marked the first major step towards aridity (Martin, 2006). The vegetation in the coastal regions would have been a more diverse woodland/forest predominantly Casuarina, with a much reduced Nothofagus content, indicating a shift to a drier climate (Martin, 2006).

The upper mid-late Miocene cooled down significantly, corresponding to global conditions, seeing a lowered sea level, major ice sheet expansion on Antarctica and the climate on land became more arid (Martin, 1982; Martin, 2006). Precipitation decreased to between 1000-1500mm, and there was a well-marked dry season which allowed burning on a regular basis (Martin, 1990). This period saw a decrease in the Brassopora type of Nothofagus however the subgenera Lophozonia and Fuscopora still persisted in highland coastal regions, most likely as a response to the cooler temperatures, but still humid climate (Martin, 2006). Eucalyptus (making its first widespread appearance on the east coast) and other Myrtaceae, Casuarinaceae and herbaceous taxa, especially of the family Asteraceae and Poaceae became more common, reflecting the increasingly drier climate (Hill, 2004; Martin, 2006). The playnofloras show most change over this time period and the rainforests that formerly blanketed the area tended to disappear, persisting only in small favourable (wetter) habitats, most likely due to an increased frequency of burning (Martin, 1990).

Pliocene

The earliest Pliocene saw a brief return to rainforests in some river valleys due to a short period of a wetter climate, however this was not maintained for long (Martin, 1990; Martin, 2006). Early marine temperatures were warm and stable, but later temperatures were cooler and quite variable (Martin, 2006). Temperatures were estimated to be about 20°C and rainfall still exceeded 1000mm, nevertheless the climate was getting dryer (Martin, 2006). This time saw the present floristic zones become established (Martin, 1982). The few remaining rainforest taxa disappeared and the vegetation became open sclerophyll forest most likely in response to large climactic swings (Martin, 1990; Hill, 2004; Martin, 2006). The late Pliocene leads to an increase in the Asteraceae and Poaceae content so that by Pleistocene time, the vegetation had become open woodland/grassland (Martin, 1990; Hill, 2004; Martin, 2006). The climactic changes at the end of the Pliocene are associated with the onset of glacial cycles and aridity in Australia, the precipitation dropped to about 500-800mm and the general climate and distribution of flora began to represent what it is today (Martin, 1982; Martin, 1990; Hill, 2004; Martin, 2006).

Pleistocene

The Pleistocene commenced with an essentially modern climatic regime operation however the precipitation was higher than that of today (Hill, 2004; Martin, 2006). This period saw a series of glacial cycles where wetter/drier episodes correspond to interglacial/glacial periods respectively. The glacial periods tended to see an increase in open vegetation and grassland and the interglacial would see more wooded vegetation (Martin, 1982). In Australia the glaciation was restricted to Tasmania and the southeastern highlands (Martin, 2006). Generally flora was much the same as it is today, eucalyptus forests generally increased along with open grasslands which became dominant in this time period (Martin, 2006). ‘The almost constantly changing climate of the Pleistocene must have had a profound effect on the biota. With each glacial/interglacial cycle, populations would have been squeezed into refugia and then when the environment became more favourable, they would have expanded their range. This almost constant shuffling must have favoured the more adaptable taxa and there would have been many local extinctions’ (Martin, 1982). There was a major change to ‘fully arid conditions’ about 0.5 Mya, thus the overall trend through the Pleistocene was decreasing precipitation (Martin, 2006).

Conclusion

This essay divulged the tendencies of the Victorian climate, the history of the climate, and the plants during these times that led to the current climate. This brief outline of Victoria’s climatic history and the floral adaptations that have occurred throughout the Cenozoic era makes it evident that climate does have a massive impact on flora. If the seasons are warm and wet, such as in the Eocene, it is likely that a rainforest style of taxa will emerge, however when the location dries out, sclerophyll or even grassland species will develop more like today. Although it is not only the climate affecting the flora, as made evident in The Azolla event, flora can indeed have an effect on the climate.