Plant macrofossils and palaeoclimates

Jack Wilkin (UK)

Palaeoclimatology is the study of past climates and environments using climate proxies, that is, the preserved physical characteristics of past, rather than using direct measurements of variables, such as temperature, levels of CO₂ and so on. Many different types of proxies are used including, but not limited to, ice cores (Petit et al., 1999), lake and ocean sediments (Cehn et al., 1999), and fossil data. Many fossil groups have specific environmental and ecological tolerances and so can be used to determine palaeotemperatures and palaeoclimates (Jones, 2006). It is the data collected using dendroclimatology and other plant macrofossils that will be examined in this article.

Dendroclimatology

Dendroclimatology is the use of tree rings to determine long-term climatic trends. This is in contrast to dendrochronology, which is dating using tree ring data. Dendroclimatology is used extensity to study the climate during the Holocene (Fig. 1) but has also been applied to the Late Cretaceous of Alaska and even the Permian of Antarctica (Taylor et al., 2009). The thickness of the tree rings helps scientists work out how much the trees had grown within a given year. Then, by comparing the rate of growth to members of the same, or closely related, genera or species, they can determine the palaeoenvironment.

Dendroclimatology can also be used to gather isotopic data. As trees grow, they assimilate carbon from the atmosphere, and hydrogen and oxygen from water in the soil. The relative abundances of different isotopes, or variations, of these elements can reveal information about ancient climates. For example, carbon has three naturally occurring isotopes: ¹⁴C, ¹³C and ¹²C. It is the ratio between ¹³C and ¹²C that is used in palaeoclimatic studies. The latter is more weakly bonded than the former, so is more likely to become incorporated in organics, such as shells and wood.

Carbon ratios tracks changes in the carbon cycle. Increases in ¹³C can be caused by the burial of organics and leads to cooling while a decrease, which is caused by the release of carbon by the melting of methane hydrates and/or volcanic active, leads to a greenhouse effect. As the temperature rises, the amount of the lighter ¹²C goes up relative to the heavier ¹³C. It is worth noting the final naturally occurring carbon isotope, ¹⁴C, is used in radiocarbon dating. Oxygen, unlike carbon, can be used to directly measure palaeotemperatures.

That is, material formed at 25°C will have a higher proportion of ¹⁶O than if it was formed at 15°C (Parrish, 1998).Isotopes can also be used to extract information from ringless tropical trees. This is important because, unlike many temperate and arid areas, trees in tropical regions continue growing throughout the year. Because growth is continuous, there is often a lack of a distinct annual ring boundaries, rendering traditional dendroclimatology useless (Gagen et al., 2011).

However, dendroclimatology is not perfect, as divergence has been observed in twentieth-century reconstructions. Usually, trees show positive responses to higher than average temperatures. However, in the 1960s and 1980s, they showed less pronounced or even negative responses to higher temperatures. This discrepancy creates a cold bias within the palaeoclimatological record, making it impossible to form any conclusions about those temperatures when compared to the rest of the historical record (Loehle, 2008). Climatic factors, such as temperature, precipitation rates and the amount of sunlight, can be limiting factors affecting tree growth and thus can lead to different interpretations of climate, even if the trees come from the same locality.

Additionally, non-climatic variables, such as soil, the tree’s age, competition for resources, pathology and numerous other factors, can lead to erroneous results. Such limitations can be resolved either by fitting spline curves, which are used to resolve the overall record, or by only studying trees of a similar age. (A spline curve is a mathematical representation that allows a user to design and control the shape of complex curves and surfaces. That is, the user enters a series of points, and a curve is constructed whose shape closely follows that sequence.) However, to be studied for evidence of past climate, the age trend – the growth curve associated with age – must be approximated and removed. The process of removing non-climatic variants from a sample is called standardisation (Sheppard, 2010).

Leaves

The anatomical structure of plants is highly responsive to changes in ambient climate and even atmospheric conditions, making them excellent palaeoclimate indicators. The study of how plants are adapted to their environments is called physiognomics (Taylor et al., 2009).

The relationship between leaf margin shape and the mean annual temperature is well documented (Foote and Miller, 2011). Leaves from cooler climates tend to have more teeth, and those from warmer climates tend to have smooth, or entire, margins (Fig. 2; Peppe et al., 2011). Plants from tropical environments have thick waxy cuticles, a drip tip to drain rainwater off the leaves, an entire margin and are evergreen (Taylor et al., 2009). Therefore, the shape of fossil leaves can provide a lot of information about palaeotemperatures and rainfall.

There is also a relationship between stomata density on the undersides of leaves and the levels of CO₂ in the atmosphere. Stomatal density decreases with an increase in CO₂ and thus temperature (Fig. 3). This trend was observed in 74% of specimens in a study by Woodward and Kelly (1995), which is more than would be expected by chance. Most climate researchers think that contemporary global warming is directly related to an increase in atmospheric CO₂ concentrations.

To help test this theory, researchers investigated changes in stomatal density over the last 200 years and found that stomatal density has indeed decreased substantially over that time. Given this strong relationship, scientists then applied the same principle to much older fossil plants to estimate changes in CO₂ levels during the Phanerozoic (Foote and Miller, 2007).

A limitation of this technique is that variables other than CO₂ can affect stomatal development, such as the availability and quality of light. Therefore, a plant can have leaves with different stomatal densities depending on its position in the canopy. Furthermore, exposure to drought during development reduces stomatal density. In fact, it appears that environmental factors may play a larger role in stomata density than CO₂ concentrations (Reid et al., 2003)

Peat bogs

One form of plant macrofossil that is often neglected is peat bogs. These can be widespread during the Holocene throughout north-western Europe and other parts of the world. Peat formed in northern temperate regions are bog-masses of the genus Sphagnum, which are important agents of environmental change both at a local and global scale.

Cores taken from bogs across Ireland and the United Kingdom have revealed distinct changes in the macrofloral assemblages, which indicate three climatic divisions during the Holocene. The early to middle Holocene, about 11,500 to 4500 BP, was dominated by monocots (that is, flowering plants whose seeds typically contain only one embryonic leaf), indicating a dry climate. The middle Holocene saw the climate getting wetter, as monocots were replaced by Sphagnum imbricatum. The final transition was the replacements of S. imbricatum by S. magellanicum,as a response to the Little Ice Age and peat cutting (Barber et al., 2003). Peat bogs are also useful palaeoenvironmental indicators because they can be used to record the effects of human activity before the direct recordings of temperatures.

Nearest Living Relative

Another important technique is the Nearest Living Relative (NLR) method. This is the oldest method of using plants as palaeoclimatic indicators. It is particularly useful when dealing with late Mesozoic and Cenozoic floras, as these are more likely to have extant relatives. The theory assumes that the environmental tolerances of fossil floras are the same as, or very close to, those of their living relatives.

A fundamental issue with using plant macrofossils as climate proxies is that, the further back, the less effective NLR becomes, as modern floras are dominated by angiosperms (flowering plants), but pre-Cretaceous ecosystems were predominantly gymnosperms (seed-bearing vascular plants, such as cycads, ginkgo, yews and conifers). Therefore, the technique is best used on Cenozoic angiosperms. In addition, NLR can only provide general estimates of climate, not absolutes as in isotopic measurements, as its use can be limited because some fossil taxa do not have the same preferences as their extant counterparts (Taylor et al., 2009).

Summary

The study of plant macrofossils is a highly useful tool for studying past climates. However, every climate proxy has its limitations so, to get the most accurate result possible, it is best to use multiple proxies. This is especially true if important variables are to be identified. Ideally, it is best to use a mixture of inorganic and organic data combined with model results (Pearson, 2012).

About the author

Jack Wilkin is a graduate researcher at the Camborne School of Mines in the United Kingdom. His research focuses on the use of isotopes from macrofossils, particularly belemnites from the Middle Jurassic of Germany, to reconstruct ancient climates.

References

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