Introduction to pollen studies for climate reconstruction

Another part of my research focused on pollen grains. Pollen is produced as part of the reproductive cycle of plants and are transported by wind and water throughout the environment. In principle, the more of a certain type of tree there is in the local environment, then the more of that plant’s pollen should be present in the surrounding landscape. When there are large shifts in the climate, then the species of trees in an environment changes. Therefore, the pollen in that environment will also change. As pollen is very resistant to degradation, analysing the pollen content of old soil (eg in a sediment core from the bottom of a lake) provides information on the vegetation present when that soil was formed.

Palynology is the study of pollen and spores and has many applications. In the context of this project, it is the palaeoclimatic significance of long-term trends in pollen and spore production that are utilised. Pollen is a microgametophyte produced on the anther of male angiosperms and gymnosperms. Spores are (mostly haploid) reproductive structures produced by peridophytes, bryophytes, algae and fungi. Both structures are dispersed widely in the biosphere, although the aim of pollen dispersal is to fertilize the stigma of another plant, whereas spores only require a suitable site for germination to continue the reproductive cycle. The dispersal of palynomorphs occurs via several pathways, although the two main mechanisms are atmospheric dispersal (anemophilous species) and animal dispersal (zoophilous species, or entomophilous species if insects are the dispersal vector). The form of an individual palynomorph is often designed to exploit the dispersal vector associated with that species. It is this specialisation that permits differentiation of palynomorphs from various source organisms (Moore, 1991). The following discussion is divided into two sections; firstly, an outline of the requirements for the successful application of palynology in theoretical terms and secondly, the reasons for the application of palynology in this project.

The extraction of a meaningful climate signal from palynological evidence requires an understanding of the biological underpinnings of both pollen and spore production and the environmental mechanisms involved with the deposition of pollen grains, as well as the diagenetic processes which may alter pollen post-depositionally and the experimental techniques used to isolate palynomorphs from other sedimentary components. When these aspects are considered and controlled, palynological data have been proven to be a sensitive climatic proxy (e.g Allen et al. 2001).

The factors which govern pollen production are multifarious, and on a local scale, patterns of pollen dispersal are highly variable. The content of pollen rain is heavily biased toward anemophilous taxa and those taxa that produce large quantities of pollen. As an example, a single shoot of Cannabis can produce ~400 million grains, compared to ~20,000 for a mature Linum catharticum (Faegri, 1989). Zoophilous species typically produce less pollen, and in certain situations, where a high degree of symbiosis has occurred, production volumes can be extremely low, with some rare cleistogamous species not releasing any pollen at all (Faegri, 1989). The physical structure of the ecosystem can affect dispersal patterns of native pollen grains, as canopy species are able to release pollen directly into the atmosphere, whereas understorey species release pollen into the trunk space, which results in more restricted dispersal. Once airborne, atmospheric circulation rapidly mixes pollen grains and transports them long distances. Sorting occurs in the atmosphere, as heavier grains settle out of the atmosphere faster than lighter and bi-saccate grains (Faegri, 1989). However, rain rapidly washes any airborne pollen into fluvial systems. These fluvial systems transport vast amounts of pollen into sea and oceans, which are further dispersed by hydrographic circulation before settling through the water column to the sea floor. It is because of these long-distance transport mechanisms that local signals merge into a broader regional signal in marine sediments. For the purposes of palaeoenvironmental analysis, this aspect is an advantage, as individual localised events do not affect the general trends. However, for the purposes of palaeoecological investigations, much of the necessary detail is lost.

Once the palynomorph is deposited, it is subject to the same remobilisation and reworking mechanisms that potentially affect all sedimentary particles. In the Black Sea, the steep submarine apron and frequent seismic activity result in numerous density flows that disturb large areas of the abyssal plain. Palynomorph degradation can occur where biogeochemical cycles result in the production of corrosive chemicals or the incidence of detritus-feeding bacteria. This process can lead to biases in the palynological data, as the exines of some species, such as Populus and Cupressaceae, are less resistant to corrosion and may be selectively removed from the sedimentary record (Faegri, 1989).

Once the pollen content of a series of samples has been determined, the pollen spectra must be attributed to a source ecosystem. This requires knowledge of the ecological preferences and tolerances of individual taxa, as well as an understanding of how certain taxa within an assemblage interact to form a distinct ecosystem. Over the course of numerous palaeoenvironmental and botanical investigations, the ecological role of many European plants has been investigated, with some plants and communities undergoing quantitative investigation. However, is is important that these generalisations are `ground-truthed’ via observations of modern environments, to ensure plants are inhabiting their assumed roles within an ecosystem. As an example, Quercus ilex can grow into a large tree when conditions are favourable, but often occurs as in the form of a shrub. Under severe grazing pressure, the plant can even occur as a ground-hugging shrub. This illustrates that although the pollen can be attributed to a certain plant, the form and ecological role of that plant may vary (Rackham, 1996). It should also be noted that that the possibility of no-analogue assemblages, relating to no-analogue environments be considered (West, 1964). Such environments, as have been identified in North America during the LGM, are the result of environmental configurations which do not currently exist and may resolve seemingly conflicting results (Jackson, 2004).

I made a dichotomous (i.e. a series of yes/no questions) to help me identify pollen grains. I have made an online version available here. Feel free to explore the key, to see some of the morphological diversity exhibited by dinoflagellates. It may even help you with your identifications. I’m sure the key is not perfect or complete, so I don’t present it with any guarantee of accuracy or completion. If you would like the original for scientific research, let me know.

I have also a useful spreadsheet of pollen grain types, and relevant information. You can also see my Ph.D thesis for a full discussion of how I used fossil pollen grains in climate reconstruction.