During the spring a lot of people suffer from a pollen allergy. As a result, many don’t look forward to this lovely season. Some of us, however, enjoy pollen season all year round — we are palynologists studying, in my case, ancient pollen, spores, and marine plankton. Our pollen samples are locked away in rock as microfossils — the perfect type of pollen for people with allergies: no sneezing, coughing, or watering eyes!
Pollen and spores have an amazingly resistant wall construction, with the result that they are often well preserved in the geological record. The particles are very small, typically becoming part of the silt-sized sediment fraction (<20 μm). We use them to date sedimentary rocks or as proxies for palaeoenvironment or palaeoclimate, since we know that plants (and plankton) are sensitive to climate.
The word pollen is derived from Latin and means flour or dust, an apt name. Pollen and spores are exchanged during the plant reproduction cycle, and plants rely on wind or other transport mechanisms for dispersal. They are distributed in enormous numbers. For people suffering from allergies, pollen distributed by wind is very troublesome but for geologists it is extremely useful. Its tendency to be carried long distances means that we find pollen far away from the areas where it was produced. For example, pollen from hinterland species can be found in offshore ocean basins.
Palynologists use a standard procedure to remove all the rock surrounding these microfossils by applying acids in a series of treatments to concentrate a sample of organic residue. When placed on a microscope slide, the pollen and spores in this residue can then be analysed by counting and recording the numbers of each pollen and spore species present. The points in geological time where we see the first occurrence and subsequent extinction of some marker species can help us identify the rock sample’s age in millions of years. This information can be extremely useful for producing correlations in time between rock outcrops and buried sequences of rock that can only be accessed by drilling expensive wells.
During the Late Triassic in the Barents Sea, an enormous delta built out filling in the vast basin that existed in this area. There are no tall plants growing in the Arctic today but, due to continental drift, this area was then situated further south in a warmer climatic zone, around 45 degrees north, and rich vegetation covered the surrounding land. As the basin filled, pollen and spores were deposited in large numbers along with the sedimentary particles that form the bulk of the rock.
A typical feature observed in outcrop and well samples from the Early Carnian, some 200 to 235 million years ago, is a monolete (elongate scar) spore called Leschikisporis which is recorded in particularly high abundance. The spores are produced by ferns and they are found together with another spore produced by a tree-like plant, lycopsid, that typically grew along rivers or in more humid areas. Due to their relatively large and dense form, spores like these were not transported very far, thereby providing information on the type of plants that grew nearby when the sediments were being deposited. As a result we know these trees and ferns must have dominated a vast delta plain that extended relatively far out into the basin.
However, above this interval, later in the Carnian, we see a change. There is more bisaccate pollen (pollen with two sacci). These pollen are similar to those produced from present-day conifers; the two sacci enables them to be transported long distances by wind and they tend to become more common relative to spores the further you are from the shoreline. This change implies that relative sea level rose and the delta plain flooded, pushing the shoreline landward. The appearance of marine plankton in the same samples supports this. So despite the fact that the sediments were deposited in a marine setting, the pollen and spores that they contain can tell us a story about the palaeovegetation on the surrounding shores — a story written millions of years ago, and read only now by palynologists.