Welcome to Plant Talk! In this new column, we’ll be looking at new and interesting pieces of plant science and research, and what they mean. This month, I’ll be telling you about how we can use fossil pollen assemblages in salt marshes to reconstruct past climate change.
The Jigsaw within the Rubik’s Cube: Using Fossilised Pollen Assemblages as an Indicator for Sea Level Change.
Science is full of puzzles. Unfortunately, in some cases many of the pieces that look like they should fit together actually come from different boxes. When dealing with something like the history of sea level change (itself part of the larger oceanographic and climate puzzles) there are contributing pieces from the geological, geographical, biological, and other jigsaw puzzles. When pieces from each of these separate jigsaws are muddled together in a single box, it can be difficult to sort out which pieces belong to which puzzle. Instead of one enormous jigsaw, it’s actually easier to think of these larger conundrums as a giant Rubik’s Cube, with each coloured square forming a piece of a disciplinary jigsaw. When each jigsaw is put together correctly, and connected to the other completed jigsaws, the Cube (and the puzzle) is complete.
One of the little squares within the Rubik’s Cube puzzle of sea level change is palynology – the study of pollen. Pollen is an exceptionally good palaeobotanical resource for several reasons. It gives an extensive record due to abundant production. Its hardened outer sheath of sporopollenin helps to protect it from damage resulting from the fossilisation process. Finally, pollen from different species is often very distinctive, allowing for easy identification. The one disadvantage to using fossilised pollen as an indicator species is dispersal – because pollen is very light and often wind-borne, it can travel large distances and spread far from its point of origin. Thus, any palaeobotanical assemblages indicated from fossilised pollen must be interpreted in the light of possible disassociation from the parent plant.
Palynology has often been used to reconstruct past vegetation patterns, which is itself an indicator for sea level and even climate change. A recent example of this can be seen in the salt marshes of South Carolina. Working with fossilised remnants, Pamela Marsh and Arthur Cohen were able to recreate assemblages that could be used to determine regional models of past sea level rise. Pollen fossilises well in estuarine sediments, and so the environment of Marsh and Cohen’s study site – the coast along South Carolina, which, like much of the south-eastern United States, is mostly characterised by tidal inlets, barrier islands and salt marshes – is ideal for recording palynological records. Unfortunately, this is complicated by the fact that salt marsh plants are often insect-pollinated, thus producing less pollen than the wind-pollinated plants seeding the salt marsh from a distance. Also, some salt marsh plants such as Juncus roemerianus reproduce primarily through rhizomes rather than pollen (while simultaneously producing pollen, flowers, and seeds). This complication can be partially mitigated by analysing palynomorphs – when pollen is extracted from a sample of sediment, other tiny organic remains such as spores, insect parts, fungal and algal remains are also extracted, and these are called palynomorphs. This broader analysis can help to provide context and supporting evidence to the vegetation reconstruction. Together, the various pieces fit together like a jigsaw to build a palynological “fingerprint” – a profile representing various ecological habitats on a local and regional scale.
Different types of vegetation can be found within the salt-marsh environment, as vegetation patterns change according to tidal movements: the lowest zones on the marsh are frequently inundated with salt water, while the highest marsh zone is only immersed in spring and storm tides. When patterns of vegetation within the marsh zone change, it can indicate a change in exposure to salt water. For instance, if the vegetation types that typically inhabit the highest zone in the salt marsh move even higher up the shore, it is an indication that sea level has risen, and the less tolerant salt marsh plants have had to migrate even further landward in order to survive.
Because the highest salt marsh plants are least exposed to the turbulence of the ocean waves, the sediment in which they grow is less disturbed than the sediment of the lower marsh plants. This means that their pollen assemblages are less muddled. If these “unmuddled” fossil assemblages can be identified, they give a means of tracing the geographic migration of the highest marsh zone – which is in this case a proxy for sea level rise. Thus, fossilised pollen from salt marsh plants can indicate the rise and fall of sea level along a coast. This is not necessarily an indicator of climate change (other factors, such as geology, may be in play) but this particular pollen “jigsaw” can mesh with other squares from the climate Rubik’s Cube.
In the South Carolina salt marshes, Marsh and Cohen tested palynomorph assemblages from three typical ecological groups, each characterised by a specific type of grassy or herbaceous vegetation: low-level salt marshes (Spartina alterniflora), high level salt marshes (Juncus roemerianus) and salt pannes (Salicornia virginica). Samples were collected from the top two centimetres of sediment, so as to represent a contemporary assemblage. Marsh and Cohen were unable to distinguish a consistent or distinctive jigsaw from either the low level-salt marsh or the salt panne ecologies. However, the high salt marsh areas with a preponderance of Juncus gave a distinct jigsaw pattern for three reasons. Firstly, there was a high diversity in palynomorphs in the Juncus dominated sediments, a diversity that was consistently almost double that found in the other marsh vegetations types. Secondly, over 10% of the palynomorph count was composed of what Marsh and Cohen referred to as Fungal Spore Type A, an unidentified spore found at all tested high-level sites (in contrast, Fungal Spore Type A was found in less than half of the low-level marsh sites, and at only one of the salt panne sites). Finally, a second fungal species, Atrotorquata lineata, was only found in sediments beneath high-level Juncus marshes.
This distinctive assemblage, found beneath Juncus grass, gives a key to a method for tracing sea-level rise over time. Given that this particular jigsaw effect – the Juncus jigsaw – exists, it can be used to identify other high-level salt marsh sites. Whenever this particular conglomeration is found within a sediment sample, there is a strong likelihood that when the palynomorphs were being deposited, they were deposited in an environment characterised by Juncus grass – in grasses that only occurred in high-level salt marsh sites.
Useful assemblages can vary from region to region. In the tropics, for example, mangrove pollen is a useful indicator of sea level change. Pollen assemblage clues like these help to establish the biological jigsaw that makes up one side of the sea level rise – and possibly the climate change – Rubik’s Cubes. And when those little bits of colour connect on the side of a very large, very complicated puzzle, we are amazed and delighted to find something as tiny and everyday as pollen has been the key.
Engelhart, S.E., Horton, B.P., Roberts, D.H., Bryant, C.L., Corbett, D.R. Mangrove pollen of Indonesia and its suitability as a sea-level indicator. Marine Geology, 242 (1-3) pp. 65-81, 2007.
Marsh, P.E., Cohen, A.D. Identifying high-level salt marshes using a palynomorphic fingerprint with potential implications for tracking sea level change. Review of Palaeobotany and Palynology, 148 (1) pp. 60-69, 2008.