"Do not date" list: radiocarbon dating of macroscopic plant remains

by Jimmy Marty

Establishing accurate sediment core chronologies is of great importance for the interpretation of paleolimnological records. Radiocarbon dating is the primary method used to create chronologies in studies focusing on the past 40-50,000 years. Described here are various macroscopic plant materials, categorized according to their suitability for radiocarbon dating. Materials were evaluated based on the characteristics of good dating material: a) all fixed CO₂ is in equilibrium with atmospheric CO₂ and b) the material accurately represents the time of sediment deposition.

Sources of error

A common age-altering error encountered in paleolimnology is the old-carbon error, which occurs when dating material has fixed carbon out of equilibrium with the atmosphere. Old carbon reservoirs are commonly associated with hard-water lakes with high concentrations of HCO₃^-, but sediment and aquatic CO₂ are often old-carbon enriched due to isotopic exchange between carbon species (Mook 1980), infrequent mixing (in lakes with high surface area-depth ratios) (Hakkanson 1979; Olsson 2009), and respired CO₂ derived from HCO₃^- fixing organisms (MacDonald et al 1987). This list does not include processes in arctic and volcanic regions, which may be old-CO₂ enriched for numerous other reasons (see Bjorck and Wohlfarth 2002).

           

Physical processes such as reworking of old sediment by means of erosion or bioturbation can redistribute plant remains up or down the sediment profile (Turney et al 2000; Bjorck and Wohlfarth 2002). Sediment disturbance is often clear through visual examination of the sediment core face, in which slumps and/or turbation may be evident. Further examination of microscopic composition using smear slides can reveal less obvious terrestrial material indicative of erosional input. Describing these macroscopic and microscopic features of the sediment is of great value to the selection of suitable emergent and floating leaved macrofossils and can help avoid erroneous dates.

Appropriate sample storage techniques should always be performed in order to avoid contamination errors. Sediment cores and macrofossil samples stored in cool and wet conditions for several months or more are susceptible to modern fungal and microbial contamination (Wohlfarth et al 1998). After isolating the macrofossil sample from the sediment, it should be stored in distilled water in a cold room for no longer than one week before submission for dating (Bjorck and Wohlfarth 2002).

Dateable: (click on heading for more comprehensive list of taxa in this category)

Terrestrial plant macrofossils

Terrestrial plants fix only atmospheric CO₂ and are considered ideal for radiocarbon dating. Care should still be taken to ensure the material has not been reworked through sediments.
 

 

Pine needle (terrestrial) of Abies balsamea. Dateable.

Seeds of terrestrial tree Betula alleghaniensis. Dateable.

Seeds of terrestrial herb Rudbeckia triloba. Dateable.

Leaf of terrestrial bog shrub Chamaedaphne calyculata. Dateable.

Bud scales of terrestrial tree Populus tremuloides. Dateable.

Charcoal (from grassland regions)

Grassland vegetation fixes only atmospheric CO₂. Grassland vegetation has a short terrestrial residence time; it is without the in-built age present in long-lived, slow-decomposing tree species that may remain on the landscape for significant lengths of time (Oswald et al 2005; Grimm 2011).

Date with caution:

Emergent-aquatic plant macrofossils

Emergent macrophytes have a strong history of providing good dates (Deevey et al 1954; Heikkinen et al 1977; Hakansson 1982; Tornqvist et al 1992; Lowe et al 2004; Grimm 2011). Most are unable to fix old carbon in the form of HCO₃^- (Spence and Maberly 1985; Sand-Jensen et al 1992; Maberly and Madsen 2002), and are only able to utilize a very small portion of CO₂ absorbed from the sediment and water. Yet there are some cases of emergent vegetation returning dates that are too old (Damon et al 1964; Haynes et al 1966; Turney et al 2000; Wasylikowa and Walanus 2004; Edwards et al 2011). Furthermore, Olsson (1983) demonstrated that emergent and floating leaved vegetation have lower ¹⁴C activities than terrestrial vegetation, though not as low as submerged aquatics.

The physiological literature shows that in some situations it is possible for old-carbon to be incorporated into emergent vegetation if there is an old-CO₂ reservoir present. Amphibious isoetids such as Lobelia dortmanna and Juncus bulbosus (the seeds of which can be easily confused with other Juncus species) acquire the majority of their CO₂ from the sediment via the roots (Wium-Anderson 1971; Winkel and Borum 2009). Recent evidence shows that root uptake of CO₂ may not only be limited to isoetids. In a hard water lake, Sparganium angustifolium utilized sediment CO₂ in amounts that significantly affected productivity (Lucassen et al 2009). Stratoides aloides is able to utilize HCO₃^- in significant amounts, the only known amphibious plant that can do so (Prins and De Guia 1986). Additionally, new studies continue to suggest that emergent plants may be able to fix aquatic CO₂ when submerged, albeit in unknown quantities. Gas film layers on leaves of emergents like Typha, Phalaris, and Phragmites allow the utilization of aquatic CO₂ (Colmer and Pedersen 2008). Morphological plasticity of leaves and roots permit other amphibious macrophytes to adjust CO₂ fixation depending on the environment (Mommer et al 2007; Rich et al 2012).

So, are the macrofossils of emergent plants dateable? While there is a short list of clearly unsuitable taxa that includes isoetids and Stratoides aloides, it appears that most other emergent macrophytes only utilize old carbon under very particular circumstances and/or in very small or unknown quantities. When determining if an emergent macrofossil is suitable for dating, it is extremely important to carefully consider the study environment and the ecophysiological characteristics of the plant material. For example, if there is old CO₂ present in a lake, the suitability of Sparganium fruits for dating is most likely dependent on whether the lake of study is hard or soft water.

Seeds of emergent aquatic macrophyte Lobelia dortmanna (water lobelia). This species absorbs the majority of  photosynthetic CO2 from the sediment (Wium-Anderson 1971). Do not date.

Seeds of emergent/submerged aquatic macrophyte Juncus pelocarpus  (brown-fruited rush).  This species of rush has both emergent and submerged forms, and a relative, Juncus bulbosus, fixes mostly sediment CO2 (Wetzel et al 1985). Date with caution.

Seeds of emergent aquatic macrophyte Schoenoplectus acutus (hard stem bulrush). A European member of this genus (Schoenoplectus tabernaemontani) internalizes large amounts of sediment CO2, but only fixes very small quantities (Singer et al 1994). Date with caution.

Fruits of emergent aquatic macrophyte Sparganium angustifolium (narrow leaf bur-reed). This species can utilize significant amounts of sediment CO₂ in hard water conditions. Date with caution.

Spikelets of emergent aquatic macrophyte Zizania palustris (northern wild rice). Species of Asian rice, of the same tribe as Z. palustris, are adept at the uptake of sediment CO₂ (Higuchi et al 1984). Date with caution.

Seeds of amphibious macrophyte Eleocharis acicularis (needle spikerush). E. acicularis can grow under submerged or emergent conditions. A similar amphibious spikerush, Eleocharis vivipara uses aquatic CO₂ when submerged (Ueno et al 1988). Date with caution.

Seeds of emergent aquatic herb Polygonum amphibium (water smartweed). P. amphibium is able to produce both floating and aerial leaves based on water level conditions. Related species of the genus Rumex can produce submerged leaves that utilize aquatic CO₂ (Mommer et al 2007). Date with caution.

 

Seeds of emergent wetland plant Carex comosa (bristly sedge). There is flow of CO₂ from root to shoot in some species of Carex, although it may not be fixed and appears to be respiratory (Koncalova et al 1988). Date with caution.

 Floating-leaved-aquatic plant macrofossils

Floating-leaved macrophytes comprise a much smaller number of taxa than emergent macrophytes, with the majority of research primarily involving nymphaeids (e.g. Nymphaea, Nuphar). Much like emergent macrofossils, macrofossils of floating leaved macrophytes have provided good dates in the past (Heikkinen et al 1977; Tornqvist et al 1992) and are unable to fix old carbon in the form of HCO₃^- (Spence and Maberly 1985; Sand-Jensen et al 1992; Maberly and Madsen 2002). This is not the only similarity nymphaeids share with emergent macrophytes; the suitability of nymphaeids for dating has also been questioned (Olsson 1983; Tornqvist et al 1992; Olsson 2009). Physiological evidence suggests that Nuphar may take up CO₂ from the sediment, which is potentially problematic if the CO₂ is derived from an old-CO₂ reservoir. In Nuphar lutea, relatively large concentrations of CO₂ move from the rhizome to the upper parts of the plant where it is then fixed (Dacey and Klug 1982). The source of the CO₂ is unknown and could be from either the sediment or a product of respiration in the rhizome, the latter of which is a product of atmospheric O₂ (Dacey and Klug 1982). Because the partial pressure of CO₂ exceeds that of the partial pressure in the rhizome, it is suggested that the CO₂ is acquired from the sediment (Dacey 1979). However, the amount of CO₂ fixed through this pathway is unknown, and may be insignificant in the context of radiocarbon dating. Like with emergent macrofossils, one must evaluate the study environment carefully to gauge the suitability of nymphaeid remains for dating. If there is an old CO₂ reservoir available, proceed dating nymphaeids with caution.

Seeds of floating-leaved aquatic macrophyte Nuphar lutea (yellow water lily, spatterdock). Date with caution.

 

Seeds of floating-leaved macrophyte Nymphaea odorata (white water lily). Date with caution.

Seeds of floating-leaved macrophyte Brasenia schreberi (watershield). Date with caution.

 

 

 

Wood charcoal

Woody plants fix only atmospheric CO₂. However, in some cases wood charcoal may provide inaccurate dates due to long terrestrial residence times (Oswald et al 2005; Grimm 2011). Woody species may take many years to decay and therefore can remain on the landscape for long after atmospheric CO₂ was originally fixed. Interpret dates from wood charcoal with care.

Wood fragments

Like wood charcoal, wood may experience long terrestrial residence times and provide inaccurate dates despite use of atmospheric CO₂ (Oswald et al 2005; Grimm 2011). Furthermore, old carbon enriched material may adhere to the rough surface texture of wood if the sample is not processes correctly (Oswald et al 2005). For the best dates of woody material, look for identifiable twigs of lakeshore species to ensure near-site deposition.

Do not date:

Submerged-aquatic plant macrofossils

It is well documented that macrofossils from submerged macrophytes and aquatic mosses return radiocarbon dates that are spuriously old (Deevey et al 1954; Hakkanson 1979; Olsson 1983; Birks 2002). Most submerged aquatic macrophytes are able to take up aquatic HCO₃^- derived from carbonate bedrock that results in radiocarbon dates many thousands of years too old (Spence and Maberly 1985; Sand-Jensen et al 1992; Maberly and Madsen 2002). Aquatic mosses and some submerged macrophytes do not utilize HCO₃^-; rather they fix CO₂ from the sediment (e.g. isoetids) and/or water (e.g. Vallisneria americana), which may still carry an old CO₂ reservoir effect (Sand-Jensen et al 1992; Maberly and Madsen 2002). In very well mixed, soft water lakes, it may be possible to date both submerged macrophytes and aquatic mosses (Hakansson 1979; Miller et al 1999; Oswald et al 2005). However, complete absence of a reservoir effect is rare, and dating macrofossils of submerged macrophytes and aquatic mosses is generally unadvisable (Olsson 2009).

 

Seeds of submerged aquatic macrophyte Potamogeton epihydrus (ribbonleaf pondweed). Do not date.

Seeds of submerged aquatic macrophyte Vallisneria americana (water celery). Do not date.

 

Seeds of submerged aquatic macrophyte Najas flexilis (nodding waternymph). Do not date.

Seeds of submerged aquatic macrophyte Callitriche palustris (water starwort). Do not date.

 

Oogonia of submerged aquatic macroalgae Chara sp. Do not date.

 

 

Seeds of submerged aquatic macrophyte Ranunculus aquatilus (white water crowfoot). Do not date.

Aquatic moss fragments (unknown species). Do not date.

References cited

Birks, H.H., 2002, Plant macrofossils. In: Smol, J.P., Birks, H.J.B., Last, W.M. (Eds.), Tracking Environmental Change Using Lake Sediments, vol. 3: Terrestrial, Algal, and Siliceous Indicators. Kluwer Academic Publishers, Dordrecht, The Netherlands, p. 49-74.

Björck, S., and Wohlfarth, B., 2002, ¹⁴C chronostratigraphic techniques in paleolimnology. In: Smol, J.P., Birks, H.J.B., Last, W.M. (Eds.), Tracking Environmental Change Using Lake Sediments, vol. 1: Basin Analysis, Coring, and Chronological Techniques. Kluwer Academic Publishers, Dordrecht, The Netherlands, p. 205-245.

Colmer, T.D., and Pedersen, O., 2008, Underwater photosynthesis and respiration in leaves of submerged wetland plants: gas films improve CO₂ and O₂ exchange: New Phytologist, v. 177, p. 918-926.

Dacey, J.W.H., and Klug, M. J., 1982, Tracer studies of gas circulation in Nuphar: ¹⁸O₂ and ¹⁴CO₂ transport: Plant Physiology, v. 56, p. 361-366.

Deevey, E.S., Gross, M.S., Hutchinson, G.E., and Kraybill, H.L., 1954, The natural C¹⁴ contents of materials from hard-water lakes: Geology, v. 40, p. 285-288.

Edwards, K.J., Schofield, J.E., Kirby, J.R., and Cook, G.T., 2011, Problematic but promising ponds? Palaeoenvironmental evidence from the Norse Eastern Settlement of Greenland: Journal of Quaternary Science, v. 26, no. 8, p. 854-865.

Grimm, E.C., 2011, High-resolution age model based on AMS radiocarbon ages for Kettle Lake, North Dakota, USA: Radiocarbon, v. 53, no. 1 , p. 39-53.

Hakansson S., 1979, Radiocarbon activity in submerged plants from various south Swedish lakes. In: Berger, R., and Suess, H.E. (Eds.), Radiocarbon Dating: Proceedings of the Ninth International Conference. University of California Press, p. 433-443.

Hakansson S., 1982, University of Lund radiocarbon dates XV: Radiocarbon, v. 24, no. 2, p. 194-213.

Heikkinen, A., and Aikaa, O., 1977, Geological survey of Finland radiocarbon measurements VII: Radiocarbon, v. 19, no. 2, p. 263-279.

Higuchi, T., Yoda, K., and Tensho, K., 1984, Further evidence for gaseous CO₂ transport in relation to root uptake of CO₂ in rice plant: Soil Science and Plant Nutrition v. 30, no. 2, p. 125-136.

Koncalova, H., Pokorny, J., Kvet, J., 1988, Root ventilation in Carex gracilis curt.: diffusion or mass flow?: Aquatic Botany, v. 30, p. 149-155.

Lowe, J.J., Walker, M.J.C., Scott, E.M., Harkness, D.D., Bryant, C.L., and Davies, S.M., 2004, A coherent high-precision radiocarbon chronology for the Late-glacial sequence at Sluggan Bog, Co. Antrim, Northern Ireland: Journal of Quaternary Science, v. 19, no. 2, p. 147-158.

Lucassen, E.C.H.E.T., Spierenburg, P., Fraaije, R.G.A., Smolders, A.J.P., and Roelofs, J.G.M., 2009, Alkalinity generation and sediment CO₂ uptake influence establishment of Sparganium angustifolium in softwater lakes: Freshwater Biology, v. 54, p. 2300-2314.

Maberly, S.C., and Madsen, T.V., 2002, Freshwater angiosperm carbon concentrating mechanisms: processes and patterns: Functional Plant Biology, v. 29, p. 393-405.

MacDonald, G.M., Beukens, R.P., Kieser, W.E., and Vitt, D.H., 1987, Comparative radiocarbon dating of terrestrial plant macrofossils and aquatic moss from the “ice-free corridor” of western Canada: Geology, v. 15, p. 837-840.

Miller, G.H., Mode, W.N., Wolfe, A.P., Sauer, P.E., Bennike, O., Forman, S.L., Short, S.K., and Stafford, T.K., 1999, Stratified interglacial lacustrine sediments from Baffin Island, Arctic Canada: chronology and paleoenvironmental implications: Quaternary Science Reviews v. 18, p. 789-810.

Mommer, L., Wolters-Arts, M., Andersen, C., Visser, E.J.W., and Pedersen, O., 2007, Submergence-induced leaf acclimation in terrestrial species varying in flooding tolerance: New Phytologist, v. 176, p. 337-345.

Mook, W.G., 1980, Carbon-14 in hydrological cylces, in Fritz, P., and Fontes, J. Ch., eds., Handbook of environmental isotope geochemistry: Amsterdam, Elsevier, p. 49-74.

Olsson, I.U., 1983, Dating non-terrestrial materials. In: Mook, W.G., and Waterbolk, H.T. (Eds.), Proceedings of the International Symposium ¹⁴C and Archaeology. PACT v. 8, p. 277-294. 

Olsson, I.U., 2009, Radiocarbon dating history: early days, questions, and problems met: Radiocarbon, v. 51, no.1, p. 1-43.

Oswald, W.W., Anderson, P.M., Brown, T.A., Brubaker, L.B., Hu, F.S., Lozhkin, A.V., Tinner, W., and Kaltenrieder, P., 2005, Effects of sample mass and macrofossil type on radiocarbon dating of arctic and boreal lake sediments: The Holocene, v. 15, no. 5, p. 758-767.

Prins, H.B.A., and De Guia, M.B., 1986, Carbon source of the water soldier, Stratiotes aloides L.: Aquatic Botany, v. 26, p. 225-234.

Sand-Jensen, K., Pederson, M.F., and Nielsen, S.L., 1992, Photosynthetic use of inorganic carbon among primary and secondary water plants in streams: Freshwater Biology, v. 27, p. 283-293.

Singer, A., Eshel, A., Agami, M., and Beer, S., 1994, The contribution of aerenchymal CO₂ to the photosynthesis of emergent and submerged culms of Scirpus lacustris and Cyperus papyrus: Aquatic Botany, v. 49, p. 107-116.

Spence, D.H.N., and Maberly, S.C., 1985, Occurrence and ecological importance of HCO₃^- use among aquatic higher plants. In: Lucas, W.J., and Berry, J.A. (Eds.), Inorganic carbon uptake by aquatic photosynthetic organisms, Proceedings of and International Workshop on Bicarbonate Use in Photosynthesis, p. 125-143.

Turney, C.S.M., Coope, G.R., Harkness, D.D., Lowe, J.J., and Walker, M.J.C., 2000, Implications for the dating of Wisconsian (Weichselian) late-glacial events of systematic radiocarbon age differences between terrestrial plant macrofossils from a site in SW Ireland: Quaternary Research, v. 53, p. 114-121.

Ueno, O., Samejima, M., Muto, S., and Miyachi, S., 1988, Photosynthetic characteristics of an amphibious plant, Eleocharis vivipara: expression of C₄ and C₃ modes in contrasting environments: PNAS, v. 85, p. 6733-6737.

Wasylikowa, K., and Walanus, A., 2004, Timing of aquatic and marsh-plant successions in different parts of Lake Zeribar, Iran, during the Late Glacial and Holocene: Acta Palaeobotanica, v. 44, no. 2, p. 129-140.

Winkel, A., and Borum, J., 2009, Use of sediment CO₂ by submersed rooted plants: Annals of Botany, v. 103, p. 1015-1023.

Wium-Andersen, S., 1971, Photosynthetic uptake of free CO₂ by the roots of Lobelia dortmanna: Plant Physiology, v. 25, p. 245-248.

Wohlfarth, B., Possnert, G., Skog, G., and Holmquist, B., 1998, Pitfalls in the AMS radiocarbon-dating of terrestrial macrofossils: Journal of Quaternary Science, v. 13, p. 137-145.