Making smear slides

by Amy Myrbo

A smear slide is a thin layer of unconsolidated sediment embedded on a glass slide for petrographic microscopic examination.  Smear slides are a powerful method for rapidly evaluating tiny quantities of sediment (mineralogy, provenance, texture, alteration) as the basis for sediment classification, and for ascertaining the presence, preservation, and diversity of microfossils.  With experience, smear slides provide surprisingly accurate percentage data useful for recognizing trends in cored sequences.  Tephras and ash falls are easily identified.

Samples for smear slides should be taken at regular intervals (somewhere between every 10 and every 100 cm, depending on sediment heterogeneity) as part of the routine examination and description of cores, and from any anomalous layers.  Smear slides can also be used to examine treated sediment to determine the efficacy of the procedures being used.  Treatments may also be used to isolate particular sediment fractions (e.g. sieving for fine fractions) for examination in smear slides.

Procedure:

1. Clean a standard (1"x3") glass slide by wiping with water or alcohol on a lint-free tissue (e.g. Kim-Wipe).  It is particularly important to remove any glass shards, as these may be mistaken for volcanic ash when viewed under the microscope.

2. Label the slide with information about the sediment sample.  This should include the core ID, sample depth in cm from the top of the core, reason for slide (e.g., coarse layer, tephra?, indurated nodule, routine), color (e.g., white lamination, black fragments), and any special treatment (e.g. "HCl" if pre-treated with acid to remove carbonate, “CF” for coarse fraction).  Include information about why the slide was taken in order to remind the analyst what he or she intended to look for (e.g., why an anomalous layer is colored as it is).

3. Turn on the hot plate to the lowest setting, or slide warmer to medium heat.  Place a bottle of Norland optical cement (the mounting medium) upside-down in a 100-mL beaker on the hot plate.  This sends the bubbles to the bottom of the bottle (away from the spout) and makes the cement less viscous.

4. Place a very small amount of sediment (e.g. toothpick tip) on the slide.  Although it is tempting to use a larger amount of sediment, individual components will be indistinguishable under the microscope if the sample is too crowded.  It’s a good idea to prepare a few slides and look at them under the microscope to adjust the amount of sediment for your own preference for crowded or sparse material.

5. Add a drop or two of deionized water or alcohol to the slide.  (Some analysts prefer to add the water/alcohol before the sediment.)  Choose water if the sediment is high in organics (alcohol will dehydrate organic matter) and alcohol if the sediment contains evaporites or other highly soluble minerals.  Using a toothpick, tiny spatula, or glass rod, disperse and spread sediment on the slide.  Fresh, wet sediment works best.  Dry sediment may need to be moistened and soften before it can be smeared, and it may also be necessary to break up lumps with the danger of damaging any microfossils. Try to make a uniform, thin coating approximately the same size as the cover slip.  Ideally grains will be more or less in a single layer and close together.  Clumpy organics will make a more sparse slide than fine clays and silts, given the same volume of material.

6. Place the slide on the slide warmer or hot-plate and allow it to dry (2-5 minutes).  The slide must be completely dry before adding the optical cement.  The cement is not miscible with water and if the sediment is still wet, the cement will form blobs and crazed patterns instead of a uniform, isotropic sheet.  Do not use high heat, which can alter minerals. 

7. Heat can change the refractive index of Norland optical cement (Rothwell, pers. comm.).  If you make use of this property, allow your dry slides to cool before mounting.

8. Drip 2-3 drops of mounting medium onto the slide (or on the cover glass).  Do not touch the dropper to the slide, to avoid contaminating the mounting medium.  Bubbles are almost unavoidable in the first slide made after removing the cap from the bottle, but should be minimal thereafter.  For this reason it’s best to wait until you have several slides ready for medium and mounting them all in sequence before recapping the bottle. 

9. Carefully place the cover slip on the slide.  Handle the cover slip by the edges to avoid fingerprints.  If you have coarse-grained sediment you may need to use more mounting medium to fill the space between the slide and cover-slip.  You should try to anticipate this in step 7 above, but it is possible to introduce additional mounting medium at this stage if necessary.  This can be done by carefully dropping medium at the edge of the cover slip where capillary action will draw the medium under the cover slip.

10. Place the slide under ultraviolet light to cure (1-2 minutes).  

Mounting media:  a variety of types are available depending upon the refraction index needed.  Water or glycerin may suffice for microfossil checks.  Petrography requires a medium with RI = 1.55-1.56 (splitting quartz and feldspars).  LacCore uses Norland Optical Cement 61, a clear one-part epoxy which cures without heat, within a few minutes under ultraviolet light at wavelengths of 320-380 nm.  This material is available in a number of configurations from www.norlandprod.com, or by the 1 oz. bottle from LacCore.

UV light sources:  LacCore uses cheap hardware-store “black light” bulbs in fluorescent desk lamp fixtures.  These bulbs emit the right wavelengths to cure the Norland optical cement (see above), and are an order of magnitude cheaper than scientific UV light fixtures.  In the most low-tech situation, smear slides mounted with Norland may also be cured on a windowsill or other location with strong sunlight.

As with glass slide shards, tiny fragments of toothpick, clothing threads, and other exotics may get into the mount.  Toothpick is the most puzzling:  fibers appear as bright, elongate, multi-colored strands that can be mistaken for odd-shaped micas or other igneous/metamorphic minerals.  Either learn to recognize and discount these, or use metal tools (pins, spatulas) instead of toothpicks.  Some toothpicks shed more than others, so changing brands may help.

Materials:

spatula

Norland cement

Wash/dropper bottles containing alcohol, acetone, or water

glass slides (standard 1"x3")

Kim-Wipes

cover slips

toothpicks

labels

Equipment:

petrographic microscope

binocular microscope (for slides of coarser material)

hot-plate/slide warmer

labels

ultraviolet light

small sieve for coarse fraction (CF)

Safety:

As inocuous as smear slide preparation may seem, there are still opportunities to hurt yourself – in this case, with broken glass.  Microscope slides and cover slips often come stuck together, and if they break while you’re trying to pry them apart, your fingers can be cut.  Don’t force them; try gently tapping the slides’ edges on a countertop, or using a tool or workgloves to pull gently on an overhanging edge of one of the two slides.  A slide dispenser can help separate slides, but it too can choke trying to overcome the attractive forces between two very flat pieces of glass.  If slides are really stuck together, discard them.

Norland Optical Cement may be irritating to the skin.  If you are sensitive, wear lab gloves or finger cots while making smear slides.  Wash your hands frequently, and especially before eating or drinking.

Note:  This text is based on an LRC/LacCore SOP originally developed by Kerry Kelts and edited over the years by Doug Schnurrenberger and Amy Myrbo.  This version was updated and prepared by Myrbo for TMI in April 2013.

"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.

Lacustrine carbonate minerals

by Mark Shapley

Carbonates are the most common of the chemically precipitated minerals occurring in lacustrine sediment.  In contrast with marine sediments, inorganically precipitated (though often biomediated) carbonates are widespread and sometimes dominant components of lacustrine sediment assemblages.  When seen in smear slides, they are even more prominent than their abundance would suggest, due to their characteristically high birefringence when viewed by transmitted, cross-polarized light. In silt size ranges, no other common mineral component of lacustrine sediment displays the bright interference colors characteristic of the carbonate minerals.  The mineral occurrence, form, size, and assemblages of associated constituents are all influenced by the chemistry and physical state of lake water from which they precipitate (as are other environmentally informative characteristics, such as isotopic composition and elemental substitution, not discernible by optical microscopy).  You will also see carbonate minerals occurring as detrital sediment components, sometimes in abundance.  

Calcite

Calcite (calcium carbonate, CaCO₃) is the predominant chemically precipitated carbonate mineral in most freshwater lakes.  Lacustrine calcite forms as a primary precipitate in the water column of lakes, and also as a diagenetic mineral at or below the sediment-water interface.  As a primary lacustrine precipitate (a process often biomediated through photosynthesis), calcite can take the form of regular rhombohedra following the crystallographic structure of the mineral, but often adopts forms that represent kinetically-controlled departures from the euhedral growth structure.  These commonly include ellipsoids, ‘footballs’, and twinned forms in which two or more grains grow in interpenetrating configurations. Although lacustrine calcite forms over a wide range of sizes, kinetics of crystal growth often result in a strong mode in grain size distribution, giving the calcite component a ‘well-sorted’ character.  A commonly observed upper limit in dimension, around 30 μm, relates to grain settling rates in the water column. 

Endogenically precipitated calcite from Lago Titicaca, Peru, in plane-polarized (top) and cross-polarized light (above). Note gradation in grain form from slightly rounded rhombs to axially symmetric ellipsoids.

One exception to this generalization is provided by charophytic algae.  Through CO₂ uptake, charophytes and other macrophytic plants stimulate localized CaCO₃ precipitation on and around plant structures; calcite crusts and scales formed in this fashion may be considerably larger than water-column precipitates.  Diagenetic calcite may appear as aggregates of fine-grained, poorly-formed grains intermingled with organic matter.  In smear slides you may see detrital calcite (or dolomite) occurring in any grain-size fraction up to sand, often in blocky rhombic forms if well-preserved, perhaps exhibiting pitting and other resorption textures if the lake environment is undersaturated with respect to the mineral phase.

Large calcite flakes, probable charophyte coatings from a shallow-water setting, Laguna Miscanti, Chile.  Macrophyte coatings may be crystallographic composites, with optical extinction occurring over local regions at different angles of stage rotation.

Anhedral, subangular detrital calcite of Mesozoic age, very fine sand size class, in plane-polarized (top) and cross-polarized (above) light.  Note the high-order pastels and lower-order interference colors along the thinner edges of the grain in the cross-polarized view.  Tahltan Lake, Canada.

The common feature of these diverse forms is the high birefringence imparted by the underlying crystal structure of the mineral.  The birefringence you observe for any given mineral in a smear slide scales by grain size, but calcite and other carbonate minerals viewed in cross-polarized light will always exhibit high-order interference colors relative to all other common constituents.  In small grains, this means carbonates will appear prominently bright with vibrant first-order interference colors.  In larger, thicker grains (>20 μm in dimension), you’ll see high optical birefringence expressed as higher-order interference colors with less vibrant, pastel appearance.  For very large grains this high-order interference can be initially difficult to recognize, however such grains will usually appear rimmed with brighter, lower-order colors where the grain thickness tapers.  

Corroded calcite with resorption accentuated along cleavage planes; plane-polarized (top) and cross-polarized (above) light.  Tahltan Lake, British Columbia, Canada.  

In plane-polarized light, lacustrine calcite may appear colorless, but it’s also common to see faint green or yellow-green hues.  Color and the details of other optical properties are influenced by the substitution of minor amounts of other doubly-charged cations (especially Mg²⁺, Fe²⁺, and Mn²⁺) for Ca²⁺ in the mineral structure.  These compositional details are best evaluated through x-ray diffraction.

Aragonite

Lacustrine calcium carbonate occurs as two distinct minerals (or polymorphs) of CaCO₃ having very different crystal structure: calcite and aragonite.  Calcite is the less soluble (more stable) mineral phase, but its formation is inhibited by elevated ion activities of Mg²⁺, SO₄^2-, and possibly other ions.  When calcite precipitation is inhibited in this manner, aragonite will form as a primary lacustrine mineral phase.   While the mechanism is complex and thresholds inexactly defined, generally lacustrine aragonite formation is recognized as an indication of elevated Mg²⁺:Ca²⁺ in lake waters, typically as a result of CaCO₃ formation and resulting solute evolution (Bischoff and Fyfe 1968; Eugster and Lawrence 1978; Kelts and Hsu 1978).  Also, partitioning of carbon and oxygen isotopes between carbonate mineral, solute, and water depends on which of the two calcium carbonate polymorphs precipitates.  Distinguishing CaCO₃ mineralogy is therefore an important step toward interpreting the depositional environment of lacustrine sediment, or inferring, for example, the isotopic composition of water.  You will need x-ray diffraction (xrd) to identify mineral phases with certainty, but smear slide analysis can provide you with valuable initial identification to be confirmed by xrd.

Aragonite shares the high birefringence and strong interference colors (relative to grain size) of calcite and other carbonate minerals, but when formed as an inorganic precipitate in natural waters, aragonite most often forms needles or elongated ellipsoids  <15 μm in length and often compared to rice grains in form.  While this describes a typical lacustrine aragonite occurrence, you may see overlap in grain form between some aragonite and ellipsoidal calcite.  Due to sediment mixing and temperature effects, you may also see aragonite and calcite co-occurring in a single smear slide, even though the favored polymorph is largely controlled by lake-water chemistry.

Aragonite grains in plane-polarized (top) and cross-polarized light (above) from the Black Sea.  Endogenic grain forms include acicular needles, rice-grain forms such as those shown here, and narrow ellipsoids.

Dolomite

Dolomite shares the extreme birefringence of other carbonate minerals, often shows rhombic habit in smear slides, and may be distinguished with difficulty from calcite of similar habit by higher relief and more equant habit.  Unambiguous distinction from other carbonate minerals is again an xrd task.  Often thought of as a secondary mineral replacing other carbonates, dolomite is known as a primary lacustrine precipitate in certain hypersaline, magnesian environments with favorable microbial ecology (Baker and Kastner 1981; Krause et al 2012), where you might expect to see it associated with other, less common Mg-carbonates such as magnesite and hydromagnesite. In the absence of hypersaline conditions, dolomite seen in smear slides is probably a detrital sediment component.

Siderite and rhodocrosite

Siderite (iron carbonate, FeCO₃) and rhodocrosite (manganese carbonate, MnCO₃) are minerals with widespread but generally less-abundant occurrence as lacustrine sediment components. Lacustrine siderite may be strongly colored in plane-polarized light, ranging from pale green to orange-brown.  Grain forms include square-ended laths, ellipsoids, and ‘footballs’ or lemon-shaped euhedra. Interpenetrating twins are common, and if you are seeing grains in the <15 μm range with a ‘bow-tie’ or ‘dog-bone’ appearance, there is a good chance you have siderite present.  Photographed at high resolution in a split core face, lamellae or nodules of siderite will sometimes appear a distinctive bronze color in a linescan image.  Rhodocrosite is greenish-brown in plane polarized light, may be strongly pleiochroic, and can display grain forms similar to siderite, including interpenetrating twins; distinguishing the two minerals reliably is a task for xrd analysis.

Siderite in plane-polarized (top) and cross-polarized light.  Grain forms include simple laths and twinned laths giving a 'bow-tie' form, accentuated under cross-polarized light.

Formation of siderite and Mn-carbonates requires the moderately negative redox potentials that occur in the near-absence of dissolved oxygen, and at commonly-encountered carbonate ion activities, also implies near-neutral pH.  These conditions normally prevail only in hypolimnetic regions of the water column or in pore-water environments, where dissolved oxygen has been consumed by microbial respiration and pH lowered by dissolution of respired CO₂. 

Simplified equilibrium phase diagram for a dilute aqueous system with
dissolved carbon dioxide, sulfur and phosphorous. The size and position of
mineral phase stability fields depends on the assumed solute concentrations,
but  crossing a lake oxycline from epilimetic (epi) to hypolimnetic (hypo)
pH/Eh conditions is commonly required for formation of siderite and
Mn-carbonates. Modified from Lemos et al 2012 and Hem 1977.  

Water-column stratification in lakes fosters spatial (position in the water column) and temporal (mixing state) gradients in the stability of carbonate minerals.  This means that the occurrence and state of preservation of different carbonate mineral phases can provide valuable clues about changes in lake mixing and other processes over time.  Calcite or aragonite precipitated in a lake’s epilimnion (mixed layer) may, upon sinking into the hypolimnion, encounter more acidic conditions resulting from the respiration of organic matter in poorly mixed conditions below the lake chemocline.  In a smear slide you may see partially dissolved (pitted) carbonate grains reflecting this water-column gradient, or you may see intervals lacking any carbonate minerals at all, perhaps reflecting complete hypolimnetic dissolution at times in lake history.   So the textural observations made may reflect the interplay of two very important components of lake carbon cycling, 1) transfer of fixed organic carbon into hypolimnetic water (by sinking and respiration of dead algal matter), and 2) transfer of inorganic carbon from mixed-layer water in exchange with the atmosphere into hypolimnetic environments isolated from the atmosphere, each involving subsequent partitioning between sediment and hypolimnetic water.  Carbon mass balance and isotopic composition depend on these interactions (Bade et al 2004; Myrbo and Shapley 2006).

Overgrowths and secondary precipitation

Through the processes of settling through the water column, changes in water column stratification, and burial below the sediment-water interface, carbonate minerals may pass from saturated to unsaturated conditions and back again over the course of sediment accumulation.  This can at times result in multiphase overgrowths, an indicator of oscillatory saturation state that may support useful environmental interpretations.  Look for secondary rims on carbonate grains, which may or may not be in crystallographic unity with the host grain (indicated by coinciding or disjunct optical extinction of the host grain and its overgrowths as you rotate the microscope stage).  In some interesting cases, a host carbonate grain may be overgrown by a different carbonate mineral reflecting equilibration in distinct parts of the lake water column (e.g., Stevens et al 2000).

References cited

Bade, D. L., Carpenter, S. R., Cole, J. J., Hanson, P. C., and Hesslein, R. H., 2004, Controls of d¹³C DIC in lakes: Geochemistry, lake metabolism and morphometry: Limnology and Oceanography, v. 49, no. 4, p. 1160-1172.
Baker, P. A., and Kastner, M., 1981, Constraints on the formation of sedimentary dolomite: Science, v. 213, p. 214-216.
Bischoff, J. L., and Fyfe, W. S., 1968, Catalysis, inhibition, and the calcite-aragonite problem: American Journal of Science, v. 266, p. 65-79.
Eugster, H. P., and Hardie, L. A., 1978, Chapter 8: Saline Lakes, in Lerman, A., ed., Physics and Chemistry of Lakes: New York, Springer-Verlag, p. 237-293.
Hem, J. D., 1977, Reactions of metal ions at surfaces of hydrous iron oxide: Geochimica et Cosmochimica Acta, v. 41, p. 527-538.
Kelts, K., and Hsu, K. J., 1978, Freshwater carbonate sedimentation, in Lerman, A., ed., Lakes: Chemistry, Geology, Physics: New York, Springer-Verlag.
Krause, S., Liebetrau, V., Gorb, S., Sanchez-Roman, M., McKenzie, J. A., and Treude, T., 2012, Microbial nucleation of Mg-rich dolomite in exopolymeric substances under anoxic modern seawater salinity: New insight into an old enigma: Geology, v. 40, no. 7, p. 587-590.
Lemos, V. P., Lima da Costa, M., Lemos, R. L., and Gomes de Faria, M. S., 2007, Vivianite and siderite in lateritic iron crust: and example of bioreduction: Quimica Nova, v. 30, no. 1, p. 36-40.
Myrbo, A., and Shapley, M. D., 2006, Seasonal water-column dynamics of dissolved inorganic carbon stable isotopic compositions (d¹³CDIC) in small hardwater lakes in Minnesota and Montana: Geochimica et Cosmochimica Acta, v. 70, p. 2699-2714.
Stevens, L. R., Ito, E., and Olson, D. E. L., 2000, Relationship of Mn-carbonates in varved lake sediments to catchment vegetation in Big Watab Lake, MN, USA: Journal of Paleolimnology, v. 24, p. 199-211.