engleski

Where to look for palynomorphs?

Palynomorphs are broadly defined as organic-walled microfossils between 5 and 500 micrometers in size. Palynomorphs may be composed of organic material such as chitin, pseudochitin, sporopollenin and dinosporin. Typically, palynomorphs are dinoflagellate cysts, acritarchs, spores, pollen, fungi, scolecodonts (scleroprotein teeth, jaws and associated features of polychaete annelid worms), arthropod organs (such as insect-mouth parts), chitinozoans and microforams. Palynomorph microscopic structures that are abundant in most sediments are resistant to routine pollen extraction including strong acids and bases, and acetolysis, or density separation. Palynomorphs are important in determining the type of prehistoric life that existed at the time the sedimentary formation was laid down. As a result, these microfossils give important clues to the prevailing climatic conditions of the time. Their paleontological utility derives from an abundance numbering in millions of cells per gram in organic marine deposits, even when such deposits are generally not fossiliferous. Palynomorphs, however, generally have been destroyed in metamorphic or recrystallized rocks. Beginning with Precambrian acritarchs up to 1.4 billion years old, palynomorphs occur in sedimentary rocks of all ages and from many different sedimentary and biological environments. They originate both on land (spores/pollen = sporomorphs) and in fresh water (Botryococcus and other algae, some dinoflagellates and a few sporomorphs) and salt water (most dinoflagellates, acritarchs, and other algal microscopic remains, the extinct chitinozoans, scolecodonts, foraminiferal test linings, a few sporomorphs) (Traverse, 2007). The whole subject of the various processes that affect palynomorphs from their shedding by the producing organism to their incorporation in sediment and on through lithification of the enclosing sediment, and even its later thermal history in the rock so formed, is all part of taphonomy (Traverse, 2007). The propensity of spores/pollen to be preserved in sediments depends largely on the amount of sporopollenin in the exine, which is partly a function of the thickness of the exine, and of the division of the exine between endexine and ektexine (the more ektexine, the more sporopollenin), and partly due to other factors. Hopkins and McCarthy (2002) have shown by experimental oxidation of dinoflagellate cysts that peridinioid (proximate) cysts were destroyed more readily than chorate forms (those with processes), which would thereby be enriched in sediments subject to oxidation. They also observed that pollen was more resistant to experimental oxidation than dinocysts and that bisaccate conifer pollen was the most resistant of all forms studied. This sort of “taphonomic effect” can obviously affect relative abundances and can even cause absence of some palynomorphs that were originally present. Sedimentary situation into which palynomorphs are deposited also has very great control over the likelihood of preservation: acid environments preserve better than alkaline, reducing environments better than oxidizing and quiet sedimentary situations better than very energetic ones. The exclusion of oxygen is especially important. However, even some seemingly unlikely sites of deposition such as soils can preserve pollen, though preservation in soils is erratic and usually poor, and thus the records are probably biased toward resistant types. Furthermore, in soils, downwashing and redistribution by vectors such as earthworms must be allowed for. Spores/pollen spectra of soils often permit drawing of conclusions about vegetational history during soil genesis (Dimbleby, 1961). However, Cushing (1966) has shown that preservability of spores/pollen in sediment is the result of complex factors, not just sporopollenin content alone. Furthermore, the destruction of exines follows various pathways in various situations: degraded exines in which the structure is generally altered are common in silty sediments, whereas superficial corrosion (pitting, etc.) is more likely to be encountered in peat, and crumpling is characteristic of copropels (caused by processing in animal guts). Spores/pollen exines (and other palynomorphs such as dinoflagellate cysts and fungal spores) when occurring in water are technically, from their size, silt or very fine sand particles, which behave in water according to the same principles as govern other clasts(Traverse, 2007). The low specific gravity (about 1.4) of palynomorphs, the fact that they contain internal space, their tendency after initial stages of preservation to be tiny disks rather than spheres, and other factors give them somewhat different settling characteristics from mineral particles (specific gravity equal to or greater than 2.4) of the same maximum size. Stanley (1965) observed that palynomorphs usually sort out in a mineral silt fraction one class smaller than would be expected from the palynomorph size, which is another way of saying that they do not settle as fast as mineral particles of the same maximum dimension. The relative proportion of various classes of palynomorphs (spores/pollen, dinoflagellates, acritarchs) in sediments is a function of distance from shore and depth of water. Pollen grains preserved in lake and bog sediment provide a record of past vegetation that has been an important source of information about climate and land cover during the Quaternary Period. The study of pollen grains in soil samples from an archaeological site provides information on ancient human use of plants and plant resources (Kipfer, 2000). The pollen grains of trees, shrubs, grasses, and flowers are preserved in either anaerobic conditions or in acid soils. Samples can be taken from the deposits by means of a core or from individual layers at frequent intervals in a section face on an archaeological site. As a dating technique, pollen has been used to identify different zones of arboreal vegetation which often correspond to climatic changes. The sediments most frequently investigated are peat and lake deposits, but the more acid soils, such as podsols, are also analyzed. Radiocarbon dates may be taken at intervals in the sequence, and it is possible to reconstruct the history of vegetation in the area around the site where the samples were taken. Coprolite analysis can provide a great deal of information on prehistoric diet and subsistence. This includes both the analysis of macrobotanical and palynological remains. While palynology can contribute to studies of diet, it can also inform on the types of economic and background pollen, seasonality of site occupation, and paleoenvironmental conditions (Bryant and Holloway 1983 ; Pearsall 2000 ; Reinhard and Bryant 1992). There have been many studies that attempt to determine the function of artefacts. These involve studies of botanical remains associated with artefacts, including pollen. An often used and potentially very important is artefact washes that involve the recovery and analysis of pollen directly from the surface of artefacts, such as complete ceramic containers and lithic artefacts, especially, grinding tools. These analyses can provide information both on the plants used at the site where the artefacts are found and on the uses of the artefacts themselves (Bryant and Holloway 1983 ; Pearsall 2000 ; Shafer 1979). Abundant pollen has been found associated with copper artefacts and deeply buried tree bark. Pollen was preserved around copper, apparently due to dissolved copper salts which act as a fungicide. Tree bark, an excellent collection surface for air-borne pollen, can yield abundant pollen when preserved (King et al, 1975). So, look everywhere for palynomorphs, take samples and palynologist will try to find the best method to extract them and analyse.

palynomorphs; methods; sediments

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