What Is a Microfossil? The Tiny Remains That Run the Oil Industry

The term covers anything requiring magnification, which groups together organisms with no relationship beyond size. Foraminifera are single-celled marine organisms that build chambered shells, mostly of calcium carbonate, and they are the workhorses of the field. Diatoms are algae with intricate silica shells, abundant in both marine and freshwater sediments. Radiolarians build silica skeletons of remarkable geometric complexity. Coccolithophores produce tiny calcite plates that accumulate in such quantities that they form chalk. Ostracods are small crustaceans with two-part shells. Conodonts are tooth-like elements from an extinct group of chordates and were a mystery for over a century before the animal producing them was identified. Pollen and spores from land plants preserve extremely well and record vegetation. Each group has its own specialists, its own preparation methods and its own strengths, and few researchers work across more than one.

The advantages over larger fossils follow directly from being small and numerous:

• •Abundance, since a small sample contains enough specimens for statistical analysis rather than one anecdote
• •Recovery from drill cores, which is decisive, since a borehole a few centimetres across would rarely intersect a large fossil and always intersects microfossils
• •Rapid evolution in several groups, which makes them precise for dating thin intervals of rock
• •Wide geographic distribution, since planktonic forms drift across oceans and allow correlation between distant places
• •Chemical records, since shells built from seawater carry isotopic and trace element signatures of the conditions at the time
• •Sensitivity to environment, since assemblages shift with temperature, salinity and depth, making them indicators of past conditions

The field grew large because of oil. Determining the age and environment of rocks encountered in a well is essential to petroleum exploration, and drill cuttings brought to the surface contain almost nothing identifiable except microfossils, which made micropalaeontologists directly useful to a large industry from the early twentieth century onward. Oil companies employed them in numbers, funded collections and research, and built the biostratigraphic frameworks that the academic field then used for other purposes. That history explains several features of the discipline, including the concentration of expertise on groups useful in marine sedimentary basins, the existence of enormous industrial collections, and a long-standing flow of people and money between commercial and academic work. It has also created a problem, since the decline of exploration in some regions has reduced training and employment, and the specialist skill of identifying these organisms is held by an ageing and shrinking group of people.

Recovering specimens is a laboratory process rather than a field one and the method depends on what the fossil is made of. Carbonate shells are separated from soft sediment by disaggregating the sample in water, sometimes with a chemical to break clay bonds, then washing it through a fine sieve and drying the residue, from which specimens are picked individually under a microscope with a fine brush. Silica-walled organisms survive treatment with acids that dissolve everything around them, which is a fast route to a clean sample. Pollen preparation uses a sequence of aggressive chemical treatments, since the outer wall of a pollen grain is among the most chemically resistant biological materials known. Conodonts are recovered by dissolving limestone in dilute acid and collecting the resistant phosphatic elements. Each route destroys the rock, so the preparation is a commitment, and picking a slide of a few hundred specimens is slow, skilled and largely unautomated work.

The scientific payoff that reaches beyond geology is the climate record. Shells of planktonic foraminifera built from seawater incorporate oxygen isotopes in ratios that depend on temperature and on how much water is locked up in ice sheets, so a core of deep-sea sediment yields a continuous temperature and ice volume record running back millions of years. Species assemblages provide an independent check, since different species prefer different temperatures. Magnesium to calcium ratios in shells give another temperature measure. Together these established the pattern of glacial cycles, tied them to variations in the Earth's orbit, and provided the long baseline against which recent warming is compared. Diatoms serve the same function in lakes, recording acidity and nutrient levels. Pollen records vegetation and therefore regional climate on land. Almost everything known in detail about climate before instrumental records rests on organisms too small to see.

Foraminifera, diatoms, radiolarians, coccoliths, ostracods, conodonts and pollen share only their size, and their abundance allows statistics where large fossils allow anecdote. They come up in drill cuttings, which is why the oil industry built the field and employed most of its specialists. Oxygen isotopes in their shells supply the long climate record that recent warming is measured against.