Biostratigraphy Party

Principles of Biostratigraphy: The ‘Biostratigraphy Party’

Biostratigraphy is the process of assigning an age to a rock based on the fossils it contains. To do this, a biostratigrapher needs to identify a particular type of fossil called an ‘index fossil’. Index fossils need to be (relatively) easily identifiable, geographically widespread and have a rapid evolutionary turnover. This is so that rocks of the same age can be easily correlated across the globe and that particular species are diagnostic of a particular time interval. Types of fossils that are useful in biostratigraphy include, but are not limited to: foraminifera, nannofossils, spores and pollen, conodonts, graptolites and ammonites.

Biostratigraphers call the length of time between a species’ first appearance (speciation) and its last appearance (extinction) in the fossil record its ‘range’. Often the range of stratigraphically useful fossils are in the region of several million years, however with high resolution dating some species’ range can be calculated as several hundred thousand years. Ideally the range of an organism is determined by its occurrence between two horizons of known age. These horizons may contain minerals that are suitable for radiometric dating such as those within volcanic ash beds (bentonites) or contain rocks that record a diagnostic isotopic signature (typically oxygen, carbon or strontium). Where this succession of rocks occurs, we may call this location the type section or stratotype for a particular species. Once the range of an organism is established from its type locality we can interpret layers of rocks that contain the same organism in different locations around the globe (e.g. where dateable bentonites are absent) to be of the same, or similar, age.

Biostratigraphers may further subdivide successions of rocks into ‘biozones’. Taken from the International Stratigraphic Guide biozones are defined as bodies of rock strata that are characterised on the basis of their contained fossils. They exist only where the particular diagnostic biostratigraphic feature or attribute on which they are based has been identified. Biozones are, therefore, descriptive units based on the identification of fossil taxa.

A biozone may be based on:

  • The appearance of a single, easily identifiable or abundant taxon
  • A combination of taxa
  • Relative abundances
  • Specified morphological features
  • Variations in features related to content and distribution of fossils in strata

There are several different types of biozone that are best described by means of analogy. Imagine geological time as a 12 hour clock and that organisms such as foraminifera are gregarious creatures that like to party. In this analogy, the foraminifera have arranged a ‘biostratigraphy party’ that begins at 6pm and ends at midnight.

Taxon-range zone

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Taxon range zone

Disco and Yab are the life and soul of any party, and when they turn up the other guests have a memorable time.  Disco arrives at the party at 6pm (his first appearance or lowermost occurrence) and leaves at 8pm (his last appearance or uppermost occurrence). Guests remember his attendance as the ‘Disco Zone’. Yab turns up to the party between 10pm and midnight, guests remember this part of the party as the ‘Yab Zone’.  This scenario represents the ‘taxon-range zone’ where the zonal boundaries are defined by the lowest and highest stratigraphic occurrence of a specified taxon in a particular stratigraphic section.

Abundance zone

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Abundance zone

As all biostratigraphers will tell you, foraminifera are huge fans of early 90’s American thrash metal. At the party, the foraminifera are divided into two groups – Megadeth fans and Pantera fans. At 7pm the DJ decides to play ‘Symphony of Destruction’ by Megadeth, at which point all the Megadeth fans come together rock out.  The guests remember this part of the evening as the ‘Megadeth Zone’. Later on, at 11pm, the DJ plays ‘Cemetery Gates’ by Pantera. As you can imagine, the Pantera fans cannot resist rocking out again to 7.03 minutes of melodic guitar riffs and soaring vocals. This period of the evening is remembered fondly as the ‘Pantera Zone’.   This scenario is referred to as the ‘abundance zone’ where boundaries are defined by a notable change in abundance of the taxon or group of taxa chosen to characterise the zone.

Concurrent range zone

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Concurrent range zone

Al arrives at the party at 6pm and stays until 10pm. Al is waiting for her friend Wilf who arrives at the party at 8pm and leaves at midnight. The only time both Al and Wilf are both at the party together is between 8pm and 10pm and guests remember this as the ‘Al and Wilf Zone’. This represents the ‘concurrent range zone’ which is defined by the lowest stratigraphic occurrence of the higher ranging of the two taxa (Wilf, 8pm) and the highest stratigraphic occurrence of the lower ranging of the two taxa (Al, 10pm).

There are many other types of biostratigraphic zones (such as lineage and interval zones) and variations of all the types mentioned. I hope this analogy gives you an insight into the different types of methods employed by biostratigraphers to determine the age of rock units.

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What are forams and why are they useful?

If you were to think about single-celled organisms, you might imagine that they could only been seen under the microscope. Foraminifera (colloquially called ‘forams’), however, are single-celled organisms that can grow from several millimetres up to the size of small frisbees. The one in the above picture, Assilina exponens (Eocene, Germany), is the same size and shape as a ready salted crisp. The largest recorded foram measured 19cm in diameter, nearly 4 times the size of the Assilina exponens pictured above.

Forams are major rock-forming organisms that contribute 43 million tons of carbonate in the oceans per year (Langer et al., 1997) and make up approximately 20% of the carbonate in the geological record. They are also major components of beach ‘sand’ especially in subtropical environments (you may have seen pictures of the beautiful variety of morphologies in foraminifera that make up beach sand that occasionally do the rounds on social media), even the pyramids of Egypt are built from Nummulitic limestone, like the one below, formed of millions of ‘tests’ of foraminifera from the genus Nummulites.

 

Nummulites gizehensis in a hand specimen of the Eocene Faumai Limestone from New Guinea
Nummulites gizehensis in a hand specimen of the Eocene Faumai Limestone from New Guinea
Nummulites gizehensis in thin section from the same sample of Eocene Faumai Limestone, New Guinea
Nummulites gizehensis in thin section from the same sample of Eocene Faumai Limestone, New Guinea

These organisms are important predators and prey in the ocean ecosystem, and make up much of the biomass of the seafloor and water column, it is even estimated that most of the seafloor is even covered by the reticulate pseudopodia of these organisms. Yet, these creatures remain almost unknown to most outside of the geological or biological communities and have yet to achieve their deserved star status having been mostly overlooked on prime-time nature documentaries. So what exactly are they? Why are they so large? And why do people like me spend all day looking at them?

Biology

The taxonomy of foraminifera is problematic and highly disputed. For now, foraminifera are placed within the Rhizaria a ‘supergroup’ of Protists, one of the ‘5 Kingdoms’, although this kingdom is currently being phased out. What is not disputed is that they are eukaryotic single-celled organisms that resemble amoebae with pseudopodia (stringy, gelatinous appendages that are used for locomotion and collecting food).

Foraminifera first appeared during the time of the ‘Cambrian Explosion’ ~542 Ma, with oldest known foraminifera being Platysolenites sp. To date there are approximately 275,000 recognised species, of these 10,000 are extant with remainder extinct and consigned to the fossil record.

There are many ways to divide forams based on their skeletal structure or mode of life. Foraminifera are often described as being ‘benthic’ – living close to the sediment-water interface, or ‘planktonic’ – living within the water column. Planktonic foraminifera account for only around 50 species of 10,000 species around today. Benthic foraminifera account for the remaining extant species, these are often further subdivided by their size into smaller and larger benthic forams, or according to their test structure.

Foraminifera secrete a shell (generally composed of calcite), which is technically called a ‘test’ because cytoplasm covers the exterior of the shell in a ‘sheath’ of ectoplasm. Foraminiferal tests may be ‘agglutinated’ where they are made from sand grains, shell fragments from other organisms, or generally whatever other detritus the foram can find within the substrate in which it lives that it can stick to itself. Foraminiferal tests may be ‘granular/porcelaneous’ such as those of miliolids that do not let light though, or ‘hyaline/glassy’ in which the orientation of crystals permits light to pass giving a transparent or translucent appearance to the test (see below). Again, foraminifera can be further subdivided and subdivided further still based on the numerous shapes, chambers, and apertural openings etc. of the test.

The differences of the major types of foramiferal test structure in cross section
The differences of the major types of foramiferal test structure in cross section

Many forams are symbiotic and grow chambered tests in which to house their symbionts, it is these that make up the larger benthic foraminifera. As their name suggests it is this group that attain the largest sizes. The first formed chamber of a larger benthic foram is called the ‘proloculus’, it is here that the foram keeps its important cellular material with cytoplasm filling the rest of the test. From the proloculus the foram produces a series of large chambers (sometimes divided into smaller ‘chamberlets’). The cytoplasm in the chambers is linked to the proloculus by a canal-like structure called a ‘foramen’ (from which the foraminifera derive their name). It takes forams such as Heterostegina, Elphidium, and Rotalia 24 hours to grow a new chamber. It then takes them a further 5-7 days to recuperate and muster the strength to add a new chamber (Myers, 1941). Within these chambers the larger benthic foraminifera ‘farm’ their symbionts, often photosynthetic cyanobacteria. Rather counter-intuitively the largest larger benthic foraminifera are found within the most oligotrophic (low nutrient) environments. This is because they must reach a large size in order to house the necessary number of cyanobacteria to keep them alive.

Larger benthic foraminifera live typically from several weeks to a couple of years. Their life span is in part dependent on their reproductive cycle. Foraminifera have a complex life cycle in which they can reproduce sexually or asexually. Asexual reproduction produces a haploid ‘Megalospheric’ morphology via meiosis that contains a single set of chromosomes. Sexual reproduction produces a diploid ‘Microspheric’ morphology that contains a pair of chromosomes (see reproductive cycle figure below). Again rather confusingly, the Megalospheric form is often smaller than the Microspheric form. The terms ‘mega’ and ‘micro’ in this case does not refer to the size of the test but of the proloculus. The Megalospheric morphology has a larger proloculus than the Microspheric morphology.

The simplified life cycle of foraminifera. Foraminifera may reproduce sexually by two haploid cells coming together to form a diploid, or asexually by dividing by either mitosis or meiosis to form two new diploid or haploid cells, respectively
The simplified life cycle of foraminifera. Foraminifera may reproduce sexually by two haploid cells coming together to form a diploid, or asexually by dividing by either mitosis or meiosis to form two new diploid or haploid cells, respectively

Uses

Foraminifera are useful for palaeontologists and geologists like me in that they provide environmental information about the rocks in which they are contained were deposited. They are also key index fossils in helping to determine at what time these rocks were laid down. As mentioned previously, many larger benthic forams contain symbiotic cyanobacteria that photosynthesise to survive. Photosynthesis requires sunlight and because of this larger benthic forams are often found within the ‘photic zone’, the zone at which sunlight can still penetrate the water column.

Some smaller benthic foraminifera such as Amphistegina house photosynthetic symbionts adapted to blue light that can penetrate greater water depths of up to 150m, although healthy populations of Amphistegina thrive in water depths between 20-30m. Amphistegina is also commonly associated with sandy sediments in warm, tropical environments with their robust tests able to withstand high energy. Other smaller benthic foraminifera can be found in bathyal environments especially where they are agglutinating and not made of calcite (see CCD below).

Planktonic foraminifera are usually only found in the inner to outer neritic environment, but never within bathyal environments due to the ‘Carbonate Compensation Depth’ (or CCD), a depth below which carbonate is dissolved, typically this boundary is found between 4-5km. Occasionally planktonic foraminifera can be washed in to much shallower environments but these environments should be interpreted based on the greater number of benthic foraminifera than planktonics within the fossil assemblage.

The environmental preferences of some Neogene foraminifera through a transect of a typical carbonate rimmed shelf within 20° latitude either side of the equator.
The environmental preferences of some Neogene foraminifera through a transect of a typical carbonate rimmed shelf within 20° latitude either side of the equator.

Species of larger benthic and planktonic foraminifera have short temporal ranges which makes them perfect index fossils in determining the age of rocks through biostratigraphy. Good index fossils must fulfill several criteria 1) they must be common and easy to find, 2) they must be cosmopolitan (although migrations must be accounted for) so they can be correlated across the globe, 3) they must be easy to identify so they can be spotted in samples with varying degrees of preservation, 4) they must have short evolutionary histories so that one (preferably several) species can be used to define a particular period of time. Many smaller benthic foraminifera are hardy organisms that have long temporal ranges which make them not particularly useful for biostratigraphic purposes but, as previously illustrated, may have palaeoenvrionmental significance.

I will give more details on the principles of biostratigraphy in a later post.

References:

Langer, M. R., Silk, M. T. B., Lipps, J. H. 1997. Global ocean carbonate and carbon dioxide production: The role of reef Foraminifera, Journal of Foraminiferal Research, 27(4), 271–277

Myers, E.H. 1941. Ecologic relationships of larger foraminifera: Report of the Committee on Marine Ecology as related to Paleontology, Division of Geology and Geography, National Research Council Annual Report, Appendix H, 1945.