How do we know that certain forams species were thriving in the past environment?


So I am doing research in paleoenvironment regarding recent forams in sea sediments. One thing which quite puzzling me is that the existence of certain forams species in relation to their past surrounding environment. For instance: how the researcher back then knew if the abundance of *Bulimina* and *Bolivina* is due to high productivity of ocean?
Thank you in advance!

In: 5

**TL;DR** the types of foram present, their internal structure, specific groupings of different species occurring together, their overall numbers, their abundance relative to each other, and the detailed chemistry of their tests (shells), all give us information about the environment they lived in.

For anybody reading who is wondering what forams are, that’s the informal name affectionately employed (by me at least) to mean foraminifera: single celled organisms that secrete a chambered calcium carbonate shell and live as planktonic or benthic (seafloor) organisms. They are mostly microscopic (less than 1 mm) and though they can be studied with simple preparation techniques and regular microscopes, they often require use of an electron microscope to see the features properly and reliably identify individual species rather than just the genera. [A selection of electron microscope images of them](, the popcorn looking ones on the right are the planktonic ones, the more varied ones on the left are benthic.

At the most basic level the fact that forams are present in sediment cores at all means there must have been decent productivity in the region. Productivity is essentially just how much biological activity is going on in the surface waters thanks to the Sun and any primary producers right? Don’t forget that only a fraction of the foraminifera get preserved in the seafloor sediments due to natural dissolution of their CaCO₃ shells, especially if their final resting place is approaching the carbonate compensation depth.

The groups you mention are benthic (sea-floor dwelling rather than surface water dwelling) so they are highly dependent on there being good productivity in the sunlit surface waters so that a healthy ecosystem exists between them and the surface or simply enough nutrients and organic detritus raining down from those productive surface waters so that the benthic communities can flourish. This is particularly important if they are deep sea benthic forams, where phytodetritus becomes a key food source. I’m not sure of the exact setting that you’re looking at though.

The structure of benthic foraminiferal communities (ie. where certain types occur, which ones occur together and which dominate) is controlled by environmental parameters, with oxygen and food (organic matter) availability being considered the most important factors. Seeing as you are looking at recent sediments, I believe it’s even possible to infer the particular kind of ecosystem that existed eg. mangrove swamp, coastal shelf, open seas, reefs, lagoons, etc. This is due to forams being a very well established paleoenvironmental proxy and so there has been lots of work done on correlating certain assemblages to certain types of ecosystems and marrying up this research with what we see operating today.

We can go even deeper and look at the chemistry of the foram shells to tell us about the environment. It’s easy to get lost in the weeds if you’re new to that side of things, but the general picture is that the conditions in air, water and soil (the latter can be found in runoff entering waters where forams live) all influence chemistry and chemical processes of foraminifera habitats. In turn, this can be seen in the chemistry of the foram tests. Substitution of elements is common in mineral growth (both in biologically produced minerals like the shells of marine organisms, and in completely inorganic situations, say where minerals are crystallising from an igneous melt) and can sometimes give an indication of temperature. Strontium and magnesium substitute for calcium in marine shells; minor and trace elements often show greater substitution at higher temperatures. Several trace elements including boron, gallium, rubidium, sodium, and strontium, are sensitive to salinity, though their concentrations are also affected by sediment source, clay mineralogy, and grain size (all of which can either help or hinder trying to reconstruct a past ecosystem!)

Isotopes are particularly valuable in environmental reconstruction. They are atoms with the same number of protons but a different number of neutrons and thus a different atomic mass. This (tiny) difference in mass means that certain chemical and physical processes separate out different isotopes very slightly. This separation leads to either an enrichment or depletion of individual isotopes compared against some standard. Oxygen isotopes ratios (written as δ¹⁸O values) in forams provide an invaluable paleothermometer. Carbon isotope ratios can be used as a proxy for specific differences in the habitat and ecological preferences, and in the total dissolved inorganic carbon (DIC) content of the ambient seawater in which the forams live. For example, different species of planktonic foraminifera have different temperature, nutrient, and light requirements and therefore can live and calcify at different vertical depths and seasons. As such their shells will reflect the vertical and seasonal variations in δ¹³C DIC within the upper ocean. Benthic foraminifera have different oxygen and food requirements and therefore can live and calcify at different depths in the sediments. For the sake of ecosystem reconstruction, its common to select species that have well-known ecological preferences that are fairly predictable across regions with different oceanographic conditions in the modern ocean; that is to say, species which are widespread, not subject to too many confounding factors and have had a lot of work done on them so that any confounding factors are at least fairly well understood.

As always, the more recent material you are working with, the less complicated it is (not that it’s straightforward, but you get the idea). There have also been many studies where the effects of controlling the nutrient and food supply to foram populations in the lab have been monitored, so we can see how it affects growth rates and abundances. Applying that to the real world helps to give more meaning to things like shell concentration counts. Similarly, looking at the particular assemblages present in modern ecosystems helps us to understand which types of ecosystems were occupied by which sorts of micro-organisms in the past. Again, things always get ropier the further back you extrapolate, but Quaternary sediments are fairly straightforward for this kind of thing, you’re not going to come across assemblages from environments that we haven’t got examples of today.

Leaving chemical analyses behind and getting back to basics (forams were the subject of many early studies that pioneered the field of micropaleontology in the after all) we can look at the different ways that forams build their tests in order to stick them into distinct categories, there are three important ones to consider:

1. Porcellaneous, randomly oriented calcite crystals which give a smooth, milky white appearance under the microscope.

2. Hyaline, formed of slightly larger calcite crystals and have a resulting glassy appearance.

3. Agglutinated, where organic and mineral matter from the sea floor is selected and bound together by a cement secreted by the foram.

The ratio of these three types to each other has been used extensively to differentiate among a range of modern environments, and I reckon this sort of thing probably gives the most relevant answer to your original question. Plotting the relative frequencies of these groups against each other can distinguish proportions for hypersaline and marine lagoons, estuaries, and open shelf seas, as shown [here.](

Fossil faunas may be plotted on these templates, and these allow palaeontologists to estimate the salinity of ancient environments (salinity being something which changes over a spectrum as we move from freshwater river section to estuary mixing to open seawater. This sort of conclusion can then be reinforced by chemical analysis of the forams, as previously mentioned certain chemistry is indicative of the seawater salinity that the forams lived in.

Other ratio based approaches are equally as useful: using benthic forams in terms of infaunal:epifaunal (living within vs directly on the seafloor sediment) has been widely used to determine the relative content of dissolved oxygen and/or organic carbon in the seafloor. Epifaunal forms occur mainly in oxygenated conditions with low amounts of organic carbon, whereas infaunal forms occur in more oxygen deficient conditions with higher organic carbon content.

Your original question made it sound like you’re doing primary research on sediment cores? I’d be interested to hear more about that, given the forams you mentioned I’d guess either somewhere in the Mediterranean or the area surrounding New Zealand? Happy to point you in the direction of some further reading if you had any other specific questions, or take a stab at explaining any other aspects of how forams are used as proxies.