by Laura Cotton
Larger benthic foraminifera are amazing but rather underappreciated fossils. They have a long geological history, ranging from the Palaeozoic to the modern day – in Okinawa, Japan, “star sand” can be bought as a souvenir, and those tiny star shaped sand grains are the larger foraminifera Calcarina and Baculogypsina. However, they are most well-known during the Eocene, where they occurred in huge, rock forming quantities and are the dominant component of many shallow water limestone deposits, including those in Florida. Large benthic foraminifera are single celled organisms with a calcareous test, or “shell,” which has a complex and often very beautiful internal structure. As their name suggests, this test can get incredibly large – up to 15 cm, and is still a single cell. One of the reasons they are thought to get so big is because they have photosynthesizing symbionts, allowing them to get more energy than from just eating. This is also the reason they developed such complex test structures, to enable symbionts to be moved around the test and help regulate the light they receive. The presence of photosymbionts means that larger benthic foraminifera favor a similar environment to corals, the shallow marine photic zone (about < 100 m) in tropical regions. It also means that they are susceptible to environmental change, making them very useful for tracking the effect of climatic changes in the shallow water through geological time.
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The Eocene (56 – 33.7 million years ago) was a dynamic interval of Earth’s climatic history. After a peak in temperatures at the beginning of the Eocene there is a gradual global cooling trend, interrupted by a short warming episode in the middle Eocene, and culminating in a rapid cooling known as the Eocene-Oligocene transition. This transition event consists of an approximately 500,000 year-long cooling interval, associated with changes in ocean circulation and the first glaciation of Antarctica (see Coxall and Pearson, 2007 for detailed review). It is also associated with a large number of extinctions in both marine and terrestrial ecosystems, including within the larger benthic foraminifera. Several genera with long global fossil records appear for the last time around this transition event. For a long time, it was thought that a sea-level fall due to water becoming locked away as ice during the Eocene-Oligocene transition was responsible for the larger benthic foraminiferal extinction. However, it is often difficult to correlate the shallow water larger benthic foraminiferal record to the deep-water climate records. This is because preservation in limestones is often not good enough for geochemical analysis and the planktonic microfossils which are frequently used to date marine sediments live in the open ocean and mostly do not occur in such shallow marine sediments. Thus the exact timing of the larger benthic foraminiferal extinction with respect to the Eocene-Oligocene Transition, and therefore the extinction mechanism, remained uncertain.
My research used material from three on-shore drill sites from the Kilwa District in southern coastal Tanzania which contain beautifully preserved small calcareous fossils, including planktonic foraminifera, molluscs, bryozoans, nannofossils, and larger benthic foraminifera. The sediments recovered in the cores continuously span the Eocene-Oligocene transition, allowing high resolution geochemical and micropalaeontological studies to be carried out across this interval. The presence of well-preserved planktonic foraminifera and nannofossils in this material, along with the larger benthic foraminifera, made it a unique site for correlating the larger benthic foraminiferal occurrences to the global climate record and therefore to gain some insight into how shallow water environments respond to major climate events. Geochemical studies using oxygen isotopes and ratios of magnesium and calcium were carried out by my colleagues on the planktonic foraminifera and small benthic foraminifera to determine the exact levels in the record that the temperature change, and sea level fall occurred (Pearson et al., 2008; Lear et al., 2008). The identification and ranges of the larger benthic foraminifera from the same samples were then determined.
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The results were surprising: rather than showing an extinction level at the same as the sea-level fall, the extinction occurs 200,000 years prior to this – during a relatively stable time in the temperature record (Cotton and Pearson, 2011). It is also almost exactly the same level as an extinction in the planktonic foraminifera, despite their inhabiting different parts of the ocean. One possible mechanism for this may be that the changes taking place in ocean circulation cause the water column to be less stratified and more nutrients to occur in the surface and shallow waters. Since both the planktonic foraminifera and larger foraminifera like low nutrient clear water environments, this may have been detrimental to them.
Recently I have been looking at the molluscs from these same samples, and unlike many other organisms, the molluscs show increasing diversity and numbers from the onset of the transition. The nannofossil record also shows a change to an assemblage that likes a more nutrient-rich environment; increased nutrients could also be a reason for the increase in molluscs and lends some support to the potential high-nutrient extinction mechanism for the foraminifera. However, more sites are needed to see if there is a similar pattern elsewhere.
In January 2016 I will be joining the Department of Geological Sciences at the University of Florida and the Florida Museum of Natural History to continue this research. Florida is full of larger benthic foraminifera from the Eocene and Oligocene, but the assemblages and extinction pattern here are quite different from other areas of the world and not well constrained compared to the climatic events. The lepidocyclinids (another group of large forams), which are common in the Ocala and Marianna limestones, survive the Eocene-Oligocene transition, while taxa with a similar morphology in the rest of the world go extinct. So, the next step in my research is to see what is happening over this interval in the Americas, how it compares to the rest of the world, and why these differences occur. Hopefully I will be able to update the myFOSSIL community on this in the future!
References Cited:
Cotton, L.J. & Pearson, P.N. 2011. Extinction of larger benthic foraminifera at the Eocene/Oligocene boundary. Palaeogeography, Palaeoclimatology, Palaeoecology 311, 281–296.
Coxall, H.K. & Pearson, P.N. 2007. The Eocene-Oligocene transition, In: Williams, M., Haywood, A.M., Gregory, F.J., Schmidt, D.N. (Eds.), Deep-time perspectives on climate change: marrying the signal from computer models and biological proxies. The Micropalaeontological Society, Special publications, London, pp. 351-387.
Lear, C.H., Bailey, T.R.,Pearson, P.N., Coxall, H.K. & Rosentha, Y., 2008. Cooling and ice growth across the Eocene-Oligocene transition. Geology 36(3), 251–254.
Pearson, P.N., McMillan, I.K., Wade, B.S., Dunkley Jones, T., Coxall, H.K., Bown, P.R.& Lear, C.H. 2008. Extinction and environmental change across the Eocene-Oligocene. Geology 36(2), 179–182.
To learn more:
Cotton, L.J., Zakrevskaya, E. Y., Boon, A. van der, Asatryan, G., Hayrapetyan, F., Israyelyan, A., Krijgsman, W., Less, G., Monechi, S., Papazzoni, C., Pearson, P. N., Razumovskiy, A., Renema, W., Shcherbinina, E., Vasilyeva, O., Wade, B. S. The integrated stratigraphy of the Priabonian (upper Eocene) of Urtsadzor section, Armenia: implications for correlation and the base Priabonian. Newsletters on Stratigraphy, DOI: 10.1127/nos/2016/0313
Renema, W. and Cotton, L.J. 2015. Three dimensional reconstructions of Nummulites tests reveal complex chamber shapes. PeerJ 3 e1072, DOI:10.7717/peerj.1072