When we think about climate change, normally what springs to mind could be rising temperatures and freak weather conditions. Our planet is definitely changing and we are pumping a lot of CO2 into the atmosphere – one big indicator of this was when the 400ppm CO2 measurement ‘milestone’ was recorded during May 20131. However, have you ever thought about what past climates might have been like and how you could find this sort of information out?
Palaeoclimatology is the study of past climates; past climates can help us to predict future climate conditions. The Intergovernmental Panel on Climate Change has a section on palaeoclimate too (see their most recent report published this year2), as this type of analysis is becoming increasingly important with predicting future climates. Microfossils, such as diatoms and foraminifera, can help us with palaeoclimatology studies. They can tell us about lots of different conditions including ocean acidification levels, sea surface temperatures, how much ice was present on Earth and how much carbon dioxide and oxygen was present in the atmosphere.
Foraminifera (forams for short) are single-celled marine protists which live in the ocean. The shapes of their tests vary greatly (see below for an example) and are made of calcite (same as the white residue you can find in your kettle) or aragonite (same as calcite but different chemical structure). Forams have two modes of life – planktonic and benthic. Planktonic forams float in the top part of the ocean and benthic forams live on the ocean floor3. There are also larger benthic forams too – the best and probably the most impressive place to find one species of these (Nummulites) are in the pyramids in Egypt – each building block contains thousands of Nummulites4! Geochemical analyses of foraminifera can help us to interpret palaeoclimatic conditions, for example, Mg/Ca ratios can be used to work out sea surface temperatures using mathematical equations, and Ba/Ca has been suggested to help infer how much freshwater was added to the ocean5. Oxygen isotope data from foraminifera can help us to understand how much ice was present on Earth and for working out sea surface temperatures too5,6.
Diatoms are microscopic plants, which are in the same group as algae, and have a shell made from silicon dioxide (glass and sand are made from this). As they are algae they photosynthesise, and can be found in any body of water (e.g. oceans, lakes, rivers). Diatoms move by secreting a moist and sticky material along a groove called a raphe. Like forams, they are planktonic but are restricted to the top 200 m of the water column7. Diatoms are particularly useful for climate studies as they are sensitive to different conditions. Nutrient availability and lake level changes are such applications, since these depend on precipitation, upwelling (deeper, colder waters rising to the surface) and wind strength, as well as solar output (how much light the sun produces) and erosion8. Silica also preserves well in the fossil record, so diatoms can be used for palaeoclimatic study too. Diatoms can help to reconstruct ice coverage too as the amount of ice present determines how much light can penetrate into a water body9. More recently, diatom geochemistry has been used to infer chemical conditions in the ocean – one study used rare earth element (such as Cerium and Europium) concentrations in diatom mats to study the evidence for reducing conditions (absence of oxygen) and higher productivity (more nutrients) in the Western Pacific during a period known as the last glacial maximum10, where the ice sheet extent was at its maximum on Earth (26, 500 – 19, 500 years ago)11. Diatoms also help to reduce global warming. When they produce oil drops through photosynthesis, dissolved carbon dioxide gas in seawater, chemical nutrients and energy from sunlight is used. This helps to reduce carbon dioxide in the air and hence stops warm air forming near the Earths' surface12.
The abundance of foram and diatom species can be used too. Water temperature determines the speed of organism growth, and specific conditions will cause particular types of species to grow, so if we pair these observations this can help us to reconstruct past climates and oceanographic conditions9.
Apart from diatoms and forams, other microfossils such as coccolithophores, acritarchs, dinoflagellates, pollen, ostracods and conodonts can help with palaeoclimatic study too. All of these tiny little fossils have a myriad of big stories to tell us, and it’s up to the next generation of climate scientists to work them out and relate them to the wider community. Maybe we’ll find out some of these from our diatom collection!
1. Welp L & Keeling R (2013) “Now what?”, [Online] Available from http://keelingcurve.ucsd.edu/now-what/ [Accessed 23/12/2013]
2. Jansen E, Overpeck J, Briffa KR, Duplessy JC, Joos F, Masson-Delmotte V, Olago D, Otto-Bliesner B, Peltier WR, Rahmstorf S, Ramesh R, Raynaud D, Rind D, Solomina D, Villalba R & Zhang D (2007) "Palaeoclimate", In: "Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change" [Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M & Miller HL (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
3. Spero, H (1998) “Life History and Stable Isotope Geochemistry of Planktonic Foraminifera”, Palaeontological Society Papers, 4, 7 – 36
4. Cassaza L (2007) “Pyramids, forams, and Red Sea reefs: Field notes from Lorraine Casazza”, University of California Museum of Paleontology [Online] Available from http://www.ucmp.berkeley.edu/science/fieldnotes/casazza_0711.php [Accessed 23/12/2013]
5. Katz ME, Cramer BS, Franzese A, Hönisch B, Miller KG, Rosenthal Y & Wright JD (2010) “Traditional and emerging geochemical proxies in foraminifera”, Journal of Foraminiferal Research, 40 (2), 165 – 192
6. Erez J & Luz B (1983) “Experimental paleotemperature equation for planktonic foraminifera”, Geochimica et Cosmochimica Acta, 47, 1025 – 1031
7. Olney M (2002) “Diatoms”, [Online] Available from http://www.ucl.ac.uk/GeolSci/micropal/diatom.html [Accessed 23/12/2013]
8. Meeker C & Stager C (2007) “Diatoms and climate change – The use of diatom analysis in reconstructing Late Holocene climate for Kigoma Region, Tanzania”, Department of Geosciences, University of Arizona
9. Mackay A, Batterbee R, Birks J & Oldfield F (2003) “Global change in the Holocene”, London, Hodder Education Publishing
10. Xiong Z, Li T, Algeo T, Chang F, Yin X & Xu Z (2012) “Rare earth element geochemistry of laminated diatom mats from tropical West Pacific: Evidence for more reducing bottomwaters and higher primary productivity during the Last Glacial Maximum”, Chemical Geology, 296 – 297, 103 – 118
11. Clark PU, Dyke AS, Shakun JD, Carlson AE, Clark J, Wohlfarth B, Mitrovica JX, Hostetler SW & McCabe Marshall A (2009) “The Last Glacial Maximum”, Science, 325 (5941), 710 – 714
12. Blinn DW & Blinn SL (2012) "Diatoms: unnoticed living jewels in the water", USA, Maple Creek Media c/o Old Line Publishing LLC
Pictures taken from http://oceanworld.tamu.edu/students/forams/images/hantkenina_small.jpg and http://www.ucmp.berkeley.edu/chromista/bacillariophyta.html respectively