Climate: Past, Present & Future

Paleoenvironment proxy

Forams, the sea thermometers of the past!

Forams, the sea thermometers of the past!
Name of proxy

Mg/Ca-SST on planktonic foraminifera shell

Type of record

Sea Surface Temperature (SST)


Marine environments

Period of time investigated

55 Million years ago to recent times

How does it work ?

Foraminifera (or Forams) are single-celled organisms varying from less than 1 mm to several cm in size. They are very abundant in the ocean floor (benthic species) or floating amongst the marine plankton (planktonic species) where they produce their shells mostly using calcite (CaCO3). The oldest fossils of benthic foraminifera date back to the Cambrian period (older than 485 million year ago (Ma)) (Armstrong and Brasier, 2005). Planktonic species are younger than the benthic group. For instance, the species Globigerina bulloides (Figure 1) range from Middle Jurassic (180 Ma) to recent times (Sen Gupta, 1999).

A large spectrum of information can be provided by the analysis of foraminifera shells, based on the chemical composition and morphology of their shells as well as the species abundance patterns. One type of proxy is the ratio between the abundance of magnesium (Mg) and calcium (Ca) (Mg/Ca ratio) present in the calcite shell. During the formation of the shell, Mg is incorporated and may weaken the shells. In some cases, it seems that foraminifera expend energy to control the incorporation of Mg (Toler et al., 2001). The substitution of Mg into calcite depends on the temperature of the seawater, so that the amount of Mg in the shell exponentially increases from cold to warm water (Lea, 1999). This means that the Mg/Ca ratio of the shells is expected to rise with increasing temperature (Rosenthal, 2007). Measuring the Mg/Ca ratio of foraminifera shells therefore allows reconstructing the sea surface temperature (SST) of the past.

What are the key findings that have been done using Mg/Ca-SST?

Past SST determination is essential for understanding past changes in climate. An advantage of the Mg/Ca ratio measured on the shells of planktonic foraminifera is that the same sample can be used for different types of analyses in order to obtain a large set of information on the past sea conditions (Elderfield and Ganssen, 2000; Barker et al., 2005). Another advantage of this Mg/Ca proxy is the possibility to reconstruct changes of temperature within the water column using multiple species living at different depths and/or coming from different seasonal habitats (Barker et al., 2005). This can give us, for example, valuable information for describing seasons in the past.

Planktonic foraminifera can survive in a wide range of environments, from polar to tropical areas, thus the analysis of their shells allows reconstructing the ocean conditions all around the world. Moreover, foraminifera are very sensitive to temperature and environmental changes therefore it is possible to reconstruct climate changes of various amplitudes and timescales, e.g. the Paleocene-Eocene Thermal Maximum (55 Ma) or the more recent climate oscillations (Zachos et al., 2003; Cisneros et al., 2016). For instance, Figure 2 shows that Mg/Ca ratio allows reconstructing the ~2ºC warming observed from the Roman Period onset to higher frequency thermal variability like those observed in the Little Ice Age (LIA).

Figure 2: Sea Surface Temperature (SST) record stack for the last 2700 years reconstructed by means of Mg/Ca analysed on the shell of the planktonic foraminifera Globigerina bulloides in the central-western Mediterranean Sea. The different historical/climate periods are indicated: TP=Talaiotic Period, RP=Roman Period, DMA=Dark Middle Ages, MCA=Medieval Climate Anomaly, LIA=Little Ice Age, IE=Industrial Era. Years are expressed as Before Common Era (BCE) and Common Era (CE). The grey shaded area integrates uncertainties of average values and represents 1 sigma of the absolute values. This uncertainty includes analytical precision and reproducibility and the uncertainties derived from the G. bulloides core-top calibration developed in the original reference. (Modified from Cisneros et al., 2016).


This article has been edited by Célia Sapart and Carole Nehme
  • Armstrong, H. and Brasier, M., Foraminifera. In: Microfossils, Blackwell Publishing, pp. 142-187.
  • Barker, S., Cacho, I., Benway, H. and Tachikawa, K., 2005. Planktonic foraminiferal Mg/Ca as a proxy for past oceanic temperatures: A methodological overview and data compilation for the Last Glacial Maximum, Quat. Sci. Rev., 24, 821–
  • Cisneros, M., Cacho, I., Frigola, J., Canals, M., Masqué, P., Martrat, B., Casado, M., Grimalt, J. O., Pena, L. D., Margaritelli, G., and Lirer, F., 2016. Sea surface temperature variability in the central-western Mediterranean Sea during the last 2700 years: a multi-proxy and multi-record approach. Climate of the Past, 12, 849-869.
  • Elderfield, H. and Ganssen, G., 2000. Past temperature and δ18O of surface ocean waters inferred from foraminiferal Mg / Ca ratios, Nature, 405, 442–
  • Lea, D.W., 1999. Trace elements in foraminiferal calcite. In: Sen Gupta, B.K., (), Modern Foraminifera, Great Britain, Kluwer Academic Publishers, pp. 259-277.
  • Rosenthal, Y., 2007. Elemental proxies for reconstructing Cenozoic seawater paleotemperatures from calcareous fossils. In: Hillaire-Marcel, C. and de Vernal, A. (), Developments in Marine Gelology, Elsevier, pp. 765-797.
  • Sen Gupta, B.K., 1999. Introduction to modern Foraminifera. In: Sen Gupta, B.K., (), Modern Foraminifera, Great Britain, Kluwer Academic Publishers, pp. 3-6.
  • Toler, S.K., Hallock, P., and Schijf, J., 2001. Mg/Ca ratios in stressed foraminifera, Amphistergina gibbosa, from the Florida Keys, Marine Micropalentology, 43, 199-206.
  • Zachos, J. C., Wara, M. W., Bohaty, S., Delaney, M. L., Petrizzo, M. R., Brill, A., Bralower, T. J., and Premoli-Silva, I., 2003. A transient rise in tropical sea surface temperature during the Paleocene–Eocene thermal maximum, Science, 302, 1551–1554.

What speleothems can tell about the past climates !

What speleothems can tell about the past climates !
Name of the proxy:

Stable isotope ratios of carbonates in speleothems

Type of proxy:

Precipitation, atmospheric circulation, CO2 availability in soil, soil productivity


Continental environments

Period of time investigated:

Present day to 10 million years

Figure 1: Cut face of the Jeita cave stalagmite covering the last 12,000 years and showing  how the stratigraphy of a speleothem can be determined (Lebanon, central-Levant) (Modified after Verheyden et al., 2008)

How does it work?

Speleothems are inorganic carbonate deposits growing in caves that form from super-saturated cave waters (with respect to CaCO3) (Figure 1). Their analysis allows recovering aspects of past changes of the cave drip water geochemical composition, which provides information on climate and environmental variations above the cave (Fairchild and Baker, 2012). Different types of speleothems (e.g. flowstone, stalagmites) are widespread in karstic cave environments, but stalagmites as well as flowstones are used mainly to reconstruct past climates, because of a well-defined stratigraphic order. The major strengths of speleothems include their suitability for accurate age determinations (U/Th for ages up to c. 500,000 years; U/Pb for ages older than 500,000 years). Moreover, the preservation of multiple quasi-independent climate and environmental proxies enables the investigation of past climate changes on orbital to seasonal scale worldwide. Some of the most used proxies of speleothem carbonates are the ratios between oxygen-18 and oxygen-16 (δ18O) and carbon-13 and carbon-12 (δ13C), which are stated as a relative deviation to the Vienna Pee Dee Belemnite (VPDB) standard.


Figure 2: A diagram illustrating the primary processes related to δ18O variations relevant to paleoclimatology using speleothem records. Variations in temperature and relative humidity affect δ18O values through various processes in the atmosphere, in in the hydrosphere, in the soil and epikarst zones, and finally in the speleothem CaCO3. Modified after Lachniet, 2009.


The δ18O values of speleothem carbonates are determined mostly by two variables: the δ18O value of the cave drip water, which in turn is related to the δ18O value of the precipitation and in-cave fractionation processes (Lachniet, 2009). The δ18O value of the precipitation is determined by the atmospheric circulation, the trajectory of the precipitation, the amount effect (describing the negative relationship between precipitation δ18O and precipitation amount) and/or the seasons. The δ13C values of speleothem carbonates are locally controlled by biogenic soil productivity associated with the vegetation type (C4- or C3-type) and density, which regulates the soil CO2 content. Furthermore, it can reflect the availability of CO2 in the soil during the dissolution of limestone, which is a function of the water level in the karst and thus of the local precipitation amounts.


Figure 3: Different types of speleothem laminas. (A) Fluorescent laminas excited by UV light. (B) Visible laminas observed under reflected-light microscopy. (C) Calcite (C) and aragonite (A) couplets, observed under transmitted-light microscopy (Modified from Tan et al. (2006) and Johnson et al. (2006)) and (D) δ18O and δ13C variations measured in different laminas, reported in permil VPDB, from the Proserpine stalagmite (Han-sur-Lesse cave, Belgium). Note that dark and compacted layers become whiter due to the translucent light of the scan while the white and porous layers become dark. Modified from Van Rambelbergh et al. (2013).

What are the key findings that have been done using speleothems?

Speleothems are growing in caves worldwide and complement marine and polar climate archives, revealing unique views onto past climates. Speleothem δ18O records were employed to study the timing and climate of glacial/interglacial transitions (Lachniet et al., 2014; Winograd et al., 1992) as well as Heinrich events of the late Pleistocene. Several speleothem δ18O records from Western Europe (Genty et al., 2003) and the Eastern Mediterranean (Unal-Imer et al., 2015) revealed Dansgaard-Oeschger (rapid climate fluctuations) oscillations and were used to precisely date these climate events (Fleitmann et al., 2009). Furthermore, speleothem δ18O records allow studying past changes of global Monsoon systems (Cruz et al., 2005; Partin et al., 2007; Wang et al., 2005) as far back as 640 thousand years (Cheng et al., 2016). Lately new efforts are undertaken by the speleothem community to map the speleothem landscape in space and time to identify the current status of speleothem-based paleoclimate reconstructions globally.

You can learn more here:

Edited by Célia Sapart

 • Cheng et al., 2016, Nature 534, 640-646.
 • Cruz et al., 2005, Nature 434, 63-66.
 • Fairchild & Baker, 2012,  John Wiley & Sons, Ltd, Chichester, UK.
 • Fleitmann et al., 2009, Geophysical Research Letters 36.
 • Lachniet et al., 2009, Quaternary Science Reviews, 28(5), 412-432.
 • Lachniet et al., 2014, Nature Communications 5, 8.
 • Genty et al., 2003, Nature, 421(6925), 833.
 • Partin et al., 2014, Pages Magazine, 22(1), 22-23.
 • Ünal-İmer et al., 2015, Scientific Reports, 5, 13560.
 • Wang et al, 2005, Science 308, 854-857.
 • Winograd et al., 1992, Science (New York, N.Y.) 258, 255-260.

Ostracods, the sentinels of past oceanic circulation

Ostracods, the sentinels of past oceanic circulation
Name of the proxy


Type of proxy

Paleoenvironment proxy


All types of aquatic environments but here we will focus on marine waters

Period of time investigated


How does it work?

Ostracoda are crustacean of millimetre size which have inhabited all types of marine environments from the Ordovician to today (e.g. Salas et al. 2007) and colonized continental water bodies during the Carboniferous (Bennett et al. 2012). They are characterised by their bivalve calcified carapace articulated dorsally which encloses and protects the soft parts and appendages of the animal (Figure 1). The majority of Ostracoda live on or in the sediments: they are consequently highly sensitive to their environment.

What are the key findings that have been done using this type of proxy?

Throughout their history, marine Ostracoda inhabiting deep seas had very different morphologies from the contemporary shallow water species: thin shells, long, hollow and delicate spines and no eye spots (although this point is discussed; Figure 2). Based on the study of sediments, associated organisms and analogies with modern-days Ostracoda, ostracodologists concluded that those animals developed in low energy environments ranging from 500 to 5000 m depth in connection with global ocean cold water supplied by ice-caps (Lethiers & Feist 1991). This discovery provided a unique window into the oceanic circulation through geological times and the existence of a cold deep-water layer. The presence and characteristics of these Ostracoda have been cornerstones in understanding that the thermohaline circulation has not been constant through the Phanerozoic but rather existed only during the Late Ordovician, the Carboniferous-Permian interval and from the Eocene to today (Benson 1975).

Figure 2. Simplified geological time scale with Eras and Periods of the Phanerozoic. On the right are reported some archetypal deep-sea Ostracoda from the literature (for all photos, scale bar is 100 µm). A: Processobairdia spinanterocerata Bless & Michel, 1987; B: Cristanaria katyae Crasquin-Soleau, 2008; C: Gencella taurensis Forel, work in progress; D: Pedicythere klothopetasi Yasuhara et al., 2009.

Today, this field of research is very active as Ostracoda are the only metazoans regularly fossilized in deep-sea sediments over an extremely long period of the history of Earth. Their long fossil record spanning 5 mass extinctions and periods of extreme climatic changes make them precious tools to unravel the response of deep-water ecosystems to past climatic changes and the rhythms of their recovery. The extreme sensitivity and history of these peculiar animals make them sentinels of deep-sea ecosystems facing ongoing global temperature increase and acidification of marine waters.

  • Bennett, C.E., Siveter, D.J., Davies, S.J., Williams, M., Wilkinson, I.P., Browne, M., Miller, C.G. 2012. Ostracods from freshwater and brackish environments of the Carboniferous of the Midland Valley of Scotland: the early colonisation of terrestrial water bodies. Geological Magazine, 149, 366-396.
  • Benson, R.H. 1975. The origin of the psychrosphere as recorded in change of deep sea Ostracode assemblages. Lethaia, 8, 69-83.
  • Bless, M.J.M., Michel, M.P. 1967. An ostracode fauna from the Upper Devonian of the Gildar-Monto region (NW Spain). Leidse Geologische Mededelingen, 39, 269-271.
  • Crasquin-Soleau, S., Carcione, L., Martini, R., 2008. Permian ostracods from the Lercara Formation (Middle Triassic to Carnian?, Sicily, Italy). Palaeontology, 51, 537-560.
  • Lethiers, F., Feist, R. 1991. Ostracodes, stratigraphie et bathymétrie du passage Dévonien–Carbonifère au Viséen Inférieur en Montagne Noire (France). Geobios, 24, 71-104.
  • Salas, M.J., Vannier, J., Williams, M. 2007. Early Ordovician Ostracods from Argentina: their bearing on the origin of Binodicope and Palaeocope clades. Journal of Paleontology, 81, 1384-1395.
  • Yasuhara, M., Okahashi, H., Cronin, T.M. 2009. Taxonomy of Quaternary Deep-Sea Ostracods from the Western North Atlantic Ocean. Palaeontology, 52, 879-931.

Written by Marie-Béatrice Forel

Edited by Célia Sapart and Caroline Jacques