The Pliocene Warm Period, an analogue of a future warmer Earth.

 

Tuktoyuktuk Beach on the Arctic Ocean (From Wikipedia)

Tuktoyuktuk Beach on the Arctic Ocean (From Wikipedia)

Microfossils from deep-sea are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments. The incorporation of Mg/Ca into the calcite of marine organisms, like foraminifera, is widely used to reconstruct the thermal evolution of the oceans throughout the Cenozoic. Planktic foraminifer Globigerinoides ruber is perhaps one of the most widely used species for reconstructing past sea-surface conditions. Additionally, Mg/Ca–oxygen isotope measurements of benthic foraminifera may be related to global ice volume and by extension, sea level (Evans et al., 2016). The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contains clues about the temperatures in which they lived.

Scanning Electron Micrographs of Globigerinoides ruber (adapted from Thirumalai et al., 2014)

Scanning Electron Micrographs of Globigerinoides ruber (adapted from Thirumalai et al., 2014)

The most recent investigations have focused on unravelling the Pliocene Warm Period, a period proposed as a possible model for future climate. The analysis of the evolution of the major ice sheets and the temperature of the oceans indicates that during the middle part of the Pliocene epoch (3.3 Ma–3 Ma), global warmth reached temperatures similar to those projected for the end of this century, about 2°–3°C warmer globally on average than today.

The mid-Pliocene is used as an analog to a future warmer climate because it’s geologically recent and therefore similar to today in many aspects like the land-sea configuration, ocean circulation, and faunal and flora distribution. Mid- Pliocene sediments containing fossil proxies of climate are abundant worldwide, and many mid- Pliocene species are extant, making faunal and floral paleotemperature proxies based on modern calibrations possible (Robinson et al., 2012).

Surface air temperature anomalies of (top) the late 21st century and (bottom) the mid-Pliocene (from Robinson et al., 2012)

Surface air temperature anomalies of (top) the late 21st century and (bottom) the mid-Pliocene (from Robinson et al., 2012)

Foraminiferal Mg/Ca data suggest that the Pliocene tropics were the same temperature or cooler than present. At high latitudes, mid- Pliocene sea surface temperatures (SSTs) were substantially warmer than modern SSTs. These warmer temperatures were reflected in the vegetation of Iceland, Greenland, and Antarctica. Coniferous forests replaced tundra in the high latitudes of the Northern Hemisphere. Additionally, the Arctic Ocean may have been seasonally free of sea-ice, and were large fluctuations in ice cover on Greenland and West Antarctica (Dolan et al., 2011; Lunt et al., 2012).  These results highlights the importance of the Pliocene Warm Period to better understand future warm climates and their impacts.

Reference:

David Evans, Chris Brierley, Maureen E. Raymo, Jonathan Erez, Wolfgang Müller; Planktic foraminifera shell chemistry response to seawater chemistry: Pliocene–Pleistocene seawater Mg/Ca, temperature and sea level change; Earth and Planetary Science Letters, Volume 438, 15 March 2016, Pages 139-148

Jochen Knies, Patricia Cabedo-Sanz, Simon T. Belt, Soma Baranwal, Susanne Fietz, Antoni Rosell-Mel. The emergence of modern sea ice cover in the Arctic Ocean. Nature Communications, 2014; 5: 5608 DOI: 10.1038/ncomms6608

Robinson, M.; Dowsett, H. J.; Chandler, M. A. (2008). “Pliocene role in assessing future climate impacts”; Eos 89 (49): 501–502.

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