Innovative method opens up new perspectives for reconstructing climatic conditions of past eras
FRANKFURT. Corals and cave carbonates are important archives
of past climate. This is because the composition of these carbonate deposits can
reveal the temperatures that prevailed at the Earth’s surface at the time they formed.
An international team of geoscientists led by Goethe University Frankfurt,
Germany, has now developed a new method that makes it possible to identify whether
the composition of these deposits was exclusively controlled by temperature, or
if the formation process itself exerted an additional control. The new method allows
scientists to determine past Earth surface temperatures more reliably and to
study the processes involved in calcareous skeleton formation of modern and
extinct species. (Nature Communications, DOI
10.1038/s41467-020-17501-0)
Corals precipitate their calcareous skeletons
(calcium carbonate) from seawater. Over thousands of years, vast coral reefs
form due to the deposition of this calcium carbonate. During precipitation,
corals prefer carbonate groups containing specific variants of oxygen (chemical
symbol: O). For example, the lower the water temperature, the higher the
abundance of a heavy oxygen variant, known as isotope 18O, within
the precipitated carbonate. Unfortunately, the 18O abundance of the
seawater also influences the abundance of 18O in the calcium
carbonate – and the contribution of 18O from seawater cannot be
resolved when determining temperatures based on carbonate 18O
abundances alone.
A great step forward was the discovery
that the isotopic composition of the precipitated carbonate allows temperature
determinations independent of the composition of the water if the abundance of
a specific, very rare carbonate group is measured. This carbonate group
contains two heavy isotopes, a heavy carbon isotope (13C) and a
heavy oxygen isotope (18O) which are referred to as “clumped
isotopes”. Clumped isotopes are more abundant at lower temperatures.
However, even with this method there was
still a problem: The mineralization process itself can affect the incorporation
of heavy isotopes in the calcium carbonate (kinetic effects). If unidentified, the
bias introduced by such kinetic effects leads to inaccurate temperature
determinations. This particularly applies for climatic archives like corals and
cave carbonates.
An international research group led by
Professor Jens Fiebig at the Department of Geosciences at Goethe University Frankfurt
has now found a solution to this problem. They have developed a highly
sensitive method by which – in addition to the carbonate group containing 13C
and 18O – the abundance of another, even rarer carbonate group can
be determined with very high precision. This group also contains two heavy
isotopes, namely two heavy oxygen isotopes (18O).
If the theoretical abundances of these two
rare carbonate groups are plotted against each other in a graph, the influence
of the temperature is represented by a straight line. If, for a given sample,
the measured abundances of the two heavy carbonate groups produce a point away
from the straight line, this deviation is due to the influence of the
mineralization process.
David Bajnai, Fiebig’s former PhD student,
applied this method to various climatic archives. Among others, he examined various
coral species, cave carbonates and the fossil skeleton of a squid-like
cephalopod (belemnite).
Today, Dr. Bajnai is a post-doctoral
researcher at the University of Cologne. He explains: “We were able to show
that – in addition to temperature – the mechanisms of mineralization also
greatly affect the composition of many of the carbonates that we examined. In
the case of cave carbonates and corals, the observed deviations from the exclusive
temperature control confirm model calculations of the respective mineralization
processes conducted by Dr. Weifu Guo, our collaborator at the Woods Hole Oceanographic
Institution in the USA. The new method, for the first time, makes it possible
to quantitatively assess the influence of the mineralization process itself. This
way, the exact temperature of carbonate formation can be determined.”
Professor Jens Fiebig is convinced that the
new method holds great potential: “We will further validate our new method and
identify climatic archives that are particularly suitable for an accurate and
highly precise reconstruction of past Earth surface temperatures. We also intend
to use our method to study the effect that anthropogenic ocean acidification has
on carbonate mineralization, for instance in corals. The new method might even allow
us to estimate the pH values of earlier oceans.” If all this succeeds, the
reconstruction of environmental conditions that prevailed throughout Earth’s
history could be greatly improved, he adds.
Publication:
David Bajnai, Weifu Guo, Christoph Spötl,
Tyler B. Coplen, Katharina Methner, Niklas
Löffler, Emilija Krsnik, Eberhard
Gischler, Maximilian Hansen, Daniela Henkel, Gregory D. Price, Jacek Raddatz,
Denis Scholz, Jens Fiebig: Dual clumped
isotope thermometry resolves kinetic biases in carbonate formation temperatures,
Nature Communications, DOI 10.1038/s41467-020-17501-0,
http://www.nature.com/ncomms
Further
information:
Professor Jens Fiebig
Department of Geosciences
Goethe University Frankfurt
Tel.: +49 (0) 69 798 40182
Jens.Fiebig@em.uni-frankfurt.de
Dr. David Bajnai
Institute of Geology and Mineralogy
University of Cologne
Tel.: +49 (0)221 470 89829
David.Bajnai@uni-koeln.de
Dr. Weifu Guo
Department of Geology and Geophysics
Woods Hole Oceanographic Institution
Woods Hole, MA
USA
Tel.:
+1 508 289 3380
wfguo@whoi.edu