Constraining Arctic glacier and ice-sheet extents during past warm periods using multiple cosmogenic isotopes

Irene Schimmelpfennig Mar 21 Mai - 11:55

The ice masses in the Arctic region, in particular the Greenland Ice
Sheet (the largest ice mass in the Northern Hemisphere), play a key role in the
global climate system, as changes in their extent and volume can alter oceanic
and atmospheric circulations and cause significant sea level variations.
Dramatic and worrisome effects of the current climate warming on certain parts
of the Arctic land ice masses have been observed over the last decade, but the
poor quantitative understanding of the rate and magnitude at which Arctic
glaciers and ice caps respond to warming hamper precise estimates of future ice
melting and thus sea level rise.

Records of past land ice extents are critical for characterizing
the significance of the ongoing changes and placing modern observations into a
long-term context. In particular, assessing the natural variability of the ice
masses during past warm periods can improve numerical ice-sheet models,
and thus help better predict short-term sea level rise with the ultimate goal
to mitigate the related potential socio-economic consequences.

However, determining past Arctic land ice change beyond the historical
record has proven difficult. Preserved glacial deposits, in particular
moraines, bear witness of larger-than-today ice extents and are subject to
extensive investigation. The response
of the ice masses to warm pulses in the past, however, is much harder to
reconstruct, because the geological evidence of ice extents during periods that were as warm
as or warmer than today such as the middle Holocene (also referred to as the
“Holocene Climatic/Thermal Optimum” in the Northern Hemisphere) have since been
overrun by late Holocene glacier re-advances.

Recent breakthroughs in multiple cosmogenic isotope techniques now afford
the investigation of sub- and pro-glacial bedrock as climate archive (Goehring
et al., 2011). Measurements of the in situ produced cosmogenic nuclides 10Be,
26Al, 36Cl and 14C in freshly deglaciated or sub-glacial bedrock can determine
if and how long these rock surfaces were ice-free and thus allow past periods
of smaller-than-today land ice mass to be constrained. While developed in
mountain glacier settings, these techniques are applicable to the Arctic as
well. The methods rely on the principle that in situ cosmogenic nuclides are produced in the first few meters of
the Earth’s surface by interactions of cosmic ray particles with target
elements in the rock material. The longer the rock surface has been exposed to
cosmic radiation, the more atoms of the nuclide accumulate. Therefore, in the
simplest case of continuous and undisturbed exposure, the knowledge of a single
nuclide’s concentration in a surface sample allows the exposure duration of the
surface to be calculated.

Glacial bedrock surfaces
have usually experienced complex exposure-burial histories (alternating periods of ice-cover and deglaciation) and therefore
require the measurement of multiple in situ cosmogenic nuclides in the same
samples to explore scenarios of ice-free periods on various time-scales. The
related approaches rely on the different half-lives of the above-mentioned
nuclides (10Be: 1.4 Ma; 26Al: 720 ka; 36Cl: 310 ka; 14C: 5.7 ka), as the different decay rates of the nuclides allow the
durations of the intermittent periods of ice-cover to be quantified.

For example, glacial surfaces
that have not or have very little been abraded during periods of ice cover
(this is the case for cold-based glaciers) partly preserve the concentrations of the
long-lived cosmogenic nuclides (10Be, 26Al and 36Cl) from past ice-free periods
over several glacial/interglacial cycles as well as from the present
interglacial. The inventories of short-lived in situ 14C only stem from
ice-free periods during the Holocene, because any 14C that accumulated during
previous interglaciations has decayed away during the glacial period ice cover.

Glacial bedrock that
has been significantly abraded during periods of ice cover (temperate glaciers)
might have lost all cosmogenic nuclide inventory from previous
interglacials during the subsequent glacial cycle and has also lost some
of the nuclide concentrations accumulated during the Holocene ice-free periods.
A new approach of pairing measurements of short-lived in situ 14C and
long-lived 10Be (Goehring et al., 2011) allows this loss during the Holocene to
be quantified and thus the Holocene ice-free duration to be determined.

At CEREGE, we routinely measure the long-lived cosmogenic nuclides 10Be,
26Al and 36Cl at the national
accelerator mass spectrometry facility ASTER. In addition, a laboratory for the
extraction of in situ 14C from terrestrial quartz as well as an accelerator
mass spectrometer dedicated to radiocarbon measurements of microsamples will be
installed this year (AixMICADAS-EQUIPEX).

The prospective study described here will be conducted in collaboration with experts in the USA, in
particular at the Lamont-Doherty Earth Observatory (LDEO, Prof. Joerg Schaefer
and Dr. Nicolas Young), whose cosmogenic nuclide team is a leading pioneer in
dating of glacial landforms in various areas in the world including the Arctic
region. The postdoc period of Dr. Irene Schimmelpfennig at LDEO allowed the
technical knowledge of in situ 14C and the paired 14C/10Be methodology to be
transferred to France.

Researchers involved at CEREGE: Irene Schimmelpfennig, Edouard Bard, Didier Bourlès, Lucilla Benedetti

Citation: Goehring, B.M., Schaefer, J.M., Schluechter, C., Lifton, N.A., Finkel, R.C., Jull, a.
J.T., Akcar, N., Alley, R.B., 2011. The Rhone Glacier was smaller than today
for most of the Holocene. Geology 39, 679–682.