# Extracting paleodust information from peat geochemistry

François de Vleeschouwer, M Ferrat, H Mcgowan, Heleen Vanneste, D Weiss

► To cite this version:

François de Vleeschouwer, M Ferrat, H Mcgowan, Heleen Vanneste, D Weiss. Extracting paleo-dust information from peat geochemistry. Past Global Changes Magazine, 2014, 22 (2), pp.88-89. 10.22498/pages.22.2.88. hal-05441979

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# Extracting paleodust information from peat geochemistry

François De Vleeschouwer¹,², M. Ferrat³, H. McGowan⁴, H. Vanneste¹,² and D. Weiss³

Peatlands are formidable geochemical archives of atmospheric dust and are increasingly being used to address major paleoclimatic questions. Here, we provide an insight into the recent advances.

Peatlands are increasingly used as terrestrial archives of atmospheric dust deposition. Peat records mostly cover the Holocene, although some may extend beyond 10 ka (e.g. Kylander et al. 2007). The global occurrence of peatlands makes them a good alternative for making inter-hemispheric comparisons of paleoclimate when other terrestrial records are not available. Europe, for example, lacks long Holocene atmospheric archives such as ice cores, but it contains widespread peat deposits. Similarly, in the Southern Hemisphere, peatlands provide an opportunity to probe the entire Holocene, a period for which dust records from polar ice and marine cores are generally absent or offer lower resolution.

Peatlands have two key advantages over many other paleoenvironmental archives: they are relatively easy to access and sample and their high accumulation rates allow records of sub-decadal scale to be developed. Ombrotrophic (i.e. atmosphere-fed) peatlands, or bogs, offer the greatest insights into past climate as they only receive input through the atmosphere, such as aerosols and rain, and thus truly reflect local to regional-scale atmospheric conditions (Le Roux et al. 2010; Marx et al. 2009). Minerotrophic (i.e. stream-fed) peatlands, or fens, can also provide valuable information in terms of dust sources (e.g. Kylander et al. 2007; Muller et al. 2008a,b).

Earlier studies on peatland chemistry focused on atmospheric trace metal contamination to reconstruct pollution histories (e.g. Lee and Tallis 1973; Shotyk et al. 1998). Today, studies focus more on the paleoclimatic and paleoenvironmental information of the dust trapped in peat. This includes developing tools to assess the integrity of the record (i.e. effects of post depositional element movements or groundwater input), to identify the origin of mineral dust particles and to quantifying flux changes through time.

## Flux calculation

Atmospheric dust fluxes in bogs are reconstructed from the vertical distribution of lithogenic trace elements such as Ti, Sc, Zr, Y or REE (Rare Earth Elements) in bulk peat

samples. These elements are not affected by post-depositional processes and do not have a significant anthropogenic source. In addition, it is essential to develop robust chronologies for the peat profiles based, for example, on ¹⁴C and ²¹⁰Pb dating, and on age modeling that takes into account dating and accumulation rate uncertainties (Blaauw and Christen 2011). Robust chronologies allow dust fluxes to be calculated accurately, and to establish specific tie-points and correlate them to known climate events. Dust flux records have been successfully used to pinpoint abrupt events such as the Younger Dryas (Shotyk et al. 2002; Weiss et al. 2002), the 8.2 ka event (Shotyk et al. 2002; Le Roux et al. 2012), and the Little Ice Age (De Vleeschouwer et al. 2009). Furthermore, with good knowledge of the geochemical properties of potential dust source areas it is possible to distinguish distal from local (sub-)catchment sources, and apportion their contributions through time, thereby providing insight into regional scale environmental change (Marx et al. 2009, 2010).

## Using dust records in modeling

The deposition rates determined from peat records offer a tool for validating atmospheric circulation model simulations, which in turn provide a platform for investigating the interactions between the atmospheric dust cycle and environmental change. Model validation of aerosol components is, however, still limited by the scarcity of past dust flux records.

A regional atmosphere-chemistry/aerosol climate model (REMOTE) has recently been successfully applied to simulate dust emissions and transport from the major Chinese deserts to its subsequent deposition over the Central Tibetan Plateau (Ferrat et al. 2013). This study showed that numerical modeling has the potential to reveal more about past and present dust cycles. The study also concluded, however, that further refinement of the dust and soil surface property parameterization is in particular needed in order to improve the climate model.

<figure><img src="image_2.png" /></figure>

**Figure 1:** Ti concentration and Nd isotopic composition (eNd) in a bog profile in Switzerland. The Younger Dryas (YD) is characterized by an increase in local dust input, reflected by high Ti concentrations. Early Holocene Vasset-Kilian eruptions (VKT) in the French Massif Central result in high eNd. Saharan dust input is identified by negative eNd around 8.4 ka. The progressive desertification of the Sahara during the Holocene decreases eNd progressively from 7.5 to 3.5 ka cal BP, when local erosion becomes dominant. The two dust peaks (yellow bars) at 5.9 and 7.6 ka remain unexplained. Modified from Le Roux et al. 2012.
<figure><img src="image_3.png" /></figure>

**Figure 2:** World coverage of dust records in peat, including already published peatland records (green stars) and ongoing works (red stars). Map drawn from Kohfeld and Tegen (2007), Mahowald et al. (2006) and Kohfeld and Harrison (2001).

Another opportunity of data-model symbi-
osis lies in quantifying abrupt changes (or
change points). The dust deposition record
is normally represented as a function of
depth or time. Trans-dimensional Markov
chain Monte Carlo analysis has recently been
explored to infer probability distributions
on the number, position and age of change
points, the mean distance between change
points, and the noise variance associated
with each dust record (Gallagher et al. 2011;
Kylander et al. 2013). Simulating noise in dust
records from bogs or ice cores can comple-
ment the observational information. Noise
estimates of proxy records are normally lim-
ited to the measurement uncertainty, and in
most cases it is not practical to repeat sam-
pling and measurements to experimentally
assess noise. Gallagher et al. (2011) analyzed
several sets of geochemical dust data from
peat cores taken from Australia, Sweden and
Tibet. They showed that their model-based
analyses are consistent with those previously
inferred qualitatively from independent data
and interpretations. Moreover, this approach
provides quantitative estimates of the
relative probability of the inferred change
points, allowing an objective assessment of
the significance of each change.

Sources

Lead isotopes were first used to identify
sources of anthropogenic and natural dust
in bogs (Shotyk et al. 1998). This opened a
field of investigation aimed at developing
proxies that better constrain the relative
contributions of local and regional dust
sources to peat deposits. However, lead has
been emitted by anthropogenic activities for
more than 5000 years (Nriagu 1983). This has
limited the potential of lead as a natural dust
tracer for the Holocene and has resulted in
uncertainty in dust provenance reconstruc-
tions (Kamber et al. 2010).

In order to define the provenance of dust
with greater certainty, other geochemical
tracers are increasingly used. Coupling REE
and Pb isotopes or lithogenic elements
allows us to determine shifts between wet
and dry climate (Kylander et al. 2007), to
discriminate local from distal dust (Marx et
al. 2009), and to reconstruct wind strength
and changes in atmospheric circulation
(Marx et al. 2011). In Europe, where Holocene
atmospheric archives are rare, Le Roux et al.
(2012) combined lithogenic elements, REE
and Nd isotopes to reconstruct the dust
flux and sources over the last 14 ka from a
bog in Switzerland (Fig. 1). In this study, the
interplay between climate and natural dust
draws a complex picture of the Holocene
dust sources, which are the Sahara desert,
volcanism in France, and local erosion. Dust
during the Younger Dryas dominantly origi-
nated from strong local erosion and mineral
input, while during early Holocene volcanic
eruptions and abrupt Saharan dust input
around 8.4 ka BP were the main sources. In
addition, the progressive desertification of
the Sahara, already suggested by Kylander
et al. (2005) using lead isotopes in a Spanish
peat core, was identified through a progres-
sive increase in Ti and a shift in εNd from 8 to
4 ka BP. With the same approach using REE
and Nd isotopes, Allan et al. (2013) identified
increased dust fluxes during cold periods
and large supplies from distal (Saharan
dust and Icelandic volcanism), regional
(European Loess) and local sources during
the Holocene.

Outlook

As an increasing number of studies have
shown, the geochemical separation of dusts
in peat and their provenance offers the
prospect of unmatched insight into paleo-at-
mospheric circulation, its variability and the
environmental consequences, primarily from

synoptic to macro-scales, and associated en-
vironmental changes (McGowan et al 2008;
Petherick et al. 2009). Accordingly, using
dusts as tracers of paleoclimate provides
an opportunity to calibrate and validate
climate models. Efforts are now being made
to refine the spatial coverage of peat dust
records from the high latitudes to the tropics
(Fig. 2) and extend the records back into
the Quaternary. Peat researchers are now
focusing on covering areas where dust re-
cords are lacking such as Canada, India and
the Southern Hemisphere (Patagonia, Tierra
del Fuego, South Africa, Falkland Islands,
Amsterdam Island).

AFFILIATIONS

CONTACT

François De Vleeschouwer: francois.devleeschouwer@ensat.fr

REFERENCES

Full reference list under:

www.pages-igbp.org/products/magazine/ref2014_2.pdf

Allan M et al. (2013) Clim Past 9: 2285-2298

De Vleeschouwer F et al. (2009) Holocene 19: 625-637

Ferrat M et al. (2012) Geochim Cosmochim Acta 91: 7-31

Le Roux G et al. (2012) Geology 40: 335-338

Marx SK et al. (2011) Quat Sci Rev 30: 3290-3305

<footnote n="1">¹EcoLab (Laboratory for Functional Ecology and Environment), Université de Toulouse, INP, UPS, ENSAT, Castanet Tolosan, France</footnote>

<footnote n="2">²EcoLab, CNRS, Castanet Tolosan, France</footnote>

<footnote n="3">³Department of Earth Sciences and Engineering, Imperial College London, London, UK.</footnote>

<footnote n="4">⁴Climate Research Group, The University of Queensland, Brisbane, Australia</footnote>