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Influence of palaeoweathering on trace metal
concentrations and environmental proxies in black
shales
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Marynowski, Leszek; Pisarzowska, Agnieszka; Derkowski, Arkadiusz; Rakociński, Michał; Szaniawski, Rafał;
Środoń, Jan and Cohen, Anthony S. (2017). Influence of palaeoweathering on trace metal concentrations and
environmental proxies in black shales. Palaeogeography, Palaeoclimatology, Palaeoecology, 472 pp. 177–191.
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2017 Elsevier B.V.
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http://dx.doi.org/doi:10.1016/j.palaeo.2017.02.023
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Accepted Manuscript
Influence of palaeoweathering on trace metal concentrations and
environmental proxies in black shales
Leszek Marynowski, Agnieszka Pisarzowska, Arkadiusz
Derkowski, Michał Rakociński, Rafał Szaniawski, Jan Środoń,
Anthony S. Cohen
PII: S0031-0182(16)30486-2
DOI: doi: 10.1016/j.palaeo.2017.02.023
Reference: PALAEO 8205
To appear in: Palaeogeography, Palaeoclimatology, Palaeoecology
Received date: 18 September 2016
Revised date: 10 February 2017
Accepted date: 11 February 2017
Please cite this article as: Leszek Marynowski, Agnieszka Pisarzowska, Arkadiusz
Derkowski, Michał Rakociński, Rafał Szaniawski, Jan Środoń, Anthony S. Cohen ,
Influence of palaeoweathering on trace metal concentrations and environmental proxies in
black shales. The address for the corresponding author was captured as affiliation for all
authors. Please check if appropriate. Palaeo(2017), doi: 10.1016/j.palaeo.2017.02.023
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ACCEPTED MANUSCRIPT
1
Influence of palaeoweathering on trace metal concentrations and environmental proxies
in black shales
Leszek Marynowski
1*
, Agnieszka Pisarzowska
2
, Arkadiusz Derkowski
2
, Michał Rakociński
1
,
Rafał Szaniawski
3
, Jan Środoń
2
, Anthony S. Cohen
4
1
Faculty of Earth Sciences, University of Silesia, Będzińska 60, 41-200 Sosnowiec, Poland
2
Institute of Geological Sciences, Polish Academy of Sciences – Research Centre in Kraków, ul. Senacka 1, 31-
002 Kraków, Poland
3
Institute of Geophysics, Polish Academy of Science, Ks. Janusza 64, 01-452 Warszawa,, Poland
4
Department of Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space and Astronomical
Research, The Open University, Milton Keynes, MK7 6AA, UK
*
- corresponding autor; e-mail: leszek.marynowski@us.edu.pl
Abstract
The mineralogical and chemical compositions of Lower Carboniferous (Tournaisian) marine black
shale from the Kowala quarry, the Holy Cross Mountains, Poland were investigated. This study
focuses on disturbances in palaeoenvironmental proxies caused by palaeoweathering, which
progressively changed the major and trace element abundances. Palaeomagnetic investigations reveal
that the Devonian – Carboniferous succession was weathered during the Permian-Triassic by the
infiltration of oxidizing fluids related to karstification following post-Variscan exhumation. The
weathering process led to vermiculitization of chlorite, partial dissolution of calcite and replacement of
pyrite by hematite and goethite. Moreover, the concentrations of some trace metals, including Co, Cu,
Pb, Mo, Ni, As and U, significantly decreased. Consequently, some elemental abundance ratios that
are used as environmental proxies, including U/Th, Ni/Co and V/Cr, were altered. Elements that are
bound to iron sulphides (e.g., Mo) appear to be especially prone to mobilization by even a lightly
weathered black shale. The documented weathering, including changes in elemental concentrations,
can create misinterpretations of the original palaeoenvironmental conditions. In addition, the
palaeoweathering of the studied samples appears to have substantially changed the carbon, oxygen,
nitrogen and molybdenum stable isotope values. The nitrogen and molybdenum stable isotope ratios,
in particular, appear to be most sensitive to the effects of weathering and therefore are good indicators
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of (palaeo)weathering processes. The major cause of these changes is the decay of organic matter and
pyrite. For the organic carbon stable isotopes ratios, the main factor that controlls this process appears
to be the preferential degradation of labile organic matter. A combination of the total organic carbon
(TOC), total sulphur (TS) content, Mo concentration and stable isotope compositions seems to be the
most useful to identify (palaeo)weathering. Our results suggest that reductions in TS and Mo in
tandem with diminished Mo stable isotope compositions in the absence of obvious changes to the
TOC content provide the most compelling evidence of (palaeo)weathering.
Key words: weathering; redox conditions; inorganic proxies; REE; U/Th; Gd/Yb; stable isotopes
1. Introduction
Black shales that are deposited in oxygen-deficient environments are typically enriched in
redox-sensitive elements, including sulphide-bound trace metals and elements that are involved in
biological cycles (Georgiev et al., 2012). Experimentally defined inorganic palaeoenvironmental
proxies (e.g., U/Th or Ni/Co ratios) are routinely used to reconstruct the redox conditions during the
deposition of organic-rich sedimentary rocks (e.g., Rimmer, 2004; Riquier et al., 2005, 2006; Racka et
al., 2010; Bond et al., 2013). Other commonly applied element-based indicators in geochemical
studies can estimate the palaeoproductivity and detrital input (e.g., P/Al, Ba/Al, Ni/Al, Cu/Al, Racki et
al., 2002, 2012; Tribovillard et al., 2006; Algeo and Ingall, 2007; Schoepfer et al., 2014), the influence
of volcanic activity (e.g., Zr/Al, Fe/Ti, Racki et al., 2002; Pujol et al., 2006), and the degree of
hydrothermal overprinting (e.g., Al/(Al+Fe+Mn), Fe/Ti, Racki et al., 2002; Brumsack, 2006). Carbon
and nitrogen stable isotope values are also regularly utilized in palaeoenvironmental studies of both
ancient and recent marine systems (e.g., Bauersachs et al., 2014; Obreht et al., 2014; Tulipani et al.,
2014). However, these proxies are often measured in sedimentary rock samples collected from
outcrops that have been exposed to atmospheric conditions for unknown periods. Weathering
processes may thus have altered the elemental abundances and isotopic ratios, potentially creating
misleading interpretations regarding the depositional redox conditions.
Previous studies have considered the loss of organic carbon induced by oxidation, the
theoretical modelling of weathering pathways, and general variations in kerogen composition
(Leythaeuser, 1973; Lo and Cardott, 1995; Bolton et al., 2006). Notably, pyrite appears to reacts more
rapidly with O
2
than organic matter (OM) does, creating a deeper pyrite oxidation front than the OM
weathering front (Petsch et al., 2000; Wildman et al., 2004; Bolton et al., 2006). Detailed studies of
changes in the molecular composition of both extractable and unextractable OM (Clayton and King,
1987; Littke et al., 1989; Petsch et al., 2000; Marynowski et al., 2011a & b; Tamamura et al., 2015)
have revealed the selective leaching of kerogen macromolecules and changes in soluble organic
compounds‟ concentrations and distributions from the initial degradation of less stable biomarkers and
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biomolecules. Numerous studies concluded that weathering processes can significantly influence the
interpretation of biomarkers as palaeoenvironmental proxies and indicators of OM sources (Clayton
and King, 1987; Petsch et al., 2000; Marynowski et al., 2011a & b).
The alteration of major and trace element compositions (including REEs) and stable isotope
ratios caused by secondary hypergenic processes, including weathering and biodegradation, remains
poorly understood. The mobility of rhenium and platinum group elements during weathering of
organic-rich deposits has been addressed by Peucker-Ehrenbrink and Hannigan, (2000) and Jaffe et al.
(2002), and Kolowith and Berner (2002) investigated the effects of weathering on phosphorus
concentrations. Still, knowledge of the behaviour of many other elements is scarce. Thus,
interpretations of palaeoenvironmental conditions based on conventional elemental proxies may be
compromised by the effects of weathering (Georgiev et al., 2012).
Here, we investigate variations in the chemical composition of a Lower Carboniferous
(Tournaisian) marine black shale horizon caused by palaeoweathering. We focus on progressive
changes in major and trace elements, with special attention on the disturbance of key
palaeoenvironmental proxies. In addition, we document the variations in δ
13
C, δ
15
N and δ
98
Mo from
unweathered black shale to highly weathered rock. This study presents a unique opportunity to
consider the effects of increasingly intense oxidative weathering along a single homogenous black
shale bed (Fig. 1).
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