c. Shells with PL, SL and TL Liothyrella neozelanica 100 µm PL SL TL 1cm b. Shells with PL and SL Terebratalia transversa 100 µm PL SL 1cm PL: Outer primary layer, made of acicular calcite. SL: Inner secondary layer, made of calcite fibers. TL: Tertiary layer, made of columnar calcite crystals. a. Shells with only PL Pajaudina atlantica 100 µm PL (all the shell) 1cm The shell microstructure Eight modern brachiopod species were selected. Using the scanning electron microscope, three main shell microstructures were identified: shells made of a) only primary layer (PL) (Pajaudina atlantica); b) primary and secondary fibrous layer (SL) (Terebratalia transversa, Magasella sanguinea, Calloria inconspicua, Notosaria nigricans and Magellania venosa) and c) primary, secondary and tertiary columnar layer (TL) (Liothyrella neozelanica and Gryphus vitreus). Oxygen isotope compositions were measured in situ using the ion microprobe technique and trace element contents by Laser ablation coupled to an ICP-MS. Material & Methods Fossil brachiopods have been extensively used to reconstruct physicochemical conditions of ancient oceans due to their extensive fossil record and shells made of stable low-Mg calcite. In this context, it is important to assess the impact of brachiopod shell biomineralization processes on geochemical proxies. In this study, we analysed the variability of δ 18 O values and trace element ratios in the shell microstructures of modern brachiopods, in order to assess which brachiopod shell portions or taxa are the most reliable for reconstructing paleoenvironmental conditions. δ 18 0, Mg/Ca, Sr/Ca (temperature) δ 13 C (DIC) δ 11 B (pH) δ 44 Ca, δ 26 Mg, δ 7 Li (weathering) δ 18 0 δ 13 C δ 7 Li δ 26 Mg δ 11 B δ 44 Ca Equilibrium? Mg/Ca Sr/Ca Brachiopod geochemistry as potential paleoenvironmental proxies Trace Element ratios (TE) in modern brachiopod shells b) Terebratalia transversa In shells made of PL or PL and SL, there is always an abrupt decrease from outer to inner part, where a constant value is reached. This is likely due to kinetic effects within the shell microstructure. The PL, SL and TL have different trace element incorporation signatures. This is likely due to chemical modifications of the internal fluid from which calcite precipitates the different shell microstructures (e.g. the biological discrimination against Mg and Na in the internal fluid, in which the SL precipitates). 0 1 2 3 0 25 50 75 100 D TE braq /D TE in.calcite Relative distance (%) Li Na Mg Sr c) Liothyrella neozelanica c) Depleted in trace elements relative to equilibrium. 0 1 2 3 0 25 50 75 100 D TE braq /D TE in.calcite Relative distance (%) Li Na Mg Sr a) Pajaudina atlantica a) Abrupt decrease from outer to inner “Steady state” zone in the innermost PL. Enriched in trace elements relative to equilibrium. b) Decrease from outer to inner “Steady state” zone in the innermost SL. PL SL TL Conclusions 1.The best shell portion to use for δ 18 O studies, when present, is the tertiary layer , and if not, the innermost secondary layer. The Tertiary layer is in δ 18 O equilibrium with seawater. The innermost secondary layer is in or near equilibrium. 2.The best shell portion to use for trace element studies is the innermost secondary layer. 3.The tertiary layer is depleted in trace elements relative to equilibrium. This part is not suitable for isotopic studies of trace elements (e.g. δ 7 Li, δ 11 B) due to its very low content 4.The primary layer has to be avoided for both, δ 18 O and trace element studies. Best parts to use as proxies PL SL TE ratios TL δ 18 O values 0 1 2 3 0 25 50 75 100 D TE braq /D TE in.calcite Relative distance (%) Li Na Mg Sr Assessing the biomineralization processes in the shell microstructure of modern brachiopods: variations in the oxygen isotope composition and minor element ratios Sara Milner 1 *, Claire Rollion-Bard 1 , Pierre Burckel 1 , Adam Tomašových 2 , Lucia Angiolini 3 and Daniela Henkel 4 (1)Institut de Physique du Globe de Paris, Paris, France, (2) Earth Science Institute, Slovak Academy of Sciences, Bratislava, Slovakia, (3) Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Milano, Italy, (4) GEOMAR Helmholtz-Zentrum für Ozeanforschung, Kiel, Germany *corresponding author: [email protected] 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 25 50 75 100 Proportion of TE relative to PL Relative distance (%) Li Na Mg Sr 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 25 50 75 100 Proportion of TE relative to PL Relative distance (%) Li Na Mg Sr δ 18 O values in modern brachiopod shells a) Pajaudina atlantica -6 -5 -4 -3 -2 -1 0 1 2 3 0 25 50 75 100 δ 18 O (‰)PDB c) Liothyrella neozelanica PL SL TL Relative distance (%) PL: depleted in 18 O relative to equilibrium in the outermost part. Towards equilibrium in the innermost part. SL: Towards equilibrium δ 18 O values. TL: Closest to equilibrium δ 18 O values. b) Terebratalia transversa Relative distance (%) There is a general trend towards equilibrium values from outer to inner part of the shell, as in Cusack et al., 2012. This isotopic variations within the same shell microstructure is likely due to kinetic effects, with δ 18 O equilibrium achieved as the shell becomes mature and precipitation rate slows. -4 -3 -2 -1 0 1 2 0 25 50 75 100 δ 18 O (‰) PDB outer layer inner layer Relative distance (%) -7 -6 -5 -4 -3 -2 -1 0 0 25 50 75 100 δ 18 O (‰)PDB Expected δ 18 O equilibrium values (Brand et al., 2013) D TEbraq , D TEin.calcite : partition coefficient of trace elements in brachiopod calcite and inorganic calcite, respectively
Assessing the biomineralization processes in the …...*corresponding author: [email protected] 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 25 50 75 100 Relative distance (%) Li Na Mg Sr 0.2 0.4 0.6
Jun 20, 2020
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c. Shells with PL, SL and TL Liothyrella neozelanica
100 µm
PL SL
TL
1cm
b. Shells with PL and SL Terebratalia transversa
100 µm
PL
SL
1cm
PL: Outer primary layer, made of acicular calcite.
SL: Inner secondary layer, made of calcite fibers.
TL: Tertiary layer, made of columnar
calcite crystals.
a. Shells with only PL Pajaudina atlantica
100 µm
PL (all the shell)
1cm
The shell microstructure
Eight modern brachiopod species were selected. Using the scanning electron
microscope, three main shell microstructures were identified: shells made of a) only
primary layer (PL) (Pajaudina atlantica); b) primary and secondary fibrous layer (SL)
(Terebratalia transversa, Magasella sanguinea, Calloria inconspicua, Notosaria
nigricans and Magellania venosa) and c) primary, secondary and tertiary columnar
layer (TL) (Liothyrella neozelanica and Gryphus vitreus).
Oxygen isotope compositions were measured in situ using the ion microprobe
technique and trace element contents by Laser ablation coupled to an ICP-MS.
Material & Methods
Fossil brachiopods have been extensively used to reconstruct physicochemical conditions of ancient oceans due to their extensive fossil record and shells made of stable low-Mg calcite. In this context, it is important to assess the impact of brachiopod shell biomineralization processes on geochemical proxies.
In this study, we analysed the variability of δ18O values and trace element ratios in the shell microstructures of modern brachiopods, in order to assess which brachiopod shell portions or taxa are the most reliable for reconstructing paleoenvironmental conditions.
δ180, Mg/Ca, Sr/Ca (temperature)
δ13C (DIC) δ11B
(pH)
δ44Ca, δ26Mg, δ7Li (weathering)
δ180 δ13C
δ7Li
δ26Mg
δ11B δ44Ca
Equilibrium?
Mg/Ca Sr/Ca
Brachiopod geochemistry as potential paleoenvironmental proxies
Trace Element ratios (TE) in modern brachiopod shells b) Terebratalia transversa
In shells made of PL or PL and SL, there is always an abrupt decrease from outer to
inner part, where a constant value is reached. This is likely due to kinetic effects
within the shell microstructure.
The PL, SL and TL have different trace element incorporation signatures. This is likely
due to chemical modifications of the internal fluid from which calcite precipitates
the different shell microstructures (e.g. the biological discrimination against Mg
and Na in the internal fluid, in which the SL precipitates).
0
1
2
3
0 25 50 75 100
DTE
bra
q/D
TEin
.ca
lcite
Relative distance (%)
Li Na
Mg Sr
c) Liothyrella neozelanica
c) Depleted in trace elements relative to equilibrium.
0
1
2
3
0 25 50 75 100
DTE
bra
q/D
TEin
.ca
lcite
Relative distance (%)
Li Na
Mg Sr
a) Pajaudina atlantica
a) Abrupt decrease from outer to inner “Steady state” zone in the innermost PL. Enriched in trace elements relative to equilibrium.
b) Decrease from outer to inner “Steady state” zone in the innermost SL.
PL SL TL
Conclusions 1. The best shell portion to use for δ18O
studies, when present, is the tertiary layer, and if not, the innermost secondary layer.
The Tertiary layer is in δ18O equilibrium w i t h s e a w a t e r. T h e i n n e r m o s t secondary layer i s in o r near equilibrium.
2. The best shell portion to use for trace element studies is the innermost secondary layer.
3. The tertiary layer is depleted in trace elements relative to equilibrium. This part is not suitable for isotopic studies of trace elements (e.g. δ7Li, δ11B) due to its very low content
4. The primary layer has to be avoided for both, δ18O and trace element studies.
Best parts to use as proxies
PL SL TE ratios TL
δ18O values
0
1
2
3
0 25 50 75 100
DTE
bra
q/D
TEin
.ca
lcite
Relative distance (%)
Li Na
Mg Sr
Assessing the biomineralization processes in the shell microstructure of modern brachiopods: variations in the oxygen isotope composition and minor element ratios
Sara Milner1*, Claire Rollion-Bard1, Pierre Burckel1, Adam Tomašových2, Lucia Angiolini3 and Daniela Henkel4 (1)Institut de Physique du Globe de Paris, Paris, France, (2) Earth Science Institute, Slovak Academy of Sciences, Bratislava, Slovakia, (3) Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Milano, Italy, (4) GEOMAR Helmholtz-Zentrum für
Ozeanforschung, Kiel, Germany *corresponding author: [email protected]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 25 50 75 100
Pro
po
rtio
n o
f TE
rela
tive
to
PL
Relative distance (%)
Li Na Mg Sr
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 25 50 75 100
Pro
po
rtio
n o
f TE
rela
tive
to
PL
Relative distance (%)
Li Na
Mg Sr
δ18O values in modern brachiopod shells a) Pajaudina atlantica
-6
-5
-4
-3
-2
-1
0
1
2
3
0 25 50 75 100
δ18 O
(‰
)PD
B
c) Liothyrella neozelanica
PL SL TL Relative distance (%)
PL: depleted in 18O relative to equilibrium in the outermost part. Towards equilibrium in the innermost part.
SL: Towards equilibrium δ18O values. TL: Closest to equilibrium δ18O values.
b) Terebratalia transversa
Relative distance (%)
There is a general trend towards equilibrium values from outer to inner part of the shell, as in Cusack et al., 2012.
This isotopic variations wi th in the same she l l microstructure is likely due to kinetic effects, with δ18O equilibrium achieved as the shell becomes mature and precipitation rate slows.
-4
-3
-2
-1
0
1
2
0 25 50 75 100
δ18 O
(‰
) PD
B
outer layer inner layer Relative distance (%)
-7
-6
-5
-4
-3
-2
-1
0
0 25 50 75 100
δ18 O
(‰
)PD
B
Expected δ18O equilibrium values (Brand et al., 2013)
DTEbraq, DTEin.calcite: partition coefficient of trace elements in brachiopod calcite and inorganic calcite, respectively
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