Assessing the biomineralization processes in the …...*corresponding author: milner@ipgp.fr 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

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: milner@ipgp.fr

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