strontium isotope composition of sedimentary rocks and its...

A N N A L E S

2013

ABSTRACT

Strontium isotope composition of seawater is homogenous in all oceans and marginal seas but

Because of a change in the strontium isotope ratio between sea- and freshwaters it can also be em-ployed for the determination of palaeosalinities or the reconstruction of diagenetic processes of marine

-

-86 87 88 87Sr is radio-

genic and forms continuously during decay of 87

of 87Rb isotope the 87 86Sr ratios of Earth crust have increased from the initial

87 86Sr ratios become increasingly higher and more and more dissimilar from the ratios of

Strontium isotope composition of sedimentary rocks and its application to chemostratigraphy and palaeoenvironmental reconstructions

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Hubert Wierzbowski

The seawater strontium isotope composition is uniform across modern 6

3

87 86

has changed through geologic history of the Earth owing to changing inputs of

87 86 -87 86

87 86Sr ratios

87 86Sr ratio of continental strontium may have varied -

87 86Sr ratio

-

Time4.55 Ga Present

MORB

0.7200

0.7100

0.70500.7025

Continentalcrust

Bulk lithosphere8786

Sr/

Sr

0.6989Primordial Earth

25Strontium isotope composition of sedimentary rocks and its application...

87 86Sr ratio regarded as a mixture of strontium contributed by

represent different 87 86

The temporal seawater strontium isotope curve is characterized by several

An almost continuous increase of the seawater 87 86Sr ratio has occurred since

The seawater strontium isotope curve may be used to date marine sedimen-

in the isotope composition of strontium may also be an indicator of diagenetic

Cont

inent

al Sr

Sr/

Sr =

0.7

11

87

86

Hydrothermal Sr

Sr/Sr = 0.703

87

86

Diagenetic SrSr/ Sr = 0.70887 86

0

10

20

30

40

50

60

70

80

90

100%0

10

20

30

40

50

60

70

80

90

100%

0102030405060708090100%

0.7050.706

0.7070.708

0.709

Modern diagenetic Sr flux

26 Hubert Wierzbowski

87 86

The aim of the present contribution is not to repeat information and interpre-tation of the 87 86Sr data given elsewhere but focus on practical aspects of using

-

from 200 to 10 000_waters may be studied based on analyses of strontium preserved in authigenic min-

-

CenozoicCretaceousJurassicTriassicPerm.Carbon.Devon.Sil.Ordov.Cambr.

0.7095

0.7090

0.7085

0.7080

0.7075

0.7070

0.7065

0 Ma50100150200250300350400450500


-ommended by some authors for impure samples to avoid dissolution of clay con-

Strontium isotope composition is traditionally reported as 87 86Sr ratio be--

fore reporting measured 87 86Sr ratios are normalized to a standard 86 88Sr ratio 86 87Sr and 88Sr isotopes

87 86Sr ratios are normalized to the recommended ratios of isotope standards analysed along with studied sam-

-

-

86 88Sr ratio of ±2 10-5 is

allow better precision of replicated strontium isotope analyses of ±3 10-6

Strontium isotope composition of sedimentary rocks may be affected by dia---

samples is therefore 1 for all chemostratigraphy applications

Skeletal calcites can be screened for the state of preservation using chemical -

tion often causes an increase in Fe and Mn contents of calcite and a decrease in its Sr contents as concentrations of these elements drastically differ between seawater

comparison between measured element concentrations and concentrations found in modern marine shells or predicted for intact marine calcites permits an assess-

indicate different threshold levels of element concentrations in well-preserved

1


Mn (ppm) Fe (ppm) Sr (ppm)

- -

-

- -

-

-

-

-

Mn2+ ions are also an activator of orange-red cathodoluminescence that is dis-tinctive of diagenetically altered calcites but high Fe2+ contents may quench their

environments where Mn2+

-nescent bands may occur in modern brachiopod shells in areas of slow growth rate

-mend staining thin sections made from calcitic fossils with potassium ferricyanide and alizarine red-S to screen for the replacement of Ca2+ by Fe2+

is because of the fact that calcite zones with higher iron contents due to its quench-

brachiopod shells

Mn (ppm) Fe (ppm) Sr (ppm)

Morrison and Brand

-

-


Low-magnesium calcite is relatively insoluble therefore shows high potential -

87 86Sr predicted for

-structure observation and comparisons to the 87 86Sr ratios of coeval samples from different sites and localities were employed to increase the reliability of bulk

Fossil biogenic apatites can be recrystallized during early or late stages of -

REE enrichment is also reported to activate orange-brown cathodoluminescence

-

exchange Sr isotopes with pore waters the reliability of conodont and fish tooth 87 86

isotope composition of well-preserved dental apatites is also found to be a proxy

-scence is activated by Mn2+


87 86Sr ratios of well-preserved samples may be used to date and correlate 87 86Sr ratio to previously construct-

precision of the strontium isotope dating depends on the analytical precision of an

-

-

Altered samples show higher and scattered 87 86

987 87 86

0.707434

0.70680

0.70685

0.70690

0.70695

0.70700

0.70705

0.70710

Parkin.

Zigzag

Tenuipl.P

rograc.S

ubcontr.M

orrisiB

remeri

Retrocost.

Discus

Herveyi

Koenigi

Callov.

Jason

Coronat.

Athleta

Lamberti

LateB

ajocian

Early

Bat honian

Middle

Bathonian

LateB

athonian

Early

Call ovian

Middle

Callovian

LateC

allovian

Chron

Substage

1

11

1

22

22

11

1

1

8786

Sr/

Sr

168.3 1.3 Ma± 166.1 1.2 Ma± 163.5 1.1 Ma±

2

195% confidence limitsLowess fitdating error

data from Callomon andDietl (2000)

data from Jones et al. (1994)

data from Wierzbowskiet al. (2012)

Diagenetically altered samples:data from Jones et al. (1994)

data from Podlaha et al. (1998)

Well-preserved samples:

2

1

0.70715


Seawater strontium isotope curves have been constructed for several strati-

-able on request for scientific purposes from in electronic

also a reason for the necessity of updating the trends with amended time scales

Strontium isotopes can be used to determine palaeosalinities of ancient brack-ish waters owing to differences in strontium isotope ratios and concentrations

87 86Sr

= 90 87 86 -

ern average CSr-

ly high 87 86

draining young volcanic arcs show low 87 86Sr ratios and high strontium con-

87 86Sr ratios and intermediate to 87 86Sr ratios and significant

The strontium isotope composition of brackish waters forms a hyperbolic re-


87 86 87 86 87 86 are strontium isotope ratios of brac-C and C concentrations of

-

87 86Sr ratios and Sr concentrations of Avon and Mur-87 86Sr ratios and Sr con-

S is a salinity factor of brackish water calculated as a fraction of freshwa-

– –

and -


87 86

CSr87 86

Sr = 90_

on 87 86 87 86

The chemistry of end-members may be assumed as similar to modern ana-

87 86Sr ratios do not allow precise re-

members mixing models are also not valid in evaporation-dominated basins due

Since 87 86Sr ratio of marine carbonates is affected by diagenetic fluids it can

and marine rocks lead to the re-equilibration of original element and isotope com-

0,707000

0.712

0

Salinity [‰]

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

0.711

0.710

0.709

0.708

8786

Sr/

Sr

87 86Sr/ Sr = 0.709174seawater



Minimal measurable effectof salinity on Sr/ Sr ratio87 86

12‰

14‰

16‰

Hubert Wierzbowski

-

18

87 86

87 86

87 86 -

An increase in 87 86Sr ratios of aragonite ammonite shells was reported to be caused by the adsorption of diagenetic strontium on aragonite with minor altera-

87 86Sr ratios of marine carbonates resulting from the non-typical alteration by hydrothermal fluids have

87 86Sr ratios of the belemnite rostra have also been used as a precise and inde-

18 87 86Sr

18 87 86Sr

recrystallization

4

0

-4

-8

ä18O

[‰]

87 86Sr/ Sr0.708 0.709 0.710

0.4

N = 410

Marinelimestone

Diageneticlimestone

50%

Water-rockinteractionK = 0.15D

Sr

Mixing of mineralend-members

1.3

180 [

‰]


The present study aims to present the most popular applications of strontium

---

Further less common aspects of strontium isotope geochemistry in marine -

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strontium isotope composition of sedimentary rocks and its...

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