Alkaline earth metal isotopes as a novel tool to constrain water sources and changes in carbonate precipitation in the lagoon-estuarine environments

Y. (Mandy) Shao1,*, J. Farkaš1,2, L. Mosley3, J. Tyler1, H. Wong4, M. Samanta1, C. Holmden5, B. M.

Gillanders3, A. Kolevica6 and A. Eisenhauer6

1Department of Earth Sciences, University of Adelaide, Australia 2Department of Environmental Geosciences, Czech University of Life Sciences, Czech Republic3Department of Ecology and Evolutionary Biology, University of Adelaide, Australia4Australian Nuclear Science and Technology Organisation, Australia5Department of Earth Sciences, University of Saskatchewan, Canada6GEOMAR, Helmholtz Centre for Ocean Research Kiel, Germany

*correspondence: yuexiao.shao@adelaide.edu.au

8th Nov. 2019

Past Modern/Future

Paleo-hydrology

Water source mixing

water cycle

water resources

environmental sustainability

Decision making

Tracers

(87Sr/86Sr,

δ88/86Sr)

Fossil shells

Refer back

Land-ocean interface

A broad picture…

• Climate change

• Anthropogenic (human)

disturbances

Alkaline earth metal elements??

Our Tools:

Strontium (Sr)

• 87Sr/86Sr – radiogenic

• 88Sr/86Sr – stable

The Coorong– a natural isotope laboratory

(Gillanders and Munro, 2012)

Lake

Alexandrina

N. Lagoon S. Lagoon

Hypersaline

Modified from Shao et al. (2018)

- Unique geomorphology and large salinity

gradient!

Sea-

water

Know the past for a better future…

• Problems of the wetland site

➢Difficulties in freshwater management

➢The Millennium Drought (late 90s to 2010)

➢Changes to ecological condition

➢Decreased tourism

• Murray-Darling Basin Plan: “$8 billion spent and still the Coorong wetland is dying”

• What was the past like? More saline or fresh?

Algae-covered ground at the South Lagoon

In this talk:

Water samples Shell samples

• Introduction to strontium isotope systems (87Sr/86Sr and δ88/86Sr)

• Application of strontium isotopes in the modern Coorong

➢Water source mixing and carbonate fluxes

➢The role of the South Coorong Lagoon as a sink of DIC – linked to blue carbon

• Application of strontium isotopes in paleo-hydrology studies

➢Calibration of δ88/86Sr against δ44/40Ca and salinity in the Coorong water

➢Calibration of δ88/86Sr in shells (A. helmsi) and local waters in the Coorong

➢87Sr/86Sr and δ88/86Sr in shells (A. helmsi) from a South Lagoon sediment core

Arthritica helmsi (aragonitic)(Chamberlayne, 2015)

Stable Strontium Isotope Systematics

Radiogenic Strontium Isotope (87Sr/86Sr) System

Tracer of radiogenic processes:

• Provenance identification

• Global weathering

• Water source mixing

Stable Strontium Isotope Systematics

Stable Strontium Isotope (88Sr/86Sr) System

d88/86Sr =

88Sr86Sr sample

88Sr86Sr standard

- 1 *103

Standard used: SRM/NBS987

Measured by TIMS with Double-Spike correction

• Sensitive to mass-dependent isotope fractionation processes

(e.g. carbonate precipitation/dissolution)

• ∆88/86Sr (δ88/86Srcarb- δ88/86Srwater) – independent of

environmental parameters (i.e., temperature, pH, salinity or

carbonate ion concentration) (Raddatz et al., 2013; Vollstaedt

et al., 2014; Fruchter et al., 2016)

• Good indicator of paleo-water Sr isotope signatures

The two-dimensional isotope tracers – 87Sr/86Sr and δ88/86Sr in the modern Coorong

• Major water components - fresh, brackish, marine and hypersaline

• 87Sr/86Sr – water source mixing

• δ88/86Sr – carbonate precipitation/dissolutionR

ad

iog

en

ic S

r is

oto

pe

ra

tio

Stable Sr isotope ratio

Carbonate fluxes in the Coorong – Saturation indices (SI) by PHREEQC

SI = 0 just saturated

SI > 0 supersaturated

SI < 0 undersaturated

Saturation Index (SI)

SI = Log

Carbonate precipitation in the Coorong – field evidence

(Haese et al., 2008)

Carbonate ‘tufa’

Policeman Point

Strand pools

Stony Well

Active seep showing

agitated sediment

south of Parnka Point

(Zeebe and Wolf-Gladrow, 2001)

The South Lagoon acts as an efficient sink of carbon

due to ongoing CaCO3 formation (mostly as aragonite)

Coorong δ88/86Sr vs δ44/40Ca and salinity

δ44/40Ca data extracted from Shao et al. (2018)

Fractionation of δ88/86Sr – biogenic carbonates (A. helmsishells) vs. water

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

35.59 35.6 35.61 35.62 35.63 35.64 35.65

δ8

8/8

6Sr

(‰

)

Latitude (S)

water

shells

∆88/86Sr (δ88/86Srcarb- δ88/86Srwater) = -0.09 ‰ (constant)

The basis of the paleo-hydrology reconstruction!

Arthritica helmsi

(aragonitic)(Chamberlayne, 2015)

Sampled by Ms. Deborah Haynes in 2005 and 2006.

Coring site C18

N. Lagoon S. Lagoon

Parnka Pt.

Age Model by C-14 dating

Coorong southern lagoon: Core C18

smooth spline 0.3

(Chamberlayne, 2015)

Salt Creek

C18

Paleo-hydrology of the Coorong

Sediment core C18

Fossil A. helmsi

• Gentle increasing trends

• ~600-year cycles?

• Has never been purely marine in the past 2500+ yrs

Age (years BP)

0.70916

0.70917

0.70918

0.70919

0.7092

0.70921

0.70922

0.70923

0.70924

0.70925

0.70926

0.32

0.34

0.36

0.38

0.40

0.42

0.44

0.46

0.48

0.50

0 10 20 30 40 50 60 70 80 90 100 110 120 130

87S

r/86S

r

δ88/8

6S

r (

‰)

Depth (cm)

Seawater

Seawater

Paleo-hydrology of the Coorong

Sediment core C18

Fossil A. helmsi

Increased continental influence, evaporation/salinity

and carbonate precipitation?

Summary

• Coupling 87Sr/86Sr and δ88/86Sr can constrain water mixing and carbonate precipitation/dissolution in the modern Coorong, also forms a two-dimensional tracers to understand the controlling processes of the local hydrology in the modern Coorong

• δ88/86Sr and mineral saturation modelling suggests that the South Lagoon acts as an efficient sink of carbon due to ongoing CaCO3 formation (mostly as aragonite)

• Similar fractionation behaviour in stable Sr and Ca isotopes (i.e, δ88/86Sr and δ44/40Ca) is confirmed, and results suggest systematic increasing trend of δ88/86Sr with salinity in the Coorong waters

• Fractionation between living A. helmsi shells and local water in the Coorong ∆88/86Sr (δ88/86Srcarb- δ88/86Srwater) = -0.09 ‰ (constant), forming the basis of paleo-hydrology reconstruction

• 87Sr/86Sr and δ88/86Sr in the A. helmsi shells from the sediment core C18 indicate more continental input and increasing carbonate precipitation in the South Lagoon since ~2000 yrs BP (more measurements to be done).

Acknowledgements

Dr. Juraj Farkaš Dr. Jonathan Tyler Prof. Bronwyn

Gillanders

Ms. Deborah

Haynes

Ms. Briony

Chamberlayne

Prof. Luke Mosley

• Adelaide Graduate

Research Scholarship

(AGRS)

• CRC LEME Regolith

Science Scholarship

• EU funding from Dr.

Juraj Farkaš

• Staff from the University

of AdelaideMr. David Bruce

Dr. Moneesha

Samanta

Dr. John Tibby

- The end ☺

Thank you for listening!!

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