Thallium (Tl) isotope cycling in a manganese-rich brackish meromictic pond
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Thallium (Tl) isotope cycling in a manganese-rich brackish meromictic pond Chadlin M. Ostrander Postdoctoral Scholar Department of Marine Chemistry and Geochemistry NIRVANA Lab Woods Hole Oceanographic Institution (WHOI) Co-authors Sune G. Nielsen (WHOI) Hayley J. Gadol (MIT) Tristan J. Horner (WHOI) Colleen M. Hansel (WHOI)
Why Tl isotopes?
Thallium isotopes have emerged as a promising
tool for tracking the history of molecular oxygen
(O2) in Earth’s ancient oceans.
The history of O2 in Earth’s oceans is of
broad interest because:
The origin and evolution Life on other planets
Right now, Earth’s
of life on Earth was likely is/was probably also
oceans are losing O2.
linked to O2 availability. linked to O2 availability.
How can Tl isotopes high-T hydrothermal fluids
sediment porewaters
be used to track past volcanic emissions
aerosols
marine O2? rivers
ε205TlIN = -1.8
• The Tl isotope composition (ε205Tl*) of
seawater today is enriched in the lighter-mass
Tl isotope relative to global Tl inputs. ε205Tlseawater = -6.0
*ε205Tl = (205/203Tlsample/205/203Tlstandard – 1) × 104 low-T basalt Mn oxide-rich open-ocean
alteration marine sediments anoxic sediments
ε205TlOUT = -7.4 +10 -6.0
Figure modified from Ostrander et al. (2020), Chemical Geology
How can Tl isotopes high-T hydrothermal fluids
sediment porewaters
be used to track past volcanic emissions
aerosols
marine O2? rivers
ε205TlIN = -1.8
• The Tl isotope composition (ε205Tl*) of
seawater today is enriched in the lighter-mass
Tl isotope relative to global Tl inputs. ε205Tlseawater = -6.0
• This is because Mn oxide-rich sediments
formed under O2 bearing bottom-waters
possess a strong preference for the heavier-
mass Tl isotope.
low-T basalt Mn oxide-rich open-ocean
alteration marine sediments anoxic sediments
ε205TlOUT = -7.4 +10 -6.0
Figure modified from Ostrander et al. (2020), Chemical Geology
How can Tl isotopes high-T hydrothermal fluids
sediment porewaters
be used to track past volcanic emissions
aerosols
marine O2? rivers
ε205TlIN = -1.8
• The Tl isotope composition (ε205Tl*) of
seawater today is enriched in the lighter-mass
Tl isotope relative to global Tl inputs. ε205Tlseawater = -6.0
• This is because Mn oxide-rich sediments
formed under O2 bearing bottom-waters
possess a strong preference for the heavier-
mass Tl isotope.
• Today’s light seawater ε205Tl value is globally
homogenous and transferred to sediments low-T basalt Mn oxide-rich open-ocean
deposited under anoxic conditions. alteration marine sediments anoxic sediments
ε205TlOUT = -7.4 +10 -6.0
Figure modified from Ostrander et al. (2020), Chemical Geology
By analogy:
Seawater ε205Tl in Earth’s past was
probably also set by global Mn oxide
burial, and this value can be reconstructed
using ancient marine sedimentary rocks
deposited under anoxic conditions.
Picture of the organic-rich ~2.5 billion-
year-old (Ga) Mt. McRae Shale (Western
Australia) recovered from drill core ABDP9
Indeed, sedimentary Tl isotope ratios follow predictable trends across
various past de-oxygenation/oxygenation events
ε205Tl
-6 -4 -2 0
422
OAE
424
ε205 Tl
Depth (m)
-6 -4 -2 0
14
426
OAE
Height (m)
12
428 10
8
430
Demerara Rise Furlo, Italy
black shale carbonate
Oceanic Anoxic Event 2 (~94 Ma) An early “whiff” of O2 (~2.5 Ga)
Ostrander et al. 2017, Science Advances Ostrander et al. 2019, Nature Geoscience
Because Tl isotopes are a fairly new ”paleoredox proxy”, some glaring knowledge gaps remain unfilled.
Knowledge gap examples
1. We don’t yet fully understand the links that connect Tl
isotopes to Mn oxide minerals.
2. We don’t yet fully understand how Tl and its isotopes
are cycled under different redox conditions today.We have attempted to fill these knowledge
gaps by studying Siders Pond.Siders Pond is a 15 meter
deep brackish meromictic
pond located on Cape Cod
in Massachusetts (USA)
Figure modified from Nalven
Image taken from Google Earth 2011, MBL Semester in
Environmental ScienceSiders Pond has a very
0 100 150 200 250 0 µM 50 100 150 200 250
particulate MnOx (μM) particulate MnOx (μM)
5 10 15 20 00
mg/L 5 10
1 15 220 0 1 2
0 temporally dynamic Mn
O2
2
O2 cycle, with variable Mn
oxide formation taking
4 place in surface waters.
depth (m)
6
Mn
9/25/19 9/25/19
7/31/20 7/31/20
8/14/20 8/14/20
8 9/17/20 9/17/20
11/5/20 11/5/20
10
H 2S H 2S
Fe
12
2 4 6 8 10 0 mM 2 4 6 8 10
500 1,000 1,500 0 µM 500 1,000 1,500This is important for our first knowledge
0 100 150 200 250 0 µM 50 100
particulate 150
MnOx 200
(μM) 250
particulate MnOx (μM) gap:
5 10 15 20 00
mg/L 5 10
1 15 220 0 1 2
0
1. We don’t yet fully understand the
O2 O2 links that connect Tl isotopes to
2
Mn oxide minerals.
4
depth (m)
6
Mn
9/25/19 9/25/19
7/31/20 7/31/20
8/14/20 8/14/20
8 9/17/20 9/17/20
11/5/20 11/5/20
10
H 2S H 2S
Fe
12
2 4 6 8 10 0 mM 2 4 6 8 10
500 1,000 1,500 0 µM 500 1,000 1,500If you would like to learn more about the Mn oxide cycle in
Siders Pond, go see Hayley Gadol’s e-presentation or attend
her lecture during the week of the conference!0 µM 50 100 150 200 250
particulate MnOx (μM)
Siders Pond is redox stratified,
0 mg/L 5 10 15 20 0 1 2
with ~4 meters of O2 bearing
0 (“oxic”) waters overlying anoxic
O2 Oxic waters that become especially
2
sulfidic (“euxinic”) in the deepest
portions of the pond.
4
depth (m)
6
Mn
Anoxic 9/25/19
7/31/20
8/14/20
8 9/17/20
11/5/20
10 Euxinic
H 2S
Fe
12
0 mM 2 4 6 8 10
0 µM 500 1,000 1,5000 µM 50 100 150 200 250
This is important for our second knowledge gap:
particulate MnOx (μM)
0 mg/L 5 10 15 20 0 1 2
0 2. We don’t yet fully understand how Tl and
O2 Oxic its isotopes are cycled under different
2
redox conditions today.
4
depth (m)
6
Mn
Anoxic 9/25/19
7/31/20
8/14/20
8 9/17/20
11/5/20
10 Euxinic
H 2S
Fe
12
0 mM 2 4 6 8 10
0 µM 500 1,000 1,500Our goals for the 2020 field season:
1. Measure ε205Tl values in surface waters and
particles on various days throughout the field
season.
To understand how Tl isotopes respond to temporal
variations in Mn oxide cycling.
2. Generate a full water column and sediment ε205Tl
profile.
To understand how Tl isotopes are cycled between waters
and sediments under different redox conditions.Our methods: Waters and particles were collected from a deep portion of the pond using a peristaltic pump and 0.22 µm filters. Sediments were collecting using a dredge and also by simply pumping material from the pond bottom using the peristaltic pump. Both methods revealed indistinguishable results.
Results: water column data
DO (mg/L) Tldissolved (pM) ε205Tldissolved
0 5 10 15 0 20 40 60 -6 -4 -2 0 2
0
3
Legend:
7/31/20
Depth (m)
6
8/14/20
9/17/20
9
11/5/20
12
15Results: particle data
MnOxparticulate (uM) Tlparticulate (pM) ε205Tlparticulate
0 1 2 0.01 1 100 -6 -4 -2 0 2
0
3
Legend:
7/31/20
Depth (m)
6
8/14/20
9/17/20
9
11/5/20
12
15Results: sediment data
DO (mg/L) Tlsediment (ng/g) ε205Tlsediment
0 5 10 15 1 100 10000 -6 -4 -2 0 2
0
3
Legend:
7/31/20
Depth (m)
6
8/14/20
9/17/20
9
11/5/20
12
15
Note: sediment depths refer to the water depth above the sediment, and all sediment Tl data are for authigenic leachates.Data summary for the 2020 field season
DO (mg/L) MnOxparticulate (uM) ε205Tldissolved ε205Tlparticulate ε205Tlsediment
0 10 20 0 1 2 -6 -4 -2 0 2 -6 -4 -2 0 2 -6 -4 -2 0 2
0
3
Depth (m)
6 Legend:
7/31/20
9 8/14/20
9/17/20
12 11/5/20
15Goal 1:
To understand how Tl isotopes
respond to temporal variations in Mn
oxide cycling.Discussion
DO (mg/L) MnOxparticulate (uM) ε205Tldissolved ε205Tlparticulate ε205Tlsediment
0 10 20 0 1 2 -6 -4 -2 0 2 -6 -4 -2 0 2 -6 -4 -2 0 2
0
3
Depth (m)
6 Legend:
7/31/20
9 8/14/20
Interestingly, particles possessed much 9/17/20
12 heavier ε205Tl values on the day with the 11/5/20
15
highest particulate Mn oxide concentrationsDiscussion
DO (mg/L) MnOxparticulate (uM) ε205Tldissolved ε205Tlparticulate ε205Tlsediment
0 10 20 0 1 2 -6 -4 -2 0 2 -6 -4 -2 0 2 -6 -4 -2 0 2
0
3
Depth (m)
6 Legend:
7/31/20
9 8/14/20
This is strong evidence for a predominant 9/17/20
12Mn oxide-control on ε205Tlparticulate values 11/5/20
15
on this day.Goal 2:
To understand how Tl isotopes are
cycled between waters and
sediments under different redox
conditions.Discussion
DO (mg/L) MnOxparticulate (uM) ε205Tldissolved ε205Tlparticulate ε205Tlsediment
0 10 20 0 1 2 -6 -4 -2 0 2 -6 -4 -2 0 2 -6 -4 -2 0 2
0
3
Depth (m)
6 Legend:
7/31/20
9 8/14/20
9/17/20
12 11/5/20
15
With the exception of some values from 8/14/2020, ε205Tldissolved are generally
indistinguishable from those found in sediments: about −4.0 epsilon units.Discussion
DO (mg/L) MnOxparticulate (uM) ε205Tldissolved ε205Tlparticulate ε205Tlsediment
0 10 20 0 1 2 -6 -4 -2 0 2 -6 -4 -2 0 2 -6 -4 -2 0 2
0
3
Depth (m)
6 Legend:
7/31/20
9 8/14/20
9/17/20
12 11/5/20
This could suggest that, despite the active Mn oxide
cycling in surface waters of Siders Pond, little to no Tl
15
isotopic fractionation effects are transferred to sediments.Discussion
DO (mg/L) MnOxparticulate (uM) ε205Tldissolved pH
0 10 20 0 1 2 -6 -4 -2 0 2 5 7 9 11
0
3
Depth (m)
6 Legend:
7/31/20
9 8/14/20
9/17/20
12 11/5/20
15
Unfortunately, it is impossible to tell just what was going on with the Tl cycle in Siders Pond on
8/14/2020 with our current data. For what it’s worth, pH, and presumably also productivity, were
especially high on this day. We hope to solve this riddle during the 2021 field season.2020 Siders Pond field season summary
• High temporal variability in water column and particle ε205Tl
values that seems at least partially driven by changes in
Mn oxide cycling.
Implications for the Tl isotope paleoredox proxy
Highlights the direct sensitivity of Tl isotopes to Mn
oxide minerals – not O22020 Siders Pond field season summary
• Minimal variability between sediment and water column
ε205Tl values, despite the active Mn oxide cycling.
Implications for the Tl isotope paleoredox proxy
May suggest that water column ε205Tl values are
transferred to sediments, or nearly so, under a
variety of local redox conditions.Siders Pond plans for 2021
• Collect full water column and particle ε205Tl profiles at least twice.
• Characterize the ε205Tl value of a freshwater inlet into the pond.
• Identify the primary host of Tl in some sediments and particles.
Broader plan
Expand our Tl isotope work to additional modern systemsSune G. Nielsen1,2 Hayley J. Gadol1,3 Tristan J. Horner1,2 Colleen M. Hansel1
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