SUPPLEMENTARY DATA

Lithium isotopes and implications on chemical weathering in the

catchment of Lake Donggi Cona, northeastern Tibetan Plateau

Marc Weynell, Uwe Wiechert, Jan A. Schuessler

Table S1 Concentration data of loess from the study area and possible long distance loess

sources (data from Tsai et al., 2014)

Table S2 Molar ratios of dissolved load species, total dissolved solids (TDS) in mg/L, data

from (Weynell et al., 2016)

S1 Rock samples

Four hand specimen including two silicate rocks (BR2 and BR4) and two limestones (BR1

and BR3), and one matrix sample of a Neogene conglomerate (BR 8) were analyzed.

Additionally fluvial pebbles (BR5 – 7) from fluvial terrace S1 were processed. The limestones

were treated by acetic acid and only the residues were analyzed on Li isotopes. The

geological map was reported in Weynell et al. (2016).

Rock sample (δ7Li in ‰):

BR1 Limestone (+4.5 ‰)

BR2 Sandstone (greywacke) (-1.0 ‰)

BR3 Permian reef limestone (+0.4 ‰)

BR4 Shale (+3.3 ‰)

BR5 Fluvial pebble – mudstone (+1.6 ‰)

BR6 Fluvial pebble – conglomerate (with large quartzite fragments) (+6.0 ‰)

BR7 Fluvial pebble – greywacke (+1.4 ‰)

BR8 Neogene conglomerate (matrix, drilled at 3 sites with micro-driller) (+4.8 ‰)

S2 Compilation of a) δ7Li values and b) Lithium concentrations of global

stream waters.

Circles represent stream waters. For Yangtze, Salween, and Mekong rivers (Liu et al., 2011)

only maximum and minimum values exist.

S3 Grain size distribution of analyzed loess samples

Measurements were performed at RWTH Aachen, Germany. For details see Stauch et al.,

2012 and Stauch et al., 2014)

S4 Calculation of contribution from carbonate and evaporite dissolution to

dissolved Li budget of samples in SE catchment and Huang He (in %)

Overall assumption:

No substantial input of lithium by wet or solid deposition or anthropogenic sources to surface

waters. Calcium is solely derived from carbonate dissolution, which is unrealistic as high

Na/Cl in the upper reaches of Dongqu River indicate dissolution of feldspars, which likely

contribute some Ca, too. Hence, the calculated carbonate contribution represents a

maximum value. The calculation follows the routine given in Dellinger et al. (2015).

Calculation of amount of Li in each sample that is contributed by dissolution of carbonates

[Li]carb = [Ca]carb • (Li/Ca)carb

(Li/Ca)carb = 0.00001 to 0.00002 (Hathorne and James, 2006; Pogge von Strandmann et al., 2013)

Calculation of amount of Li in each sample that is contributed by dissolution of evaporites

[Li]eva = [Cl]eva • (Li/Na)eva

(Li/Na)eva = 0.00003 (compiled in Dellinger et al., 2015)

Contribution in %

eva: maximum contribution from evaporites in %; carb: maximum contribution from carbonates in %

Samples from the northern catchment are discussed separately as they receive huge

amounts of Na and Cl from hydrothermal fluids. Evaporite contribution in SE streams and

sample N08-3 are from drained near surface and near lake evaporites. They are regarded as

crystallizations within sediment pores due to lake level fluctuations or stream and near

surface groundwater precipitation in arid periods.

S5 Modeling of the δ7LiNI value of the Northern Inflow

The weathering zone is regarded here as a flow-through reactor at steady state. On this

basis the lithium isotope composition of the northern inflow (δ7LiNI) can be calculated:

δ 7Li¿=δ7Liinitial−∆

7 Li•(1−f DL) (Bouchez et al., 2013; Dellinger et al, 2015)

Assumptions: The lithium isotope composition is not altered within streams (see section

5.2.1) and the δ7Li values of the streams reflect the integrated solutions of the weathering

zone.

A. Calculation of δ7Liinitial Weynell et al. (2016) showed that the chemical composition of major and minor elements in

the northern inflow can be obtained by mixing ∼70 % thermal waters and ∼30 % surface

waters (soil solution) and subsequent precipitation of calcite. Using these proportions in a

simple mixing model allows to calculate the initial Li isotope composition δ7Liinitial of the soil

solution using the δ7Li value of DC08-5, as the most pristine thermal water sample, and a

δ7Li value of +1.9 ‰ for solubilized bedrock (see section 5.1.1):

δ7Liinitial = 0.7 • δ7Lithermal waters + 0.3 • δ7Lisolubilized bedrock= 0.7 • (+10.5) + 0.3 • (+1.9) = +7.9

B. Calculation of the fraction of Li remaining in solution (fDL) Calculation of the Li concentration of a mix ([Li]mix) of ~70 % thermal waters (represented by

sample WS11-2) and ~30 % sub-surface solutions (represented by sample N08-3). The

mixing proportions are from Weynell et al. (2016).

[Li]mix = 0.69 • WS11-2 + 0.31 • N08 -3 = 13.3 µmol/L

Calculation of Li derived from the mixed solution in the respective water sample of the

northern inflow

[Li]mix = [Cl]sample • (Li/Cl)TW(µmol/L)

Calculation of Li that is removed from solution

[Li]removed = [Li]mix – [Li]sample (µmol/L)

Calculation of the fraction (fDL) of initially solubilized Li that remains in solution after removal

due to partitioning into secondary minerals (i.e. mainly illite)

fDL = 1- ([Li]removed/[Li]mix)

(for a detailed explanation see Bouchez et al., 2013; Dellinger et al., 2015)

About 30 % of the initial Li remained in the samples from the northern inflow.

C. The apparent isotope fractionation Δ7Li : Δ7Lisec-solution ~ -17.8 ± 1.7 ‰ (see section 5.2.1)

As all parameters of the equation δ7 LiNI = δ7 Liinitial - ∆7 Li •(1- f DL) are known, we can model

the δ7Li value of the samples of the northern inflow (δ7LiNI)

The modeled δ7LiNI perfectly match the measured values of the 2011 samples providing

evidence that the estimated values of δ7Liinitial, Δ7Li, and fDL are correct and the model

(described in 5.2.2) are reliable. Some 0.9 ‰ lower value for the 2008 sample (N08-2) can

be explained by slightly different mixing proportion of thermal and sub-surface-(soil) waters.

Error estimation:

A Varying of the mixing proportions between thermal waters and sub-surface solution of

±10 % results in a maximum δ7Li offset between model and measured value of ±1 ‰.

Application of the δ7Li value of the warm springs (+11.5 ‰) instead of sample DC08-5

(+10.5 ‰) results in an offset of ±0.5 ‰. Application of δ7Li values between +1 and +3 ‰ for

the bedrock (i.e. δ7Li of the sub-surface solution) results in an offset of ±0.3 ‰.

B If the fraction fDL of the remaining lithium varies around ±0.05 (i.e. 5 %) the offset between

modeled and measured δ7Li increases up to ±1 ‰.

C The estimated range of Δ7Li is -17.8 ± 1.7 ‰ (see section 5.2.1). Applying this error gives

an offset of ±1.1 ‰ for δ7LiNI.

In summary, the modeled values δ7LiNI reproduce the measured δ7Li values of the samples

from the northern inflow within errors.

S6 Modeling the lithium isotope composition of Lake Donggi Cona

Weynell et al. (2016) modeled the water budget of Lake Donggi Cona. They identified the

Dongqu River, the northern inflow, and thermal waters as the three major contributors to the

lake water budget and its chemical composition. Direct input of precipitation to the lake

surface was shown to be not substantial for the lake water budget. The same is true for input

of ions via wet or dry deposition.

The chemical composition of the lake water was explained by evaporation of 45 % of the

inflowing water and mixing of the three major contributors:

XDonggi Cona = 0.87•XDongqu River + 0.08•Xnorthern inflow + 0.05•Xthermal waters (for 2008)

XDonggi Cona = 0.94•XDongqu River + 0.01•Xnorthern inflow + 0.05•Xthermal waters (for 2011)

Substituting δ7Li values of +16.4 (for 2008) and +17.1 ‰ (for 2011) for Dongqu River, +20.5

and +20.6 ‰ for the northern inflow, and +10.5 and +11.5 ‰ for the thermal waters results in

a δ7Li of the lake of +16.3 ‰ (2008) and +17.1 ‰ (2011). This matches the measured δ7Li

values of the lake (+16.4 to +16.6 ‰) within 4 %.

S7 Calculation of FE/FW ratio and chemical weathering rate

A calculation of the ratio of Li exported from the weathering zone in solid form by physical erosion (FE) to Li exported in the dissolved form by chemical weathering (FW).The FE/FW ratio is calculated with:

FEFw

=δ7 LiW -δ7 LiBR

δ7 LiBR -δ7 LiE

The equation was developed from a simple mass balance approach (see article section 5.3

and Fig. 6a).

FBR = FE + FW

Each flux has its isotopic composition:

FBR•δ7LiBR = FE•δ7LiE + FW•δ7LiW

This equation can be rearranged:

(FE + FW)•δ7LiBR = FE•δ7LiE + FW•δ7LiW

FE•δ7LiBR + FW•δ7LiBR = FE•δ7LiE + FW•δ7LiW

FE•δ7LiBR - FE•δ7LiE = FW•δ7LiW - FW•δ7LiBR

FE • (δ7LiBR - δ7LiE) = FW • (δ7LiW - δ7LiBR)

FE

Fw =

δ7 Liw - δ7 LiBR

δ7 LiBR - δ7 LiE

The calculation requires two prerequisites:

(i) The system has to be in steady state or at least close to it. The constant δ7Li values in

most stream samples in combination with varying morphologic and hydraulic conditions

provide evidence that Li isotopes are close to steady state.

(ii) The removal of Li by incorporation in secondary weathering products has to occur with a

constant apparent isotope fractionation factor. This was proposed in section 5.2.1 and 5.2.2.

This mass balance approach is described in more detail and intensively discussed in

Bouchez et al. (2013). The approach is based on steady state mass-balance equations

balancing the different fluxes in the weathering zone. A major outcome of this study is that

the FE/FW ratio can be calculated if the δ7Li values of the bedrock, eroded sediments, and the

involved solutions are known. The advantage of this calculation approach is that no flux or

concentration data are required. A similar approach was applied and validated to quantify the

carbon cycle, for example the proportion of carbon buried as organic matter and/or is

deposited as carbonate-carbon (e.g. Hayes et al., 1999; Kump and Arthur, 1999).

Values used for calculation:

δ7LiBR = +1.9 ± 0.5 ‰ (see article section 5.1.1)

δ7LiE = -0.8 ± 0.4 ‰ (see article section 5.1.3)

δ7LiW = +16.6 ± 0.4 ‰(see article section 5.2.3)

FE

Fw =

δ7 Li W -δ7 LiBR

δ7 LiBR - δ7 LiE

=16.6-1.91.9-(-0.8)

=5.4 ± 1.3

This results in an estimate for FE/FW of 5.4 ± 1.3. We used the δ7Li values of the modern

clays as analogue for the δ7Li of the bulk eroded sediments for the calculation. The eroded

bulk sediments are a mix of modern clays and “fresh” bedrock detritus (see article Fig. 6a

and Fig. 7c), thus, the δ7Li value of the eroded sediments is likely higher than the value of

-0.8 ± 0.4 ‰. A higher value will substantially increase the FE/FW ratio as the denominator

given in the equation above will decrease. For this reason, the obtained FE/FW ratios are

minimum values.

Uncertainty considerations:

The error of 1.3 represents the calculated error from the uncertainty of the averages. Using

values of +1.4 ‰; +17 ‰, and -0.4 ‰ for the δ7Li of bedrock, solution, and eroded

sediments, respectively results in a FE/FW of 9.7. Using values of +1.4 ‰; +16.2 ‰, and

-1.2 ‰ for the δ7Li of bedrock, solution, and eroded sediments, respectively results in a FE/FW

of 4.8. The range between 4.8 and 9.7 represents the maximum and minimum values for the

FE/FW ratio. Thus, the fraction of dissolved Li that leaves the weathering zone is not

exceeding 21 % (FE/FW of 4.8). Most likely this fraction is considerably lower.

B Calculation of the chemical weathering rateIt is difficult to obtain chemical weathering rates for the catchment of Lake Donggi Cona as

no discharge data exists and the strong interannual and daily fluctuations would require

intensive monitoring of the discharge. This is difficult in such remote areas. Physical erosion

data exists for an area close to the catchment. For this reason, we assume the Li specific

FE/FW ratio is also valid for the major elements (see Gislason et al., 1996; Pogge von

Strandmann et al., 2006; Pogge von Strandmann and Henderson, 2015; Pogge von

Strandmann et al., 2016). We apply the FE/FW ratio to calculate the chemical weathering rate

(CWR) from published physical erosion rates (PER). The PER were transformed from mm/ka

to t/km2/ka using a density of 2000 kg/m3 (Sharma, 1997). This density represents a

maximum value, thus, the calculated PER between 17 and 29 t/km2/a and the resulting CWR

represent maximum values.

Applying the calculated range for the FE/FW ratio to known physical erosion rates of nearby

areas results in an average chemical weathering rate (CWR) of 4.2 t/km2/a. This chemical

weathering rate resembles weathering rates from the Lake Qinghai catchment, located

200 km northeast from the catchment (see section 5.3 in the article) and the upper Huang He

(Wu et al., 2005). The weathering rates were independently derived from continuous

concentration and discharge measurements in the streams. The similarity of the chemical

weathering rates of Lake Qinghai catchment and the upper Huang He on one hand and the

weathering rate in the Lake Donggi Cona catchment on the other hand support that Li

specific FE/FW ratio may be representative for major elements in the weathering zone as well.

S8 Developing of equation (IX)

Assumption: the whole weathering zone and its sub-systems (solution and secondary

minerals) are in steady state. Thus, all in- and outgoing fluxes are in steady state.

S9 Estimation of the average fraction favg of Li that is solubilized from bedrock

(or loess) minerals and incorporated into secondary weathering products

This estimation considers only samples that are unaffected by contamination from thermal

waters. The outcome of this estimation gives a range for the „average catchment“. The

calculation does not reflect one distinct stream or solution as we use the average δ7Li

composition of the modern secondary minerals (here illite, which are best characterized by

the lake floor sediments, see section 5.1.3), the bedrock (best characterized by loess, see

section 5.1.1), and the apparent fractionation factor α (deduced from the offset between

uncontaminated surface waters and modern clays, see section 5.2.2)

We used the standard equation for open batch flow-through systems:

δ7 Liclay = δ7 LiBR +1000 × (1- favg ) × ln αsec -sol e.g. Dellinger et al. (2014)

favg is the average fraction of Li that is solubilized from bedrock or loess minerals and

incorporated into clays. δ7LiBR is the δ7Li value of the average bedrock, i.e. loess, δ7Liclay the

average δ7Li value of the modern clays in the catchment, and ln α the apparent fractionation

factor. We rearranged the equation:

f avg=[1- [δ7 Liclay -δ7 LiBR

1000×ln( αapparent ) ] ]×100 (%)

with

δ7LiBR = +1.9 ± 0.5 ‰

δ7Liclay = -0.8 ± 0.4 ‰

α = 0.982 ± 0.002

We obtain a range for f between 78 and 91 %. Thus, most of the solubilized Li is

incorporated into secondary minerals.

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