Soluble Salts in Taylor Valley, Antarctica: Implications ... · Soluble Salts in Taylor Valley, Antarctica: Implications for Paleolakes and the Ross Sea Ice Sheet Jonathan D. Toner1;

Soluble Salts in Taylor Valley, Antarctica: Implications for Paleolakes and the Ross Sea Ice Sheet

Jonathan D. Toner1; Ronald S. Sletten1; Michael L. Prentice2 1. Earth and Space Sciences Department, University of Washington; 2. Environmental Geology Department, Indiana University

Introduction

Study Sites - Taylor Valley, Antarctica

The Chemical Composition of Salt Accumulations

Paleolakes in Western Taylor Valley

Paleolakes in Eastern Taylor Valley

Conclusions

References This material is based upon work supported by the National science

Foundation under Grant Nos. 0541054 and 0636998. Support was also

provided by the department of Earth and Space Sciences at the

University of Washington.

During the Last Glacial Maximum (LGM), the Ross Sea Ice Sheet (RSIS) expanded across McMurdo Sound

and entered the mouth of Taylor Valley. It is speculated that this ice dammed large, valley-wide proglacial

paleolakes, which were over 300 m deep in places (Stuiver et al. 1981, Hall et al. 2000). These paleolakes

have important implications for paleoclimate during and after the LGM, the sensitivity of the RSIS to deglacial

sea level rise and warming trends, and past hydrologic and climatic regimes in Taylor Valley.

To study these paleolake systems, we investigated soluble salt accumulations in Taylor Valley soils. Soluble

salts accumulate in soils from marine sea-spray aerosols and atmospheric aerosols and are preserved from

leaching due to prevailing cold-dry conditions. Soluble salts are by nature sensitive to inundation by lake

water and should reflect the extensive paleolake systems believed to have filled Taylor Valley. Lake water is

thought to influence soluble salt accumulations in Dry Valley soils by leaching inundated soils during lake

high stands and accumulating soluble salts along wetted lake margins as lake levels lower. Salts

accumulate near wetted lake margins as lake water moves along water potential gradients towards an

evaporation zone near the soil surface. This process has been termed, ‘evapoconcentration’. We infer the

extent of paleolake systems in Taylor Valley by examining the distribution of soils influenced by leaching and

evapoconcentration processes.

Methods: 89 soils were sampled throughout Taylor Valley and analyzed for soluble salts using a 1:25

soil:water extraction, with the goal of removing as much soluble salt as possible from soils. This study also

incorporated soluble salt contents determined by other researchers. Samples were collected from soils at

level sites on local topographic highs, such as paleoshorelines, moraine tops, and terrace apexes. Measured

salt concentrations were converted to total salt contents per aerial surface area using the equations:

Acknowledgements Hall, B. L., Denton, G. H., Hendy, C. H., Denton, G. H., & Hall, B. L. (2000). Evidence from Taylor Valley for a grounded ice

sheet in the Ross Sea, Antarctica. Geografiska Annaler. Series A: Physical Geography, 82(2-3), 275-303.

Stuiver, M., Denton, G. H., Hughes, T. J., & Fastook, J. L. (1981). History of the marine ice sheet in West Antarctica during

the last glaciation; a working hypothesis.

Barrett, J. E., M. A. Poage, et al. (2010). The legacy of aqueous environments on soils of the McMurdo Dry Valleys:

contexts for future exploration of Martian soils. Life in Antarctic Deserts and other Cold Dry Environments. P. T. Doran, B.

W. Lyons and D. M. McKnight. New York, Cambridge Astrobiology.

Salt contents shown here are all influenced by paleolakes, but are

higher at lower elevations indicating more stable paleolakes.

Maps showing a reconstruction of maximum paleolake extents in Taylor Valley, after Hall et al. 2000 (left panel), compared to

current lake levels (right panel).

Salt Content eq m−2 = BD hor. × thor. × Chor.×

horizon

%𝑤<2 𝑚𝑚100

ℎ𝑜𝑟.

BD (g cm−3) =

100

%w<2 mmρ<2mm

+100 −%w<2 mm

ρg

Soluble salt distributions in Bonney Basin indicate paleolake levels up to approximately 300

m elevation, which is consistent with the elevation of lacustrine strandlines and terraces.

Soluble salts below 300 m elevation are lower than in higher elevation soils due to leaching

by paleolakes. Salt contents in soils affected by paleolakes (below 300 m elevation) are a

function of paleolake level stability. Above 116 m elevation, paleolakes would have been

dammed by the RSIS (relatively unstable), while below 116 m elevation, paleolakes would

have been closed basin lakes (relatively stable). The character of paleoshorelines in Bonney

Basin provides additional evidence for paleolake stabilities. Shorelines above 116 m

elevation are faint, while shorelines below 116 m elevation are broad and well defined.

Graph showing lower salt contents in soils below 300

m elevation, due to leaching by paleolakes.

Salts in eastern Taylor Valley have

been modified by leaching and

paleolakes. Salt distributions indicate

paleolake levels up to approximately

120 m elevation, but no higher.

Paleolakes filling eastern Taylor Valley

may have been controlled by the

height of the RSIS grounding line,

which was presumably located near

the Coral Ridge moraine complex in

the valley mouth.

• Salt contents in soils are influenced by a variety

of factors including, soil age, distance from

McMurdo Sound, leaching, soil texture,

calcite/gypsum dissolution, cation exchange, and

paleolakes.

• The absence of paleolakes above 120 m in

eastern Taylor Valley suggests that paleolakes in

Bonney Basin were dammed by a lobe of the

RSIS that filled eastern Taylor Valley to at least

300 m elevation. This contrasts with previous

hypotheses that valley-wide paleolakes filled

Taylor Valley during the LGM. Instead, we

suggests that high paleolakes were restricted to

Bonney Basin and that smaller paleolakes

formed during the retreat of the RSIS in eastern

Taylor Valley.

A reconstruction of glacial and lacustrine events in Taylor

Valley; (upper panel) LGM RSIS extent, (mid panel) 120 m

high paleolakes following RSIS retreat, and (lower panel)

present day lacustrine systems.

Soils sampled in western Taylor Valley for salt contents. These

soils typically did not contain ice-cemented soil and could be

excavated to ~1 m depth. Only glacial tills were sampled.

A map showing the location of the study and the soils sampled. A total of 89 soils were

sampled. Soils were primarily sampled in eastern Taylor Valley, and only a few were

sampled in western Taylor Valley.

Soils sampled in eastern Taylor Valley. These soils all

contained ice-cemented soil at about 30 cm depth. Glacial tills

were sampled, as well as about 20 fluvial terraces, which have

been interpreted as lacustrine deltas.

Soluble Salt Distributions in Taylor Valley

The dominant characteristic of soluble salt distributions in Taylor Valley is the occurrence

of low salt contents in eastern Taylor Valley and high salt contents in western Taylor Valley.

In general, salt distributions are consistent with differences in soil age between eastern

and western Taylor Valley. In western Taylor Valley, soils are from older advances of

Taylor Glacier and alpine glaciations, while in eastern Taylor Valley, soils are primarily from

the LGM advance of the RSIS.

There are a number of exceptions to this overall trend in salt content; some older soils

have relatively low salt contents and some younger soils developing on RSIS sediments

have relatively high salt contents. In western Taylor Valley, older soils having low salt

contents are typically ice-cemented near the surface, which suggests a higher local

moisture regime that may have leached soluble salts from these soils. In eastern Taylor

Valley, ice-cemented soil is ubiquitous and salts have likely been leached from older soils

due to the wetter climate. Relatively high salt contents in young RSIS sediments are

primarily found below approximately 120 m elevation in eastern Taylor Valley. Barrett et al.

[2010] found similar high salt accumulations near Lake Fryxell and Lake Hoare and

concluded that these salt accumulations were deposited at the margins of paleolakes.

Region n pH Ca2+ Mg2+ Na+ K+ Cl- SO42- NO3

- HCO3- Total

moles m-2 eq m-2

Valley Mouth 15 9.7 1.09 1.06 12.05 1.10 5.88 0.61 0.16 8.72 34.88

Fryxell Basin 68 9.6 2.22 0.97 5.30 1.06 2.38 0.86 0.05 7.77 25.49

Bonney Basin 7 8.5 18.96 13.98 65.76 3.86 88.44 15.05 1.28 10.64 271.00

1

Graph of ion concentrations (the sum of all

sequential extractions) measured at different

soil water ratios. Extracted salts increase at

higher extraction ratios, particularly Ca2+.

Average soluble salt compositions measured in 1:25 soil:water extractions.

The composition of salts and soil pH varies with

distance from McMurdo Sound. Valley Mouth soils

are composed almost entirely of Na+ and HCO3- ions

and the average soil pH is 9.7. Further inland, in

Fryxell Basin, Na+ and HCO3- are still the dominant

ions and the average soil pH is 9.6, but soils contain

higher proportions of Ca2+ and Mg2+. In Bonney

Basin, located furthest from McMurdo Sound, soils

have high proportions of Cl- and SO42- relative to

HCO3- and the average soil pH decreases to 8.5.

Tests performed at different soil:water ratios show

that extracted soluble salts are strongly dependent

on the number of sequential extractions and the

soil:water ratio. This is consistent with a well known

effect caused by cation exchange reactions and

calcite dissolution. These reactions can be

described by the following generalized equation:

CaCO3 + H2CO3 + 2Na, 2K,Mg X2 ↔ 2Na+, 2K+, Mg2+ + 2HCO3

− + CaX2

The Na-HCO3 composition of soils in eastern Taylor Valley is likely due to cation exchange

and calcite dissolution, and reflects a high exchangeable Na component in these soils. It

is important to interpret measured salts in light of the effects of exchange reactions and

calcite dissolution.

C13A-0606