Taylor ValleyLake Fryxell

McMurdo Sound

10 km

77°40'S

163°

0'E

Contour Interval: 400 m

162°

0'E

180°

0°

90°E

90°W

80°S

70°S

Antarctica

McMurdo Dry Valleysa b

A) Location of the McMurdo Dry Valleys in Antarctica. B) Lake Fryxell at the toe of Canada Glacier within Taylor Valley.

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2 cm

Benthic Mat O2 Oases: A Modern Analog For Early EarthD. Y. Sumner*, I. Hawes^, T. J. Mackey*, A. D. Jungblut°, M. Krusor*, and P. T. Doran`,

*Department of Earth and Planetary Sciences, University of California, Davis, 1 Shields Avenue, Davis, CA 95616, dysumner@ucdavis.edu, ^Gateway Antarctica, University of Canterbury, Christchurch, New Zealand, °Department of Life Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK, `Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana, USA 70803.

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9.2

9.4

9.6

9.8

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10.4

10.6

Normalized mat type frequencyD

epth

(m)

Introduction: Cyanobacterial photosynthesis produces an “oasis” of free oxygen in benthic mats below an anoxic water column in Lake Fryxell, Antarctica. Rates of photosynthesis are slow due to low irradiance, but oxygen production seasonally exceeds con-sumption and loss to the surrounding environment, and oxygen accumulates within the mat during summer. These transient oxygen oases provide the �rst known modern analog for formation of oxygen oases during Archean time, prior to oxidation of Earth’s atmosphere. We hypothesize that once the �rst cyanobacteria evolved, they produced localized oxygen oases in benthic mats analogous to those in Lake Fryxell under a reducing atmosphere and surface waters. Then, as the e�ciency and robustness of pho-tosytem II increased, oxygen oases expanded within benthic mats and into the water column, eventually leading to oxidation of the upper oceans and the atmosphere. The presence of oxygen oases in benthic mats in terrestrial aquatic systems like those in Lake Fryxell may account for geological evidence for oxidative sulfate mineral weathering on land as early as 2.8 billion years ago.

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00 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4

Ice cover

Depth range surveyed

Conductivity (mS/cm)

Dep

th (m

)

Pitted mat

Webbed pinnacles

Honeycomb mat

Ridged prostrate

Ridged pinnacles

Prostrate

Flocculent/prostrate mix

Flocculent

Ridged mats

Morphology ColorBrown Green Gold Grey

Lake Fryxell: Lake Fryxell (75°35’S, 163°35’E) is a perennially ice-covered Antarctic lake. Density strati�cation due to increasing sa-linity with depth and ice-cover inhibit mixing so that solute trans-port below 5 m depth is dominated by di�usion. An oxycline at ~9.5 m depth separates oxygen saturated water from deeper eux-inic water. Irradiance is highly seasonal with six months of winter darkness but high irradience in the summer.

Results: In November 2012, light transmitted through the ice and water re-sulted in 1 µmol photons/m2s (the lower limit for net photosynthesis [1]) reach-ing 10.4 m depth in Lake Fryxell. However, the water column transitioned from oxygen supersaturated at 9.1 m to complete anoxia below ~9.8 m. Net oxygen production in benthic mats at 9.8 m was demonstrated by a peak in oxygen at 1 mm depth in the mat . The spatial distribution of oxygen indicates a �ux to the water column of 0.04 µmol O2/m2s and into the sediment of 0.013 µmol O2/m2s. A net export of 0.05 µmol O2/m2s is consistent with expected local net photosynthetic rates based on productivity analysis in nearby Lake Hoare [1].

8.9 m: 0.74% Incident Photosynthetically Active Radiation (PAR)

10.4 m: Minimum PAR for Net Cyanobacterial Photosynthesis (1 µmol photon/m2s daily average [1])

9.8 m: 0.27% Incident PAR (3 µmol photon/m2s daily average; 1.6 µmol/m2s November 2012 montly average)

1 cm

0 200 400 600 800 1000Dissolved O2 (µmol/L)

Upward Flux: 0.04 µmol O2/m2s

Downward Flux: 0.013 µmol O2/m2s

Net Productivity: 0.05 µmol O2/m2s

Di�usion Coe�cient: 0.13 x 10-6 cm2/s

Water column contained H2S

Mat at 9.0 m water depth

Mat at 9.8 m water depth

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from Lalonde & Konhauser, 2015

Acknowledgements: The collection of �eld data was supported by the National Science Foundation Division of Polar Programs through grant #1115245 to the McMurdo Long Term Ecological Research project. DYS and TJM were supported by NASA Astrobiology through grant NN13AI60G. We thank both the US Antarctic Program and Antarctica NZ for logistic support.

References: Lyons, T.W., Reinhard, C.T., and Planavsky, N.J., 2014. The rise of oxygen in Earth’s early ocean and atmosphere. Nature, v. 506, p. 307-315.[1] Hawes, I., Moorhead, D., Sutherland, D., Schmeling, J., and Schwarz, A.-M., 2001. Benthic primary production in two perennially ice-covered Antarctic Lakes: patterns of biomass accumulation with a model of community metabolism. Antarctic Science, v. 13, p. 18-27. [2] Buick, R., 1992. The antiquity of oxygenic photosynthesis: Evidence from stromatolites in sulphate-de�cient Archaean lakes. Science, v. 255, p. 74 –77. ye, R., and Holland, H.D., 2000. Life associated with a 2.76 Ga ephemeral pond?: Evidence from Mount Roe #2 paleosol. Geology, v. 28, p. 483-486. [3] Lalonde, S.V., and Konhauser, K.O., 2015. Benthic perspective on Earth’s oldest evidence for oxygenic photosynthesis. Proceedings of the National Academy of Sciences [USA] , v. 112, p. 995-1000.[4] Anbar, A.D., Duan, Y., Lyons,T.W., Arnold, G.L., Kendall, B., Creaser, R.A., Kaufman, A.J., Gordon, G.W., Scott, C., Garvin, J., and Buick, R., 2007. A whi� of oxygen before the great oxi-dation event? Science, v. 317, p.1903–1906.

Implications: Lake Fryxell benthic mats produce transient oxygen oases that demonstrate that cya-nobacteria are capable of producing sustained concentrations of ~50 µmol O2/L without oxidizing their environment. Observed net oxygen produc-tion rates in Lake Fryxell are consistent with models for production of terrestrial benthic Ar-chean oxygen oases as proposed by Lalonde & Konhauser (2015). Archean terrestrial environ-ments as old as at least 2.7 Ga contained benthic mats [2]. Thus, our results suggest that similar oxygen oases may have formed prior to the oxida-tion of Earth’s oceans and atmosphere. Benthic oxygen oases could have provided environments for oxidative weathering of continental minerals such as pyrite [3], creating the geochemical signa-tures indicating “whi�s of oxygen” prior to the Great Oxidation Event [4].

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from Lalonde & Konhauser, 2015Approxim

ate Lake Fryxell

Summer Oasis

Productivity

Geochemical model for oxygen production without oxidizing the Archean atmosphere. Conti-nental coverage represents microbial community abundance. Modern net photosyntehetic productivity ranges shown in blue shading with the median in the dashed line. Lake Fryxell productivity added for comparison.