# Evidence for benthic oxygen production in Neoarchean lacustrine stromatolites

Dylan T Wilmeth, Stefan V Lalonde, William M Berelson, Victoria

Petryshyn, Aaron J Celestian, Nicolas J Beukes, Stanley M Awramik, John R

Spear, Taleen Mahseredjian, Frank A Corsetti

► To cite this version:

Dylan T Wilmeth, Stefan V Lalonde, William M Berelson, Victoria Petryshyn, Aaron J Celestian, et al.. Evidence for benthic oxygen production in Neoarchean lacustrine stromatolites. Geology, 2022, 50 (8), pp.907 - 911. 10.1130/g49894.1. hal-05267480

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# Evidence for benthic oxygen production in Neoarchean lacustrine stromatolites

Dylan T. Wilmeth<fnref n="1" />,<fnref n="2" />*, Stefan V. Lalonde<fnref n="2" />, William M. Berelson<fnref n="1" />, Victoria Petryshyn<fnref n="3" />, Aaron J. Celestian<fnref n="1" />,<fnref n="4" />,
Nicolas J. Beukes<fnref n="5" />, Stanley M. Awramik<fnref n="6" />, John R. Spear<fnref n="7" />, Taleen Mahseredjian<fnref n="1" /> and Frank A. Corsetti<fnref n="1" />

## ABSTRACT

The evolution of oxygenic photosynthesis fundamentally altered the global environment, but the history of this metabolism prior to the Great Oxidation Event (GOE) at ca. 2.4 Ga remains unclear. Increasing evidence suggests that non-marine microbial mats served as localized “oxygen oases” for hundreds of millions of years before the GOE, though direct examination of redox proxies in Archean lacustrine microbial deposits remains relatively limited. We report spatially distinct patterns of positive and negative cerium (Ce) anomalies in lacustrine stromatolites from the 2.74 Ga Ventersdorp Supergroup (Hartbeesfontein Basin, South Africa), which indicate that dynamic redox conditions within ancient microbial communities were driven by oxygenic photosynthesis. Petrographic analyses and rare earth element signatures support a primary origin for Ce anomalies in stromatolite oxides. Oxides surrounding former bubbles entrained in mats (preserved as fenestrae) exhibit positive Ce anomalies, while oxides in stromatolite laminae typically contain strong negative Ce anomalies. The spatial patterns of Ce anomalies in Ventersdorp stromatolites are most parsimoniously explained by localized Ce oxidation and scavenging around oxygen bubbles produced by photosynthesis in microbial mats. Our new data from Ventersdorp stromatolites supports the presence of oxygenic photosynthesis ~300 m.y. before the GOE, and add to the growing evidence for early oxygen oases in Archean non-marine deposits.

## INTRODUCTION

The release of free oxygen via oxygenic photosynthesis has shaped Earth's surface for billions of years, yet the onset and extent of early oxygen production remains uncertain. The youngest estimate for the origin of oxygenic photosynthesis is provided by the Great Oxidation Event (GOE) at ca. 2.4 Ga, when sedimentary and geochemical proxies indicate oxidizing surface environments (e.g., Farquhar, 2000; Holland, 2002). Two prevailing hypotheses exist regarding the relationship between early oxygen production and the GOE: (1) oxygenic photosynthesis evolved hundreds of millions of years before the GOE, but nutrient limitation, ecological competition, and redox

buffering by abundant reducing compounds in Archean environments prevented oxygen accumulation in the atmosphere and oceans until ca. 2.4 Ga (Kump and Barley, 2007; Hao et al., 2020), or (2) oxygenic photosynthesis evolved at ca. 2.4 Ga, with high rates of microbial oxygen production overpowering global redox buffers to produce the GOE relatively rapidly (Fischer et al., 2016; Slotznick et al., 2022).

Models of early oxygenic photosynthesis have proposed the presence of “oxygen oases”, areas where oxygen production locally overpowered redox buffers to produce transient oxidative events in the Archean rock record (Kasting, 1991; Olson et al., 2013; Riding et al., 2014). Non-marine microbial mats are frequently proposed as oxygen oases to reconcile Neoarchean evidence for oxidative continental weathering

under a reducing atmosphere (Stüeken et al., 2012; Lalonde and Konhauser, 2015). Modern analogues for lacustrine oxygen oases occur in benthic microbial communities from Antarctic lakes, where cyanobacteria increase oxygen concentrations in mats without oxidizing the overlying water column (Sumner et al., 2015). Microbial mats preserved as stromatolites are more likely to record primary signatures for oxygenic photosynthesis than non-biogenic deposits, including oxygen bubbles in cyanobacterial cones preserved as rounded fenestrae at the tips of conical stromatolites (Bosak et al., 2009, 2010). Archean lacustrine stromatolites therefore provide clear targets to test for oxygen oases (Buick, 1992; Wilmeth et al., 2019).

The ca. 2.74 Ga Hartbeesfontein Basin (Rietgat Formation, Ventersdorp Supergroup) in South Africa (Fig. 1) is an isolated half-graben of the Ventersdorp continental rift; its lacustrine setting is further supported by frequent, meter-scale facies shifts and intercalation with subaerial volcanic deposits (Karpeta, 1989; Wilmeth et al., 2019). The basin contains extensive stromatolitic chert beds with exquisite microbial textures including abundant rounded fenestrae that are interpreted as former bubbles formed by microbial gas production (Figs. 1 and 2) (Wilmeth et al., 2019). Fenestrae occur throughout the stromatolites and are not localized at cone apices (Wilmeth et al., 2019); similar mat textures can form through various metabolisms producing oxygen, methane, or other gases (Hoehler et al., 2001; Mata et al., 2012). Therefore, while Hartbeesfontein fenestrae indicate microbial gas production, additional evidence is required to determine the specific gases. We

<footnote n="1">¹Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA</footnote>

<footnote n="2">²CNRS-UMR6538 Laboratoire Géosciences Océan, Institut Universitaire Européen de la Mer, 29280 Plouzané, France</footnote>

<footnote n="3">³Environmental Studies Program, University of Southern California, Los Angeles, California 90089, USA</footnote>

<footnote n="4">⁴Natural History Museum of Los Angeles County, Los Angeles, California 90007, USA</footnote>

<footnote n="5">⁵DST-NRF CIMERA, Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa</footnote>

<footnote n="6">⁶Department of Earth Science, University of California, Santa Barbara, California 93106, USA</footnote>

<footnote n="7">⁷Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, USA</footnote>

<footnote>*E-mail: dylan.wilmeth@univ-brest.fr</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Hartbeesfontein Basin (South Africa) stromatolite textures. (A) Location map. Stromatolites are located at 26°46′7.86″S, 26°23′37.08″E. Black square in inset represents map location. (B,C) Domal stromatolitic chert; hammer head is 2 cm tall and lens cap is 6 cm. Dashed lines highlight the contours of stromatolite domes. (D) Conical stromatolite laminae. (E) Bubble fenestrae surrounded by microbial fabrics.</figcaption></figure>

present new major and rare earth element (REE) data from Hartbeesfontein fenestral stromatolites to investigate the presence or absence of oxygenic photosynthesis in Archean lacustrine mats. We show distinct patterns of cerium (Ce) anomalies in fenestral and laminated stromatolite textures indicating localized oxygen production in microbial communities. Positive Ce anomalies around fenestrae are interpreted as oxidative Ce scavenging onto oxides around former bubbles, and negative anomalies in laminae

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Fenestral oxides. (A,B) Bubble fenestra with extensive oxides along bottom margin viewed in planepolarized and reflected light. (C,D) Fenestral (fen.) oxide viewed in reflected light. (E) Schematic of Mn and Ce oxidation and sorption onto fenestral oxides during mat growth. Qtz strom.—quartz stromatolite layers.</figcaption></figure>

indicate oxide precipitation where Ce has already been removed from oxidizing solutions.

## RESULTS

### REE Signatures of Oxides in Hartbeesfontein Stromatolites

Hartbeesfontein stromatolites contain three distinct oxide assemblages: in bubble fenestrae, stromatolite lamination, and weathering surfaces. Fenestral oxides appear orange and white in reflected light and reside at the contacts between fenestral walls and megaquartz, pore-filling cements, indicating early emplacement before cementation (Fig. 2; Fig. S3 in the Supplemental Material<fnref n="1" />; Wilmeth et al., 2019). Elemental composition of fenestral oxides measured by electron microprobe analyses (EMPA) revealed goethite, titanite, and a Mn-rich phase (Table S1). In contrast, oxides in stromatolite

laminae are composed of isolated hematite crystals with associated goethite, as identified by Raman spectroscopy, EMPA, and reflected light microscopy (Fig. 3; Fig. S3; Table S1). Laminar oxides (10–30 µm in diameter) are visually distinct from fenestral oxides and exhibit metallic luster and variable black, red, and yellow coloration (Fig. 3). EMPA backscatter imaging revealed hexagonal and rhombohedral hematite crystal habits with partial dissolution features frequently containing goethite, supported by Raman spectra and light microscopy (Fig. 3; Fig. S3; Table S1). Recent surficial weathering surfaces also contain abundant red-orange iron oxides that cross-cut through primary fabrics (Fig. 3F).

Laser ablation–high-resolution–inductively coupled plasma–mass spectrometry (LA-HR-ICP-MS) of Hartbeesfontein stromatolites revealed distinct trace element signatures that differentiate fenestral, laminar, and surficial oxides (Fig. 4; Fig. S1). Recent weathering surfaces display flat REE signatures with mild negative Ce anomalies when normalized to post-Archean Australian shales (PAAS) and are an order of magnitude higher in total REE than crustal abundances (Fig. 4A). By contrast, laminar oxides deeper in stromatolite samples

<footnote n="1">¹Supplemental Material. Supplementary discussion of geochemical and petrographic analyses, Table S1 (EMPA analyses of oxides in Hartbeesfontein stromatolites), and Table S2 (LA-ICP-MS analyses of oxides in Hartbeesfontein stromatolites). Please visit https://doi.org/10.1130/GEOL.S.19593439 to access the supplemental material, and contact editing@geosociety.org with any questions.</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Laminar and surficial oxides. (A,B) Laminar oxides viewed under reflected light. Note the difference between laminar and fenestral oxides. (C) Laminar oxide with dissolution zones filled by orange goethite shown in reflected light. (D) Electron microprobe analysis (EMPA) backscatter image of laminar oxide with dissolution zones. (E) Schematic of laminar versus fenestral oxides during mat growth. (F) Surficial oxides formed by recent weathering. Qtz cem.—quartz cement.</figcaption></figure>

are REE-poor (~0.01–0.5 × crustal values) and show strong heavy REE (HREE)/light REE (LREE) enrichments (Fig. 4A). Fenestral oxides show intermediate total REE concentrations (~0.5–5 × crustal values) and lesser degrees of HREE enrichment.

Remarkably, laminar oxides frequently exhibit strong negative Ce anomalies, as low as 0.2 Ce/Ce*, with corresponding Pr/Pr* as high as 1.74 (Fig. 4B; calculations as per Bau and Dulski, 1996). Ce/Ce*, calculated on a log-linear scale according to Lawrence and Kamber (2006), reaches 0.21. In contrast, fenestral oxides contain significant positive Ce anomalies that reach 1.56 Ce/Ce* (Pr/Pr* as low as 0.70), corresponding to 2.02 Ce/Ce* on the same log-linear scale. One ICP-MS laser shot yielded an incomplete REE data set from which Ce anomalies could not be evaluated, and three anomalous laser shots contained significantly elevated LREE and Pb concentrations, which are

not included in the figures but are noted in Table S2 and discussed in the Supplemental Material.

## DISCUSSION

### Primary Versus Secondary REE Signals

Previous petrographic analyses of Hartbeesfontein stromatolites (Wilmeth et al., 2019), such as paragenetic sequences of fenestral cements, indicate that fenestral and laminar oxides are *in situ*, derived from precursor minerals precipitated in Archean microbial mats. However, subsequent greenschist-grade metamorphism (Crow and Condie, 1988) and surficial weathering must be addressed before considering the environmental interpretations of REE signatures in oxide minerals. For example, hematite in Hartbeesfontein stromatolites is interpreted as the metamorphic product of metastable iron oxide precursors such as ferrihydrite, and goethite in laminar oxides is interpreted as secondary oxy-hydroxide precipitation in par-

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Rare earth element (REE) data from Hartbeesfontein Basin (South Africa) oxides. (A) REE + Y patterns in fenestral oxides, laminar, and surficial oxides. (B) Ce anomalies in different oxides. Areas inside dashed squares represent significant Ce anomalies. (C) Ce anomalies as a function of Mn concentration in oxides. PAAS—post-Archean Australian shales.</figcaption></figure>

tially dissolved hematite crystals during surficial weathering.

REE patterns provide a powerful metric for distinguishing chemical signatures inherited during primary iron mineral precipitation (as indicated by petrography) versus secondary alteration (as indicated by mineralogy), as REEs are relatively immobile under most post-depositional conditions. Cerium anomalies are particularly attractive as a redox proxy, as oxidation from Ce(III) to Ce(IV) renders Ce more
immobile relative to its neighbors, leading to its
enhanced removal onto reactive particles such as
Fe- and Mn-hydroxides (Byrne and Sholkovitz,
1996). The removal of Ce in oxidizing solutions
produces negative Ce anomalies in associated
chemical sediments, while Ce-scavenging par-
ticles themselves will contain positive Ce anom-
alies (Sholkovitz et al., 1994).

Alteration can produce secondary Ce anoma-
lies after deposition (Hayashi et al., 2004; Bon-
nand et al., 2020; Planavsky et al., 2020). For
example, exposed rock is more likely to con-
tain Ce anomalies than less-weathered drill
core (Planavsky et al., 2020), and Ce anoma-
lies generated by Cenozoic alteration have
been demonstrated in Paleoarchean rocks
(Hayashi et al., 2004; Bonnand et al., 2020).
Post-depositional negative Ce anomalies are
best explained by secondary LREE enrichment
from Ce-depleted aqueous solutions (Hayashi
et al., 2004; Bonnand et al., 2020). Laminar and
fenestral oxides have low LREE/HREE ratios
(~0.1-0.01), which argues against significant
post-depositional REE mobility. In contrast,
secondary weathering surfaces on Hartbeesfon-
tein stromatolites have negative Ce anomalies
but are clearly differentiated from laminar and
fenestral oxides by elevated REE concentrations
(~10× crustal values) and high LREE/HREE
ratios (Fig. 4A; Table S2). Therefore, while the
current mineralogy of iron oxides in Hartbees-
fontein stromatolites indicates a certain degree
of secondary alteration, REEs in specific stro-
matolite textures (laminae and fenestrae) appear
to maintain primary signatures of Archean lake
chemistry.

Redox Conditions in Archean Lakes

The REE signatures of fenestral and laminar
oxides in Hartbeesfontein stromatolites support
previous interpretations of a lacustrine depo-
sitional environment (Karpeta, 1989). Marine
Archean deposits typically contain yttrium/hol-
mium ratios of >45 g/g (Kamber et al., 2004;
Bolhar and van Kranendonk, 2007), and posi-
tive Eu anomalies indicate elevated hydrother-
mal input (Derry and Jacobsen, 1990). Both
features are absent in Hartbeesfontein REE
data (Fig. 4A) and other non-marine Archean
deposits such as the Fortescue Group in Aus-
tralia (Bolhar and van Kranendonk, 2007).
Unlike Fortescue Group carbonates, Hartbees-
fontein oxides are relatively depleted in LREE
(~0.01-0.1× crustal values; Fig. 4A). While
LREE depletion is more frequently observed
in marine than non-marine deposits, lakes can
exhibit a wide variety of LREE/HREE ratios,
such as LREE-depleted waters with negative Ce
anomalies in Lake Tanganyika, Africa (Barrat
et al., 2000).

Positive Ce anomalies in Hartbeesfontein
fenestral oxides indicate Ce sorption onto min-
eral surfaces under oxidizing conditions (Byrne

and Sholkovitz, 1996). Oxidative Ce scavenging
is especially prevalent on Mn- and Fe-Mn oxide
surfaces (Byrne and Sholkovitz, 1996; De Carlo
et al., 1997), and positive Ce anomalies in fenes-
tral oxides are positively correlated with Mn
concentration in Hartbeesfontein stromatolites
(Fig. 4C). Conversely, negative Ce anomalies
in laminar oxides (Fig. 4) indicate mineral pre-
cipitation in lake waters where Ce and Mn had
previously been scavenged. The presence of
positive and negative Ce anomalies in different
stromatolite fabrics has two potential explana-
tions, both of which require the presence of oxy-
genic photosynthesis in Archean lakes.

The presence of positive and negative Ce
anomalies in Hartbeesfontein stromatolites
could represent Ce shuttling across a redox-
cline in the surrounding water column. In
modern environments, Fe-Mn oxides remove
Ce from oxygen-rich surficial waters and sink
to lower depths (German et al., 1991; Byrne and
Sholkovitz, 1996). Oxide dissolution in reducing
deeper zones releases Ce back into surrounding
waters, and chemical precipitates in such Ce-
enriched areas will exhibit positive Ce anoma-
lies (Glasby et al., 1987). In such a scenario,
Hartbeesfontein stromatolites record a dynamic
lacustrine chemocline that shifted above and
below benthic microbial mats over time, where
Ce-depleted laminar oxides precipitated in oxi-
dizing lake waters, while Ce-enriched fenestral
oxides formed in reducing zones as chemoclines
shifted above mats.

Alternatively, the specific distribution of posi-
tive Ce anomalies in Hartbeesfontein fenestrae
is more parsimoniously explained by localized
Mn oxidation around oxygen bubbles entrained
in microbial mats (Figs. 2E and 3E). Mn and
Ce sequestration in fenestral oxides would
correspondingly deplete concentrations in the
surrounding layers of stromatolites, producing
negative Ce anomalies in laminar oxides farther
away from fenestrae. In modern mats, oxygen
bubbles form similar loci for oxide precipita-
tion in cyanobacterial communities (Raudsepp,
2012; Wilmeth et al., 2019). In anaerobic experi-
ments simulating Archean environments, Raud-
sepp (2012) noted a wide variability in Mn/Fe
ratios from oxides that precipitated in the same
microbial mat (between <1:1 to >5:1). There-
fore, Mn and Ce concentrations in stromatolites
can vary due to redox gradients in microbial
mats and do not necessarily require a shifting
redoxcline in lake waters.

In Hartbeesfontein stromatolites, distribu-
tions of positive Ce anomalies around bubble
fenestrae, surrounded by mat textures containing
negative Ce anomalies, support the interpreta-
tion of fenestrae as loci of intense oxygenic pho-
tosynthesis in lacustrine microbial mats. While
oxygen produced by the subaqueous stromato-
lites in our study is unlikely to have affected
the oxidation of nearby terrestrial surfaces (e.g.,

Sumner et al., 2015), evidence for oxygenic pho-
tosynthesis in Archean lakes supports the pres-
ence of microbial mats in terrestrial environ-
ments contributing to early signals of oxidative
weathering (Stüeken et al., 2012; Lalonde and
Konhauser, 2015).

CONCLUSIONS AND SIGNIFICANCE

Geochemical, petrographic, and sedimentary
data from 2.7 Ga Hartbeesfontein stromatolites
support the evolution of oxygenic photosynthe-
sis at least 300 m.y. before the GOE, which sub-
stantiates previous evidence from non-biogenic
sedimentary deposits and molecular clocks.
Localized oxygenic photosynthesis in lacus-
trine stromatolites also strengthens hypotheses
of non-marine benthic mats as oxygen oases
before the GOE, as predicted by geochemical
models, observations of modern microbial mats,
and phylogenetic analyses of cyanobacteria. The
presence of sub-meter-scale oxygen oases at ca.
2.7 Ga helps to reconcile previous Neoarchean
evidence for localized, oxidative continental
weathering under a reducing atmosphere.

ACKNOWLEDGMENTS

This work was supported by a U.S. National Science
Foundation Graduate Research Fellowship, the Lewis
and Clark Fund for Exploration and Field Research,
and a student research grant from the Geological
Society of America to D.T. Wilmeth; the European
Union's Horizon 2020 Research and Innovation Pro-
gram (716515) to S.L. Lalonde, a NASA Astrobiology
Rock Powered Life Grant and Yellowstone Center for
Resources for Research Permit (5664) to J.R. Spear,
and a NASA Exobiology grant (80NSSC19K0479)
to F.A. Corsetti. We thank the University of Johan-
nesburg (South Africa) and Alyssa Bell for fieldwork
support in summer 2014, Sami Nabhan for Raman
assistance and discussion of iron diagenesis, and the
engineers of the Pôle Spectrométrie Océan (Plouzané,
France; Bleuenn Guéguen and Marie Laure Rouget)
and Microsonde Ouest (University of Brest, France;
Jessica Laglande) for their analytical support.

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