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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.

Lamination in stromatolites (considered some of the oldest fossils on Earth) is commonly interpreted to record the periodic response of a microbial community to daily, seasonal, or perhaps yearly environmental forcing, but the inability to date ancient stromatolites precludes an understanding of the lamination formation processes. We use high-resolution 14C dating of Holocene stromatolites from Walker Lake, Nevada (United States), to construct a record of lamination rate over the course of accretion. Laminae formed with a period of 5.6 ± 1.6 yr/lamination at the base of the structure, 1.6–2.8 ± 1.9 yr/lamination in the middle, and 4.5 ± 0.8 yr/lamination at the top of the laminated portion. The predominant 4−6 yr periodicity indicates that lamination formation is likely more closely related to regional climate forcing (e.g., El Nino–Southern Oscillation) versus the typical diurnal or seasonal changes in microbial mats traditionally assumed for most ancient stromatolites. Thus, generalizations regarding the influence of microbial mats on stromatolite lamination and the use of stromatolites as biosignatures need careful consideration.

An unusual richness of biogeochemical events is recorded in Neoproterozoic‐ Cambrian strata of the Death Valley region, California, United States. Eight negative carbon isotope (d13C) excursions are found in carbonate units between 1.08 Ga and the Precambrian/Cambrian boundary; four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial ‘‘cap carbonates’’ (including one or more of the following: laminite with rollup structures, apparent ‘‘tube rocks,’’ seafloor precipitates, and sheet-crack cements). However, only two of these units, the Sourdough limestone member of the Kingston Peak Formation and the Noonday Dolomite, rest directly upon glacial strata. The basal Beck Spring Dolomite and the Rainstorm Member of the Johnnie Formation each contain negative excursions and cap-carbonate‐like lithofacies, but do not rest on known glacial deposits. If the negative d13C excursions are assumed to record depositional processes, two equally interesting hypotheses are possible: (1) The Death Valley succession records four glacial pulses in Neoproterozoic time, but glacial units are not preserved at two stratigraphic levels. (2) Alternatively, other global oceanographic processes can cause negative excursions and cap-carbonate‐ like facies in addition to, or independent of, glaciation.

Neoproterozoic “Snowball Earth” is arguably one of the most fascinating times in Earth history (e.g., [Hoffman and Schrag, 2002][1]): geologic and paleomagnetic evidence (e.g., [Evans, 2000][2]) suggests the presence of ice at low latitudes (but not without controversy: [Allen and Etienne [2008

Science has achieved tremendous success over the centuries, partly because the complexities of the Earth, the physical processes that sustain the planet, and the enormity of life were separated into disparate fi elds of study—mathematics, physics, chemistry, biology, and geology, to name only a few. Scientifi c compartmentalization was initially necessary to impart enough focus to make progress on complicated issues. However, as the knowledge base grew, it became more and more diffi cult to separate life and the history of the Earth, and vice versa. We now understand that to investigate the Earth’s surface as an abiologic system is folly: Life and Earth processes are intimately linked. Hence, a new fi eld was born at the interface between biology and geology: geobiology. As a fi eld, geobiology seeks to understand the intersection of life and the rock record across Earth’s history: how organisms infl uence the physical Earth and vice versa, and how the marriage of physical and biological processes have transformed our planet over its long history. The assessment of life’s macromolecules of DNA, RNA, polysaccharides, proteins, and lipids, and their potential recalcitrance in an ecosystem, has opened up the fi eld of geobiology to lead us toward a solid explanation of where life came from, how life has altered the planet, what may be possible for life elsewhere, and what represents one of the reasons for the explosion of geobiologic studies today. Here we outline how molecular biology has transformed our understanding of geobiology, describing a few of the essentials needed to understand geobiology and exploring an example of a modern geobiologically relevant system: a living stromatolite from the shore of a geothermal hot spring in Yellowstone National Park, Wyoming, USA.

The Neoproterozoic succession in the Death Valley region contains a physical- and chemostratigraphic record of glaciation. Direct evidence for glaciation includes dropstones, glacially influenced diamictites, and cap carbonates in the Neoproterozoic Kingston Peak Formation. Within this formation, glacially influenced deposits and cap carbonates occur at two distinct horizons, suggesting at least two glacial episodes. Cap-like carbonates and sequence boundaries elsewhere in the succession may indicate additional glacial intervals. The basal Beck Spring Dolomite has fades and isotopic characteristics commonly associated with cap carbonates, and the rest of the succession is punctuated by numerous sequence boundaries including a prominent incised horizon in the uppermost Johnnie Formation. This horizon is locally overlain by carbonate with cap-like facies and isotopic characteristics. Together, these observations indicate at least two and possibly four distinct Neoproterozoic glaciations.

The concept of a Neoproterozoic “snowball Earth”–an ice age in the tropics ([Kaufman, 1997][1])–has captured the imagination of the public and scientists alike. The idea of global-scale glaciation, indicated by the abundance of Neo-proterozoic glacial deposits (some with low-latitude

We formed calcite crusts in the presence and absence of the heterotrophic bacterium Desulfovibrio desulfuricans strain G20 to investigate microbial morphological signatures in fast-accreting carbonate precipitates. Submicrometer- to micrometer-sized pores (micropores) were present and ubiquitous in the G20 crusts but absent in abiotically precipitated crusts. Bacterial micropores resemble inclusions under transmitted light, but have distinct size, biological shapes and patterns (swirling or dendritic) and are distributed differently from common fluid inclusions. We observed similar porosity in both modern and ancient carbonate crusts of putative biotic origin. Our experiments support the microbial origin of micropores and help define specific criteria whereby to recognize these features as biosignatures in the rock record.

[Corsetti and Hagadorn (2000)][1] presented welcome carbon isotope data for two Precambrian-Cambrian transition sections from Death Valley. They argued that these data provide a “critical link toward our understanding of the correlation between siliciclastic-dominated and carbonate-dominated

The Death Valley region contains one of the best exposed and often visited Precambrian-Cambrian successions in the world, but the chronostratigraphic framework necessary for understanding the critical biologic and geologic events recorded in these sections has been inadequate. The recent discovery of Treptichnus (Phycodes) pedum within the uppermost parasequence of the lower member of the Wood Canyon Formation allows correlation of the Precambrian-Cambrian boundary to this region and provides a necessary global tie point for the Death Valley section. New carbon isotope chemostratigraphic profiles bracket this biostratigraphic datum and record the classic negative carbon isotope excursion at the boundary. For the first time, biostratigraphic, chemostratigraphic, and lithostratigraphic information from pretrilobite strata in this region can be directly compared with similar data from other key sections that record the precursors of the Cambrian explosion. Few Precambrian-Cambrian boundary sections contain both the facies-restricted boundary fossil T. pedum and carbon isotope data, as found in Death Valley. Thus, the Death Valley succession provides a critical link toward our understanding of the correlation between siliciclastic-dominated and carbonate-dominated Precambrian-Cambrian transition sections.