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The accumulation of thousands of boulder-sized clasts into boulder fields in the Atacama Desert has been linked to seismic-driven downslope transport, a rare sedimentary process corroborated by this study. We surveyed boulder arrangements occurring in the Atacama Desert and identified three accumulation types for further investigation: a small circular boulder cluster (BC), a long channelized boulder stream (BS), and a wide convex-shaped boulder field (BF). Drone-based photogrammetric techniques and field observations were used to generate high-quality digital elevation models and orthophotos to determine boulder count, size, coverage, orientation, lithology and local topography. Our data shows that the arrangement of boulder accumulations corresponds with the shape of the accommodation space and the boulder input, where BCs occur at the center of completely confined topographic depressions, BSs occur along laterally confined and incised hill slopes with boulders stacked above each other, and BFs occur on largely unconfined shallow and low-relief slopes with a distinct boulder front. A general downslope increase of average boulder size and coverage was measured in all boulder accumulations and a long-axis orientation of boulders parallel to the transport direction was observed for the BS. Based on these results and the lack of fluvial transport indicators, we conclude that transport and arrangement of boulder accumulations are largely controlled by the interplay of topography and seismic-driven boulder transport, resulting in unique landscape features present in the hyper-arid Atacama Desert. Such sedimentary transport processes are rare on Earth but potentially play a greater role on other arid planetary surfaces that are covered by boulders and subject to sufficient seismic activity.

Microbial life was well established and widespread by the Paleoarchean; however, the degree of evolutionary advancement such as microbial motility, intra- and inter-species interactions, phototropism, or oxygenic photosynthesis by that time remains highly debated. The 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB, South Africa) are Earth's oldest well-preserved siliciclastic tidal deposits. They exhibit a unique assemblage of microbial mats, providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings. The fossil mats are preserved as kerogenous laminations (0.5–1 mm thick) that can be traced laterally for ∼15 km in a ∼1000 m-thick succession of fine- to coarse-grained tidal sandstones and conglomerates. We here present a detailed stratigraphic and depositional facies analysis, documenting the association of the three principal mat morphotypes with specific environmental settings: (1) planar-type in coastal floodplain, (2) wavy-type in intertidal, and (3) tufted-type in upper inter- to supratidal facies. All mat types indicate a flourishing phototrophic biota; moreover, the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments.

The origin and evolution of life has undoubtedly had a major impact on the evolution of Earth's oceans and atmosphere. Recent studies have suggested that bioactivity may have had an even deeper impact and may have caused a change in the redox-state of the mantle and provided a path for the formation of continents. We here present a numerical model that assumes that bioactivity increases the continental weathering rate and that relates the sedimentation rate to the growth of continents and to the hydration of the mantle using elements of plate tectonics and mantle convection. The link between these factors is provided by assuming that an increase of the thickness of the sedimentary layer of low permeability on top of a subducting oceanic slab will reduce its dewatering upon subduction. This in turn leads to a greater availability of water in the source region of andesitic partial melt, resulting in an enhanced rate of continental crust production, and to an increased regassing rate of the mantle. The mantle in turn responds by reducing the mantle viscosity while increasing the convective circulation rate, degassing rate and plate speed. We use parameters that are observed for the present Earth and gauge uncertain parameters such that the present day continental surface area and mantle water concentration can be obtained. Our steady state results show two stable fixed points in a phase plane defined by the fractional continental surface area and the water concentration in the mantle, one of them pertaining to a wet mantle and the continental surface area of the present day Earth, and the other to a dry mantle and a small continental surface area. When the sedimentation rate is reduced, both fixed points move and the area of attraction of the latter fixed point increases systematically. We conclude that if the presence of life has increased the continental weathering rate, as is widely believed, and led to the observables of a wet mantle and a continental surface coverage of roughly 40%, an abiotic Earth would likely have evolved toward a dry mantle with a small continental surface area instead.

article Two Patagonian lakes studied here, Lago Sarmiento and Laguna Amarga, are located within the orographic rain shadow formed to the east of the Southern Patagonian Ice Field in the Andes Range. Major thrombolite colonies are present in Lago Sarmiento, whereas widespread stromatolites occur in Laguna Amarga. Based on the characterization of the hydrologic system of these two lakes, together with an estimation of the isotopic balance and an analysis of the equilibrium conditions between the water and biologically induced carbonates, it is concluded that the microbialites of Lago Sarmiento are better suited as paleotemperature indicators than those of Laguna Amarga. Lago Sarmiento thrombolites contain unique carbonate mineral species in which carbonate precipitation occurs close to isotopic equilibrium and where the variation in water temperature controls fractionation of the stable oxygen isotope. The results indicate that at 1215 cal yr Bp the level of the lake was at 85 m a.s.l with a temperature close to 9.3 °C, was at 82 m a.s.l. at 600 cal yr Bp with a temperature close to 8.5 °C. This coincides with the timing of the Northern Hemisphere Medieval Warming Period. At 183 cal yr Bp the level of the lake was at 80 m a.s.l with a cooler temperature close to 7.7 °C, representing a colder period coinciding with the timing of the Little Ice Age (LIA). An interesting outcome of this study is that it reinforces the idea that the δ 13 C signal in carbonate deposits can be an effective tool in distinguishing between inorganic and biologically induced precipitation.