Showing papers by "Donald R. Lowe published in 2015"

Geologic record of partial ocean evaporation triggered by giant asteroid impacts, 3.29–3.23 billion years ago

Abstract: Although lunar studies suggest that large asteroid impact rates in the inner solar system declined to their present low levels at 3.8–3.7 Ga, recent studies in greenstone belts indicate that asteroids 20 km to 70+ km in diameter were still striking the Earth as late as 3.2 Ga at rates significantly greater than the values estimated from lunar studies. We here present geologic evidence that two of these terrestrial impacts, at 3.29 Ga and 3.23 Ga, caused heating of Earth’s atmosphere, ocean-surface boiling, and evaporation of tens of meters to perhaps 100 m of seawater. Rapid ocean evaporation resulted in abrupt sea-level drops, erosion of the exposed sea floor, and precipitation of distinctive layers of laminated silica representing marine siliceous sinter. Such events would have severely affected microbial communities, especially among shallow-water and photosynthetic organisms. These large impacts profoundly affected Archean crustal development, surface environment, and biological evolution until 3.2 Ga, or even later.

Interplay between an axial channel belt, slope gullies and overbank deposition in the Puchkirchen Formation in the Molasse Basin, Austria

Abstract: Deep-water sediments in the Molasse Basin, Austria, were deposited in a narrow foreland basin dominated by a large channel belt located between the steep Alpine fold and thrust belt to the south and the gentler northern slope off the Bohemian Massif. Several gas fields occur outside the channel belt, along the outer bend of a large meander. Accumulation of these overbank sediments reflects a complicated interplay between slope accommodation and debris-flow and turbidity-flow interaction within the channel. The tectonically oversteepened northern slope of the basin (ca 2 to 3°) developed a regionally important erosional surface, the Northern Slope Unconformity, which can be traced seismically for >100 km in an east–west direction and >20 km from the channel to the north. The slope preserves numerous gullies sourced from the north that eroded into the channel belt. These gullies were ca 20 km long, 15 km long, plugged and forced the main channel to step abruptly ca 7 km to the south. This resulted in development of an abrupt turn in the channel pathway that propagated to the east and probably played a role in forming a sinuous channel later. As debris-flow topography was healed, flows spread out onto narrow area between the main channel and northern slope forming a broad fine-grained apron that serves as the main gas reservoir in this area. This model of the overbank splay formation and the resulting stratigraphic architecture within the confined basin could be applied in modern and ancient systems or for subsurface hydrocarbon reservoirs where three-dimensional seismic-reflection data is limited. This study elucidates the geomorphology of the oversteepened slope of the under-riding plate and its effects on the sedimentation.

Sedimentology and geochemistry of Archean silica granules

Abstract: The production of biogenic silica has dominated the marine silica cycle since early Paleozoic time, drawing down the concentration of dissolved silica in modern seawater to a few parts per million (ppm). Prior to the biological innovation of the first silica biomineralizing organisms in late Proterozoic time, inputs of silica into Precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes, although the mechanics of this abiotic marine silica cycle remain poorly understood. Cherty sedimentary rocks are abundant in Archean sequences, and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in Archean seawater. The recent discovery that many pure chert layers in early Archean rocks formed as sedimentary beds of sand-sized, subspherical silica granules has provided direct evidence for primary silica deposition. Here, we provide further sedimentological and geochemical analyses of early Archean silica granules in order to gain a better understanding of the mechanisms of granule formation. Silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths. The abundance and widespread distribution of silica granules in Archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column. The regular occurrence of silica granules as centimeter-scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation. Contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis. Looking to modern siliceous sinters for insight into silica precipitation, we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres. Consistent with this hypothesis, Archean ocean chemistry would have favored particle aggregation over gelling. Granule formation would have been most favorable under conditions promoting rapid silica polymerization, including high salinity and/or high concentrations of dissolved silica. Our observations suggest that granule sedimentation was often episodic, suggesting that granule formation may have also been episodic, perhaps linked to variations in these key parameters.