Triple oxygen isotope constraints on atmospheric O2 and biological productivity during the mid-Proterozoic

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (pO2) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating pO2 and productivity during the Proterozoic. O-MIF, reported as Δ′17O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS2) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air–sea gas exchange. Previous analyses of these data concluded that pO2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on pO2 is essentially unconstrained by these data. Indeed, pO2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that pCO2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O2) between the atmosphere and surface ocean is controlled more by air–sea gas transfer rates than by biological productivity. Improved estimates of pCO2 and/or improved proxies for Δ′17O of atmospheric O2 would allow tighter constraints to be placed on mid-Proterozoic pO2.

Thermal budgets of magma storage constrained by diffusion chronometry: the Cerro Galán ignimbrite

The long-term thermochemical conditions at which large bodies of silicic magma are stored in the crust is integral to our understanding of the timing, frequency, and intensity of volcanic eruptions, and provides important context for volcano monitoring data. Despite this realization, however, individual magmatic systems also may have unique time-temperature paths, or thermal histories, that are the result of many complex and, sometimes, simultaneous/competing processes, ultimately leading to an incomplete understanding of their long-term thermal evolution. Of recent interest to the volcanology community is the length of time large volumes of eruptible and geophysically detectable magma exist within the crust prior to their eruption. Here we use a combination of diffusion chronometry, trace element, and thermodynamic modeling to quantify the long-term thermal budget of the 2.08 Ma, 630km3 Cerro Galán Ignimbrite (CGI) in NW Argentina, one of the largest explosive volcanic eruptions in the recent geologic record. We find that diffusion of both Mg and Sr in plagioclase indicate that erupted magmatic material only spent decades to centuries at or above temperatures (~750°C) required to produce and store significant volumes of eruptible magma. Calculated plagioclase equilibrium liquid compositions reveal an array that is controlled overall by fractionation of plagioclase + biotite + sanidine, although high-resolution trace element transects record a diversity of long-term storage conditions with some plagioclase recording periods of co-crystallization with biotite and sanidine, while others do not. Despite these chemical differences in long-term storage, we find diffusion models record a unimodal distribution which, when combined with prior work revealing zircon residence times of ~105 years, and calculated zircon saturation temperatures of 807 ± 8°C, provide compelling evidence that the CGI magmatic system spent most of its upper crustal residence in a largely uneruptible state and was ultimately remobilized shortly before eruption.

Sand spikes pinpoint powerful palaeoseismicity

Sand spikes, pin-shaped, carbonate-cemented sandstone bodies of variable size widely interpreted as sedimentary concretions, have been enigmatic for nearly two centuries. We here present a high-energy mechanism for their formation. Two classic sand spike occurrences are found in the North Alpine Foreland Basin of Central Europe and at Mount Signal in southern California, USA. A distinct seismite horizon in Mid-Miocene Molasse sediments of southern Germany, genetically linked with the Ries impact event, exhibits dewatering structures and contains numerous sand spikes with tails systematically orientated away from the Ries crater. Sand spikes at Mount Signal, strikingly similar in shape to those found in Germany, have tails that point away from the nearby San Andreas Fault. Based on their structural and stratigraphic context, we interpret sand spikes as a new type of seismite and a promising tool to identify strong impact-induced or tectonic palaeo-earthquakes and their source regions in the geologic record.

Sink or float: microtextural controls on the fate of pumice deposition during the 2012 submarine Havre eruption

Silicic submarine volcanic eruptions can produce large volumes of pumices that may rise buoyantly to the ocean surface and/or sink to the seafloor. For eruptions that release significant volumes of pumice into rafts, the proximal to medial submarine geologic record is thus depleted in large volumes of pumice that would have sedimented closer to source in any subaerial eruption. The 2012 eruption of Havre volcano, a submarine volcano in the Kermadec Arc, presents a unique opportunity to study the partitioning of well-constrained rafted and seafloor pumice. Macro- and microtextural analysis was performed on clasts from the Havre pumice raft and from coeval pumiceous seafloor units around the Havre caldera. The raft and seafloor clasts have indistinguishable macrotextures, componentry, and vesicularity ranges. Microtextural differences are apparent as raft pumices have higher vesicle number densities (109 cm−3 vs. 108 cm−3) and significantly lower pore space connectivity (0.3–0.95 vs. 0.9–1.0) than seafloor pumices. Porosity analysis shows that high vesicularity raft pumices required trapping of gas in the connected porosity to remain afloat, whereas lower vesicularity raft pumices could float just from gas within isolated porosity. Measurements of minimum vesicle throat openings further show that raft pumices have a larger proportion of small vesicle throats than seafloor pumices. Narrow throats increase gas trapping as a result of higher capillary pressures acting over gas–water interfaces between vesicles and lower capillary number inhibiting gas bubble escape. Differences in isolated porosity and pore throat distribution ultimately control whether pumices sink or float and thus whether pumice deposits are preserved or not on the seafloor.

Removal of the Northern Paleo-Teton Range along the Yellowstone Hotspot Track

Classically held mechanisms for removing mountain topography (e.g., erosion and gravitational collapse) require 10-100 Myr or more to completely remove tectonically generated relief. Here, we propose that mountain ranges can be completely and rapidly (&lt;2 Myr) removed by a migrating hotspot. In western North America, multiple mountain ranges, including the Teton Range, terminate at the boundary with the relatively low relief track of the Yellowstone hotspot. This abrupt transition leads to a previously untested hypothesis that preexisting mountainous topography along the track has been erased. We integrate thermochronologic data collected from the footwall of the Teton fault with flexural-kinematic modeling and length-displacement scaling to show that the paleo-Teton fault and associated Teton Range was much longer (min. original length 190-210 km) than the present topographic expression of the range front (~65 km) and extended across the modern-day Yellowstone hotspot track. These analyses also indicate that the majority of fault displacement (min. 11.4-12.6 km) and the associated footwall mountain range growth had accumulated prior to Yellowstone encroachment at ~2 Ma, leading us to interpret that eastward migration of the Yellowstone hotspot relative to stable North America led to removal of the paleo-Teton mountain topography via posteruptive collapse of the range following multiple supercaldera (VEI 8) eruptions from 2.0 Ma to 600 ka and/or an isostatic collapse response, similar to ranges north of the Snake River plain. While this extremely rapid removal of mountain ranges and adjoining basins is probably relatively infrequent in the geologic record, it has important implications for continental physiography and topography over very short time spans.

A prolonged, two-step oxygenation of Earth’s early atmosphere: Support from confidence intervals

The Great Oxidation Event (GOE), among Earth’s most transformative events, marked the sustained presence of oxygen above 10–5 times the present atmospheric level. Estimates of the onset of the GOE span 2501–2225 Ma and are based primarily on the loss of mass-independent fractionation of sulfur isotopes (MIF-S) in pyrite. To better constrain the timing of the GOE, we apply probabilistic techniques to estimate the confidence intervals of four proxies: MIF-S, redox-sensitive detrital minerals, "red beds," and I/(Ca + Mg). These GOE proxies are drawn from a highly fragmentary geologic record, and consequently, estimates of the 95% confidence intervals span tens to hundreds of millions of years—orders of magnitude larger than suggested by radiometric constraints on individual successions. Confidence interval results suggest that red beds and nonzero I/(Ca + Mg) values may have appeared earlier than 2480 Ma and 2460 Ma, respectively, whereas redox-sensitive detrital minerals and MIF-S may have disappeared after 2210 Ma and 2190 Ma, respectively. These data suggest a delay of potentially &gt;300 m.y. between initial and permanent oxygenation of the atmosphere and a delay of tens of millions of years between onset of the Lomagundi-Jatuli carbon isotope excursion and permanent oxygenation of the atmosphere.

Hydrodynamic influences on sedimentology and geomorphology of nearshore parts of carbonate ramps: Holocene, NE Yucatán Shelf, Mexico

ABSTRACT Although carbonate ramps are ubiquitous in the geologic record, the impacts of oceanographic processes on their facies patterns are less well constrained than with other carbonate geomorphic forms such as isolated carbonate platforms. To better understand the role of physical and chemical oceanographic forces on geomorphic and sedimentologic variability of ramps, this study examines in-situ field measurements, remote-sensing data, and hydrodynamic modeling of the nearshore inner ramp of the modern northeastern Yucatán Shelf, Mexico. The results reveal how sediment production and accumulation are influenced by the complex interactions of the physical, chemical, and biological processes on the ramp. Upwelled, cool, nutrient-rich waters are transported westward across the ramp and concentrated along the shoreline by cold fronts (Nortes), westerly regional currents, and longshore currents. This influx supports a mix of both heterozoan and photozoan fauna and flora in the nearshore realm. Geomorphically, the nearshore parts of this ramp system in the study area include lagoon, barrier island, and shoreface environments, influenced by the mixed-energy (wave and tidal) setting. Persistent trade winds, episodic tropical depressions, and winter storms generate waves that propagate onto the shoreface. Extensive shore-parallel sand bodies (beach ridges and subaqueous dune fields) of the high-energy, wave-dominated upper shoreface and foreshore are composed of fine to coarse skeletal sand, lack mud, and include highly abraded, broken and bored grains. The large shallow lagoon is mixed-energy: wave-dominated near the inlet, it transitions to tide-dominated in the more protected central and eastern regions. Lagoon sediment consists of Halimeda-rich muddy gravel and sand. Hydrodynamic forces are especially strong where bathymetry focuses water flow, as occurs along a promontory and at the lagoon inlet, and can form subaqueous dunes. Explicit comparison among numerical models of conceptual shorefaces in which variables are altered and isolated systematically demonstrates the influences of the winds, waves, tides, and currents on hydrodynamics across a broad spectrum of settings (e.g., increased tidal range, differing wind and wave conditions). Results quantify how sediment transport patterns are determined by wave height and direction relative to the shoreface, but tidal forces locally control geomorphic and sedimentologic character. Similarly, the physical oceanographic processes acting throughout the year (e.g., daily tides, episodic winter Nortes, and persistent easterly winds and waves) have more impact on geomorphology and sedimentology of comparable nearshore systems than intense, but infrequent, hurricanes. Overall, this study provides perspectives on how upwelling, nutrient levels, and hydrodynamics influence the varied sedimentologic and geomorphic character of the nearshore areas of this high-energy carbonate ramp system. These results also provide for more accurate and realistic conceptual models of the depositional variability for a spectrum of modern and ancient ramp systems.

Extinct Style of Plate Tectonics Explains Early Earth’s Flat Mountains

The geologic record suggests that despite Earth’s hot, thin crust during the Proterozoic, mountains were still able to form thanks to an extinct style of crustal deformation.

Gravitational sliding or tectonic thrusting?: Examples and field recognition in the Miura-Boso subduction zone prism

ABSTRACT Discrimination between gravity slides and tectonic fold-and-thrust belts in the geologic record has long been a challenge, as both have similar layer shortening structures resulting from single bed duplication by thrust faults of outcrop to map scales. Outcrops on uplifted benches within the Miocene to Pliocene Misaki accretionary unit of Miura-Boso accretionary prism, Miura Peninsula, central Japan, preserve good examples of various types of bedding duplication and duplex structures with multiple styles of folds. These provide a foundation for discussion of the processes, mechanisms, and tectonic implications of structure formation in shallow parts of accretionary prisms. Careful observation of 2-D or 3-D and time dimensions of attitudes allows discrimination between formative processes. The structures of gravitational slide origin develop under semi-lithified conditions existing before the sediments are incorporated into the prism at the shallow surfaces of the outward, or on the inward slopes of the trench. They are constrained within the intraformational horizons above bedding-parallel detachment faults and are unconformably covered with the superjacent beds, or are intruded by diapiric, sedimentary sill or dike intrusions associated with liquefaction or fluidization under ductile conditions. The directions of vergence are variable. On the other hand, layer shortening structure formed by tectonic deformation within the accretionary prism are characterized by more constant styles and attitudes, and by strong shear features with cataclastic textures. In these structures, the fault surfaces are oblique to the bedding, and the beds are systematically duplicated (i.e., lacking random styles of slump folds), and they are commonly associated with fault-propagation folds. Gravitational slide bodies may be further deformed at deeper levels in the prism by tectonism. Such deformed rocks with both processes constitute the whole accretionary prism at depth, and later may be deformed, exhumed to shallow levels, and exposed at the surface of the trench slope, where they may experience further deformation. These observations are not only applicable in time and space to large-scale thrust-and-fold belts of accretionary prism orogens, but to small-scale examples. If we know the total 3-D geometry of geologic bodies, including the time and scale of deformational stages, we can discriminate between gravitational slide and tectonic formation of each fold-and-thrust belt at the various scales of occurrence.

Late Pleistocene glacial chronologies and paleoclimate in the northern Rocky Mountains

The geologic record of mountain glaciations is a robust indicator of terrestrial paleoclimate change. During the last glaciation, mountain ranges across the western U.S. hosted glaciers while the Cordilleran and Laurentide ice sheets flowed to the west and east of the continental divide, respectively. Records detailing the chronologies and paleoclimate significance of these ice advances have been developed for many sites across North America. However, relatively few glacial records have been developed for mountain glaciers in the northern Rocky Mountains near ice sheet margins. Here, we report cosmogenic beryllium-10 surface exposure ages and numerical glacier modeling results showing that mountain glaciers in the northern Rockies abandoned terminal moraines after the end of the Last Glacial Maximum around 17–18 ka and could have been sustained by −10 to −8.5 °C temperature depressions relative to modern assuming similar or drier than modern precipitation. Additionally, we present a deglacial chronology from the northern Rocky Mountains that indicates while there is considerable variability in initial moraine abandonment ages across the Rocky Mountains, the pace of subsequent ice retreat through the Lateglacial exhibits some regional coherence. Our results provide insight on potential regional mechanisms driving the initiation of and sustained deglaciation in the western U.S. including rising atmospheric CO2 and ice sheet collapse.