Estimating the mass, volume, and dispersal of the deposits of very small and/or extremely weak explosive eruptions is difficult, unless they can be sampled on eruption. During explosive eruptions of Halema‘uma‘u Crater (Kilauea, Hawaii) in 2008, we constrained for the first time deposits of bulk volumes as small as 9–300 m³ (1 x 10⁴ to 8 x 10⁵ kg) and can demonstrate that they show simple exponential thinning with distance from the vent. There is no simple fit for such products within classifications such as the Volcanic Explosivity Index (VEI). The VEI is being increasingly used as the measure of magnitude of explosive eruptions, and as an input for both hazard modeling and forecasting of atmospheric dispersal of tephra. The 2008 deposits demonstrate a problem for the use of the VEI, as originally defined, which classifies small, yet ballistic-producing, explosive eruptions at Kilauea and other basaltic volcanoes as nonexplosive. We suggest a simple change to extend the scale in a fashion inclusive of such very small deposits, and to make the VEI more consistent with other magnitude scales such as the Richter scale for earthquakes. Eruptions of this magnitude constitute a significant risk at Kilauea and elsewhere because of their high frequency and the growing number of "volcano tourists" visiting basaltic volcanoes.

The hypothesized collapse in primary productivity associated with the Cretaceous-Paleogene (K-Pg) mass extinction would have been particularly severe on suspension feeders dependent on phytoplankton. Previous research on the ecological dynamics of erect bryozoans in the Danish Basin supported the phytoplankton crash hypothesis in showing a major postextinction increase in the skeletal biomass of cyclostome bryozoans relative to the more nutrient-demanding cheilostome bryozoans. New data on the ecological dynamics between these two bryozoan clades across the K-Pg boundary are provided to determine whether the postextinction cyclostome spike is also evident in encrusting bryozoans and over a greater geographical area. Changes across the K-Pg were quantified in encrusting bryozoans using three metrics: (1) abundance of colonies; (2) area of substrate covered; and (3) colony form. Data from the southeastern United States were added to those from Denmark to control for regional factors. Our analyses indicate a globally uniform change among encrusting bryozoans but without the strong postextinction cyclostome spike seen previously in Danish erect bryozoans. An increased proportion of sheet over runner cyclostome colonies was found in the lower Danian of Denmark and the southeastern United States, despite the expectation that runners with widely dispersed zooids should fare better in low nutrient conditions. Furthermore, weighed samples of erect bryozoans from the basal Danian of Maastricht (Netherlands) and the southeastern United States failed to reveal a strong cyclostome spike. These findings question the phytoplankton crash hypothesis, or at least the extent to which this kill mechanism influenced suspension feeders during the K-Pg mass extinction.

The Afar Depression, at the northern end of the East African Rift, is the only place on land where the transition from a plume-induced continental breakup to seafloor spreading is active today. New images of seismic velocity structure, based on exceptional new data sets, show that the mantle plume that initiated rifting in Africa is absent beneath Afar today. The images are dominated by a major low-velocity feature at ~75 km depth closely mimicking the abrupt changes in rift axis orientation seen at the surface. This is likely associated with passive upwelling beneath the rift. Additional focused low-velocity anomalies show that small diapiric upwellings are present beneath major off-axis volcanoes. These multiple melting sources can explain the wide range of geochemical signatures seen in Afar. These images suggest that passive upwelling beneath Afar marks the initiation of rift segmentation as continental breakup progresses to seafloor spreading.

High-resolution lidar data reveal a prominent latest Pleistocene–Holocene scarp on the Sawtooth fault (central Idaho, United States). The fault scarp marks 55–65 km of the range front, and may comprise two segments. The scarp is 4–9 m high in latest Pleistocene glacial landforms (11–14 ka) and 2–3 m high in Holocene alluvial landforms, implying 2–3 postglacial rupture events. Patterns of fault scarp continuity, coupled with existing gravity data, suggest that active faulting may have migrated northward during Pleistocene time. Detailed comparisons of raw lidar digital elevation models (DEMs), bare-earth lidar DEMs, and field surveys indicate that the bare-earth lidar data document the fault scarp morphology accurately and allow for detailed fault analysis where field evaluation is difficult. The documentation of Holocene motion on the Sawtooth fault demonstrates that ENE-directed extension extends across central Idaho, and that the fault contributes to seismic hazards.

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-CO₂ cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here we show that the growth of garnet may be used as a proxy for devolatilization at blueschist to lower eclogite facies conditions in subduction zones. Using thermodynamic analysis including the effects of garnet fractionation and fluid removal, we show the proportional relationship between garnet and water production in two end-member crustal lithologies (pelitic sediment and hydrated mid-oceanic-ridge basalt [MORB]) in three representative subduction geotherms. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ~1.4 and 3.0 GPa where geophysical models and observations predict dehydration. The average production ratio for altered MORB compositions is 0.52 (wt% water as fluid per vol% garnet) in cooler geotherms (Honshu [Japan] and Nicaragua) and 0.27 in hotter geotherms (Cascadia [North America]), whereas for pelite the production ratios are about half (0.24 and 0.13, respectively). Garnet growth correlates with production of 3.3–5.9 wt% water in hydrated MORB and 1.8–3.1 wt% water in pelite, representing 42%–100% of the water lost between 0.5 and 6.5 GPa from a fully saturated starting material. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for progressive subduction zone dehydration.

Climate is a primary control on the chemical composition of paleosols, making them a potentially extensive archive applicable to problems ranging from paleoclimate reconstruction to paleoaltimetry. However, the development of an effective, widely applicable paleosol temperature proxy has remained elusive. This is attributable to the fact that various soil orders behave differently due to their respective physical and chemical properties. Therefore, by focusing on an individual order or a subset of the 12 soil orders whose members exhibit similar process behavior, a better-constrained paleothermometer can be constructed. Soil chemistry data were compiled for 158 modern soils in order to derive a new paleosol paleothermometry relationship between mean annual temperature and a paleosol weathering index (PWI) that is based on the relative loss of major cations (Na, Mg, K, Ca) from soil B horizons. The new paleothermometer can be applied to clay-rich paleosols that originally formed under forest vegetation, including Inceptisols, Alfisols, and Ultisols, and halves the uncertainty relative to previous approaches. A case study using Cenozoic paleosols from Oregon (United States) shows that paleotemperatures produced with this new proxy compare favorably with paleobotanical temperature estimates. Global climatic events are also evident in the Oregon paleosol record, including a 2.8 °C drop across the Eocene-Oligocene transition comparable to marine records, and a Neogene peak temperature during the Mid-Miocene Climatic Optimum.

The ca. 3 Ga Farrel Quartzite (FQ, Western Australia) contains possible organic microfossils of unusual spindle-like morphology that are surprisingly large and complex, preserved along with spheroids. The unusual nature of the possible fossils, coupled with their antiquity, makes their interpretation as biogenic difficult and debatable. Here, we report 32 in situ carbon isotopic analyses of 15 individual FQ specimens. The spheroids and the spindle-like forms have a weighted mean ¹³C value of –37, an isotopic composition that is quite consistent with a biogenic origin. Both the spheroids and the spindle-like structures are isotopically distinct from the background organic matter in the same thin section (weighted mean ¹³C value of –33), which shows that the preserved microstructures are not pseudofossils formed from physical reprocessing of the bulk sedimentary organic material. When considered along with published morphological and chemical studies, these results indicate that the FQ microstructures are bona fide microfossils, and support the interpretation that the spindles were planktonic. Our results also provide metabolic constraints that imply most of these preserved microorganisms were autotrophic. The existence of similar spindles in the ca. 3.4 Ga Strelley Pool Formation of Australia and the ca. 3.4 Ga Onverwacht Group of South Africa suggests that the spindle-containing microbiota may be one of the oldest, morphologically preserved examples of life. If this is the case, then the FQ structures represent the remains of a cosmopolitan biological experiment that appears to have lasted for several hundred million years, starting in the Paleoarchean.

Nitrogen cycling has been evaluated across a depth transect in the late Paleoproterozoic Animikie Basin (North America), spanning the end of Earth’s final period of global iron precipitation, and a major transition to euxinic conditions in areas of high productivity. Sediments from near shore, where productivity was highest, have ¹⁵N compositions up to ~3 higher than at more distal sites. This suggests that as NH₄⁺ mixed vertically upward into the oxic photic zone from the anoxic ocean interior, it was either assimilated by organisms or oxidized. Subsequent enhanced production of N₂ by denitrification or anammox (anaerobic ammonium oxidation) led to the observed increase in ¹⁵N close to shore. Any deficit in biologically available N was overcome by N₂-fixing organisms, but the input of N with low ¹⁵N from this process did not overwhelm the increase in ¹⁵N from denitrification. Because there is no evidence for conditions of severe N stress arising from trace metal limitation (particularly Mo) of N fixation during the transition to euxinic conditions, losses of N were either very small (potentially because low O₂ levels limited NH₄⁺ oxidation), or alternative pathways that retained N were important. The fact that Mo appears to have remained bioavailable for N fixation, either suggests that the extent or severity of sulfidic water column conditions was not sufficient to quantitatively sequester Mo on a global scale, or that rivers directly delivered Mo to surface waters on the inner shelf. The effects of N₂ fixation on ¹⁵N increased to more distal parts of the shelf, consistent with models invoked for modern upwelling zones over broad continental margins.

Eclogitization of the basaltic and gabbroic layer in the oceanic crust involves a volume reduction of 10%–15%. One consequence of the negative volume change is the formation of a paired stress field as a result of strain compatibility across the reaction front. Here we use waveform analysis of a tiny seismic cluster in the lower crust of the downgoing Pacific plate and reveal new evidence in favor of this mechanism: tensional earthquakes lying 1 km above compressional earthquakes, and earthquakes with highly similar waveforms lying on well-defined planes with complementary rupture areas. The tensional stress is interpreted to be caused by the dimensional mismatch between crust transformed to eclogite and underlying untransformed crust, and the earthquakes are probably facilitated by reactivation of fossil faults extant in the subducting plate. These observations provide seismic evidence for the role of volume change–related stresses and, possibly, fluid-related embrittlement as viable processes for nucleating earthquakes in downgoing oceanic lithosphere.

The sulfur (S) isotope difference between sedimentary sulfate and sulfide phases preserved in sedimentary rocks (³⁴S) has been utilized to reconstruct marine sulfate concentrations and inferentially the redox evolution of Earth’s surface. These interpretations are largely based on experimental studies that indicate that microbial sulfate reduction is accompanied by a substantial kinetic isotope effect (up to 66), but only at sulfate concentrations >~200 μM. In this study, we examine S isotope systematics in a modern, low-sulfate euxinic lake (~100–350 μM) and find that the calculated kinetic isotope effect associated with microbial sulfate reduction ( ³⁴S) is relatively large (~23.5), but preserved ³⁴S values are considerably smaller (4.7–9.9). ³⁴S values in this system are controlled by the fraction of the sulfate reservoir that is consumed during sulfate reduction and the location of pyrite formation. This reservoir effect strongly influences the S isotope composition of sulfide preserved in the rock record such that ³⁴S values increase as a function of sulfate levels, even when sulfate concentrations are >200 μM and the kinetic isotope effect is expressed. These findings have important implications for reconstructing the chemical evolution of the ocean-atmosphere system throughout Earth history—not just for the Precambrian.

Past glacial-interglacial climate transitions were accompanied by millennial-scale pulses in atmospheric CO₂ that are widely thought to have resulted from the release of CO₂ via the Southern Ocean. However, direct proxy evidence for a Southern Ocean role in regulating past ocean-atmosphere CO₂ exchange is scarce. Here we use combined radiocarbon and neodymium isotope measurements from the last deglaciation to confirm greatly enhanced overturning and/or air-sea exchange rates relative to today, in particular during the Bølling-Allerød warm interval. We show that this deglacial pulse in ocean ventilation was not driven by the North Atlantic overturning alone, and must have involved an increase in the ventilation of southern-sourced deep waters. Our results thus confirm the removal of a physical and/or dynamical barrier to effective air-sea (CO₂) exchange in the Southern Ocean during deglaciation, and highlight the Antarctic region as a key locus for global climate/carbon-cycle feedbacks.

We conducted deep seismic-reflection surveying across the Jurassic Daba Shan thrust belt of central China to investigate how and why the continued convergence between north and south China lasted ~50 m.y. after the Triassic closure of their intervening oceans. Our study, together with surface geology, gravity surveying, and magnetic observations, indicates widespread occurrence of mafic plutons below the Daba Shan thrust belt. We propose that subduction of dominantly eclogitized mafic crust of northern south China provided the driving force for continued convergence between north and south China after ocean closure.

The existence of Hesperian age (3.7–3.4 Ga) surface water bodies on Mars is a contentious issue, often conflicting with favored climate models. Extensive lakes are proposed to have filled parts of Valles Marineris during this period, yet evidence for their presence and temporal continuity is poorly constrained. Here we report geomorphic and chronologic evidence for the initiation and demise of a voluminous lake system within the basins of eastern Valles Marineris. We find that independent, kilometer-deep lakes were present here well after the wetter, global climate optimum that characterized the previous Noachian epoch (4.1–3.7 Ga). Relative and impact crater chronologies of flood channels emerging from lake basins indicate relatively late lake spillover in the Early Amazonian (ca. 3.0 Ga). Drawdown of the lake and cessation of interbasin sedimentation may be recorded by a similar Early Amazonian (ca. 3.1 Ga) crater retention age on the surface of Capri Mensa, a 4-km-tall, sulfate-bearing interior layered deposit. The topography data demonstrate that incision of the bedrock barriers between the basins during spillover was driven by a dramatic local base-level difference between the lake surface and downstream basin floors. We postulate that the lake spillover process created an integrated drainage routing system between a voluminous equatorial water supply and the northern plains basin.

Fluid mixing across unconformities between crystalline basement and overlying sedimentary basins is commonly invoked as an efficient chemical mechanism for ore deposition, but the origin of basement brines and the process of ore formation have rarely been linked by direct evidence. Using laser ablation–inductively coupled plasma–mass spectrometry microanalysis of individual fluid inclusions with an improved detection approach for anion components, we determined simultaneously the ore metal concentrations and the Cl/Br ratio in texturally well constrained inclusion assemblages from a basement-hosted quartz-fluorite-barite-Pb-Zn vein system. An inverse correlation between the Pb + Zn concentrations and the Cl/Br mass ratios in the fluid inclusions provides clear evidence for mixing of a basement-derived metal-rich brine and a metal-poor formation water that acquired its salinity from halite dissolution in Triassic evaporites of the sedimentary cover. This mixing of two distinct brines with comparable salinity is recorded during the growth of individual quartz crystals containing small galena inclusions, demonstrating the transient and episodic nature of fluid mixing during mineral deposition.

The oxidation state of Fe in garnets in a garnet peridotite xenolith from the Wesselton kimberlite (South Africa) was quantitatively mapped using X-ray absorption near edge structure (XANES) spectroscopy. Maps of Fe³⁺/Fe were produced by recording the fluorescence intensity at discrete energies rather than recording the full spectrum at each point. The intensity at each point in the map was quantitatively converted to Fe³⁺/Fe with reference to a linear calibration derived from garnet standards for which Fe³⁺/Fe had been determined previously by Mössbauer spectroscopy. The resolution of these maps approaches that of elemental maps obtained using an electron microprobe. The maps reveal zoning in Fe³⁺/Fe between the core (0.075) and rim (0.125) that correlates with zoning of other elements. The rims record an oxidizing metasomatic event in the lithospheric mantle. The oxygen fugacity (f_O2) of this metasomatism is considerably higher than expected from studies of homogeneous garnets that exhibit metasomatic signatures; such garnets may represent a re-equilibrated average of the original (core) and metasomatic (rim) f_O2 values. Metasomatism of the lithospheric mantle may thus have a greater impact on diamond stability than previously thought.

Igneous rocks record the direction of the Earth’s magnetic field as they cool through their Curie temperature. The mafic magmas of the 8-km-thick Bushveld Complex of South Africa took 65 k.y. to be emplaced, 180 k.y. to solidify (to 900 °C), and a further 500 k.y. for the entire intrusion to cool below 580 °C, the Curie temperature of magnetite. Once solid, the cooling of this intrusion occurred mainly from the top downward, with slower cooling through its floor. As a result, the upper rocks cooled through their Curie temperature before those at the base; the portion 6 km below the upper contact was the last to reach the Curie temperature. Thus, the intrusion records a mainly top-down sequence of three paleomagnetic reversals starting with N (normal direction). The last two are also recorded from the base of the mafic sequence upward as it cooled through 580 °C later than the top. The lateral variations in thickness of the Bushveld Complex are important in this interpretation, because thinner sections cooled more quickly. Hence, reversals do not always correlate with stratigraphy. Specific reversals provide a cooling marker horizon that may crosscut the stratigraphic layering. The interpretation of the order and number of paleomagnetic reversals presented here differs from previous interpretations that envisage the oldest paleomagnetic directions to be recorded sequentially from the base upward, and has implications for the interpretation of paleomagnetic results from all thick intrusions, mafic and felsic.

The accurate timing of biogeographic dispersal can be determined by examining the age of fossiliferous strata on either side of a physical barrier. Here we show that African mammals migrated to Iberia and European mammals migrated to North Africa at the same time before isolation of the Mediterranean Sea during the Messinian. The fossil site of Venta del Moro (Spain) exhibits western Europe’s most complete vertebrate fauna for the latest Miocene. Its uniquely cosmopolitan assemblage is evidence of faunal dispersals from Africa and Asia to Europe during the latest Miocene glaciation. A preliminary paleomagnetic study suggested an age of 5.8 Ma for this site, but our expanded magnetostratigraphy dates the site at 6.23 Ma. In addition, we recalibrated the paleomagnetic age of the Librilla site (Spain) and the North Africa site of Afoud-1 (Morocco) using the Astronomical Tuned Neogene Time Scale. Our results show a two-way African-Iberian mammal dispersal just before 6.2 Ma. These new ages indicate that an ephemeral land corridor existed between the two continents 250 k.y. before the onset of the Messinian Salinity Crisis, reflecting a tentative initial isolation of the Mediterranean Sea. This corridor developed after tectonics closed the Betic Seaway at 6.3 Ma and during the intensification of the latest Miocene glaciation at 6.26 Ma, when water circulation in the Mediterranean became very restricted.

The relationship between climate change and the development of Old World riverine civilizations is poorly understood because inadequate dating control has hindered effective integration of archaeological, fluvial, and climate records. This paper presents the most comprehensive and robustly dated archaeological and paleoenvironmental data sets yet compiled for the desert Nile. It focuses on the valley floor hinterland of the Kingdom of Kerma (2400–1450 B.C.) in northern Sudan. Kerma emerged as a rival to Egypt during Africa’s first "Dark Age" drought. In contrast to other irrigation-based agriculturists in Egypt and Asia, Kerma flourished during the environmental crisis ca. 2200 B.C. We have studied the stratigraphy and archaeological records of paleochannels across an 80 km reach of the Nile upstream of Kerma using optically stimulated luminescence to date when channels flowed and when they dried up. The dynamics of the local alluvial environment were critical in determining whether climatic fluctuations and changes in river flow represented an opportunity for floodwater farmers (5000–3500 B.C.), a hazard that could be managed (2400–1300 B.C.), or an environmental catastrophe that resulted in settlement abandonment (after 1300 B.C.).

Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (M_w) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an M_w 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of M_w 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ~200 m of active injection wells and within ~1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.

Many faults are characterized by naturally polished, reflective, glossy surfaces, termed fault mirrors (FMs), that form during slip. Recent experiments also find that FMs form during rapid sliding between rock surfaces, and that FM formation coincides with pronounced friction reduction. The structure of FMs and the mechanism of their formation are thus important for understanding the mechanics of frictional sliding, particularly during earthquakes. Here we characterize the small-scale structure of natural carbonate FMs from three different faults along a tectonically active region of the Dead Sea transform. Atomic force microscopy measurements indicate that the FMs have extremely smooth surface topography, accounting for their mirror-like appearance. Electron microscope characterization revealed a thin (<1 µm) layer of tightly packed nanoscale grains coating a rougher layer comprising micron-size calcite crystals. The crystals contain closely spaced, plastically formed twins that define new subgrain boundaries. The narrow subgrains are observed to break into submicron pieces near the sheared surface. This observation suggests a new brittle-ductile mechanism for nanograin formation. The role of ductility during frictional sliding, both in forming the nanograin layer, and in the deformation process of the powder, may be critical for understanding shear on geological faults.

Large explosive volcanic eruptions inject gases, aerosols, and fine ashes into the stratosphere, potentially influencing climate. Emissions of chlorine (Cl) and bromine (Br) from such large eruptions play an important role for catalytic destruction of ozone in the stratosphere, but hitherto the global effects of simultaneous catastrophic release of volcanic Br and Cl into the stratosphere have not been investigated. The Br release from 14 large explosive eruptions throughout Nicaragua covering an entire subduction zone segment in the past 70 ka was determined with petrologic methods. Melt inclusions in volcanic phenocrysts were analyzed using a new optimized synchrotron–X-ray fluorescence microprobe set-up. Single eruptions produced Br outputs of 4–600 kt, giving an average Br emission of 27 kt per eruption. Using the assumption that 10% of the emitted halogens reach the stratosphere, the average Br and Cl loading to the stratosphere would be 3 ppt and 1500 ppt, respectively, which together would account for 185% of the preindustrial equivalent effective stratospheric Cl loading. We thus conclude that many large tropical volcanic eruptions had and have the potential to substantially deplete ozone on a global scale, eventually forming future ozone holes.

Ion microprobe analyses of garnet porphyroblasts from three separate splays of the mid-crustal Walter-Outalpa shear zone, Curnamona Province, South Australia, indicate homogeneous ¹⁸O values of <3. Integrated Lu-Hf geochronology and electron microprobe compositional mapping demonstrate that closed-system growth of these isotopically light garnets initiated as early as 531 Ma, prior to peak metamorphism and deformation during the Delamerian Orogeny (514–490 Ma). We attribute this to the prograde burial and dehydration of altered fault panels under thick sedimentary sequences during pre-orogenic basin formation. Contrary to established fluid transport models, surficial fluid signatures were not imposed at depth by large fluxes of downward-penetrating fluids, but rather by the exposure and meteoric alteration of fault rocks that were subsequently buried and reactivated as ductile shear zones. The existence of low ¹⁸O values in deeply exhumed shear zones may therefore be indicative of fault structures that have a prior history of surface exposure, weathering, burial and re-exposure.

The Apennines belt of Italy undergoes a northeast-trending extension at a rate of a few millimeters per year that generates moderate to large normal-faulting earthquakes. In this paper, we show that seismicity, large earthquakes, strong gas emission, and belt topography all correlate with a broad, low Vp anomaly in the uppermost mantle. We propose that a thermal/fluid anomaly in the mantle, associated with sub-lithospheric mantle replacement after delamination of the Adria lithosphere, supports the topography of the belt and drives the extensional tectonics. The mantle anomaly is likely caused by deep fluids coming from the dehydration of the material subducted during the Europe-Adria collision and the delamination of Adria. Beneath the belt, CO₂-rich fluids are accumulated and occasionally discharged during large normal faulting earthquakes. After the replacement of sub-lithospheric mantle, the temperature at the base of the crust increases causing crustal stretching, anatexis, and strong degassing.