Recognizing seamount-forearc collisions at accretionary margins: Insights from discrete numerical simulations

We conducted discrete numerical simulations to examine the effects of seamount collisions with forearcs along actively accreting subduction margins. Modeled seamount interactions leave behind distinctive structures in overriding forearcs that differ from those found at non-accreting margins. Whereas accretion above a planar décollement produces evenly spaced thrust faults with uniform displacements, seamounts activate one or more large-offset splay faults that accommodate substantial offset. Locally oversteepened slopes develop above the seamounts, but in contrast to non-accreting margins, the steep slopes are transient. Renewed accretion following seamount passage allows the equilibrium surface slopes to recover. Seamounts also protect incoming strata in their wake, delaying formation of new thrust faults and increasing fault spacing. Weak horizons within accreting strata allow the décollement to step up above the seamount, further protecting deeper strata and vertically partitioning wedge deformation. Notably, all modeled faults form in sequence, in contrast to out-of-sequence faults found at non-accreting margins. Similar structures found at many accretionary margins, including Nankai (offshore Japan), suggest that we may underestimate the role of seamount interactions in many locations, with implications for our assessment of subduction hazards in these settings.

Xenoconformities and the stratigraphic record of paleoenvironmental change

Most interpretations of the stratigraphic record are founded on the premise that the depositional environments that produced it either have not changed appreciably through time, or else have changed very slowly. Paradoxically, some of the most important transitions in the sedimentary archive are those interpreted to reflect relatively rapid, comprehensive paleoenvironmental change. Recognition of the anomalous nature of such transitions is vital to accurately understanding their significance but is not systematically incorporated in current stratigraphic models. The new term “xenoconformity” is therefore proposed, and defined as a stratigraphic surface or gradational interval that records a fundamental, abrupt, and persistent change in sedimentary facies across basinal to global scales. Xenoconformities may mark major paleoenvironmental tipping points and signal transformations in how paleoenvironmental signals were transferred into the stratigraphic record.

El Nino-Southern Oscillation-like variability in a late Miocene Caribbean coral

Reconstructions of Pliocene sea-surface temperature (SST) gradients and thermocline depths suggest that the zonal temperature gradient of the tropical Pacific was distinct from the modern. However, the nature of any El Niño–Southern Oscillation (ENSO) variability superimposed on this mean state is difficult to determine. We developed monthly resolved multidecadal stable isotopic time series from an extremely well preserved central Caribbean coral dating to the Miocene-Pliocene transition, prior to closure of the Central American Seaway (CAS). Paleoceanographic modeling suggests that the flow of water associated with El Niño and La Niña events through the CAS allowed Caribbean corals to record the ENSO-related SST anomalies. Spectral analysis of coral oxygen isotope ratios reveals periodicities similar to modern ENSO signatures, suggesting that ENSO-like variability characterized the Miocene-Pliocene transition.

Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap

Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40–58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region’s unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.

A triple S-shaped compositional profile in a Karoo dolerite sill–Evidence of concurrent multiple fractionation processes

Thick dolerite sills show a range of vertical geochemical variation trends attributable to various processes during slow crystallization. We have identified chemical parameters in a 169-m-thick sill from the Karoo igneous province in South Africa that define three different lower crossover levels (maximum or minimum concentrations) creating S-shaped variation trends. The crossover level for whole-rock MgO is at 20 m height (due to mechanical sorting of olivine); the anorthite content of plagioclase is at 52 m (due to addition of primitive magma); and that of the incompatible trace elements is at 75 m (due to different proportions of early formed grains to trapped liquid). Each process can operate independently and concurrently, leading to their maximum effects occurring at different levels in the intrusion. The independence of these processes and the triple S-shaped geochemical profiles have not been recognized before in any mafic-ultramafic sills.

Precarious ephemeral refugia during the earliest Triassic

The term refuge describes, in both ecology and paleoecology, an ecosystem that acts as a sanctuary during times of environmental stress. This study tests the concept by examining the fate of a single community that lived ~50 k.y. after the end-Permian mass extinction (EPME). An assemblage of trace fossils, bivalves, and echinoids, living on a microbial mat in a slope environment, is preserved on a single bedding plane in the Shangsi section, south China. The microbial community was vital to the success of the refuge, acting as a stable substrate, food source, and oxygen supply. Shallow-water microbial communities have been interpreted as refugia, but this deeper site may have been critical to organisms with temperature sensitivities. Published paleotemperature calculations suggest sublethal surface temperatures of 34 °C at Shangsi. A species of cidaroid echinoid likely migrated to cooler deep waters to optimize development, suggesting that the success of this shallow-water clade is attributable to such refugia, when survival was most precarious after the EPME. The ecosystem was short lived, depending on low productivity and slow sedimentation. When conditions became suboptimal due to ash input and increasing productivity, the ecosystem quickly collapsed, allowing for colonization by opportunistic taxa including Claraia and microconchids. Elsewhere the ecosystem may have remained unchanged. Earliest Triassic refugia may have been restricted to these ephemeral environmental settings until organisms adapted to continuing harsh conditions.

Biotic effects on oxygen consumption during weathering: Implications for the second rise of oxygen

The oxygen concentration of the atmosphere likely increased substantially in the late Neoproterozoic. Although several studies have presented compelling geochemical evidence for this stepwise oxygenation, few have addressed the mechanisms behind it. Recently it was hypothesized that the advent of eukaryotic life on land, and the associated increase in soil respiration, led to a transient reduction in the supply of oxygen for rock weathering, temporarily reducing oxidative weathering rates, allowing atmospheric oxygen levels to rise to restore the oxygen supply. To evaluate this hypothesis quantitatively, we developed a simple one-dimensional diffusion-reaction soil model that reduces the many oxygen weathering sinks to one, pyrite, given that it is the dominant sink at low oxygen concentrations. In simulations with no biological respiration, pyrite weathering rates become oxygen independent at an atmospheric oxygen concentration between 10–6x the present-day atmospheric level (PAL) and 1 PAL. On the other hand, when biological respiration is considered, pyrite weathering remains oxygen dependent even at modern oxygen levels. Constrained by modern weathering profiles and soil respiration rates, we find that the atmospheric oxygen level may have increased by up to two orders of magnitude as biotic soil respiration increased. This may be sufficient to explain the second rise in atmospheric oxygen inferred for the Neoproterozoic.

Persistence of a freshwater surface ocean after a snowball Earth

Geochemical data from cap carbonates deposited above Cryogenian glacial deposits have been widely used to infer the conditions that prevailed in the aftermath of snowball Earth. However, the time scale over which these carbonates were deposited and the degree to which they record the chemistry of a globally well-mixed ocean have remained poorly constrained. During deglaciation, a large volume of meltwater entered the ocean, creating two distinct layers: the fresh, hot, and light upper layer, and the salty, cold, and dense lower layer. Here we estimate the ocean mixing time scale based on energetic constraints. We find that the mixing time scale is 104–105 yr, with a best estimate of ~5 x 104 yr, or up to 100 times longer than that of the modern ocean. Mixing of the surface temperature anomaly implies a delayed sea-level rise of 40–50 m associated with pure thermal expansion. This result reconciles geological, geochemical, and paleomagnetic data from basal Ediacaran cap carbonates with physical oceanographic theory. In particular, our model suggests that (1) the cap dolostones formed predominantly in a freshwater environment; (2) the waters in which the dolostones formed were not well mixed with saline deep water, allowing for large geochemical differences between the cap dolostones and the deep ocean; and (3) the cap carbonate sequences formed in a two-phase transgression that lasted >104 yr, which is consistent with both local sea-level records and the preservation of magnetic excursions and reversals.