BACKYARD NATURE HOME | PLANTS | ANIMALS | ECOLOGY | GEOLOGY | GARDENING | TOOLS  

Backyard Nature's Direct Feed from
The Geology Society of America's latest issue of
GEOLOGY

Excerpts from the  March 29th, 2017 issue

A previously unrecognized high-temperature impactite from the Steen River impact structure, Alberta, Canada

Here, we report a previously unrecognized impactite from the Steen River impact structure in Alberta, Canada, which was intersected by continuous diamond drill core into the allochthonous proximal deposits of this buried 25-km-diameter complex crater. A suite of high-temperature minerals defines the matrix, formed by grain growth in a solid state by static recrystallization of an originally clastic matrix, deposited at temperatures ≥800 °C. This rock type is predominantly a result of the recrystallization of target material driven by the acceleration of hot gasses from volatilized sedimentary cover mixed with variably shocked crystalline basement. Approximately one-third of terrestrial impact structures occur in mixed target rocks; therefore, this type of impactite may be more common than previously realized. Contact metamorphism between entrained sedimentary target rocks and the juxtaposed hot matrix resulted in carbonate decomposition to form a rare spinel-group mineral, magnesioferrite. In crater environments, magnesioferrite has been found in the distal Chicxulub (Mexico) ejecta and may prove a novel indicator mineral for impact into carbonate-bearing target rocks.


Recurrent hierarchical patterns and the fractal distribution of fossil localities

Understanding the spatial structure of fossil localities is critical for interpreting Earth system processes based on their geographic distribution. Coordinates of marine and terrestrial sites in the conterminous United States for 17 time bins were analyzed using point pattern statistics. Lacunarity analysis shows that the spatial distributions of sites are fractal for almost every studied interval, indicating that clumping of localities occurs at multiple scales. Random hierarchical multiplicative processes provide a theoretical null model for the distribution of collecting sites, consistent with their occurrence being a complex product of numerous biological, geological, and anthropogenic processes acting at many spatial and temporal scales. Mechanistic models for the formation, preservation, and exposure of fossil localities and other geologic entities can be tested using point pattern and related spatial statistics.


Seismic array constraints on reach-scale bedload transport

Measurements and mechanical models of heterogeneous bedload transport in rivers remain basic challenges for studies of landscape evolution and watershed management. A 700 m reach of the Trinity River (northern California, USA), a large gravel-bed river, was instrumented with an array of 76 seismographs during a dam-controlled flood and gravel augmentation to investigate the potential for out-of-stream monitoring. The temporal response to gravel augmentation during constant discharge provides strong evidence of seismic sensitivity to bedload transport and aids in identification of the seismic frequencies most sensitive to bedload in the study area. Following gravel augmentations, the seismic array reveals a period of enhanced transport that spans most or all of the reach for ~7–10 h. Neither the duration nor the downstream extent of enhanced transport would have been constrained without the seismic array. Sensitivity to along-stream transport variations is further demonstrated by seismic amplitudes that decrease between the upper and lower halves of the reach consistent with decreased bedload flux constrained by time-lapse bathymetry. Insight into the magnitude of impact energy that reaches the bed is also gained from the seismic array. Observed peak seismic power is ~1%–5% of that predicted by a model of saltation over exposed bedrock. Our results suggest that dissipation of impact energy due to cover effects needs to be considered to seismically constrain bedload transport rates, and that noninvasive constraints from seismology can be used to test and refine mechanical models of bedload transport.


The control of the tropical North Atlantic on Holocene millennial climate oscillations

Changes in ocean dynamics in the northern North Atlantic affect the thermohaline circulation that controls global climate. During glacial and deglaciation periods these dynamics are enhanced due to large variations in the surface ocean density caused by changes in glacier volumes. During full interglacial conditions, the dominant role of the northern North Atlantic on global climate is limited due to the reduced discharge of freshwater to the ocean, causing other regional dynamics to gain importance. Here we present a speleothem 18O record from the Iberian Peninsula that supports that the northern North Atlantic and tropical North Atlantic were both source regions of millennial climate oscillations during the Holocene. The speleothem 18O signal records millennial time-scale changes in the hydrological cycle as a result of persistent anomalies of the Gulf Stream–North Atlantic Current dynamics. In addition, the speleothem 18O record shows synchronous variability with records from the eastern Pacific region though the entire Holocene, whereas records from western Pacific region have limited or no correlation beyond periods of major instability of the northern North Atlantic. The discontinuous climate connection among the studied records is the result of different mechanisms affecting the climate system that originated in distant regions. We suggest that two regions, the tropical North Atlantic and northern North Atlantic, alternate their dominance as source regions causing millennial climate anomalies in large planetary regions. The duration of these persistent climate changes and the extension of the regions affected depend on the region triggering the anomaly because different mechanisms affecting the climate system are involved.


Controls and feedbacks in the coupling of mountain channels and hillslopes

Mountain channels can be strongly coupled with adjacent hillslopes, exchanging both mass and energy. However, hypotheses of the underlying cause and effect relations are based on indirect observations that do not resolve the mechanics of channel-hillslope coupling at the process scale. Here we present direct observational data of a coupled channel-hillslope system in the catchment area of the Erlenbach, a mountain stream in Switzerland. A slow-moving landslide flanking this alpine stream failed after a flood had eroded an alluvial step in the channel at its base, representing evidence for an upsystem link in channel-hillslope coupling. Progressive accumulation of landslide debris in the channel resulted in a renewed step, stabilizing the hillslope and restoring the channel step in a downsystem link. Thus, upsystem and downsystem coupling mechanisms are joined in a negative feedback cycle. In this cycle, debuttressing and rebuttressing due to channel bed erosion and alluviation are the dominant controls on hillslope stability. Based on an order of magnitude estimate it is plausible that the observed feedback mechanism is a relevant process in the production of coarse (>2 mm) sediment in the Erlenbach.


Block-controlled hillslope form and persistence of topography in rocky landscapes

Rocky hillslopes dotted with boulder-sized blocks and covered by a thin, nonuniform soil are common in both steep landscapes and arid environments on Earth, as well as on other planets. While the evolution of soil-mantled, convex-upward hillslopes in uniform lithology is reasonably well understood, the influence of heterogeneous lithology and geologic structure on hillslope form and evolution has yet to be properly addressed. Landscapes developed in layered sedimentary rocks feature sharp-edged landforms such as mesas and hogbacks that exhibit steep, linear to concave-upward ramps with scattered blocks calved from resistant rock layers overlying softer strata. Here we show that blocks can control the persistence of topography and the form and evolution of hillslopes in these landscapes. We present a numerical model demonstrating that incorporation of feedbacks between block release, interruption of soil creep by blocks, and sporadic downslope movement of blocks are necessary and sufficient to capture the morphology and evolution of these landscapes. Numerical results are reproduced by a simple analytical solution that predicts steady-state concave hillslope form and average slope angle from block size and spacing. Our results illuminate previously unrecognized hillslope feedbacks, advancing our understanding of the geomorphology of rocky hillslopes. On a landscape scale, our findings establish a quantitative method to address the migration of sharp edges and the persistence of topography in layered landscapes.


Pore fluids in Dead Sea sediment core reveal linear response of lake chemistry to global climate changes

Pore fluids extracted from a 456 m sediment core, recovered within the framework of a multinational and International Continental Scientific Drilling Program (ICDP) co-sponsored effort at the bottom of the terminal Dead Sea, recorded the chemical variations in the deep lake over the past 220 k.y. Mg2+ and Br were shown to be conservative in the pore fluids, increasing in concentration during interglacial periods, diluting during glacials, and providing excellent proxies for deep lake net water balance changes. Furthermore, the Na/Cl ratio recorded the process of halite precipitation and dissolution induced by these hydrological changes. Mg2+ and Br records follow a glacial-interglacial pattern, such as observed in atmospheric CO2 concentrations and global sea-surface temperatures, albeit with a phase offset. At the end of the last interglacial (ca. 116 ka), there is a delay in onset of dilution of the deep lake, most likely due to the limnological transition from holomictic to meromictic conditions. The increase in deep lake concentrations at Last Glacial Termination I is delayed as a result of freshwater input into the deep lake during the cooler Younger Dryas period. There is a persistent relationship between precipitation in the watershed and North Atlantic sea-surface temperatures, similar to conditions observed over the past instrumental record. Deviations from the long-term trends occurred during interglacial periods, Marine Isotope Stages MIS 5e and MIS 1, when the deep Dead Sea was significantly diluted, and coincided with Mediterranean sapropel layers S5 and S1.


Aragonitic scleractinian corals in the Cretaceous calcitic sea

Changes in seawater chemistry have affected the evolution of calcifying marine organisms, including their skeletal polymorph (calcite versus aragonite), which is believed to have been strongly influenced by the Mg/Ca ratio at the time these animals first emerged. However, we show that micrabaciids, a scleractinian coral clade that first appeared in the fossil record of the Cretaceous, when the ocean Mg/Ca ratio was near the lowest in the Phanerozoic (thus a priori favoring calcitic mineralogy), formed skeletons composed exclusively of aragonite. Exceptionally preserved aragonitic coralla of Micrabacia from the Late Cretaceous Ripley Formation (southeastern USA) have skeletal microstructures identical to their modern representatives. In addition, skeletons of Micrabacia from Cretaceous chalk deposits of eastern Poland are clearly diagenetically altered in a manner consistent with originally aragonitic mineralogy. These deposits have also preserved fossils of the scleractinian Coelosmilia, the skeleton of which is interpreted as originally calcitic. These findings show that if changes in seawater Mg/Ca ratio influenced the mineralogy of scleractinian corals, the outcome was taxon specific. The aragonitic mineralogy, unique skeletal microstructures and ultrastructures, and low Mg/Ca ratios in both fossil and living micrabaciids indicate that their biomineralization process is strongly controlled and has withstood major fluctuations in seawater chemistry during the past 70 m.y.


Sediment cycling on continental and oceanic crust

Sedimentary rocks are often described as declining in quantity with increasing age due to the cumulative effects of crustal deformation and erosion. One important implication of such a model is that the geological record becomes progressively less voluminous and less complete with increasing age. Here we show that the predictions of a model in which the destruction of sedimentary rock is the predominant process signal are borne out only among sediments deposited on oceanic crust and among sediments deposited above sea level in non-marine environments. Most of the surviving volume of sedimentary rock (~75%) was deposited in and adjacent to shallow seas on continental crust and does not exhibit any steady decrease in quantity with increasing age. Instead, shallow marine sediments exhibit large fluctuations in quantity that were driven by shifting global tectonic boundary conditions, such as those that occur during the breakup and coalescence of supercontinents. The accumulation of sediments on the continents has not been uniform in rate, but it does record a primary signal of net growth that has many implications for the long-term evolution of Earth’s surface environment.


How to make a 350-m-thick lowstand systems tract in 17,000 years: The Late Pleistocene Po River (Italy) lowstand wedge

The 350-m-thick succession of the Po River lowstand wedge (Italy) associated with the Last Glacial Maximum (deposited over ~17 k.y) contains stratal architecture at a physical scale commonly attributed to much longer time scales, with complex, systematically varying internal clinothem characteristics. This study investigated clinothem stacking patterns and controls through the integration of seismic reflection data with sediment attributes, micropaleontology, regional climate, eustacy, and high-resolution age control possible only in Quaternary sequences. Three clinothem types are differentiated based on topset geometry, shelf-edge and onlap-point trajectory, internal seismic facies, and interpreted bottomset deposits: type A has moderate topset aggradation, ascending shelf-edge trajectory, and mass-transport bottomset deposits; type B has eroded topset, descending shelf-edge trajectory, and bottomset distributary channel-lobe complexes; and type C has maximal topset aggradation, ascending shelf-edge trajectory, and concordant bottomsets. Type A and C clinothems exhibit reduced sediment bypass and delivery to the basin, whereas type B clinothems are associated with short intervals of increased sediment export from the shelf to deeper water. Clinothems individually span a range of 0.4–4.7 k.y., contemporaneous with significant eustatic and climate changes, but their stacking patterns resemble those found in ancient successions and ascribed to significantly longer durations, indicating that (1) the response time of ancient continental margin–scale systems to high-frequency variations in accommodation and sediment supply could be as short as centuries, (2) even millennial- to centennial-scale stratal units can record substantial influence of allogenic controls, and (3) sandy deposits can be compartmentalized even in a short-duration lowstand systems tract.


Events, episodes, and phases: Signal from noise in flood-sediment archives

Major floods have increased in frequency in many parts of the world, and this is often attributed to anthropogenic climate change. Because of the short length of most gauged records (~50 yr), it is unclear whether these events represent a short-term anomaly or a shift to a prolonged flood-rich period. In this paper, we use event-scale paleoflood records from upland and lowland floodplains to demonstrate the relationship between individual flood events, clusters of events in multiyear episodes, and multidecadal- to centennial-scale flood-rich phases. Catchment- and regional-scale data show that individual events and episodes generally fall within extended flood-rich phases controlled by climate. Furthermore, contrary to recent suggestions that environmental signals may be rendered incomplete in fluvial systems by autogenic processes, from a multidecadal (and longer) perspective it is clear that floodplain environments can register and preserve a useful multiscale hydromorphic signal of climate change.


Past changes in the North Atlantic storm track driven by insolation and sea-ice forcing

Changes in the location of Northern Hemisphere storm tracks may cause significant societal and economic impacts under future climate change, but projections of future changes are highly uncertain and drivers of long-term changes are poorly understood. Here we develop a late Holocene storminess reconstruction from northwest Spain and combine this with an equivalent record from the Outer Hebrides, Scotland, to measure changes in the dominant latitudinal position of the storm track. The north-south index shows that storm tracks moved from a southern position to higher latitudes over the past 4000 yr, likely driven by a change from meridional to zonal atmospheric circulation, associated with a negative to positive North Atlantic Oscillation shift. We suggest that gradual polar cooling (caused by decreasing solar insolation in summer and amplified by sea-ice feedbacks) and mid-latitude warming (caused by increasing winter insolation) drove a steepening of the winter latitudinal temperature gradient through the late Holocene, resulting in the observed change to a more northern winter storm track. Our findings provide paleoclimate support for observational and modeling studies that link changes in the latitudinal temperature gradient and sea-ice extent to the strength and shape of the circumpolar vortex. Together this evidence now suggests that North Atlantic winter storm tracks may shift southward under future warming as sea-ice extent decreases and the mid- to high-latitude temperature gradient decreases, with storms increasingly affecting southern Europe.


Dating Icelandic glacial floods using a new viscous remanent magnetization protocol

A new protocol using the viscous remanent magnetization (VRM) of boulders to date cataclysmic geological events such as tsunamis, glacial floods, and landslides is presented and its performance is assessed against two jökulhlaups (glacial floods) of known age in Iceland. High-intensity jökulhlaups have the ability to break off large boulders from bedrock and emplace and rotate them. These rocks originally carried a remanent magnetization parallel to the geomagnetic field during their formation. After being rotated by the flood, they acquire a VRM parallel with Earth’s magnetic field. In continuous thermal demagnetization experiments the unblocking temperature of the VRM can be determined, and subsequent rock magnetic VRM acquisition experiments can be used to establish a relationship between the unblocking temperature and the acquisition time, from which the time since the flood can be determined. The protocol was tested on 44 boulders from 2 historical jökulhlaups in Iceland and found to yield good order-of-magnitude estimates: 72 yr (confidence limits 11–360 yr) versus known 155 yr at the Sólheimajökull jökulhlaup and 290 yr (confidence limits 80–2300 yr) versus known 288 yr for the Kotarjökull jökulhlaup. The method can therefore be a valuable tool for future dating of cataclysmic events.


Zinc isotope evidence for intensive magmatism immediately before the end-Permian mass extinction

The end-Permian extinction is typically ascribed to massive volcanic eruptions, but direct geochemical evidence linking the two independent events is generally lacking. Zinc is an important micronutrient of marine phytoplanktons, and Zn isotope (66Zn) ratios of seawater are markedly higher than those of volcanic rocks and riverine waters. We conducted high-resolution Zn concentration and Zn isotope analyses on carbonate rocks across the Permian-Triassic boundary (PTB) in the Meishan section of south China. An abrupt increase of Zn concentration and a concomitant 0.5 decrease in 66Zn occur ~35 k.y. before the mass extinction and carbon isotope (13C) minima. Mass balance calculation demonstrates that a 0.5 negative shift in 66Zn within thousands of years requires rapid and massive input of isotopically light Zn from volcanic ashes, hydrothermal inputs, and/or extremely fast weathering of large igneous provinces. A positive 66Zn shift of as much as 1.0 following the mass extinction demonstrates that primary productivity recovered and reached a maximum in fewer than 360 k.y. Our finding provides insights into the marine Zn cycling across the PTB and clarifies the temporal relationship and duration of events, including intensive volcanism, carbon isotope excursion, mass extinction, and widespread ocean anoxia.


The overlooked human influence in historic and prehistoric floods in the European Alps

Understanding the role of climate and humans in generating mountain slope instability is crucial because such instability influences downstream fluvial activity and is a major threat to societies. Here, we use the sedimentary archive of Lake Allos (southeastern France), a mountain lake in the European Alps, to characterize mountain flood deposits and vegetation dynamics over the past 7000 yr. Our results support the interpretation of a critical threshold in catchment sensitivity to erosion at 1700 calibrated (cal.) yr B.P. (A.D. 250) probably resulting from long-term, uninterrupted impacts of human activity. The frequency and severity of floods increased dramatically after this date. These results demonstrate that underestimation of human impacts over the Holocene may pose a challenge to a clear understanding of past climate changes because paleoflood records are highly likely to have been affected by geomorphic thresholds. Natural reforestation since the end of the 19th century does not appear to be sufficient to induce a flood regime comparable to that which occurred prior to 1700 cal. yr B.P. This poses the question as to whether forest restoration in high-altitude environments is liable to foster a return to a low-erosion regime over the next decades, or whether the overall severity of soil degradation has been such as to preclude a return to previous conditions.


Isotopic signature of dissolved iron delivered to the Southern Ocean from hydrothermal vents in the East Scotia Sea

It has recently been demonstrated that hydrothermal vents are an important source of dissolved Fe (dFe) to the Southern Ocean. The isotopic composition (56Fe) of dFe in vent fluids appears to be distinct from other sources of dFe to the deep ocean, but the evolution of 56Fe during mixing between vent fluids and seawater is poorly constrained. Here we present the evolution of 56Fe for dFe in hydrothermal fluids and dispersing plumes from two sites in the East Scotia Sea. We show that 56Fe values in the buoyant plume are distinctly lower (as low as –1.19) than the hydrothermal fluids (–0.29), attributed to (1) precipitation of Fe sulfides in the early stages of mixing, and (2) partial oxidation of Fe(II) to Fe(III), >55% of which subsequently precipitates as Fe oxyhydroxides. By contrast, the 56Fe signature of stabilized dFe in the neutrally buoyant plume is –0.3 to –0.5. This cannot be explained by continued dilution of the buoyant plume with background seawater; rather, we suggest that isotope fractionation of dFe occurs during plume dilution due to Fe ligand complexation and exchange with labile particulate Fe. The 56Fe signature of stabilized hydrothermal dFe in the East Scotia Sea is distinct from background seawater and may be used to quantify the hydrothermal dFe input to the ocean interior.


Widespread compression associated with Eocene Tonga-Kermadec subduction initiation

Eocene onset of subduction in the western Pacific was accompanied by a global reorganization of tectonic plates and a change in Pacific plate motion relative to hotspots during the period 52–43 Ma. We present seismic-reflection and rock sample data from the Tasman Sea that demonstrate that there was a period of widespread Eocene continental and oceanic compressional plate failure after 53–48 Ma that lasted until at least 37–34 Ma. We call this the Tectonic Event of the Cenozoic in the Tasman Area (TECTA). Its compressional nature is different from coeval tensile stresses and back-arc opening after 50 Ma in the Izu-Bonin-Mariana region. Our observations imply that spatial and temporal patterns of stress evolution during western Pacific Eocene subduction initiation were more varied than previously recognized. The evolving Eocene geometry of plates and boundaries played an important role in determining regional differences in stress state.


Long-term glacier melt fluctuations over the past 2500 yr in monsoonal High Asia revealed by radiocarbon-dated lacustrine pollen concentrates

Long-term records of glacier mass changes are important for improving our understanding of glacier dynamics and for predicting the response of glaciers to future climate change. In contrast to moraine sequences that only record isolated stages of glacier status, proglacial lake sediments may record long-term continuous glacier activities. The melt of old glacier ice releases old pollen that may affect the radiocarbon ages of pollen in proglacial lake sediments. We define the offset between the calibrated pollen 14C ages and the sediment depositional age as the "old pollen effect" (OPE). In small catchments dominated by glaciers, the OPE may record variations in glacier melt intensity and extent, even though complex processes (e.g., modern pollen flux to a glacier or a proglacial lake, glacier flow velocities) may also impact the OPE. Using the sediments of a small proglacial lake on the southern Tibetan Plateau, we found that over the past 2.5 k.y., a weakened OPE occurred during three historical cool periods that coincided with regional glacier advances defined by moraine ages. Thus, we interpret the OPE as a new indicator of glacier melt intensity and its fluctuations. Our reconstructed glacier variability agrees well with glacier fluctuations in the European Alps and the global average temperature record, suggesting that hemispheric-scale temperature variations and/or mid-latitude Westerlies may have controlled the late Holocene glacier variability in monsoonal High Asia. We also show that the 20th century glacier melt intensity has exceeded that of two historical warm periods and is unprecedented over the past 2.5 k.y. This implies that current anthropogenic warming poses a serious threat to the survival of glaciers in monsoonal High Asia.


Stratigraphic unmixing reveals repeated hypoxia events over the past 500 yr in the northern Adriatic Sea

In the northern Adriatic Sea and in most semienclosed coastal regions worldwide, hypoxia induced by eutrophication in the late 20th century caused major die-offs of coastal marine organisms. However, ecosystem responses to hypoxia over longer centennial scales are unclear because the duration of direct observations is limited to a few decades and/or the temporal resolution of sedimentary archives is compromised by slow sedimentation and bioturbation. To assess whether perturbations of ecosystems by hypoxia recurred over centuries in the northern Adriatic Sea, we evaluate the timing and forcing of past hypoxia events based on the production history of the opportunistic, hypoxia-tolerant bivalve Corbula gibba, using 210Pb data, radiocarbon dating, amino acid racemization, and distribution of foraminifers in sediment cores that capture the past 500 yr in the Gulf of Trieste. Unmixing the stratigraphic record on the basis of 311 shells of C. gibba, we show that the reconstructed fluctuations in abundance do not correlate with abundances in the raw stratigraphic record. We find that production of C. gibba has undergone major decadal-scale fluctuations since the 18th century, with outbreaks corresponding to density of more than 1000 individuals per square meter. These outbreaks represent long-term phenomena in the northern Adriatic ecosystem rather than novel states characteristic of the 20th century eutrophication. They positively correlate with centennial-scale fluctuations in sea-surface temperature, indicating that the hypoxia events were coupled with water-column stratification rather than with nutrient enrichment.


A new model for turbidity current behavior based on integration of flow monitoring and precision coring in a submarine canyon

Submarine turbidity currents create some of the largest sediment accumulations on Earth, yet there are few direct measurements of these flows. Instead, most of our understanding of turbidity currents results from analyzing their deposits in the sedimentary record. However, the lack of direct flow measurements means that there is considerable debate regarding how to interpret flow properties from ancient deposits. This novel study combines detailed flow monitoring with unusually precisely located cores at different heights, and multiple locations, within the Monterey submarine canyon, offshore California, USA. Dating demonstrates that the cores include the time interval that flows were monitored in the canyon, albeit individual layers cannot be tied to specific flows. There is good correlation between grain sizes collected by traps within the flow and grain sizes measured in cores from similar heights on the canyon walls. Synthesis of flow and deposit data suggests that turbidity currents sourced from the upper reaches of Monterey Canyon comprise three flow phases. Initially, a thin (38–50 m) powerful flow in the upper canyon can transport, tilt, and break the most proximal moorings and deposit chaotic sands and gravel on the canyon floor. The initially thin flow front then thickens and deposits interbedded sands and silty muds on the canyon walls as much as 62 m above the canyon floor. Finally, the flow thickens along its length, thus lofting silty mud and depositing it at greater altitudes than the previous deposits and in excess of 70 m altitude.


Climate-driven thaw of permafrost preserved glacial landscapes, northwestern Canada

Ice-marginal glaciated landscapes demarcate former boundaries of the continental ice sheets. Throughout circumpolar regions, permafrost has preserved relict ground ice and glacigenic sediments, delaying the sequence of postglacial landscape change that transformed temperate environments millennia earlier. Here we show that within 7 x 106 km2 of glaciated permafrost terrain, extensive landscapes remain poised for major climate-driven change. Across northwestern Canada, 60–100-km-wide concentric swaths of thaw slump–affected terrain delineate the maximum and recessional positions of the Laurentide Ice Sheet. These landscapes comprise ~17% of continuous permafrost terrain in a 1.27 x 106 km2 study area, indicating widespread preservation of late Pleistocene ground ice. These thaw slump, relict ground ice, and glacigenic terrain associations are also evident at the circumpolar scale. Recent intensification of thaw slumping across northwestern Canada has mobilized primary glacial sediments, triggering a cascade of fluvial, lacustrine, and coastal effects. These geologically significant processes, highlighted by the spatial distribution of thaw slumps and patterns of fluvial sediment mobilization, signal the climate-driven renewal of deglaciation and postglacial permafrost landscape evolution.


Alternating Southern and Northern Hemisphere climate response to astronomical forcing during the past 35 m.y.

Earth’s climate has undergone different intervals of gradual change as well as abrupt shifts between climate states. Here we aim to characterize the corresponding changes in climate response to astronomical forcing in the icehouse portion of the Cenozoic, from the latest Eocene to the present. As a tool, we use a 35-m.y.-long 18Obenthic record compiled from different high-resolution benthic isotope records spliced together (what we refer to as a megasplice). We analyze the climate response to astronomical forcing during four 800-k.y.-long time windows. During the mid-Miocene Climatic Optimum (ca. 15.5 Ma), global climate variability was mainly dependent on Southern Hemisphere summer insolation, amplified by a dynamic Antarctic ice sheet; 2.5 m.y. later, relatively warm global climate states occurred during maxima in both Southern Hemisphere and Northern Hemisphere summer insolation. At that point, the Antarctic ice sheet grew too big to pulse on the beat of precession, and the Southern Hemisphere lost its overwhelming influence on the global climate state. Likewise, we juxtapose response regimes of the Miocene (ca. 19 Ma) and Oligocene (ca. 25.5 Ma) warming periods. Despite the similarity in 18Obenthic values and variability, we find different responses to precession forcing. While Miocene warmth occurs during summer insolation maxima in both hemispheres, Oligocene global warmth is consistently triggered when Earth reaches perihelion in the Northern Hemisphere summer. This pattern is in accordance with previously published paleoclimate modeling results, and suggests an amplifying role for Northern Hemisphere sea ice.


Atmospheric K-feldspar as a potential climate modulating agent through geologic time

Clouds and aerosols have a large, yet highly uncertain, effect on changes in Earth’s climate. A factor of particular note is the role played by ice-nucleating particles, which remains poorly understood. The mineral K-feldspar (Kfs) has recently been shown by a number of independent studies to nucleate ice in mixed-phase cloud conditions far more efficiently than other common minerals. Here, global atmospheric Kfs flux through geologic time is estimated; constrained by records of secular continental crust and biosphere evolution, plate tectonics, volcanism, glaciation, and attendant trends in land surface stability. The analysis reveals that Kfs flux today is at neither extreme of the range estimated across geological time. The present-day Kfs flux, however, is likely to be among the most spatially and temporally variable due to land surface change. The concept of an ice-nucleation efficiency factor that can be calculated from rocks, and also eolian sediments and soils, is proposed. This allows the impact of paleo-atmospheric dust to be estimated through the rock record alongside meteorological and atmospheric composition considerations. With the reasonable assumption that the ice-nucleating properties of Kfs are themselves independent of the background climate state, a better understanding of Kfs flux across a range of spatial and temporal scales will advance understanding of climate processes and interactions.


Submarine Canyons: A brief review looking forward


Wyoming on the run--Toward final Paleoproterozoic assembly of Laurentia: COMMENT


Wyoming on the run--Toward final Paleoproterozoic assembly of Laurentia: REPLY