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Mixing as a driver of temporal variations in river hydrochemistry: 2. Major and trace element concentration dynamics in the Andes-Amazon transition
No full textVariations in riverine solute chemistry with changing runoff are used to interrogate catchment hydrology and to investigate chemical reactions in Earth's critical zone. This approach requires some understanding of how spatial and temporal averaging of solute‐generating reactions affect the dissolved load of rivers and streams. In this study, we investigate the concentration‐runoff (C‐Q) dynamics of a suite of major (Na, Mg, Ca, Si, K, and SO4) and trace (Al, Ba, Cd, Co, Cr, Cu, Fe, Ge, Li, Mn, Mo, Nd, Ni, Rb, Sr, U, V, and Zn) elements in nested catchments of variable size, spanning the geomorphic gradient from the Andes Mountains to the Amazon Foreland‐floodplain. The major elements exhibit various degrees of dilution with increasing runoff at all sites, whereas the concentrations of most trace elements either increase or show no relationship with increasing runoff in the three larger catchments (160–28,000 km2 area). We show that the observed main stem C‐Q dynamics are influenced by variable mixing of tributaries with distinct C‐Q relationships. Trace element C‐Q relationships are more variable among tributaries relative to major elements, which could be the result of variations in geomorphology, lithology, and hydrology of the subcatchments. Certain trace metals are also lost from solution during in‐channel processes (possibly related to colloidal size‐partitioning), which may exert an additional control on C‐Q dynamics. Overall, we suggest that tributary aggregation effects should be assessed in heterogeneous catchments before C‐Q or ratio‐Q relationships can be interpreted as reflecting catchment‐wide solute generation processes and their relationship to hydrology
Big-time insights from a tiny bird fossil.
No full textBirds are among the most diverse and widely distributed groups of vertebrate animals. There are well over 10,000 recognized species alive today, occupying virtually every subaerial ecosystem (1). The amazing breadth of extant bird diversity is manifested in dizzying varieties of forms, colors, and lifestyles, ranging from iridescent, hovering, nectar-feeding hummingbirds to nocturnal, flightless, worm-eating kiwis. How, when, and why has this spectacular diversity arisen? The only direct evidence informing such questions can be obtained from the fossil record of the modern bird radiation, but the early fossil record of modern birds is exceedingly sparse. In PNAS, Ksepka et al. (2) help to improve our understanding of this pivotal interval of bird evolutionary history by reporting the discovery of a new fossil bird filling an important temporal gap
Ultrafast Switching in Avalanche-Driven Ferroelectrics by Supersonic Kink Movements
Full text linkDevices operating at GHz frequencies can be based on ferroelectric kink-domains moving at supersonic speed. The kinks are located inside ferroelastic twin boundaries and are extremely mobile. Computer simulation shows that strong forcing generates velocities well above the speed of sound. Kinks are accelerated from v = 0 continuously with Döring masses in the order of skyrmion masses under constant strain rates. Moving kinks emit phonons at all velocities, and the emission cones coincide with the Mach cones at supersonic speed. Kinks form avalanches with the emission of secondary kinks via a mother–daughter nucleation mechanism and may be observable in acoustic emission experiments. Supersonic kinks define a new type of material; while mobile domains are the key for ferroelastic and ferroelectric device applications at low frequencies, it is expected that fast kink movements replace such domain movements for materials applications at high frequencies
A Laurentian record of the earliest fossil eukaryotes
No full textThe oldest evidence of eukaryotes in the fossil record comes from a recurrent assemblage of morphologically differentiated late Paleoproterozoic to early Mesoproterozoic microfossils. Although widely distributed, the principal constituents of this Tappania-Dictyosphaera-Valeria assemblage have not hitherto been recognized on Laurentia. We have recovered all three taxa from a shallow-water shale succession in the early Mesoproterozoic Greyson Formation (Belt Supergroup, Montana, USA). An exceptionally preserved population of Tappania substantially expands the morphological range of this developmentally complex organism, suggesting phylogenetic placement within, or immediately adjacent to, crown-group eukaryotes. Correspondence with Tappania-bearing biotas from China, India, Australia, and Siberia demonstrates an open-ocean connection to the intracratonic Belt Basin and, along with broadly co-occurring macrofossils Grypania and Horodyskia, supports the recognition of a globally expressed biozone. The Greyson Formation, along with contiguous strata in Glacier National Park, is unique in preserving all currently confirmed taxa of early eukaryotic and macroscopic fossils
Spatial Variation of Diapycnal Diffusivity Estimated From Seismic Imaging of Internal Wave Field, Gulf of Mexico
Full text linkBright reflections are observed within the upper 1,000 m of the water column along a seismic reflection profile that traverses the northern margin of the Gulf of Mexico. Independent hydrographic calibration demonstrates that these reflections are primarily caused by temperature changes associated with different water masses that are entrained into the Gulf along the Loop Current. The internal wave field is analyzed by automatically tracking 1,171 reflections, each of which is greater than 2 km in length. Power spectra of the horizontal gradient of isopycnal displacement, , are calculated from these tracked reflections. At low horizontal wave numbers ( cpm), , in agreement with hydrographic observations of the internal wave field. The turbulent spectral subrange is rarely observed. Diapycnal diffusivity, K, is estimated from the observed internal wave spectral subrange of each tracked reflection using a fine‐scale parametrization of turbulent mixing. Calculated values of K vary between and m2 s−1 with a mean value of m2 s−1. The spatial distribution of turbulent mixing shows that m2 s−1 away from the shelf edge in the upper 300 m where stratification is strong. Mixing is enhanced by up to 4 orders of magnitude adjacent to the shoaling bathymetry of the continental slope. This overall pattern matches that determined by analyzing nearby suites of CTD casts. However, the range of values recovered by spectral analysis of the seismic image is greater as a consequence of significantly better horizontal resolution
Volatile and light lithophile elements in high-anorthite plagioclase-hosted melt inclusions from Iceland
No full textMelt inclusions formed during the early stages of magmatic evolution trap primitive melt compositions and enable the volatile contents of primary melts and the mantle to be estimated. However, the syn- and post-entrapment behaviour of volatiles in primitive high-anorthite plagioclase-hosted melt inclusions from oceanic basalts remains poorly constrained. To address this deficit, we present volatile and light lithophile element analyses from a well-characterised suite of nine matrix glasses and 102 melt inclusions from the 10 ka Grímsvötn tephra series (i.e., Saksunarvatn ash) of Iceland’s Eastern Volcanic Zone (EVZ). High matrix glass H2O and S contents indicate that eruption-related exsolution was arrested by quenching in a phreatomagmatic setting; Li, B, F and Cl did not exsolve during eruption. The almost uniformly low CO2 content of plagioclase-hosted melt inclusions cannot be explained by either shallow entrapment or the sequestration of CO2 into shrinkage bubbles, suggesting that inclusion CO2 contents were controlled by decrepitation instead. High H2O/Ce values in primitive plagioclase-hosted inclusions (182–823) generally exceed values expected for EVZ primary melts (∼∼180), and can be accounted for by diffusive H2O gain following the entrainment of primitive macrocrysts into evolved and H2O-rich melts a few days before eruption. A strong positive correlation between H2O and Li in plagioclase-hosted inclusions suggests that diffusive Li gain may also have occurred. Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd = 51–216 versus ∼∼15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al+Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill diffusion since F is highly soluble in Al-rich melts. The high S/Dy of primitive inclusions (∼∼300) indicates that primary melts were S-rich in comparison with most oceanic basalts. Cl and B are unfractionated from similarly compatible trace elements, and preserve records of primary melt heterogeneity. Although primitive plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series record few primary signals in their volatile element contents they nevertheless record information about crustal magma processing that is not captured in olivine-hosted melt inclusions suites
Magnetic Mineralogy of Meteoritic Metal: Paleomagnetic Evidence for Dynamo Activity on Differentiated Planetesimals
No full textRecord of modern-style plate tectonics in the Palaeoproterozoic Trans-Hudson orogen
Full text linkThe Trans-Hudson orogen of North America is a circa 1,800 million year old, middle Palaeoproterozoic continental collisional belt. The orogen may represent an ancient analogue to the Himalayan orogen, which began forming 50 million years ago and remains active today. Both mountain belts exhibit similar length scales of deformation and timescales of magmatism and metamorphism. A notable divergence in this correlation has been the absence of high-pressure, low-temperature metamorphic rocks in the Trans-Hudson compared with the Himalaya. It has been debated whether this absence reflects a secular tectonic change, with the requisite cool thermal gradients precluded by warmer ambient mantle temperatures during the Palaeoproterozoic, or a lack of preservation. Here we identify eclogite rocks within the Trans-Hudson orogen. These rocks, which typically form at high pressures and cool temperatures during subduction, fill the gap in the comparative geologic record between the Trans-Hudson and Himalayan orogens. Through the application of phase equilibria modelling and in situ U–Pb monazite dating we show that the pressure–temperature conditions and relative timing of eclogite-facies metamorphism are comparable in both orogenies. The results imply that modern-day plate tectonic processes featuring deep continental subduction occurred at least 1,830 million years ago. This study highlights that the global metamorphic rock record (particularly in older terrains) is skewed by overprinting and erosion
Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing
No full textOur present understanding of the mantle carbon budget is in part built upon measurements of carbon concentrations in olivine hosted melt inclusions. Only a small number of such datasets are thought to have avoided degassing, having been entrapped prior to CO2 vapour saturation, and are therefore able to constrain primary CO2 concentrations. The absence of degassing in melt inclusion datasets has been inferred from the presence of strong correlations between CO2 and trace elements. In this contribution, we demonstrate that partial degassing followed by magma mixing not only retains such positive correlations, but can enhance them. Simple models of magma mixing and degassing are used to characterise how CO2-trace element systematics respond to CO2 vapour saturation in primary mantle melts entering the crust, followed by magma mixing. Positive correlations are expected between CO2 and most trace elements, and the average CO2/Ba and CO2/Nb ratios are controlled by the pressure of magma storage, rather than the CO2 concentration in the mantle. We find that the best estimates of mantle CO2 are the maximum CO2/Ba ratios observed in melt inclusion datasets, though a large number of analyses are required to adequately characterise the maximum of the CO2/Ba distribution. Using the mixing and degassing models we estimate the number of analyses required to obtain a maximum CO2/Ba observation within 10% of the mantle value. In light of our results, we reassess existing melt inclusion datasets, and find they exhibit systematics associated with partial degassing and mixing. We argue that all the data presently available is consistent with a depleted mantle CO2/Ba ratio of ~140, and there is as yet no evidence for heterogeneity in the CO2/Ba ratio of the depleted mantle
Exceptionally preserved Cambrian loriciferans and the early animal invasion of the meiobenthos
No full textMicroscopic animals that live among and between sediment grains (meiobenthic metazoans) are key constituents of modern aquatic ecosystems, but are effectively absent from the fossil record. We describe an assemblage of microscopic fossil loriciferans (Ecdysozoa, Loricifera) from the late Cambrian Deadwood Formation of western Canada. The fossils share a characteristic head structure and minute adult body size (~300 μm) with modern loriciferans, indicating the early evolution and subsequent conservation of an obligate, permanently meiobenthic lifestyle. The unsuspected fossilization potential of such small animals in marine mudstones offers a new search image for the earliest ecdysozoans and other animals, although the anatomical complexity of loriciferans points to their evolutionary miniaturization from a larger-bodied ancestor. The invasion of animals into ecospace that was previously monopolized by protists will have contributed considerably to the revolutionary geobiological feedbacks of the Proterozoic/Phanerozoic transition