Thermochronologic constraints on the origin of the Great Unconformity

The origin of the phenomenon known as the Great Unconformity has been a fundamental yet unresolved problem in the geosciences for over a century. Recent hypotheses advocate either global continental exhumation of more than 3–4 km during Cryogenian (717–635 Ma) snowball Earth glaciations, or alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1000–540 Ma) due to plate tectonic reorganization from supercontinent Rodinia assembly and breakup. To test these hypotheses, the temporal pattern of Neoproterozoic thermal histories were evaluated for four North American locations using previously published medium-to-low temperature thermochronology and geologic information. We present inverse time-temperature simulations within a Bayesian modelling framework that record a consistent signal of relatively rapid, high magnitude cooling of ~120–200°C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ~3–5 km of unroofing during snowball Earth glaciations, but also demonstrate that plate tectonic drivers, with the potential to cause both exhumation and burial, may have significantly influenced the thermal history in regions that were undergoing deformation concomitant with glaciation. In the cratonic interior, however, glaciation remains the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion revealed by our thermochronological inversions. To obtain a full picture of the extent and synchroneity of such erosional exhumation, studies on stable cratonic crust below the Great Unconformity must be repeated on all continents.

Oceanic environment changes caused the Late Ordovician extinction: evidence from geochemical and Nd isotopic composition in the Yangtze area, South China

The Ordovician–Silurian (O–S) transition was a critical interval in geological history. Multiple geochemical methods are used to explore the changes in oceanic environment. The Nd isotopic compositions in the Yangtze Sea are controlled by two sources: the continental erosion and the Panthalassa Ocean. High εNd(t) values during the Katian, late Hirnantian and Rhuddanian intervals are associated with the high sea level, which resulted in less terrestrial input based on the low Ti/Al and Zr/Al ratios. In contrast, low εNd(t) values during the early Hirnantian interval are related to the sea-level fall; in this case, the exposure of submarine highs and the growth of Yangtze Oldlands could lead to more continental materials being transported into the Yangtze Sea based on high Ti/Al and Zr/Al ratios. In addition, the negative εNd(t) excursion can also be attributed to the weak circulation between the Yangtze Sea and Panthalassa Ocean when sea level was low. Furthermore, the sea-level eustacy plays a significant role in the changes in redox water conditions. The redox indices, mainly UEF, Ce/Ce* and Corg/PT, across the O–S transition show a predominance of anoxic ocean over the Yangtze Sea during the Katian, late Hirnantian and Rhuddanian intervals, and an oxygenated episode was briefly introduced during the early Hirnantian period because of the fall in sea level. The Late Ordovician biotic crisis was marked by two-phase extinction events, and the change in sea level and redox chemistry may be the important kill mechanisms.

Scientometric analysis of the activities of the International Commission for Continental Erosion (ICCE)

The article is dedicated to the analysis of the International Commission of Continental Erosion (ICCE) of International Association of Hydrological Sciences (IAHS) activity since foundation in 1981. The empirical data is the collections of scientific articles, published in IAHS Red Books after symposiums and workshops, organized by the ICCE. 2460 authors published articles in Red Books, 81 % of them have publication in the one issue. The most active contributors (who have articles in 7 or more issues) compose a group of 39 scientists (1.6 % from the total number of authors). Researchers from 89 countries were published articles in the ICCE issues of Red Books. The activity of the ICCE can be described by a cyclic model of growth. It is shown that the ICCE scientific community is becoming progressively global.

Short Communication: Earth is (mostly) flat, but mountains dominate global denudation: apportionment of the continental mass flux over millennial time scales, revisited

Carbon dioxide consumption by silicate mineral weathering and the subsequent precipitation of carbonate sediments sequesters CO2 over geologic timescales. The rate of this carbon sequestration is coupled to rates of continental erosion, which exposes fresh minerals to weathering. Steep mountain landscapes represent a small fraction of continental surfaces but contribute disproportionately to global erosion rates. However, the relative contributions of Earth's much vaster, but more slowly eroding, plains and hills remain the subject of debate. Recently, Willenbring et al. (2013) analyzed a compilation of denudation rates and topographic gradients and concluded that low-gradient regions dominate global denudation fluxes and silicate weathering rates. Here, we show that Willenbring et al. (2003) topographic and statistical analyses were subject to methodological errors that affected their conclusions. We correct these errors, and reanalyze their denudation rate and topographic data. In contrast to the results of Willenbring et al. (2013), we find that the denudation flux from the steepest 10% of continental topography nearly equals the flux from the other 90% of the continental surface combined. This new analysis implies global denudation fluxes of ∼23 Gt yr−1, roughly five times the value reported in Willenbring et al. (2013) and closer to previous estimates found elsewhere in the literature. Although low-gradient landscapes make up a small proportion of the global fluxes, they remain important because of the human reliance, and impact, on these vast areas.