Description: In austral winter, biological productivity at the Angolan shelf reaches its maximum. The alongshore winds, however, reach their seasonal minimum suggesting that processes other than local wind‐driven upwelling contribute to near‐coastal cooling and upward nutrient supply, one possibility being mixing induced by internal tides (ITs). Here, we apply a three‐dimensional ocean model to simulate the generation, propagation, and dissipation of ITs at the Angolan continental slope and shelf. Model results are validated against moored acoustic Doppler current profiler and other observations. Simulated ITs are mainly generated in regions with a critical/supercritical slope typically between the 200‐ and 500‐m isobaths. Mixing induced by ITs is found to be strongest close to the coast and gradually decreases offshore thereby contributing to the establishment of cross‐shore temperature gradients. The available seasonal coverage of hydrographic data is used to design simulations to investigate the influence of seasonally varying stratification characterized by low stratification in austral winter and high stratification in austral summer. The results show that IT characteristics, such as their wavelengths, sea surface convergence patterns, and baroclinic structure, have substantial seasonal variations and additionally strong spatial inhomogeneities. However, seasonal variations in the spatially averaged generation, onshore flux, and dissipation of IT energy are weak. By evaluating the change of potential energy, it is shown, nevertheless, that mixing due to ITs is more effective during austral winter. We argue that this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing.

Description: Previous studies show accelerations of West Antarctic glaciers, implying that basal melt rates of these glaciers were previously small and increased in the middle of the 20th century. This enhanced melting is a likely source of the observed Ross Sea (RS) freshening, but its long-term impact on the Southern Ocean hydrography has not been well investigated. Here, we conduct coupled sea ice-ice shelf-ocean simulations with different levels of ice shelf melting from West Antarctic glaciers. Freshening of RS shelf and bottom water is simulated with enhanced West Antarctic ice shelf melting, while no significant changes in shelf water properties are simulated when West Antarctic ice shelf melting is small. We further show that the freshening caused by glacial meltwater from ice shelves in the Amundsen and Bellingshausen seas can propagate further downstream along the East Antarctic coast into the Weddell Sea. The freshening signal propagates onto the RS continental shelf within a year of model simulation, while it takes roughly 5-10 and 10-15 years to propagate into the region off Cape Darnley and into the Weddell Sea, respectively. This advection of freshening modulates the shelf water properties and possibly impacts the production of Antarctic Bottom Water if the enhanced melting of West Antarctic ice shelves continues for a longer period.

Description: The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 W m−2 over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic.

Description: Antarctica has traditionally been considered continental inside the coastline of ice and bedrock since Press and Dewart (1959). Sixty years later, we reconsider the conventional extent of this sixth continent. Geochemical observations show that subduction was active along the whole western coast of West Antarctica until the mid-Cretaceous after which it gradually ceased towards the tip of the Antarctic Peninsula. We propose that the entire West Antarctica formed as a back-arc basin system flanked by a volcanic arc, similar to e.g. the Japan Sea, instead of a continental rift system as conventionally interpreted. Globally, the fundamental difference between oceanic and continental lithosphere is reflected in hypsometry, largely controlled by lithosphere buoyancy. The equivalent hypsometry in West Antarctica (−580 ± 335 m on average, extending down to −1.6 km) is much deeper than in any continent, but corresponds to back-arc basins and oceans proper. This first order observation questions the conventional interpretation of West Antarctica as continental, since even continental shelves do not extend deeper than −200 m in equivalent hypsometry. We present a suite of geophysical observations that supports our geodynamic interpretation: a linear belt of seismicity sub-parallel to the volcanic arc along the Pacific margin of West Antarctica; a pattern of free air gravity anomalies typical of subduction systems; and extremely thin crystalline crust typical of back-arc basins. We calculate residual mantle gravity anomalies and demonstrate that they require the presence of (1) a thick sedimentary sequence of up to ca. 50% of the total crustal thickness or (2) extremely low density mantle below the deep basins of West Antarctica and, possibly, the Wilkes Basin in East Antarctica. Case (2) requires the presence of anomalously hot mantle below the entire West Antarctica with a size much larger than around continental rifts. We propose, by analogy with back-arc basins in the Western Pacific, the existence of rotated back-arc basins caused by differential slab roll-back during subduction of the Phoenix plate under the West Antarctica margin. Our finding reduces the continental lithosphere in Antarctica to 2/3 of its traditional area. It has significant implications for global models of lithosphere-mantle dynamics and models of the ice sheet evolution.

Description: Highlights • Temporally close-spaced double eruption within a couple of hundreds of years. • Magmas are variably tapped from zoned magma chambers during eruptions due to changing magma discharge rates and/or vent migration. • Eruptions started with a series of fallouts featuring stable eruption columns followed by fluctuating and partially collapsing eruption columns. • Eruptive volumes sum up to a total of 25.6 km3 and 40.5 km3 tephra volume, eruption column heights have been between 20–33 km. • Potential hazards from similar sized eruptions around Coatepeque Caldera are indicated even in the distal regions around San Salvador. Abstract The Coatepeque volcanic complex in El Salvador produced at least four Plinian eruptions within the last 80 kyr. The eruption of the 72 ka old Arce Tephra formed the Coatepeque Caldera and was one of the most powerful explosive eruptions in El Salvador. Hitherto it was thought that the Arce tephra had been emplaced only by one, mostly Plinian, eruptive event that ended with the deposition of a thick ignimbrite. However, our stratigraphic, geochemical, and zircon data reveal a temporally closely- spaced double eruption separated by a gap of only a couple of hundred years, and we therefore distinguish Lower and Upper Arce Tephras. Both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions. The final phase of the Upper Arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse. Mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the Pacific Ocean, the Guatemalan lowlands and the Caribbean Sea, result in 25.6 km3 tephra volume, areal distribution of 4 × 105 km2 and eruption column heights between 20–33 km for the Lower Arce eruption, and 40.5 km3 tephra volume, including 10 km3 for the ignimbrite, distributed across 6 × 105 km2 and eruption column heights of 23–28 km for the Upper Arce eruption. These values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at Coatepeque Caldera, especially for this kind of double eruption.

Description: The Yedoma region is unique in the permafrost region of the Northern Hemisphere and is characterized by a particularly high ground ice content in the sediment. These frozen deposits store a large amount of carbon and thus have the potential to influence the global climate. Especially the upper layers are susceptible to thaw processes, as they are exposed to increasingly rising mean annual air temperatures. The Northeastern Siberian Yedoma domain is of particular interest in this study. The morphology of ground ice is highly variable and the exact abundance and distribution is still unknown in large parts of Siberia. For an accurate overview of the distribution of intrasedimentary ground ice content, data from 26 sites in Northeastern Siberia were examined. The data were taken from data repositories (e.g., PANGAEA), expedition reports, scientific papers etc. and has been synthesized in a template in Excel. Of relevance was the absolute ice content (wt%) at different depths. Five depth classes were investigated: depth class 1: 0-0.99m; depth class 2: 1-1.99m; depth class 3: 2-2.99m; depth class 4: 3-24.99m; depth class 5: 25-65m. Using the mean absolute ice content for each depth class, ArcGIS was used to create a map for the distribution of ice content. R was applied to represent the ground ice content distribution at the different depths. Furthermore, the focus was on other parameters such as stratigraphy, total organic carbon content and landscape types, which were also examined with respect to the absolute ice content. The ice content is distributed very heterogeneously in Northeastern Siberia, averaging between 30 and 60 wt% over all depths. In large parts of the study area, the ice content in the upper three meters is with 40 to 65 wt% much higher than in the deeper sediment layers. In the depths of 3-65m, the ice content ranges from 20 to 50 wt%. Investigations of the age classes showed that the mean absolute ice content in thermokarst deposits (MIS 1) is with 48.60 wt% higher than in older sedimentary units. The TOC content also decreases significantly with depth. The Yedoma sediment composition and depositional regimes are highly variable. Even on a small scale, large differences in ice content could be observed. With the given data basis, no concrete statements about the vertical and horizontal ice content could be made for the whole study area. The model created in this study can be applied to model the absolute ground ice content based on the TOC content. Assessing the nature and content of ground ice in the upper layers in Northeastern Siberia is fundamental to environmental assessment and important for quantifying carbon fluxes and understanding permafrost response to climate change.

Description: This study was undertaken, in April 1964, at the suggestion of Prof. Dr. J. Lanjouw. The need for such a revision, was, however, stressed by the authors of the subtribe, viz. PAX et K. HOFFMANN, who, while delimiting these genera, remarked that the Systematics of both the (main) genera of this subtribe (viz. Paronychia and Herniaria) required a thorough study. By that remark they implicitly wanted to convey the difficulties encountered in studying the two genera. As a matter of fact theirs is the only account covering the whole group. There is no previous review of all the species of Paronychia, and CORE\xe2\x80\x99s account (1941 of the 21 North American species is the only study of this genus. Herniaria has been studied by F. N. WILLIAMS (1896) and F. HERMANN (1937), but both the accounts are brief and sketchy reviews, and cannot be described as revisions. The present study was, therefore, undertaken to clear up the taxonomic position of these genera. The study was greatly facilitated by the extensive collections made by various workers (in some areas for the first time in history) in recent years, particularly during the past two or three decades. It is indeed immensely gratifying to note that well over 13500 herbarium sheets (about 10500 of them obtained on loan from nearly 45 herbaria) were available for study. A thorough examination of such a vast collection has necessitated a re-evaluation most of the infra-generic groupings, and, moreover, resulted in a considerable increase in the number of species belonging to the two main genera. Of the nine genera originally included by PAX et HOFFMANN in this subtribe, viz. Sphaerocoma, Corrigiola, Gymnocarpos, Lochia, Sclerocephalus, Paronychia, Siphonychia, Herniaria and Philippiella, only eight are retained in this account, the ninth (i.e. Siphonychia) is merged with Paronychia. The latter is subdivided (for the first time) into three subgenera (Siphonychia, Paronychia and Anoplonychia); the subgenus Paronychia includes 57 species of which all but one belong to the section Paronychia. The latter in turn comprises 5 sub-sections and 8 series, all of them being set up for the first time. The second major subgenus, Anoplonychia, includes 48 species grouped into two sections and four subsections. In all there are 29 species being described for the first time (20 in Paronychia, 7 in Herniaria and 2 in Corrigiola) in addition to a number of infra-specific taxa. These figures, however, do not include the taxa published by me in two earlier papers (1966, 1967).\nIn spite of the extensive collecting, however, most of the new species as well as the infra-specific taxa are known from one or a few gatherings only, and are, accordingly, described in this account as endemics. Obviously, in some of these cases more material is highly desirable. Some regions are still rather poorly known or botanized. This is particularly true of Central and Eastern Turkey, NE Spain, and parts of Morocco, Ethiopia, Peru and Chile. Moreover, leaving aside matters of taxonomic interest, which I have endeavoured to clarify in this account, there still remain some problems which require further investigation. The main genera require population studies, and the evidence available so far for hybridization needs to be substantiated. Similarly, cytotaxonomic and experimental studies are needed to help solve some of the problems concerning the delimitation of the infra-specific taxa, especially in the genus Herniaria. Another highly intriguing matter that requires further investigation concerns the correct interpretation of the petals which in most of the genera of this group are filiform structures resembling staminodia, but in two of the genera (viz. Corrigiola and Sphaerocoma) they are quite well-developed. Yet another issue that still remains to be settled, though on a much broader scale, is the long-standing controversy as to whether or not these genera, along with the other members of the subfamily Paronychioideae, are sufficiently distinct from the rest of the family Caryophyllaceae so as to justify their inclusion in a separate family. This is a matter that cannot be settled by the study of a small group of genera, or a small tribe of a large family. I have, therefore, not gone into this problem, and have followed the treatment of PAX & HOFFMANN in the disposition of this group of genera.