Manifestation of the Late Famennian Dasberg Event in the shelf-batial transition (Pai-Khoi sequences)

Research subject. Regional manifestations of the Dasberg eustatic event in the shelf and bathyal Pai-Khoi successions. The event appears in the Lower-Middle expansa zones interval (Upper Devonian, Famennian).Aim. To evaluate the manifestations of the event in the realm of transition from the shallow-water shoal succession of the Pai-Khoi carbonate parautokhtone towards the deep-water (bathyal) successions of the Kara shale allokhtone.Materials and methods. A number of successions comprising different facies and located in different parts of Pai-Khoi were studied: the Silova-Yakha River section and Tal’beyshor Creek section (south-western Pai-Khoi), the Lymbad’yakha section and the Peschanaya River section (northern Pai-Khoi). The interpretation of facies and the reconstruction of transgression-regression couplets were conducted based on the previously developed models of shoal and bathyal sedimentation. The stratigraphic framework comprised data on conodonts, transgression acmes, and carbonate carbon isotopic record.Results. The Pai-Khoi successions comprising Lower–Middle expansa zones demonstrate four transgression-regression cycles. The transgression acme of the third cycle marks the Dasberg eustatic event. The absence of anoxia is characteristic of this event in the region under consideration. The carbonate carbon isotope record of the Silova-Yakha River section shows a structure similar to that of North American successions. Variations in δ13Cкарб were likely to be caused by climate changes and perturbations of the global oceanic circulation.Conclusions. The stratigraphic interval comprising Lower–Middle expansa or Lower–Upper expansa (expansa s. l.) zones is detected clearly in different facies. A more detailed subdivision and correlation in the region under consideration is possible on the basis of evaluating manifestations of the Dasberg eustatic event: a characteristic eustatic succession and variations of the carbonate carbon isotopic composition.

Review article: Existing and potential evidence for Holocene grounding-line retreat and readvance in Antarctica

Widespread existing geological records from above the modern ice-sheet surface and outboard of the current ice margin show that the Antarctic Ice Sheet (AIS) was much more extensive at the Last Glacial Maximum (~20 ka) than at present. However, whether it was ever smaller than present during the last few millennia, and (if so) by how much, is known only for a few locations because direct evidence lies within or beneath the ice sheet, which is challenging to access. Here, we describe how retreat and readvance (henceforth “readvance”) of AIS grounding lines during the Holocene could be detected and quantified using subglacial bedrock, subglacial sediments, marine sediment cores, relative sea-level (RSL) records, radar data, and ice cores. Of these, only subglacial bedrock and subglacial sediments can provide direct evidence for readvance. Marine archives are of limited utility because readvance commonly covers evidence of earlier retreat. Nevertheless, stratigraphic transitions documenting change in environment may provide support for direct evidence from subglacial records, as can the presence of transgressions in RSL records. With independent age control, past changes in ice structure and flow patterns revealed by radar can be used to infer ice volume changes commensurate with readvance. Since ice cores capture changes in surface mass balance, elevation, and changes in atmospheric and oceanic circulation that are known to drive grounding-line migration, they also have potential for identifying readvance. A multidisciplinary approach is likely to provide the strongest evidence for or against a smaller-than-present AIS in the Holocene.

The Role of Continental Topography in the Present-Day Ocean’s Mean Climate

Climate models of varying complexity have been used for decades to investigate the impact of mountains on the atmosphere and surface climate. Here, the impact of removing the continental topography on the present-day ocean climate is investigated using three different climate models spanning multiple generations. An idealized study is performed where all present-day land surface topography is removed and the equilibrium change in the oceanic mean state with and without the mountains is studied. When the mountains are removed, changes found in all three models include a weakening of the Atlantic Meridional Overturning Circulation and associated SST cooling in the subpolar North Atlantic. The SSTs also warm in all the models in the western North Pacific Ocean associated with a northward shift of the atmospheric jet and Kuroshio current. In the ocean interior, the magnitude of the temperature and salinity response to removing the mountains is relatively small and the sign and magnitude of the changes generally varies among the models. These different interior ocean responses are likely related to differences in the mean state of the control integrations due to differences in resolution and associated sub-grid scale mixing parametrizations. Compared to the results from 4xCO2 simulations, the interior ocean temperature changes caused by mountain removal are relatively small, however, the oceanic circulation response and Northern Hemisphere near-surface temperature changes are of a similar magnitude to the response to such radiative forcing changes.

Applications of unmanned aerial vehicles in Antarctic environmental research

Antarctica plays a fundamental role in the Earth's climate, oceanic circulation and global ecosystem. It is a priority and a scientific challenge to understand its functioning and responses under different scenarios of global warming. However, extreme environmental conditions, seasonality and isolation hampers the efforts to achieve a comprehensive understanding of the physical, biological, chemical and geological processes taking place in Antarctica. Here we present unmanned aerial vehicles (UAVs) as feasible, rapid and accurate tools for environmental and wildlife research in Antarctica. UAV surveys were carried out on Deception Island (South Shetland Islands) using visible, multispectral and thermal sensors, and a water sampling device to develop precise thematic ecological maps, detect anomalous thermal zones, identify and census wildlife, build 3D images of geometrically complex geological formations, and sample dissolved chemicals (< 0.22 µm) waters from inaccessible or protected areas.

Climate and ice sheet evolutions from the last glacial maximum to the pre-industrial period with an ice-sheet–climate coupled model

The last deglaciation offers an unique opportunity to understand the climate–ice-sheet interactions in a global warming context. In this paper, to tackle this question, we use an Earth system model of intermediate complexity coupled to an ice sheet model covering the Northern Hemisphere to simulate the last deglaciation and the Holocene (26–0 ka). We use a synchronous coupling every year between the ice sheet and the rest of the climate system and we ensure a closed water cycle considering the release of freshwater flux to the ocean due to ice sheet melting. Our reference experiment displays a gradual warming in response to the forcings, with no abrupt changes. In this case, while the amplitude of the freshwater flux to the ocean induced by ice sheet retreat is realistic, it is sufficient to shut down the Atlantic meridional overturning circulation from which the model does not recover within the time period simulated. However, with reduced freshwater flux we are nonetheless able to obtain different oceanic circulation evolutions, including some abrupt transitions between shut-down and active circulation states in the course of the deglaciation. The inclusion of a parameterisation for the sinking of brines around Antarctica also produces an abrupt recovery of the Atlantic meridional overturning circulation, absent in the reference experiment. The fast oceanic circulation recoveries lead to abrupt warming phases in Greenland. Our simulated ice sheet geometry evolution is in overall good agreement with available global reconstructions, even though the abrupt sea level rise at 14.6 ka is underestimated, possibly because the climate model underestimates the millennial-scale temperature variability. In the course of the deglaciation, large-scale grounding line instabilities are simulated both for the Eurasian and North American ice sheets. The first instability occurs in the Barents–Kara seas for the Eurasian ice sheet at 14.5 ka. A second grounding line instability occurs ca. 12 ka in the proglacial lake that formed at the southern margin of the North American ice sheet. With additional asynchronously coupled experiments, we assess the sensitivity of our results to different ice sheet model choices related to surface and sub-shelf mass balance, ice deformation and grounding line representation. While the ice sheet evolutions differ within this ensemble, the global climate trajectory is only weakly affected by these choices. In our experiments, only the abrupt shifts in the oceanic circulation due to freshwater fluxes are able to produce some millennial-scale variability since no self-generating abrupt transitions are simulated without these fluxes.

Control of oceanic circulation on sediment distribution in the southwestern Atlantic margin (23 to 55° S)

In this study, we interpret the role played by ocean circulation in sediment distribution on the southwestern Atlantic margin using radiogenic Nd and Pb isotopes. The latitudinal trends for Pb and Nd isotopes reflect the different current systems acting on the margin. The utilization of the sediment fingerprinting method allowed us to associate the isotopic signatures with the main oceanographic features in the area. We recognized differences between Nd and Pb sources to the Argentinean shelf (carried by the flow of Subantarctic Shelf Water) and slopes (transported by deeper flows). Sediments from Antarctica extend up to the Uruguayan margin, carried by the Upper and Lower Circumpolar Deep Water. Our data confirm that, for shelf and intermediate areas (the upper 1200 m), the transfer of sediments from the Argentinean margin to the north of 35∘ S is limited by the Subtropical Shelf Front and the basin-wide recirculated Antarctic Intermediate Water. On the southern Brazilian inner and middle shelf, it is possible to recognize the northward influence of the Río de la Plata sediments carried by the Plata Plume Water. Another flow responsible for sediment transport and deposition on the outer shelf and slope is the southward flow of the Brazil Current. Finally, we propose that the Brazil–Malvinas Confluence and the Santos Bifurcation act as boundaries of geochemical provinces in the area. A conceptual model of sediment sources and transport is provided for the southwestern Atlantic margin.

Marine carbon cycle response to a warmer Southern Ocean: the case of the Last Interglacial

Recent studies investigating future warming scenarios have shown that the ocean carbon sink will weaken over the coming century due to ocean warming and changes in oceanic circulation. However, significant uncertainties remain regarding the magnitude of the oceanic carbon cycle response to warming. Here, we investigate the Southern Ocean’s (SO, south of 40° S) carbon cycle response to warmer conditions, as simulated under Last Interglacial boundary conditions (LIG, 129–115 thousand years ago, ka). We find a ∼150 % increase in carbon dioxide (CO2) outgassing over the SO at the LIG compared to pre-industrial (PI), due to a 0.5 °C increase in SO sea surface temperatures. This is partly compensated by an equatorward shift of the Southern Hemisphere westerlies and weaker Antarctic Bottom Water formation, which lead to an increase in dissolved inorganic carbon (DIC) in the deep ocean at the LIG compared to PI. These deep ocean DIC changes arise from increased deep and bottom water residence times, and higher remineralization rates due to higher temperatures. While our LIG simulation features a large reduction in SO sea-ice compared to PI, we find that changes in sea ice extent exert a minor control on the marine carbon cycle. Our results thus suggest that the projected poleward intensification of the SH westerlies, coupled with warmer conditions at the surface of the SO would weaken the SO carbon uptake over the coming century.

Lagrangian betweenness as a measure of bottlenecks in dynamical systems with oceanographic examples

The study of connectivity patterns in networks has brought novel insights across diverse fields ranging from neurosciences to epidemic spreading or climate. In this context, betweenness centrality has demonstrated to be a very effective measure to identify nodes that act as focus of congestion, or bottlenecks, in the network. However, there is not a way to define betweenness outside the network framework. By analytically linking dynamical systems and network theory, we provide a trajectory-based formulation of betweenness, called Lagrangian betweenness, as a function of Lyapunov exponents. This extends the concept of betweenness beyond the context of network theory relating hyperbolic points and heteroclinic connections in any dynamical system to the structural bottlenecks of the network associated with it. Using modeled and observational velocity fields, we show that such bottlenecks are present and surprisingly persistent in the oceanic circulation across different spatio-temporal scales and we illustrate the role of these areas in driving fluid transport over vast oceanic regions. Analyzing plankton abundance data from the Kuroshio region of the Pacific Ocean, we find significant spatial correlations between measures of diversity and betweenness, suggesting promise for ecological applications.