3D geological modelling of the South Orkney Microcontinent (southern Scotia Arc, Antarctica) from seismic and potential field data

<p>The South Orkney Microcontinent (SOM) is located in the central sector of the South Scotia Arc, at the Weddell Sea northern edge. The SOM is the largest continental block in the southern Scotia Arc with a surface of more than 70.000 km<sup>2</sup>. Its current location is the result of the continental break-up from the Antarctic Peninsula related to the Powell Basin opening, considered one of the first steps in the formation of the Drake Passage during the Eocene-Oligocene.</p><p>In this work we present a 3D geological model of the SOM built with Geomodeller® using free-air gravity anomaly data from Topex and magnetic data from WDMAM. To obtain a reliable result, some constrains have been taken into account: (1) GEBCO data are used to establish the bathymetric level, (2) basement depth and geometry is calculated from multi-channel seismic profiles over the study area obtained from the Seismic Data Library System (SDLS), and (3) the analytic signal of total field magnetic anomalies has been used to limit the extension of the bodies that cause the PMA (Pacific Margin Anomaly).</p><p>All these data, together with additional geological and geophysical interpretation, have allowed to build the 3D model. The characterization of the sedimentary basins shape, the deep crust structure and Moho geometry, the volume of the magnetic bodies and the nature and geometry of the SOM margins will provide a better understanding of the complex SOM structure resulting from different tectonic phases since the Mesozoic and related to the Scotia-Drake opening.</p><p>The preliminary result shows a good fit between the observed and calculated gravimetric anomaly. We are currently working on the gravimetric inversion to obtain an optimal adjustment.</p>

The role of transform faults during back-arc spreading centre jumps

<p>Jumps in the location of back-arc spreading centres are a common feature of back-arc basins, but the controlling factors are not understood. In several narrow subduction zones with a long subduction history, such as the Scotia arc or Tyrhennian Sea, several spreading centres have been active in the course of history with regular, quasi-instantaneous jumps towards the retreating trench. A prominent feature of these regions are large bounding transform (‘STEP’) faults. However, whether STEP faults influence the (unknown) dynamics spreading centre jumps remains to be explored.</p><p> </p><p>We therefore run 3D-models to simulate a long narrow subducting slab, bound by continents, which retreats and creates necessary STEP-faults self-consistently. The results offer a new mechanism for back-arc spreading jumps: After the creation of a back-arc spreading centre in the retreating subduction system, transform faults between trench and back-arc basin form. Spreading jumps are thus a consequence of the fact that these constantly elongating transform faults, which decouple the overriding plate from neighbouring plates, fail to remain active once a threshold length (~1.3x plate width) is reached. Subsequently, the back-arc basin and neighbouring plates are strongly coupled, and ongoing trench retreat localizes stresses and rapidly ruptures the overriding plate closer to the trench while the old spreading centre is abandoned.  In a parameter study, the results further explain why the narrowest subduction zones, such as the Calabrian Arc, experience more frequent and closer spreading jumps than the long-period jumps of a wider subduction zone such as the Scotia Arc. The widest subduction zones should not undergo any back-arc spreading jumps with this mechanism, consistent with other natural examples.</p>

A newly discovered radiation of endoparasitic gastropods and their coevolution with asteroid hosts in Antarctica

Background Marine invertebrates are abundant and diverse on the continental shelf in Antarctica, but little is known about their parasitic counterparts. Endoparasites are especially understudied because they often possess highly modified body plans that pose problems for their identification. Asterophila, a genus of endoparasitic gastropod in the family Eulimidae, forms cysts in the arms and central discs of asteroid sea stars. There are currently four known species in this genus, one of which has been described from the Antarctic Peninsula (A. perknasteri). This study employs molecular and morphological data to investigate the diversity of Asterophila in Antarctica and explore cophylogenetic patterns between host and parasite. Results A maximum-likelihood phylogeny of Asterophila and subsequent species-delimitation analysis uncovered nine well-supported putative species, eight of which are new to science. Most Asterophila species were found on a single host species, but four species were found on multiple hosts from one or two closely related genera, showing phylogenetic conservatism of host use. Both distance-based and event-based cophylogenetic analyses uncovered a strong signal of coevolution in this system, but most associations were explained by non-cospeciation events. Discussion The prevalence of duplication and host-switching events in Asterophila and its asteroid hosts suggests that synchronous evolution may be rare even in obligate endoparasitic systems. The apparent restricted distribution of Asterophila from around the Scotia Arc may be an artefact of concentrated sampling in the area and a low obvious prevalence of infection. Given the richness of parasites on a global scale, their role in promoting host diversification, and the threat of their loss through coextinction, future work should continue to investigate parasite diversity and coevolution in vulnerable ecosystems.

Conservation implications of spatial genetic structure in two species of oribatid mites from the Antarctic Peninsula and the Scotia Arc

Mitochondrial and nuclear sequence data from two Antarctic ameronothroid mites, Halozetes belgicae and Alaskozetes antarcticus, were used to address three key questions important for understanding both the evolution of biodiversity and its future conservation in the Antarctic Peninsula Region: i) Do populations of mites across the Antarctic Peninsula and Scotia Arc constitute distinct genetic lineages? ii) What implications does the spatial genetic structure in these species have for current understanding of the region’s glacial history? iii) What are the conservation implications of these findings? Our results indicate that both mite species have been present in the Antarctic since at least the Pliocene. At the regional scale, both species are comprised of a number of divergent, but sympatric, lineages that are genetically as distinct as some species within the genera Halozetes and Alaskozetes. At the local scale, complex structure suggests limited and stochastic post-Holocene dispersal. For both species, considerable spatial genetic structure exists across the region, similar to that found in other terrestrial invertebrates. These results support the implementation of stringent biosecurity measures for moving between the Scotia Arc islands and the Antarctic Peninsula, and throughout the latter, to conserve both evolutionary history and future evolutionary trajectories.

Description of Antho (Plocamia) bremecae sp. nov. and checklist of Microcionidae (Demospongiae: Poecilosclerida) from Burdwood Bank and neighboring areas, SW Atlantic Ocean

In this contribution, we describe a new Demospongiae species, Antho (Plocamia) bremecae sp. nov., from the west slope of Burdwood bank, a poorly studied region in the SW Atlantic Ocean. We also recorded for the first time in the region two other microcionid species, Clathria (Axosuberites) nidificata and Clathria (Microciona) antarctica. In addition, a regional checklist of Microcionidae from Burdwood Bank and neighboring areas, including Malvinas (Falkland) Islands, Tierra del Fuego Province and the North of the Scotia Arc (South Georgia and Shag Rocks) is provided.