Stratigraphy of the Pleistocene, phonolitic Cão Grande Formation on Santo Antão, Cape Verde

Highlights: • Two new phonolitic tephra units complementing the two previously known. • First radiometric ages of the CGF. • Contemporaneously evolution of the CGF and the Tope de Coroa. • Marine correlations improve tephra volume estimations for CG I and II. Abstract: The Cão Grande Formation (CGF) on the western plateau of Santo Antão Island is part of the younger volcanic sequence that originated from both, basanitic and nephelinitic magmatic suites, respectively called COVA and COROA suites. Based on our detailed revised stratigraphy of the CGF, including two yet unknown tephra units, we can show that both suites produced multiple, highly differentiated eruptions over a contemporaneous period. Correlations of CGF tephras with marine ash layers provide distal dispersal data for Cão Grande I (CG I) and also identify two highly explosive, phonolitic eruptions that pre-date the CGF tephra deposits known on land. Within the CGF, the lowermost, 220±7 ka old unit Canudo Tephra (CT; COVA suite) comprises phonolitic fall deposits and ignimbrites; it is partly eroded and overlain by debris flow deposits marking a hiatus in highly differentiated eruptions. The phonolitic CG I Tephra (COROA suite) consists of an initial major plinian fall deposit and associated ignimbrite and terminal surge deposits. This is immediately overlain by the phonolitic to phono-tephritic Cão Grande II (CG II; COVA suite), a complex succession of numerous fallout layers and density-current deposits. CG I and CG II have radiometric ages of 106±3 ka and 107±15 ka, respectively, that are identical within their error limits. The youngest CGF unit, the Furninha Tephra (FT; COROA suite), consists of three foidic-phonolitic fall deposits interbedded with proximal scoria deposits from a different vent. The phonolitic eruptions switched to and fro between both magmatic suites, in each case with a stronger first followed by a weaker second eruption. Each eruption evolved from stable to unstable eruption columns. During their terminal phases, both magma systems also leaked evolved dome-forming lavas next to the tephras. Distal ashes increase the CG I tephra volume to ~ 10 km3, about twice the previously published estimate. The tephra volume of CG II is ~ 3 km3; CT and FT are too poorly exposed for volume estimation. The characteristics of the CGF tephra units outline hazard conditions that may be expected from future evolved explosive eruptions on the western plateau of Santo Antão

New approaches to high-resolution mapping of marine vertical structures

Vertical walls in marine environments can harbour high biodiversity and provide natural protection from bottom-trawling activities. However, traditional mapping techniques are usually restricted to down-looking approaches which cannot adequately replicate their 3D structure. We combined sideways-looking multibeam echosounder (MBES) data from an AUV, forward-looking MBES data from ROVs and ROV-acquired videos to examine walls from Rockall Bank and Whittard Canyon, Northeast Atlantic. High-resolution 3D point clouds were extracted from each sonar dataset and structure from motion photogrammetry (SfM) was applied to recreate 3D representations of video transects along the walls. With these reconstructions, it was possible to interact with extensive sections of video footage and precisely position individuals. Terrain variables were derived on scales comparable to those experienced by megabenthic individuals. These were used to show differences in environmental conditions between observed and background locations as well as explain spatial patterns in ecological characteristics. In addition, since the SfM 3D reconstructions retained colours, they were employed to separate and quantify live coral colonies versus dead framework. The combination of these new technologies allows us, for the first time, to map the physical 3D structure of previously inaccessible habitats and demonstrates the complexity and importance of vertical structures

Pleistocene to Holocene offshore tephrostratigraphy of highly explosive eruptions from the southwestern Cape Verde Archipelago

Thirteen sediment gravity cores from the seafloor around the southern Cape Verdean islands Fogo and Brava as well as the Cadamosto seamount recovered 43 mafic and 5 phonolitic Pleistocene to Holocene primary ash layers. Twelve of these layers could be identified in several cores; they evidently cover areas of at least 6200-17,650km2 corresponding to minimum tephra volumes of ~1km3 (Volcanic Explosivity Index (VEI) 5), and thus are attributed to eruptions of sub-Plinian to Plinian dimensions. Provenance analyses based on geochemical compositions and geological evidence link the mafic tephras to eruptions on Fogo island. The detailed foraminiferal δ18O stratigraphy of one core yields a high-resolution age scale that can be applied to other cores via correlated ash beds and background sediment intervals. Sedimentation rates of the background sediment then constrain ages of other ash beds in the cores. The resulting temporal distribution of the marine tephras indicates that relatively large magnitude, highly explosive eruptions occurred about every 3000years on Fogo during the last 150kyrs; during the past 30kyrs the rate has increased to one in 2000years. In addition, we identified a 117kyr old thick turbidite sequence containing both mafic and phonolitic ash. We interpret this as the deposit of density currents generated by the Monte Amarelo flank collapse on Fogo and its associated tsunami.On the other hand, only a single widespread phonolitic tephra layer (145. kyrs old) could be correlated to Brava island indicating that highly explosive activity, producing deposits spread widely beyond the island's shores, did not occur anymore over the last 145. kyrs. No equivalents of the caldera filling ignimbrites on Brava could be found in the cores and therefore we infer that the caldera formed earlier than the 155. ka reached by coring. Two widespread phonolitic tephra layers, 17. ka and 40. ka old, are correlated to the Cadamosto seamount and thus derived from eruptions that occurred at >. 1380. m water depths where thermal granulation producing blocky glass shards was a major fragmentation process. We interpret that these eruptions produced huge submarine volcanic ash-water plumes that spread widely across the seafloor.In conclusion, our marine tephrostratigraphy provides the first evidence of frequent highly explosive volcanic eruptions at the southwestern part of the Cape Verdes during the past 155. kyrs. A somewhat surprising result is that such large eruptions occurred much more frequently from mafic alkalic magmas at Fogo than from highly evolved phonolitic magmas on Brava

The discovery and preliminary geological and faunal descriptions of three new Steinahóll vent Sites, Reykjanes Ridge, Iceland

During RV MS Merian expedition MSM75, an international, multidisciplinary team explored the Reykjanes Ridge from June to August 2018. The first area of study, Steinahóll (150–350 m depth), was chosen based on previous seismic data indicating hydrothermal activity. The sampling strategy included ship- and AUV-mounted multibeam surveys, Remotely Operated Vehicle (ROV), Epibenthic Sledge (EBS), and van Veen grab (vV) deployments. Upon returning to Steinahóll during the final days of MSM75, hydrothermal vent sites were discovered using the ROV Phoca (Kiel, GEOMAR). Here we describe and name three new, distinct hydrothermal vent site vulnerable marine ecosystems (VMEs); Hafgufa, Stökkull, Lyngbakr. The hydrothermal vent sites consisted of multiple anhydrite chimneys with large quantities of bacterial mats visible. The largest of the three sites (Hafgufa) was mapped, and reconstructed in 3D. In total 23,310 individual biological specimens were sampled comprising 41 higher taxa. Unique fauna located in the hydrothermally venting areas included two putative new species of harpacticoid copepod (Tisbe sp. nov. and Amphiascus sp. nov.), as well as the sponge Lycopodina cupressiformis (Carter, 1874). Capitellidae Grube, 1862 and Dorvilleidae Chamberlin, 1919 families dominated hydrothermally influenced samples for polychaetes. Around the hydrothermally influenced sites we observed a notable lack of megafauna, with only a few species being present. While we observed hydrothermal associations, the overall species composition is very similar to that seen at other shallow water vent sites in the north of Iceland, such as the Mohns Ridge vent fields, particularly with peracarid crustaceans. We therefore conclude the community overall reflects the usual “background” fauna of Iceland rather than consisting of “vent endemic” communities as is observed in deeper vent systems, with a few opportunistic species capable of utilizing this specialist environment