Description: Remotely Operated Vehicles (ROVs) have proven to be highly effective in recovering well localized samples and observations from the seafloor. In the course of ROV deployments, however, huge amounts of video and photographic data are gathered which present tremendous potential for data mining. We present a new workflow based on industrial software to derive fundamental field geology information such as quantitative stratigraphy and tectonic structures from ROV-based photo and video material. We demonstrate proof of principle tests for this workflow on video data collected during dives with the ROV Kiel6000 on a new hot spot volcanic field that was recently identified southwest of the island of Santo Antão in the Cape Verdes. Our workflow allows us to derive three-dimensional models of outcrops facilitating quantitative measurements of joint orientation, bedding structure, grain size comparison and photo mosaicking within a georeferenced framework. The compiled data facilitate volcanological and tectonic interpretations from hand specimen to outcrop scales based on the quantified optical data. The demonstrated procedure is readily replicable and opens up possibilities for post-cruise “virtual fieldwork” on the seafloor.

Description: Highlights • The Red Sea Rift (RSR) comprises the typical terrain of slow MOR axes seen elsewhere. • Submarine salt glaciers occur extensively along the RSR and blanket parts of the RSR. • Inter-trough zones are not continental, but oceanic crust covered by evaporite flows. • We see a global mechanism for spreading initiation and no need for a “multi node” model. • We see prospects for large mineral deposits at passive margins that host evaporites. Abstract The transition from continental rifting to seafloor spreading is presently occurring at only a few places on Earth, such as the Red Sea or the Woodlark Basin. Competing theories for how spreading begins (either by quasi-instantaneous formation of a whole spreading segment or by initiation of spreading at multiple discrete “nodes” separated by thinned continental lithosphere) have been put forward. The major evidence for the nodes theory comes from the Red Sea and geophysical surveys carried out there in the “multi-deeps region” during the 1970's and 1980's. We present new high-resolution multibeam bathymetric information over the same region, which, when combined with acoustic backscatter data, seafloor sampling and magmatic geochemical information appears to provide no support for the nodes model. We show that, although the discrete deeps undoubtedly exist, they are not separated from one another by tectonic boundaries but rather represent “windows” onto a continuous spreading axis which is locally inundated and masked by massive slumping of sediments and evaporites from the rift flanks. The geophysical data that was previously used to support the presence of continental crust between the “nodes” can be equally well explained by processes related to the sedimentary blanketing and sub-sedimentary hydrothermal alteration. A single, “quasi-instantaneous segment formation” model would appear to be all that is required to explain observations from present-day rifting/spreading transitions globally.

Description: Highlights • The Red Sea Rift overall morphology is typical for (ultra)slow-spreading ridges. • Distribution of various volcanic morphotypes correlates with mantle temperatures. • Spreading perpendicular ridges indicate stable magma focusing over 8–12 My. • Warm mantle under the RSR prohibits the occurrence of oceanic core complexes. • Specific characteristics of the RSR morphology are related to its young age. Abstract Continental rifting and ocean basin formation is occurring today in the Red Sea, providing a possible modern analogue for the creation of mid-ocean ridges. Yet many of the seafloor features observed along the axis of the Red Sea appear anomalous compared to ancient and modern examples of mid-ocean ridges in other parts of the world, making it unclear, until recently, whether the Red Sea is truly analogous. Recent work suggests that the main morphological differences between the Red Sea Rift (RSR) and other mid-ocean ridges are due to the presence and movement of giant, submarine salt flows, which blanket large portions of the rift valley and thereby the oceanic crust. Using ship-based, high-resolution multibeam bathymetry of the central RSR between 16.5°N and 23°N we focus here on the RSR volcanic terrains not covered by salt and sediments and compare their morphologies to those observed along slow and ultra-slow spreading ridges elsewhere. Regional variations in style and intensity of volcanism can be related to variations in volcanic activity and mantle heat flow. The Red Sea oceanic seafloor shows typical features of mature (ultra)slow-spreading mid-ocean ridges, such as 2nd order discontinuities (overlapping spreading centres) and magma focussing in the segment centres (forming spreading-perpendicular volcanic ridges of thick oceanic crust). The occurrence of melt-salt interaction at locations where salt glaciers blanket the neovolcanic zone, and the absence of large detachment faults are unique features of the central RSR. These features can be related to the young character of the Red Sea and may be applicable to all young oceanic rifts, associated with plumes and/or evaporites. Thus, the RSR falls in line with (ultra)slow-spreading mid-ocean ridges globally, which makes the Red Sea a unique but highly important type example for initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research.

Description: 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.

Description: 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.

Description: Precipitation processes in hydrothermal fluids exert a primary control on the eventual distribution of elements, whether that sink is in the subseafloor, hydrothermal chimneys, near-field metalliferous sediments, or more distal in the ocean basin. Recent studies demonstrating abundant nanoparticles in hydrothermal fluids raise questions as to the importance of these nanoparticles relative to macro minerals, as well as the fate of such particles in hydrothermal systems. Here we evaluate the particle geochemistry of black smoker fluids from Niua South vent field, including nanoparticles and macro minerals, in order to consider how the processes of mineral precipitation affect mineral size and morphology, and how this mineral precipitation may dictate element sinks as hydrothermal fluids begin to mix with seawater. We find that the Niua vent fluids are dominated by sulfide and sulfate minerals, with the mineralogy of major and minor minerals changing with temperature, degree of mixing with seawater and rate of precipitation. The majority of particles are submicron in size, and sulfide minerals become larger and exhibit more crystalline morphology with increasing seawater content in the fluids. Minor minerals include gold and bismuth tellurides, and nanoparticulate chalcopyrite and nano-zinc sulfide occur. These findings are consistent with major mineral classes and precipitation processes observed in other systems, while providing further insight into the details of mineral precipitation at Niua including the separate and combined influences of boiling, mixing and cooling during hydrothermal fluid transport and initial interactions with seawater. This work demonstrates that boiling and rapid mixing encourages the formation of nanoparticles, whereas conductive cooling encourages particle growth. Further, these data demonstrate that the possible influence of nanoparticles in hydrothermal systems are not restricted to enhancing element transport, but may also include restricting mineral growth and affecting physicochemical properties of hydrothermal chimneys.