High-resolution multibeam bathymetry of the northern Mid-Atlantic Ridge at 45–46° N: the Moytirra hydrothermal field

ABSTRACT This work presents a new high-resolution multibeam bathymetric map of a segment of active deep sea-floor spreading in the Atlantic Ocean, the northern Mid-Atlantic Ridge (MAR) at 45–46º N. New high-resolution bathymetry data were acquired using an Atlas multibeam echosounder onboard the research vessel Sarmiento de Gamboa during the EXPLOSEA-2 survey in 2019. The final map of the MAR (50 m cell grid size) at the original scale of 1:200,000 shows a segment of 140 × 35 km of the MAR, at water depths from 715 to 3700 m. This new high-resolution bathymetric map allows to better defining the submarine morphology of the Moytirra hydrothermal active field, the only high-temperature field identified between the Azores Archipelago (Portugal) and Iceland. ROV submarine observations reaching the deepest part of the system for the first time show giant anhydrite-sulfide chimneys up to 20 m high, active strong black smokers and polymetallic massive sulfides.


Introduction
Mid-ocean ridges (MORs) constitute the lithospheric plate boundary where the oceanic crust, which covers 60% of the solid Earth's surface, is generated. MORs are linked to seafloor spreading and plates drift, and are thus features of first order importance in the Earth system. MORs illustrate well how volcanic, tectonic, hydrothermal and sedimentary processes sculpt geomorphology in the deep ocean (e.g. Mitchell, 2018;Searle, 2013). Despite their poor accessibility (lying 2700 m below sea level on average) and remoteness, the development and deployment of new technologies have been important for the discovery and investigation of new features on these geologic structures. Each ocean basin contains a MOR, and most of them are named for the basin they are in, as the Mid-Atlantic Ridge (MAR).
High-resolution seabed mapping is fundamental for the knowledge and understanding of deep-seafloor morphology, key geological processes, including sedimentary depositional and erosional processes, habitat distribution and typology and availability of mineral deposits and energy resources, among other aspects. It is also valuable for more applied purposes such as siting submarine cables and pipelines, detection of marine geohazards, development of early warning systems for volcanic eruptions, tsunamis and earthquakes, pollution prevention, fisheries management, analysis of potential environmental impacts associated with deep-sea exploration and exploitation, and for safer shipping.
Despite many years of mapping effort, only a small fraction of the world´s ocean seafloor has been sampled at a resolution higher than one nautical mile (one depth measurement every 1852 m), greatly limiting our ability to explore and understand critical ocean and seafloor processes (e.g. Ryan et al., 2009). Nevertheless, there are several recent international cooperation initiatives promoting seafloor mapping at high resolution. At global scale, the Nippon-Foundation GEBCO (The General Bathymetric Chart of the Oceans) Seabed 2030 has the goal of mapping the entire world´s ocean floor using Multi Beam Echo Sounders (MBES) data by 2030. The resolution of the global MBES seafloor mapping strongly depends on the water depth: 100 m resolution at water depths between 0 and 1500, 200 m resolution between 1500 and 3000, and 400 m resolution between 3000 and 4000 m (Mayer et al., 2018). The total coverage of ocean floor has significantly increased from 6% in 2014-15% in 2020 (https://www.gebco.net).
Otherwise, mapping seabed and habitat of the whole North Atlantic Ocean at high resolution is a challenge that is necessary to overcome the present knowledge on the Atlantic seabed. This task is being promoted by the Atlantic Seabed Mapping International Working Group (ASMIWG) as part of the Tri-Lateral Galway Statement agreed, between USA, European Union and Canada for the Atlantic Ocean Research Alliance (AORA, https://www.atlanticresource.org/aora/ mapping-our-ocean).
The northern MAR is considered as a slow-spreading MOR with average spreading rates of 20-50 km Ma −1 (Lonsdale, 1977). Seawater penetrating the crust and heated by magma at depth creates hydrothermal spring in the way back to surface such as spectacular 'black smokers' and associated massive sulfide deposits (e.g. Mitchell, 2018). More than eleven high temperature vent fields have been confirmed in the axial valley of the MAR between the Equator and the Azores Archipelago (Portugal) at ∼ 39°N (Inter-Ridge Vents Database, https://vents-data.interridge. org/; Baker, 2017;Beaulieu et al., 2013).
However, between the Azores Archipelago and the Charlie-Gibbs Fracture Zone at 52°25´N, only one high-temperature hydrothermal field developing active black smokers has been reported at ∼ MAR 45°N (Wheeler et al., 2013). In 2019 (11 June to 27 July 2019), the EXPLOSEA-2 survey (Somoza et al., 2020) aboard the RV Sarmiento de Gamboa carried out a complete seabed mapping of the axial valley of the segment of MAR where the Moytirra field was identified ( Figure 1). Furthermore, the deepest part of the Moytirra hydrothermal field was surveyed for the first time down to the bottom of the MAR axial valley, at more than 3000 m water depth, using the ROV Luso, rated to 6000 m depth, whereas previously ROV used by the Irish-UK team was limited (Wheeler et al., 2013).
The northern MAR beyond the areas of national jurisdiction has been recently proposed by the International Seabed Authority (ISA) as a priority area for the development of a Regional Environmental Management Plan (REMP). REMPs have been interpreted as an essential tools, since they provide regional-specific information that facilitates the decision-making for present exploration and future exploitation of mineral resources in order to ensure the effective protection of the marine environment according to Article 145 of the United Nations Convention on the Law of the Sea (UNCLOS). In preparing the North Atlantic REMP, the procedure should involve the use of an extensive multidisciplinary scientific dataset including information such as high-resolution bathymetry mapping, habitat types, biodiversity, active vents, distribution and characterization of massive sulfide mineralizations as essential tools for ensuring the effective management and protection of the marine environment. The surveyed area is included in the extended continental shelf proposal submitted by Portugal to the United Nations Commission on the Limits of the Continental Shelf.
This work presents a new high resolution multibeam bathymetric map of a segment of the northern Mid-Atlantic Ridge (MAR) between 45°20´N and 46°20Ń obtained during the EXPLOSEA-2 survey (http:// www.igme.es/explosea), which includes the Moytirra active hydrothermal vent field, the only identified high-temperature hydrothermal field along the northern MAR between the Azores Archipelago and Iceland.

Acoustic data acquisition and processing
High-resolution bathymetry data were acquired using an Atlas DS 1 × 1 MBES onboard the RV Sarmiento de Gamboa during the EXPLOSEA-2 survey in 2019. The Atlas MBES operating at a frequency between 14 and 16 kHz, has an angular coverage of 140°, allowing a maximum of 360 beams per ping with an angular distance of 1° (Table 1). At average water depths of 1700 m, the real Atlas MBES coverage is four times the water depth with a resolution of 0.2% of the water depth. During the EXPLOSEA-2 survey we used a frequency of 15 kHz, a power of 236 dB, a pulse length of 30 ms, a gain of −132 dB and an average 313 beams per ping. The Atlas MBES collected bathymetry and backscatter data, together with water column data. We used the NAVIPAC (EIVA) navigation system to design bathymetric lines and to visualize 3D models of the multibeam bathymetry data in real time. Positioning was assured by a GPS system with an estimated error below 2 m.
MBES data were processed using CARIS HIPS&-SIPS™ software. The MBES bathymetry data were used to generate digital terrain models (DTM) at spatial resolutions between 30 and 60 m. The multiresolution DTMs were also used to generate regional sun-shaded image renders and perspective views and to extract margin-wide bathymetric profiles using Fle-dermaus™ software to interpret the submarine landscapes. The rugosity map calculated in Fledermaus Version 7 is based on Jenness (2004) where rugosity is represented as the ratio between the surface and planar areas of each cell in the input bathymetric data set.
DTMs were exported as ASCII ESRI files to Arc-Gis TM desktop v.12.4. ASCII ESRI files were transformed to raster files to generate derivative products such as slope angle maps in order to design the ROV tracks. In this way, Figure 2 shows the multi-resolution MBES data overlapped to Global Multi-Resolution Topography GMRT v3.7 bathymetric data available in this area (Ryan et al., 2009).
Sound Velocity Profiles (SVPs) used for correction of the MBES data consisted of real-time synthetic profiles derived from the CTDs data and Expendable Bathy Thermograph (XBT) probes T5, that were compared with the synthetic profiles from the WOA13 sound speed database (Masetti et al., 2018). Single beam bathymetric tracks were also collected using an EA600 echosounder in order to calibrate the gravimeter data. No further processing was applied to this data.   (Somoza et al., 2020). The black rectangle marks the location of the mapped segment of the Mid-Ocean Ridge around the Moytirra hydrothermal field. ROV dive sites and multibeam tracks are also shown. Global Multi-Resolution Topography (GMRT) was used as regional background map (Ryan et al., 2009).

Results and discussion
All available data were merged to create a bathymetric   (Ryan et al., 2009). Multibeam bathymetry tracks are also shown as red lines. Red dots mark location of CTDs casts for water column parameters measurements.

Geological interpretation of the segmentation of spreading centers
Most of the MAR segments and other slow-spreading centers display an Axial Volcanic Ridge (AVR) aligned approximately normal to the spreading direction and containing the youngest lavas and a rugged topography (e.g. Searle, 2013). These AVRs are displaced by second-order and third-order discontinuities that take the form of oblique offsets (e.g. Searle & Laughton, 1977). The structural analysis of the high-resolution bathymetry map highlights the complexity of the spreading centers and ridge segmentation controlled by these systems of faults oblique to the spreading direction (Figure 3). The map legend used in the geological interpretation of the high-resolution map is based on the established classification of mid-ridge segmentation seafloor structures and volcanic geomorphology (e.g. Anderson et al., 2017;Macdonald et al., 1991;Searle, 2013) (Figure 4).
In the mapped area, we identified 5 segments along the mid-Atlantic spreading center that are between 26 and ∼40 km long and ∼9-20 km wide (Figure 3). These Individual segments are separated by secondorder oblique faults (D2 in Figure 3), including major shear zones offsets (SZO), and numerous minor discontinuities (D3) according to the classification proposed by Macdonald et al. (1991) for discontinuities segmenting mid-ocean ridges. According this classification, D1 are the main transform faults, D2 corresponds to major oblique-slip normal faults offsetting between 5 and 9 km the secondorder spreading segments (S2), and D3 are the minor oblique faults that only offset the axial volcanic valley forming third-order ridge segments (S3). The five segments identified can be grouped into northern (segments I and II) and southern (segments III, IV and V) ones separated by a complex shear zone offset (SZO in Figure 3) formed by a couple of D2 oblique slip normal faults, which offsets up to 9 km the axial ridge. This system of D2 oblique-slip normal faults affects mainly the western side of the AVR showing mainly N-70°azimuths (Figure 3), whereas the eastern side of the southern AVR is only affected by a couple of also D2 faults trending N-120°. The western D2 system shows an angle of 70°oblique to the AVR whereas the eastern D2 system shows an angle of 60°oblique to the AVR. A third-order system of oblique-slip faults (D3) was identified affecting only the AVR and not its flanks. As occurs with the D2 faults, the D3 discontinuities show the same two main trends: N-70°and N°1 20°N. This type of discontinuities creates segmentation of S2 ridges generating S3 ridges as observed in segment IV (Figure 3).

Morphology types and volcanic edifices of the ridge segments
The northern segments (I and II), are 35-40 km long, 4 km wide and 400 m high, with the axial ridge located at the center of the median valley constituted by a depression of 10-12 km wide ( Figure 5A The southern segments (III to V) vary from 26 to 37 km long, are 2 km wide and 500 m high, and are located at the center of the 8-12 km wide median valley ( Figure 5B). Both, the axial ridge and the median valley are narrower than the northern ones ( Figure 5B). The median valley is bounded by large fault scarps 800-1300 m high, forming terraces along their flanks. The eastern flank of the southern median valley shows the shallowest water depths of the area reaching values of 750 m. Along the intermediate terraces of segment IV in this eastern flank ( Figure 5B), flat topped volcanoes with sub-circular morphology, 1200 m in diameter, 150 m height and steep slopes up to 25°are also identified. Flat topped volcanoes apparently form a distinct class of volcanoes, usually along slow spreading centers. Clague et al. (2000) proposed that they are formed by long-lived eruptions producing a lava lake and surrounding levee.

Morphology and structures controlling the Moytirra hydrothermal venting
The hydrothermal field of Moytirra is located at water depths between 2750 and 3060 m along the western flank of a ridge detached from the main eastern flank of segment IV (Figure 6). This ridge, which we termed as Moytirra ridge, extends ∼10 km, in a sub-parallel  trend to the direction of the AVR but detached from the flanks of the median valley ( Figure 6B).
The rugosity map of the Moytirra region shows that the Moytirra hydrothermal vent field is located at the intersection between D2 oblique faults with N-120°o rientation and the eastern flank of the axial valley (Figure 7). At same time, the rugosity map shows that the Moytirra ridge is composed of a series of sub-circular structures interpreted as volcanic edifices (Figure 7).
The map of the Moytirra hydrothermal field at 1:2000 scale based on the observations from the ROV images shows the distribution of active and inactive chimneys as well as active black smokers along the field (Figure 8). The mapped area has an extension of ∼24,000 cubic meters ( Figure 8B). ROV observations showed that the active hydrothermal field is composed of an elongated mound of sea-floor massive sulfides topped by numerous anhydrite-sulfide chimneys up to 40 m high ( Figure 9A) and active black smokers ( Figures 9B,C). The active hydrothermal field extends ∼125 m along a N-120°orientation covering an extension of ∼5580 cubic meters ( Figure 8B). The distribution of the cluster of chimneys fits with the orientation of the D2 oblique faults as observed on the rugosity map of the Moytirra region at regional scale ( Figure 7). Furthermore, the uppermost boundary of the hydrothermal field is composed of steep walls of basalts showing evidence of fault planes. Based on these data, we interpret that Moytirra hydrothermal field develops from tensional gashes formed by D2 oblique faults with N-120°orientation that intersect the flanks of the axial valley allowing fluids to rise at high-temperatures (Figure 8).
The active hydrothermal field is surrounded by a talus extending from 3000 to 3038 m water depths. This talus is constituted by blocks of weathered sulfide, variably covered by hemipelagic and hydrothermal sediments, forming chaotic talus of debris sourced from the destruction of the active vents. Hydrothermally extinct seafloor massive sulfide deposits forming chimneys and mounds also occur westwards at deeper waters from 3030to 3040 m ( Figure 8B). This area is composed of an accumulation of weathered sulfides and iron-rich sediments with a discontinuous pelagic sedimentary cover. The talus of debris has a bulk extension of ∼24,000 square meters ( Figure 8B).
Processes of crustal accretion in the mapped segments of this slow-spreading mid-Atlantic ridge show certain similarities with back-arc ridges as the Mariana arc, both being characterized by strong ridge segmentation along shear zones offsets and nontransforms ridge-axis discontinuities (Anderson et al., 2017). In these slow-spreading mid-ocean ridges, second-order discontinuities take the form of oblique offsets in the spreading axis and median valley, such as occurs in the MAR at Kurchatov Fracture Zone (Searle & Laughton, 1977) and at 33.5°S (Grindlay et al., 1991). The main structures controlling ridge segmentation are these second-order systems of oblique-slip normal faults and tension gashes that offset the main segments of spreading ridges. Moreover, we interpret that active hydrothermal venting as the Moytirra fieldis associated with the occurrence of these second-order oblique-slip normal faults (D2) as they intersect the outer flanks of the AVR, in contrast to back-arc ridges where hydrothermal venting is mainly associated with recent magmatism located along the axis of the AVR (e.g. Anderson et al., 2017).

Conclusions
The new high-resolution bathymetric map presented here allows to better define the submarine morphology and structure of a segment up to 140 km in length of the northern MAR and their relationship with the occurrence of active hydrothermal vent fields and massive sulfide sites.
The morphological and geological interpretation of the high-resolution bathymetric map of the northern MAR between 45°20´N and 46°20´N which comprises the Moytirra active hydrothermal vent field allows to present the following conclusions: . We identified 5 segments along mid-Atlantic spreading center that are between 26 and ∼40 km long and ∼9-20 km wide, offsets by second-order oblique slip normal faults (Figure 3). Two main orientations of these faults are identified: (a) a 70°t rending system affecting mainly the western AVR; and (b) a 120°trending system affecting mainly the eastern AVR. . The five segments identified can be grouped into northern (segments I and II) and southern (segments III, IV and V) ones separated by a WSW trending shear zone offset formed by a couple of  1:2000 scale (Figure 8), we interpret that the Moytirra hydrothermal field is related to N-120°trending oblique slip-normal faults intersecting the steep flanks of the axial valley. We infer that these oblique faults are acting as tension gashes allowing fluids to circulate from the flanks of the former ridges to the AVR forming high-temperature fluid venting at the intersection with the flanks of the valley.
This study on the Moytirra hydrothermal site reached the deepest part of the system for the first time. The new bathymetric map of Moytirra segment of the MAR presented here, represents the basis for future investigation on the occurrence of further hydrothermal fields and seafloor massive sulfide sites along the Northern MAR.

Software
MBES data were processed using CARIS HIPS&-SIPS™ software. Sound Velocity Profiles (SVPs) used for correction of the MBES data consist of real time synthetic derived from the CTDs data and Expendable Bathy-Thermograph (XBT) probes T5, that were compared with the synthetic profiles from the WOA13 sound speed database. Multi-resolution DTMs were also used to generate the Main Map and regional sun-shaded image renders and perspective views and to extract margin-wide bathymetric profiles using FLEDERMAUS™ software.