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.

Iceland-Scotland Overflow Water Transport Variability through the Deep Bight Fracture Zone

<p>Iceland Scotland Overflow Water (ISOW), a component of the deep limb of the Atlantic Meridional Overturning Circulation (AMOC), is the equilibrated product of dense overflow into the eastern North Atlantic basin.  Modeling results and recent observations have suggested that a significant westward transport of ISOW (~1x10<sup>6</sup> m<sup>3</sup>s<sup>-1</sup>) may occur through the Bight Fracture Zone (BFZ) near 57°N, the first major channel through the Reykjanes Ridge where ISOW can cross into the Irminger Sea.  The remaining denser (and deeper) ISOW has been shown to leave the Iceland Basin westward via the Charlie-Gibbs Fracture Zone near 53°N, or southward into the West European Basin. Until now, there have been no measured time series in the BFZ to validate model results. Single moorings placed in the north and south channels of the BFZ from summer 2015 to summer 2017 were used to estimate a mean combined transport across the fracture zone of 0.8 ± 0.4 x10<sup>6</sup> m<sup>3</sup>s<sup>-1</sup> westward, with each channel contributing about half of the mean transport. Variability between the two channels on shorter (month-long) times scales can be extreme: in March of 2016, for example, north channel transport was ~0.4 x10<sup>6</sup> m<sup>3</sup>s<sup>-1</sup> eastward, while south channel transport was ~0.8 x10<sup>6</sup> m<sup>3</sup>s<sup>-1</sup> westward.  For this 2-year period, transport is stronger in the summer (0.9-1.2 x10<sup>6</sup> m<sup>3</sup>s<sup>-1</sup>) than in winter (0.5-0.7 x10<sup>6</sup> m<sup>3</sup>s<sup>-1</sup>), where large fluctuations including complete reversals suggest transport variability may be affected by winter storms.  This mooring record also shows a fresh anomaly in ISOW beginning in early 2017, which has been shown by others to originate from the surface waters near the Grand Banks region of the western north Atlantic.  Transport variability in this two-year record is examined in the context of the transport variability of the OSNAP mooring arrays on the east and west flanks of the Reykjanes Ridge just north of BFZ during the same time period.  An observationally-based understanding of how the Iceland and Irminger basins communicate with each other via the deep limb of the AMOC through the BFZ will provide fundamental insight into the pathways and processes that define the subpolar AMOC system.</p>

The circulation near the Reykjanes Ridge in summers 2015, 2016 and 2017

<p>Located south of Iceland, the Reykjanes Ridge is a major topographic structure of the North Atlantic Ocean that strongly influences the spatial distribution and circulation of the North Atlantic Subpolar Gyre water masses. Around the ridge, the circulation is composed of two main along-ridge currents, the southwestward East Reykjanes Ridge Current (ERRC) in the Iceland Basin and the northeastward Irminger Current (IC) in the Irminger Sea. To study the along Reykjanes Ridge flow variability and the inter-basin connection through the ridge and connections with the interior of each basin, volume and water mass transports over the Reykjanes Ridge during summer 2015, 2016 and 2017 are analyzed. Data used are velocity and hydrographic measurements carried out along and perpendicular to the crest of the Reykjanes Ridge during the RREX (Reykjanes Ridge Experiment Project) cruises in June–July 2015 and June–July 2017 and BOCATS cruise in July 2016. The new circulation scheme in the area described in 2015  by Petit et al. (J. Geophys. Res., 2018) with flows connecting the ERRC and IC branches at specific locations set by the bathymetry of the ridge is again observed  in 2016 and  2017, with variations concerning the connections with the interiors of the basins. The data set reveals remarkable changes in the hydrological properties and transports of the ERRC, IC and cross ridge flows. The westward transport across the ridge, which represents the subpolar gyre intensity, was estimated at -19.6±3.4 Sv in 2015 and -35.2±3 Sv in 2017. A freshening and a decline in density mainly affecting the Subpolar Mode Water was observed in 2017. It was associated with a lower mode water  transport partly compensated by a higher transport of intermediate and Arctic waters. We further document each water mass contribution to the westward flow of the gyre and the structure of the ERRC and IC.</p>

Remote-predictive geologic mapping of the Reykjanes Ridge: Implications for the volcanic and structural evolution of a slow-spreading Mid-Ocean Ridge

<p>The Reykjanes Ridge is a spreading center that presents an opportunity to track the dynamic formation of structural and volcanic features at an asymmetric slow-spreading plate boundary. The ridge spans the northern ~1000 km of the Mid Atlantic Ridge and has been spreading at a full spreading rate of ~20 mm/year [1]. The characteristic along-ridge basement depth, crustal thickness, and chemical gradient have been variably attributed to an active mantle plume beneath Iceland, or a passive mantle anomaly pre-dating the rifting [1]. A unique feature of the ridge is that it spreads obliquely to the spreading axis: a consequence of the change in spreading direction from ~125<sup>o </sup>to ~100<sup>o</sup> due to the failure of the triple junction between the Greenland, Eurasian, and North American plates 37 Mya [2]. Along with the sudden change in orientation, disjunct ridge segments were formed and separated by transform faults which have been continuously eliminating from north to south, thereby re-establishing the original linear geometry of the ridge [1]. The Bight Transform Zone is the final remaining transform fault and constitutes the boundary between the southern Reykjanes Ridge and the northern Mid-Atlantic Ridge. Despite the termination of strike-slip transform fault motion, the ridge remains in a state of active tectonic deformation as demonstrated by the time-dependant orientations of linear structures, lengths of spreading segments, and deviation from the previously asserted linear continuity of the ridge. Investigating the relationship between structures, volcanism, and regional geodynamics is possible with the application of a novel remote-predictive geological mapping method based on interpretations from newly acquired bathymetric and acoustic backscatter data. Notably, the bathymetric data provides significant high-resolution coverage of both on-axis and off-axis regions, allowing us to track the evolution of the ridge for up to 13 Mya. The acoustic backscatter data aids in the interpretation of geologic features and terrains whose distribution and morphology reflect both present-day and historic ridge dynamics. This analysis will produce new insight into the on-going first and second-order deformation of the Reykjanes Ridge, its controls, and its effects on diffuse low-temperature vs. focused high-temperature hydrothermal venting.</p><p>[1] Martinez et al., 2020. Reykjanes Ridge evolution: Effects of plate kinematics, small-scale upper mantle convection, and a regional mantle gradient. Earth-Science Reviews.</p><p>[2] Jones, Stephen M., 2003. Test of a ridge–plume interaction model using oceanic crustal structure around Iceland. Earth and Planetary Science Letters.</p>

Tidal and near-inertial internal waves over the Reykjanes Ridge

Internal waves in the semi-diurnal and near-inertial bands are investigated using an array of seven moorings located over the Reykjanes Ridge in a cross-ridge direction (57.6-59.1°N, 28.5-33.3°W). Continuous measurements of horizontal velocity and temperature for more than two years allow us to estimate the kinetic energy density and the energy fluxes of the waves. We found that there is a remarkable phase locking and linear relationship between the semi-diurnal energy density and the tidal energy conversion at the spring-neap cycle. The energy-to-conversion ratio gives replenishment time scales of 4-5 days on the ridge top vs 7-9 days on the flanks. Altogether, these results demonstrate that the bulk of the tidal energy on the ridge comes from near local sources, with a redistribution of energy from the top to the flanks, which is endorsed by the energy fluxes oriented in the cross-ridge direction. Implications for tidally-driven energy dissipation are discussed. The time-averaged near-inertial kinetic energy is smaller than the semi-diurnal kinetic energy by a factor 2-3, but is much more variable in time. It features a strong seasonal cycle with a winter intensification and sub-seasonal peaks associated with local wind bursts. The ratio of energy to wind work gives replenishment time scales of 13-15 days, which is consistent with the short time scales of observed variability of near-inertial energy. Finally, in the upper ocean (1 km), the highest levels of near-inertial energy are preferentially found in anticyclonic structures, with a twofold increase compared to cyclonic structures, illustrating the funneling effect of anticyclones.