3He along the ultraslow spreading AMOR in the Norwegian-Greenland Seas

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3He along the ultraslow spreading AMOR in the Norwegian-Greenland Seas Anne Stenslanda,∗, Tamara Baumbergera,b,c, Kjell A. Morkd, Marvin D. Lilleye, Ingunn H. Thorsetha, Rolf B. Pedersena a

K.G. Jebsen Centre for Deep Sea Research/ Centre for Geobiology, Department of Earth Science, University of Bergen, Norway Cooperative Institute for Marine Resources Studies, Oregon State University, Newport, OR, USA NOAA Pacific Marine Environmental Laboratory, Newport, OR, USA d Institute of Marine Research and Bjerknes Centre for Climate Research, P.O. Box 1870 Nordnes, N-5817 Bergen, Norway e School of Oceanography, University of Washington, Seattle, WA, USA b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Mid-ocean ridges 3 He Hydrothermal processes and products Nordic Seas

The majority of undiscovered hydrothermal vent fields are thought to be located on slow and ultraslow spreading ridges. Locating new vent sites on these ridges is important due to their tendency to host large seafloor massive sulfide deposits. It is also important as it can increase our understanding of the tectonics at ultraslow spreading ridges as well as giving us an understanding of how hydrothermal venting in these regions affect and interact with the surrounding water column. To assess the hydrothermal productivity of the slow to ultraslow spreading Mohns and Knipovich ridge segments of the Arctic Mid-Ocean Ridge we used the primordial isotope 3 He. Due to the conservative nature of 3He, this isotope can function as a tracer both for hydrothermal activity and on the ocean current pathways. In addition to assessing the hydrothermal productivity of the ridges we also studied the impact the ridges have on ocean circulation. Between 2006 and 2016, 400 water samples were collected along the Mohns Ridge and the Knipovich Ridge and 117 samples were collected in two transects crossing each ridge. The results show that the surface mixed layer (500–0 m depth) directly above each ridge has higher than equilibrium values of δ3He, which we interpret as an effect of the bottom topography on the vertical mixing. We also observe direct indications of at least two undiscovered vent sites along the Mohns Ridge, making the total count of 7 vent fields along this 550 km ridge segment, which gives a vent field frequency (Fs) of 1.27 sites/100 km. This number is comparable to the vent field frequency on other ultraslow ridges. By comparing the current speed to non-buoyant plume measurements obtained at the previously discovered Loki's Castle and the Jan Mayen vent fields (JMVF) we made an estimate of the 3He flux at these vent fields. This estimate gave a flux of 0.22 mmol/km/mm/yr from Loki's Castle, which is well below the world's average of 0.33 mmol/km/mm/yr. However, the flux rate estimated here is comparable to estimates previously calculated from the ultraslow Gakkel ridge. The northeastern termination of the Mohns Ridge, where Loki's Castle is located, is also the thinnest part of the ridge segment indicating a low magmatic influence. The flux rate of the JMVF was 0.40 mmol/km/mm/yr and thus higher than the global average. This could indicate an influence from the Jan Mayen hot spot as similar flux rates has been found at other hot spot influenced vent sites. This study not only illuminates the hydrothermal productivity of ultraslow spreading ridges, but also adds a considerable dataset of helium isotopes in Nordic Seas.

1. Introduction Beaulieu et al. (2015) predicts that around 400 vent fields remain undiscovered along the worlds spreading ridges. Of these undiscovered sites the majority are believed to be hosted in slow spreading (less than 55 mm/yr) to ultraslow spreading (less than 20 mm/yr) regimes (Dick et al., 2003; Beaulieu et al., 2015). This is supported by increasing evidence, showing that vent sites are more abundant on slow and

∗

ultraslow spreading ridges then predicted (Baker and German, 2004; German et al., 2016). This discrepancy between predicted and observed vent field densities implies that non-magmatic heat sources are common along these ridges (Escartin et al., 2008; Rona et al., 2010). Identification of new vent sites along these ridges also has socioeconomic impact due to their tendency to be co-located with seafloor massive sulfide (SMS) deposits (Hannington et al., 2011; Beaulieu et al., 2015; German et al., 2016). Mining of SMS deposits has become

Corresponding author. E-mail address: [email protected] (A. Stensland).

https://doi.org/10.1016/j.dsr.2019.04.004 Received 19 September 2018; Received in revised form 5 April 2019; Accepted 5 April 2019 0967-0637/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Anne Stensland, et al., Deep-Sea Research Part I, https://doi.org/10.1016/j.dsr.2019.04.004

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increasingly significant as the world searches for new ways to retrieve metals which fuel emerging technologies (e.g. Boschen et al., 2013). Unlike on fast spreading ridges (80–180 mm/yr), SMS deposits on ultraslow spreading ridges can occur in hydrothermal settings driven by magma or tectonics (German et al., 2016). In this study we focus on the ultraslow Mohns and Knipovich ridge segments of the Arctic Mid-Ocean Ridge (AMOR), with the aim of quantifying the hydrothermal input in this region by applying the primordial isotope 3He as a hydrothermal tracer. 3 He is introduced into the water column through two different processes. (1) Radiogenic 3He is produced via decay of tritium (e.g. Jenkins et al., 1980). The tritium input to the world's oceans was minor before the nuclear age when it increased dramatically as a result of abundant testing of thermonuclear weapons in the atmosphere (Craig and Lal, 1961; Eriksson, 1965). (2) Primordial 3He is introduced in the deep sea through hydrothermal activity along active plate boundaries and Hot Spots (Clarke et al., 1969). Primordial 3He enters the oceans through mantle degassing and mixing with hydrothermal fluids which are discharged at the ocean floor at hydrothermal vent sites and submarine volcanoes (e.g. Clarke et al., 1969; Lupton and Craig, 1981; Lupton, 1998; Baumberger et al., 2014). Once discharged, the heated hydrothermal fluids, enriched in 3He, will rise upwards in the water column due to density differences between the hydrothermal fluid and the ambient seawater (e.g. Lupton, 1995). Once density equilibrium is achieved the diluted hydrothermal fluids will spread out laterally in the water column and form a hydrothermal non-buoyant plume (e.g. Lupton, 1995). Although the origin of 3He may differ in the water column, the chemical behavior of the isotope remains the same. Once introduced into the water column 3He concentrations in the nonbuoyant plume will only decrease by dilution allowing non-buoyant plumes to be traced over thousands of kilometers (e.g. Lupton, 1983). In this study 3He/4He is expressed by the delta notation, δ 3He (%) = (R/ Ra-1) x 100, which describes the 3He/4He ratio in the seawater sample (R) relative to the ratio in the atmosphere (Ra = Rair = 1.38 × 10−6). The 3He input to ocean basins varies regionally, resulting in nonuniform background values for the deep ocean. Due to widespread hydrothermal activity and differences in abyssal ventilation rates, the Pacific Ocean has the highest 3He enrichment of the ocean basins with maximum values close to 25–30% (Lupton and Craig, 1981; Lupton, 1998) the Indian ocean has an intermediate δ3He enrichment (13–15%) (Srinivasan et al., 2004) whereas the Atlantic Ocean has the lowest 3He enrichment in the deep water with values between 5 and 8% (Top et al., 1987; Rüth et al., 2000). The water masses in the Norwegian and Greenland Sea are divided into cold, less saline Arctic water in the Greenland Sea and warm, saline Atlantic water in the upper layer of the Norwegian Sea (Figs. 1 and 2). The Mohns and Knipovich Ridges function as borders between these two water masses, allowing 3He distributions to provide information about their Arctic and Atlantic 3He inventory. We assess 3He in these water masses which provides insights into the productivity of the ridges and the ridges impact on the water mass distribution column.

Fig. 1. Map of the study area and all sample locations obtained from 2006 to 2016. The Loki's Castle (LC), the Ægir, the Jan Mayen Vent Fields (JMVF) and the Seven Sisters (7S) vent fields are indicated.

and Pedersen, 2005). Presently, six hydrothermal vent fields have been discovered in the Norwegian Exclusive Economic Zone, the Seven Sisters vent site on the northern termination of the Kolbeinsey Ridge (Marques et al., in review), the Troll Wall, Soria Moria and Perle & Bruse vent sites on the southern termination of the Mohns Ridge (described collectively as the JMVF) (Pedersen et al., 2010a; Dahle et al., 2018), the Ægir vent field on the central Mohns Ridge and lastly the Loki's Castle vent field on the northern termination of the Mohns Ridge (Fig. 1) (Baumberger et al., 2016a, 2016b; Pedersen et al., 2010b). 3. Materials and methods 3.1. Sampling Water column samples were collected using a CTD (conductivity, temperature, depth) probe (911plus Seabird) with a Niskin water bottle (10 L) rosette. The samples were collected between 2006 and 2016 in the Norwegian and Greenland Seas (Fig. 1; Tables S1 and S2). The water column above the JMVF was sampled in the years 2006, 2011, 2012 and 2013 (82 samples). The water column between the JMVF and the northern termination of the Mohns Ridge was sampled in the years 2013 and 2014 (177 samples). The water column above the Loki's Castle vent field was sampled in the years 2007, 2008 and 2009 (116 samples). In 2015, two transverses were conducted crossing the Mohns Ridge from the Norwegian Sea to the Greenland Sea (66 samples) and crossing the Knipovich Ridge from the Greenland Sea towards the Bear Island (51 samples). In addition, two water column profiles were obtained above the Knipovich Ridge in 2016 (23 samples). One water column profile was obtained above the Seven Sisters vent field on the northern termination of the Kolbeinsey Ridge in 2013.

2. Regional setting The AMOR is defined as the ridge segment north of the Arctic circle (66°N), consisting of the Kolbeinsey, the Mohns, the Knipovich, the Molloy and the Gakkel Ridges as well as the Lena Trough (Pedersen et al., 2010a). This study focuses on a portion of the AMOR within the Norwegian Exclusive Economic Zone, primarily the ultraslow spreading Mohns and Knipovich Ridges (Fig. 2). The Mohns Ridge is a 550 km long ridge stretching from the Jan Mayen Fracture Zone in the south to the Knipovich Ridge in the north (Pedersen et al., 2010a) (Fig. 1). The Mohns Ridge is spreading at an estimated rate of 15 mm/yr (Vogt, 1986; Pedersen et al., 2010a) and thus is defined as an ultra-slow spreading ridge (Dick et al., 2003). The 500 km long Knipovich Ridge is spreading with a rate below 10 mm/yr (Okino et al., 2002; Hellevang

3.2. Measurements Immediately upon recovery of the underwater sampling package, air-free water samples were flushed through 24-inch long sections of refrigeration grade Cu tubing with duplicate half-sections cold-weld sealed for later laboratory determinations of He concentrations and isotope ratios at the NOAA/PMEL Helium Isotope Laboratory in 2

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Fig. 2. (a) Schematic map of the surface circulation in the Nordic Seas and (b) gridded spatial temperature distribution at 500 m depth, averaged over spring 1995–2017, using combined hydrographic observations from the ICES (International Council for the Exploration of the Sea) archive and Argo data from the Global Data Assembly Centre (http://www.coriolis.eu.org/).

concentrations of helium were determined using a 21-cm radius, dualcollector, sector-type mass spectrometer specially designed for helium isotope analyses. The mass spectrometer inlet is fitted with a low temperature cryotrap system that can separate helium from other noble gases before the sample is analyzed. Helium isotope ratios were analyzed with a precision of 0.2% in δ3He, where δ3He is the percentage

Newport, OR, USA (Young and Lupton, 1983; Jenkins et al., 2010). The gas was extracted at the lab where a high vacuum sample preparation system and activated charcoal at liquid nitrogen temperature were used for the extraction of the dissolved gases from the seawater collected in the copper tubing samples. The gas phase was then sealed in an aluminosilicate glass vial for subsequent analyses. Isotope ratios and 3

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Fig. 3. δ3He from water column profiles from (a) the Mohns Ridge (b) transect crossing the Knipovich Ridge from the Greenland Sea into the Norwegian Sea towards the Bear Island and (c) transect crossing the Mohns Ridge from the Greenland Sea into the Norwegian Sea to the Lofoten Basin.

three other areas of elevated δ3He values were identified at similar depths between 72 °N and 73 °N along the ridge section (Fig. 3a), with maximum δ3He values of 8.3%, 11.4% and 7.9%, respectively (Table S1). Venting at the JMVF, at the southern termination of the Mohns Ridge, occurs at 580 m depth. The JMVF are the shallowest vent fields located on this ridge segment to date. The maximum δ3He value recorded in this area was 103% directly over the Troll Wall vent field (Table S2). The water column deeper than 350 m was enriched in δ3He with values ranging from 7 to 103%. The shallow water column (0–250 m) displayed values between −1.74 and 6.4%.

deviation of 3He/4He from the atmospheric ratio and a concentrations accuracy of 1% relative to a laboratory air standard. 4. Results 4.1. δ3He characteristics 4.1.1. The Mohns Ridge The water column above 1000 m water depth has δ3He values between 2 and 3%, indicating lower δ3He values compared to the intermediate and deeper water layers (1000–2400 m) with average values between 5 and 6% (Fig. 3a) (Table S1). The deep water column below 2400 m has an average between 6 and 7%. Elevated δ3He values (maximum 154.4%) were found at approximately 2000 m water depth above the Loki's Castle vent field at the northern termination of the Mohns Ridge (Fig. 3a) (Table S2). Along the 500 km long Mohns Ridge

4.1.2. The Knipovich Ridge All samples collected from the Knipovich Ridge come from depths greater than 2100 m. The average δ3He value of this deep layer is 7% (Fig. 3b) (Table S1). 4

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5. Discussion

4.1.3. Cross sections The average δ3He values in the deep water (5–7%) were also found below 1000 m depth in the two cross sections (Fig. 3b and c) (Table S1). In the cross section of the Mohns Ridge, elevated δ3He values were found in the intermediate/deep layer in the Lofoten Basin (7.16%), over the Mohns Ridge (7.52%) and in the Greenland Sea (7.54%) (Fig. 3c; Table S1). The upper layer (above 1000 m water depth) in the Norwegian Sea had δ3He values between -2 and 0%. Towards the Greenland Sea the δ3He values in the upper layer increased to a maximum of 2.84% directly above the Mohns Ridge. In the cross section of the Knipovich Ridge the water column below 2500 m water depth was enriched in δ3He (7–7.5%) over the ridge and outwards into the Greenland Sea (Fig. 3b). Water depth decreases on the Norwegian Sea side of the ridge, northeast towards the Bear Island. The upper water column east and west of the Knipovich Ridge had δ3He values between −1.82 and 0. Directly over the ridge segment the δ3He values of the upper water column reached 2.40% (Fig. 3b).

5.1. Ridges as a control of circulation The air-sea exchange driving the 3He isotope anomaly towards equilibrium produces a δ3He value close to −2% (Weiss, 1970; Benson and Krause., 1980; Fuchs et al., 1987) in the surface mixed layer. This value is similar to values measured in the Norwegian and Greenland Seas surface waters in this study (Fig. 3) and also to previous studies conducted in the region (Heinze et al., 1990; Jenkins et al., 2015). However, this value does not correspond to the values measured in the upper water column above the ridge segments (Fig. 3). Above the ridges the δ3He values were close to 4% higher than the air-sea exchange equilibrium, implying that the equilibrium has not been reached. Positive δ3He values in surface waters are expected to be found in areas where upwelling occurs, as the Southern Ocean, the tropical Pacific and the tropical Indian Ocean (Jenkins et al., 2015; Schlitzer, 2016). This disequilibrium is even more pronounced above the Seven Sisters vent field where the δ3He value in the surface was 6% higher than the equilibrium value. Disequilibrium between the atmosphere and the surface mixed layer, as observed above these locations, can be established through vertical movements of water masses from the deep towards the surface (Klein and Rhein, 2004). The results therefore indicate that the primordial 3He produced at the ridges is transported not only laterally along the ridges but also vertically towards the surface. The Mohns Ridge generates a front of Arctic water in the Greenland Sea and Atlantic water front between southward moving Arctic water in the Greenland Sea and northwards moving Atlantic water in the Norwegian Sea (Fig. 2a). This creates a unique environment in the water column above the mid-ocean ridge resulting in horizontal and vertical transport of water masses responsible for the enrichment of 3He in the upper water column. This can in part be explained by buoyancy of the hydrothermal fluids, however, additional vertical mixing is required to make the 3He continue to rise above the level of neutral buoyancy. The calculated geostrophic velocities from the densely sampled survey along the Mohns Ridge reveal alternating westward and eastward flow along the ridge. This indicates the presence of local cyclonic and anticyclonic baroclinic eddies with horizontal scales in the order of 20–50 km (Fig. 7). In comparison, Van Aken et al. (1995) estimated the horizontal range of the baroclinic eddies in the same region to be 40–50 km using a combination of current measurements and CTDs. These eddies may not only contribute to cross-frontal exchange of water masses but also to vertical mixing (e.g., Wunsch and Ferrari, 2004). In addition, Naveira Garabato et al. (2004) estimated enhanced turbulent diapycnal mixing above the Mohns Ridge. We documented this oceanographic phenomenon in Nordic Seas, however, further investigations are required to assess the impact on this oceanographic phenomenon on the biosphere and surrounding water masses in the Nordic Seas.

4.1.4. The Kolbeinsey Ridge One water column profile was obtained above the Seven Sisters vent field on the northern termination of the Kolbeinsey ridge. This is the shallowest part of the study area, where venting occurs between 170 and 100 m water depth. A maximum of 108% δ3He was measured in the deepest sample at 170 m depth. In the shallowest sample at 72 m depth, the δ3He value was 4.4% (Table S1).

4.2. Hydrography Vertical profiles of temperature and salinity were obtained with the CTD probe. Maximum salinity (> 35 PSU) and temperature (> 3 °C) were found in the upper water column (above 700 m depth) in the Norwegian Sea (Figs. 2b, 5 and 6). Along the Mohns Ridge this warm, saline layer is shallower (< 500 m) and shallowest in the southern end (Fig. 4). The surface layer of the Greenland Sea is colder and fresher than the Norwegian Sea (Figs. 2b, 4 and 5). The water column below 1500 m depth has a salinity of about 34.91–34.92 PSU and temperatures below −0.5 °C (Figs. 4–6). The geostrophic currents (vg) along the Mohns Ridge were calculated from hydrographic data obtained in the summer of 2013 using the thermal wind balance (e.g., Pond and Picard, 1978; Cushman-Roisin, 1994)

vg (z ) = vg (−H ) −

g fρ0

z

∫ ∂∂xρ dz −H

The x-axis is directed along the section, from southwest to northeast, and the velocity component vg is directed across the section, f is the Coriolis parameter, g is the acceleration of gravity, ρ is the density and ρ0 is a reference density (1030 kg m-3). z is the vertical axis with z = 0 at the sea surface and z = -H at the bottom. Assuming the currents weaken with greater depths, the reference level was set near the bottom (at the deepest measurements) where the velocities between each station par, vg (-H), were set to zero. Thus, the calculated geostrophic velocities, which we term as baroclinic velocities, are relative to the bottom velocities that we assume are small. The data reveal several geostrophic mesoscale eddies, both cyclonic and anticyclonic, occurring along the ridge segment (Fig. 7). Vertical shear in the currents are observed, while below 1500 m velocities are weak and about constant with depth. The integrated geostrophic velocities in the upper 1500 m depth show a total westward flow 3.2 Sverdrup (Sv; 1 Sv = 106m3s−1) and a total eastward flow of 2.6 Sv in the upper 1500 m. Thus, velocities in the upper 1500 m give a net volume transport of 0.6 Sv directed into the Greenland Sea (Fig. 7).

5.2. The Norwegian-Greenland Sea- δ3He characteristics Atlantic water in the Norwegian Sea is an upper water mass and is generally characterized by a salinity higher than 35.0 and temperatures higher than 3 °C (Fig. 2b) (e.g. Mauritzen, 1996: Orvik. 2004). This water mass is easily identified as the layer above about 800 m water depth in the Lofoten Basin, the northern most basin in the Norwegian Sea (Figs. 2a, 4–6). This water mass corresponds to a layer with δ3He values around −2%, which has been established as the air-sea exchange equilibrium value for δ3He. The water column between 1000 m and 1500 m is characterized with δ3He between 2 and 4% in the Norwegian Sea, which likely defines the Norwegian Sea Arctic Intermediate Water (NSAIW) and represents a mixture between Atlantic water and Arctic deep waters originating from the Greenland Sea (Fig. 2a) (Blindheim and Østerhus, 2005). Similar δ3He values have been found further south along the mid-Atlantic ridge, which were interpreted as the North Atlantic Deep Water (NADW) (Rüth et al., 2000). Deeper than 5

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Fig. 4. Vertical distribution of (a) salinity (psu) with a contour spacing of 0.01 and (b) temperature (°C) with a contour spacing of 0.5 along the Mohns Ridge.

(Fig. 4). There is a distinct enrichment of δ3He over the ridges, and as discussed in the previous section, this feature extends vertically to the upper water column. The areas along the ridge segments with positive anomalies in the middle of the water column, represent the nonbuoyant plume of vent fields situated below. These anomalies will be further discussed in the following section.

1500 m the δ3He values are around 6% with a few exceptions (Fig. 3). The deep water of the Norwegian Sea is largely occupied by the Norwegian Sea Deep Water (NSDW), which is a mixture of Arctic deep water masses and Greenland Sea Deep Water (GSDW) (Aagaard et al., 1985; Swift and Koltermann, 1988; Schlosser et al., 1995; Blindheim and Østerhus, 2005). The positive anomaly (7.16%) in the Lofoten Basin (Fig. 3c), is consistent with previous findings in other Atlantic deep ocean basins which has been interpreted as 3He enriched Antarctic bottom water (AABW) (Rüth et al., 2000). Here, however, this enriched deep water may represent the GSDW as this water mass has higher δ3He values (Schlosser et al., 1995). The negative anomaly (−2 – 1%) found on the slope from the Knipovich ridge up towards the Bear Island (Fig. 3b) can likely be ascribed to an overflow of bottom water forming in the Barents Sea and flowing down-slope into the Norwegian Sea (Blindheim and Østerhus, 2005). The water column in the Greenland Sea has slightly higher δ3He values at all depths compared to the Norwegian Sea (Fig. 3b and c). Previous studies have found that the GSDW, which constitutes most of the deep water component in the Greenland Sea, has a higher δ3He value due to higher input of radiogenic 3He compared to the NSDW (Schlosser et al., 1995). The high radiogenic input originates from the high tritium component in Arctic river runoff (Schlosser et al., 1995). The Jan Mayen Current is a branch of the East Greenland Current, bringing cold and fresh waters eastwards from the Greenland Sea into the Jan Mayen fracture zone and up along the Mohns Ridge before returning into the Greenland Sea Gyre (Fig. 2a) (Bourke et al., 1992). Due to this current there is a transition in the surface water along the Mohns Ridge from cold, less saline Arctic water to gradually more Atlantic dominated water towards the northern end of the ridge

5.3. Hydrothermal vent fields 5.3.1. Loki's castle The most profound δ3He enrichment was found on the northern termination of the Mohns Ridge and represents the non-buoyant plume from the Loki's Castle vent field (Fig. 3a). The hydrothermal plume is characterized by a δ3He enrichment close to 2000 m water depth, indicating a rise height of about 400 m above the seafloor (Table S2). In high-temperature hydrothermal fluids the concentrations of Mg and SO4 will be close to zero due to the formation of anhydride and fixation of Mg through precipitation of chlorite and albite (Mottl and Holland, 1978; Alt, 1995; Shanks et al., 1995). Thus, a zero Mg fluid provides a chemical representation of the pure endmember fluid, which can be graphically obtained through common extrapolation methods. The endmember fluid composition from Baumberger et al. (2016a, b) shows a 3He concentration of 9.4–10.1 pmol/kg and a 4He concentration of 913–1066 nmol/kg giving a 3He/4He ratio of 7.4 (R/Racorr). The maximum 3He concentration measured in the hydrothermal plume above Loki's Castle was 8.4 fM (corresponding to a δ3He value of 154%) (Table S1). This corresponds to an enhancement of 5.8 fM compared to background, which gives a local dilution factor of 1700. A theoretical dilution factor for non-buoyant plumes in the Pacific Ocean has been 6

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Fig. 5. Vertical distribution of (a) salinity (psu) with a contour spacing of 0.01 and (b) temperature (°C) with a contour spacing of 0.5 in a transect crossing the Mohns Ridge from the Greenland Sea to the Lofoten Basin in the Norwegian Sea.

5.3.4. The Ægir vent field The newly discovered Ægir vent field is responsible for the second highest δ3He (11.4%) positive anomaly at 2050 m depth, on the central part of the Mohns Ridge (Fig. 1). This anomaly discovered in 2013 led to the discovery of a new field in 2014 and the chemistry of the vent fluids has yet to be described.

calculated to be between 8000 and 10000 (Speer and Rona, 1989; McDuff, 1995; Field and Sherrell, 2000). This indicates that the maximum value measured above the Loki's Castle was from the buoyant plume. Samples at the same depth, but further away from the venting show an enrichment of 1.1 fM above the background, indicating a dilution factor of 8900, which is more in line with previous findings at other vent fields.

5.3.5. Potential vent sites The two other positive δ3He anomalies found along the Mohns Ridge are not associated with any known vent sites (Fig. 3a). The distance from these anomalies to other known vent fields in the area is over 100 km, which indicates that these signals could represent undiscovered vent sites (Fig. 3). What is important to note is that both the anomalies were found at a depth of 2050 m, similar to the height of the NBP from the Loki's Castle and Ægir vent field. This is an indicator of the homogenous deep water column above the Mohns Ridge. The two anomalies are small (8.3, 7.9%), however, they represent a local enrichment of 3He in the water column. Such a local enrichment of 3He is unlikely to represent radiogenic helium and can thus only be explained by hydrothermal processes. Apart from the four positive anomalies (Loki's Castle, Ægir and the two undiscovered vent fields) along the Mohns Ridge, the remainder of the deep water column has similar values (5–7%) to that of the Norwegian and Greenland Sea. The two anomalies are small (8.3, 7.9%) and.

5.3.2. The JMVF The JMVF are situated on the southern termination of the Mohns Ridge and consist of three separated areas of venting: The Troll Wall, Soria Moria and Perle & Bruse vent fields (Fig. 1). Unlike the NBP from the Loki's Castle vent fields, the plumes from the JMVF can be observed as δ3He enrichments ranging from the seafloor (580–700 m depth) and up towards a plume centre at 500 m water depth (Table S1). The areas around the larger chimneys are characterized by several locations with diffuse venting and as a result we observe 3He, CH4 and dissolved metal enrichments close to the seafloor below the NBP (Stensland et al., 2019). The NBP rise height of approximately 100 m is about ¼ of the rise height of the plume at the Loki's Castle vent field. The end-member concentration of 3He was 11.4 pmol/kg at the Troll Wall field (Baumberger et al., in prep) and the NBP shows an enrichment of 1.2 fM above background indicating a dilution factor of 9500.

5.4. Productivity of ultraslow spreading ridges 5.3.3. The seven sisters vent field Only one water column profile was obtained above the Seven Sisters vent field and this profile revealed higher than background δ3He from 170 m water depth up to 72 m depth. This indicates that the shallow venting from the Seven Sisters affects the entire water column above the vent site. The endmember composition showed 3He/4He between 7.8 and 8.6 (R/Ra corr) (Marques et al., in review).

In this study we find direct indication of at least two undiscovered vent sites along the 550 km long Mohns Ridge, which implies at least 7 vent fields in total along the ridge. The site frequency (Fs = sites/ 100 km ridge segment) of the Mohns Ridge can thus be estimated to be 1.27 sites/100 km. This Fs value is comparable to the Fs of the ultraslow spreading Gakkel Ridge (1.1–1.2 sites/100 km) and the eastern South 7

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Fig. 6. Vertical distribution of (a) salinity (psu) with a contour spacing of 0.01 and (b) temperature (°C) with a contour spacing of 0.5 in a transect crossing the Knipovich Ridge from the Greenland Sea into the Norwegian Sea towards the Bear Island.

due to the bubbles released at these sites. In these shallow areas venting has proved highly abundant with 3 sites within one neo-volcanic zone as seen on the Jan Mayen plateau. The two other sites previously discovered along the Mohns Ridge, the Loki's Castle and Ægir vent sites, are located on deep neo-volcanic zones and were identified by water column anomalies. However, the potential presence of more vent fields on and surrounding these neovolcanic zones requires a more detailed survey. Baker et al. (2016) found that discharge sites were 3–6 times more frequent on fast spreading ridges than previously estimated using conservative tracers. They also hypothesized that the amount of vent fields on slow spreading ridges has been similarly underestimated (Baker et al., 2016). 5.5. Hydrothermal flux estimates The end-member concentrations for Loki's Castle and the JMVF are known, but these end-member components only represent the fluid composition from the focused flow venting and exclude the diffuse components. By utilizing helium isotope measurements in the plume one can make an estimate of the 3He flux from the vent sites in the region. To make this estimate some assumptions need to be made, specifically in regards to the rift valley water flux. Many measurements of the currents and their fluxes has been conducted in the Nordic Seas (e.g. (Woodgate et al., 1999, 2001; Orvik and Niiler, 2002; Nøst and Isachsen, 2003; Orvik, 2004; Quadfasel and Käse, 2007; Voet et al., 2010), but no specific measurement of the deep current fluxes have been obtained in the venting areas. The hydrographical data from the 2013 cruise along the Mohns Ridge show a net volume flux of 0.6 Sv

Fig. 7. Geostrophic velocity (cm/s) along the Mohns Ridge, from south to north between 4° 30′ W and 3° E. Positive values (red colour) is westward flow (i.e., directed into the Greenland Sea). Note that colour scale is not linear. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

West Indian Ridge (1.3 sites/100 km) (Baker et al., 2004) and is congruent to a linear function of Fs versus spreading rate reported by Beaulieu et al. (2015) and German et al. (2016). However, the vent field abundance reported here could be a low estimate for this part of the ridge system. All three of the JMVF as well as the Seven Sisters vent field were discovered in part by hull-mounted acoustic sonar systems 8

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is approximately 4 km, which is 2–3 km thinner compared to the world average (Klingelhöfer et al., 2000). The flux estimate for the JMVF plot above the global average, as do flux estimates at the Rainbow and the Lucky Strike vent fields situated further south on the mid-Atlantic Ridge (Jean-Baptiste et al., 2004b). These estimates reveal a flux of 0.5 mmol/km/mm spreading rate for Rainbow and 0.45 mmol/km/mm spreading rate for Lucky Strike, both estimates well above the global average (Fig. 8). The higher than average flux estimated at these sites has been attributed to the proximity to the Azores hot spot (German et al., 1996; Jean-Baptiste et al., 2004b). This is also likely the cause for the elevated 3He inventory at the JMVF, where there is an influence from the nearby Jan Mayen hot spot (Elkins et al., 2016). The hot spot influence at the JMVF is likely also responsible for the unusually thick oceanic crust at the southwestern part of the Mohns Ridge (Kandilarov et al., 2012). 6. Conclusions Fig. 8. The annual flux of 3He per kilometer of ridge as a function of spreading rate. The dashed line gives the global circulation model line with an average of 0.33 mmol/km/mm/yr (Farley et al., 1995; Dutay et al., 2004). The calculations and data for the Lucky Strike, the Red Sea, the Rainbow site and the Gakkel Ridge can be found in Jean-Baptiste et al. (1998), Jean-Baptiste et al. (2004c), Jean-Baptiste et al. (2004b) and Jean-Baptiste and Fourré (2004). This figure is modified from Jean-Baptiste and Fourré (2004). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

• Along

•

current flux in the upper 1500 m of the water column. The flux below 1500 m depth is not known, but is thought to be less in the deeper water column due to the weaker currents there (Fig. 7). The water flux in the deeper rift valley further south on the Mid-Atlantic Ridge has shown a remarkable stability with a flux of 0.07 Sv (Thurnherr et al., 2002), and we therefore can make the assumption that the flux rate is stable and low in the deeper water column. However, not knowing the exact flux of the water masses in the rift valley means that the estimates calculated here have a large uncertainty. The average δ3He in the plume (the water column below 1900 m water depth) of Loki's Castle is 11.74% and the average of the JMVF plume (the water column below 300 m water depth) is δ3He 17.3%. The 3 He flux transported by the plume has been expressed as;

•

the ultraslow spreading Knipovich Ridge and the slow spreading Mohns Ridge five vent sites have previously been discovered. In this study we show direct indications of at least two additional vent fields on these ridge segments. The vent field frequency of these ridges is thus 1.27 sites/100 km, which is similar to estimated vent field frequencies of other ultraslow spreading ridges. Above the ridges we observed a δ3He disequilibrium between the atmosphere and the surface mixed layer. This disequilibrium is created due to a combination of buoyancy of the hydrothermal fluid and vertical mixing above the ridges bringing enriched 3He water from the deep up towards the surface. The 3He in the non-buoyant plume gives a flux of 0.26 mmol/km/ mm/yr from Loki's Castle and 0.48 mmol/km/mm/yr from the JMVF. The flux estimate for the Loki's Castle vent field is below the world average of 0.33 mmol/km/mm/yr and correlates to estimates for the ultraslow spreading Gakkel Ridge. The flux estimate for the JMVF is higher than the global average indicating a hot spot influence.

Acknowledgments We thank W. Jenkins, R. Newton and the two other anonymous reviewers for their valuable input; E. Olson and A. Schaen for obtaining the samples in 2006 and 2016, respectively; The crew of R/V G.O Sars and R/V Johan Hjort for assisting us at sea; NOAA for letting us borrow their crimper from 2011 to 2014; J.E Lupton and the NOAA/PMEL Helium Isotope Laboratory in Newport, OR, USA for the shore-based analysis and a thank you to the Institute of Marine Research who allowed us to join their cruise in 2015. This work received funding from the Norwegian Research Council (RCN) through the Centre for Geobiology, UiB, and by Stiftelsen Kristian Gerard Jebsen (project no. 179560). This is PMEL contribution 4816.

Q3He = Φ x[4He]xR a x ( δ− δBG)/100 (Jean-Baptiste et al., 2004b), where 4He is the helium concentration measured in the plume; 1.87 μmol/m3 for the Loki's Castle vent field and 1.91 μmol/m3 for the JMVF, (δBG) is 5%, and Ra is 1.38 × 10−6 and is the atmospheric ratio. This gives a Q3He = 12.18 nmol/s for the plume from Loki's Castle and Q3He = 22.69 nmol/s for the JMVF. This flux can be described as a flux per kilometer of ridge per spreading rate. The spreading rate of the Mohns Ridge is about 15 mm/year (Vogt, 1986; Pedersen et al., 2010a). The average spacing between vent fields at ultraslow spreading ridges has been estimated to be slightly above 1/ 100 km ridge (Baker and German, 2004). This gives a flux of 0.26 mmol/km/mm/yr from Loki's Castle and 0.48 mmol/km/mm/yr from the JMVF. Although these estimates of the hydrothermal flux along the Mohns Ridge have a high uncertainty, the value for the Loki's Castle area corresponds to values calculated from the Gakkel Ridge by JeanBaptiste and Fourré (2004a) (0.15–0.32 mmol/km/mm spreading rate) and to fluxes estimated in the same region by Bönisch and Schlosser (1995) (Fig. 8). Bönisch and Schlosser (1995) observed through their calculations that the flux of 3He from the ridge below was indeed lower than the world average 0.33 mmol/km/mm/yr (Farley et al., 1995; Dutay et al., 2004). The northeastern part of the Mohns Ridge, where Loki's Castle is located, seems to share this feature. This is consistent with the fact that from the central Mohns Ridge and northeastwards the crustal thickness

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