Iceland Basin Research Articles

Description of four new species from the Haploniscus unicornis Menzies, 1956 complex (Isopoda: Asellota: Haploniscidae)

Based on the adult males the four species Haploniscus bihastatus n. sp., H. monoceros n. sp., H. machairis n. sp. and H. angolensis n. sp. are described from the deep sea of the Southeast Atlantic. These belong to a species complex, which also includes H. unicornis Menzies from the Puerto Rico Trench and H. aduncus Lincoln from the Iceland Basin and shares several characters with species from the genus Mastigoniscus Lincoln. While males of the species complex can be reasonably well distinguished by their habitus and pleopods 1 and 2, the discrimination of female specimens is hampered by the pronounced sexual dimorphism. The Haploniscus unicornis complex has a wide distribution in the Atlantic Ocean, but appears to be absent from basins south of the Walvis Ridge and also from the southwest Atlantic basins.

Wyville Thomson Ridge Overflow Water: Spatial and temporal distribution in the Rockall Trough

Wyville Thomson Ridge Overflow Water (WTOW), which is the only part of the outflow from the Norwegian Sea not to directly enter the Iceland Basin, is shown to be a significant water mass in the northern Rockall Trough. It is found primarily at intermediate depths (600–1200 m) beneath the northward flowing warm Atlantic waters, and above recirculating Mediterranean influenced waters and Labrador Sea Water (LSW). The bottom of the WTOW layer can be identified by a mid-depth inflexion point in potential temperature–salinity plots. An analysis of historical data reveals that WTOW has been present in all but eight of the last 31 years at 57.5°N in the Rockall Trough. A denser component of WTOW below 1500 m has also been present, although it appears to be less persistent (12 out of the 31 years) and limited to the west of the section. The signature of intermediate WTOW was absent in two periods, the mid-1980s and early 1990s, both of which coincided with a freshening, and probable increase in volume, of LSW in the trough. Potential temperature–salinity diagrams from historical observations indicate that WTOW persists at least as far south as 55°N (and as far west as 20°W in the Iceland Basin) although its signature is quickly lost on leaving the Rockall Trough. We suggest that a transport of WTOW down the western side of the trough exists, with WTOW at intermediate depths entering the eastern trough either via a cyclonic recirculation, or as a result of eddy activity. Further, WTOW is seen on the Rockall–Hatton Plateau and in the deep channels connecting with the Iceland Basin, suggesting additional possible WTOW transport pathways. These suggested transport routes remain to be confirmed by further observational or modelling studies.

Coccolithophore dynamics in non‐bloom conditions during late summer in the central Iceland Basin (July‐August 2007)

Measurements of primary production (PP), calcification (CF), and coccolithophore abundance were made during late summer (July‐August 2007) in the Iceland Basin. Low numbers of coccolithophore cells and detached coccoliths (< 1 × 103 cells mL−1 and 1‐15 × 103 coccoliths mL−1, respectively) indicated a non‐bloom community, with Emiliania huxleyi as the dominant coccolithophore in terms of abundance, coccolithophore organic biomass, and cell calcite. PP ranged from 0.1 to 2 [mol C m−3 d−1, while CF ranged from 10 to 250 [mol C m−3 d−1, with both typically decreasing with depth. Coccolithophores were estimated to contribute 10‐20% toward total chlorophyll a, phytoplankton carbon, and PP within the euphotic zone. In these non‐bloom conditions, ~ 30‐60% of the total calcite in the water column was present as detached coccoliths rather than whole cells. Both cell numbers and variability in cell‐normalized CF controlled the magnitude of total CF, and hence both physiological limits to cell CF and growth, as well as mortality factors, need to be taken into account when examining oceanic coccolithophore communities. Combining cell‐normalized CF with an estimate of coccolith calcite gave coccolith production rates (0.4‐1.8 h−1) similar to those reported in the literature for laboratory cultures of E. huxleyi. None of the factors currently associated with coccolithophore blooms (irradiance, mixed‐layer depth, nitrate, phosphate, or calcite saturation) showed a clear correlation with community or cellular CF. Hence, although mortality is likely to control cell numbers, other factors such as trace metal (iron) availability may influence coccolithophore physiology in the central Iceland Basin during late summer.

Surface and deep ocean coupling in the subpolar North Atlantic during the last 230 years

The subpolar North Atlantic Ocean (SPNA) is of key importance for modulating the climate of NW Europe because of heat loss to the atmosphere from the North Atlantic Current. Although hydrographic properties of the surface SPNA vary on interannual to multidecadal timescales, hydrographic time series scarcely extend back beyond the 1950s. We present a 230 year long record of SPNA surface conditions reconstructed from a very high accumulation rate core that also registers changes in deep flow speed in the Iceland Basin. A lagged correlation is observed between the records of deep flow speed and stable oxygen isotopic composition of the surface SPNA (d 18 O w ), with strongest correlation when the paleoflow speed record leads by 15–20 years. This offset may to some extent reflect size‐selective biological mixing of the sediment. Nonetheless, these records reveal a decadal‐scale coupling between surface and deep ocean variability over the past 230 years, possibly driven by the North Atlantic Oscillation, with implications for North Atlantic circulation and climate.

Trends of anthropogenic CO<sub>2</sub> storage in North Atlantic water masses

Abstract. A high-quality inorganic carbon system database, spanning over three decades (1981–2006) and comprising of 13 cruises, has allowed the applying of the φC°T method and coming up with estimates of the anthropogenic CO2 (Cant) stored in the main water masses of the North Atlantic. In the studied region, strong convective processes convey surface properties, like Cant, into deeper ocean layers and grants this region an added oceanographic interest from the point of view of air-sea CO2 exchanges. Generally, a tendency for decreasing Cant storage rates towards the deep layers has been observed. In the Iberian Basin, the North Atlantic Deep Water has low Cant concentrations and negligible storage rates, while the North Atlantic Central Water in the upper layers shows the largest Cant values and the largest annual increase of its average concentration (1.13 ± 0.14 μmol kg−1 yr−1). This unmatched rate of change in the Cant concentration of the warm upper limb of the Meridional Overturning Circulation decreases towards the Irminger basin (0.68 ± 0.06 μmol kg−1 yr−1) due to the lowering of the buffering capacity. The mid and deep waters in the Irminger Sea show rather similar Cant concentration rates of increase (between 0.33 and 0.45 μmol kg−1 yr−1), whereas in the Iceland basin these layers seem to have been less affected by Cant. Overall, the Cant storage rates in the North Atlantic subpolar gyre during the first half of the 1990s, when a high North Atlantic Oscillation (NAO) phase was dominant, are ~48% higher than during the 1997–2006 low NAO phase that followed. This result suggests that a net decrease in the strength of the North Atlantic sink of atmospheric CO2 has taken place during the present decade. The changes in deep-water ventilation are the main driving processes causing this weakening of the North Atlantic CO2 sink.

Geomagnetic paleointensity deduced for the last 300 kyr from Unzen Volcano, Japan, and the dipolar nature of the Iceland Basin excursion

We have conducted paleointensity measurements of lavas and pyroclastic flows from Unzen Volcano, Japan. They have been shown in a previous study to contain a record of Brunhes Chron secular variation, with one flow recording a transitional direction (Senbongi geomagnetic excursion; 191±17ka). Various rock magnetic analyses indicate that the main magnetic carriers of samples are Ti-poor titanomagnetites, which have mixtures of single domain (SD) (and/or pseudo-SD) and multi-domain grain sizes. The low-temperature demagnetization (LTD)-double heating test (DHT) Shaw method yielded 209 successful results from 56 sites, giving 27 acceptable site-mean paleointensities for the past 5–300kyr. When excluding the Senbongi excursion data and two other data points, the data give an average virtual dipole moment (VDM) of 7.69×1022Am2 with a standard deviation of 2.51×1022Am2 (N=24). This is consistent with the selected 80 site-mean Thellier paleointensity data from the latest paleointensity database, with an average VDM of 7.64×1022Am2 and a standard deviation of 2.56×1022Am2. Paleointensity measurements on samples that record the Senbongi excursion, which is claimed to correlate with the Iceland Basin excursion, provide a value of 7.74–9.37μT using the LTD-DHT Shaw method. This corresponds to an average (standard deviation) VDM of 2.19×1022Am2 (0.21×1022Am2). When calibrating sedimentary records that capture the Iceland Basin excursion by averaging the LTD-DHT Shaw VDMs obtained from the pre- and post-excursional period, the records give consistent VDM values with the volcanic (Senbongi) record during the excursion. This indicates that variation of the geomagnetic field direction and intensity during the Iceland Basin excursion can be well represented by a simple dipolar variation.

Sedimentation of acantharian cysts in the Iceland Basin: Strontium as a ballast for deep ocean particle flux, and implications for acantharian reproductive strategies

Acantharian cysts were discovered in sediment trap samples from spring 2007 at 2000 m in the Iceland Basin. Although these single‐celled organisms contribute to particulate organic matter flux in the upper mesopelagic, their contribution to bathypelagic particle flux has previously been found negligible. Four time‐series sediment traps were deployed and all collected acantharian cysts, which are reproductive structures. Across all traps, cysts contributed on average 3–22%, and 4–24% of particulate organic carbon and nitrogen (POC and PON) flux, respectively, during three separate collection intervals (the maximum contribution in any one trap was 48% for POC and 59% for PON). Strontium (Sr) flux during these 6 weeks reached 3 mg m−2 d−1. The acantharian celestite (SrSO4) skeleton clearly does not always dissolve in the mesopelagic as often thought, and their cysts can contribute significantly to particle flux at bathypelagic depths during specific flux events. Their large size (~ 1 mm) and mineral ballast result in a sinking rate of ~ 500 m d−1; hence, they reach the bathypelagic before dissolving. Our findings are consistent with a vertical profile of salinity‐normalized Sr concentration in the Iceland Basin, which shows a maximum at 1700 m. Profiles of salinity‐normalized Sr concentration in the subarctic Pacific reach maxima at ≤ 1500 m, suggesting that Acantharia might contribute to the bathypelagic particle flux there as well. We hypothesize that Acantharia at high latitudes use rapid, deep sedimentation of reproductive cysts during phytoplankton blooms so that juveniles can exploit the large quantity of organic matter that sinks rapidly to the deep sea following a bloom.

Sedimentation of acantharian cysts in the Iceland Basin: Strontium as a ballast for deep ocean particle flux, and implications for acantharian reproductive strategies

Acantharian cysts were discovered in sediment trap samples from spring 2007 at 2000 m in the Iceland Basin. Although these single-celled organisms contribute to particulate organic matter flux in the upper mesopelagic, their contribution to bathypelagic particle flux has previously been found negligible. Four time-series sediment traps were deployed and all collected acantharian cysts, which are reproductive structures. Across all traps, cysts contributed on average 3–22%, and 4–24% of particulate organic carbon and nitrogen (POC and PON) flux, respectively, during three separate collection intervals (the maximum contribution in any one trap was 48% for POC and 59% for PON). Strontium (Sr) flux during these 6 weeks reached 3 mg m−2 d−1. The acantharian celestite (SrSO4) skeleton clearly does not always dissolve in the mesopelagic as often thought, and their cysts can contribute significantly to particle flux at bathypelagic depths during specific flux events. Their large size (~ 1 mm) and mineral ballast result in a sinking rate of ~ 500 m d−1; hence, they reach the bathypelagic before dissolving. Our findings are consistent with a vertical profile of salinity-normalized Sr concentration in the Iceland Basin, which shows a maximum at 1700 m. Profiles of salinity-normalized Sr concentration in the subarctic Pacific reach maxima at ≤ 1500 m, suggesting that Acantharia might contribute to the bathypelagic particle flux there as well. We hypothesize that Acantharia at high latitudes use rapid, deep sedimentation of reproductive cysts during phytoplankton blooms so that juveniles can exploit the large quantity of organic matter that sinks rapidly to the deep sea following a bloom.

Cessation and partial reversal of deep water freshening in the northern North Atlantic: observation-based estimates and attribution

Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait OverflowWater salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland—Scotland Overflow Water (ISOW) and in the deep water column (σ0 > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced contraction of the subpolar gyre and corresponding northwestward advance of subtropical waters that followed the NAO decline in the mid-1990s and continued through the mid-2000s. Remarkably, the ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland—Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.

Intense mixing of the Faroe Bank Channel overflow

The continuous, swift flow of cold water across the sill of the Faroe Bank Channel, the deepest passage from the Nordic Seas to the North Atlantic Ocean, forms a bottom‐attached dense plume (overflow). The amount and distribution of entrainment and mixing that the overflow encounters during its descent influence the ventilation of the deep North Atlantic, however, remain poorly known due to lack of direct measurements. Using the first direct turbulence measurements, we describe the dynamic properties and mixing of the overflow plume as it descends toward the Iceland Basin. The vigorously turbulent plume is associated with intense mixing and enhanced turbulent dissipation near the bottom and at the plume‐ambient interface, but with a quiescent core. Our measurements show a pronounced transverse circulation consistent with rotating plume dynamics, a strong lateral variability in entrainment velocity, and a vertical structure composed of order 100 m thick stratified interface and comparably thick well‐mixed bottom boundary layer with significant transport and entrainment.

Distribution of calcifying and silicifying phytoplankton in relation to environmental and biogeochemical parameters during the late stages of the 2005 North East Atlantic Spring Bloom

Abstract. The late stage of the North East Atlantic (NEA) spring bloom was investigated during June 2005 along a transect section from 45 to 66° N between 15 and 20° W in order to characterize the contribution of siliceous and calcareous phytoplankton groups and describe their distribution in relation to environmental factors. We measured several biogeochemical parameters such as nutrients, surface trace metals, algal pigments, biogenic silica (BSi), particulate inorganic carbon (PIC) or calcium carbonate, particulate organic carbon, nitrogen and phosphorus (POC, PON and POP, respectively), as well as transparent exopolymer particles (TEP). Results were compared with other studies undertaken in this area since the JGOFS NABE program. Characteristics of the spring bloom generally agreed well with the accepted scenario for the development of the autotrophic community. The NEA seasonal diatom bloom was in the late stages when we sampled the area and diatoms were constrained to the northern part of our transect, over the Icelandic Basin (IB) and Icelandic Shelf (IS). Coccolithophores dominated the phytoplankton community, with a large distribution over the Rockall-Hatton Plateau (RHP) and IB. The Porcupine Abyssal Plain (PAP) region at the southern end of our transect was the region with the lowest biomass, as demonstrated by very low Chla concentrations and a community dominated by picophytoplankton. Early depletion of dissolved silicic acid (DSi) and increased stratification of the surface layer most likely triggered the end of the diatom bloom, leading to coccolithophore dominance. The chronic Si deficiency observed in the NEA could be linked to moderate Fe limitation, which increases the efficiency of the Si pump. TEP closely mirrored the distribution of both biogenic silica at depth and prymnesiophytes in the surface layer suggesting the sedimentation of the diatom bloom in the form of aggregates, but the relative contribution of diatoms and coccolithophores to carbon export in this area still needs to be resolved.

Surface and deep-water hydrography on Gardar Drift (Iceland Basin) during the last interglacial period

Changes in surface and deep-water hydrography were inferred from variations in stable isotopes and sortable silt mean grain size, respectively, on the southern Gardar Drift in the subpolar North Atlantic. The bathymetric δ 13C gradient during the penultimate glaciation was similar to the last glaciation with high- δ 13C Glacial North Atlantic Intermediate water above ∼ 2000 m, and low- δ 13C water derived from the Southern Ocean below. During Termination II, low- δ 13C water was present throughout the water column with minimum values at intermediate depths (∼ 1500–2000 m) and below 3000 m. This pattern continued well into the early part of the Last Interglacial (LIG) period. Sortable silt mean size at 3275 m suggests that deep-water circulation on Gardar Drift was relatively weak during the earliest part of the LIG (128 to 124.5 ka) when planktonic δ 18O was at a minimum, reflecting warming and/or reduced salinity. We suggest that low- δ 13C water and slow current speed on Gardar Drift during the early part of the LIG was related to increased melt water fluxes to the Nordic Seas during peak boreal summer insolation, which decreased the flux and/or density of overflow to the North Atlantic. The resumption of the typical interglacial pattern of strong, well-ventilated Iceland Scotland Overflow Water was delayed until ∼ 124 ka. These changes may have affected Atlantic Meridional Overturning Circulation.

Iron limitation of the postbloom phytoplankton communities in the Iceland Basin

Measurements performed on a cruise within the central Iceland Basin in the high‐latitude (>55°N) North Atlantic Ocean during late July to early September 2007 indicated that the concentration of dissolved iron (dFe) in surface waters was very low, with an average of 0.093 (<0.010–0.218, n = 43) nM, while nitrate concentrations ranged from 2 to 5 μM and in situ chlorophyll concentrations ranged from 0.2 to 0.4 mg m−3. In vitro iron addition experiments demonstrated increased photosynthetic efficiencies (Fv/Fm) and enhanced chlorophyll accumulation in treatments amended with iron when compared to controls. Enhanced net growth rates for a number of phytoplankton taxa including the coccolithophore Emiliania huxleyi were also observed following iron addition. These results provide strong evidence that iron limitation within the postspring bloom phytoplankton community contributes to the observed residual macronutrient pool during summer. Low atmospheric iron supply and suboptimal Fe:N ratios in winter overturned deep water are suggested to result in the formation of this seasonal high‐nutrient, low‐chlorophyll (HNLC) condition, representing an inefficiency of the biological (soft tissue) carbon pump in the region.

Evidence of Greenland Sea water in the Iceland Basin

This study evaluates the presence of intermediate water from the Greenland Sea in the Iceland Basin deduced from the observed excess of the tracer sulphur hexafluoride (SF6), released in the central Greenland Sea in 1996. The large tracer release experiment has served a unique opportunity to follow the spread of Greenland Sea intermediate water to the adjacent basins of the Nordic Seas and to the areas bordering this region. In the present study, using data from May–June 2001, the released tracer was detected at the sill in the Faroe Bank Channel and at several locations in the Iceland Basin of the North Atlantic, just downstream the sill and southeast of Iceland. The estimated excess of the released tracer at the Icelandic slope combined with reported values of the volume flow at this location suggest an annual transport rate of approximately 1.4 kg excess SF6. The results suggest an upper transit time from the central Greenland Sea to the area southeast of Iceland of approximately 4 years.

Upper ocean heat content in the Nordic seas

Seven years of temperature profiles are obtained from the Argo float project and are used to study the vertically integrated heat content for the Nordic seas which is broken up into the Norwegian Basin, Greenland Sea, Lofoten region, and Iceland Plateau. The World Ocean Atlas 2005 (WOA05) is used as the climatological baseline for temperature and hence heat content. Temporally and spatially averaged temperature profiles within the Norwegian and Iceland basins show good agreement between the Argo and WOA05 data sets. For the Greenland and Lofoten basins, the WOA05 data show cooler surface temperatures (by 1–2°C) but still lie within 1 standard deviation of each other over the 7‐year period. The Argo float data show circulation within each of the basins that is counterclockwise, and some of the floats eventually disperse toward the outer edges of the basins. Vertically integrated heat content from 0 to 1200 m shows similar characteristics as the WOA05 results including the timing and strength of the seasonal variability. The anomalous heat content for the Greenland and Norwegian sea basins is calculated in depth bins of 400 m. The Greenland Sea shows a warming trend over the 7 years of study, which corresponds to a temperature increase of (4.1 ± 0.3) × 10−2 °C/yr. The Norwegian Sea basin shows interannual variability in heat content for the surface layer (0–400 m) that may be related to the North Atlantic Oscillation (NAO). During an NAO+ phase, the Norwegian Basin shows a warm phase, and during an NAO− phase, the Norwegian Basin is in a cold phase; the correlation is small (r = 0.32) but different from zero correlation only at the 90% level, implying that the variability in the AHC may be due to (1) local atmospheric forcing and hence changes in heat loss, (2) atmospheric pressure differences that are not resolved by the NAO index, and (3) exchange of heat with the basin boundaries.

Warming and salinification of Labrador Sea Water and deep waters in the subpolar North Atlantic at 60°N in 1997–2006

A quantitative estimate of the temperature and salinity variations in the Labrador Sea Water (LSW), the Iceland-Scotland Overflow Water (ISOW), and the Denmark Strait Overflow Water (DSOW) is given on the basis of the analysis of repeated observations over a transatlantic section along 60°N in 1997, 2002, 2004, and 2006. The changes distinguished in the research evidence strong warming and salinification in the layers of the Labrador Sea Water and deep waters at the latitude of the section. The maximum increments of the temperature (+0.35°C) and salinity (+0.05 psu) were found in the Irminger Basin in the core of the deep LSW, whose convective renewal in the Labrador Sea stopped in the mid-1990s. The long-term freshening of the ISOW, which started in the mid-1960s, changed in the mid-1990s to a period of intense stable warming and salinification of this water. By 2005, the salinity in the core of the ISOW in the Iceland Basin increased to the values (∼34.99 psu) characteristic of the mid-1970s. In 2002, the warming “signal” of the ISOW reached the Irminger Basin. From 1997 to 2006, the warming and salinification of the columns of the Labrador Sea Water and deep waters became as high as 0.2°C and 0.03 psu, respectively. The character of the long-term variations in the thermohaline properties of the LSW and ISOW from the 1950s evidence that these variations were nearly in-phase and correlated with the low-frequency component of the North Atlantic Oscillation.

Vertical distribution and seasonal dynamics of mesozooplankton in the Iceland Basin

The abundance and seasonal vertical distribution of dominant zooplankters in the Iceland Basin was studied from data collected on four cruises between November 1996 and June 1997. Copepods were by far the most abundant group, making up ∼70% or more of the mesozooplankton. Two species constituted >90% of the copepod biomass: Calanus finmarchicus and Pareuchaeta norvegica. A seasonal migration pattern was evident in C. finmarchicus, P. norvegica, Oithona spp., and Metridia lucens: from December to February they inhabited the deeper layers, whereas from April to June they were most abundant in the upper layers. The vertical distribution of Pleuromamma robusta was rather similar year round. Oncaea spp. stayed deep during winter, but only a limited part of the population rose to upper waters during the summer. The reproduction of C. finmarchicus took place in May in the surface layers and it was linked to the phytoplankton spring bloom. In contrast, the reproduction of P. norvegica occurred at depth in February and was uncoupled to the spring bloom. Calanus hyperboreus is an expatriate in the Iceland Basin that does not reproduce there. The data on Oithona spp. and Oncaea spp. indicate that the former group reproduces between April and June in the upper layers, whereas the latter appears to reproduce year round at depth. As judged by the incidence of adults of both sexes the reproduction of M. lucens took place in spring and summer. The breeding of P. robusta was probably not confined to a particular season of the year.

Influence of margin segmentation upon the break-up of the Hatton Bank rifted margin, NE Atlantic

The Hatton Bank margin, flanking the Iceland Basin, is an example of a volcanic rifted margin and has been studied to examine the along margin tectono-magmatic variability. Integration of 5660 km of new seismic reflection profiles with > 60,000 km 2 of new multibeam bathymetry has allowed the margin to be divided into three segments, each of which are flanked by oceanic crust. The southernmost segment is characterised by a series of inner and outer seaward dipping reflector (SDR) packages, which are separated by an “Outer High” feature. The outer SDRs are truncated by Endymion Spur, a chain of steep sided, volcanic cones connected by narrow septa or necks. The central segment has no Inner SDR package and is characterised by the presence of a continental block, the Hatton Bank Block (HBB). The northern segment is adjacent to Lousy Bank, with a wider region of SDRs recognised than to the south, and characterised by many volcanic cones. The variations in the distribution of the SDRs along the margin, the presence of the HBB and Endymion Spur all suggest that the break-up process was not a uniform smooth process along-strike. Structural segmentation controlled the variations along the margin with break-up initiated in the south, producing the SDR packages. The HBB prompted the focus of break-up to relocate outboard of the block. The northern segment was closest to the Iceland “hot-spot”, and regular seafloor spreading did not become established until Chron 21. Shortly after break-up, the eruption of Endymion Spur occurred and may have been triggered by the passage of a pulse of hot asthenospheric material along the margin. The margin segmentation pattern we describe controlled the location of the enhanced volcanism along the Endymion Spur to the southern sector. In addition the segmentation has influenced the break-up style (presence or absence of SDR) and also the location and nature of post break-up volcanism.

Morphology of the Iceland Basin Excursion from a spherical harmonics analysis and an iterative Bayesian inversion procedure of sedimentary records

Based on 5 published marine high-resolution sedimentary records of the Iceland Basin Excursion [IBE; Channell, J.E.T., Hodell, D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and ∂ 18O at ODP Site 983/Gardar Drift, North Atlantic since 350 ka. Earth Planet. Sci. Lett. 153, 103–118; Laj, C., Kissel, C., Roberts, A., 2006. Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: a simple transitional field geometry? Geochem. Geophys. Geosystems. 7, Q03004, doi:10.1029/2005GC001122] dated around 186–190 kyr, we present models of the excursional geomagnetic field at the Earth's surface using two different approaches. First a spherical harmonics analysis is performed after synchronization of the records using their paleointensity profiles. Second, we have used an iterative Bayesian inversion procedure, calibrated using the single volcanic data available so far. Both modeling approaches suffer from imperfections of the paleomagnetic signals and mostly from the still poor geographical distribution of detailed records, presently available only from the North Atlantic and the West Pacific. For these reasons, our modeling results should only be regarded as preliminary models of the geomagnetic field during the IBE, susceptible to improvements when including results from future paleomagnetic studies. Nevertheless, both approaches show distinct similarities and are stable against moderate variations of modeling parameters. The general picture is that of a dipole field undergoing a strong reduction, but remaining higher than the non-dipole field all through the excursional process, except for a very short interval of time corresponding to the dipole minimum at the center of the excursion. On the other hand, some differences exist between the results of the two models with each other and with the real data when the virtual geomagnetic pole (VGP) paths are considered. The non-dipole field does not appear to undergo very significant changes during the excursion except for a slight increase just at the dipole minimum. The width of mid-height of the dipole minimum, which can be considered as an approximate measure of the duration of the IBE is of the order of 3 kyr according to the SPECMAP age model, consistent with a suggestion by Gubbins [Gubbins, D., 1999. The distinction between geomagnetic excursions and reversals. Geophys. J. Int. 137, F1–F3]. These results are compared to those obtained for the Matuyama-Brunhes geomagnetic reversal [Leonhardt, R., Fabian, K., 2007. Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: iterative Bayesian inversion and independent verification. Earth Planet. Sci. Lett. 253, 172–195].

Interannual variability of the Subpolar Mode Water properties over the Reykjanes Ridge during 1990–2006

Combining hydrographic data from the OVIDE (Observatoire de la Variabilité Interannuelle à Decennale/Observatory of the Interannual to Decadal Variability) section (Greenland‐Portugal) with Argo and historical CTD data over the period 1990–2006, we estimate the variability of the core properties of a variety of Subpolar Mode Water (SPMW) observed on the eastern flank of the Reykjanes Ridge. This SPMW acquires its core properties in the winter mixed layer along the eastern side of the Reykjanes Ridge. We find that the February sea surface temperature along the ridge is a proxy for its core temperature. The sources of this mode water are water masses advected by the mean cyclonic circulation in the Iceland Basin. A density compensated tendency for cooling and freshening of the SPMW core properties is observed in the early 1990s. It stops in 1996 and is followed by an increase in temperature and salinity (+1.41°C and +0.11 psu) and a decrease in density (−0.12 kg m−3) until at least 2003. During the entire period, the data do not show any significant modification in the depth of the mode water core while they suggest that the thickness of the layer shrank. The variability of the local air‐sea freshwater and heat fluxes cannot explain the observed salinity and temperature variations. They are most likely related to the modifications of the properties of the SPMW sources due to the recently evidenced changes, driven by the North Atlantic Oscillation, in the relative contributions of subtropical waters and subpolar waters in the Iceland Basin.