Introduction

Meltwater Pulse 1a

A meltwater pulse (MWP) is a short-lived, global acceleration in sea-level rise resulting from intense front- and/or subglacial meltwater release, and/or surging ice streams into oceans and iceberg discharge during ice sheets disintegration [9]. It has been calculated that the rate of global sea-level rise during meltwater pulses could have been as high as 60 mm a−1 during less than 500 years [29].

The existence of a pulsing mode for global sea-level rise after LGM, was discovered through the study of drowned late Quaternary reef-crest sequences cored in tropical areas (Barbados, [29]; Caribbean-Atlantic reef province, [10]; Tahiti, [3, 63]; Sunda Shelf, [32], Hawaii, [92]; among others). Although high-resolution radiocarbon and 230Th dating of coral skeletons permit precise dating of the events, diffuse neo-tectonic uplift of these areas often led to controversial correlating results and difficult calculation of the events’ magnitude.

Four main meltwater pulses have been pointed to have occurred during the last deglaciation phase: MWP-19 ka, also known as MWP-1a0, about 19 cal ka BP [15, 93]; MWP-1a, 14.650–14.310 cal a BP [24]; MWP-1b, 11.500–11.000 cal a B.P. [4]; and MWP-1c at about 8.000 cal a BP [11, 33]. Of these events, MWP-1a was possibly the most prominent leading to a global sea-level rise of about 20 m in the course of 340 a (5.9 cm a−1, [24]).

Evidences of the existence MWP-1a have been found in many low-latitude areas but straightforward evidence is still lacking in polar areas where the event is thought to have originated.

An on-going controversy concerns the identification of the trigger area and mechanisms responsible for MWP-1a ([1, 5, 6, 16–18, 20, 30, 31, 47, 65, 71, 81, 84, 89, 91], among others). Four ice sheets are considered as possible candidates responsible for shaping the global sea-level curve: the Laurentide, Fennoscandian, Barents and Antarctic ice sheets. Simulation of ocean-ice sheet interactions indicates that different meltwater sources should leave a geographically distinctive ‘sea-level fingerprint’ due to ice unloading histories and gravitational pull between the shrinking ice masses and oceanic water-masses variations [18, 22, 62].

Scarce and ambiguous evidences of MWP-1a imprints in the polar areas are puzzling and do not help resolve the controversy on its origin.

In this study, we present marine sedimentary evidence of MWP-1a recorded in the deep-sea sedimentary sequence of the Trough Mouth Fan (TMF) offshore the Storfjorden and Kveithola palaeo ice streams (Fig. 1). The deep-sea areas of polar continental margins, beyond the continental shelf brake controlled by ice grounding, are characterized by low-energy continuous sediment accumulation unaffected by glacial erosion, thus providing a valuable marine sedimentary record of ice sheet dynamics. While Lucchi et al. [58] gave a complete account of all the recovered sedimentary units in the studied area, we here focus on the interlaminated plumite unit and its significance as sedimentological signature of Meltwater Pulse 1a.

Fig. 1
figure 1

Location map of the studied area with indication of the main palaeo ice streams and sediment cores location. Gray hued bathymetry was acquired during the SVAIS and EGLACOM cruises that is superimposed on IBCAO data [41]. Adapted from Camerlenghi et al. [12]

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Geological and oceanographic characteristics of the study area

The Storfjorden and Kveithola TMF depositional systems are located south of the Spitsbergen Island in the NW Barents Sea [38, 51], (Fig. 1). Seismo-stratigraphic information in this area suggests that the onset of glacially influenced sedimentation occurred since about 1.8 Ma, when the Barents Sea Ice Sheet reached the continental shelf edge [39, 45]. Reconstruction of the late Pleistocene and Holocene depositional processes of the marine sedimentary record of the Storfjorden and Kveithola TMFs has been described in Lucchi et al. [58] and it is briefly summarized in the following (Fig. 2).

Fig. 2
figure 2

Sediment facies and related depositional processes on the Storfjorden and Kveithola TMFs after LGM. The figure reports a synthetic interpretative lithological log, the radiographs and photos of the identified sediment facies after [58]

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The older parts of the studied sediment cores contain mass transport deposits (MTD) emplaced during LGM, consisting of highly consolidated glacigenic diamicton (Fig. 2a) and normally consolidated debris flows derived from the down-slope transport of glacigenic sediments locally incorporating older stratigraphic intervals.

Above the LGM deposits, are proximal glacimarine sediments containing abundant Ice Rafted Debris (IRD) deposited between 20–19 and 17–16 cal ka BP (Fig. 2b). The former interval contains an oxidized layer at its base holding evidences of sea-ice free/seasonal conditions (abundant planktonic foraminifera).

The glacimarine sediments are overlain by a progressively fining-up sequence of interlaminated sediments consisting of laminated mud interbedded with sand/silt layers (Fig. 2c). The interlaminated sediments are several m-thick on the upper slope, and only a few cm-thick on the middle slope. Sedimentological and compositional characteristics suggest that deposition occurred under the effects of extensive subglacial meltwaters (plumites, [58]).

On the upper slope, the top boundary of the interlaminated sediments has a sharp contact with bioturbated and slightly laminated distal glacimarine sediments (Fig. 2d) indicating the presence of bottom currents and palaeo-environmental conditions favorable to bioactivity. On the contrary, a massive IRD layer and distal glacimarine sediments overlie the interlaminated facies on the middle slope. The Holocene sequence on the middle slope consists of diatom-bearing crudely laminated sediments (Fig. 2e), and foraminifera/nannofossils-rich, heavily bioturbated sediments both deposited under the effect of bottom currents (contourites, Fig. 2f) associated with the warm, moderately saline Western Spitsbergen Current (WSC) deriving from the North Atlantic Current. This current flows northward along the continental slope transporting heat to the Arctic area.

Materials and methods

Bathymetric, sub-bottom profiling data, and sediment cores are the result of the merging of two data sets collected during two cruises.

BIO Hespérides IPY Cruise SVAIS (Longyearbyen, July 29–August 17, 2007). Bathymetry was acquired with multi-beam echo-sounder Simrad EM1002S. Data were logged using Simrad’s Mermaid system and processed with Caris HIPS and SIPS V 6.1.

Sub-bottom profiling employed a hull-mounted Kongsberg TOPAS PS 18 parametric profiler using two primary frequencies ranging between 16 and 22 kHz. Cores were obtained with piston coring.

R/V OGS Explora IPY Cruise EGLACOM (Kristiansund, July 07–August 04, 2008). Bathymetry was acquired with multi-beam echo-sounder Reson MB8150 operating at a mean frequency of 12 kHz, and MB8111, operating at a mean frequency of 100 kHz. Data were logged and processed with the Reson PDS2000V 2.5.3.2 software. Sub-bottom profiling employed a hull-mounted Benthos CAP-6600 profiler, using a sweep of acoustic frequencies ranging between 2 and 7 kHz. Cores were obtained by gravity coring.

The final bathymetric digital terrain model was produced from the combined data set with 75 m grid spacing. Sub-bottom profiles from both surveys were imported in Seismic Micro-Technology’s Kingdom Suite 8.3 software. In addition, regional bathymetric imaging was obtained from the Olex Ocean DTM®.

The sediment cores (Table 1) were logged through an X-ray CT scan and an Avaatech XRF-core scan for chemical composition of the sediments. For this contribution we considered the down-core trends of Si (silica) content as proxy of the quartz content (terrigenous input) rather than amorphous silica forming diatoms frustules (assumption made after sediments microscope investigation); the Cl (chlorine) content was associated with the water content; the Sr (strontium) content was used as indicator of bio-productivity in place of barium that in the Arctic areas is often of detrital origin; and the S (sulfur) content, commonly associated to oxygen depleted environments.

Table 1 Core location
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Discrete samples collected at every 5–10 cm resolution were analyzed for water content, clay mineral analyses, total and organic carbon (Ctot, Corg) and total nitrogen (Ntot) content. For this study, we considered only the distribution of the smectite content indicated to be, in the studied area, a proxy of Atlantic Water strength [43], whereas the Corg/Ntot ratio was used to distinguish between marine and continental-derived organic matter according to Meyers [61]. Details on the analytical procedure applied for geochemistry and clay mineral analyses are indicated in Lucchi et al. [58].

Palaeomagnetic investigations were performed on u-channels collected from undisturbed sediments. The low-field magnetic susceptibility (k) and the natural remanent magnetization (NRM) were measured at 1 cm spacing. The NRM was then stepwise demagnetized by alternating field (AF) up to 100 mT. The characteristic remnant magnetization (ChRM) has been isolated for each measured interval and its direction determined by fitting a least-square line on stepwise AF data with principal component analysis, according to Kirschvink [44]. The maximum angular deviation was then computed for each determined ChRM direction. Since the cores were not azimuthally oriented, the ChRM declination trends obtained for each u-channel have been arbitrarily rotated to align their average value with the true north.

From individual characteristic remnant magnetization directions, we calculated the corresponding Virtual Geomagnetic Pole (VGP), under the assumption of a geocentric axial dipole field.

The VGP scatter (S) during an established time interval was estimated by the angular standard deviation of the VGP distribution during an established interval of time. In order to filter out large deviations that may not be representative of genuine PSV, we applied a cut-off angle to the computed VGPs following the iterative approach described by Vandamme [86]. Additional details on the palaeomagnetic analytical methodology applied are reported in [73].

The age model used for this study follows Sagnotti et al. [73], and Lucchi et al. [58], relied on rocks palaeomagnetic parameters and radiocarbon dating analyses (Table 2) calibrated with the calibration software program Calib 6.0 [82], using the marine09 calibration curve [70], and applying an average marine regional reservoir effect ΔR = 84 ± 23 (south of Svalbard). The mean values of the calibrated age range of ±1σ were then normalized to calendar year.

Table 2 Radiocarbon dates (after Lucchi et al. [58])
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Results

Outer shelf iceberg scours

Sets of large parallel furrows are observed on the northern flank of Spitsbergen Bank in water depths between 350 and 390 m (Fig. 3). The furrows are 10–15 m deep with respect to the surrounding seafloor and are bound by elongated berms about 500 m wide and about 10 m high (Fig. 3). Three or four parallel furrows compose each set of bottom scours. Outer shelf furrows bear two elongation directions: NE–SW that coincides with the main direction of the Storfjorden glacial trough axis, and E–W. Lower resolution Olex bathymetry suggests that NE–SW furrows extend further landwards in water as shallow as 200 m in the southwestern edge of the Storfjorden glacial trough marking the northwestern flank of Spitsbergenbanken (Figs. 1, 3). The largest set of furrows observed is 35 km long and 1 km wide (Fig. 3a).

Fig. 3
figure 3

Seabed morphology evidence of multi-keel mega-icebergs scours. a SVAIS-EGLACOM bathymetry superimposed to the Olex Ocean DTM® outlining the regional extent of the furrows beyond the SVAIS-EGLACOM bathymetric coverage. Large parallel furrows are outlined by thick black lines; b DETAILED bathymetry (SVAIS-EGLACOM data sets) of multi-keel mega-icebergs scours. c 3D view and topographic profile across the multi-keel mega-icebergs scours

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Similar furrows have been identified on many polar continental margins in water depth reaching 1000 m [25], such as on the Yermak Plateau north of Svalbard [27], offshore West Greenland [49], the Canadian margin [66], in Pine Island Bay, Antarctica [40], and on the Argentine continental margin [55]. They are produced by the seafloor scouring of mega-scale iceberg keels (megabergs, [27, 87]).

These sets of large parallel furrows are not associated to the sets of smaller (4–5 m deep and on average 300 m wide), V-shaped, linear, subdued ridges and groves, with a mean NE-SW orientation, conformable to the main Storfjorden Trough axis, traceable for up to 20 km inland (Fig. 3a, b). These sets of large furrows are rather interpreted as Mega Scale Glacial Lineations (MSGL) produced by streaming ice and, like in adjacent Kveithola Trough [69, 72], loose their morphological expression near the continental shelf edge due to the presence of moraine banks intensely scoured by icebergs.

The large parallel furrows on the northern flank of Spitsbergen Bank are instead interpreted being produced by large, multi-keel icebergs released during break-off of the Storfjorden sub-ice stream III fed by the Spitsbergenbanken ice cap. Such icebergs were concentrated on the southeaster part of the Storfjorden Trough because the source of continental ice for this sub-ice stream was closes the continental shelf break [64, 58]).

Sediment geochemistry and composition

The characteristics for the sediments facies observed in the upper and middle slope are very much consistent. For this reason, in the following we will show only the down-core sedimentological characteristics of cores SV-02 and SV-04 as the most representative for the upper and middle slope, respectively (Fig. 4).

Fig. 4
figure 4

Down-core distribution of sedimentological and compositional parameters measured in the studied cores. The depositional significance of the synthetic lithological log is explained in Fig. 2. Black bold numbers on the down-core logs refer to ages (cal ka BP) according to the age model of Sagnotti et al. [73] and Lucchi et al. [58]. Seismic Units A1, A2, B according to Pedrosa et al. [64]

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Core SV-02 is characterized by a high Corg/Ntot ratio always largely exceeding the boundary value of 10 (except for the interval above the oxidized layer) indicating predominant continental-derived organic matter input. The smectite content in the sediments located above the glacigenic diamicton increases gradually up-core, whereas it is virtually absent in the LGM sediments.

The Si content is generally high throughout the core but decrease in the upper 25 cm (distal glacimarine sediments). Lower values are observed also between the oxidized layer and the interval characterized by the presence of marine-derived organic matter (388–400 cm bsf). The down-core trend of Cl distribution is in agreement with the water content measured on individual samples with Cl peaks corresponding to the sandy layers in the interlaminated sediments. The boundary between LGM sediments and the proximal glacimarine sediments is characterized by a salient change in Cl content. This boundary is also characterized by abrupt changes in the other parameters including the S content with high values within LGM sediments that contain partially pyritized carbonate rock fragments with framboid pyrite.

Core SV-04 is characterized by predominant marine-derived organic matter input except for the proximal glacimarine sedimentation (massive IRD input) and the interlaminated sediment facies. The trend of smectite content is characterized by a progressive increase from the base of post-LGM deposition (260 cm bsf), to the beginning of the distal glacimarine sedimentation (ca 200 cm bsf) after which the values are maintained stables with only minor fluctuations (minimum value at 202 cm bsf can be related to local sampling of IRD). The trend of Si content presents three main steps: the glacial, proximal glacimarine and interlaminated sediments have the higher Si content, whereas minor values characterize the distal glacimarine sedimentation. In the upper part of the core, the Si content decreases within the crudely layered sediments and became almost stable after about 9 cal ka BP. The Sr content is very low in the proximal glacimarine and interlaminated sediments with only slightly higher values in the distal glacimarine sediments. On the contrary, the Sr content increases considerably in the upper part of the core within the heavily bioturbated sediments having a trend very similar to that of Ca content. The S content is generally high within the glacimarine and interlaminated facies as well as the older interval of the crudely layered sediments. A sharp decrease of S values occurs at around 9 cal ka BP after which the values increases again towards the top of the core. The S content in the middle slope cores is, however, one-order of magnitude lower than the one measured in the upper slope cores.

Sediment palaeomagnetic properties

The geomagnetic angular dispersion related to Palaeosecular Variations (PSV) should increase with latitude [21, 60, 23]. At fixed latitudes (i.e., at a core site), the sampled time interval is another factor affecting the measured Virtual Geomagnetic Pole (VGP) dispersion. In general, samples spanning a temporally large interval (e.g., >10 ka) that fully sample the geomagnetic PSV define a large scatter of the measured VGPs, whereas samples referring to temporally short events under-represent the geomagnetic PSV resulting in a limited VGP scatter (S).

Within the interlaminated sediments, the VGP scatter computed from characteristic remnant magnetization directions of SV-02 is very limited, with an S value of 9.3° (Fig. 5a), whereas in core SV-03 the S value is as low as 8.5° (Fig. 5b). The VGP scatter in the interlaminated sediments of core SV-05 is, instead, as large as 15.8° (Fig. 5c).

Fig. 5
figure 5

Distribution of the Virtual Geomagnetic Pole (VGP) within the interlaminated sediments (yellow area) of cores a SV-02, b SV-03, c SV-05, and within the whole sedimentary sequence of cores d SV-04 (last 18 cal ka BP), e EG-02 (last 16 cal ka BP), and f EG-03 (last 12 cal ka BP). S = VGP scatter

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In core SV-04, spanning about the last 18 cal ka BP (or up to 25 cal ka BP, considering the possibly reworked sediments at the bottom of the core), the VGP scatter is rather large, with an S value of 17° (Fig. 5d), that is comparable with the VGP scatter determined for core EG-02 (16.9°, Fig. 5e), spanning a slightly shorter time (last 16 cal ka BP). In core EG-03, that contains an expanded Holocene sequence containing the record of the Younger Dryas (YD) at its base, the VGP scatter is relatively small (12.1°, Fig. 5f) with respect to core SV-04 and EG-02, but still much higher than the values determined for the plumites of cores SV-02 and SV-03.

Discussion

The NW Barents Sea sedimentary record of MWP-1a

The over 4.5 meter thick interlaminated plumites derived from meltwater discharge from the Storfjorden-Kveithola glacial system, was deposited in about 130 years at a very high sedimentation rate of 3.4 cm a−1, at about 7 km from the present shelf break. This sedimentary signature has been correlated to the MWP-1a event by Lucchi et al. [58] based on sedimentological, geochemical and chronostratigraphic analysis.

Initial uncertainties on the stratigraphic assignment of such deposit were related to (a) the slightly older ages determined in core SV-03 (15,061 ± 146–14,929 ± 141 cal a BP), with respect to the establish timing for the meltwater event (14,650–14,310 cal a BP, [24]), and (b) the age range assigned to the plumites (about 130 a) that is comparable with the calibrated age error (±143 a).

The slightly older ages of the Storfjorden-Kveithola upper slope plumites can be explained with an underestimation of the local regional reservoir correction applied to the radiocarbon calibration of mixed benthic and planktonic foraminifera. Sarnthein [75], Rae et al. [67], and Thornalley et al. [84], indicated that deep ocean waters after the LGM and during deglaciation were up to 1–2 ka older than they are today determining a higher local reservoir age. In our case the aging effect of the benthic and planktonic foraminifera mix to explain the correlation to the MWP-1a is only 400–600 a.

The plumites recorded in the SVAIS-EGLACOM cores are constrained by an excellent palaeomagnetic and stratigraphic correlation with the sedimentary sequences described west of Svalbard and neighboring glacial depositional systems [42] where radiocarbon ages align with the timing of MWP-1a as defined in tropical areas.

Stratigraphic equivalent deposits have been reported from other areas offshore the West and North Svalbard margin including the Yermak Plateau [7, 13, 28, 42, 68, 72], and the southern Barents Sea [88], indicating a nearly synchronous regional event responsible for a massive sediment input likely accompanied by a huge flux of fresh meltwater into the northern Atlantic and Arctic Oceans.

The rapid emplacement of the interlaminated plumites is supported beyond the radiocarbon dating by sediments palaeomagnetic characteristics. According to various models of geomagnetic Palaeosecular Variation (e.g., [23, 60, 83]), the Virtual Geomagnetic Pole (VGP) scatter representing the full geomagnetic Palaeosecular Variation (PSV) spectrum of variability at the geographic latitudes of the sampled cores, should be between 15° and 20°. At fixed latitude (i.e., at a core site), the sampled time interval is another factor affecting the retrieved VGP dispersion. In general, samples spanning a temporally large interval (e.g., >10 ka) fully sample geomagnetic PSV and provide a reliable estimate of the VGP scatter, whereas samples referring to temporally short events under-represent geomagnetic PSV resulting in a limited VGP scatter. The very low VGP scatter measured in the interlaminated sediments of cores SV-02 and SV-03 indicates that this stratigraphic interval spans a time period that is not long enough to fully represent the overall spectrum of variability of geomagnetic PSV, notwithstanding a thickness of 3–4 m (Fig. 6). This inference is consistent with the very high sedimentation rate indicated by radiocarbon dating. The higher values measured in the plumite record of core SV-05, located at the head-scar of a landslide, were related to minor syn-sedimentary re-depositional events occurred during the plumites emplacement Sagnotti et al. [74].

Fig. 6
figure 6

Models of VGP scatter values (S) with respect to geographic latitude according to Vandamme [86], McElhinny and McFadden [59], Tauxe and Kent [83]. Yellow circles indicate the S values from the plumite interval of cores SV‐02, SV‐03 and SV‐05, whereas red circles indicate the S values for the sedimentary sequence of cores SV-04, EG-02, and EG-03

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The plumite layer is easily tracked on in the sub-bottom profiler record of the 300 km long Storfjorden-Kveithola margin. The thickness is estimated, using a 1500 m s−1 average sound velocity in such water-rich sediments, to be over 20 m-thick in the SE end of the Storfjorden TMF. The thickness decreases rapidly down-slope where it becomes negligible at about 35–40 km form the present shelf break in about 1800 m water depth. The NW end of the Storfjorden TMF is characterized by a thinner plumite sequence due to faster withdrawal of the ice stream in that area [58].

The extent of the deglaciation seismic Unit A was mapped on the Storfjorden-Kveithola TMFs and the de-compacted sedimentation rate averaged over 19.5 ka was calculated to be 0.6 kg m−2 a−1 [54]. If we scale these values to the MWP-1a depositional event, volumetrically forming about 90 % of seismic Unit A, and considering a duration of about 130 years, as measured in our cores, the sediment mass accumulation rate results as high as 78 kg m−2 a−1, corresponding to a total sediment mass accumulation of 1.1 × 1011 tonnes on the upper continental slope.

These values further stress the extreme nature of the MWP-1a sedimentary event recorded on the upper slope of Storfjorden-Kveithola TMFs.

Forcing mechanisms and environmental conditions during MWP-19 ka and MWP-1a

The presence of clustered glacigenic debrites forming the TMF of the North-western Barents Sea continental margin confirms fully glaciated conditions of the shelf area during the LGM [2, 50, 64]. Dating of these deposits indicate the grounded ice streams reached the shelf edges at ca. 24 cal ka BP ([26, 42], Fig. 7a).

Fig. 7
figure 7

Schematic graphic representation of the three main depositional events recording the Storfjorden and Kveithola ice stream dynamics in the SVAIS-EGLACOM sediment cores. a LGM, b inception of ice stream decay during MWP-19 ka; c ice stream collapse during/after MWP-1a

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There is a large consensus on considering enhanced summer insulation as the primary mechanism determining the onset of the northern hemisphere deglaciation. Intense ice-melting from the northern hemisphere ice sheets and mountain glaciers produced a large volume of meltwaters with deposition of a several m-thick laminated sequence on the northern continental margins (e.g., [36, 37, 52, 85]). This event produced a first abrupt global sea-level rise (MWP-19 ka) and a renewal of the Atlantic Meridional Overturning Circulation [15, 19, 34].

The oxidized and massive IRD layer overlying LGM sediments in the SVAIS-EGLACOM cores, records the Storfjorden ice stream response to MWP-19 ka responsible for breakdown of the outer part of the grounded ice shelf (Fig. 7b). Oxidized sediments, characterized by low sulfur content, presence of planktonic foraminifera, and marine-derived organic matter indicate deep ocean ventilation with ice-free (or seasonal) surface conditions. These environmental characteristics support the re-establishment of a vigorous thermohaline circulation in the north Atlantic after the LGM.

According to Llopart et al. [53], the rapid sea-level rise during MWP-19 ka was also responsible for local slope instability due to destabilized distribution of slope interstitial pore pressure along the Storfjorden-Kveithola TMFs, due to the rapid increase of sea-hydrostatic pressure (slump/debrites involving, or among, the oxidized layer in cores SV-05 and SV-04).

Detachment and lift-off of the marine-based grounded ice streams on the deep Antarctic shelves of the South-eastern Weddell Sea, Antarctic Peninsula and Amundsen Sea in response to sea-level forcing during MWP-19 ka, lead to the deposition of a thick laminated sequence underneath the Antarctic ice shelves [35, 90, 91]. Antarctic meltwater release was in turn responsible for (1) a further strengthening of the Atlantic meridional overturning circulation accelerating the climate warming of the North Atlantic region (onset of the Bølling-Allerød warm interval, [89]) and (2) promoted sea-level rise forcing lift-off of shallow marine grounded ice streams including the Storfjorden and Kveithola ice streams in the northern margins. Evidence of renewed North Atlantic warm inflows at the onset of the Bølling warm interval were reported in the south Svalbard [42, 68], west and northern Svalbard margins [13, 14, 46, 79, 80], the Franz Victoria Trough [48, 56] and the NW Yermak slope [7].

Ice shelves are more sensitive to warm sub-glacial inflows forcing their rapid melting and retreat [76]. Re-enhanced warm North Atlantic inflows were responsible for fast decay of the Storfjorden and Kveithola ice streams with extensive ice-melting and retreat to an inner grounding line, contributing significantly to global sea-level rise during MWP-1a (Fig. 7c).

Contemporaneous intense calving from the retreating ice stream in the Storfjorden Trough is inferred to have generated the release of multi-keel megabergs responsible for the mega-scours visible on the Storfjorden outer continental shelf in water depth as deep as 390 m below present-day sea level. The chronology of this event fits well with the modeled 15,000 ka peak flux of iceberg across the Storfjorden Trough [77].

Melting of the main Storfjorden ice streams (I and II) occurred with a rapid retreat to the shallower grounding zone wedges located on the middle shelf (thinner plumites on the slope facing lobe I and II, Fig. 1). Diffuse melting and rapid retreat in this area was likely responsible for a thinner ice shelf, producing thinner icebergs unable of deep scouring the seafloor. On the contrary, the Kveithola and the south-western Storfjorden ice stream III, were probably active longer as they were fed by a closer ice catchment area (Spitsbergenbanken ice cap), producing a larger sequence of plumites on the continental slope facing lobe III and the Kveithola Trough.

The high sulfur content measured in the Storfjorden interlaminated sediments indicates reduced sea bottom ventilation likely caused by strong water stratification under extensive fresh water release during MWP-1a. It is also possible that the presence of surface fresh water enhanced multi-year sea ice formation during the plumite deposition that would explain the little occurrence of IRD within the interlaminated sediments. Strong water stratification would contribute to reduce nutrient availability hampering the bio-productivity and to promote poor deep-water ventilation affecting the preservation of calcareous foraminifera tests in the sediments, already diluted by the strong detrital input (barren sediments).

In addition, surface water stratification would enhance the areal distribution and sediment dispersion of the meltwater plumes by reducing sediment flocculation in the fresh water and particle settling across the halocline surface (c.f. [36, 57]). This explains the wide spread occurrence of fine-grained laminated sediments having considerable thickness only on the upper slope of TMFs in the area close to the sediment source output.

The interlaminated plumite sequence is sharply topped by a massive IRD layer that records the collapse of the outer part of the Storfjorden and Kveithola ice shelves (ca 14.2 cal ka BP, [8, 72]). The abrupt disappearance of the main TMFs source of terrigenous input is responsible for a strong change in the sedimentation rate with starving TMFs receiving sediments only through icebergs, wind, and along-slope contour currents (sharp decrease in Si content at 14.3 cal ka BP, Fig. 4). The sediment composition changes rapidly with increasing content of smectite and a swift change in organic matter composition being of marine origin (Corg/Ntot ratio <10). We think that the abrupt “switch-on” of the thermohaline circulation described to occur after melt water pulses went together with the “shut-down” of the sediment discharge and starving of TMFs affecting the Svalbard margins [13, 42, 78], the northern Barents Sea (Franz Victoria Trough, [48]) and part of the Arctic Ocean (NW Yermak slope, [7]) suggesting a major sudden decay of the ice sheet in the NW Barents Sea.

Conclusions

Decay and retreat of the Storfjorden and Kveithola palaeo-ice streams after the LGM were reconstructed through sedimentological and geochemical analysis of deep-sea marine sediments and new multi-beam and sub-bottom profiler data analysis.

The sedimentary record on the upper continental slope contains a several m-thick sequence of plumites deposited under an extensive meltwater event associated to the palaeo-ice streams decay. Radiometric dating and rock palaeomagnetic characteristics indicate a very fast event (ca. 130 years) responsible for the settling of about 1.1 × 1011 tonnes of sediments on the upper slope of the Storfjorden-Kveithola TMFs with an extreme sedimentation rate of 3.4 cm a−1. New data constrains confirm an early interpretation that related the event to the Meltwater Pulse 1-a.

New sedimentological and geochemical insights indicate that strong seawater stratification during extensive meltwater release promoted a wide dispersion of fine-grained sediments over the continental margin. Consistent thicknesses of the plumites are recorded only on the upper slope of TMFs, close to the sediment source output when the Storfjorden and Kveithola ice streams were grounded at the continental shelf brake during LGM. This would explain the difficulties encountered to find a substantial “MWP-1a deposit” in different settings of the Arctic, as well as Antarctic, areas.

A major ice sheet collapse at the end, or during, the MWP-1a produced multi-keel megabergs scouring the outer Storfjorden continental shelf, followed by TMFs sediment starving that signed the reprise of the biogenic activity and strengthening of the Termohaline Circulation (THC). The whole sequence of THC “swich on”—extreme short-term accumulation of fine sediments with apparent THC “shut-down”—restart of the THC and reduction of sediment delivery to the slope, described offshore the Storfjorden-Kveithola glacial troughs, was observed in many other areas of the NW Barents Sea and NW Yermak slope, suggesting the whole margin underwent an almost synchronous depositional fate with a major sudden decay of the ice sheet after, or during, the MWP-1a event.