Sea ice trajectories

To determine sea ice drift trajectories we developed a Lagrangian approach (ICETrack) that traces sea ice backward or forward in time using a combination of satellite-derived low resolution drift products. So far, ICETrack has been used in a number of publications to examine sea ice sources, pathways, thickness changes and atmospheric processes acting on the ice cover4,5,19,20,47.

Sea ice motion information is provided by different institutions, obtained from different sensors, and for different time intervals28,15,28,29,30. In this study we apply a combination of three different products: (i) motion estimates based on a combination of scatterometer and radiometer data provided by the Center for Satellite Exploitation and Research (CERSAT)32, (ii) the OSI-405-c motion product from the Ocean and Sea Ice Satellite Application Facility (OSISAF)33, and (iii) Polar Pathfinder Daily Motion Vectors from the National Snow and Ice Data Center (NSIDC)48.

The tracking approach works as follows: An ice parcel is traced backward or forward in time on a daily basis. Tracking is stopped if a) ice hits the coastline or fast ice edge, or b) ice concentration at a specific location drops below 20% and we assume the ice to be melted (forward tracking) or formed (backward tracking). A weighted approach is used to determine the motion product that is applied for the tracking: The algorithm first checks for the availability of CERSAT motion data within a predefined search range. CERSAT provides the most consistent time series of motion vectors starting from 1991 to present and has shown good performance on the Siberian shelves17,31. During summer months (June–August) when drift estimates from CERSAT are missing, motion information is bridged with OSISAF (2012 to present). Prior 2012, or if no valid OSISAF motion vector is available within the search range, NSIDC data is applied. For a detailed description of the differences between the CERSAT, OSISAF and NSIDC products we refer to28.

Experimental setup for backward tracking of sea ice leaving Fram Strait

In order to determine drift patterns and source areas of sea ice that exits the Arctic between Greenland and Svalbard, backward trajectories were calculated starting from six positions located in northern Fram Strait (81.3°N, 10°W–15°E, shown in Fig. 1b, dark grey circles). Trajectories are computed every two weeks between 1998 and 2017 (January–December) with a maximum duration of seven years. Backward trajectories of all sea ice exported through Fram Strait during the 1998–2017 period are shown in Fig. S2a).

Experimental setup for forward-tracking of sea ice formed on the Siberian shelf

The survival rate of sea ice formed on the Siberian shelf sea is investigated by tracking sea ice in a forward direction starting from the 32 points shown in Fig. 1b). The 32 points are located along the Siberian coast offshore the fast ice edge where waters are between 25–30 m deep. Sea ice was tracked at two-week intervals from October–April between 1992 and 2017 until next freeze-up. Forward trajectories of all sea ice formed on Siberian Shelf seas during the 1992–2017 period are shown in Fig. S2b).

Water depth, sea ice concentration and thickness along trajectories

Water depth along trajectories is extracted from the International Bathymetry Chart of the Arctic Ocean (IBCAO)49. The applied sea ice concentration product is provided by CERSAT and is based on 85 GHz SSM/I brightness temperatures, using the ARTIST Sea Ice (ASI) algorithm. The product is available on a 12.5 km × 12.5 km grid50. Sea ice thickness information of the winter month between October and April (Fig. 3c) is based on the sea ice thickness climate data record from the ESA Climate Change Initiative (CCI). The gridded thickness fields are derived from freeboard trajectory data from the radar altimeters onboard the Envisat and CryoSat-2 satellite platforms in the period from 2002 to 2017. The climate data record provides the mean sea ice thicknesses of the ice-covered area in each grid cell with a spatial resolution of 25 km and a duration of 1 month. The spatial coverage of Envisat sea ice thickness observation is limited to latitudes below 81.45°N due to the orbit parameters of the satellite, while the orbit of CryoSat-2 allows observations up to a latitude of 88°N. The data sets and algorithm documentation are available at the CCI Data Portal51,52.

Thermodynamic ice growth along trajectories

Along trajectories, a simple one-dimensional thermodynamic model calculates sea ice growth and melt. The model was developed by53 and has been used by54 to reconstruct the surface hydrography of the Arctic Ocean interior. Here we use the model output to number accumulated melt rates and identify potential sites where intensified sedimentation on Siberian Shelf Seas and in the central Arctic Ocean takes place (Fig. 3d,e). The model computes thickness changes at daily increments based on surface air temperature, ocean heat flux and snow cover. Surface air temperature, together with surface level pressure and 10 m wind, are extracted along the ice trajectories from NCEP re-analysis55. Snow depth is computed from the Warren climatology56, while the ocean heat flux is assumed to be constant at 2 W/m². The model was validated against ice-mass-balance buoys57 providing an accuracy of a few centimetres during the growth phase. While such one-dimensional models perform very well on simple ice growth and melt of typical Arctic sea ice, they are not suited to reproduce strong regional melting features as observed in Fram Strait58. Hence, we limit model application to the forward tracking of sea ice starting from the Siberian shelf seas (Fig. S2b).

Sea ice area flux

Ice area flux estimates provided in Fig. 1c are calculated using CERSAT motion estimates together with CERSAT ice concentration information. Fluxes are estimated along a zonal gate positioned at 82.5°N between 60°E and 180°E for the period 1992–2017 (October–April). The ice area flux at the gate is the integral of the product between the meridional (V) and zonal (U) ice drift and ice concentration. A positive (negative) sign refers to northward (southward) transport of sea ice. Transport (flux) rates are given in km².

Electromagnetic (EM) sea ice thickness measurements

Electromagnetic (EM) ice thickness observations were carried out during summer in northern Fram Strait and the southern part of the Nansen Basin within the framework of the AWI IceBird program. Measurements (shown in Fig. 1e) were obtained in the months of July and August of 2001, 2004, 2010–2012, and 2016–2018 during two cruises of the German ice-breaker RV Polarstern and six airborne campaigns with the German DC3-T research aircraft Polar-5 and Polar-6. EM ice thickness measurements utilize the contrast of electrical conductivity between sea water and sea ice to determine the distance of the instrument to the ice-water interface59. In 2001 only ground-based EM (GEM) data were obtained using an instrument pulled on a sledge across the ice60. In 2004, measurements were made with an airborne EM (AEM) system towed by a helicopter 20 m above the ice surface. Surveys performed after 2010 were conducted with the research aircraft Polar-5 or Polar-6 operating from the Danish Station Nord in Nord-East Greenland20. The accuracy of the EM measurements is in the order of ±0.1 m over level sea ice61. The AEM thickness data enables us to determine the general thermodynamic and dynamic boundary conditions of ice formation62,63. The most frequently occurring ice thickness, the mode of the distribution, represents level ice thickness and is the result of winter accretion and summer ablation. We assume the bias that arises from the unknown snow thickness to be negligible, since temperatures above freezing had certainly led to a significantly reduced snow cover or no snow cover at all56. For details about data processing and handling we refer to20,59.

Sedimentary budget

Quantification of sediment uptake and transport by sea ice is challenging due to the poorly understood entrainment processes and patchy distribution of matter12. A few studies exist that provide values for sediment content in sea ice obtained during expeditions carried out in early to mid 1990s64,65. Based on data from various authors66 postulates that 10–50% of the total Arctic sea ice is covered by dirty sea ice, and that the average sediment content in Arctic sea ice is ~20–30 mg l−1. In the central Arctic and Fram Strait sediment concentrations of up to 40% were found, which emphasizes the significance of sea ice as a long-distance transport agent67.

Following3, the annual sea ice sediment uptake in the Kara Sea and East Siberian Sea is around 2.4 × 106 t y−1 and 1.5 × 106 t y−1, respectively (see Fig. S4). The Laptev Sea was found to be the most important source region for sediment-laden ice (10 × 106 t y−1), but a more recent study of 8 could show that values are much larger and roughly 20–30 × 106 t y−1 may be exported from the Laptev Sea towards the central Arctic Ocean. Estimates are based on particle concentrations in ice samples from near-coastal sites and quantifications of new ice formation rates in winter. The authors assume that most sediment-laden ice produced in leads and polynyas reach areas north of the mean multiyear ice edge, and thus will not melt during summer.

Here we use our estimates of FYI survival rate from the forward tracking approach (Tables S1 and S2) together with estimates of sediment uptake by3,8 to quantify the amount of sediments released in the marginal shelf seas and central Arctic Ocean before and after 2004 (Fig. S4). Assuming a constant annual flux of matter from the marginal seas, we find that the total amount of sediments released from FYI increased by 24% between 1998–2003 and 2004–2017. This is equivalent to 4.8 × 106 t y−1 of mainly fine-grained material with high silt content12.