Separating individual contributions of major Siberian rivers in the Transpolar Drift of the Arctic Ocean

Abstract

The Siberian rivers supply large amounts of freshwater and terrestrial derived material to the Arctic Ocean. Although riverine freshwater and constituents have been identified in the central Arctic Ocean, the individual contributions of the Siberian rivers to and their spatiotemporal distributions in the Transpolar Drift (TPD), the major wind-driven current in the Eurasian sector of the Arctic Ocean, are unknown. Determining the influence of individual Siberian rivers downstream the TPD, however, is critical to forecast responses in polar and sub-polar hydrography and biogeochemistry to the anticipated individual changes in river discharge and freshwater composition. Here, we identify the contributions from the largest Siberian river systems, the Lena and Yenisei/Ob, in the TPD using dissolved neodymium isotopes and rare earth element concentrations. We further demonstrate their vertical and lateral separation that is likely due to distinct temporal emplacements of Lena and Yenisei/Ob waters in the TPD as well as prior mixing of Yenisei/Ob water with ambient waters.

Figure 1

Map of the study region. Station locations along the GEOTRACES GN04 cruise transect in the central Arctic Ocean as turquoise dots. Black squares mark the locations of stations sampled for REEs^,,,,,. Green triangles mark the location of data for the Laptev Shelf (for the Lena plume in 2013 and 2014 and the Vilkitsky Strait). Dark red arrows mark the areas of river input, the boxes provide river water Nd concentrations, ε_Nd composition^, and average annual river discharge. Arrows show the schematic circulation of surface water. TPD: Transpolar Drift. The map was produced using Ocean Data View (version 5.1.7, https://odv.awi.de/) and modified manually.

Figure 2

Vertical distribution of Nd concentration and Nd isotope composition. Water column profiles of Nd concentrations (left) and ε_Nd (right) for all stations along the transect. Stations outside the TPD are shown in brownish colors, stations within the TPD in blueish colors. Error bars are external errors for Nd concentrations (1SD, error bars usually smaller than the symbols) and propagated errors for ε_Nd (2SD).

Figure 3

Distributions of REE concentrations, ε_Nd and meteoric fraction along the cruise track with salinity as contours. Transect from station 32 to 117 for the upper 200 m of dissolved (a) Nd concentrations, (b) ε_Nd, (c) PAAS-normalized HREE/LREE ratios and (d) the fraction of meteoric water in color and salinity (Rabe et al., 2016³³) as contours. Station numbers and basins are given on top of the transects, the transect is shown in the insert in panel (d). The figure was produced using Ocean Data View (version 5.1.7, https://odv.awi.de/) and modified manually.

Figure 4

Mixing plots of ε_Nd vs. salinity. Data from samples from 10 to 200 m water depth with [Nd] in color (upper panel) and the meteoric fraction in color (lower panel). Error bars show the propagated errors (2SD, ε_Nd). Grey lines represent mixing between the endmembers, grey squares show 10% mixing intervals, grey squares at the end of the mixing lines show the endmember Atlantic and Pacific. The black arrows mark the shift in ε_Nd from the very unradiogenic (Lena dominance) to more radiogenic (Yenisei/Ob dominance) values with water depths for selected stations. Grey triangles show samples from the Laptev Shelf close to Vilkitsky Strait. The ‘radiogenic band’ comprises the samples at st. 69 10–100 m water depth, st. 81 50–100 m water depth and st. 96, 101 and 125 at 100 m water depth. Lena and Yenisei/Ob endmembers have a salinity of zero and thus are not shown but arrows point in their direction. Endmember values can be found in Table S2. The figure was produced using Origin (version 2019, https://www.originlab.com/) and modified manually.