Grain size separation and sediment mixing in Arctic Ocean sediments: evidence from the strontium isotope systematic

The Rb-87/Sr-86 and Sr-87/Sr-86 ratios of Laptev Sea sediments, of Arctic O cean sediments and of suspended particulate matter (SPM) from Siberian rive rs (Lena and Khatanga) form 'pseudo-isochrons' due to grain-size separation processes which are referred to as 'Lena Mixing Envelope' (LME) and as 'Fl ood Basalt Envelope' (FBE). At the land-ocean transition the reduction of t he particle velocity causes a deposition of coarser grained material and th e contact with saline water enhances a precipitation of finer-grained mater ial. The coarse-grained material is enriched in Sr showing less radiogenic Sr-87/Sr-86 ratios whereas fine grained material is depleted in Sr relative to Rb showing more radiogenic Sr-87/Sr-86 ratios, The experimentally deter mined spread of the Rb-87/Sr-86 and Sr-87/Sr-86 ratios as a function of gra in size in one sediment sample is on the same order as the natural spread o f the Sr-87/Sr-86 ratios observed in all samples from the Arctic Ocean. Che mical Index of Alteration (CIA) for the Lena river SPM tend to confirm prev ious observations that chemical alteration is negligible in the Arctic envi ronment. Thus, these 'pseudo-isochrons' reflect an average age and the aver age isotope composition in the river drainage area. Calculated apparent age s from the FBE reflect the age of the Siberian flood basalt of about 220 Ma and the initial ratio of 0.707(1) reflects their mantle origin. The age ca lculated from the LME of about 125 Ma reflects accidentally the Jurassic an d Cretaceous age of the sediments drained by the Lena river and the initial ratio of 0.714(1) reflects the crustal origin of their source rocks. Compa rison of geographical locations reveals that all samples from the eastern L aptev Sea (east of 120 degrees E) fall along the LME whereas all samples fr om the western Laptev Sea (west of 120 degrees E) fall between LME and FBE. Mixing calculations based on Nd-143/Nd-144 measurements, not influenced by grain size, show that about 75% of the western Laptev Sea sediments origin ate from the Lena drainage area whereas about 25% of the sediments are deli vered from the Siberian flood basalt province. Sediments from the central A rctic Ocean are isotopically related to the Lena drainage area and the Sibe rian flood basalt province. However, sediments from the Arctic Ocean margin s close to Novaya Semlya, Greenland, the Fram Strait and Svalbard originate from sources not yet identified. (C) 1999 Elsevier Science B.V. All rights reserved.