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The combined use of Lu-Hf and Sm-Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf-Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (εNd ~ − 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by εHf values (from − 1.1 to + 1.3) far more radiogenic than associated sediments (from − 7.1 to − 12.0) and turbidite sands (from − 27.2 to − 31.6). εHf values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between εHf of secondary clay minerals and chemical weathering intensity.These results combined with data from the literature have global implications for understanding the Hf-Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the 'seawater array' (i.e. the correlation defined by deep-sea Fe-Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the 'seawater array', which we refer to as the 'zircon-free sediment array' (εHf = 0.91 εNd + 3.10). Finally, we show that the Hf-Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (εHf = 1.80 εNd + 2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust.

Chloride and δ 18 O compositions of interstitial water extracted from a long sediment core retrieved from the NW coast of the Black Sea allowed us to constrain the main hydrologic changes of the Back Sea during the Late Pleistocene and Holocene. Prior to its reconnection with the Mediterranean Sea (through the Marmara Sea) at approximately 9000 calendar yr before present (9 ka cal BP), the Black Sea has evolved as a fresh to brackish water lake. At the time of reconnection, hydrologic changes were drastic. Bottom water salinities changed from a few psu (practical salinity unit) to 22 psu. Since solutes in the interstitial water column within sediments are advected and diffused the measured concentrations do not reflect those of past bottom waters. In order to reconstruct these former concentrations, we used an advection/diffusion model. Different scenarios of bottom water chloride and δ 18 O variations were accounted for in this model in order to simulate "present day" vertical profiles for concentrations of interstitial water in order to compare them to measured ones. The comparison suggests that the glacial Black Sea was a homogeneous freshwater lake (with a δ 18 O of − 10‰ and a salinity of 1 psu). Modern hydrologic conditions would only have been reached at 2 ka cal BP, concomitant with the onset of coccolith-rich thin layers that characterize modern basin sediments. Such delayed salinization (over 7000 yr) in the basin may have been due to higher precipitation during the early Holocene. We also simulated the impact of a catastrophic reconnection and a smoother reconnection. Both salinity scenarios lead to undistinguishable modelled "present day" profiles, indicating that the precise impact of the last reconnection was lost due to the advection/diffusion processes.