Lake Sihailongwan - A Natural Monitor Of Dust Deposition For NE-China. The Pb, Nd, and Sr Isotopic Record.

The dimictic Lake Sihailongwan (SHL, 42^o17'N, 126^o25'E) fills a Quaternary volcanic crater. The siliciclastic sediment fraction of Holocene SHL sediments accounts for c. 75 wt-% at 0.3 mm mean annual sediment accumulation and is largely represented by aeolian influx of clay to silt sized debris of remote provenance. The major mineral constituents of the silt are quartz and feldspar. Because of the low influx of solutes, the geochemical signatures of the lacustrine record closely correspond with those of the siliciclastic input. Carbonate, if present in the aeolian input, is not preserved in the local soils and in the Holocene SHL sediments.In the ⁸⁷Sr/⁸⁶Sr - ¹⁴³Nd/¹⁴⁴Nd diagram, local alkalibasalts and SHL-sediments define narrow fields that differ from each other. The sediment cluster falls in the variation field of loess and desert sands from North China. The Sr isotope composition of the clay fraction of loess is characterised by a more radiogenic composition than the silt, whereas the Nd isotope characteristics for clay and silt are similar for individual sampling sites. Leaching experiments of loess and sand samples from China demonstrate that the carbonate fraction of loess is characterised by lower ⁸⁷Sr/⁸⁶Sr values than the siliciclastic residue. The Sr isotope values of the SHL sediments may be therefore slightly shifted towards higher ⁸⁷Sr/⁸⁶Sr values by post depositional carbonate dissolution. Since we have independent evidence for the low concentration of alkalibasaltic debris and the clay/silt balance of the SHL sediments, the mixing trends between the end members (i) alkalibasalt [⁸⁷Sr/⁸⁶Sr= 0.7048, ¹⁴³Nd/¹⁴⁴Nd= 0.51266], (ii) dust-silt, and (iii) dustclay could be used to get a close approximation for the isotopic composition of the dust end members in the SHLsediments: (ii) [⁸⁷Sr/⁸⁶Sr= 0.710, ¹⁴³Nd/¹⁴⁴Nd= 0.51267], (iii) [⁸⁷Sr/⁸⁶Sr= 0.717, ¹⁴³Nd/¹⁴⁴Nd= 0.51268]. The ɛ _Nd values for the above end member compositions (-7.2, -7.0) show a close correspondence with the ɛ _Nd values of aeolian deposits between 42-43^oN, 250 to 1000 km west of the lake and are less negative than typical ɛ _Nd values of loess or sand samples from the southern Gobi, the Loess Plateau, the Takla Makan desert, and of loess west and north of Beijing. Local alkalibasalts define a mixing trend between ²⁰⁶Pb(²⁰⁸Pb)/ ²⁰⁴Pb= 17.6(38.3) and 18.0(39.0). The SHL sediments plot distinct off this trend with a more radiogenic composition [²⁰⁶Pb/²⁰⁴Pb= 18.6-18.7, ²⁰⁸Pb/²⁰⁴Pb= 38.8-38.9] close to a narrow field defined by the clay fractions of loess samples from North China. The Pb-concentrations of the silt (16.4 ppm) and clay sized dust influx (30.8 ppm) exceed that of the alkalibasalts (4 ppm). This makes the mixing trend between alkalibasalt Pb and remote Pb less sensitive for minor variations in the influx of alkalibasalt debris. The SHL sediments largely record variations in the Pb isotope signatures of the dust influx. Our data suggest that silt sized dust is characterized by a higher variability in its Pb isotope composition than the clay sized influx. Recent SHL sediments, which record the sedimentation history during the last 50 years, document an increase in the Pb-influx and show a shift to a less radiogenic Pb composition. The Pb isotope composition of soil samples from the vicinity of the lake falls between the trend defined by alkalibasalts and the field defined by the SHL sediments.