Dating recent lake sediment records yielding disturbed 210Pb profiles has been a problem of wide interest in palaeoclimatic and palaeoseismic studies over the last few centuries. When applied to an alpine lake sedimentary record, a high‐resolution sedimentological study reveals that the 210Pb profile is disturbed by the occurrence of single‐event deposits triggered by two different mechanisms: flood events deposits and gravity reworking. Removing disturbed layers from the 210Pb profile yields a logarithmic depth–activity relationship. Using a simple 210Pb decay model (CFCS) provides an assessment of mean accumulation rate of `continuous sedimentation', as opposed to `event‐linked sedimentation'. The correlation of the thickest four gravity‐reworked deposits with historically known earthquakes permits both validation and refinement of the age–depth relationship. This refinement highlights variations in accumulation rate consistent with post‐Little Ice Age climatic variations.

To cite this version:Fabien Arnaud, Michel Revel, Emmanuel Chapron, Marc Desmet, Nicolas Tribovillard. 7200 years of Rhône river flooding activity in Lake Le Bourget, France: a high-resolution sediment record of NW Alps hydrology.. Holocene, SAGE Publications, 2005, 15, pp.3, 420-428. 10 C dated core from Lake Le Bourget (Savoie, France), spanning the last 7000 years. The strong correlation (R > 0.85) of the MS with the silicateborne suite of elements (Si, Al, Fe, Mg, K) and anti-correlation with the carbonate content (R = -0.87) allows to use it as a proxy for the fluctuations of the abundance of river-borne clastic fraction versus authigenic carbonates in sediment. As the Rhône River is the only one bringing a significant amount of silicate minerals to the coring site, the MS downstream is interpreted as a proxy of the Rhône suspended load discharge in Lake Le Bourget. This is confirmed over the last 3000 years by the good match with the evolution of hydrographical activity of the Rhône river as it is known through geomorphological studies of well-dated archaeological sites (Bravard et al., 1992; Bravard, 1996). Over the last 7200 years, the record is consistent with the regional evolution of lake water-level fluctuations (Magny, 2004). Hence, while the intensity of the MS signal seems to be widely affected by the human impact on soil stability, the timing of the period of enhanced hydrological activity appears to be mostly climate related and should thus constitute a first step toward a high resolution (< 8 yrs) continuous history of hydrological conditions in NW Alps.

Abstract. Holocene eastern Mediterranean Sea sediments contain an organic-rich sapropel S1 layer that was formed in oxygen-depleted waters. The spatial distribution of this layer revealed that during S1 deposition, deep waters were anoxic below a depth of 1800 m. However, whether this boundary permanently existed from the early to the mid-Holocene has not been examined yet. To answer this question, a multiproxy approach was applied to a core retrieved close to the 1800 m boundary (at 1780 m). We measured the bulk sediment elemental composition, the stable isotopic composition of the planktonic foraminifer Globigerinoides ruber and the abundance of benthic foraminifera since the last deglaciation. The result indicates that authigenic U and Mo accumulation began around 13-12 cal ka BP, in concert with surface water freshening estimated from the G. ruber δ 18 O record. The onset of bottom and pore water oxygen depletion occurred prior to S1 deposition inferred from barium enrichment. In the middle of the S1 deposition period, reduced authigenic V, Fe and As contents and the Br / Cl ratio indicated short-term bottom-water re-oxygenation. A sharp Mn peak and maximal abundance for benthic foraminifera marked a total recovery for circulation at approximately 7 cal ka BP. Based on our results and existing data, we suggest that S1 formation within the upper 1780 m of the eastern Mediterranean Sea was preconditioned by reduced ventilation, resulting from excess freshwater inputs due to insolation changes under deglacial conditions that initiated between 15 and 12 cal ka BP within the upper 1780 m. Short-term re-oxygenation in the Levantine Basin is estimated to have affected bottom water at least as deep as 1780 m in response to cooling and/or the reduction of freshwater inputs. We tentatively propose that complete ventilation recovery at the S1 termination was depth-dependent, with earlier oxygenation within the upper 1780 m. Our results provide new constraints on vertical water column structure in the eastern Mediterranean Sea since the last deglaciation.

This paper presents a lake-level record established for the last millennium at Lake Saint-Point in the French Jura Mountains. A comparison of this lake-level record with a solar irradiance record supports the hypothesis of a solar forcing of variations in the hydrological cycle linked to climatic oscillations over the last millennium in west-central Europe, with higher lake levels during the solar minimums of Oort (around AD 1060), Wolf (around AD 1320), Spörer (around AD 1450), Maunder (around AD 1690), and Dalton (around AD 1820). Further comparisons of the Saint-Point record with the fluctuations of the Great Aletsch Glacier (Swiss Alps) and a record of Rhône River floods from Lake Bourget (French Alps) give evidence of possible imprints of proxy sensitivity on reconstructed paleohydrological records. In particular, the Great Aletsch record shows an increasing glacier mass from AD 1350 to 1850, suggesting a cumulative effect of the Little Ice Age cooling and/or a possible reflection of a millennial-scale general cooling until the mid-19th century in the Northern Hemisphere. In contrast, the Saint-Point and Bourget records show a general trend toward a decrease in lake levels and in flood magnitude anti-correlated with generally increasing solar irradiance.