Juan Pablo Corella

Three millennia of heavy rainfalls in Western Mediterranean: frequency, seasonality and atmospheric drivers

Documenting subdecadal-scale heavy rainfall (HR) variability over several millennia can rarely be accomplished due to the paucity of high resolution, homogeneous and continuous proxy records. Here, using a unique, seasonally resolved lake record from southern Europe, we quantify temporal changes in extreme HR events for the last 2,800 years in this region and their correlation with negative phases of the Mediterranean Oscillation (MO). Notably, scarce HR dominated by a persistent positive MO mode characterizes the so-called Migration period (CE 370–670). Large hydroclimatic variability, particularly between CE 1012 and 1164, singles out the Medieval Climatic Anomaly, whereas more stationary HR conditions occurred between CE 1537 and 1805 coinciding with the Little Ice Age. This exceptional paleohydrological record highlights that the present-day trend towards strengthened hydrological deficit and less HR in the western Mediterranean is neither acute nor unusual in the context of Late Holocene hydrometeorological variability at centennial to decadal time scales.

Spatial heterogeneity of benthic methane dynamics in the subaquatic canyons of the Rhone River Delta (Lake Geneva)

Abstract Heterogeneous benthic methane (CH4) dynamics from river deltas with important organic matter accumulation have been recently reported in various aquatic and marine environments. The spatial heterogeneity of dissolved CH4 concentrations and associated production and diffusion rates were investigated in the Rhone River Delta of Lake Geneva (Switzerland/France) using sediment cores taken as part of the éLEMO Project. Benthic CH4 dynamics within the complex subaquatic canyon structure of the Rhone Delta were compared (1) between three canyons of different sedimentation regimes, (2) along a longitudinal transect of the ‘active’ canyon most influenced by the Rhone River, and (3) laterally across a canyon. Results indicated higher CH4 diffusion and production rates in the ‘active’ compared to the other canyons, explained by more allochthonous carbon deposition. Within the active canyon, the highest diffusion and production rates were found at intermediate sites further along the canyon. Stronger resuspension of sediments directly in front of the river inflow was likely the cause for the variable emission rates found there. Evidence also suggests more CH4 production occurs on the levees (shoulders) of canyons due to preferred sedimentation in those locations. Our results from the heterogeneous Rhone delta in Lake Geneva further support the concept that high sedimentary CH4 concentrations should be expected in depositional environments with high inputs of allochthonous organic carbon.

Historical shifts in oxygenation regime as recorded in the laminated sediments of lake Montcortès (Central Pyrenees) support hypoxia as a continental-scale phenomenon

Recent expansion of anoxia has become a global issue and there is potential for worsening under global warming. At the same time, obtaining proper long-term instrumental oxygen records is difficult, thus reducing the possibility of recording long-term changes in oxygen shifts that can be related with climate or human influence. Varved lake sediments provide the better time frame to study this phenomenon at high resolution. We tracked the oxic/anoxic shifts of the varved Lake Montcortès since 1500CE, and tried to recognise anthropogenic and climatic influences combining biological and geochemical proxies. Four main scenarios emerged: 1) years with abrupt sediment inputs (A); 2) years with outstanding mixing and oxygenation of the water column (B); 3) years with strong stratification, anoxia, intense sulfur bacterial activity and increased biomass production (C); 4) years with stratification and anoxia, but relatively less biomass production (D). In line with current limnologic trends, high supra-annual variability in the occurrence of oxygenation events was observed. Interestingly, at least 45.3% of the years were mixing years and, like the meromictic ones, were mostly clustered into groups of consecutive years, thus alternating years of monomixis with years of meromixis. Most years of D belong to the period 1500-1820CE, when human activities were the most intense. Most years of A belonged to the climatic unstable period of 1850-1899CE. Years of B were irregularly distributed but were best represented in the period 1820-1849CE. Most years of C belonged to the 20th century. More than 90% of the years with climatic instrumental records belonged to B and C. Current climate warming seems to be taking control over the oxygenation capacity of the lake, especially since the second half of the 20th century. Our results support recent findings related to hypoxia spreading at the global scale.

Trace metal enrichment during the Industrial Period recorded across an altitudinal transect in the Southern Central Pyrenees

Abstract The study of three lacustrine sedimentary archives along an altitudinal transect in the Southern Central Pyrenees - lakes Estanya, Basa and Marbore- has provided a unique record of changes in anthropogenic trace metal concentrations over the last six centuries in NE Iberian Peninsula. Although site-specific processes influence metals enrichments in each lacustrine system, significant enrichments of Hg and Pb and minor to moderate enrichments of Cu, Cd, and Zn with respect to baseline (Pre-industrial) concentrations highlight intensive release of anthropogenic trace metals with the advent of the Industrial Revolution leading to maximum values during the 20th century. The largest trace metal pollution occurred between 1840s and 1920s CE mainly derived from the increasing demand of ore resources in Southern Europe during the Industrialization. A second, less distinct pollution phase occurred between 1950s and 1990s, associated with the “Great Acceleration” and increased trace metal emissions related to road-transport, use of fertilizers in agriculture and the global boost of the Chemical Industry. Enrichment of mercury during the Industrial Period correlates well with Hg production in Spanish Almaden mines and global emission inventories. Local mining in the Pyrenees and regional smelting activities in Spain and Southern France may explain the enrichment of lead (and associated by-products cadmium and zinc) during the first pollution phase while the use of leaded gasoline since the mid-20th century drives the higher Pb enrichment factor found till the late 20th century. This investigation demonstrates that environmental regulations controlling emissions of hazardous metals during the last decades have greatly contributed to a significant reduction of these anthropogenic trace metals enrichments in natural ecosystems although they still double pre-industrial levels. This study also exemplifies the different sensitiveness of lacustrine systems to record past atmospheric pollution phases and highlights the need of multi-archive studies to conduct regional (rather than local) pollution reconstructions.