Particulate organic matter in rivers of Fukushima An unexpected carrier phase for radiocesiums

International audienceThe role of particulate organic matter in radiocesium transfers from soils to rivers was investigated in areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. Suspended and deposited sediments, filtered water, macro organic debris and dead leaves were sampled along the six most contaminated coastal river catchments of the Fukushima prefecture in the early autumns 2013 and 2014. Radiocesium concentrations of river samples and total organic carbon concentrations in suspended and deposited sediments were measured. Radiocesium concentrations of suspended and deposited sediments were significantly correlated to 137Cs inventories in soils and total organic carbon. The distributions of radiocesium between the organic and mineral phases of both types of sediment were assessed by using a modelling approach. The results suggest that, during the early autumn season, the organic fraction was the main phase that carried the radiocesiums in deposited sediments and in suspended sediments for suspended loads andlt; 25 mg·L− 1. For higher suspended loads like those occurring during typhoon periods, the mineral fraction was the main carrier phase. Thus, high apparent distribution coefficient values noted by various authors in Fukushima could be attributed to the high radiocesium contents of particulate organic matter. Since it is well known that organic compounds generally do not significantly adsorb radiocesium onto specific sites, several hypotheses are suggested 1) Radiocesiums may have been absorbed into organic components at the early stage of atmospheric radioactive deposits and/or later due to biomass recycling and 2) Those elements would be partly carried by glassy hot particles together with organic matter transported by rivers in Fukushima. Both hypotheses would lead to conserve the amount of radiocesiums associated with particles during their transfers from the contaminated areas to the marine environment. Finally, such organically bound radiocesium would lead to significant deliveries of bioavailable radiocesium for living organisms at Fukushima. © 2016 Elsevier B.V

A millennial summer temperature reconstruction for northeastern Canada using oxygen isotopes in subfossil trees

Climatic reconstructions for northeastern Canada are scarce such that this area is under-represented in global temperature reconstructions. To fill this lack of knowledge and identify the most important processes influencing climate variability, this study presents the first summer temperature reconstruction for eastern Canada based on a millennial oxygen isotopic series (δ¹⁸O) from tree rings. For this purpose, we selected 230 well-preserved subfossil stems from the bottom of a boreal lake and five living trees on the lakeshore. The sampling method permitted an annually resolved δ¹⁸O series with a replication of five trees per year. The June to August maximal temperature of the last millennium has been reconstructed using the statistical relation between Climatic Research Unit (CRU TS3.1) and δ¹⁸O data. The resulting millennial series is marked by the well-defined Medieval Climate Anomaly (MCA; AD 1000–1250), the Little Ice Age (AD 1450–1880) and the modern period (AD 1950–2010), and an overall average cooling trend of −0.6 °C millennium⁻¹. These climatic periods and climatic low-frequency trends are in agreement with the only reconstruction available for northeastern Canada and others from nearby regions (Arctic, Baffin Bay) as well as some remote regions like the Canadian Rockies or Fennoscandia. Our temperature reconstruction indicates that the Medieval Climate Anomaly was characterized by a temperature range similar to the one of the modern period in the study region. However, the temperature increase during the last 3 decades is one of the fastest warming observed over the last millennium (+1.9 °C between 1970–2000). An additional key finding of this research is that the coldest episodes mainly coincide with low solar activities and the extremely cold period of the early 19th century has occurred when a solar minimum was in phase with successive intense volcanic eruptions. Our study provides a new perspective unraveling key mechanisms that controlled the past climate shifts in northeastern Canada

Bayesian multiproxy temperature reconstruction with black spruce ring widths and stable isotopes from the northern Quebec taiga

International audienceNortheastern North America has very few millennium-long, high-resolution climate proxy records. However, very recently, a new tree-ring dataset suitable for temperature reconstructions over the last millennium was developed in the northern Quebec taiga. This dataset is composed of one δ18O and six ring width chronologies. Until now, these chronologies have only been used in independent temperature reconstructions (from δ18O or ring width) showing some differences. Here, we added to the dataset a δ13C chronology and developed a significantly improved millennium-long multiproxy reconstruction (997–2006 CE) accounting for uncertainties with a Bayesian approach that evaluates the likelihood of each proxy model. We also undertook a methodological sensitivity analysis to assess the different responses of each proxy to abrupt forcings such as strong volcanic eruptions. Ring width showed a larger response to single eruptions and a larger cumulative impact of multiple eruptions during active volcanic periods, δ18O showed intermediate responses, and δ13C was mostly insensitive to volcanic eruptions. We conclude that all reconstructions based on a single proxy can be misleading because of the possible reduced or amplified responses to specific forcing agents

Modelling tree ring cellulose δ\delta18^{18}O variations in two temperature-sensitive tree species from North and South America

International audienceOxygen isotopes in tree rings ($\delta$$^{18}$O$_{TR}$) are widely used to reconstruct past climates. However, the complexity of climatic and biological processes controlling isotopic fractionation is not yet fully understood. Here, we use the MAIDENiso model to decipher the variability in $\delta$$^{18}$O$_{TR}$ of two temperature-sensitive species of relevant palaeoclimato-logical interest ($Picea\ mariana$ and $Nothofagus\ pumilio$) and growing at cold high latitudes in North and South America. In this first modelling study on $\delta$$^{18}$O$_{TR}$ values in both northeastern Canada (53.86 • N) and western Argentina (41.10 • S), we specifically aim at (1) evaluating the predictive skill of MAIDENiso to simulate $\delta$$^{18}$O$_{TR}$ values, (2) identifying the physical processes controlling $\delta$$^{18}$O$_{TR}$ by mechanistic modelling and (3) defining the origin of the temperature signal recorded in the two species. Although the linear regression models used here to predict daily $\delta$$^{18}$O of precipitation ($\delta$$^{18}$O$_P$) may need to be improved in the future, the resulting daily $\delta$$^{18}$O$_P$ values adequately reproduce observed (from weather stations) and simulated (by global circulation model) $\delta$$^{18}$O$_P$ series. The $\delta$$^{18}$O$_{TR}$ values of the two species are correctly simulated using the $\delta$$^{18}$O$_P$ estimation as MAIDENiso input, although some offset in mean $\delta$$^{18}$O$_{TR}$ levels is observed for the South American site. For both species, the variability in $\delta$$^{18}$O$_{TR}$ series is primarily linked to the effect of temperature on isotopic enrichment of the leaf water. We show that MAIDENiso is a powerful tool for investigating isotopic fractionation processes but that the lack of a denser isotope-enabled monitoring network recording oxygen fractionation in the soil–vegetation–atmosphere compartments limits our capacity to decipher the processes at play. This study proves that the eco-physiological modelling of $\delta$$^{18}$O$_{TR}$ values is necessary to interpret the recorded climate signal more reliably