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The large volcanic eruptions of AD 536 and 540 led to climate cooling and contributed to hardships of Late Antiquity societies throughout Eurasia, and triggered a major environmental event in the historical Roman Empire. Our set of stable carbon isotope records from subfossil tree rings demonstrates a strong negative excursion in AD 536 and 541-544. Modern data from these sites show that carbon isotope variations are driven by solar radiation. A model based on sixth century isotopes reconstruct an irradiance anomaly for AD 536 and 541-544 of nearly three standard deviations below the mean value based on modern data. This anomaly can be explained by a volcanic dust veil reducing solar radiation and thus primary production threatening food security over a multitude of years. We offer a hypothesis that persistently low irradiance contributed to remarkably simultaneous outbreaks of famine and Justinianic plague in the eastern Roman Empire with adverse effects on crop production and photosynthesis of the vitamin D in human skin and thus, collectively, human health. Our results provide a hitherto unstudied proxy for exploring the mechanisms of 'volcanic summers' to demonstrate the post-eruption deficiencies in sunlight and to explain the human consequences during such calamity years.

Various studies report substantial increases in intrinsic water-use efficiency (Wi), estimated using carbon isotopes in tree rings, suggesting trees are gaining increasingly more carbon per unit water lost due to increases in atmospheric CO2. Usually, reconstructions do not, however, correct for the effect of intrinsic developmental changes in Wi as trees grow larger. Here we show, by comparing Wi across varying tree sizes at one CO2 level, that ignoring such developmental effects can severely affect inferences of trees' Wi. Wi doubled or even tripled over a trees' lifespan in three broadleaf species due to changes in tree height and light availability alone, and there are also weak trends for Pine trees. Developmental trends in broadleaf species are as large as the trends previously assigned to CO2 and climate. Credible future tree ring isotope studies require explicit accounting for species-specific developmental effects before CO2 and climate effects are inferred. [ABSTRACT FROM AUTHOR]

Abstract: The carbon isotope composition (δ13C) of tooth enamel of woolly mammoth molars from Estonia, Latvia, Lithuania, Denmark and southern Poland was analysed and the data were complemented with previously published δ13C values from Sweden, Finland and northwestern Russia, Switzerland and Britain. The δ13C values display a NE- to SW-trending geographical pattern, with more depleted compositions in the northeast, suggesting differences in the diet consumed by mammoths in the northeastern parts of the study area relative to mammoths in the southwesterly regions. While the pattern is probably a reflection of a number of controlling environmental parameters, with possible additional contribution from physiological factors, statistically significant correlations of the δ13C values to δ18O data from the same specimens and to palaeotemperature estimates imply a strong influence of climate over the δ13C values. The δ13C data do not provide convincing evidence of any temporal changes in diet. [Copyright &y& Elsevier]