Evidence of long-term NAO influence on East-Central Europe winter precipitation from a guano-derived \u03b415N record

Daniel M Cleary,Jonathan G Wynn,Bogdan P Onac,Monica Ionita,Ferenc L Forray

doi:10.1038/s41598-017-14488-5

Daniel M Cleary, Jonathan G Wynn + Show 3 more

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Currently there is a scarcity of paleo-records related to the North Atlantic Oscillation (NAO), particularly in East-Central Europe (ECE). Here we report δ15N analysis of guano from a cave in NW Romania with the intent of reconstructing past variation in ECE hydroclimate and examine NAO impacts on winter precipitation. We argue that the δ15N values of guano indicate that the nitrogen cycle is hydrologically controlled and the δ15N values likely reflect winter precipitation related to nitrogen mineralization prior to the growing season. Drier conditions indicated by δ15N values at AD 1848–1852 and AD 1880–1930 correspond to the positive phase of the NAO. The increased frequency of negative phases of the NAO between AD 1940–1975 is contemporaneous with higher δ15N values (wetter conditions). A 4‰ decrease in δ15N values at the end of the 1970’s corresponds to a strong reduction in precipitation associated with a shift from negative to positive phase of the NAO. Using the relationship between NAO index and δ15N values in guano for the instrumental period, we reconstructed NAO-like phases back to AD 1650. Our results advocate that δ15N values of guano offer a proxy of the NAO conditions in the more distant past, helping assess its predictability.

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Global atmospheric circulation has a number of preferred patterns of variability, all of which have expressions in surface climate
The fractionations occurring during the metabolic processes within bats and insects as nitrogen is transferred from plant to guano remain fixed through time. Since these fractionations follow conservative pathways, variation in the nitrogen isotopic composition of guano can be related to changes that occur in the soil inorganic nitrogen reservoir from which plants access nitrate and ammonia[28,29]
A nitrogen isotopic proxy record of guano provides new information regarding the effect of DJF hydroclimate system on the nitrogen cycle (N-cycle) and the influence of the winter North Atlantic Oscillation (NAO) on the East-Central Europe (ECE)

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Global atmospheric circulation has a number of preferred patterns of variability, all of which have expressions in surface climate. We use coupled carbon and nitrogen isotope ratio data (δ13C and δ15N values) of a 286 cm guano core from MC in ECE, in an attempt to characterize the hydroclimate influence on the N-cycle since the latest part of the Little Ice Age (LIA; AD 1650 to 1850) and up to AD 2012. We first compare these data to the December-January-February (DJF) NAO index that represents the strength of the system during the winter months. Such circumstances offer additional information with respect to local cave environmental changes while guano was being deposited[16,17]

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