Spatial and temporal long-term variation of environmental isotopes composition of precipitation and climate change in Azores Archipelago (Portugal)

Azores is located in the middle North Atlantic Ocean, between the latitudes 37º-40ºN and longitudes 25º-31º W and are divided into three geographical groups (Western, Central and Eastern Group). Since February 2012 until January 2014, it was install rain water collectors in Terceira and São Miguel Islands to collect monthly meteoric water. GNIP data was incorporated for the long term study. The main goal of the study was to determine spatial and temporal isotopic variations of meteoric water and long-term changes of isotopic composition related with climate change. Determination of environmental isotopes can be a valuable tool to understand the interconnections among climate, hydrology and water resources at local scale for further studies.

Azores Local Meteoric Water line can be defined as δ²H = 6.84δ¹⁸O + 7.51 (R² = 0.85). The δ¹⁸O annually average value in Azores is in accordance with the world-wide distribution based on the GNIP data set.

Current monthly distribution patterns of δ¹⁸O and δH concentrations in meteoric water shows a close relation among surface air temperature and amount precipitation. The slope of the linear fit between δ¹⁸O concentration in precipitation and temperature is 0.27‰/ºC for South station (Terceira Island). Data show a depletion of isotopic composition of precipitation that coincides with the seasonal occurrence of the frequent crossing of low pressure systems associated with the polar front. The highest values of d-excess are related with storms and are possible to identify different values between North and South stations related with the amount of precipitation, orography and the relative humidity. Short sampling interval during the wet season shows a large variation in isotopic composition of precipitation.

Long term sampling reveals an increase of δ18O concentration values in precipitation that coincides with the temperature increase at the same period. The decreases of d-excess values suggest a drop in the amount of moisture in the atmosphere. Although, the annual average of precipitation shows a relative increase over the last decade δD monthly average concentration exhibits the largest variation during the year. The result can be related with climate change suggested by the IPCC scenarios.