Palynological characterization of gassy sediments in the inner part of Ría de Vigo (NW Spain). New chronological and environmental data

Palynological characterization of the pollen assemblages from core ZV-01 offers a reliable age record of shallow ‘gas seal’ and ‘gas reservoir’ sediments from the Ría de Vigo (NW Spain). Gassy sediments show a direct correspondence with the palynological assemblage zones (LPAZ) 2, 3 and 4, which range in age from 950 to 1510 a.d. for the interval 81–176 cm where gas is found. The facies acting as a seal would correspond to the sedimentation in the ria starting from 1510 a.d. These assemblages also provide significant data which make it possible to reconstruct the palaeoenvironmental changes recorded in the sedimentary infill of the Ría de Vigo. The gassy horizon corresponds to the phase when riparian and mesophilous forest were better represented. This coincides with a relative high abundance of Lingulodinium machaerophorum, which indicates that the waters of the ria had been less renewed by oceanic waters and had a plentiful supply of continental nutrients. At that time, anoxic conditions resulted from restricted vertical circulation of seawater and/or high biological productivity. The dinocyst assemblages overlying and underlying the gassy horizon in core ZV-01 are clearly different from those concomitant with the accumulation of the gassy sediments. This suggests that oceanographic conditions in the Ría de Vigo may have changed several times during the time interval covered by our record. The weaker relationship between L. machaerophorum/Spiniferites spp., and simultaneous significant increases in Impagidinium spp. and Bitectatodinium tepikiense may indicate a greater intrusion of colder, more oceanic waters into the Ría de Vigo, which may have provoked intensification in upwelling during two periods, ca. 700–850 a.d. and ca. 1500–1750 a.d.