Sediment recycling during the Holocene marine transgression in Ría de Vigo (NW Iberia): multiproxy evidence and environmental implications

Abstract

A new multiproxy study of a 155-cm sediment core located in Ría de Vigo (NW Iberia) recording the transgressive and highstand system tract of the Holocene sea-level rise is presented. New data described include grain size, geochemical analyses, high-resolution qualitative/quantitative analyses of pollen, non-pollen palynomorphs and dinoflagellate cysts content, the qualitative evaluation of diatoms at different key levels and the reconsideration of some seismic interpretations. Besides, 14 radiocarbon dates were obtained from shells and pollen extracts. This new evidence revealed that notable changes in the sedimentation rates and the relative abundance of the different types of palynomorphs occurred during the Holocene. The sediment core studied shows anomalous pollen successions, including unexpected Pinus pollen peaks, during the Early and Middle-Holocene. Differential dating revealed that those anomalous successions correspond to conspicuously aged sediment that has inverted pollen chronologies. Thus, these facies may constitute evidence of redeposition of ancient upland sediments, formed above the ancient coastline at different intervals comprised between the end of the Last Glacial Maximum and the early stages of the Lateglacial, which were rich in very resistant pollen types (Pinus). Dinoflagellate cyst records in combination with the non-reworked pollen evidence reveal a reliable climatic seesaw during the Holocene, includingwet stormy periods of prevailing mixed waters, with strong development of Lingulodinium and high accumulation rates in dinoflagellate cysts, which alternate with other sparsely stormy and drier phases, characterised by the coastal upwelling intensification, well-stratified waters, and increases in Bitectatodinium and Spiniferites spp. but low accumulation rates in dinoflagellate cysts. These climatic dynamics, including a millennial drier cool period that occurred just after the Thermal Optimum are consistent with previous evidence obtained in other limnetic systems in the Iberian Peninsula. Furthermore, there is a delay between the two phases with the highest marine contribution to sediment (ca. 11.0-8.2 and ca. 4.0–3.0 cal ka BP) and the subsequent warm stages when the relative sea level stabilises: the Thermal Optimum (8.0–6.0 ka BP) and the 2.5-0.5 ka BP interval, respectively. Changes in Pinus pollen concentrations and grain size recorded in the postglacial sediments suggest that some upland ancient (Lateglacial) pollen evidence might be remobilised, especially during stages of upwelling intensification, and then resedimented in the seabed. Further studies will be needed to determine if the significant differences existing between the Pinus pollen grain sizes respond to climatic variations affecting the region or have been the result of any replacement of pine species in the area. Nonetheless, Pinus pollen evidence almost disappeared with the first signs of anthropisation in the area, and only recovered after the modern repopulations. Our findings open up new possibilities to further our understanding of sediment reworking and recycling mechanisms through the integration of multiproxy studies encompassing pollen, seismic and sedimentary data.