Versªo online: www.lneg.pt / www.scielo.pt Comunicaıes Geolgicas (2011) 98, 5-106 ISSN: 0873-948X; e-ISSN: 1647-581X A multidisciplinary study of an organic-rich mudstone in the Middle Holocene on the northern coast of Portugal Estudo multidisciplinar e um argilito rico de matØria orgnica do HolocØnico MØdio na costa Norte de Portugal H. Ribeiro 1* , A. Bernal 2 , D. Flores 1,3 , J. Pissarra 2,4 , I. Abreu 1,2 , J.V. Romani 5 & F. Noronha 1,3 Recebido em 06/06/2011 / Aceite em 21/07/2011 Disponvel online em Dezembro de 2011. ' 2011 LNEG ! Laboratrio Nacional de Geologia e Energia IP Abstract: A multidisciplinary study was carried out in an organic-rich mudstone level from Castelo do Neiva (Viana do Castelo) beach usually covered by gravels and sands in the intertidal zone. Palynomorphs (pollen and foraminifera) and wood macrofossil remains were analyzed and a petrographic study of dispersed organic matter was performed. The formation was dated, by 14C, from 5880–60 BP, corresponding to middle Holocene. The palynomorphs analysis revealed the absent of foraminifera. Pollen results showed a clear dominance of Quercus associated with Corylus and Alnus, as well as the presence of hydrophytes species, such as Myriophyllum and Typha angustifolia, and the Lycophyte Isoetes. The macroremains analysis was coherent with pollen results, showing the presence of Quercus robur L. wood. The organic matter of this level is at an early evolutionary stage compatible with a peat stage, which deposition was taken in a forest swamp. The results showed the presence of a fresh water environment occupying the area of the nowadays beach, suggesting different climatic conditions and sea level relatively to present times. At that time sea level was lower and climate conditions would correspond to a period of colder winters and hotter summers than present. Keywords: Castelo do Neiva, organic-rich deposit, plant macroremains, pollen, petrography. Resumo: Foi estudada uma camada rica em matØria orgnica localizada na zona intertidal, habitualmente coberta por cascalhos e areia, na praia de Castelo do Neiva (Viana do Castelo). Efectuaram-se anÆlises de macrorestos de madeira e palinomorfos (plenes e foraminferos) e foi feito um estudo petrogrÆfico da matØria orgnica dispersa. A unidade deu dataıes 14C de 5880 – 60 BP, correspondendo ao HolocØnico mØdio. A anÆlise de palinomorfos revelou a inexistŒncia de foraminferos. A nvel polnico verificou-se predomnio de Quercus associado a Corylus e Alnus a presena de espØcies hidrfitas, como Myriophyllum e Typha angustifolia, e de Isoetes. Os macrrorestos identificados correspondem a madeira de Quercus robur L.. A nvel petrogrÆfico verificou-se que a matØria orgnica estÆ numa fase evolutiva inicial compatvel com uma turfa. Os resultados mostraram a existŒncia de um ambiente de Ægua doce onde hoje se localiza a praia, sugerindo diferentes condiıes climÆticas e um nvel do mar mais baixo do que na actualidade. Palavras-chave: Castelo do Neiva, matØria orgnica, macrorrestos de plantas, plenes, petrografia. 1 Centro de Geologia da UP, Rua do Campo Alegre, 687, 4169-007 Porto, telephone:+351220402775; 2 Departamento de Biologia da Faculdade de CiŒncias da UP, Rua do Campo Alegre, s/n, 4169-007 Porto 3 Departamento de GeociŒncias, Ambiente e Ordenamento do Territrio da Faculdade de CiŒncias da UP 4 BIOFIG ! Centro de Biodiversidade e Genmica Integrativa & Funcional 5 Instituto UniversitÆrio de Xeoloxia Isidro Parga Pondal, Universidade da Coruæa. Spain *Corresponding author /Autor correspondente: helena.ribeiro@fc.up.pt 1. Introduction Over the years intensive research has been performed using a multiproxy approach in order to understand how past climate changes affected sea level dynamics, ecosystems and to reconstruct existing palaeoenvironments. A multidisciplinary approach using, among others, geological records, botanical remains or pollen distribution patterns allows to have more precise indicators to reconstruct environment changes and mean high tide level in the course of the Holocene (Freund et al., 2004). The Holocene is an epoch of particular interest since climatic changes and human actions played a role in landscape modifications (Messerli et al., 2000; Roberts et al., 2011). During this time period, coastal zones were particularly vulnerable to sea level fluctuations which influenced the available land for plants (Garca-Amorena et al., 2007). Therefore cold climatic episodes caused loss of diversity in northern Europe but allowed a refuge for biota survival in central and southern parts of this continent (Cheddadi et al., 1997; Carrin et al., 2010). Due to its vast coast, Portugal occidental margin is an adequate local to the study of sea-level fluctuations and impacts during the Holocene. However, due to human development and land use intensification it is not always easy to find undisturbed Holocene environments. In the northern coast of Portugal, several Holocene remains of peat bog sediments, ancient lagoon bottoms (Tijucas), and soils of ancient forest (podsoils) have been studied aiming to characterize the past vegetation and climatic conditions and to relate them with sea-level fluctuations (Granja & de Groot, 1996; Granja, 1999; Gmez-Orellana et al., 2001; Garca-Amorena et al., 2007; Granja et al., 2010). The aim of this work was study one organic-rich mudstone found in the coastal region of Castelo do Neiva (Viana do Castelo), where nowadays the beach is present. Through geochronological, paleobotanical and petrographic studies it was characterized the existing past vegetation, contributing to the knowledge of mid-Holocene vegetation dynamics in Portuguese coastal areas. 2. Study area The studied organic-rich mudstone (Fig.1) from Castelo do Neiva (Viana do Castelo) beach (N 41” 37,472; W 8” 49,046 and N 41” 37,353; W 8” 48,936) at north of the Neiva River was observed in a low tide and a marine over-washing occurred, as it is located on Artigo original Original article