Estuaries are sensitive to changes in global to regional sea level, to climate-driven variation in rainfall and to fluvial discharge. In this study, we use source and environmentally sensitive proxies together with radiocarbon dating to examine a 7-m-thick sedimentary record from the Sado estuary accumulated throughout the last 3.6 kyr. The lithofacies, geochemistry and diatom assemblages in the sediments accumulated between 3570 and 3240 cal. BP indicate a mixture between terrestrial and marine sources. The relative contribution of each source varied through time as sedimentation progressed in a low intertidal to high subtidal and low-energy accreting tidal flat. The sedimentation proceeded under a general pattern of drier and higher aridity conditions, punctuated by century-long changes of the rainfall regime that mirror an increase in storminess that affected SW Portugal and Europe. The sediment sequence contains evidence of two periods characterized by downstream displacement of the estuarine/freshwater transitional boundary, dated to 3570–3400 cal. BP and 3300–3240 cal. BP. These are intercalated by one episode where marine influence shifted upstream. All sedimentation episodes developed under high terrestrial sediment delivery to this transitional region, leading to exceptionally high sedimentation rates, independently of the relative expression of terrestrial/marine influences in sediment facies. Our data show that these disturbances are mainly climate-driven and related to variations in rainfall and only secondarily with regional sea-level oscillations. From 3240 cal. BP onwards, an abrupt change in sediment facies is noted, in which the silting estuarine bottom reaches mean sea level and continued accreting until present under prevailing freshwater conditions, the tidal flat changing to an alluvial plain. The environmental modification is accompanied by a pronounced change in sedimentation rate that decreased by two orders of magnitude, reflecting the loss of accommodation space rather than the influence of climate or regional sea-level drivers.
AndersonNJVosP (1992) Learning from the past: Diatoms as palaeoecological indicators of changes in marine environments. Netherlands Journal of Aquatic Ecology26(1): 19–30.
2.
AntunesMPaisJGonçalvesF (1991) Carta Geológica de Portugal 1:50000 Folha 39-D Torrão. Lisboa: Serviços Geológicos de Portugal.
3.
AzevêdoICBordaloAADuartePM (2010) Influence of river discharge patterns on the hydrodynamics and potential contaminant dispersion in the Douro estuary (Portugal). Water Research44: 3133–3146.
4.
BaptistaJMartinhoFDolbethMet al. (2010) Effects of freshwater flow on the fish assemblage of the Mondego estuary (Portugal): Comparison between drought and non-drought years. Marine and Freshwater Research61: 490–501.
5.
BettencourtAGomesVDiasAet al. (2003) Estuários Portugueses. Lisboa: Direcção dos Serviços de Planeamento, Instituto da Água, Ministério das Cidades, Ordenamento do Território e Ambiente.
6.
BlaauwM (2010) Methods and code for ‘classical’ age-modelling of radiocarbon sequences. Quaternary Geochronology5(5): 512–518.
7.
BritoP (2009) Impactos da elevação do nível médio do mar em ambientes costeiros: o caso do estuário do Sado. PhD Thesis, Departamento de Geologia da Faculdade de Ciências, Universidade de Lisboa, Lisboa.
8.
BrownAG (1997) Alluvial Geoarchaeology: Floodplain Archaeology and Environmental Change (Cambridge Manuals in Archaeology). London: Cambridge University Press.
9.
CarriónJSFernándezSGonzález-SampérizPet al. (2010) Expected trends and surprises in the Lateglacial and Holocene vegetation history of the Iberian Peninsula and Balearic Islands. Review of Palaeobotany and Palynology162: 458–475.
10.
ChabaudLSánchez GoñiMFDespratSet al. (2014) Land–sea climatic variability in the eastern North Atlantic subtropical region over the last 14,200 years: Atmospheric and oceanic processes at different timescales. The Holocene24(7): 787–797.
11.
ChurchJAClarkPUCazenaveAet al. (2013) Sea level change. In: StockerTFQinDPlattnerG-Ket al. (eds) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge and New York: Cambridge University Press, pp. 1137–1216.
12.
ColmanSMBaucomPCBrattonJFet al. (2002) Radiocarbon dating, chronologic framework, and changes in accumulation rates of Holocene estuarine sediments from Chesapeake Bay. Quaternary Research57: 58–70.
13.
CooperSR (1999) Estuarine palaeoenvironmental reconstructions using diatoms. In: StoermerEFSmolJP (eds) The Diatoms: Applications for the Environmental and Earth Sciences. Cambridge: Cambridge University Press, pp. 352–373.
14.
CostaACSoaresA (2009) Trends in extreme precipitation indices derived from a daily rainfall database for the South of Portugal. International Journal of Climatology29: 1956–1975.
15.
CostaMJVasconcelosRCostaJLet al. (2007) River flow influence on the fish community of the Tagus estuary (Portugal). Hydrobiologia587: 113–123.
CostasSNaughtonFGobleRet al. (2016b) Windiness spells in SW Europe since the last glacial maximum. Earth and Planetary Science Letters436: 82–92.
18.
CostasSRebêloLBritoPet al. (2015) The joint history of Tróia Península and Sado Ebb-Delta. In: RandazzoGJacksonDWTCooperJAG (eds) Sand and Gravel Spit. New York: Springer, pp. 79–102.
19.
CoutinhoMTP (2003) Comunidade fitoplanctónica do estuário do Sado. Estrutura, Dinâmica e Aspectos Ecológicos. Research Thesis, Instituto Português do Mar e da Atmosfera, Lisbon.
20.
DanielsenRCastilhoAMDinisPAet al. (2012) Holocene interplay between a dune field and coastal lakes in the Quiaios-Tocha region, central littoral Portugal. The Holocene22(4): 383–395.
21.
DenysLDe WolfH (1999) Diatoms as indicators of coastal paleoenvironments and relative sea-level change. In: StoermerEFSmolJP (eds) The Diatoms: Applications for the Environmental and Earth Sciences. Cambridge: Cambridge University Press, pp. 277–297.
22.
DinizMAriasP (2012) O povoamento humano do paleo-estuário do Sado (Portugal): Problemáticas em torno da ocupação dos concheiros Mesolíticos. In: AlmeidaACBettencourtAMSMouraDet al. (eds) Mudanças ambientais e interação humana na fachada atlântica ocidental. Coimbra: APEQ/CITCEM/CEGT/CGUP/CCT, pp. 139–157.
23.
EllwoodBBHarroldFBBenoistSLet al. (2001) Paleoclimate and intersite correlations from Late Pleistocene/Holocene cave sites: Results from Southern Europe. Geoarchaeology: An International Journal16(4): 433–463.
24.
FletcherWJBorkiTMouraD (2007) Palynological evidence for environmental and climatic change in the lower Guadiana valley, Portugal, during the last 13000 years. The Holocene17(4): 481–494.
25.
FletcherWJDebretMSanchez GoñiMF (2013) Mid-Holocene emergence of a low-frequency millennial oscillation in western Mediterranean climate: Implications for past dynamics of the North Atlantic atmospheric westerlies. The Holocene23(2): 153–166.
26.
FreitasMCAndradeCCrucesA (2002) The geological record of environmental changes in southwestern Portuguese coastal lagoons since the Lateglacial. Quaternary International93–94: 161–170.
27.
García-ArtolaAStephanPCearretaAet al. (2018) Holocene sea-level database from the Atlantic coast of Europe. Quaternary Science Reviews196: 177–192.
28.
GonçalvesFAntunesMT (1992) Notícia explicativa da Folha 39-D Torrão, Carta Geológica de Portugal 1:50000. Lisboa: Serviços Geológicos de Portugal.
29.
HasleGRSyvertsenEE (1996) Marine diatoms. In: TomasCR (ed.) Identifying Marine Phytoplankton. San Diego, CA: Academic Press, pp. 53–83.
30.
HayesWM (ed.) (2015) CRC Handbook of Chemistry and Physics 96th Edition. New York: CRC Press.
INE (2007) Anuário Estatístico da Região do Alentejo. Lisboa: INE.
33.
Jiménez CisnerosBEOkiTArnellNWet al. (2014) Freshwater resources. In: FieldCBBarrosVRDokkenDJet al. (eds) Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects (Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change). Cambridge and New York: Cambridge University Press, pp. 229–269.
34.
JohnsonMEBaarliBGCachãoMet al. (2012) Rhodoliths, uniformitarism and Darwin: Pleistocene and recent carbonate deposits in the Cape Verde and Canary archipelagos. Palaeogeography, Palaeoclimatology, Palaeoecology329–330: 83–100.
35.
KovatsRSValentiniRBouwerLMet al. (2014) Europe. In: BarrosVRFieldCBDokkenDJet al. (eds) Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects (Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change). Cambridge and New York: Cambridge University Press, pp. 1267–1326.
36.
KristensenE (1990) Characterization of biogenic organic matter by stepwise thermogravimetry (STG). Biogeochemistry9(2): 135–159.
37.
LambALWilsonGPLengMJ (2006) A review of coastal palaeoclimate and relative sea-level reconstructions using δ13C and C/N rations in organic material. Earth-Science Reviews75: 29–57.
38.
LarioJZazoCGoyJLet al. (2011) Holocene palaeotsunami record of SW Iberia. Quaternary International242: 196–200.
39.
LeiraMFreitasMCFerreiraTet al. (2019) Holocene sea level and climate interactions on wet dune slack evolution in SW Portugal: A model for future scenarios?The Holocene. 29(1): 26–44.
40.
LeorriECearretaAMilneG (2012) Field observations and modelling of Holocene sea-level changes in the southern Bay of Biscay: Implication for understanding current rates of relative sea-level change and vertical land motion along the Atlantic coast of SW Europe. Quaternary Science Reviews42: 59–73.
41.
LittleSWoodPJElliottM (2017) Quantifying salinity-induced changes on estuarine benthic fauna: The potential implications of climate change. Estuarine, Coastal and Shelf Science198(pt. B): 610–625.
42.
LiuW-CChenW-BChengRTet al. (2007) Modeling the influence of river discharge on salt intrusion and residual circulation in Danshuei River estuary, Taiwan. Continental Shelf Research27: 900–921.
43.
MatosJXOliveiraV (2003) Mina do Lousal (Faixa Piritosa Ibérica) – Percurso geológico e mineiro pelas cortas e galerias da antiga mina. Instituto Geológico y Minero de España. Publicaciones Museo Geominero, N2: 117–128.
44.
MoreiraS (2016) Contributo da geoquímica e sedimentologia na caracterização de influências antrópicas em ambientes estuarinos. PhD Thesis, Universidade de Lisboa, Lisboa.
45.
Peyroteo-StjernaR (2016) On death in the mesolithic. or the mortuary practices of the last hunter-gatherers of the South Western Iberian Peninsula, 7th–6th Millennium BCE (Occasional papers in archaeology 60). PhD Thesis, Uppsala University, Uppsala.
46.
PimentelNLVPimentelPRVAzevêdoTMet al. (2001) Estudo sedimentológico e geoquímico de depósitos holocénicos do rio Sado. Actas da V Reunião do Quaternário Ibérico / I Congresso do Quaternário dos Países de Língua Portuguesa. Lisboa: 125–127.
47.
QueirozP (1999) Ecologia histórica da paisagem do noroeste alentejano. PhD Thesis, Universidade de Lisboa, Lisboa.
48.
ReimerPJBardEBaylissAet al. (2013) IntCal13 and Marine13 Radiocarbon Age calibration curves 0-50,000 Years cal. BP. Radiocarbon55(4): 1869–1887.
49.
RenbergI (1990) A procedure for preparing large sets of diatom slides from the sediment cores. Journal of Paleolimnology4: 87–90.
50.
RobinsPELewisMJFreerJet al. (2018) Improving estuary models by reducing uncertainties associated with river flows. Estuarine, Coastal and Shelf Science207: 63–73.
51.
Rodríguez-RamírezAPérez-AsensioJNSantosAet al. (2015) Atlantic extreme wave events during the last four millennia in the Guadalquivir estuary, SW Spain. Quaternary Research83: 24–40.
52.
SabatierPDezileauLColinCet al. (2012) 7000 years of paleostorm activity in the NW Mediterranean Sea in response to Holocene climate events. Quaternary Research77: 1–11.
53.
SantosJFPulido-CalvoIPortelaMM (2010) Spatial and temporal variability of droughts in Portugal. Water Resources Research46: W03503.
54.
SchubelJR (1982) Estuarine sedimentation. In: SchwartzM (ed.) Beaches and Coastal Geology: Encyclopedia of Earth Science. Boston, MA: Springer, p. 324.
55.
ShahaDCChoY-K (2016) Salt plug formation caused by decreased river discharge in a multi-channel estuary. Nature Scientific Reports6: 27176.
56.
SousaPMTrigoRMAizpuruaPet al. (2011) Trends and extremes of drought indices throughout the 20th century in the Mediterranean. Natural Hazards and Earth System Science11(1): 33–51.
57.
VisG-JCorneliusK (2009) Late Quaternary valley-fill succession of the Lower Tagus valley, Portugal. Sedimentary Geology221: 19–39.
58.
VisG-JCornelisKKroonDet al. (2015) Time-integrated 3D approach of late Quaternary sediment-depocenter migration in the Tagus depositional system: From river valley to abyssal plain. Earth-Science Reviews153: 192–211.
59.
VisG-JKasseCVandenbergheJ (2008) Late Pleistocene and Holocene palaeogeography of the Lower Tagus Valley (Portugal): Effects of relative sea level, valley morphology and sediment supply. Quaternary Science Reviews27: 1682–1709.
60.
VosPCDe WolfH (1988) Methodological aspects of paleoecological diatom research in coastal areas of the Netherlands. Geologie en Mijnbouw67: 31–40.
61.
VosPCDe WolfH (1993) Diatoms as a tool for reconstructing sedimentary environments in coastal wetlands: Methodological aspects. Hydrobiologia269–270: 285–296.
62.
WeckströmK (2006) Assessing recent eutrophication in coastal waters of the Gulf of Finland (Baltic Sea) using subfossil diatoms. Journal of Paleolimnology35: 571–592.
63.
WongPPLosadaIJGattusoJ-Pet al. (2014) Coastal systems and low-lying areas. In: FieldCBBarrosVRDokkenDJet al. (eds) Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects (Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change). Cambridge and New York: Cambridge University Press, pp. 361–409.
