Roberto Bao

Upwelling influence on the Galician coast: silicate in shelf water and underlying surface sediments

Abstract Local characteristics in the upwelling of Galicia (NW Iberian Peninsula) are studied in terms of dissolved silicate patterns in the seawater column and opal distribution in the sediments. Freshwater input, upwelling, remineralization and sedimentation are considered. The freshwater silicate input is not important during the upwelling season. Upwelling is the main process controlling silicate biogeochemistry activity in the coastal zone. Areas of silicate remineralization in the seawater column and opal abundance in surface sediment practically coincide. These together define the coastal limit of upwelling influence and its diverse local effects. Galician upwelling is important in the area surrounding three well-defined zones; Cape Finisterre, Cape Prior and La Corun˜a Canyon. South of Finisterre upwelling is more intense and closer to the coast. To the north, it is discontinuous and keeps distant from the coast, being near to the edge of the continental shelf.

Separating eustatic from local environmental effects: a late-Holocene record of coastal change in Albufeira Lagoon, Portugal

Diatom and sedimentological analyses of a 7.6-m core of the Albufeira coastal lagoon, western Portugal, provide information about the forcing factors and environmental history of the evolution of this lagoon during the last 2500 years. At this timescale, eustasy is considered to be a minor environmental control on the dynamics of the barrier-lagoonal system. Instead, these dynamics are forced by local factors, such as changes in the sand-barrier permeability. Lithological and diatom facies are compared with present-day environ ments and show that the lagoon evolved essentially as a slightly brackish water body with a salinity of around 1‰. This pattern was disturbed by three major episodes of sedimentation associated with threshold responses in the permeability of the barrier. The first (c. 2370 BP) did not produce a specific lithological signal and is characterized by a peak in freshwater euplanktonic diatoms, implying an almost permanent isolation of the basin and freshwater flooding. A second episode was identified at c. 1600 BP, when a dramatic opening of the tidal inlet provoked a significant change in the sedimentation regime, which became dominated by inorganic, minerogenic sediments and allowed marine/brackish epiphytic diatoms to flourish together with allochthonous marine planktonic species. Finally, a third threshold occurred at c. 1225 BP corresponding to the establishment of the present-day muddy or sandy-muddy sedimentation pattern. Both the sedimentological features and the diatom assemblages indicate the establishment of a more tidally influenced environment. This reflects regular artificial breaching of the barrier associated with increasing anthropogenic intervention in the system.

Lateglacial and Holocene environmental changes in Portuguese coastal lagoons 2: microfossil multiproxy reconstruction of the Santo André coastal area

Benthic foraminiferal, ostracod, calcareous nannoplankton and diatom assemblages have been analysed from the 25.45 sedimentary succession of a borehole drilled in the Santo André lagoon (southwestern Portugal). The zones and subzones recognized according to their microfossil content have been used to define six palaeoenvironmental stages extending from the Lateglacial to the present. Stage A and the lower part of stage B (14.2 to 10.0 ka) represent a postglacial terrestrial environment when sea level was below present. Following eustatic sea-level rise, a normal marine, open-bay environment established during the upper part of stage B (10.0 to 5.4 ka). Before 5.4 ka, a coastal lagoon developed as a consequence of a sandy barrier formation. Since then, natural variation in the permeability of the detrital barrier through time has been the primary control of the physical, chemical and biological features of the lagoonal environment: during stage C (5.4 to 3.6 ka) as a freshwater-dominated lagoon, stage D (3.6 to 1.6 ka) as a brackish lagoonal environment, and stage E (1.6 to 0.5 ka) as a return to a freshwater-dominated lagoon. Finally, in the last 400 years (stage F), human intervention on the sandy barrier (artificial open- and closed-inlet conditions) has provoked rapid and dramatic environmental changes in Santo André.

Mesoscale distribution patterns of diatoms in surface sediments as tracers of coastal upwelling of the Galician shelf (NW Iberian Peninsula)

Abstract Diatom distribution on continental shelf sediments from the upwelling area off Galicia (NW Iberian Peninsula) has been determined in 78 surface sediment samples. Three well defined biofacies with a close relationship to upwelling influence along the coast have been distinguished from both the absolute diatom abundances in the sediments and assemblage diatom composition based on multivariate statistics: Rias Baixas and the shelf areas south and north to Cape Finisterre. Chaetoceros resting spores as well as highest absolute diatom content in the sediments are recorded in the highly productive Rias Baixas where most of the primary production is due to intense upwelling phenomena. The western shelf, south of Cape Finisterre, is characterized by Thalassionema nitzschioides and Thalassiosira cf. leptopus . Both taxa reflect a minor influence of upwelling conditions and productivity related to more persistent nutrient input due to coastal outwelling rather than upwelling. Minor and patchy upwelling conditions as well as lower productivity in the northern shelf are characterized by the lowest absolute diatom content in the sediments and the important increase in relative abundance of the resistant Paralia sulcata . Caution must be taken in the interpretation of Paralia sulcata dominated biofacies in downcore studies since a juxtaposition of discontinuous upwelling conditions and dissolution effects may be the factors responsible for the relative increase on the taxon in the sediments. The obtained results show the extent and limitations of fossil diatom distribution in surface sediments as tracers of regional coastal upwelling conditions in continental shelves where the existence of extensive areas of relict sediments and transported diatoms may mask the present-day upwelling signal of the photic zone.

Challenging Easter Island"s collapse: the need for interdisciplinary synergies

The reigning paradigm holds that Easter Island suffered a socio-ecological collapse (ecocidal or not) sometime in the last millennium, prior to European contact (AD 1720). We discuss some novel paleoecological and archaeological evidence that challenges this assumption. We use this case study to propose a closer collaboration between archaeology and paleoecology. This collaboration allows us to unravel historical trends in which both environmental changes and human activities might have acted, alone or coupled, as drivers of ecological and social transformations. We highlight a number of particular points in which scholars from disparate disciplines, working together, may enhance the scope and the soundness of historical inferences. These points are the following: (1) the timing of the initial Easter Island colonization and the origin of the settlers, (2) the pace of ecological and social transformations since that time until the present, and (3) the occurrence of potential climate-human synergies as drivers of socio-ecological shifts.

Lake-level and salinity reconstruction from diatom analyses in Quillagua formation (late Neogene, Central Andean forearc, northern Chile)

Abstract A sedimentary and micropaleontological study of the Quillagua Formation provides a detailed paleohydrological reconstruction of the lacustrine system which occupied the present-day hyperarid Quillagua–Llamara fore-arc Basin (Northern Chile) from lattermost Miocene (5.8±0.4 Ma) to Early Pliocene times. Diatom and lithofacies analyses were carried out in two correlated stratigraphic sections of the lacustrine system. The Quebrada Temblor section is located at the southern margin of the ancient lake and is directly influenced by the freshwater inputs of a northward flowing fluvial system draining the Precordillera and Calama regions. The Cerro Mogote section occupies a western marginal location in the northern zone sheltered from direct fluvial inputs from the south but subjected to the activity of the alluvial fan systems of the Coastal Range. A mostly shallow oligosaline waterbody occupied the basin during the interval studied, though with fluctuations in salinity and the extent of the inner, open waters. The southern margin sector — represented by Quebrada Temblor — had, in general terms, a palustrine oligosaline character with almost freshwater conditions during certain periods, which favoured the establishment of semi-permanent to permanent freshwater plumes overlying a saline waterbody. Development of shoreline facies subjected to desiccation events was also characteristic at the top of this section. The innermost shallow lacustrine areas — represented by Cerro Mogote — maintained more homogeneous oligosaline characteristics and more persistent open waters. They were also subjected to freshwater pulses mediated in this case by the activity of the terminal alluvial fan zones. Paleohydrological evolution of the basin was strongly conditioned not only by shifts in the climatic–tectonic system but by variations in the local hydrological parameters. Four hierarchical orders of variability in the lake level of the basin were distinguished both by stratigraphic analysis of lithofacies and from changes undergone by the diatom record. Strong intrasample mixing of diatoms of incompatible salinity spectra and the presence of fine laminated lacustrine facies in some terms define the highest order short-term intra- or inter-annual pulses experienced by the lacustrine system (higher than 6th order). Diatom-based punctuated interruptions of the minor order bathymetrical trends highlight the high frequency variability in the basin (probably 6th order, 0.001–0.01 Ma), while the deepening/shallowing facies defined by the arrangement of the decimetre thick lithological sequences revealed a lower order of variability (5th order, 0.01–0.1 Ma). Combined lithofacies and diatom analyses delineate the lowest order of variability (4th order, 0.1–1 Ma) allowing the reconstruction of two well-established highstand and lowstand situations which implied the existence of a regressive trend between two transgressive trends. This order of variability is related to the interplay of tectonic events in the Calama Basin, and the regional climatic evolution during late Neogene to Pliocene times.

THE BIOGEOCHEMICAL CYCLING OF DISSOLVED SILICATE IN A GALICIAN RIA

Abstract The annual variation of the dissolved silica contributions to the ria of Vigo and its spatial distributions on the silicate sediment is considered using a box model. During winter, the silicate freshwater flux is high, about 10 mol Si s-l. This mixes with the ria water and is exported to the ocean. During the rest of the year, the most important source of dissolved silicate is the subsurface incoming flux of marine water to the ria which provides from 52% to 65% of the total. In these seasons, the redissolution of biogenic silicate is the second source and causes up to 46% of silicate inputs to the photic zone. The utilisation of dissolved silicate by the diatoms was 1 mol Si s-1, but increased to 11 mol Si s-1 in the event of upwelling. The abundance of silicate in sediments in different parts of the ria of Vigo is a consequence of the cited fluxes. The bottom of the inner middle zone in the ria receives diatoms in spring and summer when no upwelling occurs (from 50 to 120 mg Si m-2 d-1), and si...

Biogeography and lake morphometry drive diatom and chironomid assemblages’ composition in lacustrine surface sediments of oceanic islands

Subfossil biotic assemblages in lakes’ surface sediments have been used to infer ecological conditions across environmental gradients. Local variables are usually the major drivers of assemblage composition, but in remote oceanic islands biogeographic filters may play a significant role. To assess the contribution of local and regional filters in the composition of subfossil diatom and chironomid assemblages in surface sediments, 41 lakes in Azores archipelago were studied and related to environmental variables. Ordination techniques were used to identify the forcing factors that best explain the composition of these assemblages. Both assemblages are influenced by multiple limnological variables (conductivity, pH and nutrients). However, diatom assemblages differed mainly in the proportion of planktonic versus benthic species along lakes’ depth gradient while chironomids differed significantly among islands but not among lake depths. Thus, biogeographic filters play an important role in shaping islands’ freshwater communities, particularly insect ones, more influenced by geographic variables. Results demonstrate the accuracy and potential of biotic remains in sediments for applied studies of lake ecology, trophic status, climatic trends and ecological reconstruction and evolution of lakes. In the Azores, the application of this information for the development of inference models is envisaged as a further step to accomplish these goals.

8000 years of environmental evolution of barrier–lagoon systems emplaced in coastal embayments (NW Iberia)

The rocky and indented coast of NW Iberia is characterized by the presence of highly valuable and vulnerable, small and shallow barrier–lagoon systems structurally controlled. The case study was selected to analyse barrier–lagoon evolution based on detailed sedimentary architecture, chronology, geochemical and biological proxies. The main objective is to test the hypothesis of structural control and the significance at regional scale of any high-energy event recorded. This work is also aimed at identifying general patterns and conceptualizing the formation and evolution of this type of coastal systems. The results allowed us to establish a conceptual model of Holocene evolution that applies to rock-bounded barrier–lagoon systems. The initial stage (early Holocene) is characterized by freshwater peat sedimentation and ended by marine flooding. The timing of the marine flooding depends on the relation between the elevation of the basin and the relative mean sea-level position; the lower the topography, the earlier the marine inundation. Thus, the age of basin inundation ranged from 8 to 4 ka BP supporting significant structural differences. Once marine inundation occurred, all systems followed similar evolutionary patterns characterized by a phase of landward barrier migration and aeolian sedimentation towards the back-barrier (i.e. retrogradation) that extended circa 3.5 ka BP. The later phases of evolution are characterized by a general trend to the stabilization of the barriers and the infilling of the lagoons. This stabilization may be temporally interrupted by episodes of enhanced storminess or sediment scarcity. In this regard, washover deposits identified within the sedimentary architecture of the case study explored here suggest pervasive high-energy events coeval with some of the cooling events identified in the North Atlantic during the mid- to late Holocene.

Holocene sea level and climate interactions on wet dune slack evolution in SW Portugal: A model for future scenarios?

We examine the Holocene environmental changes in a wet dune slack of the Portuguese coast, Poço do Barbarroxa de Baixo. Lithology, organic matter, biological proxies and high-resolution chronology provide estimations of sediment accumulation rates and changes in environmental conditions in relation to sea-level change and climate variability during the Holocene. Results show that the wet dune slack was formed 7.5 cal. ka BP, contemporaneous with the last stages of the rapid sea-level rise. This depositional environment formed under frequent freshwater flooding and water ponding that allowed the development and post-mortem accumulation of abundant plant remains. The wetland evolved into mostly palustrine conditions over the next 2000 years, until a phase of stabilization in relative sea-level rise, when sedimentation rates slowed down to 0.04 mm yr−1, between 5.3 and 2.5 cal. ka BP. Later, about 0.8 cal. ka BP, high-energy events, likely due to enhanced storminess and more frequent onshore winds, caused the collapse of the foredune above the wetlands’ seaward margin. The delicate balance between hydrology (controlled by sea-level rise and climate change), sediment supply and storminess modulates the habitat’s resilience and ecological stability. This underpins the relevance of integrating past records in coastal wet dune slacks management in a scenario of constant adaptation processes.