Browsing by Author "Fletcher, William"
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- Chronology of the sedimentary processes during the postglacial sea level rise in two estuaries of the Algarve coast, Southern PortugalPublication . Boski, T.; Camacho, Sarita; Moura, Delminda; Fletcher, William; Wilamowski, A.; Veiga-Pires, C.; Correia, Victor; Loureiro, C.; Santana, PauloFour profiles of estuarine sediments obtained from boreholes drilled in the Algarve, Southern Portugal were studied in order to reconstruct the process of sediment accumulation driven by the postglacial sea level rise. In addition to the sedimentological analysis, the Foraminifera Index of Marine Influence (FIMI) permitted assessment of the nature and organization of sedimentary facies in the BelicheeGuadiana and Gilão-Almargem estuaries. The Beliche- Guadiana CM5 and Almargem G2 profiles accumulated in a sheltered environment, with the former presenting an almost continuous record of the sea level rise since ca 13 000 cal yr BP. The G1 and G3 profiles from the Gilão-Almargem area represent a more discontinuous record of the last 8000 years, which accumulated in the more dynamic environment of an outer estuary. The integration of all radiocarbon ages of dated levels, led to an estimate of sediment accumulation rates. Assuming a constant position of the sediment surface with respect to the tidal range and a negligible compaction of sediment, the sea level rose at the rate of 7 mm yr ^-1 in the period from 13 000 to 7500 cal yr BP. This process slowed down to ca 0.9 mm yr 1 from 7500 cal yr BP until the present. The marked historical change in the rate of sediment accumulation in these estuaries also occurred with the accumulation of organic matter and is, therefore, important data for global biogeochemical models of carbon. The main obstacle to obtain higher temporal resolution of the sedimentary processes was the intense anaerobic respiration of organic matter via sulphate reduction, which did not allow any accumulation of peat and, furthermore, led to erasure of the palaeontological record by acid formed from the subsequent oxidation of sulphides.
- Does the glacial/interglacial sea-level changes modulate the atmospheric CO2 variations?Publication . Boski, T.; Moura, Delminda; Veiga-Pires, C.; Fletcher, William; Correia, VictorIt is postulated the rate of the terrestrial organic carbon (OC) retention around the moving ocean/continent interface is strongly affected by the sea-level.
- Palynological evidence for environmental and climatic change in the lower Guadiana valley, Portugal, during the last 13 000 yearsPublication . Fletcher, William; Boski, T.; Moura, DelmindaPollen analysis of a 48 m AMS radiocarbon-dated sediment sequence from the Guadiana estuary provides the first record of Lateglacial and Holocene vegetation history in the Algarve province of Portugal. This paper focuses on the record of terrestrial pollen taxa, which document a series of forest expansions and declines during the period 13 000 cal. BP to 1600 cal. BP and provide insights into climate evolution in southwestern Iberia. The main vegetation phases identified in the Guadiana valley are (1) Lateglacial interstadial (Allerød) forest with Quercus and Pinus under a temperate, moist, continental climate; (2) a Younger Dryas forest decline (Quercus) and expansion of pinewoods, xeric scrub and open ground habitats (with Juniperus, Artemisia, Ephedra distachya type, Centaurea scabiosa type) under arid and cold conditions; (3) an early Holocene forest/scrub/open-ground vegetation mosaic developing under a warm, dry and continental climate; (4) a maximum of Quercus forest and thermomediterranean evergreen taxa (Olea, Phillyrea, Pistacia) reflecting a warm, moist oceanic climate between c. 9000 cal. BP and c. 5000 cal. BP; and (5) the expansion of shrublands with Cistaceae and Ericaceae under a drier climatic regime and increasing anthropogenic activity since c. 5000 cal. BP. Holocene episodes of maximum climatic aridity are identified in the record of xerophytic taxa (Juniperus, Artemisia, Ephedra distachya type) centred around 10 200 cal. BP, 7800 cal. BP, 4800 cal. BP, 3100 cal. BP and 1700 cal. BP. Regional comparisons suggest a correlation of arid phases across southern Iberia and northwest Africa, which can be related to abrupt North Atlantic coolings (Bond events).
- Rising atmospheric CO2 concentrations: the overlooked factor promoting SW Iberian Forest development across the LGM and the last deglaciation?Publication . Domingues Gomes, Sandra; Fletcher, William; Stone, Abi; Anica Claro Rodrigues, Teresa Isabel; Rebotim, Andreia; Oliveira, Dulce; Goñi, Maria Sánchez; Abrantes, Fatima; Naughton, FilipaA cross the last deglaciation, the atmospheric CO2 concentration (CO2) increased substantially from ∼ 180 to ∼ 280 ppm, yet its impact on vegetation dynamics across this major climatic transition remains insufficiently understood. In particular, Iberian pollen records reveal an intriguing feature that can be related to an often-overlooked role of CO2 in shaping vegetation responses during the last deglaciation. These records reveal the near disappearance of forests during the cold Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS1) phases and an unexpected recovery during the Younger Dryas (YD) cold phase when CO2 increased. Here, we present high-resolution tracers of terrestrial (pollen, C29: C31 organic biomarker) and marine (alkenone-derived Sea Surface Temperature, C37: 4 %, and long-chain n-alkanes ratios) conditions from the southwestern (SW) Iberian margin Integrated Ocean Drilling Program Site U1385 (“Shackleton site”) for the last 22 cal kyr BP. This direct land-sea comparison approach allows us to investigate how the Iberian Peninsula vegetation responded to major global CO2 changes during the last deglaciation. Our results show that cool and moderately humid conditions of the LGM supported a grassland-heathland mosaic ecosystem, but low CO2 likely caused physiological drought and suppressed forest development. HS1, the coldest and most arid period, combined with sustained low CO2 values, almost suppressed forest growth in favour of Mediterranean steppe. In contrast, the warmer Bølling-Allerød, characterised by a temperature optimum and variable but generally wetter conditions, along with the rise of CO2 above 225 ppm at ∼ 15 cal kyr BP, contributed to substantial forest development. During the YD, sufficient moisture combined with increasing CO2 enabled the persistence of a mixed grassland-forest mosaic despite cooler temperatures. Our study suggests that during cool and humid periods (LGM and YD) different pCO2 values led to contrasting SW Iberian vegetation responses. In contrast, during periods of relatively high CO2, temperature and precipitation played the main role in shaping the distribution and composition of the vegetation.