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... They constitute a succession of gravelly mud and gravelly muddy sand corresponding to a general increase in energy (Fig. 5). This is probably related to river infill with reworked former fluvial/marine sediments and mud coming from soil erosion, under warm and wet conditions (insolation maxima and precession minimum, Gomes et al., 2020) and sea level rise. This succession presents two levels of finer sediments concerning events of quitter dynamics, one before 9963 cal BP and the other after 9680 cal BP (Fig. 2). ...
... The period 9800-7400 cal BP is characterized by optimum temperature and precipitation conditions in NW Iberia (Gomes et al., 2020), which would have favoured the increase of fluvial flow. During this period, the climatic sequences register two abrupt cooling phases, dated around 9300 cal BP and 8200 cal BP (Rasmussen et al., 2014). ...
... In NW Iberia, Romanization is related to a highly anthropized landscape with very few areas of woodland and other pristine territorial environments (Ramil-Rego et al., 2009, 2015Ramil Rego and Gómez-Orellana, 2016). In other sequences of this period in central and northern Portugal, there is a similar situation (Mateus, 1992;Ramil-Rego et al., 1996Gómez-Orellana et al., 2001;2010;Granja et al., 2016). ...
Article
This paper aims to contribute to the understanding of the Holocene evolution of the Cávado estuarine coastal system (Portugal) and the adjacent terrestrial areas, using a multidisciplinary approach, which includes geomorphology, sedimentology, palynology, radiocarbon and history. During the Early Holocene, the Cávado environment was characterized by the dominance of coarse sediments especially in the most westward areas, corresponding to fluvial energetic dynamics. During the Middle Holocene, the presence of wetlands dominated especially in the most sheltered areas. The pollen data reflect the predominance of forests during the initial phases of the introduction of agriculture, with a significant presence of humid forests. Between 4240 and 3980 cal BP, a sudden marine flooding took place over the peat. During the Late Holocene, there was a succession of low, high and again low energy fluvial environments in the eastward sheltered areas, while high energy dominated in the Fão channel entrance, with marine influence on the uppermost units. The pollen content reflects a situation of high human influence, with a poor representation of natural forests and a predominance of agrosystems, related to the rise of the Cávado estuary as a port during the heyday of the city of Braga during the Roman and Suevian periods, as described in historical texts and archaeological research. In connection with this growth, the pollen data show the existence of local Pinus plantations related to shipbuilding and repair in the port. During these times, the estuary was larger and open to the sea at Fão. After the 15th century, with the general silting of coastal areas, the inlet closed and the river mouth was displaced to the North.
... Indeed, western Mediterranean Late Holocene is characterized by prevailing drier climatic conditions since ca. 5000-4000 cal BP (Fletcher et al., 2007(Fletcher et al., , 2013Carrión et al., 2010;Chabaud et al., 2014;Gomes et al., 2020) with peaks of high fluvial activity that promote regular to sporadic inundation of distal estuarine areas in South Iberia (Vis et al., 2010, Fig. 10). The availability of sediment can also result from vegetation retreat due to the establishment of drier climatic conditions (e.g. ...
... The availability of sediment can also result from vegetation retreat due to the establishment of drier climatic conditions (e.g. Fletcher et al., 2007Fletcher et al., , 2013Carrión et al., 2010;Chabaud et al., 2014;Gomes et al., 2020). In Unit 2, deposited between 4350 and 4300 cal BP, coarse materials decrease to the top such as the magnetic materials (Fig. 4A) while organic compounds increase, reaching to maximum values in OM, C org (Fig. 5) and N (Fig. 6), reflecting a low energy environment with limited fluvial inputs. ...
... However, most probably, small MSL oscillations under a prevailing drier climate (e.g. Fletcher et al., 2007;Chabaud et al., 2014;Gomes et al., 2020) and low fluvial activity will contribute to enlargement of the estuarine area and to the retreat of the fluvial-estuarine boundary. It is also worth noting that a shoreline retreat was identified in the Tróia spit at ca. 4000 years ago (Costas et al., 2015). ...
Article
The aims of this work are to characterize the palaeoenvironmental evolution and the sedimentary fluvial responses to climate and river flow changes of the upstream reaches of the Sado estuary since the Mid-Late Holocene. By combining the previous information, we also intent to define the limit of the fluvial-estuarine boundary and maximum limit of tidal influence of the estuarine area during the last 4500 years. To accomplish these objectives a 10.5 m-long sediment core was collected at ca. 65 km upstream the Sado estuary inlet and studied using multiproxy analyses and radiocarbon dating. In addition, multi-proxy data from sediment cores and superficial samples collected downstream, in the alluvial plain and intertidal areas near the present-day fluvial-estuarine boundary, were combined and contributed to the interpretations. We concluded that: i) the organic matters is mostly sourced in terrestrial/freshwater environments even during periods with marine influence, as suggested by the occurrence of marine benthic diatoms between 4300 and 4200 cal BP; ii) the fluvial-estuarine boundary retreat during the Midle to Late Holocene transition (4350-4000 cal BP) reaching areas of the Sado river (Laxique), located ca. 15 km upstream of the present-day fluvial-estuarine boundary; iii) modifications on the location of the fluvial-estuarine boundary are mostly derived from drier climatic conditions and consequent flow river discharges; iv) the aggradation and progradation of the alluvial plain started at ca. 4000 cal BP at Laxique (and at ca. 3240 cal BP at Arapouco); and v) the abrupt change in the sedimentation rates and sedimentation pattern before and after the beginning of the alluvial plain aggradation process (4000 cal BP at Laxique and 3240 cal BP at Arapouco) reflects essentially changes in the fluvial activity and the loss of accomodation space.
... A similar pattern is observed in the total pollen concentration record of nearby marine core MD95-2042 (site shown in Fig. 1; Chabaud et al., 2014), which shows an abrupt decline at~12 cal ka BP and lowest values between~8 and 5 cal ka BP. MD01-2444 also documents a reduced pollen concentration from the YD into the Holocene, while core D13882 (Gomes et al., 2020), located on the continental shelf near the mouth of the Tagus (site shown in Fig. S1), shows lower pollen concentrations after 10.6 ka ( Fig. 5(g)), remaining low until 5.5 cal ka BP, increasing thereafter. ...
... After 2.7 cal ka BP, pollen concentrations and PARs rise slightly, which coincides with a significant rise in SAR and a decline in ln(Ca/Ti). Other records from this region also show a rise in sedimentation rates after~2 cal ka BP, resulting from the impact of anthropogenic land-use change (Vis et al., 2016;Gomes et al., 2020). At the very top of the SHAK06-5K record, after 0.6 cal ka BP, SAR declined, coinciding with reduced Mixed forest Slight decline in temperate taxa (~61%), primarily due to a decline in deciduous Quercus (~45%). ...
... Iberian Margin SSTs remain high (~17.5°C) (Martrat et al., 2007;Rodrigues et al., 2010;Ausín et al., 2019b;Gomes et al., 2020), although displaying a small gradual decreasing trend from the mid-Holocene. The heathland expansion coincides with a boreal insolation minimum, associated with reduced precipitation seasonality and increased summer water availability. ...
Article
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We present new high‐resolution pollen records combined with palaeoceanographic proxies from the same samples in deep‐sea cores SHAK06‐5K and MD01‐2444 on the southwestern Iberian Margin, documenting regional vegetation responses to orbital and millennial‐scale climate changes over the last 28 ka. The chronology of these records is based on high‐resolution radiocarbon dates of monospecific samples of the planktonic foraminifera Globigerina bulloides, measured from SHAK06‐5K and MD01‐2444 and aligned using an automated stratigraphical alignment method. Changes in temperate and steppe vegetation during Marine Isotope Stage 2 are closely coupled with sea surface temperature (SST) and global ice‐volume changes. The peak expansion of thermophilous woodland between ~10.1 and 8.4 cal ka bp lags behind the boreal summer insolation maximum by ~2 ka, possibly arising from residual high‐latitude ice‐sheets into the Holocene. Rapid changes in pollen percentages are coeval with abrupt transitions in SSTs, precipitation and winter temperature at the onset and end of Heinrich Stadial 2, the ice‐rafted debris event and end of Heinrich Stadial 1, and the onset of the Younger Dryas, suggesting extrinsically forced southwestern Iberian ecosystem changes by abrupt North Atlantic climate events. In contrast, the abrupt decline in thermophilous elements at ~7.8 cal ka bp indicates an intrinsically mediated abrupt vegetation response to the gradually declining boreal insolation, potentially resulting from the crossing of a seasonality of precipitation threshold.
... A similar pattern is observed in the total pollen concentration record of nearby marine core MD95-2042 (site shown in Fig. 1; Chabaud et al., 2014), which shows an abrupt decline at~12 cal ka BP and lowest values between~8 and 5 cal ka BP. MD01-2444 also documents a reduced pollen concentration from the YD into the Holocene, while core D13882 (Gomes et al., 2020), located on the continental shelf near the mouth of the Tagus (site shown in Fig. S1), shows lower pollen concentrations after 10.6 ka ( Fig. 5(g)), remaining low until 5.5 cal ka BP, increasing thereafter. ...
... After 2.7 cal ka BP, pollen concentrations and PARs rise slightly, which coincides with a significant rise in SAR and a decline in ln(Ca/Ti). Other records from this region also show a rise in sedimentation rates after~2 cal ka BP, resulting from the impact of anthropogenic land-use change (Vis et al., 2016;Gomes et al., 2020). At the very top of the SHAK06-5K record, after 0.6 cal ka BP, SAR declined, coinciding with reduced Mixed forest Slight decline in temperate taxa (~61%), primarily due to a decline in deciduous Quercus (~45%). ...
... Iberian Margin SSTs remain high (~17.5°C) (Martrat et al., 2007;Rodrigues et al., 2010;Ausín et al., 2019b;Gomes et al., 2020), although displaying a small gradual decreasing trend from the mid-Holocene. The heathland expansion coincides with a boreal insolation minimum, associated with reduced precipitation seasonality and increased summer water availability. ...
Thesis
Full-text available
This thesis aims to enhance the current understanding of the response of SW Iberian ecosystems to abrupt and orbital-scale climate changes. The last ~28 thousand years can provide such insight, containing several abrupt North Atlantic climate events superimposed on orbital-scale global changes. This study presents new high-resolution pollen and leaf-wax n-alkane records combined with palaeoceanographic proxies from the same deep-sea cores (SHAK06-5K and MD01-2444) on the Southwestern (SW) Iberian Margin. The chronology of these records is based on high-resolution Accelerator Mass Spectrometry radiocarbon dating of planktonic foraminifer Globigerina bulloides from cores SHAK06-5K and MD01-2444. Changes in temperate and steppe records during the Last Glacial Maximum and subsequent deglaciation are closely coupled with changes in sea surface temperatures (SSTs), and global ice volume. This coupling continues during the onset of the Holocene, with the peak in thermophilous woodland lagging the boreal insolation maxima by ~2 kyr. This possibly arises from the persistence of residual high-latitude ice-sheets into the Holocene. A close correlation between rapid oscillations in pollen percentages and millennial/centennial-scale variations in SSTs, planktonic d18O, and lithology suggests extrinsically-forced SW Iberian ecosystem changes in response to abrupt North Atlantic climate events. In contrast, the abrupt thermophilous woodland decline at ~7.8 thousand years before present (cal ka BP) indicates an intrinsically-mediated abrupt vegetation response to the gradually declining boreal insolation, resulting in the crossing of an ecological threshold. The leaf-wax n-alkane d13C record from SHAK06-5K combined with the pollen record from the same core and modern leaf-wax n-alkane d13C data from SW Iberia suggest that this geochemical proxy is directly or indirectly driven by SW Iberian climate variations. Two potential mechanisms are proposed: i) n-alkane d13C is directly controlled by changes in regional moisture availability; or ii) climate change leads to a turnover of plant species with inherently different n-alkane d13C signatures.
... At ca. 10-9 ka BP an abrupt increase in moisture was identified in several lake records of the Western Mediterranean [93], and the progressive expansion and maximum values of oakwoods between ca. 10 to 7 ka BP seem to confirm this trend. Despite slight differences in the chronologies, this rise of temperate and Mediterranean forest taxa is also registered in other marine cores drilled in the Atlantic margin of southwestern Iberia [101][102][103], as well as in some continental deposits of this area [91]. During this period, two peaks of microcharcoals were recorded and interpreted as episodes of increased regional wildfires at 8.8 and 8.4 ka BP (Figure 7). ...
... 8.6-8 ka BP, as well as in the Alborán Sea between ca. 8.3-8 ka BP [101][102][103]106,107]. In continental deposits, some forest setbacks and expansion of open-ground taxa were recorded in the Guadiana Valley and the Medina Lagoon at ca. 7.8 and 8.2 ka BP, respectively [91,108]; however, several sequences do not mirror any vegetal changes for this period [15,109]. ...
... Another important crisis was also identified in the GeoB235-19-01 core at ca. 7.5 ka BP, defined by a visible peak of xerophytes and an abrupt drop of mesophilous trees, riparian woodland, and Mediterranean woodland and shrubland, pointing to a rapid decrease of moisture availability (Figures 6e and 7). Diverse episodes of forest contractions were identified between 7.5 and 7 ka BP in different cores of the Atlantic Iberian Margin and the Alborán Sea, as well as in continental sequences [91,[101][102][103]106,107,109]. Diverse palaeorecords from SE Iberia suggest changes towards increased aridity between 7.8-7.3 ...
Article
Full-text available
The SW coast of the Iberian Peninsula experiences a lack of palaeoenvironmental and archaeological data. With the aim to fill this gap, we contribute with a new palynological and geochemical dataset obtained from a sediment core drilled in the continental shelf of the Algarve coast. Archaeological data have been correlated with our multi-proxy dataset to understand how human groups adapted to environmental changes during the Early-Mid Holocene, with special focus on the Mesolithic to Neolithic transition. Vegetation trends indicate warm conditions at the onset of the Holocene followed by increased moisture and forest development ca. 10–7 ka BP, after which woodlands are progressively replaced by heaths. Peaks of aridity were identified at 8.2 and 7. 5 ka BP. Compositional, textural, redox state, and weathering of source area geochemical proxies indicates abrupt palaeoceanographic modifications and gradual terrestrial changes at 8.2 ka BP, while the 7.5 ka BP event mirrors a decrease in land moisture availability. Mesolithic sites are mainly composed of seasonal camps with direct access to the coast for the exploitation of local resources. This pattern extends into the Early Neolithic, when these sites coexist with seasonal and permanent occupations located in inland areas near rivers. Changes in settlement patterns and dietary habits may be influenced by changes in coastal environments caused by the sea-level rise and the impact of the 8.2 and 7.5 ka BP climate events.
... It is punctuated by a sharp trough between 12.9 and 11.8 kyr corresponding to the Younger Dryas (YD) and relatively low values between 10 and 8 kyr, both periods characterised by a high elongation ratio ( Fig. 3.9g) (Daniau et al., 2007). From these data we infer the progressive increase of biomass burning during the deglaciation and the interglacial interrupted by two periods of burning decrease during the cold and dry YD and the warm and wet climatic optimum in the Iberian Peninsula (Chabaud et al., 2014;Gomes et al., 2020). The comparison with pollen-based vegetation changes detected in the same core (Chabaud et al., 2014) confirms that the sharp decrease in biomass burning, associated with elongated particles during the YD, is associated with the expansion of semi-desert vegetation (Fig. 3.9f). ...
... It is punctuated by a sharp trough between 12.9 and 11.8 kyr corresponding to the Younger Dryas (YD) and relatively low values between 10 and 8 kyr, both periods characterised by a high elongation ratio (Fig. 9g) (Daniau et al., 2007). From these data we infer the progressive increase of biomass burning during the deglaciation and the interglacial interrupted by two periods of burning decrease during the cold and dry YD and the warm and wet climatic optimum in the Iberian Peninsula (Chabaud et al., 2014;Gomes et al., 2020). ...
Thesis
Les microcharbons ont été analysés dans des sédiments marins de surface prélevés dans l’Océan Atlantique Est au large de la Péninsule Ibérique et en Mer Méditerranée Occidentale. Ils ont également été analysés dans un enregistrement sédimentaire du Golfe du Lion couvrant les derniers 8500 ans. Les paramètres de concentrations et de morphométries des microcharbons des sédiments de surface ont été comparés aux facteurs pouvant influencer la production, le transport et le dépôt de microcharbons dans l’océan. A l’échelle de la Péninsule Ibérique, les concentrations élevées de microcharbons fortement allongés dans les sédiments de surface indiquent des incendies rares, de grandes tailles et de forte intensité. Des concentrations faibles de microcharbons faiblement allongés indiquent des incendies fréquents, plus petits et de plus faible intensité. A l’échelle de la Méditerranée Occidentale, des concentrations fortes de microcharbons faiblement allongés indiquent des feux de forte intensité sous des conditions climatiques fraiches ou des feux de faible intensité se propageant dans une végétation fermée et arbustive sous des conditions climatiques chaudes et sèches. Des concentrations plus fortes de microcharbons fortement allongés indiquent des incendies dans une végétation ouverte et mixte sous des conditions chaudes et sèches. Les résultats observés à l’échelle de la Péninsule Ibérique ont été appliqués à un enregistrement sédimentaire marin couvrant l’Holocène dans le sud-ouest de la Péninsule Ibérique. Ainsi de rares et grands feux de forte intensité sont décrits depuis 8000 ans coïncidant avec l’ouverture progressive de la végétation. Les résultats observés à l’échelle de la Méditerranée, ont été appliqués à un enregistrement sédimentaire marin dans le sud-est de la France. Ainsi entre 8500 et 2700 ans de rares petits feux de forte intensité sont décrits tandis que depuis 2700 ans est décrite une alternance de rares petits feux de forte intensité et feux fréquents et larges de faible intensité. L’occurrence des feux dans le sud-est de la France pourraient être liée à des évènements froids, à la diminution de l’activité solaire et aux modes négatifs de l’Oscillation Nord Atlantique et de la condition atmosphérique de l’East Atlantic.
... The overall climatic trends in the Iberia have been discussed in three main intervals: an early-Holocene humid period (~ 11.5 -7.0 ka), a mid-Holocene characterized by an increase in climatic variability, linked to decreasing Northern Hemisphere insolation (~ 7.0 -5.5 ka) with a relative humidity maximum been pointed to coastal Western Iberia (Roberts et al., 2011;Mauri et al., 2015;Walczak et al., 2015) with more Atlantic climate characteristics (Gomes et al., 2020), and a generalized late-Holocene period of aridification (since ~ 5.5 ka) (Jalut et al., 2009;Pérez-Obiol et al., 2011;Walczak et al., 2015). These terms refer to the Holocene's time intervals, such as 'Early Holocene', 'Middle or Mid Holocene', and 'Late Holocene' are often used (e.g., Walker et al., 2012;Magny et al., 2013;Thatcher et al., 2020a). ...
... The IP have experienced a wide spectrum of climatic and microclimate conditions, a result of the effects such as latitude (e.g., Moreno, et al., 2014a;Morellón et al., 2018;Naughton et al., 2019) and the orographic complexity and proximity to the ocean (continentality). These regional diversities and their interactions with the above-mentioned climate drivers could explain the spatiotemporal climate variability observed in the Iberia throughout the Holocene (e.g., Carrión et al., 2010;Tarroso et al., 2014;Sánchez-López et al. 2016;Abrantes, et al., 2017;Morellón et al., 2018;Baldini et al., 2019;Thatcher et al., 2020a;Naughton et al., 2019;Gomes et al., 2020). Consequently, although challenging due to the diversity of competing for climatic factors, the paleoclimate studies of this region can provide key insights into how the North Atlantic zone, particularly the Eastern North Atlantic, Southwestern European, and Western Mediterranean areas, respond to global climate change. ...
... These results seem to relate to the generalised forest decline identified in diverse sequences of SW Iberia from 7000 cal yrs. B.P. onwards (Cambourieu-Nebout et al., 2009;Chabaud et al., 2015;Fletcher et al., 2007;Fletcher and Sánchez Goñi, 2008;Gomes et al., 2020;Vis et al., 2010). ...
Article
In this paper we present the results of a multidisciplinary study performed in the Carrasqueira valley, a tributary of the River Sado (SW Portugal), aimed at characterising the Holocene environmental conditions during the late Mesolithic occupation of this valley. Our findings are based on a 13.5 m long sediment core (Arez3) collected on the alluvial plain close to a late Mesolithic shell midden, the Arapouco site. The results of the multiproxy analyses (texture, magnetic susceptibility, organic composition and chemistry, n-alkanes and palynology) point to a greater marine influence between ca. 8850 cal yrs. B.P. (at the core base) and ca. 7450 cal yrs. B.P. (at 750 cm below mean sea level (MSL)) and the existence of an environment similar to the present-day central estuarine basin. At this point in time, sedimentation rates were lower than the rate of sea-level rise, resulting in the formation of a drowned area with intertidal environments developing on the less incised margins. After 7040 cal yrs. B.P. the contribution of organic matter from terrestrial plants and freshwater phytoplankton to the sediment increased, reflecting a change in the sedimentary pattern, with the estuarine environments progressively giving way to freshwater environments. After the Middle Holocene (ca. 6530 cal yrs. B.P.), negative shifts of δ¹⁵N to values ~0‰ point to hyper-eutrophication and cyanobacteria bloom episodes under backswamp conditions. According to these results, the estuarine environment prevailed in the area until 7040 cal yrs. B.P. (5090 cal yrs. B.C.; 390 cm below MSL), i.e., during the Mesolithic occupation of the valley, allowing for the occurrence and for the exploitation of marine shellfish and fish by these hunter-gatherer communities at the proximity of the downstream occupation areas.
... These latter periods are characterised by a global trend of stabilisation under warmer and wetter climatic conditions (Cacho et al. 2001), interrupted by several cooling events, the most notable being the 8.2 kya climatic event (López-Sáez et al. 2007). These climatic changes had a transformative effect on the environment with a sea level rise (Peltier 2002) and the expansion of deciduous and thermophilous forests (Carrión et al. 2010;Gomes et al. 2020). ...
Preprint
Archaeologists have been reconstructing interactions among hunter-gatherer populations for a long time. These exchanges are reflected in the movements of raw materials and symbolic objects which are found far from their original sources. A social network, i.e., the structure constituted by these interactions, is a well-established concept in archaeology that is used to estimate the connectivity of hunter-gatherer populations. The heuristic potential of formal network analysis, however, has been scarcely exploited in prehistoric hunter-gatherer archaeology. In this work, we use Social Network Analysis to analyse the interactions among hunter-gatherers on the Iberian Peninsula in the Early and Late Mesolithic (10.200 to 7600 cal BP). Ornaments are accepted markers of non-utilitarian mobility and exchange. We thus used ornaments as proxies for social interaction and constructed one network for each phase of the Iberian Mesolithic. We applied three levels of analysis: first, we characterised the overall structure of the networks. Second, we performed node-level analysis to uncover the most relevant nodes in each network. Finally, we conducted an exploratory analysis of the networks’ spatial characteristics. No significant differences were found between the overall network topology of the Early and Late Mesolithic. This suggests that the interaction patterns among human groups did not change significantly on the Iberian Peninsula. Moreover, the spatial analysis showed that most interactions between human groups took place over distances under 300 km, but that specific ornament types such as C. rustica and Trivia sp. were distributed over more extensive distances. To summarise, our findings suggest that Iberian Mesolithic social networks were maintained through a period of environmental, demographic, and cultural transformation. In addition, the interactions took place at different scales of social integration.
... sedimentological records of lakes in the Sierra Nevada that mark the beginning of this aridification around 7 ka BP (Mesa-Fernández et al., 2018), but pollen records from this area as well as those collected from marine sequences of the Alboran Sea place it later (Fig. 2.6), in c. 5 ka BP (Fletcher et al., 2013;Ramos-Román et al., 2018). Available pollen records from marine cores of the western margin of the peninsula show a very different evolution, with the development of a Mediterranean temperate forest in the early Holocene while evolving from 7.5 ka BP to more Atlantic or oceanic conditions (Gomes et al., 2020). This reflects, on the one hand, the diversity of rainfall patterns throughout the Iberian geography and, on the other hand, the different sensitivities of the applied proxies to seasonality and other environmental parameters. ...
Chapter
This chapter reviews the evolution of the glacial–interglacial cycles of the Quaternary on a global scale and then focuses on the context of the Iberian Peninsula, mainly reviewing studies based on marine records and complemented by some terrestrial records, particularly those of lakes and speleothems. After briefly describing some of the most remarkable results from records covering several glacial–interglacial cycles, this chapter analyzes the climate variability of the Last Glacial Cycle, deglaciation, and the Holocene. The discussed results focus on analyzing the changes in the evolution of the marine temperature around the peninsula and its hydrologic availability. These results are complemented with other relevant variables to understand the functioning of the oceanography of the North Atlantic and the Mediterranean. This chapter tries to frame the changes described for the Iberian Peninsula with the climatic evolution on a global scale.
... It is punctuated by a sharp trough between 12.9 and 11.8 kyr corresponding to the Younger Dryas (YD) and relatively low values between 10 and 8 kyr, both periods characterised by a high elongation ratio (Fig. 9g) (Daniau et al., 2007). From these data we infer the progressive increase of biomass burning during the deglaciation and the interglacial interrupted by two periods of burning decrease during the cold and dry YD and the warm and wet climatic optimum in the Iberian Peninsula (Chabaud et al., 2014;Gomes et al., 2020). ...
Article
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Marine microcharcoal records provide invaluable information to understand changes in biomass burning and its drivers over multiple glacial and interglacial cycles and to evaluate fire models under warmer climates than today. However, quantitative reconstructions of burnt area, fire intensity and frequency from these records need calibration studies of the current fire-microcharcoal relationship. Here, we present the analysis of microcharcoal concentration and morphology in 102 core-top sediment samples collected in the Iberian margin and the Gulf of Cádiz. We show that microcharcoal concentrations are influenced by the water depth or the distance from the river mouth. At regional scale, the mean microcharcoal concentrations and microcharcoal elongation (length to width ratio) show a marked latitudinal variation in their distribution, primarily controlled by the type of burnt vegetation in the adjacent continent. High microcharcoal concentrations in marine sediments represent rare, large and intense fires in open Mediterranean woodlands. Based on these results, the increasing trend of microcharcoal concentrations recorded since 8 ka in the well-known marine sedimentary core MD95-2042 off the Iberian margin indicates the occurrence of large and infrequent fires of high intensity due to the progressive degradation of the Mediterranean forest and the expansion of shrublands.
... The impact of precession is also clearly noted in the development of the Mediterranean sclerophylls, although tenuous, during the intervals of precession minima and in the expansion of heathlands under precession maxima (Fig. 3). This vegetation response is consistent with the influence of precession on the seasonal contrast over the Mediterranean region, south of 40 N, (Ruddiman and McIntyre, 1984), and its imprint on the SW Iberian vegetation across the middle-to late Pleistocene and Holocene (e.g., Oliveira et al., 2017;S anchez Goñi et al., 2018S anchez Goñi et al., , 2019Gomes et al., 2020). ...
Article
Marine Isotope Stage (MIS) 13,~533e478 ka, has received particular attention due to the unexpected enhancement of monsoon systems under a cool climate characterized by lower atmospheric CO 2 and larger ice volume than many other interglacials. Key questions remain about its regional expression (intensity, climate variability, length), and underlying forcing factors, in particular at the mid-latitudes. Here we examine the SW Iberian vegetation, terrestrial climate and sea surface temperature (SST) variability during MIS 13 by combining pollen and biomarker data from IODP Site U1385 with climate-model experiments. We show, for the first time, that despite strong precessional forcing, MIS 13 stands out for its large forest expansions with a reduced Mediterranean character alternating with muted forest contractions, indicating that this stage is marked by a cool-temperate climate regime with high levels of humidity. Results of our data-model comparison reveal that MIS 13 orbitally driven SW Iberian climate and vegetation changes are modulated by the relatively strong ice-sheet forcing. We find that the Northern Hemisphere ice-sheets prescribed at the MIS 13 climate optimum reinforce the insolation effect by increasing the tree fraction and both winter and summer precipitation. We propose that the interactions between ice-sheets and major atmospheric circulation systems may have resulted in the persistent influence of the mid-latitude cells over the SW Iberian region, which led to intensified moisture availability and reduced seasonality, and, in turn, to a pronounced expansion of the temperate forest.
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The relative importance of climate change and human activities in influencing regional fire regimes during the Holocene is still a matter of debate. The introduction of agriculture during the Neolithic provides an opportunity to examine the impact of human activities on fire regimes. Here, we examine changes in fire regimes across Iberia between 10,000 and 3500 cal. BP, reconstructed using sedimentary charcoal records. We compare the regional fire history with estimates of changes in population size, reconstructed based on summed probability distributions of radiocarbon dates on archaeological material. We also compare the fire records and population reconstructions with the timing of the onset of agriculture across the region as indicated by archaeological data. For Iberia as a whole, there are two intervals of rapid population increase centred on ca. 7400 and ca. 5400 cal. BP. Periods of rapid population growth, either for the region as a whole or more locally, do not closely align with changes in charcoal accumulation. Charcoal accumulation had already begun to increase ca. 400 years prior to the onset of the Neolithic and continued to increase for ca. 750 years afterwards, indicating that changes in fire are not directly associated with the introduction of agriculture. Similarly, there is no direct relationship between changes in charcoal accumulation and later intervals of rapid population growth. There is also no significant relationship between population size and charcoal accumulation across the period of analysis. Our analyses show that the introduction of agriculture and subsequent increases in population are not directly linked with changes in fire regimes in Iberia and support the idea that changes in fire are largely driven by other factors such as climate.
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Cooling during most of the past two millennia has been widely recognized and has been inferred to be the dominant global temperature trend of the past 11,700 years (the Holocene epoch). However, long-term cooling has been difficult to reconcile with global forcing, and climate models consistently simulate long-term warming. The divergence between simulations and reconstructions emerges primarily for northern mid-latitudes, for which pronounced cooling has been inferred from marine and coastal records using multiple approaches. Here we show that temperatures reconstructed from sub-fossil pollen from 642 sites across North America and Europe closely match simulations, and that long-term warming, not cooling, defined the Holocene until around 2,000 years ago. The reconstructions indicate that evidence of long-term cooling was limited to North Atlantic records. Early Holocene temperatures on the continents were more than two degrees Celsius below those of the past two millennia, consistent with the simulated effects of remnant ice sheets in the climate model Community Climate System Model 3 (CCSM3). CCSM3 simulates increases in 'growing degree days'-a measure of the accumulated warmth above five degrees Celsius per year-of more than 300 kelvin days over the Holocene, consistent with inferences from the pollen data. It also simulates a decrease in mean summer temperatures of more than two degrees Celsius, which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records. Despite the differing trends, pollen- and marine-based reconstructions are correlated at millennial-to-centennial scales, probably in response to ice-sheet and meltwater dynamics, and to stochastic dynamics similar to the temperature variations produced by CCSM3. Although our results depend on a single source of palaeoclimatic data (pollen) and a single climate-model simulation, they reinforce the notion that climate models can adequately simulate climates for periods other than the present-day. They also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11,000 years.
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Pollen from deep-sea sedimentary sequences provides an integrated regional reconstruction of vegetation and climate (temperature, precipitation, and seasonality) on the adjacent continent. More importantly, the direct correlation of pollen, marine and ice indicators allows comparison of the atmospheric climatic changes that have affected the continent with the response of the Earth's other reservoirs, i.e., the oceans and cryosphere, without any chronological uncertainty. The study of long continuous pollen records from the European margin has revealed a changing and complex interplay between European climate, North Atlantic sea surface temperatures (SSTs), ice growth and decay, and high-and low-latitude forcing at orbital and millennial timescales. These records have shown that the amplitude of the last five terrestrial interglacials was similar above 40 • N, while below 40 • N their magnitude differed due to precession-modulated changes in seasonality and, particularly, winter precipitation. These records also showed that vegetation response was in dynamic equilibrium with rapid climate changes such as the Dangaard-Oeschger (D-O) cycles and Heinrich events, similar in magnitude and velocity to the ongoing global warming. However, the magnitude of the millennial-scale warming events of the last glacial period was regionally-specific. Precession seems to have imprinted regions below 40 • N while obliquity, which controls average annual temperature, probably mediated the impact of DO warming events above 40 • N. A decoupling between high-and low-latitude climate was also observed within last glacial warm (Greenland interstadials) and cold phases (Greenland stadials). The synchronous response of western European vegetation/climate and eastern North Atlantic SSTs to DO cycles was not a pervasive feature throughout the Quaternary. During periods of ice growth such as MIS 5a/4, MIS 11c/b and MIS 19c/b, repeated millennial-scale cold-air/warm-sea decoupling events occurred on the European margin superimposed to a long-term air-sea decoupling trend. Strong air-sea thermal contrasts promoted the production of water vapor that was then transported northward by the westerlies and fed ice sheets. This interaction between long-term and shorter timescale climatic variability may have amplified insolation decreases and thus explain the Ice Ages. This hypothesis should be tested by the integration of stochastic processes in Earth models of intermediate complexity.
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The suitability of MIS 11c and MIS 19c as analogues of our present interglacial and its natural evolution is still debated. Here we examine the regional expression of the Holocene and its orbital analogues over SW Iberia using a model–data comparison approach. Regional tree fraction and climate based on snapshot and transient experiments using the LOVECLIM model are evaluated against the terrestrial–marine profiles from Site U1385 documenting the regional vegetation and climatic changes. The pollen-based reconstructions show a larger forest optimum during the Holocene compared to MIS 11c and MIS 19c, putting into question their analogy in SW Europe. Pollen-based and model results indicate reduced MIS 11c forest cover compared to the Holocene primarily driven by lower winter precipitation, which is critical for Mediterranean forest development. Decreased precipitation was possibly induced by the amplified MIS 11c latitudinal insolation and temperature gradient that shifted the westerlies northwards. In contrast, the reconstructed lower forest optimum at MIS 19c is not reproduced by the simulations probably due to the lack of Eurasian ice sheets and its related feedbacks in the model. Transient experiments with time-varying insolation and CO2 reveal that the SW Iberian forest dynamics over the interglacials are mostly coupled to changes in winter precipitation mainly controlled by precession, CO2 playing a negligible role. Model simulations reproduce the observed persistent vegetation changes at millennial time scales in SW Iberia and the strong forest reductions marking the end of the interglacial “optimum”.
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The coastal vegetation of the Iberian Peninsula (including the Balearic Islands) is represented by a set of plant communities that colonize biotopes as diverse as sea cliffs, beaches, dunes, marshes and estuaries. From a geographical point of view, we have differentiated the following sections: in the north, the coastal strip extending from the mouth of the Bidasoa River, the political border with France, up to Ría de Aveiro in Portugal integrates into the Atlantic-European biogeographic Province (covering the sectors Cantabrian-Basque, Galician-Asturian and Galician-Portuguese) and exhibits a great diversity in the vegetation of coastal cliffs. Dune vegetation is well represented in Corrubedo, Liencres, Somo, Zarautz and other places; communities of marshes and estuaries also show strong contrasts (Galician Rías Bajas, Santoña, Urdaibai) and constitute biogeographic boundaries of species and plant communities with the rest of the Iberian coasts.
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The following biogeographic units for the Iberian Peninsula and Balearic Islands have been established according to the conceptual geobotanical proposals of Rivas-Martínez et al. (Parte I. Itinera Geobotanica 17:5–436, 2007; Parte II. Itinera Geobotanica 18 (1):5–424, 2011a; Glob Geobot 1(1):1–634, 2011b; Int J Geobot Res 1(1):21–40, 2011c and Int J Geobot Res 4(1):1–64, 2014): one kingdom (Holartic), two regions (Eurosiberian, Mediterranean), eight provinces, 16 subprovinces, 49 sectors and 264 districts. The potential natural vegetation: climatophilous, climato-temporihygrophilous, xerophilous, hygrophilous sigmetum or geopermasigmetum (series and geopermaseries) are shown for each province and sector. We also point out some of their specific features. Biogeographic maps up to district level are shown.
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Quaternary records provide an opportunity to examine the nature of the vegetation and fire responses to rapid past climate changes comparable in velocity and magnitude to those expected in the 21st-century. The best documented examples of rapid climate change in the past are the warming events associated with the Dansgaard–Oeschger (D–O) cycles during the last glacial period, which were sufficiently large to have had a potential feedback through changes in albedo and greenhouse gas emissions on climate. Previous reconstructions of vegetation and fire changes during the D–O cycles used independently constructed age models, making it difficult to compare the changes between different sites and regions. Here, we present the ACER (Abrupt Climate Changes and Environmental Responses) global database, which includes 93 pollen records from the last glacial period (73–15 ka) with a temporal resolution better than 1000 years, 32 of which also provide charcoal records. A harmonized and consistent chronology based on radiometric dating (¹⁴C, ²³⁴U∕²³⁰Th, optically stimulated luminescence (OSL), ⁴⁰Ar∕³⁹Ar-dated tephra layers) has been constructed for 86 of these records, although in some cases additional information was derived using common control points based on event stratigraphy. The ACER database compiles metadata including geospatial and dating information, pollen and charcoal counts, and pollen percentages of the characteristic biomes and is archived in Microsoft AccessTM at https://doi.org/10.1594/PANGAEA.870867.
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Climate seasonality is an essential element in the Earth system. Long-term global climate change is largely forced, through seasonal scale processes and feedbacks, by changes in the seasonal distribution of the solar flux of energy on the Earth surface. Still, and for a variety of reasons, annual means of climate variables are being reconstructed in most paleoclimate studies, although climate is properly defined by the annual cycle of these variables. This results in an incomplete and sometimes biased documentation of the climate natural variability. We present here a brief overview of the significance of climate seasonality in the study of long-term climate change, of the techniques that have been developed to reconstruct climate seasonality, and the associated issues and challenges. We argue here that getting to the next level of understanding of natural climate variability requires a larger effort into the reconstruction of past climate seasonality.
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We created a new dataset of spatially interpolated monthly climate data for global land areas at a very high spatial resolution (approximately 1 km 2). We included monthly temperature (minimum, maximum and average), precipitation, solar radiation, vapour pressure and wind speed, aggregated across a target temporal range of 1970–2000, using data from between 9000 and 60 000 weather stations. Weather station data were interpolated using thin-plate splines with covariates including elevation, distance to the coast and three satellite-derived covariates: maximum and minimum land surface temperature as well as cloud cover, obtained with the MODIS satellite platform. Interpolation was done for 23 regions of varying size depending on station density. Satellite data improved prediction accuracy for temperature variables 5–15% (0.07–0.17 ∘ C), particularly for areas with a low station density, although prediction error remained high in such regions for all climate variables. Contributions of satellite covariates were mostly negligible for the other variables, although their importance varied by region. In contrast to the common approach to use a single model formulation for the entire world, we constructed the final product by selecting the best performing model for each region and variable. Global cross-validation correlations were ≥ 0.99 for temperature and humidity, 0.86 for precipitation and 0.76 for wind speed. The fact that most of our climate surface estimates were only marginally improved by use of satellite covariates highlights the importance having a dense, high-quality network of climate station data.
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Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability, which is notoriously difficult for models to reproduce. We propose here a new proxy-based climate synthesis synthesis and its comparison – at a regional (∼ 100 km) level – with a regional climate model to examine (i) opposing northern and southern precipitation regimes and (ii) an east-to-west precipitation dipole during the Holocene across the Mediterranean basin. Using precipitation estimates inferred from marine and terrestrial pollen archives, we focus on the early to mid-Holocene (8000 to 6000 cal yr BP) and the late Holocene (4000 to 2000 cal yr BP), to test these hypotheses on a Mediterranean-wide scale. Special attention was given to the reconstruction of season-specific climate information, notably summer and winter precipitation. The reconstructed climatic trends corroborate the north–south partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the south–central and eastern Mediterranean regions, while drier conditions prevailed from 45° N northwards. These patterns then reverse during the late Holocene. With regard to the existence of a west–east precipitation dipole during the Holocene, our results show that the strength of this dipole is strongly linked to the reconstructed seasonal parameter; early-Holocene summers show a clear east–west division, with summer precipitation having been highest in Greece and the eastern Mediterranean and lowest over Italy and the western Mediterranean. Summer precipitation in the east remained above modern values, even during the late-Holocene interval. In contrast, winter precipitation signals are less spatially coherent during the early Holocene but low precipitation is evidenced during the late Holocene. A general drying trend occurred from the early to late Holocene, particularly in the central and eastern Mediterranean. For the same time intervals, pollen-inferred precipitation estimates were compared with model outputs, based on a regional-scale downscaling (HadRM3) of a set of global climate-model simulations (HadAM3). The high-resolution detail achieved through the downscaling is intended to enable a better comparison between site-based paleo-reconstructions and gridded model data in the complex terrain of the Mediterranean; the model outputs and pollen-inferred precipitation estimates show some overall correspondence, though modeled changes are small and at the absolute margins of statistical significance. There are suggestions that the eastern Mediterranean experienced wetter summer conditions than present during the early and late Holocene; the drying trend in winter from the early to the late Holocene also appears to be simulated. The use of this high-resolution regional climate model highlights how the inherently patchy nature of climate signals and paleo-records in the Mediterranean basin may lead to local signals that are much stronger than the large-scale pattern would suggest. Nevertheless, the east-to-west division in summer precipitation seems more marked in the pollen reconstruction than in the model outputs. The footprint of the anomalies (like today, or dry winters and wet summers) has some similarities to modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive Arctic Oscillation–North Atlantic Oscillation (AO–NAO)) and a weak westerly circulation in summer associated with anticyclonic blocking; however, there also remain important differences between the paleo-simulations and these analogues. The regional climate model, consistent with other global models, does not suggest an extension of the African summer monsoon into the Mediterranean. Therefore, the extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question.
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Climatic variability of Marine Isotope Stage (MIS) 11 is examined using a new high-resolution direct land–sea comparison from the SW Iberian margin Site U1385. This study, based on pollen and biomarker analyses, documents regional vegetation, terrestrial climate and sea surface temperature (SST) variability. Suborbital climate variability is revealed by a series of forest decline events suggesting repeated cooling and drying episodes in SW Iberia throughout MIS 11. Only the most severe events on land are coeval with SST decreases, under larger ice volume conditions. Our study shows that the diverse expression (magnitude, character and duration) of the millennial-scale cooling events in SW Europe relies on atmospheric and oceanic processes whose predominant role likely depends on baseline climate states. Repeated atmospheric shifts recalling the positive North Atlantic Oscillation mode, inducing dryness in SW Iberia without systematical SST changes, would prevail during low ice volume conditions. In contrast, disruption of the Atlantic meridional overturning circulation (AMOC), related to iceberg discharges, colder SST and increased hydrological regime, would be responsible for the coldest and driest episodes of prolonged duration in SW Europe.
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Abrupt changes in Western Mediterranean climate during the last deglaciation (20 to 6 cal ka BP) are detected in marine core MD95-2043 (Alboran Sea) through the investigation of high-resolution pollen data and pollen-based climate reconstructions by the modern analogue technique (MAT) for annual precipitation ( P <sub>ann</sub>) and mean temperatures of the coldest and warmest months (MTCO and MTWA). Changes in temperate Mediterranean forest development and composition and MAT reconstructions indicate major climatic shifts with parallel temperature and precipitation changes at the onsets of Heinrich stadial 1 (equivalent to the Oldest Dryas), the Bölling-Allerød (BA), and the Younger Dryas (YD). Multi-centennial-scale oscillations in forest development occurred throughout the BA, YD, and early Holocene. Shifts in vegetation composition and ( P <sub>ann</sub> reconstructions indicate that forest declines occurred during dry, and generally cool, episodes centred at 14.0, 13.3, 12.9, 11.8, 10.7, 10.1, 9.2, 8.3 and 7.4 cal ka BP. The forest record also suggests multiple, low-amplitude Preboreal (PB) climate oscillations, and a marked increase in moisture availability for forest development at the end of the PB at 10.6 cal ka BP. Dry atmospheric conditions in the Western Mediterranean occurred in phase with Lateglacial events of high-latitude cooling including GI-1d (Older Dryas), GI-1b (Intra-Allerød Cold Period) and GS-1 (YD), and during Holocene events associated with high-latitude cooling, meltwater pulses and N. Atlantic ice-rafting. A possible climatic mechanism for the recurrence of dry intervals and an opposed regional precipitation pattern with respect to Western-central Europe relates to the dynamics of the westerlies and the prevalence of atmospheric blocking highs. Comparison of radiocarbon and ice-core ages for well-defined climatic transitions in the forest record suggests possible enhancement of marine reservoir ages in the Alboran Sea by 200 years (surface water age 600 years) during the Lateglacial.
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SUMMARY. Vegetation Analysis, Pollen Analysis, and Pollen Morphology have been considered together to study the present-day and past Holocene vegetation in the Carvalhal region and surrounding landscape. In part IV - Pollen Morphology of Portuguese Ericales - we propose a new terminology for describing tetrads of Ericales. Later, 14 pollen types (out of the 18 species recognized in the Portuguese Flora) were established and characterized. Finally, a dichotomous key was prepared, permitting detailed identification of the Ericales pollen in the cores studied. The importance of this result in palaeoecological research is exceptional, for it permitted the identification of three heathland types, preponderant elements of the past vegetation of NW Alentejo. Preliminary vegetation analysis was done on the upland (dry-land) vegetation of the NW Alentejo dune district. The Braun-Blanquet approach was used, but no definitive syntaxonomic hierarchical classification was considered. A provisional classification scheme is proposed, based on the informal "community" and "variant". 16 vegetation communities and 12 variants are described from the littoral unstable dunes and from the inland old dune region: Communities occurring on the littoral unstable dunes (AMMOPHILETEA) 1. the Elymus farctus - Otanthus maritimus community, an AMMOPHILION high beach prairie on predunes or incipient dunes. 2. the Artemisia crithmifolia - Ammophila arenaria community, a AMMOPHILION first dune prairie. 3. the Artemisia crithmifolia - Helichrysum italicum community, a AMMOPHILION community on the exposed flanks of the second dunes; 4. the Helychrysum italicum - Pancratium maritimum community, a AMMOPHILION "compression" community between the AMMOPHILION and the COREMION vegetation on overlapping dune systems. 5. the Helichrysum italicum - Ononis ramosissima community, a COREMION community on the exposed flanks of the third dunes (occurring also in the first dune depressions). 6. the Juniperus phoenicea - Corema album community, a COREMION scrub on the protected flanks of the third dunes. Communities occurring on the inland old dunes: 7. the Halimium halimifolium - Cistus salvifolius community, a STAURACANTHO GENISTOIDES - HALIMIETALIA COMMUTATI degraded shrubland on regosols and eroded podzols. 8. the Santolina impressa - Stauracanthus genistoides community, a STAURACANTHO GENISTOIDES - HALIMIETALIA COMMUTATI degraded "charneca". 9. the Thymus capitellatus - Corynephorus canescens community, a TUBERARIETEA GUTTATAE developed prairie on long-term fallows. 10. the Vulpia alopecurus - Bromus rigidus community, a TUBERARIETEA GUTTATAE weed community on fallows. 11. the Arctotheca calendula - Hordeum murinum ssp. leporinum community, a CHENOPODIETALIA ruderal (nitrophilous) prairie. 12. the Erica scoparia - Halimium lasianthum community, a CALLUNO - ULICETALIA high heathland in old dune depressions. 13. the Calluna vulgaris - Ulex parviflorus community, a CALLUNO - ULICETALIA low heathland on (degraded?) podzols. 14. the Cistus psilosepalus - Scirpus holoschoenus community, a CALLUNO - ULICETALIA dawrfshrubland vegetation on the margins of the wetlands. 15. the Scirpus pseudosetaceus - Eleocharis multicaulis community, an ISOETONANO-JUNCETEA low prairie on small dune depressions, with temporary wet soils. 16. the Anagallis monelli - Scirpus holoschoenus community, a AMMOPHILETEA prairie on the coastal lagoon margins, subjected to seasonal flooding. Following the unedited results of QUEIROZ (manuscr.) a concise description of the main vegetation units of the NW Alentejo wetlands is provided. This scheme is valuable for dealing with the palaeoecological reconstruction of past wetland vegetation of the basins. Palaeoecological studies included pollen analysis of three cores - Lagoa Travessa I (LT.I), Lagoa Travessa II (LT.II), and Figueira de Baixo (FIG). In addition to pollen, non-pollinic microfossils were systematically counted in the FIG profile. Special emphasis was directed to the pollen identification procedure itself: We systematically used 1000 x oil immersion observation of different aspects of the pollen grains, which could be rotated by locally pressing a needle on the coverglass, This time-consuming method proved quite useful for recording a rich pollen flora in the diagrams and for solving difficult pollen identification problems (frequent in the case of the Ericales, Quercus, or Umbelliferae, among others...). The three diagrams are only partly synchronous. With the help of a set of 18 C14 dates, a "regional pollen-zone correlation scheme" was reconstructed. It includes 8 regional pollen zones and several subzones. These zonation units are not all homogeneous in character. They correspond to chronozones, regional or site assemblage pollen zones, or a hybrid between both. Main palaeoecological results are summarized in part III and are only referred here briefly: Palaeovegetation units: Prior to the description of the diagrams, zone by zone, or before any interpretation of past vegetation history, eco-stratigraphical pollen groups were identified and a first scheme of palaeovegetation units was erected accordingly. Main palaeovegetation units identified in the NW Alentejo are: In the dry-lands: - Maritime pinewoods; especially abundant in the Early Holocene, gradually declining after 6000 BP. - A Quercus faginea marcescent forest. Extensive formations before 3000 PB, drastically declining afterwards and relictual nowadays. - An Erica scoparia heathland (UCI-A), preclimacic, mesomediterranean humid/subhumid. Extensive during zone CAR-D (roughly from 4000 to 2000 BP), now relictual. - A more xeric heathland type (UCI-B) related to our present-day Calluna vulgaris - Ulex parviflorus community (Erica umbellata variant). The UCI-B heathland expands after man's clearance of forests and preclimacic heathlands (through cutting, fire) and is favoured by the expansion of degraded podzols. - A "charneca" vegetation (COR-B) (disturbance shrubland dominated by Cistaceae and spiny Leguminosae). COR-B charneca expands under strong human impact on the vegetation and subsequent deep podzol erosion. In the littoral dunes: - A COR-A scrub, a Corema - Juniperus formation, which we relate to our present-day COREMION Juniperus phoenicea - Corema album community. Especially abundant in the Carvalhal coastal dry-lands from 6000 to 4000 BP. In the freshwater wet-lands: - A Alnus/Salix riparian forest; edaphic climax on the fluvial wetlands. Extensive formations before 3000 BP, residual nowadays. - An Erica erigena wet heathland (from the ERICETO SPHAGNETALIA). Edaphic climax on (oligohaline?) coastal freshwater lagoons. - A Myrica gale wet scrub (replacement community on the wet peat marshes). Especially expanding on eutrophic conditions, enhanced by strong upland and lowland human impact (mainly after the Bronze Age). - A MOLINIETALIA related fen vegetation (another replacement community). The MOL fen expands under strong zoo-anthropogenic pressure in the wetlands, most probably on grazed wet marshes and moors. In the littoral lowlands: - An estuarine vegetation especially characterizing the very high salt-marshes of the storm flooding zone. This PAV/CHE palaeovegetation is especially evident in the Carvalhal Palaeoestuary from around 5 700 to 4000 BP. Two main themes conduct our interpretation on vegetation history and evolution: Littoral palaeoecology and Human impact. Littoral palaeoecology: Main palaeovegetation events related to the littoral history are ascribed to littoral transgressions and regressions in strict palaeoecological terms. These are pollen-zone episodes characterized respectively by an increased or decreased marine influence in the ecosystems. Littoral transgressions are generally defined by the simultaneous 1) expansion and/or inland ingression of the salt-marsh palaeovegetation (CHE and PAV), 2) drastic changes in the local conditions of the coastal mires (fluvio-marine clastic deposition on fluvial mires; inundation episodes in the marginal basins, occasionally with brackish water), and 3) decline (and/or inland retreat) of freshwater riparian woods. Littoral regressions are defined by the simultaneous 1) decline and/or seaward retreat of the salt marsh palaeovegetation (CHE and PAV), 2) terrestrialization processes in the coastal mires, and 3) expansion (and/or seaward advance) of freshwater riparian woods. Four littoral transgressions and three littoral regressions have been traced. The expansion and/or inland ingression of the littoral dune palaeovegetation occurs mainly during phases of increased proximity of the littoral zone to the LT and FIG coring sites (such as the Carvalhal Estuary Phase); their fluctuating patterns, however, do not strictly follow the transgression and regression episodes. The palaeoecological interpretation of these patterns remains complex. In addition to the transgressions and regressions, four main phases in the evolution of the Carvalhal littoral ecosystems have been considered: 1) The Coastal Retreat Phase - Coast line migrating inland to the vicinity of the LT basin. 2) The Carvalhal Estuary Phase - Important salt-marshes developed in a "mature" estuary system north of Lagoa Travessa. 3) The Travessa III Regression Phase ("Lagoa Travessa Hiatus") - The Carvalhal palaeoestuary is de-activated and a massive coastal sand barrier is formed north of Lagoa Travessa. 4) Barbaroxa I Transgression Phase - New inland ingressions of the littoral tide zone occur, but far from Lagoa Travessa. Human impact: Although still preliminary, palaeoecological results concerning human impact on the vegetation agree well with the archaeological evidence and with the chronology of cultural phases generally accepted for South Portugal. Before the Bronze Age, human impact in the Carvalhal region and surrounding landscape was diffuse and sometimes difficult to distinguish from other major factors of vegetation change such as climate or littoral dynamics. Early/Middle Neolithic communities may have had a certain influence in the old dune interfluves, clearing pinewoods and preclimacic heathlands. Late Neolithic and Calcolithic communities had a more extensive impact, affecting ecosystems in the interfluves, in the valley margins, and in the wetlands. The impact of human communities of the Southwest Bronze II culture was remarkable, affecting all types of ecosystems, and it may be related to the spread of small family-type settlements in NW Alentejo. There must have been a certain continuity in land-use during the Second Iron Age and the Roman Republican period, when a certain emphasis may have been given to the use of upland pastures. The land use of the "Early Roman Empire" was again remarkably strong and modern in character: expansion of wetland and upland pastures, emphasis on cultivated land (mainly cereal fields), exploitation of exotic tree crops. During the "Middle Roman Empire" this rural productive landscape declined strongly and was partly abandoned, to re-expand later on during the "Late Roman Empire". The last 1600 years are either missing from the Carvalhal pollen records or correspond to disturbed peat sequences, unsuitable for interpretation. An abundant and diversified non-pollinic microfossil assemblage (microremains of algae, fungi, invertebrata, and unknown remains) was recorded in the Figueira de Baixo profile. The highly dynamic and distinct patterns shown by these microfossil curves throughout the organic sequence are symptomatic of the importance of these assemblages for the reconstruction of local mire environments. A preliminary catalogue of types is presented in part V, following the approaches of Bas van Geel and collaborators from the Hugo de Vries Laboratory (Amsterdam).
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RESUMO Esta dissertação procura contribuir para o conhecimento da evolução da paisagem e da vegetação da planície litoral norte alentejana nos últimos quinze mil anos (Tardiglacial e Holocénico). Este estudo, embora parta duma investigação sobre os ecossistemas do presente, tem por centro a análise palinológica e macropaleobotânica de séries sedimentares orgânicas micro-estratificadas, preservadas nas lagoas e turfeiras existentes numa estreita faixa litoral entre Lisboa e Sines, com incidência nas regiões das lagoas de Albufeira e Stº André. Procura-se uma abordagem multidisciplinar, integrando temas de pesquisa, métodos, e linguagens da Ecologia, Geobotânica, Geomorfologia, Paleoecologia e Paleobotânica, numa estratégia abrangente, no âmbito de uma Ecologia Histórica da Paisagem. Como ponto de partida procura descrever-se a ecofisiografia da paisagem actual - formas de relevo (unidades geomórficas) e zonação fito-fisionómica (formações vegetais) da região, com particular ênfase nos habitats de solos húmidas (não salgados). Definem-se assim os principais tipos de turfeiras litorais existentes no nosso país – turfeiras interfluviais e turfeiras perimarinhas (interdunares, fluviais adjacentes e vestibulares), com base nas suas características ecohidrológicas e, principalmente tendo em conta a sua génese. É também delineada uma zonação hidrosserial (sucessão ecológica palustre) para as turfeiras litorais do noroeste alentejano, partindo-se de uma caracterização duplamente fito-fisionómica e hidrogeomórfica. Quatro principais estádios hidrosseriais (ou zonas hidrotopográficas) são aqui reconhecidos: a) zona de sedimentação límnica; b) zona de sedimentação telmática; c) zona de sedimentação semiterrestre e d) zona de sedimentação terrestre. Estas zonas dispõem-se frequentemente em anéis concêntricos no seio das bacias, ocupando áreas variáveis sazonalmente, dependentes das variações do nível das águas. O capítulo 4 constitui um ensaio de geobotânica regional exclusivamente centrado na vegetação actual dos ecossistemas palustres do litoral norte alentejano. Inclui uma descrição detalhada das comunidades aquáticas e higrófilas, um primeiro esquema de classificação tipológica de âmbito fitossociológico territorial, finalmente um conjunto de modelos interpretativos ao nível da sinecologia das comunidades. Este estudo é baseado numa amostragem de 388 relevés de vegetação realizados segundo os métodos da escola de Braun-Blanquet, e suportado por técnicas numéricas de ordenação (programa DECORANA) e de classificação hierárquica (programa TWINSPAN). Para além da afinidade florística-sociológica o esquema final de hierarquização das comunidades vegetais considera ainda a sua estrutura fisionómica e a zonação hidrosserial, hidrogenética e ecoterritorial proposta para a região, elegendo 8 principais grupos ecofisionómicos: 1) vegetação límnica - a) natural e b) eutrofizada; 2) vegetação das zonas telmáticas - a) natural e b) artificializada; 3) vegetação das zonas de sedimentação semiterrestre - a) prados e juncais turfícolas seminaturais e b) ervedos altos artificializados; 4) prados arenícolas temporários; 5) matagais alagados_; 6) salgueirais semiterrestres; 7) vegetação das zonas de sedimentação terrestre - a) urzal húmido turfícola, b) mata climácica ribeirinha; e 8) vegetação das zonas de transição das depressões húmidas. Inclui-se finalmente uma referência à vegetação de influência salobra. Para além da vegetação pretende-se caracterizar de forma preliminar a diversidade ecológica dos habitats palustres da região – lagoas, turfeiras e depressões húmidas temporárias. Agregando vegetação, hidrologia, solos e unidades geomórficas descrevem-se assim zonas ecológicas no seio das bacias. Esta zonação é ilustrada por meio de pequenos transectos de vegetação, representativos da sucessão local. No caso da região de Fernão Ferro esta descrição é acompanhada pela cartografia dos habitats palustres (nas escalas 1:25000 e 1:2500) produzida com o apoio de técnicas de SIG, foto-interpretação e análise digital de aéro-fotos. O estudo paleoecológico baseia-se na análise palinológica de quatro sequências turfosas: Lagoa do Golfo (Fernão Ferro - Sesimbra); Estacada (Lagoa de Albufeira - Sesimbra); Poço do Barbaroxa de Cima (Santo André - Santiago do Cacém); e Vale da Carregueira (Santo André - Santiago do Cacém); Inclui sete diagramas polínicos, cinco diagramas de microfósseis não polínicos (principalmente algas e fungos) e um diagrama de macrorrestos. Os depósitos turfo-lodosos foram sondados e amostrados, recolhendo-se uma coluna de amostras cobrindo a totalidade da sequência sedimentar. Destas séries foram datadas 26 amostras pelo método do radiocarbono. A realização de concentrações polínicas seguiu os procedimentos laboratoriais tradicionais. Na identificação e contagem procurou-se optimizar a resolução taxonómica através da observação de grãos de pólen, com possibilidade de rotação, a ampliações de 1000 x; Procurou-se ainda contar um número alargado de grãos de pólen por amostra, com vista a obter diagramas com uma boa diversidade de tipos polínicos. A interpretação paleoecológica parte do reconhecimento de entidades de paleovegetação – comunidades vegetais do passado. Estas emergem do agrupar dos tipos polínicos que evidenciam afinidade simultânea de comportamento estratigráfico e ecológico (à luz do reconhecimento geobotânico actual). A descrição dos diagramas polínicos passa pela definição de dois sistemas de zonação polínica – um regional, referindo-se ao desenvolvimento da paisagem exterior à bacia palustre, ou seja aos “ecossistemas de solos secos”; outro local, considerando o desenvolvimento da vegetação higrófila da própria turfeira. A zonação polínica segue dois critérios complementares (um estático, outro dinâmico): 1) zonas de “conjunto polínico” definidas a partir de um conteúdo polínico homogéneo e mais ou menos estático; e 2) zonas de comportamento polínico baseadas nos padrões dinâmicos das curvas polínicas. O conjunto das conclusões cobre quatro temas principais de síntese paleoecológica – evolução paleoclimática, desenvolvimento litoral, história do impacte humano e, finalmente, evolução da vegetação natural. A síntese paleoclimática tem por base critérios de bioclimatologia e de paleolimnologia e estabelece uma periodização climática para o Tardiglacial e Holocénico. Os dados disponíveis para o Tardiglacial são escassos; permitem, no entanto, definir quatro principais períodos onde condições de secura e frio alternam com períodos de melhoramento climático – a uma fase de características frias/temperadas; sucede um período de agravamento climático; segue-se uma nova fase denotando um rápido e franco aquecimento; por sua vez seguida por um novo retorno a condições muito frias (Dryas Recente) que imediatamente antecede o início do Holocénico. O Holocénico é, dividido em três grandes períodos – Holocénico Antigo, Médio e Recente. Estes períodos, particularmente o Holocénico Médio e Recente, correspondem virtualmente a ciclos climáticos que se iniciam com fases húmidas a hiper-húmidas, e terminam com momentos de maior secura. No que respeita à evolução ecológica do litoral norte alentejano, consideram-se seis principais fases de evolução relativa do nível do mar e influência marinha nos ecossistemas costeiros corroborando e complementando uma primeira síntese de MATEUS (1992): uma fase de ingressão marinha, até cerca de 5500 BP, caracterizada pelo rápido recuo da linha de costa; uma fase de estabilização da linha de costa, até cerca de 4150 BP; uma fase regressiva, de progradação da linha de costa e terrestrialização dos sistemas perimarinhos interdunares (entre 4150 e 3250 BP); uma nova fase transgressiva, caracterizada por grandes inundações (pontualmente salobras) nos sistemas perimarinhos (até 1200 BP); seguida de uma nova fase de estabilização; finalmente (últimos 600 anos) uma nova fase de regressão litoral. O prevalecimento simultâneo de um clima húmido e um regime transgressivo tem como consequência a abertura ao mar (entre 3250 e 600 BP) de parte das ribeiras, hoje endorreicas. A história do impacte humano na paisagem da planície litoral norte alentejana apresenta fases marcadas quer por momentos de grande pressão antrópica nos ecossistemas, quer por períodos de abrandamento do impacte. Nesta tese complementa-se a periodização de MATEUS (1992) dando ênfase aos últimos 1500 anos, até aqui pouco documentados. Os primeiros sinais de impacte humano, ainda fraco e difuso, remontam a cerca de 6500 BP. Este vai gradualmente crescendo em expressão quantitativa e maior abrangência ecológica para culminar na Idade do Bronze numa fase de forte impacte, visível em todas as zonas ecológicas. Um refortalecimento progressivo dos sinais de impacte humano torna-se evidente, após um declínio datável da transição “Bronze - Ferro”, para se atingir um clímax de transformação da paisagem durante o Alto Império Romano. Após o declínio territorial do que MATEUS (1992) designa por “Império (Romano) Médio”, a que se sucede um retomar progressivo dos padrões de ocupação, é sobretudo a partir do século XIV que a transformação da paisagem reganha uma expressão dominante, desta vez com o carácter de um pastoralismo extensivo. Os séculos XVIII e XIX trazem a reflorestação e agricultura extensiva. Finalmente, a partir dos fins do século passado, a ocupação da paisagem especializa-se: florestação intensiva nos interflúvios arenícolas, agricultura especializada e intensiva nas baixas drenadas e terraços baixos dos vales. Durante os últimos 14 milénios assistimos nesta planície litoral à sucessão climaticamente ditada de diferentes pinhais e carvalhais na sua bipartição paisagística “vales - interflúvios”. Nos interflúvios do Tardiglacial dominam os pinhais silvestres; no interior dos vales os carvalhais (provavelmente negrais) e os vidoais. Com o início do Holocénico os pinhais bravos instalam-se em detrimento dos silvestres, a mata decídua (de carácter supramediterrâneo) refugia-se; dominam agora os carvalhais marcescentes (de Quercus faginea) e os amieirais (no fundo dos vales). Durante o Holocénico Médio (entre 8 e 3 Ka BP.), à medida que o clima tende para a secura, as árvores perenifólias esclerófilas ganham terreno e provavelmente individualidade vegetacional: o zambujal, o sobreiral, o pinhal manso - processo que é favorecido pelas comunidades humanas do Calcolíticas e do Bronze. Embora o Holocénico Recente se inicie sob os auspícios de um clima húmido, propício à “mesicidade” mesomediterrânea o papel do homem é agora dominante como factor directo e indirecto na modelação de uma paisagem vegetal de características xeromórficas. Do ponto de vista dos matos permanentes o início do Holocénico faz desaparecer os zimbrais crioxéricos e a lande temperada fria para promover os urzais-maquis altos de Erica scoparia e Quercus coccifera. Em situações oligotróficas as turfeiras oceânicas dão agora lugar ao urzal turfícola de Erica erigena. O acentuar do impacte humano conjugado com a secura climática propiciam a expansão da “charneca” xeromórfica (STAURACANTHOHALIMIETALIA), em detrimento dos urzais (CALLUNO-ULICETALIA), e ainda a eutrofização geral da vegetação das baixas. Dois estudos de Morfologia Polínica complementam esta tese: No primeiro, de natureza mais exploratória, investiga-se a morfologia polínica dos pinheiros de Portugal resultando a definição de quatro subtipos polínicos, exclusivamente baseados nas características morfológicas dos grãos de pólen. No segundo é apresentada uma análise detalhada da morfologia polínica das cistáceas portuguesas. Deste trabalho resulta a definição de nove tipos e sete subtipos polínicos dentro desta família. A história do pinheiro em Portugal e o papel dos estevais e sargaçais como componentes chave da evolução do coberto vegetal justificam esta direcção prévia da pesquisa como necessário prólogo à investigação paleoecológica. Por fim a tese inclui uma primeira proposta de léxico português de morfologia polínica. ABSTRACT This thesis aims to contribute to the reconstruction of the landscape and vegetation evolution in the north littoral of Alentejo, during the last fifteen thousand years (Lateglacial and Holocene). Although starting from the present-day ecosystems, the investigation is focused on the palynological and macropalaeobotanical analysis of organic sedimentary series, preserved in lakelet and peatmire basins of the littoral plane between Lisbon and Sines, especially those occurring on the wetland systems of Lagoa de Albufeira and Lagoa de Santo André. A multidisciplinary approach is intended, under the integration framework of the Historical Landscape Ecology, evoking the aims, methods and specific languages of the Ecology, Geobotany, Geomorphology, Palaeoecology and Palaeobotany. As a starting point, the present day landscape ecophysiography is approached, embracing together geomorphic (relief) and phyto-physiognomic (vegetation) units, with a special emphasis on the wetland habitats. Both the interfluvial and perimarine peatmires in the region are typologically defined based on their eco-hydrological status and origin - the latter including three types - interdunal, fluvial (backswamps mires) and vestibular). The main ecological succession schemes for the north-west Alentejo wetlands are described and sumarized, following a phyto-physiognomic and hidro-geomorphic zonation of the basins. Four main successional stages (or hydro-topographic zones) are recognised, named according to their depositional environment: a) the limnic sedimentation zone; b) the telmatic sedimentation zone; c) the semiterrestrial sedimentation zone, and e) the terrestrial sedimentation zone. These zones are usually present as concentric rings in the basins, with variable surfaces, depending seasonally on water level changes. Chapter 4 consists on a regional geobotanical survey, exclusively focused on the present day vegetation of the north-west Alentejo fresh-water mires. It includes a detailed description of aquatic and hygrophilous plant communities, a first typological classification considered on a strict territorial basis, finally, a set of interpretative synecological models for the communities. This research is based on a set of 388 vegetation relevés, following the floristic-sociological inventory techniques of the Braun-Blanquet’s school, and is supported by numeric analysis – ordination (DECORANA) and hierarchical classification (TWINSPAN). The final hierarchical scheme for the vegetation entities is mainly based on floristic affinity patterns, but it also considers the physiognomic structure and the three-fold ecophysiographic zonation (successional, hydrogenetic and ecoterritorial) established for the habitats. Eight major ecophysiognomic vegetation groups are proposed: 1) limnic vegetation – a) natural and b) eutrophic; 2) telmatic vegetation – a) natural and b) disturbed; 3) semiterrestrial vegetation – a) seminatural prairies on peaty soils and b) disturbed fens; 4) seasonal prairies on sand; 5) wet scrubs; 6) semiterrestrial willow carrs; 7) terrestrial vegetation – a) humid heathland on peat, and b) fluvial humid forest; and 8) humid vegetation of the transition zones boundaring the wetland basins. A quick reference to the brackish vegetation of the lagunar littoral basins is also included. Besides the description of the plant communities, we aimed to assess, at a preliminary level, the ecological diversity of the wetland habitats in the region – lakelets, mires and temporary wet depressions. For each basin a ecological zonation is considered, based on vegetation, hydrology, soils and geomorphic units. This zonation is illustrated by a series of short vegetation transects summarizing the local succession. For the Fernão Ferro region, a cartographic survey of the wetland habitats is included (at 1:25000 and 1:2500 scales). The production of these maps was supported by GIS techniques, photo-interpretation, and digital analysis of airphotos. The palaeoecological investigation is based on the palynological analysis of four organic profiles: Lagoa do Golfo (Fernão Ferro – Sesimbra); Estacada (Lagoa de Albufeira – Sesimbra); Poço do Barbaroxa de Cima (Santo André – Santiago do Cacém); and Vale da Carregueira (Santo André – Santiago do Cacém). It includes seven pollen diagrams, five diagrams of other (non-pollinic) microfossils (mainly fungi and algae) and one diagram of macroremains. The “peat” profiles were cored with piston corers (Dachnowsky and Livingstone), recovering the complete organic sequence at each site. From these cores, a set of 26 “bulkpeat” samples were dated by radiocarbon standard methods. Apart form percentage data, pollen concentration values were obtained using the exotic spores addition method. During pollen counting and identification procedures we aimed to optimise the taxonomic resolution by the systematic use of optical high magnification (x 1000) coupled with the technique of rotating the pollen grains under observation. A large number of pollen grains counted per sample was achieved, in order to obtain a good representation and an enlarged number of different pollen types. The palaeoecological interpretation of the diagrams follows the recognition of palaeovegetation entities – the past plant communities. These result from the grouping of pollen types accordingly to their affinities, both stratigraphical and ecological (as suggested by the present day geobotanical results). The description of the pollen diagrams is supported by the definition of two pollen zonation schemes: one - regional - referring to the palaeovegetation development taking place on the non-hygrophilous habitats (“dry soils”) surrounding the peatmire; the other - local - referring to the evolution of the hygrophilous vegetation inside the mire itself. Pollen zonation is following two complementary criteria (one static, the other dynamic): 1) assemblage pollen zones defined by an homogeneous and more or less static pollen content; and 2) pollen peak zones based on the dynamic patterns of pollen curves. Four major themes on Landscape Evolution are explored in this thesis – Climate Change, Littoral Dynamics and Evolution, the History of Human Impact, and, finally, the Evolution of Natural Vegetation. The palaeoclimatic synthesis is based on the interpretation of the diagrams by bioclimatic and limnologic analog criteria and considers a preliminary climato-chronostratigraphy for the Lateglacial and Holocene. The data available for the Lateglacial is still scarce; nevertheless it can be used to define four main phases where colder and drier conditions alternate with the evidence of climate improvement – a) a first phase of cold/temperate conditions is succeeded by b) a period of climatic degradation; c) a new phase of quick and noticeable warming is again followed by d) a phase of return to “glacial” cold conditions (Younger Dryas), just before the beginning of the Holocene. The Holocene is divided in three main periods – Early, Middle and Late Holocene. These periods, especially the Middle and Late Holocene, correspond to climatic cycles, started with humid to hyperhumid phases, and ending with drier phases. In what concerns the evolution of the littoral ecosystems in the North-west Alentejo, six main phases are established, which can be related to moments of sea level change and its influence on the coastal ecosystems. This synthesis supports and complements the first synthesis of MATEUS (1992): a first phase of marine ingression till around 5500 BP, characterised by a fast coastal retreat; a phase of coastal line stabilisation, until around 4150 BP; a regressive phase, when coastal progadation and terrestrialization of the interdunal perimarine mires prevails (between 4150 and 3250 BP); a new transgressive phase, characterised by huge inundations (sometimes brackish) on the perimarine systems (until 1200 BP); followed by a new phase of coastal stabilisation; finally (during the last 600 years) a new phase of littoral regression. The conjunction of a humid climate phase and a transgressive littoral regime (between 3250 and 600 BP) results on the opening to the ocean of part of the rivulets of NW Alentejo, whose sea outflows are nowadays blocked by sand bars (endorreal systems). The history of human impact on the North Alentejo littoral plane is on the whole characterised by an increase of anthropic influence on the ecosystems through time - yet an increase not gradual in character, but rather marked by the alternation of periods of progressive impact with periods when man’s ecological influence in the landscape declines, more or less markedly. On this study the chronological scheme proposed by MATEUS (1992) is further elaborated with special emphasis for the last 1500 years. The first signs of human impact, still rather weak and diffuse, are indicated at around 6500 BP. Since then, the human impact is gradually increasing both in strength and ecological range (influencing a broader spectrum of habitats), to culminate, during the Bronze Age, at a phase of strong impact, active in all the ecological zones of the region. A phase of decline in the human pressure occurs at the transition “Bronze – Iron Age”, followed by a new progressive reinforcing of the human impact which results at this phase in a climax of landscape transformation, corresponding to the Early Roman Empire. After a eco-territorial decline named by MATEUS (1992) “Middle Empire”, a progressive increase of territorial occupation is evident. After the 14th century, the expression of the human strong landscape occupation regains dominance in the diagrams, this time with the signs of extensive pastoralism. The 18th and 19th centuries are characterised by reforestation and extensive agriculture. Finally, since late 19th century, the land use specialisation which characterizes present-day landscape become evident: intensive pine (and eucaliptus) forestation on the poor sandy soils of the interfluves; specialised and intensive agriculture on the drained wetlands and low fluvial terraces. During the last 14 millennia, we can reconstruct for the NW Alentejo a climatically induced succession of different pinewoods and oaklands, following strictly the “valleyinterfluves” landscape duality. On the Lateglacial interfluves Pinus sylvestris woods are dominant; on the valley systems oakwoods (probably of Quercus pyrenaica) and Betula groves are present. With the Holocene, Pinus pinaster forests gain dominance in detriment of the previous sylvester pinewoods; the deciduous forest (with a supramediterrenean character) retreats and is replaced by marcescent oakwoods (of Quercus faginea) and alder (Alnus) carrs (in the valley bottoms). During the Middle Holocene (between 8 and 3 Ky BP), as climate becomes more dry, the sclerophyllous evergreen trees tend to prevail, probably gaining vegetational individuality: the wild Olea forests, the Quercus suber oakwoods and the Pinus pinea pinewoods – a pattern which is also partly induced by the human communities of the Calcolithic and Bronze Age. Although the Late Holocene starts with a humid climatic phase, favourable to the mesomediterranean “mesic” vegetational elements, the role of man is now dominant both as a direct and indirect factor for modulating the landscape, an influence which co-favours its xeromorphic character. Considering now the permanent shrublands - the beginning of the Holocene induces the retreat of the cryo-xeric juniper scrubs and the cold/temperate low heathlands, and favours the expansion of the heath-macquis vegetation of Erica scoparia and Quercus coccifera. In humid oligotrophic conditions, the oceanic peatmire heaths give place to the high Erica erigena wet heathland. With the on going of the Holocene, the joint effect of increasing human pressure and climatic dryness, favours the expansion of the xeromorphic dwarfshrublands (STAURACANTHO-HALIMIETALIA), at the expense of the heathlands (CALLUNOULICETALIA). At the same time eutrophization in the wetlands is now the dominant pattern. Two studies on Pollen Morphology complement this thesis: The first one considers the pollen morphology of portuguese pine species, resulting in the definition of four subtypes, exclusively based on the morphological characters of the pollen grains. The second study consist on a detailed analysis of the pollen morphology of the portuguese Cistaceae. Nine pollen types and seven subtypes are defined, considering this plant family. The privileged history of pinewoods in Portugal and the role of the Cistaceae dominant scrubs as key components of the plant cover evolution are the main justification for this pollen morphological research, as a necessary preamble to palaeoecological investigation. Finally, this thesis includes a first proposal for a portuguese terminology for pollen morphology.
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Many habitat specialist species are originally composed of small, discontinuous populations because their habitats are naturally fragmented or patchy. They may have suffered the long-term effects of natural patchiness. Mediterranean heathlands, a representative habitat in the Strait of Gibraltar region, are associated with nutrient-poor, acidic sandstone soils. Sandstone soil patches in the African side of the Strait (Tangier) are, in general, smaller and more scattered than in the European side (Algeciras). In this study, we analyze the effect of this sandstone patchiness on the population genetic diversity and structure of two Erica species from these Mediterranean heathlands that differ in their edaphic specificity, E. australis, sandstone specialist, and E. arborea, generalist. Average levels of within-population genetic diversity and gene flow between populations were significantly lower in Tangier (high sandstone patchiness) than in Algeciras (low patchiness) for the sandstone specialist, whereas no differences between both sides of the Strait were detected in the edaphic generalist. Since most endemic species in Mediterranean heathlands of the Strait of Gibraltar are sandstone specialists, these results highlight an increased vulnerability to loss of genetic diversity and local extinction of the heathland endemic flora in the Tangier side of the Strait of Gibraltar.
Preprint
Paleoclimate reconstructions suggest that the complex variability within the Greenland stadial 1 (GS-1) over western Europe was governed by coupled ocean and atmospheric changes. However, few works from the North Atlantic mid-latitudes document both the GS-1 onset and its termination, which are often considered as single abrupt transition events. Here, we present a direct comparison between marine (alkenone-based sea surface temperatures) and terrestrial (pollen) data, at very high resolution (28 years mean), from the southwestern Iberian shelf record D13882. Our results reveal a rather complex climatic period with internally changing conditions. The GS-1 onset (GS-1a: 12890-12720 yr BP) is marked by a progressive cooling and drying; GS-1b (12720-12390 yr BP) is the coldest and driest phase; GS-1c (12390-12030 yr BP) is marked by a progressive warming and increase in moisture conditions; GS-1 termination (GS-1d: 12030-11770 yr BP) is marked by rapid switches between cool wet, cold dry and cool wet conditions. Although hydroclimate response was very unsteady throughout the GS-1 and in particular during its termination phase, the persistence of an open temperate and Mediterranean forest in southwestern Iberia during the entire episode suggests that at least some moisture was delivered via the Westerlies. We propose coupled ocean and atmospheric mechanisms to reproduce these scenaria. Changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) as well as variations in the North Atlantic sea-ice growth have favoured the displacement of the polar jet stream's latitudinal position and contributed to a complex spatial pattern and strength of the Westerlies across western Europe.
Article
A informação paleobotânica e cronológica obtida em diversos depósitos limnéticos de natureza poligênica situados na actual área litoral Atlântica (N de Portugal e S da Galiza) permite avaliar as mudanças da paisagem vegetal desse território, correlacionando-as com a dinâmica climática global estabelecida para o Sudoeste da Europa. O início da sequência situar-se-ia no âmbito do lnterestagial Würmiano (Estágio Isotópico 3), durante o qual se constata a presença, na área litoral, de formações arbóreas. A posterior deterioração climática representada pelo Estágio Würmiano Final aparece refletida no desenvolvimento de formações arbustivas, cujo predomínio se estende até o início do actual interglacial. A dinâmica aqui obtida é compatível cronológica e ecologicamente com as seqüências regionais obtidas em outras áreas do Noroeste Ibérico. A transgressão Flandriana provocou grandes modificações no espaço litoral do Noroeste Ibérico. Como conseqüência, os depósitos limnéticos formados no final do Pleistocénieo e/ou no início do Holocénico, em áreas distantes da costa, ficaram situados próximos a esta ou submergidos no prelitoral. Esses processos virào ligados, em muitos casos, ao recobrimento dos sedimentos por materiais detríticos de natureza continental e/ou marinha, o que provoca a cobertura da maioria dos depósitos mais antigos. As mudanças geomorfológicas repercutem na representatividade polínica dos sedimentos recuperados nesses depósitos, de modo que a partir de meados do Holocénico aprecia-se uma óptima representatividade da dinâmica das comunidades litorâneas (vegetação halófila, matorrais, úmidas), enquanto que as comunidades de carácter climatófilo, próprias de meios continentais, mantêm um mínimo reflexo, em consonância com a sua escassa significação na paisagem.
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The Holocene is probably the most intensively studied series/epoch within the geological record, and embodies a wide array of geomorphological, climatic, biotic and archaeological evidence; yet little attention has hitherto been paid to a formal subdivision of this series/epoch. Here we report a tripartite division of the Holocene into the Greenlandian, Northgrippian and Meghalayan stages/ages and their corresponding Lower/Early, Middle, Upper/Late subseries/subepochs, each supported by a Global Boundary Stratotype Section and Point (GSSP). The GSSP for the lowermost stage, the Greenlandian, is that of the Holocene as previously defined in the NGRIP2 Greenland ice core, and dated at 11,700 yr b2k (before 2000 CE). The GSSP for the Northgrippian is in the NGRIP1 Greenland ice core, and dated at 8236 yr b2k, whereas that for the Meghalayan is located in a speleothem from Mawmluh Cave, Meghalaya, northeast India with a date of 4250 yr b2k. The proposal on which this subdivision is based was submitted by the Subcommission on Quaternary Stratigraphy, approved by the International Commission on Stratigraphy, and formally ratified by the Executive Committee of the International Union of Geological Sciences on 14 th June 2018. © 2018 International Union of Geological Sciences. All Rights Reserved.
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Based on sixty Holocene sections, the most important North American pollen types are: Pinus, Picea, Ephedra, Tsuga canadensis-heterophylla, Pseudotsuga-Larix, Juniperus-Taxodium, Cyperaceae, Gramineae, Betula, Carya, Alnus, Ulmus, Chenopodiineae, Liquidambar, Acer, Quercus, Nyssa, Fagus, Artemisia, and Ambrosia. The distribution of these taxa is employed to describe six floristic provinces-the Boreal, Lakes, Southeast, Pacific, Plains, and Mountain provinces. These provinces are present during earlier Pleistocene interglacial times, but they are not apparent in glacial-age pollen assemblages, which are dominated by conifers and herbs.
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We analyzed a 10-m sediment core retrieved at 82 m water depth off the coast of the Tagus River (Western Iberian Margin, Portugal) to investigate a linkage between variations in benthic foraminiferal assemblages and Tagus River discharge over the last 5700 years. Benthic foraminiferal assemblages were studied at high resolution in combination with the stable carbon and oxygen isotopic composition of fossil shells of Nonion scaphum, bulk and molecular organic matter properties (TOC, TN, C/N ratio, δ¹³CTOC, δ¹⁵Nbulk, and BIT index), magnetic susceptibility, and XRF analyses. Three periods of environmental changes were identified: 1) high Tagus River discharge in 5750-2200 calendar year before present (cal yr BP), 2) lower discharge characterized by intense upwelling conditions (2250-1250 cal yr BP), and 3) both intense upwelling and Tagus River discharge (1250 cal yr BP-present). The data reveal alternating intense upwelling periods, as shown by the dominance of Cassidulina carinata, Valvulineria bradyana, or Bulimina marginata, whereas periods of increased river discharge are indicated by increase of N. scaphum, Ammonia beccarii, and Planorbulina mediterranensis. The Tagus River discharge was the strongest during the first period, transporting riverine material further offshore and preventing the establishment of a mud belt on the mid-shelf (around 100 m depth). During the second period, a decrease in Tagus River discharge favored the formation of the Tagus mud belt and strongly influenced the benthic environment by creating an organic matter stock. During the third period, intense upwelling and increased Tagus River discharge were recorded by benthic foraminiferal distribution, with an increase of terrestrial elements present in the mud belt. Furthermore, our results showed that variations in benthic foraminiferal assemblages corresponded to the well-known climatic periods in the study area, such as the Roman Period, the Dark Ages, the Medieval Warm Period, and the Little Ice Age. Our study strongly suggests that benthic foraminiferal assemblages can be used as a bio-indicator to trace the influence of past river discharge.
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Simulations by climate models show that Earth warmed during the Holocene epoch, whereas ocean sedimentary cores suggest that global cooling occurred. An analysis of fossil pollen samples now sides with the models.
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Abstract Comparison of selected, well-dated, lacustrine, speleothem and terrestrial pollen records spanning the Holocene onset and the Early Holocene (ca. 11.7–8 cal kyrs BP) in the Iberian Peninsula shows large hydrological fluctuations and landscape changes with a complex regional pattern in timing and intensity. Marine pollen records from Alboran, the Mediterranean and off shore Atlantic sites show a step-wise increase in moisture and forest during this transition. However, available continental records point to two main patterns of spatial and temporal hydrological variability: i) Atlantic-influenced sites located at the northwestern areas (Enol, Sanabria, Lucenza, PRD-4), characterized by a gradual increase in humidity from the end of the Younger Dryas to the Mid Holocene, similarly to most North Atlantic records; and ii) continental and Mediterranean-influenced sites (Laguna Grande, Villarquemado, Fuentillejo, Padul, Estanya, Banyoles, Salines), with prolonged arid conditions of variable temporal extension after the Younger Dryas, followed by an abrupt increase in moisture at 10-9 cal kyrs BP. Different local climate conditions influenced by topography or the variable sensitivity (gradual versus threshold values) of the proxies analyzed in each case are evaluated. Vegetation composition (conifers versus mesothermophilous taxa) and resilience would explain a subdued response of vegetation in central continental areas while in Mediterranean sites, insufficient summer moisture availability could not maintain high lake levels and promote mesophyte forest, in contrast to Atlantic-influenced areas. Comparison with available climate models, Greenland ice cores, North Atlantic marine sequences and continental records from Central and Northern Europe and the whole Mediterranean region underlines the distinctive character of the hydrological changes occurred in inner Iberia throughout the Early Holocene. The persistent arid conditions might be explained by the intensification of the summer drought due to the high seasonality contrast at these latitudes caused by the orbital-induced summer insolation maximum. New records, particularly from western and southernmost Iberia, and palaeoclimate models with higher spatial resolution would help to constrain these hypotheses.
Chapter
The present study attempts to illustrate the large phytocoenotical diversity of forests, pre-forest, high-scrub and most of the corresponding sub-seral communities of the Lusitania territories and enables the identification of their clumped pattern of occurrence as landscape mosaics. These communities are syntaxonomically included in the Quercetea ilicis, Querco-Fagetea sylvaticae, Salici purpureae-Populetea nigrae, Alnetea glutinosae, Nerio-Tamaricetea, Calluno vulgaris-Ulicetea minoris, Cisto-Lavanduletea stoechadis and Rosmarinetea officinalis vegetation classes and span a large spectrum of environmental variables, being present in a wide range of bioclimatic stages and edaphic conditions. The grasslands, that represent seral stages of the Quercion broteroi and Quercion pyrenaicae and are included in the Stipo giganteae-Agrostietea castellanae class, are also described. In Lusitania, on soils rich in bases, without hydromorphy and salinity, dry grasslands are widespread in the potential areas of Quercus suber, Q. rotundifolia and/or Q. faginea, and are included in the Festuco-Brometea vegetation class; associations of the Arrábida mountain and Algarve are especially rich in endemic species. The ephemeral pioneer communities are dominated by non-nitrophilous, small therophytes included in the Helianthemetea guttati class. Grasslands linked to heavy grazing are also described. In this group we include pastures, with adequate sheep pasture, grazed and manured, dominated by dwarf perennial grasses and other nutritious prostrate chamaephytes and hemicryptophytes, which encompass the associations of the Poetea bulbosae class. Other herbaceous communities, dependent on anthropic influence, are those dense meadows and reed-beds included in the Molinio-Arrhenatheretea class.The tall herbs of nitrified wood fringes and other semi-shaded anthropic biotope communities belong to the Galio-Urticetea class. The herbaceous ruderal vegetation is also described. Lastly, the coastal vegetation of sea cliffs and lithosols, dunes, and estuaries and saltmarshes is referred to, as well as the interior sandy soil or semi-fixed dune grasslands.
Chapter
The Iberian Cantabrian Atlantic biogeographical territory is a narrow strip of land, fallen L-shaped, which runs parallel to the coast from Pamplona (Spain) to near Aveiro (north Portugal), characterized by a wet and warm climate, with smooth winters and slight or absent drought in summer. Its relief is very varied and includes from coastal and inland plains to mountainous terrains with altitudes up to 1700 m. Homo sapiens has been present in this territory since the Upper Pleistocene but its influence on vegetation cover seems to have been very low until the Climate Optimum of the Holocene. Since this period human activities increased progressively and led to a wide deforestation of the territory, the expansion of non-arboreal seral communities and, during the last century, the introduction of a great variety of alien species for timber production and ornamental use. In spite of this great influence of man on the vegetal cover of the territory, some facts reveal its similarity to the rest of Atlantic Europe: (1) Supremacy of deciduous forests dominated by pedunculate oak and beech which are replaced by ash, maple, elm and linden trees in mixed forests, or alder, birch and willow in alluvial forests. (2) Replacing forests, seral scrub of thorny bushes or broom scrub (Cytisus sp. pl.) occur, with further degradation giving way to heathlands dominated by ericoid species but also with gorses (Ulex sp. pl.) and other thorny leguminosae (Genista sp. pl.). (3) Several types of meadows and other grasslands play a relevant role in the landscape and in traditional agricultural systems. (4) Different vegetation complexes typical of sandy deposits, rocky coasts and saltmarshes merge along its extensive shoreline. Nevertheless, there is a particular fact that differentiates these Atlantic territories from others located further north: the existence of many examples of evergreen vegetation, such as forests of holm oak (Quercus ilex, Q. rotundifolia), cork oak (Quercus suber) and laurel tree (Laurus nobilis), or scrub dominated by Arbutus unedo or Phillyrea sp. pl., due to their proximity to the Mediterranean Region.
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Marine Isotope Stage 31 (MIS 31) is an important analogue for ongoing and projected global warming, yet key questions remain about the regional signature of its extreme orbital forcing and intra-interglacial variability. Based on a new direct land-sea comparison in SW Iberian margin IODP Site U1385 we examine the climatic variability between 1100 and 1050 ka including the " super interglacial " MIS 31, a period dominated by the 41-ky obliquity periodicity. Pollen and biomarker analyses at centennial-scale-resolution provide new insights into the regional vegetation, precipitation regime and atmospheric and oceanic temperature variability on orbital and suborbital timescales. Our study reveals that atmospheric and SST warmth during MIS 31 was not exceptional in this region highly sensitive to precession. Unexpectedly , this warm stage stands out as a prolonged interval of a temperate and humid climate regime with reduced seasonality, despite the high insolation (precession minima values) forcing. We find that the dominant forcing on the long-term temperate forest development was obliquity, which may have induced a decrease in summer dryness and associated reduction in seasonal precipitation contrast. Moreover, this study provides the first evidence for persistent atmospheric millennial-scale variability during this interval with multiple forest decline events reflecting repeated cooling and drying episodes in SW Iberia. Our direct land-sea comparison shows that the expression of the suborbital cooling events on SW Iberian ecosystems is modulated by the predominance of high or low-latitude forcing depending on the glacial/interglacial baseline climate states. Severe dryness and air-sea cooling is detected under the larger ice volume during glacial MIS 32 and MIS 30. The extreme episodes, which in their climatic imprint are similar to the Heinrich events, are likely related to northern latitude ice-sheet instability and a disruption of the Atlantic Meridional Overturning Circulation (AMOC). In contrast, forest declines during MIS 31 are associated to neither SST cooling nor high-latitude freshwater forcing. Time-series analysis reveals a dominant cyclicity of about 6 ky in the temperate forest record, which points to a potential link with the fourth harmonic of precession and thus low-latitude insolation forcing.
Article
A multi-proxy characterization of the uppermost sedimentary infill of an Iberian alpine lake (Cimera, 2140 m a.s.l.) was performed to establish the climatic and environmental conditions for the Iberian Central Range (ICR) over the last two millennia. This multi-proxy characterization was used to recon- struct the intense runoff events, lake productivity and soil erosion in the lake catchment and interpret these factors in terms of temperature and precipitation variability. The Roman Period (RP; 200 BCE e 500 CE) beginning was characterized by an alternation between cold and warm periods as indicated by short- lived oscillations of intense runoff conditions and soil erosion, although warm conditions dominated the end of the period and the Early Middle Age (EMA; 500e900 CE) onset in the ICR. A noticeable decrease in intense runoff events and a progressive decrease in soil erosion during the late EMA indicated a shift to colder temperatures. In terms of precipitation, both the RP and EMA climate periods displayed a tran- sition from dry to wet conditions that led to a decrease in lake productivity. The Medieval Climate Anomaly (MCA; 900e1300 CE) was characterized by warm and dry conditions with frequent intense runoff episodes and increases in lake productivity and soil erosion, whereas the Little Ice Age (LIA; 1300 e1850 CE) showed the opposite characteristics. The Industrial Era (1850e2012 CE) presented an increase in lake productivity that likely demonstrates the influence of global warming. The spatio-temporal integration of the Cimera record with other Iberian reconstructions has been used to identify the main climate drivers over this region. During the RP and EMA, NeS and EeW humidity gradients were dominant, whereas during the MCA and LIA, these gradients were not evident. These differences could be ascribed to interactions between the North Atlantic Oscillation (NAO) and East Atlantic (EA) phases. During the RP, the general warm conditions and the EeW humidity gradient indicate a dominant interplay between a negative NAO phase and a positive EA phase (NAOÀeEAþ), whereas the opposite conditions during the EMA indicate a NAOþeEAÀ interaction. The dominant warm and arid conditions during the MCA and the cold and wet conditions during the LIA indicate the interplay of the NAOþeEAþ and NAOÀeEAÀ, respectively. Furthermore, the higher solar irradiance during the RP and MCA may support the predominance of the EAþ phase, whereas the opposite scenario during the EMA and LIA may support the predominance of the EAÀ phase, which would favour the occurrence of frequent and persistent blocking events in the Atlantic region during these periods.
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This was the third cruise using the new SOC giant piston corer. The objectives were to complete the testing of the giant piston corer which was begun on cruise D219 in November/December 1995 and cruise D225 in February/March 1997, and to collect giant piston cores from the Gulf of Cadiz on sites selected from the TOBI survey carried out on cruise D244 in December 1999. As on previous cruises we had problems with both the ships equipment (outboard sheave on coring gantry), and the giant piston corer. The sheave problem necessitated 2.5 days in port plus transit time. This combined with some weather downtime severely limited the amount of work which could be carried out. Nevertheless, we resolved some coring issues, especially proving that elastic rebound of the kevlar is not a major problem. We also managed to take a consistent series of good cores with a 15 metre barrel, and although all the 27 metre barrel cores bent, the bending took place above the sediment surface leaving a good quality core below. The longest core recovered was 18.1 metres and 21 cores were obtained in all. The cruise also involved testing the scatterometer. This proved to be a failure due to multiple electronic problems and no data was recorded in any of its two deployments.
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Pollen analysis of peat cores in a valley adjacent to the village of Alpiarca, where numerous artifacts from prehistoric times are known to exist, reflects a relationship with the human occupation on the hill. The most intensive relationship with the activities of man is shown in the pollen assemblages. All the poillen diagrams show from the bottom to the top of the core a decrease in the pollen values of Pinus, while the values of the nonarboreal pollen types increase. The deposits offer an excellent opportunity to study the influence of man's activities on the vegetation in a small area that was occupied in prehistoric and historic times. -from Authors
Article
The direct comparison between marine and terrestrial data from the NW Iberian margin, core MD03-2697, allows us to accurately evaluate, without chronological ambiguity, the vegetation response to North Atlantic climate events across the last deglaciation. Comparison of MD03-2697 data with other marine and terrestrial records from a vast area stretching from the Azores to western (W) France, Iberia and its margin, the W Mediterranean and NW Africa reveals the importance of enhanced winter North Atlantic westerlies episodes in driving a heterogeneous regional climatic signal during particular events of the last deglaciation. Heinrich Stadial 1 (HS1)/Oldest Dryas is a complex event marked by three synchronous main phases (a: extremely cold/relatively wet; b: cool/dry; c: relatively warmer/increasing moisture availability) in regions directly influenced by the North Atlantic while it is characterized by a single phase (cold and dry) in most inland and high altitude areas. Changes in the strength and position of North Atlantic westerlies could explain the variability in moisture during HS1 from W Pyrenees to W Mediterranean. The Bølling-Allerød (B-A) event is marked by a synchronous progressive increase of ocean and atmospheric temperatures and precipitation from the Bølling to the Allerød in W Iberia and W Pyrenees contrasting with the Greenland temperature pattern. Mid-to high latitudes thermal contrast and the gradual strengthening of the Atlantic Meridional Overturning Circulation (AMOC) triggered the continuing enhancement of westerlies, and moisture, along this period.
Article
AimOur aim was to discriminate different species of Pinus via pollen analysis in order to assess the responses of particular pine species to orbital and millennial-scale climate changes, particularly during the last glacial period.LocationModern pollen grains were collected from current pine populations along transects from the Pyrenees to southern Iberia and the Balearic Islands. Fossil pine pollen was recovered from the south-western Iberian margin core MD95-2042.Methods We measured a set of morphological traits of modern pollen from the Iberian pine species Pinus nigra, P. sylvestris, P. halepensis, P. pinea and P. pinaster and of fossil pine pollen from selected samples of the last glacial period and the early to mid-Holocene. Classification and regression tree (CART) analysis was used to establish a model from the modern dataset that discriminates pollen from the different pine species and allows identification of fossil pine pollen at the species level.ResultsThe CART model was effective in separating pollen of P. nigra and P. sylvestris from that of the Mediterranean pine group (P. halepensis, P. pinea and P. pinaster). The pollen of Pinus nigra diverged from that of P. sylvestris by having a more flattened corpus. Predictions using this model suggested that fossil pine pollen is mainly from P. nigra in all the samples analysed. Pinus sylvestris was more abundant in samples from Greenland stadials than Heinrich stadials, whereas Mediterranean pines increased in samples from Greenland interstadials and during the early to mid-Holocene.Main conclusionsMorphological parameters can be successfully used to increase the taxonomic resolution of fossil pine pollen at the species level for the highland pines (P. nigra and P. sylvestris) and at the group of species level for the Mediterranean pines. Our study indicates that P. nigra was the dominant component of the last glacial south-western/central Iberian pinewoods, although the species composition of these woodlands varied in response to abrupt climate changes.
Article
There is strong proxy and model evidence of precession- and obliquity-induced changes in the freshwater budget over the Mediterranean Sea and its borderlands, yet explanations for these changes vary greatly. We investigate the separate precession and obliquity forcing of the freshwater budget over the Mediterranean using a high-resolution coupled climate model, EC-Earth. At times of enhanced insolation seasonality, i.e. minimum precession and maximum obliquity, the area was wetter and the Mediterranean Sea surface was less saline. The latter has been attributed to increased runoff from the south as a consequence of a strengthened North African monsoon, as well as to increased precipitation over the Mediterranean Sea itself. Our results show that both mechanisms play a role in changing the freshwater budget. Increased monsoon runoff occurs in summer during times of enhanced insolation seasonality, especially minimum precession, while increased precipitation is important in winter for both precession and obliquity. We relate changes in winter precipitation to changes in the air-sea temperature difference and subsequently, convective precipitation. The freshening in the minimum precession and maximum obliquity experiments has a strong effect on Mediterranean sea surface salinity and mixed layer depth, thereby likely influencing deep sea circulation and sedimentation at the ocean bottom.
Article
We present a new gridded climate reconstruction for Europe for the last 12,000 years based on pollen data. The reconstruction is an update of Davis et al. (2003) using the same methodology, but with a greatly expanded fossil and surface-sample dataset and more rigorous quality-control. The modern pollen dataset has been increased by more than 80%, and the fossil pollen dataset by more than 50%, representing almost 60,000 individual pollen samples. The climate parameters reconstructed include summer/winter and annual temperatures and precipitation, as well as a measure of moisture balance, and growing degree-days above 5 °C. Confidence limits were established for the reconstruction based on transfer function and interpolation uncertainties. The reconstruction takes account of post-glacial isostatic readjustment which resulted in a potential warming bias of up to +1–2 °C for parts of Fennoscandia in the early Holocene, as well as changes in palaeogeography resulting from decaying ice sheets and rising post-glacial sea-levels. This new dataset has been evaluated against previously published independent quantitative climate reconstructions from a variety of archives on a site-by-site basis across Europe. The results of this comparison are generally very good; only chironomid-based reconstructions showed substantial differences with our values. Our reconstruction is available for download as gridded maps throughout the Holocene on a 1000-year time-step. The gridded format makes our reconstructions suitable for comparison with climate model output and for other applications such as vegetation and land-use modelling. Our new climate reconstruction suggests that warming in Europe during the mid-Holocene was greater in winter than in summer, an apparent paradox that is not consistent with current climate model simulations and traditional interpretations of Milankovitch theory.
Article
1 Pollen diagrams from four peatbogs and a marshland located in the Atlantic and Mediterranean zones of north-central Spain are described, and their correlation is used to define a Holocene pollen reference sequence for the region. 2 Two main variants may be defined for the reference sequence: a northern variant, characterized by the predominance of deciduous trees, corresponds to sites under Atlantic temperate and moist climatic influence and a southern variant, with lower deciduous tree diversity, corresponds to sites under Mediterranean and continental climatic conditions. A vegetation gradient from the Atlantic to the Mediterranean side may be inferred from the pollen analysis. 3 A compilation of $^{14}C$ dated pollen diagrams from the northern Iberian Peninsula is used to plot maps for the Holocene spread of the principal tree genera. 4 Quercus, Corylus, Alnus, Taxus and Pinus had a wide distribution in the northern Iberian Peninsula during pre-Holocene times, as patchy refugia situated at medium elevations. They spread from these refugia at different times depending on climatic, geomorphological and successional conditions. Fagus, Abies and Carpinus could have spread through the Pyrenees. 5 Anthropogenic disturbance at the range limit of Fagus, Abies and Carpinus is probably responsible for their present geographical distribution. 6 The present-day Pinus silvestris forests growing in the Iberian Cordillera, southern Pyrenees and Cantabrian Cordillera have a local origin and may constitute the Mediterranean mountain pine forest altitudinal zone, not previously described for the Iberian Peninsula.
Article
Maps of continental-scale land cover are utilized by a range of diverse users but whilst a range of products exist that describe present and recent land cover in Europe, there are currently no datasets that describe past variations over long time-scales. User groups with an interest in past land cover include the climate modelling community, socio-ecological historians and earth system scientists. Europe is one of the continents with the longest histories of land conversion from forest to farmland, thus understanding land cover change in this area is globally significant. This study applies the pseudobiomization method (PBM) to 982 pollen records from across Europe, taken from the European Pollen Database (EPD) to produce a first synthesis of pan-European land cover change for the period 9000 bp to present, in contiguous 200 year time intervals. The PBM transforms pollen proportions from each site to one of eight land cover classes (LCCs) that are directly comparable to the CORINE land cover classification. The proportion of LCCs represented in each time window provides a spatially aggregated record of land cover change for temperate and northern Europe, and for a series of case study regions (western France, the western Alps, and the Czech Republic and Slovakia). At the European scale, the impact of Neolithic food producing economies appear to be detectable from 6000 bp through reduction in broad-leaf forests resulting from human land use activities such as forest clearance. Total forest cover at a pan-European scale moved outside the range of previous background variability from 4000 bp onwards. From 2200 bp land cover change intensified, and the broad pattern of land cover for preindustrial Europe was established by 1000 bp. Recognizing the timing of anthropogenic land cover change in Europe will further the understanding of land cover-climate interactions, and the origins of the modern cultural landscape.
Article
A recent temperature reconstruction of global annual tempera-ture shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [Marcott SA, et al., 2013, Science 339(6124):1198–1201]. This global cooling is puzzling be-cause it is opposite from the expected and simulated global warm-ing trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models. global temperature | Holocene temperature | model-data inconsistency
Article
We have analysed alkenones in 149 surface sediments from the eastern South Atlantic in order to establish a sediment-based calibration of the U37K′ paleotemperature index. Our study covers the major tropical to subpolar production systems and sea-surface temperatures (SST’s) between 0° and 27°C. In order to define the most suitable calibration for this region, the U37K′ values were correlated to seasonal, annual, and production-weighted annual mean atlas temperatures and compared to previously published culture and core-top calibrations. The best linear correlation between U37K′ and SST was obtained using annual mean SST from 0 to 10 m water depth (U37K′ = 0.033 T + 0.069, r2 = 0.981). Data scattering increased significantly using temperatures of waters deeper than 20 m, suggesting that U37K′ reflects mixed-layer SST and that alkenone production at thermocline depths was not high enough to significantly bias the mixed-layer signal. Regressions based on both production-weighted and on actual annual mean atlas SST were virtually identical, indicating that regional variations in the seasonality of primary production have no discernible effect on the U37K′ vs. SST relationship. Comparison with published core-top calibrations from other oceanic regions revealed a high degree of accordance. We, therefore, established a global core-top calibration using U37K′ data from 370 sites between 60°S and 60°N in the Atlantic, Indian, and Pacific Oceans and annual mean atlas SST (0–29°C) from 0 m water depth. The resulting relationship (U37K′ = 0.033 T + 0.044, r2 = 958) is identical within error limits to the widely used E. huxleyi calibrations of Prahl and Wakeham (1987) and Prahl et al. (1988) attesting their general applicability. The observation that core-top calibrations extending over various biogeographical coccolithophorid zones are strongly linear and in better accordance than culture calibrations suggests that U37K′ is less species-dependent than is indicated by culture experiments. The results also suggest that variations in growth rate of algae and nutrient availability do not significantly affect the sedimentary record of U37K′ in open ocean environments.