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Alluvial records of the African Humid Period from the NW African highlands (Moulouya basin, NE Morocco)
Abstract
This paper presents Early to Late-Holocene sedimentary deposits from the Moulouya catchment, the main Mediterranean river basin of Morocco. Within this region, recent studies have focused particularly on marine or lacustrine archives. Studies on fluvial archives are thus important for improving our understanding of palaeoenvironmental responses to Holocene climatic changes at regional and supra-regional scales. Here, we studied alluvial archives from two basins 300 km apart in the mid-Moulouya basin and High Plateaus region located in the rarely studied NW African highlands, which are currently defined by arid and cold climatic conditions. Strong similarities between these deposits in terms of morpho-pedosedimentary and chronostratigraphical evolution demonstrate a regional morphogenic pattern controlled by Holocene climatic changes. Using a multi-proxy approach, we document rapid palaeoenvironmental changes and orbital-scale morphogenic changes. Four particular phases of regional geomorphic stability have been dated to ca. 10800–10500, 10200–9900, 9300, and 8900 cal. BP. Enhanced flooding periods that reflect times of climatic aridification may correlate with Early and Mid-Holocene Rapid Climatic Changes, especially during the 9.2 ka event. Furthermore, the Moulouya alluvial archives record the development of an extensive wetland formation and tufa sedimentation within the alluvial plains between ca. 10.8 to 6-5 cal ka BP. This formation is recognised at a regional scale across the upper and mid-Moulouya catchments, whereas it is missing from other Moroccan fluvial archives. These lasting humid climatic conditions (ca. 10.8–6/5 ka cal. BP.) are consistent with those recorded in the sub-Saharan records during the last African Humid Period (AHP), and suggest a potential linkage between the NW African highlands and the Saharan domain. Hence, we assume that this fluvio-palustrine wetland formation located at 32°–34°N is consistent with the northern archives recording AHP wet conditions.
... Therefore, distinguishing direct relationships and causalities can be overprinted and delicate due to the high number of involved controls (Cordier et al., 2014;Hoffmann et al., 2009;Pope and van Andel, 1984;Schumm, 1973;Vandenberghe et al., 2010;Vandenberghe and Maddy, 2001). However, the ability to convincingly interpret climatic information from fluvial records has been demonstrated previously (Depreux et al., 2021;Fletcher and Zielhofer, 2013;Törnqvist, 2007;von Suchodoletz et al., 2015;Wolf et al., 2013). As a result, alluvial records can be interpreted in terms of climate variations, local or related to larger-scale atmospheric circulation. ...
... As a result, alluvial records can be interpreted in terms of climate variations, local or related to larger-scale atmospheric circulation. This sensitivity of the fluvial domain has been demonstrated by several authors on all continents and in particular in Mediterranean regions (e.g., Bergillos et al., 2016;Besset et al., 2017;Blanchet et al., 2021;Depreux et al., 2022Depreux et al., , 2021Diodato et al., 2022Diodato et al., , 2021Dusar et al., 2012;Faust and Wolf, 2017;Macklin et al., 2002). Recent literature evidences the capacity of Rapid Climate Change and North Atlantic cooling events to control the large and long-scale climatic modes over Late Quaternary time within the Mediterranean realm (e.g., Faust and Wolf, 2017;Fletcher and Zielhofer, 2013). ...
... In Morocco, some recent works have tried to determine the climatic variations, tectonic evolution, and human influence in the rivers during the Late Pleistocene and Holocene. Most of the important similar research have been concentrated in the valleys of the wadis of the littoral northwestern, southern, and eastern inland areas: El Kert (Barathon et al., 2000;El Amrani, 2007), Rheris (Boudad et al., 2003), El Hay and Isly (Wengler et al., 1994), and Moulouya valley in eastern Morocco (Depreux et al., 2021;Ibouhouten et al., 2010;Zarki et al., 2004;Zielhofer et al., , 2010; Assaka and some other rivers to the south (Weisrock et al., 2006;Wengler et al., 2002). Therefore, there is a notable absence of similar comprehensive studies focusing on the central northern part of Morocco, representing a significant gap in our understanding of the region's paleoenvironmental history. ...
This study investigates the paleoenvironmental changes and fluvial dynamics in the Inaouène River Valley of central northern Morocco over the last 22,000 years. Through comprehensive field study and radiocarbon dating of Upper Pleistocene and Holocene alluvial deposits, the research identifies six main alluvial units reflecting distinct phases of fluvial activity and environmental conditions. These phases include coarse gravel aggradation between ∼22 and 15 cal kaBP, indicating a braided channel system, followed by multiple fine-dominated alluviation phases occurring at ∼13, ∼10, ∼6–5, ∼3, ∼0.7–0.3, and ∼0.1-0 cal kaBP. The trunk channel evolved from a low-sinuosity wandering style in the Early-Middle Holocene to a high-sinuosity pattern in the Late Holocene. Periods of landscape stability, evidenced by soil formation, were dated around 0.3 and 0.8 cal kaBP, with additional episodes estimated just after ∼12 cal kaBP and around 3.8 cal kaBP. Notably, the study also uncovered evidence of significant human intervention during the Middle Holocene.
A comparison with other archives enabled us to reconstruct late Pleistocene and Holocene palaeoenvironmental conditions on both regional and supraregional scales, linking specific stages of floodplain development to prevalent influencing factors. Emphasizing the fluvial system response, we present a cause-effect model that focuses on long-lasting climatic phases (e.g., Last Glacial Maximum, MIS2-MIS1 transition, African Humid Period) as well as sub-millennial to centennial Rapid Climate Changes, North Atlantic cooling events, and solar activity minima. Human intervention played a key role in the Inaouène's evolution during the Middle Holocene, as evidenced by clear anthropogenic markers found in the associated deposits. These findings not only contribute to our understanding of the area's geomorphological history but also highlight its significant archaeological potential, opening new avenues for interdisciplinary research in this understudied region.
The study offers new insights into central northern Morocco's late Quaternary alluvial geochronology and paleoenvironmental evolution, aligning with regional findings and contributing to a broader understanding of climate-driven landscape changes and human-environment interactions across North Africa and the Mediterranean.
... Over the last 20 years, the collection and cumulative analysis of radiocarbon dates from river series has become generalized as a method to document the activity and stability phases of rivers on a catchment scale and to deduce long-term changes in river dynamics (Macklin et al., 2010). This probabilistic method, known as the cumulative probability density function (CPDF), has previously been used in many studies on the Mediterranean area (Zielhofer and Faust, 2008;Sancho et al., 2008;Berger et al., 2016;Benito et al., 2015;Faust and Wolf, 2017;Depreux et al., 2021), and will also be used in this study. ...
... Holocene formations in Morocco and the Maghreb. In contrast, the northern part of Morocco benefits from more stringent documentation of the reconstruction of Holocene paleoenvironments from river archives, especially within the watershed of the Moulouya River, which has been well-documented for about fifteen years Depreux et al., 2021Depreux et al., , 2022. The watershed of the Drâa River that we have chosen to investigate in this study is located in the southeast of Morocco. ...
... The atmospheric regimes of the Mediterranean, Atlantic Ocean, and Sahara Desert have exerted a complex and alternating influence on North-West Africa, an area that has been strongly influenced by the welldescribed southward retreat of the monsoon since the end of the Middle Holocene and the African Humid Period (Krö;pelin et al., 2008;Zielhofer et al., 2017;Tierney et al., 2017;Depreux et al., 2021), According to recent paleoclimatic data from stalagmites in North-West Africa, two periods of the Green Sahara during Marine Isotope Stages (MIS) 5a and early Holocene have been identified . ...
The alluvial filling of the middle basin of the Drâa River (southeastern Morocco) has recorded hydrogeomorphological responses to environmental changes. A systematic study of the Holocene fluvial terraces along an upstream-downstream section of about a hundred kilometers, combined with a sedimentary multiproxy study, was carried out on the alluvial archives of the Drâa's river. The stratigraphic field studies, combined with granulometric and geochemical X-ray Fluorescence analyses and radiocarbon dating, enable us to propose, for the first time, a reconstruction of the major stages in the morpho-hydrodynamic and paleo-environmental evolution of the middle Drâa river over the past 3500 years. Our results highlight six morpho-hydrodynamic and paleoenvironmental phases. Between 3500–2700 and 1800–1600 cal BP, the Drâa river was highly active, associated with torrential activity, reflecting arid climatic conditions. The lack of sedimentary record observed between 2800 and 2350 cal BP points either to a phase of erosion, or to a minimal and discontinuous activity that left no traces in the studied archives. The periods 2350–1800 (Roman Warm Period) and 1600–550 cal BP (Medieval Climate Anomaly) are characterized by strong fine and more regular alluviation punctuated by episodes of low energy of the Drâa floodplain, sometimes favoring fluviosols development that showed similar characteristics in the three outcrops studied. Finally, from 550 cal BP the fluvial records generally shows signs of anthropization, marked by the formation of anthrosols characteristic of fluvial oasis construction, and associated with the presence of ceramics and hearths, in which eolization features are frequent. The comparison of Drâa evolution in a broader paleohydrological and climatic context, integrating other Moroccan studies and regional data, shows solid connection with the river, lakes, and marine archives from the southern Mediterranean to the Western Sahelian steppe zones, revealing a clear response of this large hydrosystem to regional climatic variations.
... In detail, at least two main phases of deposition occurred during MIS 5, at 120-130 ka (i.e. during MIS 5e) and 90-74 ka ( Fig. 4; Boudad et al., 2003;Falguères et al., 2016;Weisrock et al., 2008). Few Holocene tufas have been reported in Morocco so far, except in the Ksabi basin (e.g. at Blirh and Aït Blal; Lefèvre, 1989;Limondin-Lozouet et al., 2013;Vaudour, 1986) and the Aïn Beni Mathar basin (Depreux et al., 2021;Wengler and Vernet, 1992), in the Moulouya catchment, where they are dated from the Early Holocene ( Figs. 1 and 4). Later tufa deposits were recently dated from the last 5000 years at Imouzzer Kandar (Azennoud et al., 2022), only a few kilometres from ASI ( Figs. 1 and 4). ...
... On both sides of the western Mediterranean, there is thus no evidence of strong aridity that would have led to evaporation processes and less negative δ 13 C values in tufa (Andrews, 2006). This is strongly consistent with the prior assumption that, in pluvial and subtropical areas, tufa deposits are observed during humid (interglacial or interstadial) phases (Butzer et al., 1978;Delgado Castilla, 2009;Depreux et al., 2021;Hamdan and Brook, 2015;Nicod, 2000). ...
... Chronological distribution of tufa deposits from southern Spain(Andrews et al., 2000;Delannoy et al., 1993;Delgado Castilla, 2009;Durán, 1996;García-García et al., 2014;Martìn- Algarra et al., 2003;Wolf et al., 2021) and from Morocco(Azennoud et al., 2022;Boudad et al., 2003;Depreux et al., 2021;Falguères et al., 2016;Lefèvre, 1989;Mercier et al., 2009;Rousseau et al., 2006;Weisrock et al., 2008) including ASI (this study, in red) compared to the evolution of Greenland δ 18 O (North Greenland Ice Core Project members, 2004) and Mediterranean forest at the SW Iberian margin(MD95-2042;Sánchez Goñi et al., 2008) during the last climatic cycle. MIS: Marine Isotopic Stage. ...
In the last few decades, multidisciplinary research on
calcareous tufas as palaeoenvironmental and palaeoclimatic records has
intensively grown, which has provided an increasing number of well-documented
sites. Consequently, inter-site comparisons and regional- to
continental-scale reviews have developed, discussing the link between tufa
distribution and climate or providing diachronic comparisons of climatic
and environmental conditions prevailing during Quaternary interglacials
(and interstadials). This paper proposes such a review for the southeastern
Mediterranean area, including new dating and isotopic data from Aït
Said ou Idder (northern Morocco) to be compared with available regional
data, in order to discuss the intensity of some humid periods of the last
125 kyr.
According to several radiocarbon and U–Th dates, three chronological phases
are indeed identified at Aït Said ou Idder: the Holocene, the
Dansgaard–Oeschger (D–O) interstadial 8 and the Marine Isotopic Stage (MIS) 5e. Similarly, other tufa
deposits from both Morocco and southern Spain (mostly Andalusia) appear to
have preferentially developed during interglacial or interstadial periods, marked
by maximal developments of the Mediterranean forest as reported in the
palynological records from regional marine cores. Furthermore, isotopic data
(δ18O and δ13C) from Aït Said ou Idder (and
from other southeastern Mediterranean tufa deposits where available) suggest no
significant difference in terms of temperature or air mass circulation
between the Holocene, D–O 8 and MIS 5e. In terms of humidity
conditions, no evidence of strong aridity is recorded even if D–O 8
appears drier than both interglacials. Conditions seems slightly
wetter during the Holocene than during MIS 5e, but δ13C
values at Aït Said ou Idder could also reflect strong differences in
the seasonality of these interglacials. We demonstrate that calcareous tufa
deposits have promising potential for discussing, in both space and time,
the climate variability in the southeastern Mediterranean area, but new
investigations, including dating and stable isotopes, are required to
accurately feed such discussions.
... They systematically appear in the fluvial archives of the NW Mediterranean basin (contrasting with the Rhone river), and in the rivers of southern England and Germany (Figure 2), and all except the 6200 BCE event are recorded in the Atlantic Iberian river basins. Tunisian, Moroccan, and southern Spanish fluvial archives do not exhibit a fluvial response at this time, probably because it has not yet been identified or at least not in the same way (Depreux et al., 2021). In the two Spanish basins (Guadalete and Jarama rivers) intensively studied by Wolf and Faust (2016), EH-MH pedosedimentary archives are weak, and a general lack of absolute chronology is observed, which is particularly damaging for the discussions concerning the Neolithisation period. ...
... The SPD for the entire 14 C data confirms a lack of absolute chronology in the Jarama river basin from 5400 to 3300 BCE and in the Medjerda river basin (Tunisia) between 9800 and 4700 BCE, where there is a very slow and fine aggradation trend (Zielhofer & Faust, 2008). In the slightly better documented Moulouya basin, recent geoarchaeological studies by Depreux et al. (2021) complement Ziehofer's data and document a chronological hiatus between 6600 and 5400 BCE despite extensive and systematic surveys. We can question whether this situation results from the absence of 14 C material or from the destruction of the sedimentary and archaeological archives synchronous with the EN, probably because of a deep and long entrenchment of the river systems. ...
... The question of water supply for the first Neolithic communities in this southern part of the Mediterranean basin must also be considered, when all the rivers cut deeply, simultaneously, and durably into their beds. The ecological changes, and thus the resources available, observed are also very important and undoubtedly require adaptations (Depreux et al., 2021). Further detailed fluvial geomorphology work is required to confirm this trend. ...
Climate change is still a subject of debate for archaeologist-neolithicists. Its exact chronology, internal pattern, variations in space and time, and impacts on sites and ecosystems and on coastal dynamic and river systems have yet to be assessed. Only a strict comparative approach at high chronological resolution will allow us to make progress on the causality of the socio-environmental processes at work during Neolithisation. Post-depositional impacts on the Early Neolithic hidden reserve also remain underestimated, which has led to the perpetuation of terms such as “Macedonian desert” and “archaeological silence” in the literature on the Neolithic. Off-site geoarchaeological and paleoenvironmental approaches provide some answers to these questions and opens up new research perspectives.
... However, in the Galera Valley, Vallonia enniensis occurred almost continuously from the Early to Late Holocene ( Figs 11 ,14 ,15 ). Furthermore, in Holocene records, Vallonia enniensis is very common from Spain ( Preece, 1991a ;Taylor et al. , 1998 ) to Morocco ( Limondin-Lozouet et al. , 2013 ;Wackenheim et al. , 2020 ;Depreux et al. , 2021 ;Depreux et al. , 2022 ), which indicates a wider Holocene distribution of the species compared to the modern biogeographic range. Reviewing and considering fossil data from Vallonia enniensis would overcome the deficient modern data to assess its status and argue for conservation measures. ...
... Indeed, the climatic and environmental change across the Holocene at Galera offers insights into the palaeobiogeographic history of terrestrial molluscs prior to the aridification trends and the strong anthropogenic impact on ecosystems. The rich Middle Holocene wet mollusc assemblage at Galera shows strong similarities with faunas of the same period from Morocco ( Wackenheim et al. , 2020 ;Depreux et al. , 2021 ;Depreux et al. , 2022 ). However, in Morocco, the modern distribution of faunas has been less studied than in Spain and is therefore not suitable for an extensive palaeobiogeog raphical discussion. ...
In Spain, Holocene deposits are widespread, and although they have been extensively investigated by quaternarists for several decades, their malacological content has remained scarcely studied. In the framework of a German–French–Spanish collaboration, alluvial and tufa sequences in the Baza Basin, Andalusia, Spain are analysed with a multidisciplinary approach, including the analysis of the molluscan successions. Holocene fossil molluscs of the studied sequences reveal a rich assemblage of 53 species, among which 33 are terrestrial. Between 10,000 and 9,000 cal. yr BP, the Galera succession indicates strong river dynamics that prevented the establishment of adjacent terrestrial habitats. The Early to Middle Holocene transition reflects the continuity of humid environments. The Middle Holocene fauna reveals a decrease in humidity accompanied by slight evidence of drier habitats in the near periphery of the floodplain/tufa deposits from 5,900 cal. yr BP onwards. Late Holocene deposits (around 3,000 to 300 cal. yr BP) point to dry and open biotopes indicated by the appearance of xero-resistant and mesophilous gastropods. The palaeoenvironmental reconstruction inferred from the malacocenoses allows discussing local to regional responses of the malacofauna to environmental changes and is consistent with past regional environmental and climatic trends identified in southeastern Spain. The fossil record at Galera offers new evidence to question and clarify the modern range of endemic taxa whose distributions are fragmented. By following a ‘conservation paleobiology’ approach, the analysis of the mollusc fossil record at Galera provides (1) baselines to determine the natural variability of ecosystems and their response to environmental changes, (2) data to discuss and clarify species distribution evolution and (3) arguments to support conservation priorities.
... Subsequent Quaternary incision of the Oued el Haï renewed the relief and alternated with depositional phases, creating a sequence of five fluvial terraces, situated from +20-22 m to the modern river, and probably formed from the Middle Pleistocene to the Holocene ( Figure 2) (Benito-Calvo et al. 2020). Wengler and Vernet (1992) proposed a Holocene age for three lower terraces based on C 14 dates, while Depreux et al. (2021) described the alluvial architecture in the lowest terrace as nested and cut and fill alluvial formations ranging from the Early to Late Holocene (11,C 14 ). Other works indicate the presence of older terraces (Benito-Calvo et al. 2020), in a catchment with low incision rates (Bartz et al. 2019), and low gradient longitudinal profiles (Pastor et al. 2015). ...
... This suggests a Late Pleistocene age for these sediments of terrace T4 (+8-12 m) (Benito-Calvo et al. 2020), which is older than previous Holocene ages provided by Wengler and Vernet (1992). Following the cut and fill alluvial architecture proposed by Depreux et al. (2021), Late Pleistocene deposits could form the base of the fluvial terrace, before the incision-aggradation phases described for the Holocene. The sucession of different incision-aggradation phases whitout significant bedrock incision would agree with the low incision rates described in the Moulouya catchment (Bartz et al. 2019), and the low gradient longitudinal profiles described in the study area (Pastor et al. 2015). ...
Most biogeographers considered the Maghreb to be part of the Palearctic biogeographic region, though it is relatively recently that the proportion of Palearctic species increased there. How and when exactly these biogeographic changes occurred is not well understood, but they are probably the result of the increasing aridification of the Sahara and decreasing global temperatures. Fossils of Bos primigenius and Ceratotherium simum from a new fossil locality in a terrace of the Oued el Haï (NE Morocco) contribute to our understanding of some of these biogeographic processes and their timing. They also suggest an age between ~57 and ~100 ka for the terrace. The same evolutionary change in Bos in Europe and the Maghreb suggests geneflow as the most parsimonious explanation, though parallel evolution is an alternative possibility. Oued el Haï has the oldest well-documented record of C. simum in the Maghreb. The dispersal of this species to North Africa, where it replaced C. mauritanicum, may have occurred during a ‘Green Sahara Period’ between 80–85 or 100–105 ka, when the Maghreb had acquired already a Palearctic character. The study of the biogeography and requirements of the large mammals of the Maghreb may provide information to calibrate the region’s climate modelling.
... Fluvial systems occupy a key position in paleoenvironmental research, providing important archives of environmental and geomorphic changes, especially during the late Pleistocene and Holocene (e.g., Arbogast et al., 2008;Zielhofer et al., 2008;Kirchner et al., 2015Kirchner et al., , 2022von Suchodoletz et al., 2015;Depreux et al., 2021). In the last century, global climatic changes and increased human pressure on fluvial systems and their catchments have spurred interest in understanding the long-term dynamics of fluvial systems, particularly during the late Pleistocene and the Holocene. ...
High-resolution sedimentological and geochronological investigations of paleochannel systems in the Ried Central d'Alsace (northeastern France) allow for the reconstruction of the late glacial and Holocene fluvial evolution of this section of the Upper Rhine alluvial plain. During the Oldest Dryas, the landscape featured a dominant braided Rhine system and, to a lesser extent, a braided Fecht system. The shift to the Bølling-Allerød saw a narrowing of the Rhine's active channel belt, the development of a complex channel pattern, and the genesis of the Ill River. The river channel patterns remained unchanged during the Younger Dryas. In the Early Holocene, the Rhine's active belt narrowed further, and the Rhine and Ill Rivers developed braided-anastomosing and anastomosing channel patterns, respectively. Throughout the Holocene, both rivers maintained their channel patterns while migrating east and west across the alluvial plain, respectively. In the late glacial, fluvial dynamics in this section of the Upper Rhine plain were primarily influenced by climate-related environmental and hydrogeomorphological changes. Conversely, during the Holocene, the evolution of the fluvial hydrosystems was driven by a complex interaction of climatic and non-climatic factors, including human activity at the catchment scale, alluvial plain architecture, and local neotectonics.
... 40 Chavaillon J. 1964.41 Depreux B. et al. 2021.42 Bouzouggar A. et Barton R. N. E. 2012.43 ...
The Aïn Beni Mathar – Guefaït (ABM-GFT) region in Eastern Morocco is the object of anarchaeological, palaeontological, geological and geochronological research project, led by an interna-tional team since 2006. The research in this former fluvio-lacustrine basin, roughly 2000 km2, hasrevealed a significant number of Pleistocene and Holocene sites. Here we introduce the research pro-ject, that we conduct in the region, the main issues it aims to address, and the results already obtained
(18) (PDF) Pleistocene and Holocene peopling of Jerada province, eastern Morocco: introducing a research project. Available from: https://www.researchgate.net/publication/358736473_Pleistocene_and_Holocene_peopling_of_Jerada_province_eastern_Morocco_introducing_a_research_project [accessed Mar 11 2022].
... 40 Chavaillon J. 1964.41 Depreux B. et al. 2021.42 Bouzouggar A. et Barton R. N. E. 2012.43 ...
The Aïn Beni Mathar – Guefaït (ABM-GFT) region in Eastern Morocco is the object of an
archaeological, palaeontological, geological and geochronological research project, led by an international team since 2006. The research in this former fluvio-lacustrine basin, roughly 2000 km2, has
revealed a significant number of Pleistocene and Holocene sites. Here we introduce the research project, that we conduct in the region, the main issues it aims to address, and the results already obtained.
Keywords: Pleistocene, Holocene, Human peopling, Eastern Morocco, Green Sahara
The African Humid Period (AHP, ~14.8–5.5 ka BP) is a key time period of the lateglacial and early Holocene when, under a wetter climate regime, areas of the central Sahara that are now dry once experienced flowing rivers and permanent lakes. More extensive vegetation and fauna then developed and this was able to sustain a greater intensity of and likely more sedentary patterns of human activity. For this reason, this period is known informally as the ‘Green Sahara’. Evidence for the AHP comes from a range of geomorphic, sedimentary, biological, and archaeological data from across North Africa in particular. In the central Sahara, this evidence provides insight into AHP climates, environments, and the ways in which prehistoric human activity exploited these environmental conditions. This chapter reviews the AHP as a time period, including its onset and termination, the evidence for climatic and environmental conditions during this period, and the richness and diversity of the archaeological record. The AHP provides a good example of the close interrelationships between climate change, land surface processes and human activity, and as such may usefully inform on the sensitivity of both environmental and human systems to climate forcing.
This work focuses on litho-stratigraphic and malacological studies of the fluvial deposits of the Middle Moulouya, which are used to reconstruct the paleoenvironments and paleoclimates of Morocco during the Holocene. The objectives were to deduce environmental and climatic changes from the study of fluvial deposits known for their richness in paleontological remains and their spatial and temporal variations. The chronostratigraphic framework of the fluvial record is based on the radiocarbon dating of a dozen charcoal assemblages.
Sidi Bouramdan is located in the Midelt region in the Ksabi basin (the Middle Moulouya) of Morocco. The Holocene plain morphodynamics are characterised by a cut-and-fill terrace system associated with a progressive deepening of the riverbed. At the level of Sidi Bouramdan, a geomorphological and lithological study identified five Holocene nested terraces of over 10-m sedimentary thickness, dated to between 10248 and 10649 cal BP and 782–921 cal BP. These show a strong chronostratigraphic discontinuity with formations of the middle Holocene period being locally absent, probably because they were destroyed by the evolution of the alluvial plain.
Forty-four mollusc species were identified in the malacological study of this fluvial Holocene sequence, with changes in the malacological communities reflecting environmental variations in the fluvial system. The Upper Holocene terrace T0 was characterized by marshland-preferring species, thereby revealing a wet environment. In the other three lower Late Holocene terraces, aquatic species tended to disappear, and terrestrial species predominated, indicating the appearance and development of a dry environment in phase with paleoenvironmental data from various regional marine and continental sedimentary archives.
The upper Unstrut River is located in Germany at the modern Central European climate boundary of Cfb and Dfb climate. The river drains a loess landscape, which has experienced important environmental changes throughout the last 12,000 years. To evaluate the impacts of these changes on floodplainevolution, a multi-proxy research program, consisting of 2D electrical resistivity tomography profiling (ERT), vibracoring, and sedimentological investigations, 14C and OSL dating were applied. From base to top the investigations the following fluvial deposits were revealed: (1) gravels embedded in a fine-grained sediment matrix (interpreted as fluvial bedload deposits); (2) silty sediment with pedogenic features (interpreted as overbank floodplain deposits); (3) peat and tufa deposits (interpreted as wetland deposits) intercalated by pedogenetically influenced silty sediments (interpreted as overbank deposits); (4) humic silty sediment with some pedogenic features (interpreted as overbank floodplain deposits); and (5) silty sediments (interpreted as overbank deposits). Radiocarbon and luminescence dates yielded the following periods for sediment formation: (1) Younger Dryas to Preboreal period (around 11.6 cal ka BP); (2) Preboreal to early Atlantic period (approx. 11.6 to 7.0 cal ka BP); (3) early Atlantic to late Subboreal period (approx. 7.3 to 3.4 cal ka BP); (4) late Subboreal to early Subatlantic period (2.9 to 2.3 cal ka BP); and (5) late Subatlantic period (approx. 1.0 to 0.6 cal ka BP). The results suggest that floodplain development during the latest Pleistocene and early Holocene (approx. 11.6 to 7.0 cal ka BP) was considerably controlled by climatic conditions and short-term climate variabilities, which caused gravel deposition and overbank sedimentation. Afterwards floodplain conditions varied between rather stable (peat and tufa development, initial soil formation) and active periods (deposition of overbank fines). In this context, active periods with increased sediment input prevailed from approx. 5.1 to 3.4 cal ka BP, 2.9 to 2.3 cal ka, and 1.0 to 0.6 cal ka BP, temporally corresponding well with increased land-use phases of the past. In conclusion this study demonstrates that the investigated Unstrut catchment has reacted very sensitively to natural and human-induced changes during the latest Pleistocene and Holocene. Consequently, this high vulnerability to external changes should be considered in future river predictions or river management.
The Early–Mid Holocene transition is a period of profound changes in climatic mechanisms and hydrological features in Europe and North Africa. The melting of the Laurentide ice sheet led to an oceanic and atmospheric reorganisation in the North Atlantic, while the Mediterranean underwent a major hydrological shift. The impacts on Mediterranean rivers remain unclear, as there are few records documenting responses to the 8.2 ka event (the main Holocene climatic degradation). We present a fluvial record from Eastern Morocco documenting detailed hydrological variations from 8200 to 7500 cal. BP and their climatic forcing. A major hydrogeomorphic evolution of the Charef River occurred at that time, marked by two major incision stages close in time, under hyper-arid conditions at 8200 and ca. 7500 cal. BP. The impacts of these phenomena on the alluvial plains and associated archaeological records during Neolithisation, a major process in human history, currently remain unidentified. This new record sheds light on the fluvial response to the 8.2 ka event in North Africa and why other records are missing. We also bring new insights into the hydrological disruption at the Early–Mid Holocene transition, which was driven by the end of deglaciation combined with insolation and solar forcing. Furthermore, centennial solar variability may have paced river activity in the Moulouya basin and arid regions of North Africa.
Radiocarbon (C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.
Enhanced summer insolation during the early and mid-Holocene drove increased precipitation and widespread expansion of vegetation across the Sahara during the African humid period (AHP). While changes in atmospheric dynamics during this time have been a major focus of palaeoclimate modelling efforts, the transient nature of the shift back to the modern desert state at the end of this period is less well understood. Reconstructions reveal a spatially and temporally complex end of the AHP, with an earlier end in the north than in the south and in the east than in the west. Some records suggest a rather abrupt end, whereas others indicate a gradual decline in moisture availability. Here we investigate the end of the AHP based on a transient simulation of the last 7850 years with the comprehensive Earth system model MPI-ESM1.2. The model largely reproduces the time-transgressive end of the AHP evident in proxy data, and it indicates that it is due to the regionally varying dynamical controls on precipitation. The impact of the main rain-bringing systems, i.e. the summer monsoon and extratropical troughs, varies spatially, leading to heterogeneous seasonal rainfall cycles that impose regionally different responses to the Holocene insolation decrease. An increase in extratropical troughs that interact with the tropical mean flow and transport moisture to the western Sahara during the mid-Holocene delays the end of the AHP in that region. Along the coast, this interaction maintains humid conditions for a longer time than further inland. Drying in this area occurs when this interaction becomes too weak to sustain precipitation. In the lower latitudes of west Africa, where the rainfall is only influenced by the summer monsoon dynamics, the end of the AHP coincides with the retreat of the monsoonal rain belt. The model results clearly demonstrate that non-monsoonal dynamics can also play an important role in forming the precipitation signal and should therefore not be neglected in analyses of north African rainfall trends.
We present new high‐resolution oxygen isotope (δ¹⁸O) records from three NW African speleothems located at ~31°N. The present‐day rainfall patterns at 31°N in NW Africa are linked to negative winter North Atlantic Oscillation phases. However, on multimillennial time scales, our δ¹⁸O records, together with other hydroclimate records, provide new evidence of humid conditions during the mid‐Holocene, a period that was presumably characterized by arid climate. Thus, the apparent increase in moisture during the mid‐Holocene is interpreted better as an increase in summer rainfall. This is most likely linked to the expansion of the West African summer monsoon fringe during the African Humid Period, which terminated in our record abruptly around 4 Kyr BP. The temporospatial difference with speleothem records from N Morocco suggests that the High‐Atlas Mountains might have been a topographic barrier to further expansion of the West African summer monsoon fringe into higher latitudes.
http://wiredspace.wits.ac.za/handle/10539/28129
Wonderkrater in the Limpopo Province in South Africa is a late Quaternary archaeological site with peat deposits extending back more than 30 000 years before the present. Palaeoclimatic indices based on multivariate analysis of pollen spectra reflect a decline in temperature identifiable with the Younger Dryas (YD). A prominent spike in platinum is documented in a Wonderkrater sample (5614) with a mean date of 12 744 cal yr BP using a Bayesian model, preceding the onset of the YD cooling event. The YD platinum spike at
Wonderkrater is the first to be observed in Africa in the southern hemisphere, supplementing new discoveries from Patagonia in South America, in addition to more than 25 sites with such platinum anomalies in the northern hemisphere. The observations from South Africa serve to strengthen ongoing assessments of the controversial YD Impact Hypothesis, whereby it is proposed that a meteorite or cometary impact contributed to a decline in temperature, associated inter alia with dispersion of atmospheric dust, mammalian extinctions and cultural changes.
http://wiredspace.wits.ac.za/handle/10539/28129
Plain Language Summary
During the Holocene, periods of enhanced ice‐rafting, associated with cooling and sea ice expansion in the North Atlantic high latitudes, have been recognized over distant regions. While the causes of these events are still a matter of debate, changes in the atmospheric circulation have been proposed as a potential trigger or amplifier. Here, we use speleothems to establish a precisely dated record of rainfall variability in the Western Mediterranean, a highly sensitive region to the westerly winds belt modulated by the North Atlantic Oscillation. Our results reveal new insights about the dynamics of NAO‐like patterns during the Holocene. On centennial to millennial timescales, prolonged periods of both north and south shifts of the westerly winds belt might trigger or amplify the slowdown of the Atlantic Meridional Overturning Circulation and the North Atlantic ice‐rafting. However, not all ice‐rafting events are associated with either positive or negative NAO‐like conditions.
Gerard C. Bond established a Holocene series of North Atlantic ice-rafted debris events based on quartz and haematite-stained grains recovered from subpolar North Atlantic marine cores. These so-called “Bond events” document nine large-scale and multi-centennial North Atlantic cooling phases that might be linked to a reduced thermohaline circulation. Regardless of the high prominence of the Holocene North Atlantic ice-rafted debris record, there are critical scientific comments on the study: the Holocene Bond curve has not yet been replicated in other marine archives of the North Atlantic and there exist only very few palaeoclimatic studies that indicate all individual Bond events in their own record. Therefore, evidence of consistent hydro-climatic teleconnections between the subpolar North Atlantic and distant regions is not clear. In this context, the Western Mediterranean region presents key hydro-climatic sites for the reconstruction of a teleconnection with the subpolar North Atlantic. In particular, variability in Western Mediterranean winter precipitation might be the result of atmosphere–ocean coupled processes in the outer-tropical North Atlantic realm.
Based on an improved Holocene δ18O record from Lake Sidi Ali (Middle Atlas, Morocco), we correlate Western Mediterranean precipitation anomalies with North Atlantic Bond events to identify a probable teleconnection between Western Mediterranean winter rains and subpolar North Atlantic cooling phases. Our data show a noticeable similarity between Western Mediterranean winter rain minima and Bond events during the Early Holocene and an opposite pattern during the Late Holocene. There is evidence of an enduring hydro-climatic change in the overall Atlantic atmosphere–ocean system and the response to external forcing during the Middle Holocene. Regarding a potential climatic anomaly around 4.2 ka (Bond event 3) in the Western Mediterranean, a centennial-scale winter rain maximum is generally in-phase with the overall pattern of alternating “wet and cool” and “dry and warm” intervals during the last 5000 years.
We present new global maps of the Köppen-Geiger climate classification at an unprecedented 1-km resolution for the present-day (1980–2016) and for projected future conditions (2071–2100) under climate change. The present-day map is derived from an ensemble of four high-resolution, topographically-corrected climatic maps. The future map is derived from an ensemble of 32 climate model projections (scenario RCP8.5), by superimposing the projected climate change anomaly on the baseline high-resolution climatic maps. For both time periods we calculate confidence levels from the ensemble spread, providing valuable indications of the reliability of the classifications. The new maps exhibit a higher classification accuracy and substantially more detail than previous maps, particularly in regions with sharp spatial or elevation gradients. We anticipate the new maps will be useful for numerous applications, including species and vegetation distribution modeling. The new maps including the associated confidence maps are freely available via www.gloh2o.org/koppen.
The Sahara is the world's largest dust source with significant impacts on trans-Atlantic terrestrial and large-scale marine ecosystems. Contested views about a gradual or abrupt onset of Saharan aridity at the end of the African Humid Period dominate the current scientific debate about the Holocene Saharan desiccation. In this study, we present a 19.63 m sediment core sequence from Lake Sidi Ali (Middle Atlas, Morocco) at the North African desert margin. We reconstruct the interaction between Saharan dust supply andWestern Mediterranean hydro-climatic variability during the last 12,000 yr based on analyses of lithogenic grain-sizes, XRF geochemistry and stable isotopes of ostracod shells. A robust chronological model based on AMS 14C dated pollen concentrates supports our multi-proxy study. At orbital-scale there is an overall increase in southern dust supply from the Early Holocene to the Late Holocene, but our Northern Saharan dust record indicates that a gradual Saharan desiccation was interrupted by multiple abrupt dust increases before the ‘southern dust mode‘ was finally established at 4.7 cal ka BP. The Sidi Ali record features millennial peaks in Saharan dust increase at about 11.1, 10.2, 9.4, 8.2, 7.3, 6.6, 6.0, and 5.0 cal ka BP. Early Holocene Saharan dust peaks coincide with Western Mediterranean winter rain minima and North Atlantic cooling events. In contrast, Late Holocene dust peaks correspond mostly with prevailing positive phases of the North Atlantic Oscillation. By comparing with other North African records, we suggest that increases in Northern Saharan dust supply do not solely indicate sub-regional to regional aridity in Mediterranean Northwest Africa but might reflect aridity at a trans-Saharan scale. In particular, our findings support major bimillennial phases of trans-Saharan aridity at 10.2, 8.2, 6.0 and
4.2 cal ka BP. These phases coincide with North Atlantic cooling and a weak African monsoon.
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.
During the “Green Sahara” period (11,000 to 5000 years before the present), the Sahara desert received high amounts of rainfall, supporting diverse vegetation, permanent lakes, and human populations. Our knowledge of rainfall rates and the spatiotemporal extent of wet conditions has suffered from a lack of continuous sedimentary records. We present a quantitative reconstruction of western Saharan precipitation derived from leaf wax isotopes in marine sediments. Our data indicate that the Green Sahara extended to 31°N and likely ended abruptly. We find evidence for a prolonged “pause” in Green Sahara conditions 8000 years ago, coincident with a temporary abandonment of occupational sites by Neolithic humans. The rainfall rates inferred from our data are best explained by strong vegetation and dust feedbacks; without these mechanisms, climate models systematically fail to reproduce the Green Sahara. This study suggests that accurate simulations of future climate change in the Sahara and Sahel will require improvements in our ability to simulate vegetation and dust feedbacks.
The limited availability of high-resolution continuous archives, insufficient chronological control, and complex hydro-climatic forcing mechanisms lead to many uncertainties in palaeo-hydrological reconstructions for the Western Mediterranean. In this study we present a newly recovered 19.63 m long core from Lake Sidi Ali in the North African Middle Atlas, a transition zone of Atlantic, Western Mediterranean and Saharan air mass trajectories. With a multi-proxy approach based on magnetic susceptibility, carbonate and total organic C content, core-scanning and quantitative XRF, stable isotopes of ostracod shells, charcoal counts, Cedrus pollen abundance, and a first set of diatom data, we reconstruct Western Mediterranean hydro-climatic variability, seasonality and forcing mechanisms during the last 12,000 yr. A robust chronological model based on AMS ¹⁴C dated pollen concentrates supports our high-resolution multi-proxy study. Long-term trends reveal low lake levels at the end of the Younger Dryas, during the mid-Holocene interval 6.6 to 5.4 cal ka BP, and during the last 3000 years. In contrast, lake levels are mostly high during the Early and Mid-Holocene. The record also shows sub-millennial- to centennial-scale decreases in Western Mediterranean winter rain at 11.4, 10.3, 9.2, 8.2, 7.2, 6.6, 6.0, 5.4, 5.0, 4.4, 3.5, 2.9, 2.2, 1.9, 1.7, 1.5, 1.0, 0.7, and 0.2 cal ka BP. Early Holocene winter rain minima are in phase with cooling events and millennial-scale meltwater discharges in the sub-polar North Atlantic. Our proxy parameters do not show so far a clear impact of Saharan air masses on Mediterranean hydro-climate in North Africa. However, a significant hydro-climatic shift at the end of the African Humid Period (∼5 ka) indicates a change in climate forcing mechanisms. The Late Holocene climate variability in the Middle Atlas features a multi-centennial-scale NAO-type pattern, with Atlantic cooling and Western Mediterranean winter rain maxima generally associated with solar minima.
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 ann) 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 ann 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.
Pollen and clay mineralogical analyses of a Holocene
sequence from Sebkha Boujmel (southern Tunisia) trace the climatic and
environmental dynamics in the lower arid bioclimatic zone over the last 8000 years.
During the mid- to late Holocene transition, between ca. 8 and 3 ka BP,
a succession of five wet–dry oscillations is recorded. An intense arid
event occurs between ca. 5.7 and 4.6 ka BP. This episode marks the onset of
a long-term aridification trend with a progressive retreat of Mediterranean
woody xerophytic vegetation and of grass steppes. It ends with the
establishment of pre-desert ecosystems around 3 ka BP. The millennial-scale
climate change recorded in the data from Sebkha Boujmel is consistent with
records from the south and east Mediterranean, as well as with climatic
records from the desert region for the end of the African Humid Period
(AHP). Eight centennial climatic events are recorded at Sebkha Boujmel and
these are contemporary with those recorded in the Mediterranean and in the
Sahara. They indicate a clear coupling between the southern Mediterranean
and the Sahara before 3 ka BP. The event at 4.2 ka BP is not evidenced and
the link between events recorded in Sebkha Boujmel and the North Atlantic cooling events is clearer from ca. 3 ka BP onwards. These variations
indicate the importance of climatic determinism in the structuring of
landscapes, with the establishment of the arid climatic conditions of the
late Holocene. It is only from ca. 3 ka BP onwards that the dynamic of plant
associations is modified by both human activity and climatic variability.
The climatic episodes identified during the historic period indicate strong
regionalisation related to the differential impact of the North Atlantic
Oscillation (NAO) and the Mediterranean Oscillation (MO) on the
Mediterranean Basin. The local human impact on regional ecosystems is
recorded in the form of episodes of intensification of pastoral and/or
agricultural activities. The development of olive production and of several
taxa associated with agriculture attest to increasing sedentism among human
populations during classical antiquity. The significant increase in
Artemisia (wormwood) between ca. 1.1 and 0.8 ka BP (850–1150 AD) is linked to
intensive pastoral activity, associated with heightened interannual and/or
seasonal climatic instability. A complete reshaping of the landscape is
recorded during the 20th century. The remarkable expansion of the olive
tree, and the deterioration of regional ecosystems with the spread of desert
species, is linked to recent local socio-economic changes in Tunisia.
The West African Monsoon (WAM) is crucial for the socio-economic stability of millions of people living in the Sahel. Severe droughts have ravaged the region in the last three decades of the 20th century, highlighting the need for a better understanding of the WAM dynamics. One of the most dramatic changes in the West African Monsoon (WAM) occurred between 15000–5000 yr BP, when increased summer rainfall led to the so-called “Green Sahara” and to a reduction in dust emissions from the region. However, model experiments are unable to fully reproduce the intensification and geographical expansion of the WAM during this period, even when vegetation over the Sahara is considered. Here, we use a fully coupled simulation for 6000 yr BP (Mid-Holocene) in which prescribed Saharan vegetation and dust concentrations are changed in turn. A closer agreement with proxy records is obtained only when both the Saharan vegetation changes and dust decrease are taken into account. The dust reduction strengthens the vegetation–albedo feedback, extending the monsoon's northern limit approximately 500 km further than the vegetation-change case only. We therefore conclude that accounting for changes in Saharan dust loadings is essential for improving model simulations of the WAM during the Mid-Holocene.
When dealing with current and past landscape evolution, a key issue addresses responses of geomorphic systems to the large number of influencing variables. Identifying morphodynamic phases and revealing interrelations with specific driving forces are demanding tasks for Quaternary research. In this paper, we present late Pleistocene and Holocene fluvial sedimentation patterns of three Western Mediterranean river catchments, namely Jarama River, Guadalete River and Guadalquivir River that extent along a climatic transect from semi-humid SW-Spain to semi-arid central Spain. These studies are based on extensive fieldwork conducted on 36 exposures and 13 drillings in floodplain positions. Field data is supported by geochemical analyses, while the chronological framework was obtained from the analyses of 70 radiocarbon samples. Results show distinct patterns of fluvial sedimentation as well as soil formation linked to floodplain stability for each river catchment. On regional or catchment scale, pollen stratigraphical correlation and comparison with lacustrine records show that fluvial dynamics have a strong reaction to climatic shifts, with phases of high fragility characterized by catchment erosion and floodplain sedimentation in response to climatic aridification events and phases of climate change in general. The comparison of the examined river systems reveals that periods of supra-regional floodplain sedimentation in several catchments occurred from 8.0 to 7.0, 5.0 to 3.8, 2.2 to 1.5, and around 1.0 as well as 0.4 ka cal. BP, while we found periods of supra-regional soil formation from 13.3 to 12.7, 7.0 to 5.1 (with a short interruption around 6.0 to 5.5 ka), 2.8 to 2.3 ka, 1.4 to 1.2 ka, and 0.8 to 0.5 ka cal. BP. Beside these consistencies we found deviating dynamic patterns that are apparently expressed in terms of differing onset and offset, differing durations, or even the lack of fluvial system response. The main reasons for this can be seen in different regional climate condition and impacts of further influencing factors, or in different levels of sensitivity of the river catchments that may be controlled by initial hydrological conditions, catchment size, or the degree of anthropogenic influence. A larger scale assessment shows that fluvial dynamic patterns are hardly comparable across entire Spain due to strong spatial heterogeneity of physiographic and climatic conditions on the Iberian Peninsula, in particular when areas are influenced by different circulation systems (e.g. regions influenced by the Atlantic Ocean vs. regions influenced by the Mediterranean Sea). However, the consideration of North Atlantic marine records reveals a certain coupling between North Atlantic coolings, atmospheric processes leading to arid climate over large parts of Spain, as well as increased landscape instability including strong fluvial sedimentation activity. Attendant atmospheric conditions are discussed.
Dans les zones arides et semi-arides de l’Afrique du Nord, notamment au Maroc, la notion de précarité des ressources en eau n’est pas récente. Cependant, la sécheresse qui a affecté les pays du Bassin méditerranéen lors des dernières décennies s’est manifestée d’une manière particulièrement sévère, persistante et caractérisée par une intensité remarquable avec des conséquences dramatiques liées aux déséquilibres importants des ressources en eau aussi bien quantitativement que qualitativement.
Dans ce contexte, nous nous proposons, d’étudier à différentes échelles, l’évolution récente et la variabilité spatio-temporelle des pluies au Maroc, de caractériser la sècheresse climatique et d’évaluer son impact sur la disponibilité des ressources en eau. La dernière phase de ce travail est consacrée à la réactualisation de la carte des précipitations au Maroc.
Les données de 312 postes climatologiques, géographiquement représentatifs des grands domaines climatiques du pays, sont traités, homogénéisées et validées sur la période de mesures la plus complète disponible : 1935-2005.
Les variabilités, structurelle et relative ainsi que les tendances sont analysées et étudiées aux pas de temps annuel et saisonnier. Les ruptures sont détectées par trois tests statistiques recommandés par l’OMM. On diagnostique ainsi, les années sèches et non sèches par application de l’indice pluviométrique standardisé (IPS). La méthode des moyennes mobiles centrées (9 ans) est utilisée pour rechercher les éventuelles tendances internes et identifier les périodes sèches et humides. Enfin, des causes possibles de cette évolution sont proposées.
Il ressort que les pluies au Maroc se caractérisent par une grande variabilité inter-annuelle et intra-annuelle. La majorité des stations ont signalé une tendance à la baisse des précipitations à l’exception de la région Saharienne et la frange littoral, Casablanca-Safi, qui ont montré une tendance à la hausse des pluies statistiquement non significative. La période de rupture se situe entre 1970 et 1980. Les régions montagneuses ont subi les réductions pluviométriques les plus importantes avoisinant 20 %. Tandis que le territoire marocain méditerranéen a subi une réduction pluviométrique de 17.8 %. A l’échelle nationale, la réduction est de l’ordre de 13 %. L’application de l’indice pluviométrique standardisé, nous a permis de déceler une phase déficitaire avant 1956, une phase normale à excédentaire jusqu’en 1970 et au-delà de cette date, la sècheresse s’est installée d’une manière significative sur l’ensemble du territoire. L’effet combiné de l’oscillation Nord-Atlantique (ONA) au Nord et de la progression de la désertification par le Sud influence sans doute le régime pluviométrique au Maroc et par conséquent l’extension spatio-temporelle de la sécheresse.
Enfin, nous avons essayé de réactualiser la carte des précipitations au Maroc en utilisant des mesures sur une période assez longue allant de 1935-2005. Ceci a été réalisé en combinant deux approches ; géostatistique, et empirique.
Apport du modèle géologique 3D à l'étude de la structure de l'aquifère profond d'Aïn Béni Mathar, Maroc Oriental Contribution of the 3D geological model to the study of the Aïn Béni Mathar deep aquifer structure, Eastern Morocco. Résumé. L'aquifère profond d'Aïn Béni Mathar est un important réservoir d'eau au Maroc. Il est formé de calcaires dolomitiques du Jurassique et mis en charge sous un recouvrement généralement argilo-marneux d'âge mio-plio-quaternaire. Vu que c'est la seule ressource en eau d'une région connue par son climat aride et la pénurie de ses eaux de surface, la nappe est de plus en plus sollicitée à des fins multiples. Il en résulte une baisse lente mais continue de son niveau piézométrique, constituant ainsi un problème non encore résolu. Les informations sur cette nappe sont encore fragmentaires et insuffisantes, en raison de sa large étendue et de sa profondeur importante. Dans le but d'améliorer la connaissance de cet aquifère, un modèle géologique 3D a été élaboré dans cette étude. Il a été construit à partir des informations fournies par les cartes géologiques et par 174 sondages mécaniques. Les forages compilés ont été analysés, encodés et intégrés dans le logiciel Groundwater Modeling System (GMS). Pour ce faire, quatre ensembles lithostratigraphiques ont été considérés : le Paléozoïque, le Trias, le Jurassique et le Mio-Plio-Quaternaire. Le modèle élaboré permet de visualiser la géométrie générale du bassin et plus particulièrement celle de l'aquifère jurassique qui s'enfonce progressivement vers le Nord et vers le Nord-Ouest où il dépasse les 800 m, au niveau la zone d'effondrement de l'Oued El Haï. Mots-clés : Maroc oriental, Aïn Béni Mathar, aquifère profond, modèle géologique 3D. Abstract. The deep aquifer of Aïn Béni Mathar is an important groundwater reservoir in Morocco. It is composed of Jurassic dolomitic limestone and covered by a clay-marly impermeable layer of Mio-Plio-Quaternary age. Since this is the only water source in an area known by its arid climate and surface water shortage, groundwater is increasingly sought for multiple purposes. This is reflected by a slow but continuous decline of the piezometric level. It constitutes a problem that is not yet solved. The knowledge of this aquifer is still fragmentary and very insufficient, due to its wide extent and its important depth. In order to improve this knowledge, a 3D geological model was elaborated in this study. It was elaborated on the basis of geological maps and geological data extracted from 174 boreholes. Compiled wells data were analyzed, encoded and embedded with the software Groundwater Modeling System (GMS). Four litho-stratigraphical groups were considered, namely: Paleozoic, Triassic, Jurassic and Mio-Plio-Quaternary. The model allows visualizing the general architecture of the Jurassic aquifer which gradually sinks toward the North and NorthWest , where it exceeds 800 m in deep at the collapse area of the Oued El Haï. Keywords : Eastern Morocco, Aïn Béni Mathar basin, deep aquifer, 3D geological model.
During the African Humid Period about 14,800 to 5,500 years ago, changes in incoming solar radiation during Northern Hemisphere summers led to the large-scale expansion and subsequent collapse of the African monsoon. Hydrologic reconstructions from arid North Africa show an abrupt onset and termination of the African Humid Period. These abrupt transitions have been invoked in arguments that the African monsoon responds rapidly to gradual forcing as a result of nonlinear land surface feedbacks. Here we present a reconstruction of precipitation in humid tropical West Africa for the past 20,000 years using the hydrogen isotope composition of leaf waxes preserved in sediments from Lake Bosumtwi, Ghana. We show that over much of tropical and subtropical Africa the monsoon responded synchronously and predictably to glacial reorganizations of overturning circulation in the Atlantic Ocean, but the response to the relatively weaker radiative forcing during the African Humid Period was more spatially and temporally complex. A synthesis of hydrologic reconstructions from across Africa shows that the termination of the African Humid Period was locally abrupt, but occurred progressively later at lower latitudes. We propose that this time-transgressive termination of the African Humid Period reflects declining rainfall intensity induced directly by decreasing summer insolation as well as the gradual southward migration of the tropical rainbelt that occurred during this interval.
Crater sediments from the Rodderberg Volcanic Complex (East Eifel, Germany) provide a climate and environmental record spanning several glacial-to-interglacial cycles. We document and describe the lithology and demonstrate that the composition of this sediment record can be interpreted
with regard to environmental variations as well as to changes in lacustrine productivity. Based on major lithological changes and preliminary pollen data, three interglacials are preserved in superposition revealing fluctuations of physical and chemical properties during the Middle to Late
Pleistocene.
German
Die Sedimente aus dem Krater des Rodderberg-Vulkankomplexes (Osteifel) stellen ein Umwelt- und Klimaarchiv zur Verfügung, das mehrere Glazial-Interglazial-Zyklen umfasst. Wir dokumentieren die Lithologie und zeigen, dass der Aufbau dieser Sedimentsequenz Umweltveränderungen
und Schwankungen der lakustrinen Produktivität belegt. Basierend auf lithologischen Veränderungen und vorläufigen Pollenanalysen können drei übereinander liegende Interglaziale nachgewiesen werden, die Schwankungen der physikalischen und geochemischen Eigenschaften
während des Mittel- bis Spätpleistozäns zeigen.
Historically CoDaPack 3D has intended to be a software of Compositional Data with an easy and intuitive way of use. For this reason from the beginning it has been associated to Excel, a software known and used for many people. However, over the years different versions of Excel and Windows have been appeared and CoDaPack has had to be adapted to these new versions due to some incompatibilities. For this reason, and also because of CoDaPack only works with Excel under windows, the Girona Compositional Data Group has decided to implement a new software with at least the same capabilities and the same profile of users but independent of any other software. The graphical user interface has three different areas: The variables area, the data area and the results area which has a textual output window and independent graphical output. Also, because the new CoDaPack is being developed under Java code, the final software is going to work in any platform having a Java Virtual Machine: Windows, Linux and other Unix based systems.
monsoon precipitation experienced a dramatic change in the course of the Holocene. The pace with which the African monsoon shifted from a strong early to middle to a weak late Holocene is critical for our understanding of climate dynamics, hydroclimate-vegetation interaction, and shifts of prehistoric human settlements, yet it is controversially debated. On the basis of planktonic foraminiferal Ba/Ca time series from the eastern Mediterranean Sea, here we present a proxy record of Nile River runoff that provides a spatially integrated measure of changes in East African monsoon (EAM) precipitation. The runoff record indicates a markedly gradual middle to late Holocene EAM transition that lasted over 3500 years. The timing and pace of runoff change parallels those of insolation and vegetation changes over the Nile basin, indicating orbitally forced variation of insolation as the main EAM forcing and the absence of a nonlinear precipitation-vegetation feedback. A tight correspondence between a threshold level of Nile River runoff and the timing of occupation/abandonment of settlements suggests that along with climate changes in the eastern Sahara, the level of Nile River and intensity of summer floods were likely critical for the habitability of the Nile Valley (Egypt).
CLIMATE records from lake sediments in tropical Africa, Central America and west Asia show several century-scale arid intervals during the Holocene1–10. These may have been caused by temporary weakening of the monsoonal circulation associated with reduced northward heat transport by the oceans7 or by feedback processes stimulated by changes in tropical land-surface conditions10. Here we use a lake-sediment record from the montane Mediterranean zone of Morocco to address the question of whether these events were also felt in temperate continental regions. We find evidence of arid intervals of similar duration, periodicity and possibly timing to those in the tropics. But our pollen data show that the forest vegetation was not substantially affected by these events, indicating that precipitation remained adequate during the summer growing season. Thus, the depletion of the groundwater aquifer that imprinted the dry events in the lake record must have resulted from reduced winter precipitation. We suggest that the occurrence of arid events during the summer in the tropics but during the winter at temperate latitudes can be rationalized if they are both associated with cooler sea surface temperatures in the North Atlantic.
Fluvio-lacustrine Holocene sequences from the semi-arid region of eastern Morocco are rich in shells and
show similar sedimentary levels that could correspond to synchronous environmental variations. The high
diversity of Moroccan continental malacofaunas makes them a very suitable fossil group for palaeoenvironmental
reconstructions and palaeobiodiversity implications. The malacological study of Blirh is
the first in Morocco to be based on a systematic sampling providing a complete faunal record in order to
reconstruct palaeoenvironmental successions. Three biozones have been recognized. They illustrate
occurrence of a marsh and a lake from shortly before 10,600 cal. BP followed by an abrupt drying up
around 5000 cal. BP. This dramatic environmental change is not synchronous with lithological variations
as stated by previous studies based on geological and palynological records. The Blirh lacustrine phase fits
into the humid period that follows full glacial hyperarid conditions during the latest Pleistocene in North
Africa from 14,800 cal. BP. The abrupt shift to dry conditions around 5000 cal. BP inferred from the
malacological succession correlates with the contraction of Saharan lakes. During the last biozone, despite
disappearance of the main water-body, marshy species persist and demonstrate occurrence of very wet
environments in the Middle Moulouya plateau until historical times. Several species, currently absent
from the region, were found within the Holocene assemblages. At least two new species of aquatic
molluscs (Hydrobiidae) have been discovered. Sampling of modern malacofaunas from wells and springs
in the area has demonstrated that one of them is still living in the water-table. The Blirh malacological
record provides an insight of the palaeo-biogeographical changes affecting this group during the Holocene
and demonstrates the very recent increase of human impact on the regional biodiversity.
In comparison to the rest of Europe, Africa, and Asia, most rivers arising and flowing within the Mediterranean watershed typically drain small catchments with mountainous headwaters. The hydrology of Mediterranean catchments is strongly influenced by the seasonal distribution of precipitation, catchment geology, vegetation type and extent, and the geomorphology of the slope and channel systems. It is important to appreciate, as the preceding chapters have shown, that the area draining to the Mediterranean Sea is large and enormously variable in terms of the key controls on catchment hydrology outlined above, and it is therefore not possible to define, in hydrological terms, a strict single Mediterranean river type. However, river regimes across the basin do have a marked seasonality that is largely controlled by the climate system (Chapter 3) and, in most basins, the dominant flows occur in winter—but autumn and spring runoff is also important in many areas. These patterns reflect the general water balance of the basin as a whole, but there are key geographical patterns in catchment hydrology and sediment yield and a marked contrast is evident between the more humid north and the semi-arid south and east (Struglia et al. 2004; Chapter 21). Also, because of the long history of vegetation and hillslope modification by human activity and the more recent and widespread implementation of water resource management projects, there are almost no natural river regimes in the Mediterranean region, especially in the middle and lower reaches of river catchments (Cudennec et al. 2007). Runoff generation on hillslopes in the Mediterranean is very closely related to rainfall intensities and land surface properties as discussed in Chapter 6. While this is probably true of most catchments, runoff generation in the Mediterranean is very sensitive to vegetation cover because of the seasonal dynamics of rainfall and the role played by extreme events. The cumulative effect of these characteristics is a specific set of management problems and restoration issues and, although these are rather different in the various socio-political regimes of the region, it can be argued that they are in many ways unique to Mediterranean catchments.
On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north– south transect, data collected in the central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and temporal coherency. Contrasting patterns of palaeohydrological changes have been evidenced in the central Mediterranean: south (north) of around 40◦ N of latitude, the middle part of the Holocene was characterised by lake-level maxima (minima), during an interval dated to ca. 10 300–4500 cal BP to the south and 9000–4500 cal BP to the north. Available data suggest that these contrasting palaeohydrological patterns operated throughout the Holocene, both on millennial and centennial scales. Regarding precipitation seasonality, maxi- mum humidity in the central Mediterranean during the middle part of the Holocene was characterised by humid winters and dry summers north of ca. 40◦ N, and humid winters and summers south of ca. 40◦ N. This may explain an apparent conflict between palaeoclimatic records depending on the proxies used for reconstruction as well as the synchronous expansion of tree species taxa with contrasting climatic requirements. In addition, south of ca. 40◦ N, the first millennium of the Holocene was characterised by very dry climatic conditions not only in the eastern, but also in the central- and the western Mediterranean zones as reflected by low lake levels and delayed reforestation. These results suggest that, in addition to the influence of the Nile discharge reinforced by the African monsoon, the deposition of Sapropel 1 has been favoured (1) by an increase in winter precipitation in the northern Mediterranean borderlands, and (2) by an increase in winter and summer precipitation in the southern Mediterranean area. The climate reversal following the Holocene climate optimum appears to have been punctuated by two major climate changes around 7500 and 4500 cal BP.
In the central Mediterranean, the Holocene palaeohydrological changes developed in response to a combination of orbital, ice-sheet and solar forcing factors. The maximum humidity interval in the south-central Mediterranean started ca. 10 300 cal BP, in correlation with the decline (1) of the possible blocking effects of the North Atlantic anticyclone linked to maximum insolation, and/or (2) of the influence of the remnant ice sheets and fresh water forcing in the North Atlantic Ocean. In the north-central Mediterranean, the lake-level minimum interval began only around 9000 cal BP when the Fennoscandian ice sheet disap- peared and a prevailing positive NAO-(North Atlantic Oscillation) type circulation developed in the North Atlantic area. The major palaeohydrological oscillation around 4500– 4000 cal BP may be a non-linear response to the gradual de- crease in insolation, with additional key seasonal and inter- hemispheric changes. On a centennial scale, the successive climatic events which punctuated the entire Holocene in the central Mediterranean coincided with cooling events associated with deglacial outbursts in the North Atlantic area and decreases in solar activity during the interval 11700– 7000 cal BP, and to a possible combination of NAO-type circulation and solar forcing since ca. 7000 cal BP onwards. Thus, regarding the centennial-scale climatic oscillations, the Mediterranean Basin appears to have been strongly linked to the North Atlantic area and affected by solar activity over the entire Holocene.
In addition to model experiments, a better understanding of forcing factors and past atmospheric circulation pat- terns behind the Holocene palaeohydrological changes in the Mediterranean area will require further investigation to establish additional high-resolution and well-dated records in selected locations around the Mediterranean Basin and in adjacent regions. Special attention should be paid to greater precision in the reconstruction, on millennial and centen- nial timescales, of changes in the latitudinal location of the limit between the northern and southern palaeohydrological Mediterranean sectors, depending on (1) the intensity and/or characteristics of climatic periods/oscillations (e.g. Holocene thermal maximum versus Neoglacial, as well as, for instance, the 8.2 ka event versus the 4 ka event or the Little Ice Age); and (2) on varying geographical conditions from the west- ern to the eastern Mediterranean areas (longitudinal gradi- ents). Finally, on the basis of projects using strategically located study sites, there is a need to explore possible influences of other general atmospheric circulation patterns than NAO, such as the East Atlantic–West Russian or North Sea– Caspian patterns, in explaining the apparent complexity of palaeoclimatic (palaeohydrological) Holocene records from the Mediterranean area.
To understand the effects of future climate change on the ecology of the central Mediterranean we can look to the impacts of long-term, millennial to centennial-scale climatic variability on vegetation in the basin. Pollen data from the Adriatic Marine core MD 90-917 allows us to reconstruct vegetation and regional climate changes over the south central Mediterranean during the Holocene. Clay mineral ratios from the same core reflect the relative contributions of riverine (illite and smectite) and eolian (kaolinite) contributions to the site, and thus act as an additional proxy with which to test precipitation changes in the Holocene. Vegetation reconstruction shows vegetation responses to the late-Glacial Preboreal oscillation, most likely driven by changes in seasonal precipitation. Pollen-inferred temperature declines during the early-mid Holocene, but increases during the mid-late Holocene, similar to southern-western Mediterranean climatic patterns during the Holocene. Several short climatic events appear in the record, indicating the sensitivity of vegetation in the region to millennial-scale variability. Reconstructed summer precipitation shows a regional maximum between 8000 and 7000 cal yr BP similar to the general pattern across southern Europe. Two important shifts in vegetation occur at 7700 and between 7500 and 7000 yr. These vegetation shifts are linked to changes in seasonal precipitation and are correlated to increased river inputs respectively from the north (7700 event) and from the central Adriatic borderlands (7500–7000 event). These results reinforce the strengths of multi-proxy analysis and provide a deeper understanding of the role of precipitation and particularly the seasonality of precipitation in mediating vegetation change in the central Mediterranean during the Holocene.
Astronomically forced insolation changes have driven monsoon dynamics and recurrent humid episodes in North Africa, resulting in green Sahara Periods (GSPs) with savannah expansion throughout most of the desert. Despite their potential for expanding the area of prime hominin habitats and favouring out-of-Africa dispersals, GSPs have not been incorporated into the narrative of hominin evolution due to poor knowledge of their timing, dynamics and landscape composition at evolutionary timescales. We present a compilation of continental and marine paleoenvironmental records from within and around North Africa, which enables identification of over 230 GSPs within the last 8 million years. By combining the main climatological determinants of woody cover in tropical Africa with paleoenvironmental and paleoclimatic data for representative (Holocene and Eemian) GSPs, we estimate precipitation regimes and habitat distributions during GSPs. Their chronology is consistent with the ages of Saharan archeological and fossil hominin sites. Each GSP took 2-3 kyr to develop, peaked over 4-8 kyr, biogeographically connected the African tropics to African and Eurasian mid latitudes, and ended within 2-3 kyr, which resulted in rapid habitat fragmentation. We argue that the well-dated succession of GSPs presented here may have played an important role in migration and evolution of hominins.
Palaeoenvironmental significance and climate change in the Maghreb. Eastern Morocco in the late Pleistocene.
After the discovery of new prehistoric and geological sites in eastern Morocco, multidisciplinary studies were made on a regional scale about prehistoric cultures, sediments, grains charcoal, spores and pollen, fauna and chronology. They allowed treating the following topics:
-•the evolution of flora and climates during the Rharbian (= Holocene) and Soltanian (= Würm) periods, i.e. about one hundred thousand years, according to altitude and latitude;
- the palaeoenvironmental meaning of glacis-alluvial terrace systems, calcretes and sheltered deposits.
The palaeoflora are the same as actually and only the vegetation boundaries fluctuate. These variations decrease from North to the South. The Soltanian is a dry period with three humid interstages during which the bioclimate is semi-arid to subhumid. The Rharbian is principally humid with three short arid periods.
The glacis-terrace systems are built during the arid-humid transition and also during the humid interstadial period like the calcretes at the top of the soltanian formations. The sheltered deposits have recorded both arid and humid stages.
The simplex plays an important role as sample space in many practical situations where compositional data, in the form of proportions of some whole, require interpretation. It is argued that the statistical analysis of such data has proved difficult because of a lack both of concepts of independence and of rich enough parametric classes of distributions in the simplex. A variety of independence hypotheses are introduced and interrelated, and new classes of transformed‐normal distributions in the simplex are provided as models within which the independence hypotheses can be tested through standard theory of parametric hypothesis testing. The new concepts and statistical methodology are illustrated by a number of applications.
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.
This paper presents data from 42 new samples yielding Late Pleistocene cosmogenic ¹⁰Be and ³⁶Cl exposure ages of moraine boulders across a series of glaciated valleys in the Toubkal Massif (4167 m a.s.l.), High Atlas, Morocco. This represents the first comprehensive Pleistocene glacial chronology in North Africa and one of the largest datasets from the Mediterranean region. The timing of these glacier advances has major implications for understanding the influence of Atlantic depressions on moisture supply to North Africa and the Mediterranean basin during the Pleistocene. The oldest and lowest moraines which span elevations from ∼1900 to 2400 m a.s.l. indicate that the maximum glacier advance occurred from MIS 5 to 3 with a combined mean ¹⁰Be and ³⁶Cl age of 50.2 ± 19.5 ka (1 SD; n = 12, 7 outliers). The next moraine units up-valley at higher elevations (∼2200–2600 m a.s.l.) yielded exposure ages close to the global Last Glacial Maximum (LGM) with a combined mean ¹⁰Be and ³⁶Cl age of 22.0 ± 4.9 ka (1 SD; n = 9, 7 outliers). The youngest exposure ages are from moraines that were emplaced during the Younger Dryas with a combined mean ¹⁰Be and ³⁶Cl age of 12.3 ± 0.9 ka (1 SD; n = 7, no outliers) and are found in cirques at the highest elevations ranging from ∼2900 to 3300 m a.s.l. From moraines predating the Younger Dryas, a large number of young outliers are spread evenly between 6 and 13 ka suggesting a continuing process of exhumation or repositioning of boulders during the early to mid-Holocene. This attests to active seismic processes and possibly intense erosion during this period.
Fluvial archives, in particular, late glacial to Holocene floodplain records may provide valuable information regarding past environmental conditions and stages of landscape evolution. In view of the high significance of floodplain dynamics for the development of entire landscapes, the number of studies that have been performed on floodplain sediments remains comparatively low, especially regarding the Western Mediterranean region. However, one of the reasons could be seen in the high complexity and diversity of processes and factors that control and influence fluvial activity that often hampers a straightforward interpretation of floodplain dynamics. Therefore, a basic demand on fluvial archive research is to address the complexity of the factors that control the characteristics of fluvial sequences in order to provide a robust basis for their interpretation.
As a starting-point for discussion this paper aims to give an overview of fluvial dynamic patterns in the Western Mediterranean for the last 15 ka in order to examine their relevance for palaeoenvironmental reconstructions. The basis for this is provided by previous investigations on four different river systems in Spain and northern Africa the results of which are herein synthesized in order to propose a regional Late Pleistocene and Holocene fluvial history for the Western Mediterranean realm. Basic results are related to alluvial floodplain deposits and visible features within them such as alluvial soils, incision marks and aggradation phenomena. Since fluvial systems are open systems, we discuss our findings against the background of different influencing factors that could modify fluvial architecture and may restrict palaeoenvironmental reconstructions. A more comprehensive interpretation focuses on signals that are common to each of the studied river systems. In this respect, we critically discuss the customary nature of cumulative probability functions for the identification of regional flooding episodes and point out the benefits of a stratigraphy-supported approach for characterizing regional floodplain dynamics. Finally, three alluvial soil formation periods were found in all settings: a first phase after 15 ka, ending with the Younger-Dryas Event; a second one from 7 to 5 ka with a break between 6.5 and 5.5 ka, and a third phase between 3 and 2 ka. These soil formation periods were interrupted and framed by fluvial dynamic phases accompanied by channel aggradation, floodplain deposition, floodplain erosion and/or river incision. In particular, after 5 ka, around 1.6 and at 1 ka, and during the Little Ice Age (LIA) floodplain aggradation affected river systems in Iberia and northern Africa as well. A cautious assessment of these results and extensive comparisons with secondary archive information prompts us to relate the aggradation periods with climate forcing by means of a supra-regional aridification that effected large areas of the Western Mediterranean. In contrast, the three mentioned soil formation periods can be linked with landscape stability, most probably triggered by favorable climate conditions in the Western Mediterranean.
Apart from these large-scale patterns we discuss the reliability of information emerging from floodplain records against the background of individualistic river behavior and self-organization. Regarding future work we want to emphasize the great potential of yet rarely applied system-oriented approaches that also attach importance to sub-catchment dynamics as a link between catchment slopes and the main river floodplain.
In lake sediments where terrestrial macrofossils are rare or absent, AMS radiocarbon dating of pollen concentrates may represent an important alternative solution for developing a robust and high resolution chronology suitable for Bayesian modelling of age-depth relationships. Here we report an application of the heavy liquid density separation approach (Vandergoes and Prior, Radiocarbon 45:479–492, 2003) to Holocene lake sediments from karstic Lake Sidi Ali, Morocco. In common with many karstic lakes, a significant lake ¹⁴C reservoir effect of 450–900 yr is apparent, evidenced by paired dates on terrestrial macrofossils and either aquatic (ostracod) or bulk sediment samples. AMS dating of 23 pollen concentrates alongside laboratory standards (bituminous coal, anthracite, IAEA C5 wood) was undertaken. Concentrates were prepared using a series of sodium polytungstate (SPT) solutions of progressively decreasing density (1.9–1.15 g/cm³) accompanied by microscopic analysis of the resulting residues to allow quantification of the terrestrial pollen content. The best fractions (typically precipitating at 1.4–1.2 g/cm³) yielded dateable samples of 0.5–5 mg (from sediment samples of ∼15 g), with C content typically ∼50% by weight. Terrestrial pollen purity ranges from 29% to 88% (μ = 67%), reflecting the challenge of isolating pollen grains from common aquatic algae, e.g. Pediastrum and Botryococcus. A Poisson-process Bayesian depositional model incorporating radiocarbon (pollen and macrofossil) and ²¹⁰Pb/¹³⁷Cs data is employed. As all pollen samples incorporate some non-terrestrial organic matter, we assume an exponential outlier distribution treating each pollen concentrate datum as an old outlier and terminus post quem. This approach yields strong data-model agreement, and differences between the prior and posterior age distributions are furthermore consistent with theoretical offsets anticipated for the known reservoir ages and sample-specific terrestrial content. This application of the pollen concentrate dating approach reinforces the importance of microscopic inspection of the residues during the separation and sieving stages. Sample specific differences mean that the pollen concentrate preparation cannot be reduced to a simplistic “black box” protocol, and dating and subsequent age-model development must be supported by detailed analysis of the microfossil content of the sediments.
Our study focuses on the ephemeral stream deposits of Wadi Selloum to identify phases of morphodynamic stability (pedogenesis) and activity (flooding) in the direct vicinity of the rock shelter of Ifri n'Ammar (NE Morocco). As one of the oldest settlement sites of anatomically modern humans (AMH) in North Africa, Ifri n'Ammar documents periodical occupations since ~ 170 ka. Since the discontinuous settlement record may reflect climate forcing with subsequent landscape changes, we aim at reconstructing the palaeoenvironmental variability recorded in the ephemeral stream deposits. In addition to the use of micromorphological, sedimentological, geochemical and mineralogical methods, the geochronological framework of ephemeral stream deposits was established by the application of different luminescence dating techniques.
The Schwalbenberg II loess-paleosol sequence (LPS) denotes a key site for Marine Isotope Stage (MIS) 3 in Western Europe owing to eight succeeding cambisols, which primarily constitute the Ahrgau Subformation. Therefore, this LPS qualifies as a test candidate for the potential of temporal high-resolution geochemical data obtained by X-ray fluorescence (XRF) scanning of discrete samples providing a fast and non-destructive tool for determining the element composition. The geochemical data is first contextualized to existing proxy data such as magnetic susceptibility (MS) and organic carbon (C-org) and then aggregated to element log ratios characteristic for weathering intensity [LOG (Ca/Sr), LOG (Rb/Sr), LOG (Ba/Sr), LOG (Rb/K)1 and dust provenance [LOG (Ti/Zr), LOG (Ti/Al), LOG (Si/Al)1. Generally, an interpretation of rock magnetic particles is challenged in western Europe, where not only magnetic enhancement but also depletion plays a role. Our data indicates leaching and top -soil erosion induced MS depletion at the Schwalbenberg II LPS. Besides weathering, LOG (Ca/Sr) is susceptible for secondary calcification. Thus, also LOG (Rb/Sr) and LOG (Ba/Sr) are shown to be influenced by calcification dynamics. Consequently, LOG (Rb/K) seems to be the most suitable weathering index identifying the Sinzig Soils S1 and S2 as the most pronounced paleosols for this site. Sinzig Soil S3 is enclosed by gelic gleysols and in contrast to S1 and S2 only initially weathered pointing to colder climate conditions. Also the Remagen Soils are characterized by subtle to moderate positive excursions in the weathering indices. Comparing the Schwalbenberg II LPS with the nearby Eifel Lake Sediment Archive (ELSA) and other more distant German, Austrian and Czech LPS while discussing time and climate as limiting factors for pedogenesis, we suggest that the lithologically determined paleosols are in -situ soil formations. The provenance indices document a Zr-enrichment at the transition from the Ahrgau to the Hesbaye Subformation. This is explained by a conceptual model incorporating multiple sediment recycling and sorting effects in eolian and fluvial domains.
The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia. The index that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear. A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic freshened surface waters, affecting the strength of the meridional overturning circulation. Here we present a Holocene rainfall record from northwest Africa based on speleothem 18 O and compare it against a speleothem-based rainfall record from Europe. The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets.
A new fossil record from the southern Middle Atlas spans continuously the last 25 000 years and provides evidence of an increased amount of snow precipitation during the last glacial period and a warm early Holocene with rather dry climate conditions. This environmental reconstruction is based on a multi-proxy approach that integrates pollen, micro-charcoals, grain size and geochemical analysis. During the last glacial period we observe a strong presence of aquatic plants species that today flower during late spring and summer. These occurrences are related to an increased amount of snow precipitation on the surrounding mountains which fed the marsh during the summer season. Although the early Holocene reveals a slight and steady expansion of Mediterranean oaks, the semi-arid Artemisia steppe remained dominant in the landscape until 6.8 ka cal BP. Thus, the early Holocene seems to have been less humid than elsewhere in North Africa. The Atlas cedars began to establish around 6 ka cal BP. This indicates that the amount of annual rainfall increased after the mid-Holocene. The late Holocene is marked by an increase of fire events, which may be related to a strengthening of the dry season and/or a higher seasonality.
XRF core scanners represent a major innovation in the analysis of cored sediment sequences and have revolutionised palaeoenvironmental research over the last decade. Such scanners provide capability to rapidly and non-destructively record element proxy variations at decadal, annual and even sub-annual scales. Their use, initially by the marine science community, was soon taken up by lake core researchers, particularly after the advent of high-resolution models incorporating x-radiography, particularly suited for analysis of varved sequences. Their impact on the environmental sciences is seen in the exponential rise in research papers published since 2005 involving their use. Although their main application has been in the study of Quaternary marine and lake cores, they have also been used in the analysis of terrestrial hard rock cores in mining applications, analysis of loess cores, speleothems, cores from peat bogs and river banks and cores collected for environmental forensics and pollution studies. Further, an important cohort of papers has addressed interpretation and calibration issues, increasing the robustness of acquired datasets. In this paper we review marine and lacustrine applications of XRF core scanning, together with environmental forensics applications and research into data optimisation and calibration presented in the current volume. We provide synopses of the principal findings and a concise summary of the current work.
Mediterranean fluvial hydrology is characterised by decadal-to-multi-centennial length wet and dry episodes with abrupt transitions related to changes in atmospheric circulation. Since the mid-1990s site-based flood chronologies from slackwater deposits in bedrock rivers and regionally aggregated flood histories from alluvial deposits have developed increasingly higher resolution chronological frameworks, although regional coverage is still uneven. This paper analyses the spatial and temporal distribution of extreme Holocene hydrological events recorded in fluvial stratigraphy in the Iberian Peninsula (Spain and Portugal), southern France, southern Italy, Northern Africa (Morocco and Tunisia) and eastern Mediterranean (Greece, Crete, Turkey, Cyprus and Israel). This study constitutes the most comprehensive investigation of Holocene river flooding ever undertaken in the Mediterranean and is based on the analysis of 515 14C and 53 OSL dates. It reveals that flood periods in different regions cluster into distinct time intervals, although region-wide flooding episodes can be identified at 7400–7150, 4800–4600, 4100–3700, 3300–3200, 2850–2750, 2300–2100, 1700–1600, 1500–1400, 950–800, ca. 300, 200–100 cal. BP. Periods with more frequent floods in the western Iberian region coincide with transitions to cool and wetter conditions and persistent negative NAO mode. In Northern Africa increased flood frequency coincides with periods of generally drier climate, while in the eastern Mediterranean there is a higher incidence of extreme flood events under wetter conditions. Our meta-data analysis identifies an out-of-phase pattern of extreme events across the Mediterranean over multi-centennial timescales, which is particularly evident between the western Iberian and eastern Mediterranean regions. This centennial-to-multi-centennial see-saw pattern in flooding indicates that bipolar hydroclimatic conditions existed in the Mediterranean during the Holocene.
In only a few decades, the ecological balance has been deeply endangered in the High-Plateaux of Eastern Morocco as a result of the excessive and irrational exploitation of soil and vegetation, which has been caused by the socio-spatial mutations of the XXth century relating to the colonialist economy. Several projects of intervention have succeeded in this area, which is pastoral by excellence. Studies have especially been concerned with the productivity of biotic resources and the socio-economic aspects of stock-breeding. The degradation of the ecosystems is approached essentially in its biocenotic context, while the processes, the forms and the extent of erosion and desertification are marginalized. Such a situation for such a sensitive and degraded environment entails the apprehension of those problems by systemic and largely pluridisciplinary approaches.
A new record from the heart of the Moroccan Middle Atlas cedar forests spans the last 18 000 years and provides valuable insight into our understanding of the natural vegetation and environmental changes. The approach is based on the study of pollen content, geochemical elements and grain size analysis. The pollen data indicate that the vegetation was dominated by herbaceous plants until 9000 BP. Such open landscape allowed greater soil erosion and an input of chemical elements from the watershed. After 9000 BP, tree cover, mainly oak, increased slightly and was accompanied by a higher taxonomic diversity. However, several steppe elements remain well represented in the area until 5000 BP, which suggests that the climate was rather dry during the first part of the Holocene. After 6000 BP, the climate became more favourable to expansion of the forest ecosystems, including Cedrus atlantica, thereby reducing erosion. A strong reduction of the tree pollen percentages is recorded after 2000 BP, which may be related to increasing human activities during the Roman period. These forest changes are concomitant with an increase of lead and copper concentrations in the record, probably related to Roman metalworking activities.
Projected future changes in climate call for more definite information on the magnitude–frequency relationships of major floods than can be obtained from short instrumental or historical records alone. Floodplains represent a sedimentary archive of unrecorded flood events, which can potentially be used to extend existing flood records by centuries or millennia. In this study a c. 3750-year flood record was produced using ln(Zr/Rb) profiles from two 4-m-deep cores through floodplain silts at the Roundabout in the upper Severn catchment, UK. Sediment geochemical profiles were obtained using high resolution (500 μm) XRF core scanning. Ages were assigned to the flood record using 14C dating and contaminant Pb profiles. Comparison of sediment ln(Zr/Rb) with sediment grain size showed that ln(Zr/Rb) increases with increasing grain size and indicates that it can be used as a grain size proxy at this site. Within the ln(Zr/Rb) flood record, two floods that occurred at c. 235 and c. 10 cal. BC were probably the largest that have occurred at the site during the past 3750 years while the historical flood of A.D. 1795 appears to be the largest since c. AD 200. The record shows that, during the past c. 3750 years, multicentennial periods characterized by the occurrence of high magnitude floods (return periods > c. 30 years) have alternated with periods of similar length without such floods. These periods correspond to large-scale variations in hydroclimate recorded in the North Atlantic region. However, the incidence of the highest magnitude floods appears to be unaffected by changes in catchment land use. This study provides information about flood magnitude–frequency relationships and their controls at a local level that could be used to help catchment managers in assessing future flood risk at a time of rapidly changing climate.