The aggradation of alluvial fans in response to monsoon variability over the last 400 ka in the Hajar Mountains, south-east Arabia

The preservation and interpretation of past external forcings within sedimentary archives can be complicated. Marine proxy records show that, throughout the Mid-Late Quaternary, monsoon rainfall in south-east Arabia has varied at precessional timescales (∼ 23 ka). By contrast, terrestrial environmental records from the region, such as speleothems and palaeolake sequences, generally only capture rainfall variability at eccentricity (∼ 100 ka) timescales and geomorphological archives rarely record events over multiple glacial cycles. Previous work has suggested that the alluvial fan systems of the Hajar Mountains record aggradation coincident with precessional peaks because of more northerly Indian Ocean Summer Monsoon (IOSM) rainfall, relative to present. However, the alluvial record is complex: there is a growing body of ‘anomalous’ aggradation age data from periods of precessional minima and it is spatially uneven, mainly being derived from fans at either the northern- or southern-most extents of the western side of the mountains. We present optically stimulated luminescence (OSL) ages of alluvial aggradation from new sites in the central portion of the western and eastern sides of the Hajar mountains. Sedimentary units that show evidence of more sustained flow conditions, such as coarse conglomerate deposition and the development of thick floodplain deposits, typically date to periods of higher IOSM rainfall at precessional timescales. Sites where there is evidence of deposition under more ephemeral flow conditions, like today, yield ages corresponding to precessional minima. We synthesise these data with similarly interpreted alluvial and lacustrine deposits from across the region, employing a hierarchical clustering approach to generate clusters of ages representing periods of increased hydrological activity. This objective approach to regional chronological data suggests that ten periods of more sustained hydrological activity are evidenced across the Hajar region, broadly aligning to precessional and eccentricity peaks over the last 400 ka. These clusters show that Hajar fans predominately aggrade because of increasing rainfall over their catchments, with the central ages of clusters often aligning with the onset of precessional forcing. Hajar alluvial systems preserve important records of Quaternary climate variability over long timescales in an arid region where terrestrial records are scarce due to preservation issues

Orbital-scale climate variability in Arabia as a potential motor for human dispersals

The Arabian Peninsula is situated at an important crossroads for the movement of Pleistocene human populations out of, and into, Africa. Although the timings, routes and frequencies of such dispersals have not yet been confirmed by genetic, fossil or archaeological evidence, expansion into Arabia would have been facilitated by humid periods driven by incursions of monsoon rainfall, potentially from both Indian Ocean and African monsoon systems. Here we synthesise terrestrial and marine core palaeoclimatic data in order to establish the spatial and temporal variability of humid periods in Arabia between late Marine Isotope Stage (MIS) 7 and 3. Incursions of monsoon rainfall occurred during periods of insolation maxima at ca. 200–190, 170, 155, 130–120, 105–95, 85–75 and 60–55 ka, providing multiple ‘windows’ of favourable climatic conditions that could have facilitated demographic expansion through Arabia. Strong summer monsoons are generally associated with mid-high latitude interglacials, however, enhanced monsoon convection also brought rainfall into Arabia during global glacial phases, possibly due to a strengthened winter monsoon and a greater influence of southern hemispheric temperature changes. Key periods for dispersal into northern regions of Arabia correspond with the synchronous intensification of both eastern Mediterranean and monsoon rainfall systems at insolation maxima during MIS 7 and MIS 5, which may have facilitated demographic connectivity between the Levant and the Arabian interior. Environmental conditions throughout southern and southeast regions were also favourable to expansion during these times, although strong monsoons in these regions during MIS 6 and MIS 3 suggest further opportunities for demographic expansion and exchange. Terrestrial and marine evidence show that during early MIS 3 (ca. 60–50 ka), a strengthened monsoon led to the activation of interior drainage systems and increased productivity in coastal zones, indicating that favourable environmental conditions existed along both coastal and interior routes at that time

RELACIÓN ENTRE EL RENDIMIENTO ACADÉMICO DE LOS INGRESANTES Y EL ABANDONO DE LOS ESTUDIOS UNIVERSITARIOS: COMPARACIÓN ENTRE CARRERAS DE INGENIERÍA Y ECONOMICAS

La preocupación por orientar adecuadamente los recursos que se afectan a los programas de apoyo a los estudiantes de primer año de las carreras, nos llevó a trabajar intentando generar un modelo que sea capaz de determinar, anticipadamente, que estudiantes se encontrarían en situación de abandonar sus estudios. En una primera aplicación del modelo, se analizó la correlación entre el rendimiento académico y el abandono para diez cohortes de ingresantes, de las dos carreras de Ingeniería que ofrece la Universidad Nacional de Luján, observándose una significativa correlación entre el rendimiento académico del estudiante en el primer cuatrimestre y el abandono de los estudios durante el segundo, tercero y cuarto cuatrimestre. En este trabajo, presentamos los resultados que se alcanzaron al aplicar el mismo modelo a las diez cohortes, equivalentes, de estudiantes de las dos carreras del campo de las Ciencias Económicas que se ofrecen en la misma Institución: Licenciatura en Administración y Licenciatura en Comercio Internacional. Las principales conclusiones a las que arribamos es que el modelo se valida positivamente al cambiar el origen disciplinar de las carreras en las que se aplica. Por otra parte, existe mayor correlación entre el rendimiento académico en el primer cuatrimestre y el momento en que se abandonan los estudios para los estudiantes de carreras de Ciencias Económicas que la observada en los de Ingeniería. Se repite el fenómeno observado para las carreras de Ingeniería, de disminuir la correlación entre el rendimiento académico y el abandono para la cohorte 2002. También se detecta una diferencia significativa respecto de la moda para la cantidad de asignaturas aprobadas durante el primer cuatrimestre para las cuatro carreras

Alluvial fan records from southeast Arabia reveal multiple windows for human dispersal

The dispersal of human populations out of Africa into Arabia was most likely linked to episodes of climatic amelioration, when increased monsoon rainfall led to the activation of drainage systems, improved freshwater availability, and the development of regional vegetation. Here we present the first dated terrestrial record from southeast Arabia that provides evidence for increased rainfall and the expansion of vegetation during both glacial and interglacial periods. Findings from extensive alluvial fan deposits indicate that drainage system activation occurred during Marine Isotope Stage (MIS) 6 (ca. 160–150 ka), MIS 5 (ca. 130–75 ka), and during early MIS 3 (ca. 55 ka). The development of active freshwater systems during these periods corresponds with monsoon intensity increases during insolation maxima, suggesting that humid periods in Arabia were not confined to eccentricity-paced deglaciations, and providing paleoenvironmental support for multiple windows of opportunity for dispersal out of Africa during the late Pleistocene

An early MIS 3 pluvial phase in Southeast Arabia: climatic and archaeological implications

Climatic changes in Arabia are of critical importance to our understanding of both monsoon variability and the dispersal of anatomically modern humans (AMH) out of Africa. The timing of dispersal is associated with the occurrence of pluvial periods during Marine Isotope Stage (MIS) 5 (ca. 130–74 ka), after which arid conditions between ca. 74 and 10.5 ka are thought to have restricted further migration and range expansion within the Arabian interior. Whilst a number of records indicate that this phase of aridity was punctuated by an increase in monsoon strength during MIS 3, uncertainties regarding the precision of terrestrial records and suitability of marine archives as records of precipitation, mean that the occurrence of this pluvial remains debated. Here we present evidence from a series of relict lake deposits within southeastern Arabia, which formed at the onset of MIS 3 (ca. 61–58 ka). At this time, the incursion of monsoon rainfall into the Arabian interior activated a network of channels associated with an alluvial fan system along the western flanks of the Hajar Mountains, leading to lake formation. Multiproxy evidence indicates that precipitation increases intermittently recharged fluvial systems within the region, leading to lake expansion in distal fan zones. Conversely, decreased precipitation led to reduced channel flow, lake contraction and a shift to saline conditions. These findings are in contrast to the many other palaeoclimatic records from Arabia, which suggest that during MIS 3, the latitudinal position of the monsoon was substantially further south and did not penetrate the peninsula. Additionally, the occurrence of increased rainfall at this time challenges the notion that the climate of Arabia following MIS 5 was too harsh to permit the further range expansion of indigenous communities

Developing a framework of Quaternary dune accumulation in the northern Rub' al-Khali, Arabia

Located at the crossroads between Africa and Eurasia, Arabia occupies a pivotal position for human migration and dispersal during the Late Pleistocene. Deducing the timing of humid and arid phases is critical to understanding when the Rub' al-Khali desert acted as a barrier to human movement and settlement. Recent geological mapping in the northern part of the Rub' al-Khali has enabled the Quaternary history of the region to be put into a regional stratigraphical framework. In addition to the active dunes, two significant palaeodune sequences have been identified. Dating of key sections has enabled a chronology of dune accretion and stabilisation to be determined. In addition, previously published optically stimulated luminescence (OSL) dates have been put in their proper stratigraphical context, from which a record of Late Pleistocene dune activity can be constructed. The results indicate the record of dune activity in the northern Rub' al-Khali is preservation limited and is synchronous with humid events driven by the incursion of the Indian Ocean monsoon

Palaeoenvironmental and sea level changes during the Holocene in Eastern Saudi Arabia and their implications for neolithic populations

This paper presents the key findings of a multidisciplinary study investigating the nature and timing of coastal landscape evolution in eastern Saudi Arabia during the Holocene. To date, most sea level reconstructions for the Arabo-Persian Gulf are based on uncalibrated 14C ages without correction for marine reservoir effects, or lack precision with regard to the effects of neotectonic changes, indicators of sea level used, errors in elevation of sedimentary units used, and the relationship with actual tides. As a consequence, the nature and timing of relative sea level (RSL) changes during the Late Pleistocene and Holocene remain poorly understood. To help address this, we use sea level index points (SLIPs) based on calibrated 14C ages to present a RSL curve for the central-southern Gulf of Saudi Arabia from coastal sabkha deposits near the archaeological site of Dosariyah. The sediments record rapid transgression during the early Holocene with a midHolocene high-stand immediately prior to 6880–6560 cal. BP when the upper limit for the palaeo Mean Highest High tide water (MHHW) was 2.8–3.10 m above present day mean sea level. Transgression continued until shortly after 5575–5310 cal. BP with an upper limit to the palaeo-MHHW of 3.75 m above present sea levels. Thereafter a fall in RSL was recorded, with the regression leading to the progradation of the coastal system and the development of coastal sabkhas. Nonetheless later transgressions are recorded in the region between 4848–4536 and 4335–3949 cal BP. Radiometric dating results from archaeological excavations at Dosariyah, one of the most important Neolithic coastal sites in the Gulf, suggest that occupation of the site during the Neolithic coincides with the mid-Holocene marine transgression (ca. 7200 – 6500 cal. BP). Whilst the close proximity of the site to the sea may have facilitated maritime exchange activities, occupation of the site was short-lived and the phase of abandonment occurred during a period of rapid RSL rise, which would have transformed the area around Dosariyah into an island or certainly cut it off tidally from the mainland

Multiple phases of human occupation in Southeast Arabia between 210,000 and 120,000 years ago

Changing climatic conditions are thought to be a major control of human presence in Arabia during the Paleolithic. Whilst the Pleistocene archaeological record shows that periods of increased monsoon rainfall attracted human occupation and led to increased population densities, the impact of arid conditions on human populations in Arabia remains largely speculative. Here, we present data from Jebel Faya in Southeast (SE) Arabia, which document four periods of human occupation between c. 210,000 and 120,000 years ago. The Jebel Faya record indicates that human occupation of SE Arabia was more regular and not exclusively linked to major humid periods. Our data show that brief phases of increased rainfall additionally enabled human settlement in the Faya region. These results imply that the mosaic environments in SE Arabia have likely formed a population refugia at the end of the Middle and the beginning of the Late Pleistocene.

Middle-late Quaternary palaeoclimate variability from lake and wetland deposits in the Nefud Desert, Northern Arabia

Records of former lake and wetland development in present day arid/hyper-arid environments provide an important source of information for palaeoclimatic and palaeoenvironmental studies. In Arabia, such records are typically confined to eccentricity-modulated insolation maxima, and are often spatially and temporally discontinuous. Here we present records from a single locality in Northern Arabia of wetter interludes during both global interglacial and glacial conditions, providing a unique opportunity to examine the nature of these events in a common setting. At Jubbah, in the southern Nefud Desert, lake and wetland deposits reveal the repeated formation of a water body within a large endorheic basin over the past ca. 360 kyr. Lake/wetland formation occurred during MIS 11/9, 7, 5, 3 and the early Holocene, assisted by local topographic controls, and spring recharge. Palaeoenvironmental and palaeoecological data reveal the existence of a large still water body formed during either MIS 11 or 9 (ca. 363 ka), and basin wide alluviation followed by lake formation during MIS 7 (ca. 212 ka). During MIS 5e (ca. 130 ka) a large freshwater lake occupied the basin, while during MIS 5a (ca. 80 ka) the basin contained a shallow wetland and freshwater lake complex. Lake/wetland formation also occurred during early MIS 3 (ca. 60 ka), at the Terminal Pleistocene-Holocene transition (ca. 12.5 ka), and the early-middle Holocene (ca. 9–6.5 ka). Phases of lake and wetland development coincided with human occupation of the basin during the Middle Palaeolithic, Epipalaeolithic and Neolithic periods, highlighting the significance of the region for early demographic change