PP23B-1469
Evaluating Vegetation Feedbacks at 6kyBP Across Northern Africa in a GCM.
Geological evidence indicate that the mid-Holocene experienced stronger than present day monsoon circulations, particularly across northern Africa. Sahara saw more in the way of a Sahelian vegetation, rather than the present day hyper arid conditions. General Circulation Models notoriously underestimate the northwards expansion of the increase in the Saharan moisture budget at 6kyBP. Here results are presented from a model (HadSM3) run in which the vegetation has been altered across northern Africa and the Middle East to evaluate the vegetation -- climate feedbacks. In addition to altering the orbital boundary conditions to 6k settings, the vegetation is changed from desert to rain forest, as opposed to a more realistic 6kyBP vegetation cover of steppe. This is to tease out a stronger signal and see if the modelled climate can be kicked out of its regional dry regime.
PP23B-1470
A humid corridor across the Sahara for the migration "Out of Africa" of early modern humans 120,000 years ago
The climatic history of the Sahara in the Quaternary is likely to have been a key control on the history of human migrations out of Africa. For example, it is widely accepted that modern humans originated in sub- Saharan Africa around 150-200 thousand years ago (ka), but their route of dispersal across the currently hyper-arid Sahara remains controversial. Given that the first modern humans north of the Sahara are found in the Levant around 120-90 ka, northward dispersal likely occurred during the humid episode in the Sahara within Marine Isotope Stage (MIS) 5e (130-117 ka). The obvious dispersal route, the Nile, may be ruled out by notable differences between archaeological finds in the Nile Valley and the Levant at the critical time. Further west, space-born radar images reveal networks of - now buried - fossil river channels that extend across the desert to the Mediterranean coast, which represent alternative dispersal corridors. This would explain scattered findings at desert oases of Middle Stone Age (MSA) Aterian lithic industries with bifacial and tanged points that can be linked with industries further to the east and as far north as the Mediterranean coast. We have previously used neodymium and oxygen isotope data to suggest that there was enhanced Nile outflow to the Levantine Sea during Saharan humid periods in Stage 5e and the Holocene1. Here, however, we present further geochemical data which suggest that the freshwater signal from Africa was much stronger further west in the Ionian Sea. Furthermore, analyses of shell samples retrieved from the Libyan fossil river channels traces the geochemical signature found in the Ionian Sea to the Tibesti Mountains in the southern Sahara. These data demonstrate that water in the fossil river systems of the Sahara derived from the south during wet episodes in general, and penetrated all the way to the Mediterranean during MIS 5e in particular, confirming the existence of an uninterrupted freshwater corridor across a currently hyperarid region of the Sahara at a key time for early modern human migrations to the north and out of Africa. 1Scrivner, A.E. et al. (2004) Geology 32, 565-568.
PP23B-1471
Interglacial Surface Ocean Temperatures Reveal Strong Ocean-Atmosphere Linkages Between the Subtropical and Subpolar Regions
Investigating ocean-atmospheric processes of past interglaciations may provide a better understanding of
the climatic development of the present warm period. Here we present a detailed reconstruction of the
meridional sea surface temperature (SST) across MIS 11 using three sites: ODP 958 (off NW Africa), ODP
975 (western Mediterranean Sea) and M23414 (subpolar NE Atlantic). Sea surface temperatures (SST) were
derived from foraminiferal assemblages and alkenones, further supported by planktic and benthic O-isotope
records as well as by iron content, the latter as indicator for Saharan dust export and continental dryness. On
a large scale, reconstructed SSTs reveal a close synchronism of environmental changes between all three
sites. It shows that, before full interglacial conditions became established, there was a long-lasting, glacial-
interglacial transition with a total temperature increase of about 10°C in the subtropical as well as in the
subpolar North Atlantic. After the main glacial-interglacial transition was over, SSTs continued to rise and
reached their maximum during, what could be considered, the upper main phase of high sea-level stand in
MIS 11. In the western Mediterranean SSTs show several cooling events with amplitudes of up to 4°C
during the early full-interglacial warm period. Off NW Africa this phase is characterized by rather humid
conditions, as inferred from strongly decreased values of iron in the sediments, and successive changes in
the foraminiferal assemblages. We assume that these rather wet conditions at the lower latitudes were
related to an intensification of the West African monsoon that increased summer moisture flux over Northwest
Africa and runoff into the Mediterranean. At the same time increased abundance of subtropical, deep-
dwelling foraminiferal species are found in the subpolar region implying a strong linkage between the
subtropical atmospheric circulation regime over NW Africa and the northward propagation, of relatively warm
water masses from the south. The demise of peak interglacial conditions was associated at all sites with a
progressive surface ocean cooling, altogether indicating a remarkable good coherency of the overall climate
development within this North Atlantic sector during MIS 11.
http://www.clim-
past.net/4/181/2008/cp-4-181-2008.html
PP23B-1472
Ups and Downs of Palaeolake Megachad
Palaeolake Megachad had an area of at least 360,000 km2 which is greater than all the Great Lakes of North America combined. At its peak in it was around 170m deep, 1000 km long and 600 km wide, which would have made it the greatest freshwater lake in Africa. Today Lake Chad is greatly reduced and its northern sub-basin the Bodele Depression is dry. We report the results of optical dating of high-stand beach deposits from around the lake shores in Chad and Nigeria, and the ages of low-stand dune sands preserved within lake sediments on the basin floor. Optical ages from barchan dune sands preserved beneath lacustrine diatomite indicate that the lake bed was flooded at 9.1 ± 0.5 ka and again at 7.3 ± 0.4 ka to 7.4 ± 0.5 ka. Optical ages from shoreline deposits around the lake confirm highstands at 14.5 ± 0.8 ka, 11.4 ± 0.7 ka, 6.7 ± 0.7 ka, 6.0 ± 0.3 ka, 5.9 ± 0.5 ka, and 2.4 ± 0.1 ka. These ages indicate at least five different lake transgressions, one in the late Pleistocene (14.5 ka), another in the early Holocene (9.1 ka) another at 7.3-7.4 ka and a fourth at 6.3ka, and a later high-stand around 2.4 ka, although the latter is at a lower elevation than the earlier beach deposits indicating that the lake basin was only partially filled. The change from dry desert dunes to lake appears to have been extremely rapid and suggests abrupt changes from arid to humid conditions in an area which is now part of the semiarid to hyperarid Sahara desert.
PP23B-1473
Palaeohydrology of the Fazzan Basin, Libyan Sahara: Evidence for multiple phases of North African humidity.
The Fazzan is a large closed basin with an area of 450,000 km2, located in south-western Libya. The present-day climate is hyper-arid. Modern human occupation is restricted to oases, notably at the foot of the Messak Sattafat, a Nubian Sandstone escarpment which divides the Fazzan into two sub-basins. However widespread lithic scatters with varying inferred ages, and numerous Holocene hearth fields, indicate that the Fazzan has a long history of human occupation. In addition, regionally extensive lake deposits attest to the presence of a large palaeolake, here termed Lake Mega-Fazzan, at various periods in the past. Although the Sahara contains evidence for several other large palaeolakes, Lake Mega-Fazzan is the only one fed exclusively by rivers draining the Sahara proper. Thus, the Lake Mega-Fazzan sediments and archaeological remains provide an important resource for advancing our understanding of climate change in this part of the Sahara. Optically stimulated luminescence (OSL) dating techniques have been applied to a range of lacustrine deposits, and more experimentally to the Holocene hearth fields, to provide an internally consistent chronology for the Fazzan record. Results indicate that sediments within the Fazzan Basin record a very long history of palaeohydrological change. The oldest lacustrine sediments are beyond the range of conventional OSL dating techniques, but younger humid periods during oxygen isotope stages 11, 5 and 1 are recognised. Initial results from one hearth field also indicate that OSL has the potential to reveal human responses to changes in hydrology during the Holocene. These results, when compared with similar studies of adjacent closed basins, indicate that the Sahara may not always have provided as formidable a barrier to faunal migration as it does at present. The implications of this finding for our understanding of North African palaeoclimate and biogeography will be discussed.
PP23B-1474 INVITED
An Oceanic Perspective on the African Humid Period
Data gathered from cores drilled off NW Africa have been instrumental in understanding recent climate change in northern Africa, including the most recent African Humid Period (AHP). Early work by deMenocal et al. (2000, Quat. Sci. Rev. 19: 347-361) showed that dust (as terrigenous percent) increased and then decreased abruptly at the onset and termination of the AHP, and linked that on/off switch to a threshold crossing in precessional index. However, this perspective differs from recent lacustrine and palynological evidence from Lake Yoa in the eastern Sahara suggesting a more gradual transition out of the AHP (Kroepelin et al., 2008, Science 320: 765-768). We will present major and trace element and radiogenic isotope data from the last ~25 kyr at ODP 658C off Mauritania. Large and abrupt shifts, notably in Sr isotope ratios, are contemporaneous with changes observed in dust flux and cannot be explained simply by changes in geologic source terrane or grain size sorting that might result from a shift in wind direction or wind speed. We envisage the addition of highly weathered, authigenic mineral phase(s) formed in lakes during the AHP. Additionally, we will present new data from the recent CHEETA (Changes in the Holocene Environment of the Eastern Tropical Atlantic) sediment coring cruise, a transect of 28 coring stations from Gibraltar to Senegal. Preliminary results suggest that the abrupt termination of the AHP near 5.5 ka BP can be traced from Senegal to the Canary Islands (roughly 18-28°N), suggesting that the end of the AHP was indeed abrupt along the western African margin whereas it was more gradual in the eastern Sahara.
PP23B-1475
Climate Change and Human Occupation of the Sahara
The Sahara was void of settlement throughout the Last Glacial Maximum (LGM) and terminal Pleistocene (Kuper and Kreopelin, 2006). During early Holocene abrupt arrival of monsoon rains ~10,500 yrs BP produced a savanna allowing Human occupation. Between ~9.5 and ~7 kyrs BP was the wettest phase of the last 25,000 years across much of the Levant and Eastern Mediterranean (Robinson et. al., 2006). Kuper and Kreopelin, (2006) show timing of human occupation via archaeological data from Egypt, Sudan, Libya and Chad, with maximum human habitation in the Sahara North of 25 degrees at ~ 7.7 kyrs BP. Details of temporal variations in the Sun, and influence of these variations on the Earth's climate, remain contentious. None-the-less, analyses of paleoclimatic data and modeling (c.f. Braconnot et. al., 2007a, 2007b, 2008) indicate notable effects on human activities. We explore relationship of human occupation of the Sahara and long-term solar irradiance variations synergetic with atmospheric-ocean circulation patterns. The non-linear nature of climate change diminishes value of linear regression methods often used. To address these issues of non-stationary and non-linearity we applied wavelet decomposition and a new method, Empirical Mode Decomposition (EMD, Huang et al., 1998), designed account for non-linear/non-stationary features of the signals. EMD represents the data as a sum of a small number of orthogonal empirical modes that have time- variable amplitudes and frequencies. A mode has a symmetric envelope defined by the local maxima and minima so mean amplitude is zero everywhere. Mean period is determined by counting the number of peaks (maxima). Each mode is equivalent to an adaptively filtered signal in empirically determined (not imposed) frequency range. The last mode characterizes a nonlinear trend in the signal. The orthogonality of EMD modes prevents power leakage into a selected mode from other modes, an improvement over other types of filtering. We employ records taken over the globe (e.g. Greenland, Cariaco Basin, and African Ocean cores) to identify timing and spatial patterns affecting Saharan climate. Space based images provide a geographically comprehensive geomorphic overview (Farr, et. al., 2008). Such coverage is becoming available from the Japanese ALOS PALSAR radar system (Paillou, et. al., 2007, 2008), which can guide fieldwork to collect archaeological and climatic data to further constrain climate change and models.
PP23B-1476
Terrestrial constraints on the timing and magnitude of Saharan humid phases
Spring carbonates (tufas), lacustrine sediments, and fluvial deposits from the Western Desert of Egypt provide evidence for the periodic 'greening' of the Sahara during the Mid-Pleistocene. However, as these deposits are spatially and temporally discontinuous, a comprehensive record of the timing, magnitude, and spatial synchronicity of North African pluvial phases is difficult to construct from individual terrestrial localities. A compilation of over 200 published dates on deposits indicative of pluvial conditions from across the Sahara, plus 29 new U/Th dates on tufas from the Western Desert of Egypt, suggests that a precession-scale signal, previously primarily documented in marine proxies of North African humidity, can be detected in terrestrial proxy data as well. However, frequency distributions of pluvial phase dates contain age-related preservational biases; thus direct sedimentological and geochemical data are better equipped to address the relative magnitude of successive humid phases. Uranium, carbon, and oxygen isotope and minor element analyses of tufas and lake sediments from the Western Desert of Egypt suggest roughly similar conditions during pluvial phases recorded by carbonate deposition, with the exception of the MIS6/5e humid event. Paleolake sediments from Kharga and Dakhleh Oases also provide paleoenvironmental constraints upon Mid-Pleistocene pluvial conditions in the Western Desert; water balance models necessitate the presence of significant surface water input to lakes in addition to inflow from the Nubian aquifer (rainfall estimates 410-670 mm/yr), consistent with recovered savanna fauna remains (c.f. Churcher et al., 1999). The potential for hominin occupation of North Africa would be expected to increase significantly during humid phases every ~20 ka. The frequency of Saharan humid phases and their temporal relationship to climatic events elsewhere (e.g., tropical African 'megadroughts') has implications for understanding hominin migration through these regions.
PP23B-1477
Rapid Onset of the African Humid Period 14.5--11ka BP: External Forcing and Feedback Mechanisms.
Using transient glacial-interglacial simulations conducted with two different climate-vegetation models we elucidate the mechanisms that control the onset of the African Humid Period (AHP) 14,500 to 11,000 years ago. Our analysis suggests that locally increased boreal summer solar radiation leads to a northward shift of the ITCZ. Vegetation in northern Africa adjusts to wetter conditions and provides a positive albedo-vegetation feedback that is further accelerating the northward migration of the ITCZ. Sensitivity experiments reveal that the albedo-vegetation feedback and its effect on the large-scale atmospheric circulation together with the CO2-fertilization effect provide a mechanism for generating multiple states of early to mid-Holocene vegetation over northern Africa. Our model simulations document that not only orbitally-driven insolation changes played a key role in controlling the onset of the AHP, but also the presence of the remnant ice-sheet over Europe, variations of the Atlantic Meridional Overturning circulation during the Younger Dryas and increasing levels atmospheric CO2. The model results presented here do not lend support to the notion that simple insolation thresholds govern the abrupt transitions of north African vegetation during the early to middle Holocene.
PP23B-1478
Thermocline-West African Monsoon-Coupling Recorded in Guinea Basin Sediments
The South Atlantic is a key region in understanding ocean circulation and heat budget not only for the Atlantic region but, as a strong coupling through the Thermohaline Circulation and atmospheric linkage is existent, for the whole ocean system. The presented study supports the idea of centennial- to millennial scale changes in the African monsoon during the Late Holocene based on marine proxy records from the abyssal (>5000 m) Guinea Basin. Changes in the surface water stratification and thermocline depth during the Late Holocene are evident in equatorial East Atlantic stable oxygen isotope records. The reconstruction of the thermocline in the Guinea Basin was based on the use of differences in δ18O (d δ18O) between shallow (Sphaeroidinella dehiscens), intermediate (Globorotalia truncatulinoides) and deep dwelling (Globorotalia crassaformis) planktonic foraminiferal species. This multi-species approach turned out to be most adequate to ensure the results. Our studies give clear evidence for fluctuations of the thermocline depth in the EEA during the Late Holocene. A shallow thermocline was observed at about 3.5 ka, between 2.8 and 2.4 ka and again at about 1.9 ka possibly related to times of low precipitation in the West African Monsoon Belt. Increasing d δ18O trends between S. dehiscens and G. truncatulinoides point to an intensification of equatorial upwelling during the Late Holocene in general which may be linked to a decreasing West African monsoon. This trend is interrupted by a sharp reduction of d δ18O around 3.2 ka which can be explained by increasing monsoon activity in West Africa and an aligned deepening of the thermocline. The following increasing d δ18O trend correlates with the final aridification of the Sahara. This observation corresponds with results of various studies of terrestrial and shallow marine records and supports the hypothesis of centennial to millennial fluctuations of the West African monsoon system.
PP23B-1479
A Numerical Study of Precessional Forcing on the Water Budget of the Mediterranean Sea and Northern Africa During the Messinian Salinity Crisis
During the Messinian Salinity Crisis plate tectonics restricted Atlantic water from entering the Mediterranean Sea (MS). Evaporative drawdown created a subaerial depression 1500-2000m deep (Ryan, 1978) and deposited a thick layer of evaporites on the MS floor (Hsu, 1973). The Messinian sequences consist of cyclical deposits of evaporites and non-evaporites that are related to variations in the precessional cycle (Krijgsman et al., 1999). Recent studies have been unable to achieve realistic evaporite thicknesses and found the degree of halite saturation is critically dependent on the freshwater budget (E-P-R) (Meijer and Krijgsma, 2005). This study utilizes an atmosphere-slab ocean general circulation model to examine precessional variations on the water budget to improve paleo-hydrological fluxes. In our MSC simulations, the MS level is reduced up to 1500 m, a conservative estimate of sea level fall. In the Lowered Sea precession maximum (LSP+) run, the longitude of perihelion is set to 90°; in the Lowered Sea precession minimum (LSP-) run it is set to 270°. Obliquity is held at its current value of 23.45° and eccentricity is increased to 0.05 to account for maximum seasonality in precession. In each simulation the Mediterranean Sea is isolated from the Atlantic Ocean by eliminating the Q-flux, which takes into account the horizontal transport of heat in the slab ocean. Area averages of seasonal and annual mean hydrological components (E, P, and R) are computed over the MS (5°W-35°E and 30°N-44°N) and North Africa (15°W-35°E and 20°N- 36°N). The atmospheric water budget (E-P) is calculated over sea points for the MS average and over land points for the North Africa average. Percent differences are calculated based on our control run. Over North Africa, precession minimum results in a strong North African monsoon that leads to a 186% increase in total runoff and a 237% increase in E-P. Precession maximum leads to a 58% reduction in total runoff and a 64% reduction in E-P. Over the MS, the percent difference in annual mean runoff is 154% higher and E-P is 11% lower during precession minimum compared to the control. This is due to increased precipitation over southern Europe and a North African summer monsoon. Under precession maximum the annual mean runoff is 21% higher and the annual mean atmospheric water budget is 7% higher. These results may elucidate the high frequency salinity variations that occurred during the MSC.