Mike Rogerson

A humid corridor across the Sahara for the migration of early modern humans out of Africa 120,000 years ago

It is widely accepted that modern humans originated in sub-Saharan Africa ≈150–200 thousand years ago (ka), but their route of dispersal across the currently hyperarid Sahara remains controversial. Given that the first modern humans north of the Sahara are found in the Levant ≈120–90 ka, northward dispersal likely occurred during a 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. These corridors would explain scattered findings at desert oases of Middle Stone Age 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. Here we present geochemical data that demonstrate that water in these fossil systems derived from the south during wet episodes in general, and penetrated all of the way to the Mediterranean during MIS 5e in particular. This proves 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.

Reconstructing past planktic foraminiferal habitats using stable isotope data: a case history for Mediterranean sapropel S5

Abstract A high-resolution stable O and C isotope study is undertaken on all planktic foraminiferal species that are reasonably continuous through an Eemian sapropel S5 from the western side of the eastern Mediterranean. The data are considered within a context of high-resolution isotope records for two further S5 sapropels from the central and easternmost sectors of the basin, alkenone-based sea surface temperature records for all three sapropels, and planktic foraminiferal abundance records for the same sample sets through all three sapropels. Results are compared with similar data for Holocene sapropel S1. The adopted approach allows distinction between species that are most suitable to assess overall changes in the climatic/hydrographic state of the basin, including depth-related differentiations and the main seasonal developments, and species that are most affected by variable biological controls or local/regional and transient physico–chemical forcings. It is found that a-priori assumptions about certain species’ palaeohabitats, based on modern habitat observations, may become biased when non-analogue conditions develop. In the case of Mediterranean sapropel S5, these consisted of enhanced freshwater dilution, elevated productivity, shoaling of the pycnocline between intermediate and surface waters, and stagnation of the subsurface circulation. Under these conditions, some species are found to ‘shift’ into habitat settings that differ considerably from those occupied today. The present multiple-species approach can identify such ‘anomalous responses’, and thus offers a sound background for further shell-chemistry investigations and quantitative interpretation of the isotopic profiles. We capitalise on the latter potential, and offer the first quantitative estimates of monsoon flooding into the Mediterranean during the deposition of Eemian sapropel S5.

Promotion of meridional overturning by Mediterranean-derived salt during the last deglaciation

We demonstrate that changes in the behavior of the Mediterranean Outflow Water (MOW) prior to and through the last deglaciation played an important role in promoting Meridional Overturning Circulation (MOC). Estimation of past MOW salt and heat fluxes indicates that they gradually increased through the last deglaciation. Between 17.5 and 14.6 thousand years ago (ka B.P., where B.P. references year 1950), net evaporation from the Mediterranean exported sufficient fresh water from the North Atlantic catchment to cause an average salinity increase of 0.5 psu throughout the upper 2000 m of the entire North Atlantic to the north of 25

Enhanced Mediterranean-Atlantic exchange during Atlantic freshening phases

The Atlantic-Mediterranean exchange of water at Gibraltar represents a significant heat and freshwater sink for the North Atlantic and is a major control on the heat, salt and freshwater budgets of the Mediterranean Sea. Consequently, an understanding of the response of the exchange system to external changes is vital to a full comprehension of the hydrographic responses in both ocean basins. Here, we use a synthesis of empirical (oxygen isotope, planktonic foraminiferal assemblage) and modeling (analytical and general circulation) approaches to investigate the response of the Gibraltar Exchange system to Atlantic freshening during Heinrich Stadials (HSs). HSs display relatively flat W–E surface hydrographic gradients more comparable to the Late Holocene than the Last Glacial Maximum. This is significant, as it implies a similar state of surface circulation during these periods and a different state during the Last Glacial Maximum. During HS1, the gradient may have collapsed altogether, implying very strong water column stratification and a single thermal and δ18Owater condition in surface water extending from southern Portugal to the eastern Alboran Sea. Together, these observations imply that inflow of Atlantic water into the Mediterranean was significantly increased during HS periods compared to background glacial conditions. Modeling efforts confirm that this is a predictable consequence of freshening North Atlantic surface water with iceberg meltwater and indicate that the enhanced exchange condition would last until the cessation of anomalous freshwater supply into to the northern North Atlantic. The close coupling of dynamics at Gibraltar Exchange with the Atlantic freshwater system provides an explanation for observations of increased Mediterranean Outflow activity during HS periods and also during the last deglaciation. This coupling is also significant to global ocean dynamics, as it causes density enhancement of the Atlantic water column via the Gibraltar Exchange to be inversely related to North Atlantic surface salinity. Consequently, Mediterranean enhancement of the Atlantic Meridional Overturning Circulation will be greatest when the overturning itself is at its weakest, a potentially critical negative feedback to Atlantic buoyancy change during times of ice sheet collapse.

A dynamic explanation for the origin of the western Mediterranean organic‐rich layers

The eastern Mediterranean sapropels are among the most intensively investigated phenomena in the paleoceanographic record, but relatively little has been written regarding the origin of the equivalent of the sapropels in the western Mediterranean, the organic-rich layers (ORLs). ORLs are recognized as sediment layers containing enhanced total organic carbon that extend throughout the deep basins of the western Mediterranean and are associated with enhanced total barium concentration and a reduced diversity (dysoxic but not anoxic) benthic foraminiferal assemblage. Consequently, it has been suggested that ORLs represent periods of enhanced productivity coupled with reduced deep ventilation, presumably related to increased continental runoff, in close analogy to the sapropels. We demonstrate that despite their superficial similarity, the timing of the deposition of the most recent ORL in the Alboran Sea is different than that of the approximately coincident sapropel, indicating that there are important differences between their modes of formation. We go on to demonstrate, through physical arguments, that a likely explanation for the origin of the Alboran ORLs lies in the response of the western Mediterranean basin to a strong reduction in surface water density and a shoaling of the interface between intermediate and deep water during the deglacial period. Furthermore, we provide evidence that deep convection had already slowed by the time of Heinrich Event 1 and explore this event as a potential agent for preconditioning deep convection collapse. Important differences between Heinrich-like and deglacial-like influences are highlighted, giving new insights into the response of the western Mediterranean system to external forcing.

Were Rivers Flowing across the Sahara During the Last Interglacial? Implications for Human Migration through Africa

Human migration north through Africa is contentious. This paper uses a novel palaeohydrological and hydraulic modelling approach to test the hypothesis that under wetter climates c.100,000 years ago major river systems ran north across the Sahara to the Mediterranean, creating viable migration routes. We confirm that three of these now buried palaeo river systems could have been active at the key time of human migration across the Sahara. Unexpectedly, it is the most western of these three rivers, the Irharhar river, that represents the most likely route for human migration. The Irharhar river flows directly south to north, uniquely linking the mountain areas experiencing monsoon climates at these times to temperate Mediterranean environments where food and resources would have been abundant. The findings have major implications for our understanding of how humans migrated north through Africa, for the first time providing a quantitative perspective on the probabilities that these routes were viable for human habitation at these times.

Testing benthic foraminiferal distributions as a contemporary quantitative approach to biomonitoring estuarine heavy metal pollution

Biomonitoring of estuarine pollution is the subject of active research, and benthic foraminifera are an attractive group to use for these purposes due to their ubiquitous presence in saline water and wide diversity. Here, we describe a case study of biomonitoring using benthic foraminifera in the French Mediterranean lagoon, Bages-Sigean lagoon. In this case, the major pollutants of interest are heavy metals in the sediment, particularly contaminated by Cu and Cd derived from industrial and agricultural sources. The foraminiferal assemblages of the Bages-Sigean lagoon are typical of normal paralic environments, but unusually almost completely lack agglutinated forms. The density of benthic foraminifera was shown to be more influenced by the sediment characteristics rather than heavy metal pollution. However, the relative abundance of Quinqueloculina bicostata was shown to increase in the most polluted areas and we propose that this taxon may be used as an indicator of heavy metal pollution.

In vitro investigations of the impact of different temperature and flow velocity conditions on tufa microfabric

Abstract A series of experiments on freshwater carbonates (tufas) involving biofilm colonization in both fast-flow and slow-flow mesocosms was carried out in order to assess the changing nature of biofilm and associated precipitates under contrasting conditions. A thin biofilm developed over 14 weeks during the ‘summer’ experimental run contained a basal calcite layer overlain by small calcite crystals suspended within the Extracellular Polymeric Substances (EPS). The ‘autumn’ biofilm, however, showed the development of multi-laminated calcite precipitates within the EPS despite constant environmental conditions throughout the run. The experiments also showed that the largest volume of calcite precipitate developed in the fast-flow flumes regardless of temperature control. Development of an extensive calcite layer at the base of EPS in conditions of complete darkness within the sump was also observed. This study provides increased weight for the concepts: (1) that fresh- and saltwater stromatolites appear to be highly comparable multi-laminated systems with precipitation strongly influenced by both phototrophic and heterotrophic microbes; and (2) that microbial precipitation may be more common within aphotic (including cave, lake bottom and soil) systems than has previously been considered.

Introduction to tufas and speleothems

Ambient temperature freshwater carbonates precipitate as surface deposits within karstic stream, lake and swamp environments (tufas) and in subterranean situations (speleothems), where they line vadose caves and fracture systems. Although physico-chemical mineral precipitation contributes significantly to both kinds of deposit, there is a clear spatial association between the development of tufa deposits and the occurrence of microbial biofilms. This fact, and the recent discovery that the occurrence of certain heterotrophic bacteria promote precipitation onto the surface of stalactites (Cacchio et al. 2004), strongly implicates a degree of microbial influence in the calcite precipitation process, regardless of the environmental context. To add to the inherent complexity of these systems, there is considerable interplay between biological and physical processes to consider. Water velocity and turbulence will strongly affect biofilm colonization and may damage the community, thereby affecting carbonate precipitation rates, in addition to regulating important kinetic limitations on precipitation via modifications of the calcium ion delivery rate. Exchange of CO2 gas at the air–water interface is an important conditioner for precipitation in vadose systems but will also occur within surface systems as a consequence of photosynthesis. It is only by considering karst hydrological systems holistically that these processes can be untangled. Tufas and speleothems share the same soilderived meteoric water supply, represent zones of deposition of calcium ions chemically eroded from the same geological sources and produce laminated deposits which are superficially similar. In passing from cave environments via resurgences (Fig. 1) into surface waterways, individual packages of water pass down an interconnected hydrological system at any point in which the conditions necessary for calcite precipitation may be achieved. Within the deposits that this precipitation creates, it is apparent that there is a progressive gradation from massive, laminated speleothems fabrics into stromatolitic, biofilm dominated tufas fabrics. In fact, speleothems and tufa represent two end members within a continuum of freshwater carbonate reflecting different balances of physicochemically and biologically controlled precipitation. On a regional scale the occurrences of tufas and speleothems are both controlled by water table fluctuations. Typically, tufa deposition is associated with predominantly high water tables and although tufas enjoy global distributions from the tropics to polar regions, they are most effective as bioconstructors where spring fed streams are not subjected to spate conditions. Similarly, tufa developments are severely impaired by fluctuating water tables associated with increasingly arid climatic cycles. Limitation on surface carbonate precipitation is consequently derived from the necessity for biofilm development combined with the equal necessity of adequate supply of dissolved calcium and carbon, which must be present at least in part as carbonate. The latter requirement of sufficient Ca(aq) and CO3 22 (aq) ionic activity demands that these ions are not lost from solution before resurgence, making it likely that tufas will develop best where caves are flooded, thereby minimizing the distribution of the subterranean vadose environment where speleothems develop most abundantly. Curiously, these elevated tables are frequently encouraged by the tufa growth itself as a consequence of the valley bottom ponding and back flooding caused by barrage development. Conversely, as lower water tables become established and the subterranean vadose environment becomes more important, speleothems will become established. Conceptually, the occurrence of abundant tufa or speleothem deposition simply reflects the position of a hydrochemical ‘knick-point’, which occurs when sufficient CO2 has been lost from solution for carbonate ions to become abundant, for example when the thermodynamic gradient promoting precipitation (Gibbs Free Energy) exceeds the barrier presented by the activation energy. This knick-point may occur either above or below ground depending on the height of the water table. As part of the same hydrochemical system, tufas and speleothems offer an inseparable duo when exploring the climatic archive, and will reflect the same processes within the catchment. Much palaeo-environmental information in tufas and speleothems can be extracted from geochemical time series created from these deposits. However, one of the greatest obstacles to collective use of these materials in ‘climate’ reconstruction is the

Microbial influence on macroenvironment chemical conditions in alkaline (tufa) streams: perspectives from in vitro experiments

Abstract Tufas represent a palaeoclimatic archive of potentially global significance. However, uncertainty remains over the exact process of calcite precipitation from these systems, inhibiting our ability to decipher the precise meaning of geochemical records. For example, field studies of alkaline stream systems are unable to disentangle the influence of temperature and photosynthesis on ambient hydrochemistry on diurnal and annual timescales. This report describes a series of flume experiments in which temperature and light conditions are manipulated separately. These experiments reveal that precipitation of calcite occurs preferentially under conditions of rising pH, and consequently at the night–day transition. The amplitude of diurnal changes is regulated by the buffering capacity (i.e. alkalinity) of the ambient water and by the daytime balance of photosynthesis and respiration. Respiration is shown to be strongly affected by temperature, whereas photosynthesis is found to be limited by nutrient and/or DIC availability making temperature impacts minor. Consequently, macroenvironment pH during both day and night-time tend to be higher under lower temperatures, in contrast to expectation. These observations may have potential implications for the isotopic geochemistry of tufa carbonate, promoting slightly lower δ18O, due to the carbonate ion effect, and more significantly negative δ13C, due to incorporation of respired CO2 accumulated during the night. The observation that long periods of daylight are not necessarily needed for photosynthetically induced precipitation to occur confirm previous arguments that seasonal lamination requires either strong variability in ambient physicochemical activity or an ecological change in the microbial assemblage, and cannot be ascribed to reduced temperature and light intensity.