H. Faure

Impacts of climatic change on carbon storage in the Sahara–Gobi desert belt since the Last Glacial Maximum

Abstract Reconstructions of palaeolandscapes for intervals with different climatic conditions help define regional trends in palaeobiomass and carbon storage due to global climatic change. The Sahara–Gobi desert belt stretches for about 15,000 km from the Atlantic coast to Northern China. Natural vegetation zones have undergone a number of significant shifts and complex qualitative changes under the contrasting climatic conditions of the Last Glacial Maximum (LGM) and the Holocene Climatic Optimum (HCO). The results presented here are based on palynological, pedological and sedimentological evidence, which indicate that the amount of carbon stored in vegetation and soils would have been much smaller during the Glacial Maximum than in the interglacial and post glacial times. Comparison of a set of palaeogeographic maps of this region for the chosen time-slices (ca. 20–18 ka, 9–8 ka and the present) allows us to discuss land biomass changes. Dry and cool conditions during the LGM resulted in the spread of arid and semi-arid ecosystems at northern and southern margins of the desert belt. The southern limit of the Sahara migrated southward at least 400 km relative to its present position, and almost 1000 km south compared to the mid-Holocene. The northern margin of the temperate deserts and dry steppes of Central Asia shifted northward for not less than 200–300 km over Kazakhstan, southern Siberia and Mongolia. In this study we have quantified variations of the main ecosystems from the LGM to the HCO in terms of changes in carbon storage. Each vegetation zone has been assigned a carbon density for living and dead (soil) organic matter. During the last world deglaciation, the Sahara–Gobi desert belt was a sink for approximately 200 Gt of atmospheric carbon, but since the mid-Holocene, it has been a source of carbon.

Pleistocene evolution of the Red Sea coastal plain, Egypt: Evidence from uranium-series dating of emerged reef terraces

Abstract Eight emerged reef terraces (I–VIII) belonging to three cycles of reef formation were recognized along the Red Sea coastal plain of Egypt. The oldest cycle is represented by terrace VIII at altitude varying between +9 and +35 m, the middle cycle is represented by terraces VII-V whose altitudes vary between +22 and +32 m, while the youngest one is represented by terraces IV-I at altitudes of +9, +6, +3 and +2 m, respectively. The three lowermost terraces (I–III) of the youngest cycle give 230 Th 234 U ages between 72.1 ± 2.5 and 131.2 ± 4.4 ka BP which are distributed in three ranges; 72.1 ± 2.5–87.6 ± 2.2, 112.1 ± 3.3–113.2 ± 4.1 and 123.6 ± 4.7–131.2 ± 4.4 ka BP. These age ranges coincide with the chronology of the last interglacial cycle but their correlation with the Oxygen Isotope Substages 5a, 5c and 5e, respectively, is not evident. This suggests that the three dated terraces, combined with terrace IV, were built during the maximum high sea level stand of the last interglacial cycle (Oxygen Isotope Substage 5e) while their surfaces are the product of short still stands during a continuously falling sea level. However, the comparison of the stratigraphic relations of the older terraces, made up of recrystallized corals, with both the oxygen isotope record and the curve for solar insolation received by the earth at latitude 65°N, allowed the estimation of their minimum ages. The terraces of the middle cycle were considered to have formed during Oxygen Isotope Stage 7 (170–230 ka BP) while that of the oldest cycle was formed during Stage 9 (300–330 ka BP). The present-day altitudes of these terraces are the product of eustatic sea level fluctuations and differential tectonic uplift.

Estimates of methane emission during the last 125,000 years in Northern Eurasia

The concentration of methane in the atmosphere has varied considerably during the last 125,000 years. Boreal wetlands represent one of the main sources of methane emissions into the atmosphere, the rate of which is largely controlled by climate. Changes in climate (mainly in the duration of the frost-free period) and in the extent of wetlands presumably caused variations in the methane production from boreal ecosystems. We chose Northern Eurasia to estimate both climatic changes and the area of methane-producing ecosystems, as it plays a leading role in methane emission. Palaeobotanic and palaeocryological data were used for the reconstruction. The two most recent warm stages: the Holocene Optimum (5500–6000 years BP) and the Last Interglacial Optimum (ca. 125,000 years BP) were studied. During these warm periods, both an area of tundra and the proportion of the wetlands within the boreal forest zone were considerably reduced. On the other hand, a longer frost-free period and higher precipitation would have caused higher methane production. The precipitation rise was apparently in part compensated by an increase in potential evaporation due to higher summer temperatures. Compared to methane emissions of about 9×106 t per year from modern forests of Northern Eurasia, emissions amounted to 86 and 44% of modern values for the region during the Holocene Optimum and Last Interglacial Optimum respectively. Under the greenhouse warming expected early in the 21st century, the climatic conditions may lead to a considerable increase of methane emission.

Recent Crustal Movements Along the Red Sea and Gulf of Aden Coasts in Afar (Ethiopia and T.F.A.I.

Abstract Faure, H., 1975. Recent crustal movements along the Red Sea and Gulf of Aden coasts in Afar (Ethiopia and T.F.A.I.). In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectcnophysics, 29 (1—4): 479—486.

Greenhouse warming and the Eurasian biota: are there any lessons from the past?

Abstract Climate models predict a rise in global mean temperature of around 2–4°C by the end of the next century, with far greater rises in the high latitudes. Mean annual temperature rises of 6–8°C are predicted for 65°N, and as much as 10–12°C for above 70°N. There can be little doubt that such changes will have profound effects on boreal and arctic ecosystems, both through the temperature effects themselves and through associated changes in water balance. There is abundant evidence of climatic change in the high latitudes from the last 2.4 million years of the Quaternary. In a succession of glacial-interglacial cycles, high latitude temperatures seem to have fluctuated overall by about the same amount as is projected for the next century. Perhaps it is possible to use our knowledge of such past changes to understand what might happen to the high latitude ecosystems once the future greenhouse warming gets under way? There are many potential pitfalls in using data from the past to attempt to predict the future. In addition to the limitations in the data, there are also many important differences in the rate and setting of changes that should be borne in mind. With regard to climatic time-scale, the biogeographical patterns which we observe for the past are far more likely to represent equilibrium situations than those which we will observe in the future. Equilibrium data can itself be useful in that it provides indications of the distribution of climate conditions towards which the Earth will move. For example, it provides support for the notion that the climatic models are indeed correct in predicting that the strongest warming will occur in the high latitudes. Even following the relatively slow climate changes of the Quaternary high latitudes, there is abundant evidence of disequilibrium in tree species migrations, lasting for millennia in some cases. The survival of nearly all the high-latitude forms of plants and animals known from the Pleistocene fossil record—despite the repeated climatic fluctuations—may provide reassuring evidence of their future resilience. However, the extinctions of many large arctic mammals at around the time of the most recent warming phase may provide warning of what will occur in the future to certain species whose populations are already depleted by human activity. The exceptions to this pattern of gradual change are the sudden climatic shifts which have occurred in the North Atlantic region on several occasions during the late Quaternary. These may offer the closest analogues that we have to the effects of a future greenhouse warming on high-latitude plant and animal communities. It seems that some groups of organisms, such as insects, molluscs and water plants were able to respond rapidly to the climate warming, perhaps on the timescale of decades. However, tree populations were left far behind and took centuries or milennia to catch up with the changed climate, resulting in unfamiliar ecological scenarios in the mid and high latitudes.

The Sunda and Sahul continental platform: Lost land of the Last Glacial Continent in S.E. Asia

Abstract Global climate change is the most significant phenomenon that may control the global variations of sea level over the coming thousands of years. During the alternate glacial and interglacial periods, ice-cap melting and ice accumulation in the high latitudes change the ocean water volume, which causes the sea level oscillations. For the longer periods, the change of sea level is due to the change of the basin volume following basin uplift or subsidence and the tectonic opening of the ocean floor due to plate motion. Some maximum glacial periods were marked by the very low sea level, about 125 m below the present sea level during the last glacial maximum, drying up and exposing the continental platform that was quickly covered by humid lowland tropical forest. The following rapid sea level rise due to the melting of the ice cap submerged the continent, transferring most of the carbon to the atmosphere. During the very low sea level, the deep pass Indian-Pacific Ocean Gateways remained open, allowing the global ocean current to go through the corridor between the two exposed platforms, Sunda in the West and Sahul in the East of the Indonesian Archipelago. Data obtained from these platforms will be important in order to understand the global climatic pattern from the Last Glacial Maximum (L.G.M. 18,000 BP) which was followed by a rapid sea level rise.

Changes in moisture balance between glacial and interglacial conditions: influence on carbon cycle processes

Abstract During the arid Late Glacial and Last Glacial Maximum (between approximately 30 000 and 13 000 calendar years ago), vegetation cover retreated and large areas of the continents were occupied by desert and semi-desert vegetation. The result of this general decrease in biological activity would have been a decrease in the size of the land carbon reservoir, and a decrease in the rate of chemical rock weathering. By contrast, during the early-mid-Holocene, conditions in many areas seem to have been moister than today due to a more active hydrological cycle. All of these processes would have affected the global carbon cycle and altered the amplitude and timing of the climate fluctuations themselves. In effect, the climatic shift between glacial and interglacial conditions creates a very large ‘missing source’ of carbon, perhaps amounting to thousands of gigatonnes, to account for the carbon uptake by the land system during the present interglacial, and thus carbon cycle models of the late Quaternary may need to be revised extensively.

CARBON STORAGE AND CONTINENTAL LAND SURFACE CHANGE SINCE THE LAST GLACIAL MAXIMUM

Abstract Estimates of the storage and flux of shelf carbon in vegetation, soils, carbonates, and organic matter during the period of the marine transgression since the Last Glacial Maximum (LGM) 18 ka are presented. Whereas at present each square metre of land on the planet carries about 10.65 kg of carbon in vegetation and soils, during the LGM most areas of exposed continental shelf carried relatively little carbon, probably about 5.86 kg C m −2 , but this increased to a maximum density of 15.49 kg C m −2 after 10 ka when conditions generally favoured peat deposition and forest development. In the ensuing sea level rise up until mid-Holocene time this large store of carbon was displaced. Assuming an average value of 10.65 kg m −2 carbon (combining the land lost to sea-level rise before and after 13 ka), a transgression covering 15–23×10 12 m 2 would mean that 160 to 245 Gigatons of Carbon (1 Gt=10 12 kg) were lost from the terrestrial system at the same time that the remainder of the terrestrial biosphere was still taking up organic carbon. This additional and opposite flux from the land system must be taken into account when considering changes in the global carbon cycle and CO 2 fluxes. Moreover, it complicates the interpretation of the ocean carbon isotope record.

Limnogeologic studies on an intertrappean continental deposit from the northern Ethiopian Plateau (37°03′E, 12°25′N)

Abstract The Chilga lacustrine deposit from a small graben in the heart of the Northwestern Ethiopian Highlands (37°E, 12°N) is one of the ubiquitous intertrappean continental sedimentations on the Plateau. The basalt layer which makes the bottom of the basin has been dated as 8 Ma. The lacustrine sedimentation occurred contemporaneously with active volcanic phases in the region. Silicic aggregates are common in the cements and as important mineralogic constitutents of these phases whereas periods of calm sedimentation are characterized by thick lignite seams and the presence of authigenic minerals such as pyrite and vivianite. The sequence shows a general upward fining and evolution from shallow fluviatile to reduced lacustrine basin. The palynoflora from the sequence has a unique palaeofloral assemblage where the abundance of Guineo-Congolian-like pollen taxa, pteridophytes and absence of conifers imply a regional palaeoaltitude much lower than at present. The uplift of the Plateau at a rate of 0.1 mm yr −1 similarly suggests palaeotitudes of ca 900–1000 m.

The global carbon cycle and its changes over glacial–interglacial cycles

Carbon is an essential element for life, food and energy. It is also a key component of greenhouse gases and, thus, plays an important role in past and present climatic changes.