Pollen analyses of sediment core GeoB9508-5

Millennial-scale dry events in the Northern Hemisphere monsoon regions during the last Glacial period are commonly attributed to southward shifts of the Intertropical Convergence Zone (ITCZ) associated with an intensification of the northeasterly (NE) trade wind system during intervals of reduced Atlantic meridional overturning circulation (AMOC). Through the use of high-resolution last deglaciation pollen records from the continental slope off Senegal, our data show that one of the longest and most extreme droughts in the western Sahel history, which occurred during the North Atlantic Heinrich Stadial 1 (HS1), displayed a succession of three major phases. These phases progressed from an interval of maximum pollen representation of Saharan elements between ~19 and 17.4 kyr BP indicating the onset of aridity and intensified NE trade winds, followed by a millennial interlude of reduced input of Saharan pollen and increased input of Sahelian pollen, to a final phase between ~16.2 and 15 kyr BP that was characterized by a second maximum of Saharan pollen abundances. This change in the pollen assemblage indicates a mid-HS1 interlude of NE trade wind relaxation, occurring between two distinct trade wind maxima, along with an intensified mid-tropospheric African Easterly Jet (AEJ) indicating a substantial change in West African atmospheric processes. The pollen data thus suggest that although the NE trades have weakened, the Sahel drought remained severe during this time interval. Therefore, a simple strengthening of trade winds and a southward shift of the West African monsoon trough alone cannot fully explain millennial-scale Sahel droughts during periods of AMOC weakening. Instead, we suggest that an intensification of the AEJ is needed to explain the persistence of the drought during HS1. Simulations with the Community Climate System Model indicate that an intensified AEJ during periods of reduced AMOC affected the North African climate by enhancing moisture divergence over the West African realm, thereby extending the Sahel drought for about 4000 years.

Isopollen maps for 18,000 years B.P. of the offshore of Northwest Africa: Evidence for paleowind circulation

Distribution patterns of the most important pollen types from southern European and northwest African source areas for the 18,000 years B.P. time slice are reconstructed from pollen records of 14 well-dated deep-sea cores located between 37° and 9°N and compared with the modern pollen distribution in this area. It is concluded that the belt with maximum African Easterly Jet transport did not shift latitudinally during the last glacial-interglacial transition but remained at about 20°N. Furthermore, it is substantiated that the trade winds did not shift latitudinally during the last glacial-interglacial transition. This evidence is not compatible with an atmospheric circulation model that assumes a zone of surface westerlies in the northern part of northwest Africa. Trade winds during glacial episodes did, however, intensify, especially from about 36° to 24° N.

Pollen analysis, particle size analysis, end-member modelling and x-ray fluorescence measurements from sediment cores off Cape Ghir, Morocco

Understanding past human-climate-environment interactions is essential for assessing the vulnerability of landscapes and ecosystems to future climate change. This is particularly important in southern Morocco where the current vegetation is impacted by pastoralism, and the region is highly sensitive to climate variability. Here, we present a 2000-year record of vegetation, sedimentation rate, XRF chemical element intensities, and particle size from two decadal-resolved, marine sediment cores, raised from offshore Cape Ghir, southern Morocco. The results show that between 650 and 850 AD the sedimentation rate increased dramatically from 100 cm/1000 years to 300 cm/1000 years, and the Fe/Ca and pollen flux doubled, together indicating higher inputs of terrestrial sediment. Particle size measurements and end-member modelling suggest increased fluvial transport of the sediment. Beginning at 650 AD pollen levels from Cichorioideae species show a sharp rise from 10% to 20%. Pollen from Atemisia and Plantago, also increase from this time. Deciduous oak pollen percentages show a decline, whereas those of evergreen oak barely change. The abrupt increase in terrestrial/fluvial input from 650 to 850 AD occurs, within the age uncertainty, of the arrival of Islam (Islamisation) in Morocco at around 700 AD. Historical evidence suggests Islamisation led to population increase and development of southern Morocco, including expanded pastoralism, deforestation and agriculture. Livestock pressure may have changed the vegetation structure, accounting for the increase in pollen from Cichorioideae, Plantago, and Artemisia, which include many weedy species. Goats in particular may have played a dominant role as agents of erosion, and intense browsing may have led to the decline in deciduous oak; evergreen oak is more likely to survive as it re-sprouts more vigorously after browsing. From 850 AD to present sedimentation rates, Fe/Ca ratios and fluvial discharge remain stable, whereas pollen results suggest continued degradation. Pollen results from the past 150 years suggest expanded cultivation of olives and the native argan tree, and the introduction of Australian eucalyptus trees. The rapidly increasing population in southern Morocco is causing continued pressure to expand pastoralism and agriculture. The history of land degradation presented here suggests that the vegetation in southern Morocco may have been degraded for a longer period than previously thought and may be particularly sensitive to further land use changes. These results should be included in land management strategies for southern Morocco.

Occurrence of nannoplankton observed in Early Pliocene samples from Zanclean stratotype of Capo Rossello, Sicily (Table I)

Calcareous nannoplankton biostratigraphy has been worked out in the eastern Mediterranean utilizing deep-sea sediments recovered from DSDP Leg 42A Sites 375 and 376. These two drill sites were located approximately 55 km west of Cyprus on the Florence Rise. Sediments, ranging in age from early Miocene (Helicosphaera ampliaperta Zone) through Holocene, contain sufficient age-diagnostic species to recognize essentially all of the lowlatitude nannoplankton zones described by Bukry, although regional, secondary marker species are needed to define some zonal boundaries. Reworked Cretaceous and Paleogene nannoplankton occur throughout the stratigraphic interval studied, but not in quantities large enough to mask indigenous species. Sedimentation rates at Sites 375 and 376 were highest in the late Miocene and late Pleistocene. Open-marine, warm-water species of discoasters are present in significant numbers throughout the Miocene and Pliocene. Earliest Pliocene assemblages contain numerous specimens of ceratoliths. Nannoplankton in post-Messinian sediments at the drill sites and the Zanclean stratotype at Capo Rossello, Sicily, indicate that the base of the Amaurolithus tricorniculatus Zone (base of Triquetrorhabdulus rugosus Subzone) corresponds with the Miocene-Pliocene boundary.

Calcareous nannofossils of ODP Leg 103 holes

Ocean Drilling Program Leg 103 occupied five sites on the Galicia margin, northwest of the Iberian Peninsula. Two holes (Holes 637A and 638B) yielded significant Cenozoic sedimentary sections ranging from late Miocene to late Pleistocene in age. From the nannofossil stratigraphy, one hiatus is recognized in Hole 637A (2.35-2.4 Ma), whereas two hiatuses (one at 1.9-2.6 Ma and another at 3.5-3.7 Ma) are recognized in Hole 638B. Sediment-accumulation rates for the Cenozoic portions of these two holes have been calculated based on the nannofossil datums. The abundance ratios of Coccolithus pelagicus to Discoaster brouweri for Hole 637A show relatively low values and small fluctuations from 2.5 to 6.5 Ma but sharply increase and then widely fluctuate beginning at about 2.5 Ma. This may indicate relatively warmer, more stable surface-water temperatures from 2.5 to 6.5 Ma and cooler, variable surface-water temperatures after 2.5 Ma at Site 637. C. pelagicus/D. brouweri ratios from Hole 638B also show a trend of increasing values with time from late Miocene to late Pliocene, but with more fluctuations and a different pattern from that of Hole 637A.

Biostratigraphical investigations of Lago d'Averno, near Naples

Remains of diatoms, molluscs, ostracods, foraminifera and pollen exines preserved in the sediments of Lago d'Averno, a volcanic lake in the Phlegrean Fields west of Naples, allowed us to reconstruct the changes in the ecological conditions of the lake and of the vegetation around it for the period from 800 BC to 800 AD. Lago d'Averno was at first a freshwater lake, temporarily influenced by volcanic springs. Salinity increased slowly during Greek times as a result of subsidence of the surrounding land. Saline conditions developed only after the lake was connected with the sea by a canal, when Portus Julius was built in 37 BC. The first post-Roman period of uplift ended with a short freshwater phase during the 7th century after Christ. Deciduous oakwoods around the lake was transformed into a forest of evergreen oaks in Greek times and thrived there - apparently almost uninfluenced by man - until it was felled, when the Avernus was incorporated into the new Roman harbour in 37 BC, to construct a shipyard and other military buildings there. Land-use was never more intense than during Roman times and weakest in Greek and Early Roman times, when the Avernus was considered a holy place, the entrance to the underworld.

Palynological records of sediments from the Odderade type locality, north Germany

Reinvestigation of the Odderade Interstadial in its type locality led to an augmentation of the flora list and correction of some misinterpretations (e.g. Omorica, Frangula). The Eemian, stadials FW 1 and 3, and the interstadials Amersfoort/Broerup and Odderade have been manifested by pollen analyses. FW 1 and FW 3 are probably not completely free from forested areas. The new pollen diagrams considered with older data from Odderade, and in comparison with other regions in Central Europe, fit essentially with the classification and development of Vegetation during the Early Weichselian in Oerel by Behre & Lade (1986).

Calcareous nannofossils of the Soithern Coral Sea

Leg 90 of the Deep Sea Drilling Project drilled 18 holes at eight sites (Sites 587-594) on several shallow-water platforms in the southern Coral Sea, Tasman Sea, and southwestern Pacific Ocean. The results from an additional hole (Hole 586B) drilled at Site 586 during Leg 89 are included in this report. Together, these sites form a latitudinal traverse which extends from the equator (Site 586) to 45°S (Site 594) and includes all the major water masses from tropical to subantarctic. Samples recovered at these sites range in age from middle Eocene to late Quaternary. The calcareous nannoplankton biostratigraphy for Leg 90 has divided into two parts: part 1, the Neogene and Quaternary of Sites 586-594. (this chapter); and part 2, the Paleogene of Sites 588, 592, and 593 (Martini, 1986). A slightly modified version of the Martini (1971) standard Tertiary and Quaternary zonation scheme was used to make age determinations on over 700 samples. All of the relevant Neogene and Quaternary zone-defining nannoplankton are present at Sites 586-591 (0°-30°S) but become increasingly rare or are absent at Sites 592-594 (35°-45°S). Species diversity increases southward from the equator (Site 586) and reaches a peak at 20°S (Site 587). A decrease at 25°S (Site 588) and 30°S (Sites 589-591) is followed by an increase in species diversity at 35°S (Site 592). South of 35°S, species diversity again decreases and reaches a low at 45 °S (Site 594). Species diversity for all sites as a group generally increases through the early, middle, and late Miocene, reaches a peak in the early Pliocene, then gradually decreases through the late Pliocene and Quaternary

Pollen analysis of modern mangrove systems

Three sediment cores from the Bragança Peninsula located in the coastal region in the north-eastern portion of Pará State have been studied by pollen analysis to reconstruct Holocene environmental changes and dynamics of the mangrove ecosystem. The cores were taken from an Avicennia forest (Bosque de Avicennia (BDA)), a salt marsh area (Campo Salgado (CS)) and a Rhizophora dominated area (Furo do Chato). Pollen traps were installed in five different areas of the peninsula to study modern pollen deposition. Nine accelerator mass spectrometry radiocarbon dates provide time control and show that sediment deposits accumulated relatively undisturbed. Mangrove vegetation started to develop at different times at the three sites: at 5120 14C yr BP at the CS site, at 2170 14C yr BP at the BDA site and at 1440 14C yr BP at the FDC site. Since mid Holocene times, the mangroves covered even the most elevated area on the peninsula, which is today a salt marsh, suggesting somewhat higher relative sea-levels. The pollen concentration in relatively undisturbed deposits seems to be an indicator for the frequency of inundation. The tidal inundation frequency decreased, probably related to lower sea-levels, during the late Holocene around 1770 14C yr BP at BDA, around 910 14C yr BP at FDC and around 750 14C yr BP at CS. The change from a mangrove ecosystem to a salt marsh on the higher elevation, around 420 14C yr BP is probably natural and not due to an anthropogenic impact. Modern pollen rain from different mangrove types show different ratios between Rhizophora and Avicennia pollen, which can be used to reconstruct past composition of the mangrove. In spite of bioturbation and especially tidal inundation, which change the local pollen deposition within the mangrove zone, past mangrove dynamics can be reconstructed. The pollen record for BDA indicates a mixed Rhizophora/Avicennia mangrove vegetation between 2170 and 1770 14C yr BP. Later Rhizophora trees became more frequent and since ca. 200 14C yr BP Avicennia dominated in the forest.