Land-sea interactions are recorded in palynological and geochemical records of the eastern South Atlantic. A sediment core from the Congo Deep-Sea Fan provides a record of fluctuations in river discharge and the extent of the tropical rain forest. One from the Angola margin outside the influence of river discharge registers fluctuations in wind strength and the extent of the Afromontane forest. During warm and humid Marine Oxygen Isotope Stages (MIS), MIS 1 and 5e, dinoflagellate cyst flux decreased in relation to increased freshwater input to the eastern South Atlantic. Southeast trade wind vigour was low. At the same time on the continent, tropical lowland forest expanded. During the cool stadials of MIS 5, MIS 5b and 5d, having intermediate levels of sea surface temperature and river discharge, the Afromontane forest represented by Podocarpus pollen expanded to lower altitudes and occupied former areas of the lowland rain forest. During the glacial Stages, MIS 6 and 2, trade winds became strong, but freshwater run-off into the eastern South Atlantic decreased. Representation of forest elements decreased and that of grasses increased indicating more savanna and/or less dense forests. The fresh-water discharge of the Congo is related to the intensity of the West African monsoon modulated by low-latitude precessional insolation changes. Spectral analysis indicate that the reponses between sea-surface temperatures (recorded by alkenones), chemical weathering and run-off (indicated by K/Al-ratios), salinity fluctuation of marine surface waters (indicated by dinoflagellate cysts), and the vegetation in the Congo basin (recorded by pollen and spores) are more or less direct. Fluctiations in the Southeast trade wind system, however, show a typical glacial-interglacial rhythm.
A global network of well-dated cores, which have been studied with high resolution, is of prime importance for our understanding of timing and extent of short-term climatic change. To date, this network is still fragmentary with most high-resolution studies concentrated in a few areas on the Northern Hemisphere. Here, we present a high resolution (one sample per 130 yrs) palaeoenvironmental reconstruction for the interval from 20-8 ka B.P. on a well-dated piston core off the Congo River mouth (SE Atlantic). A palaeo-environmental reconstruction is made on the profiles of marine, and river derived organic biomarkers and their carbon isotopes as well changes in the planktic foraminifera assemblage and the oxygen isotope record of some of its species. The changes in sea surface temperature, upwelling, and aridity, reconstructed from these proxies indicate from the glacial maximum to the early Holocene, sea surface temperatures increased by only 2°C, upwelling strongly decreased and humidity in tropical Africa strongly increased. On top of this general glacial to interglacial transition, two short intervals with warmer surface waters, increased humidity and less upwelling occur. These intervals are time equivalent with the amphi-North-Atlantic cooling events: Heinrich Event 1 and the Younger Dryas. For both events a reduction of the formation of North Atlantic Deep Water has been suggested. This reduction may have led to a less efficient northward interhemisphere heat transport causing accumulation of heat in the equatorial Atlantic. The resulting relocation of the atmospheric pressure field in turn, enhanced precipitation in tropical Africa and reduced coastal upwelling.
During the last glacial the Northern Hemisphere ice sheet collapsed several times. Consequently, large amounts of icebergs were released into the northern North Atlantic. Their presence is expressed in the sedimentary record of the North Atlantic as layers bearing Ice Rafted Debris (IRD), known as Heinrich events. Through atmospheric and oceanic heat transport patterns of the northern North Atlantic, the largest climatic impact of Heinrich events is expected on regions downwind of the North Atlantic: Europe and the NW African Margin. For a better understanding of this impact, high-resolution comparison is needed of palaeoceanographic and terrestrial climate records at the time of the deposition of the IRD layers. However, linking of these records is often hampered by dating problems.
In this study, two marine cores from offshore Southwest Portugal (SO75-6KL) and south of Spain (KS310), respectively, are used to investigate the impact of the last Heinrich event (H1; 12-15 ka BP) on the climate of Iberia. The former core records IRD attributed to H1 between 14.3-13.5 ka BP. Problems with linking records of marine and terrestrial palaeoenvironments are avoided by combined study of dinoflagellate cyst assemblages (marine algae), and of pollen and spores from the same samples. The latter group of fossils reflects the vegetation on land, providing information on climate variables like temperature and precipitation. The dinoflagellate cyst record is used to reconstruct surface water mass properties, such as Sea Surface Temperature (SST), salinity and oceanic productivity. The dinocyst records from both cores indicate an onset of sea surface cooling preceding IRD deposition offshore Portugal. Productivity patterns differ in both cores due to different oceanographic settings. In core SO75-6KL, primary productivity inferred from dinocysts peaks immediately after the presence of IRD, while in core KS-310 productivity is high over most of the interval of H1 (12-14 ka BP). Pollen data of both cores indicate arid conditions on the Iberian Peninsula coinciding with the cooling related to H1.
The results from this study confirm previous ideas that a lowering of the SST in the northern North Atlantic led to an arid climate of the Western European margin. Moreover, they show the leads and lags among the different components of the climate system such as SST, precipitation and air temperatures.
Traps are known as the most important volcanic manifestations. They correspond typically to an aerial emission of more than 106 km3 of magma in 1 Myr. The potential global impact of such emission make the precise correlation of traps with the global magnetobiochronologic scale a very important task. Our study is focused on the Ethiopian traps which correspond to the birth of the Afar hot spot at the triple point: Red Sea, Aden Gulf and East African Rift. The Ethiopian traps exhibits significant acidic component (about 10% of the whole traps volume) providing a stratospheric aerosol diffusion more efficient than the main basaltic eruptions. Furthermore, their magnetostratigraphy is well established : trap activity began in Chron C11r.2r and ended in C11r.1r or C10r (Rochette et al., 1999), with well clustered Ar/Ar ages at 30 ±0.5 Myr.
Prominent magnetic susceptibility peaks identified as tephra layers occur in four Oligocene sections of ODP sites from leg 115, drilled near the Madingley rise were described. In order to demonstrate that these tephra layers are related to the Ethiopian traps a high resolution study of site 709 and 711 have been undertaken, involving magnetostratigraphy, nannofossil and isotopic stratigraphy, geochemical characterization of the tephra.
Geochemical analyses of the glass shards have permitted us to characterize an acid continental source for these tephras excluding a Reunion hot-spot source and compatible with the Ethiopian signature.
The 711A borehole provides a very reliable magnetostratigraphy for the whole Oligocene sections (Chron 13 to 9). The first tephra level occurs in Chron C11n.1n and the last one in C11n.1r in perfect agreement with the acidic layers position in the traps magnetostratigraphy. Calcareous nannofossil stratigraphy confirms the magnetostratigraphic interpretation, with the NP23-24 limit right within the tephra layers. The 709B borehole (depth: 3040.8 m) confirms 711A (depth: 4429.8 m) results and provides a better isotopes stratigraphic record due to better preservation of carbonates. Large fluctuations of 18O may suggest a complex climatic response to traps emissions, compatible with the conflicting short term cooling effect of aerosols and longer term warming effect of volcanic C02.
Our results confirms the earlier proposal (Hoffman et al., 1997) that the onset of Ethiopian traps emission is synchronous with the global climatic event Oi2 .
Hoffmann and al, Nature, 389, 838-841, (1997).
Rochette and al, Earth and Planet, In press, (1999).
During Cruise 40/4 of the RV Meteor in January / Febuary 1998, 6 piston and 5 gravity cores (45.43 m and 25.7 m total length) were recovered from 6 sites in the Aegean and Levantine Sea. The lowermost sapropel penetrated by core #71 is S9, which is astronomically callibrated at 242 ka (Lourens et al., 1996). Since late Quarternary times the Eastern Mediterranean has been a site of extensive explosive volcanism. The eruption centers are located in the Hellenic arc (Milos, Santorini, Kos, Nisyros and Yali). Several prominent tephra layers, mainly from Santorini and Kos could be identified in the cores. One tephra (Y-5) is from an Italian eruption center (Grey Campanian Ignimbrite). Additionally, so far undescibed tephra layers were found, e.g. within sapropels S3 and S7. All tephra layers are of calc-alkaline composition containing plagioclase, clinopyroxene, orthopyroxene, and in the case of W-3 (rhyolitic Kos Plateau Tuff) biotite and sanidine.
A tephrostratigraphical standard profile for the Eastern Mediterranean is established. Major explosive events documented in the cores are correlated with major eruption centers in the Hellenic arc. Especially Core #65, located 20 km SE of Santorini is correlated with plinian fall deposits from the Thera Formation on Santorini (Druitt et al., 1989). The correlation is based by chemical and morphological analyses on glass shards with the electron microprobe and the SEM, and by XRF-analyses on pumice lapilli.
To determine the age of the deep sea tephra layers two approches were applied:
1) sapropel chonology (Lourens et al., 1996)
2) published K/Ar and Ar/Ar age data from the terrestrial record on correlated units were imported into the marine sedimentary record. The astronomically calibrated ages are derived by assuming a linear sedimentation rate between the individual sapropels used for interpolation. In order to establish a reliable tephrochronology laser Ar/Ar age determinations on minerals direct from the deep sea ash layers will be the main part of our future work.
Druitt TH, Mellors RA, Pyle DM, Sparks RSJ, Geol. Mag., 126(2), 95-126, (1989).
Lourens LJ, Antonarakou A, Hilgen FJ, Van Hoof AAM, Vergnaud-Grazzini C, Zachariasse WJ., Paleoceanography, 11(4), 391-413, (1996).
The formation of sapropel, dark and organic-rich sediment interbedded in hemipelagic sediments in the eastern Mediterranean Sea, is thought to be related to change in the climatic and oceanographic systems at summer insolation maxima. It is significant to understand that how the terrigenous material flux varied temporally and spatially back in time concerning with sapropel formations. One of our objectives in this study is to reveal variations of clay minerals in the eastern Mediterranean Sea during 120Ky as an indicator of changes of terrigenous input. And the other one is to understand the relationship between sapropel formations and terrigenous input.
Two piston cores, ODP Leg 160 967D-1H from northern Erathosthenes seamount and 969F-1H from central Mediterranean ridge were analyzed for clay mineralogy by X-ray diffraction method. Procedure of semi-quantitative clay mineral analysis was based on that of Petschick et al. (1996).
Smectite content in Core 967D-1H and illite content in Core 969F-1H are higher at insolation maxima. These fact support the recent idea on sapropel formation that the increase of river discharge and/or precipitation according to monsoonal intensification and related climatic change at insolation maxima are important paleoceanographic conditions for sapropel formation. Kaolinite content of both cores shows very similar which is the lowest at LGM and generally higher at insolation maxima. This variation probably indicates increase of river discharge and precipitation in interglacial period from Sahara due to global climatic change. There were no sapropels at some of insolation maxima, however, although terrigenous input increased in both cores. In Core 967D-1H, smectite is generally more abundant than illite and chlorite is scarcely identified. According to Venkatarathnam and Ryan (1971), this shows the influence of Nile discharge. On the other hand, the variation of illite, which is more abundant than smectite thorough the Core 969F-1H, shows the influence of surface water flooding from Black Sea.
The variations of clay minerals had been closely correlated with summer insolation change during 120 Ky and they were interpreted as the variations of river discharge and/or precipitation related to African monsoonal change. We concluded that intensification of terrigenous input is sufficient but not necessary condition for sapropel formation.
Petschick R, Kuhn G & Gingele F, Mar. Geol, 130, 203-229, (1996).
Venkatarathnam K & Ryan WBF, Mar. Geol, 11, 261-282, (1971).
A distinct feature of the Pleistocene and Holocene sediments of the Eastern Mediterranean Sea are the sapropels, distinct, black, organic-rich thin layers that were deposited on a basin-wide scale as a marine response to enhanced rainfall and fresh water run off from the continent. The ages of the four sapropel layers in the Eastern Mediterranean Sea, labeled as S1, S3, S4, and S5 and the hydrological regime during their formation were reevaluated. We use the isotopic composition of low magnesium calcite (LMC) speleothems deposited in the Soreq Cave, Israel over the last 140 Ky to provide tighter constraints on the formation age of the sapropels and the paleohydrological regime at the time of their deposition. During periods in which low 18O values of low magnesium calcite are observed, an increase in the annual rainfall in the Eastern Mediterranean region is inferred. The low 18O events are contemporaneous with the ages of the sapropels, and both were formed as a result of enhanced hydrological activity; therefore we suggest that they are related. Pollen data from the sapropel layers and the isotopic pattern of the speleothems indicate that during sapropels S5 and S1 there was a deluge period. There was a major decrease in the sea surface salinity, coupled with frost-free winter and drought-free summers. Sapropels S4 and S3 were formed during relatively wet periods, but in a semi-arid climate similar to that of the present-day. It is not clear whether the continental paleoclimate in the vicinity of the Soreq cave was directly influenced by the monsoon rain or that the changes in the salinity of the Mediterranean Sea affected the vapor source and caused changes in the hydrological conditions. The accurate ages of the isotopic events in the speleothems suggest that the ages of the sapropel formation are: S5 from 124 to 120 Ky, with the main peak at 122 Ky; S4 from 108 to 100 Ky, with two peaks at 107 and 102 Ky; S3 from 85 to 80 Ky, the main peak occur at 80 Ky. Sapropel S1 started at 10 Ky, but the major events occurred between 8.5 to 7.0 Ky, with the main peaks at 8.5 and 7.0 Ky.
Occurrence and content of Mediterranean sapropels from the last few hundred thousand years are powerful tools for investigating climate correlations between low and high latitudes in the North Hemisphere as well as for investigating origin (and related questions) of atmospheric changes. These investigations are based on two assumptions developped during the last decade: Mediterranean sapropel deposits are correlated to summer maxima insolation in the northern hemisphere and sapropel occurrence is driven by low latitude summer maximum insolation. A compilation of recent results is presented reinforcing the validity of these assumptions.
Based on these two assumptions the time occurrence of sapropel deposits together with their content (which give information on climatic conditions) draws attention to the key period around 175 kyr Bp with unusual situation caracterized by the occurrence of a cold Mediterranean sapropel deposit. This reveales that at that time high and low latitude climates are decoupled: high and middle NH latitudes stay glacial but low NH latitudes climate experienced a normal response to the insolation cycle as indicated by a strenghthening of the summer monsoon around 175 kyr Bp.
This decoupling gives informations concerning the origin of change of the atmospheric gas content as revealed by the ice core air bubble analyses, CH4, CO2 and the isotopic signature of O2. In particular it calls into question the role of changes in d18O (sea-water) as the driving mechanism for 18O (atmospheric O2). We present arguments for other source of the 18O (atmospheric O2) (Melieres et al.,1997).
Melieres M-A, Rossignol-Strick M & Malaize B, Geophysical Research Letter, 24, 1235-1238, (1997).
The Lower Paleocene series of the Aix-en-Provence basin are composed of a succession of carbonate-rich paleosols which developed either on floodplain alluvium or on marginal lacustrine carbonates (palustrine facies. In these continental formations, the paucity of fossils prevents to establish a detailed biostratigraphic framework and to propose correlation with the marine realm.
This lack of detailed biostratigraphic information has been overcome by combining other methods such as a magnetostratigraphy and a carbon isotopic stratigraphy :
The magnetostratigraphy is largely based on the palustrine facies. When compared to the red siltstones, these facies show a signal of low magnitude but do not possess any overprint component. The major Chrons were identified and helped to constraint the stratigraphic framework given by the biota.
The carbon isotopic stratigraphy is constructed with the data obtained from the paleosols. Correlation of 13C isotopic curves from deep sea sediments and that of paleosols in terrestrial setting is based on the general equilibrium that exists in the main CO2 reservoirs: oceans, atmosphere, biomass and soils.
The sampled interval in the Provence section covers the Upper Maastrichtian up to the Early Eocene (Lutetian). The integrated paleomagnetic and 13C isotopic stratigraphy obtained from these continental series is well in accordance with the marine data.It is marked by a long-term shift from low values in the early Paleocene to higher values in the late Paleocene and then by another shift towards values that are 2 to 3‰ lower in the Early Eocene. Beside these long term evolutions, the short term 13C spike associated to the LPTM (near the Paleocene/Eocene boundary) could also be identified.
These results show the potential of the 13C isotopic stratigraphies from carbonate-rich paleosols to establish detailed correlations with the marine realm.
Bering Strait is the last major interoceanic seaway whose age is not accurately known and determing its precise geological age has proved to be an enduring quest. A closed strait (i.e., the Bering Land Bridge) allowed the interchange of terrestrial animals and plants between Eurasia and North America, and an open strait provided a marine connection between the North Pacific and North Atlantic-Arctic oceans. Despite the biogeographic and paleogeographic importance of Bering Strait's first opening, determining its age has proved difficult owing to the apparent absence of a stratigraphic sequence containing both a faunal signal of an open strait and age-diagnostic fossils. One of the signals that Bering Strait opened is the appearance of the bivalve mollusc Astarte in the North Pacific Neogene. Astarte dwelled in the North Atlantic and Arctic Oceans throughout the Cenozoic, but was absent from the North Pacific until Bering Strait formed.
Neogene marine diatoms have been found at two Astarte-bearing horizons in the Bear Lake Formation of the Alaska Peninsula, SW Alaska. Diatoms that directly occur with Astarte in the Bear Lake Formation have well-documented ages in North Pacific region. A diverse marine diatom flora from the Bear Lake Formation represents Subzone "b" of the North Pacific Neodenticula kamtschatica diatom Zone according to Barron & Gladenkov (1995). This Subzone has an age range of 5.5 Ma to 4.8 Ma, based on correlations with the geomagnetic polarity time scale of Berggren et al. (1995). Its base is defined by the first occurrence of Thalassiosira oestrupii at 5.5 Ma, and the top by the last occurrence of Thalassiosira insigna at 4.8 Ma. Both these species as well as other forms typical of Subzone "b" are presented in the samples from the Bear Lake Formation. These new paleontological data from Alaska are comparable to molluscan and diatom data from approximately coeval strata on Karaginsky Island, northeastern Kamchatka and Hokkaido Island, Japan. The first appearance of Astarte here is approximately coeval with that in the Bear Lake Formation. This new paleontological evidence implies a greater age than previously proposed for the initial opening of Bering Strait.
The Lena river draining to the Siberian Arctic is considered to be the main sediment source for the Laptev Sea. Nevertheless, the delta of the Lena river occupying an area of 28.000 km2 is a poorly studied and very complicated region of Land-Ocean-Interactions in the Arctic. To understand the sedimentation and environmental history of the Lena delta different geological and geophysical approaches were chosen. To identify sedimentary and permafrost structures within the Lena delta sampling of sedimentary sequences by shallow coring and through natural exposures, ground penetrating radar and shallow seismic studies have been carried out. Age determinations are based on radiocarbon and OSL datings.
Mineralogy and geochemistry of the sediments show details about the processes controlling the late Quaternary conditions of accumulation and deposition. The geophysical methods of sub-bottom profiling were two-fold: (1) A RAMAC impulse radar system proved to be a viable technique for mapping subsurface structures on land. The 100 MHz radar signal penetrated the permafrost down to 30 m at maximum showing periglacial features like ice wedges and ice layers. Drilling results were used to determine the geologic composition. (2) A sediment echo sounder was used as high-frequency pulse source for seismic surveying of sediments of Nikolay lake in the western Lena Delta. It was possible to characterize the geometry of basin fills and changes in lake sedimentation as well as to identify the permafrost table below talik zones.
Amsterdam-Terminal is a cored bore-hole made in 1997 at the southern end of the Amsterdam basin. The basin is one of a series of depressions created by the Saalian ice-mass in the central part of The Netherlands and was a major area of deposition during Late Saalian and Eemian times. The bore-hole was made as an additional reference section for the Eemian. The sedimentary record was expected to be more continuous than in Amersfoort-1, the stratotype for the Eemian.An odd forty metres of Eemian sediments were penetrated and core-recovery appeared to be exceptionally good. The cores are being studied by a wide array of disciplines. Age control is as yet only provided by pollenanalysis. The results of U-Th and stabile isotope analysis are to be expected soon. Amino-acid and TL analysis is planned. The complete section is normally magnetized.
Integrated facies analysis resulted in a detailed reconstruction of the depositional history during Late Saalian and Eemian times. At the end of the Saalian a shallow lake existed in which diatom-rich silts and sands were deposited. The lake rapidly deepened at the beginning of the Eemian and slightly less than 1.5 m of diatomites were deposited during the E1-3 phases. Deposition of diatomite continued until the early Corylus-phase (E4a), but the environment was no longer strictly lacustrine because the sea occasionally entered the basin. Shortly after, the deposition of diatomites ended and the basin came under the permanent influence of the sea. A restricted marine, lagoon-type of environment came into being in which clays rich in organic matter were formed. Initially the watercolumn had a strong salinity-stratification which led to oxygen deficiency and adverse bottom conditions. As the influence of the sea increased, the salinity contrast gradually diminished, the conditions at the bottom improved and foraminifers and molluscs could live at least periodically on the basinfloor. Exchange of water with the open sea intensified and at the very beginning of the Taxus-phase (E4b) there is no longer an actual brackish water-layer at the surface. A weak salinity-stratification seems to have existed for some time, but well before the end of the Taxus-phase it had disappeared completely. Hitherto the changes in the environment were coupled to an increase of waterdepth, the common cause of which is to be found in a rising sea-level. Depositional depth reached an apparent maximum early in the Carpinus phase (E5) and thereafter a shallowing trend set in which culminated in the deposition of sands in tidal channels during a late stage of the Carpinus-phase. The details of the depositional history during this part of the Eemian are as yet poorly understood.
In the last few decades, our knowledge of the Pleistocene climate has substantially increased, and several factors have been forwarded as important controls on glacial-interglacial cyclicity, such as changes in deep-water formation and ocean circulation, events of fresh-water input into the North Atlantic, changes in atmospheric CO2, and Milankovitch cyclicity. Much of the progress achieved is based on research on marine sediment cores and continental ice cores, whereas other continental records have been integrated to a lesser extent, mainly because of their incomplete character.
In our project, we propose to trace continent-ocean links in marine Pleistocene sediment records by means of P-sequential analysis, stable nitrogen isotope analysis, and XRD-mineral characterization. Targeted sites are ODP sites 658, East equatorial Atlantic; 680, offshore Peru; 724, Arabian Sea, offshore Oman; 798, Japan Sea; 806, Ontong-Java Plateau; 907, North Atlantic Gateways, which are characterized by different oceanographic and climatic conditions, and well calibrated by stable oxygen isotopes.
We use P as a proxy of continental weathering and marine productivity, because 1) the primary source of P is continental weathering, and 2) P is an essential nutrient with limiting capacities in marginal marine settings. N is equally an essential nutrient and nitrogen isotope fractionation is mainly caused by phytoplankton nutrient uptake and denitrification, and stable nitrogen isotopic composition of sediment is for this reason, an useful indicator of nutrient utilization and productivity. Mineralogical analyses are used to constrain and characterize detrital flux rate.
In a first approach, we concentrated on ODP Sites 680B and 724C (offshore Peru and Oman). The first series of data sets measured and to be presented during the meeting shows variations, which are consistent with the climate record, as time-series analyses confirm. These data suggest that productivity and carbon storage diminished in the two upwelling centers offshore Peru and Oman during glacial periods, as has been shown in previous studies.
The only curve of sea-level change after 18000 BP available for the Portuguese western coast was proposed by Dias (1985, 1987 in Freitas, 1995) and it is a useful tool to frame the information collected from several cores obtained in the coastal lagoons of Albufeira, Melides and Santo André. Albufeira - The sedimentary record starts approximately at 7000 BP and until 5000 BP indicates a shallow, fresh-water depositional environment located far from any significant littoral influence. Between 5000-1600 BP several flooding episodes occurred, alternating with terrestrialization episodes (Freitas, C., 1995), which are interpreted as resulting from changes in the permeability of the barrier. Between 1600-1225 BP a dramatic opening of the tidal inlet is recorded (Bao et al., in press). After 1225 BP, the sedimentation reflect the regular breaching of the barrier. The last 600 years of sedimentation illustrate the anthropogenic intervention in the system. Melides - The lower unit of the sedimentary record (4540 - 3700 BP) corresponds to the distal deposits of flood tidal deltas merged with washovers, indicating dependency of an open, high energy inlet channel (Freitas & Andrade, 1997). However, the very base of this unit reflects mainly a deep lagoonal environment, dominated by fresh to brackish water (Queiroz & Mateus, 1994) and implies the former definition of a detrital barrier. The intermediate unit (3700 - 700 BP), was deposited in a strict lagoonal environment in a closed-inlet situation (Queiroz & Mateus, 1994; Freitas & Andrade, 1997). The upper unit corresponds to clean barrier sands and indicates the shorewards progradation of the barrier. Santo André - One single core was taken that crossed the whole Holocene sequence; it includes a lower lithostratigraphic unit, dated from circa 14000 to 5000 BP. It consists of coarse sands and gravels in its base, followed by alternating layers of muddy and sandy sediments. Between circa 5000-1600 BP a thick sequence of muddy and organic lagoonal sediments (intermediate unit) was deposited, implying the presence of an effective barrier. The top unit is of fluvial facies.
Data obtained from these three coastal lagoons suggest that a detrital barrier was formed in each of these systems circa 5000 BP. Sea-level information indicates that the barriers were defined in a short period of marked sea-level rise deceleration. Since 5000 BP until present the sheltering provided by the barriers became the main environmental control of the lagoonal dynamics. The topmost sedimentary units found in any of these lagoons correspond to present-day sedimentation and show clear signals of anthropogenic influence.
Bao, R, Freitas, MC, Andrade, C, Separating eustatic from local environmental effects: a late holocene record of coastal change in Albufeira lagoon, Portugal. The Holocene, in press, (1998).
Freitas, MC, A laguna de Albufeira (Península de Setúbal), sedimentologia, morfologia e morfodinâmica. Tese de Doutoramento. Lisboa, 337, (1995).
Freitas, MC & Andrade, C, Evolução Holocénica da barreira de Melides (SW Portugal). I Jornadas do Quaternário de Portugal. Braga. (1997).
Queiroz, P & Mateus, J, - Preliminary palynologic investigation on the Holocene deposits of Lagoa de Albufeira and Lagoa de Melides, Alentejo (Portugal). Revista de Biologia, 15, 15-27, (1994).
On Sonne-cruise 115 'SUNDAFLUT' we have cored a profile easterly and southerly Natuna Island off Peninsula Malaysia / Borneo at about 50 sites to follow the post-Pleistocene transgression. Supported by seismic profiles and precise AMS-14C dating the major changes in the paleo-environment could be reconstructed by sedimentological and geochemical methods. Extremly high sedimentation rates as well as large time gaps required combined studies on several cores.
The different sedimentary facies types represent upper paleo-floodplain to paleo-prodelta deposits. In the late Quaternary, 4 periods of depositional and transforming processes can be distinguished:
* 55-30 ka: prograding prodelta sequence
* ~ 20 ka: pedogenesis
* 16-9 ka: nearshore transgressive system deposits
* < 11 ka: Holocene foraminiferal sands
During the regression preceding the Last Glacial Maximum the Sunda-Shelf became widely exposed. A large coastal plain evolved drained by the North Sunda 'Molengraaff' river system into the South China Sea. Beside the central river valley a net of associated channels and creeks is responsible of the large small-scale variability in the depositional system. Sea-level lowstand and stillstand during the LGM led to the formation of a widely distributed pedogenic horizon.
The overlaying Transgressive System Tract is mainly reduced to the early phase of sediment accumulation according to the rapid drowning combined with reworking of the substratum by coastal erosion and with the subsequent collaps of sediment supply from the hinterland. The preserved nearshore deposits originated in calm water bodies (deltafront / terrestrial depressions), in areas with sediment-catching vegetation (mangrove), and by overbanking.
Furthermore, a swampy facies (mangrove) with an increasing marine influence in the upper part indicates directly the position of the paleo-shoreline in several cores. A sea-level curve for this tectonically stable area is under construction using precise AMS-14C dating. Especially, the nearshore swamp deposits, which contain high quantities of in situ plant remnants, enables us to develop a new detailed sea-level curve indicating an early sea-level rise of the time interval 16,5-11,5 ka 14C-years BP. The spatial-temporal successive evolution reflects a retrograde movement of the facies arrangement due to this rapid postglacial sea-level rise.
Investigations of heavy mineral assemblages from this borehole offer the possibility of obtaining evidence on the origin of material that filled the southern part of the glacial basin beneath Amsterdam. The following succession could been established. It comprises (in stratigraphival order):
Drente Formation. The basal sandy infilling shows a mixture of material from the nearby floor of the basin and glacial sediment material. Gravel in the size-fraction 3-5 mm shows glacier-derived material exclusively. In the central parts of the glacial basin the silts that form the main part of the Saalian infilling yield higher values for alterites, indicating more influence from ice-pushed deposits, bordering the basin. Deposits of the early Middle Pleistocene Sterksel Formation (sediments of the river Rhine) seem to have acted as source material.
Eem Formation. The sandlayer at the base of the diatomite contains material from another source. This source is, most probably, to be found in the northern part of the North Sea Basin. It is characterized by high values for hornblende and epidote, together with low values for alterites and garnet. This can be seen to indicate a primary Scandinavian origin of the sand-fraction for the sediment. This association continues up to the top of the clayey deposits with the exception of two samples. One sample (63.53-63.56 m) shows an admixture with fresh material from the basin fringes, the other (39.51-39.57 m) presents an association characterized by an extremely high frequency of garnet. The may have been caused by a strong selective sorting of heavy minerals. The upprmost sandy beds of the marine sequence show a strong increase in alterite with a maximum of volcanic minerals, indicating the influence of the Rhine system. However, this influence is short-lived and basin-derived sediments occur again.
Twente formation with the exception of the typical garnet-rich cover-sand facies at the top, the sediment of this formation seems for the greater part to be derived from local sources.
The succession presented here is characteristic of the infilling of the southern part of the Amsterdam glacial basin. The northern part has a somewhat different history, of which only very little is known due to lack of investigations.
Renewed mapping of the subsurface of the Amsterdam area has resulted in new cored drillings from the Glacial Basin of Amsterdam. In combination with older available data of the basin (Zagwijn, 1983), the borehole Amsterdam-Terminal was cored in the deeper part of the glacial basin in 1997.
The sequence starts with a warved clay at the base of the infilling which, according to the pollen association, was of Late Saalian age at the top. During the pollen zones E1 (Betula), E2 (Pinus) and E3 (Quercus) diatomite was deposited under fresh water conditions. During the first period of the Corylus phase (E4) the depositional conditions changed fundamentally. A gradual increase of organic material resulted in an organic rich layer (sapropel). This deposit is known as the Harting Layer. It is assigned to pollen zone E4a, which is characterized by increasing percentages of Corylus and a still higher frequency of Quercus pollen. The presence of the algae of Pediastrum and Tetraedron is a striking phenomena in the Harting Layer. During the Corylus phase the input of clastic sediment increased and rhythmically layered clays were deposited. The Taxus zone (E4b) is represented by a sticky crumbly clay. During the Carpinus zone (E5) clay and sand were laid down. Except for the upper part, where some thin clay layers are present, the infilling of the glacial basin ends predominantly with sands. The relatively high percentages of Chenopodiaceae in the intercalated clay indicate the presence of tidal flats and perhaps salt marshes not far off in the basin. The development of these tidal flats and salt marshes took place during the Carpinus phase (E5) of the Eemian.Because of the percentages of Juniperus and Hippophao, found at a higher level near the top of the infilling, the presence of a habitat for the pioneer vegetation of 'sand dunes' is supposed. This sediment is probably already of Early Weichselian age.
Borehole Amsterdam Terminal was cored in 1997 in order to serve as an additional source of information on the Eemian deposits in the type-area. Up till now borehole Amersfoort I (van Voorthuyzen, 1958 and Zagwijn, 1961) was used as the stratotype for this chronostratigraphical unit. As shown by Zagwijn (1996) the record of the Eemian sediments in the Amsterdam glacial basin is more complete. In the present borehole an Eemian sequence, generally composed of very fine-grained deposits, reaches a thickness of at least 35 meter. The sediments have been studied by a wide range of disciplines.
The first, although weak marine indications are found at 62.07 and 61.73 m. At 61.26 m the marine influence is distinct based on an assemblage strongly predominated by heterotroph dinoflagellates of the Brigantedinium plexus, reflecting nutrient rich conditions. Samples from 61.03 to 32.65 m are characterized by lagoonal dinocyst associations (Diplopelta symmetrica, Lingulodinium machaerophorum and Tuberculodinium vancampoae). Moreover, the two last mentioned species indicate (sub-) tropical sea-surface temperatures. In this interval three subzones can be distinguished: 61.03 - 57.76 m: specimens of L. machaerophorum with small processes indicate decreased salinity. In particular around 60.50 m anaerobic bottom conditions prevail. 55.79 - 41.20 m: gradual increase of the marine influence as exemplified for example by Bitectatodinium tepikiense 37.93 - 32.65 m: possibly more open-marine influence.
From 52.28 m and upward Tertiary reworking is noted, and thought to be introduced by North Sea near-coastal transport into the Amsterdam Basin.
A comparison with Eemian/Ipswichian assemblages from NW Europe is difficult because of uncertain age determinations, particularly from the British Isles. North Atlantic associations (Eemian or Isotope Stage 5) are characterized by colder and full marine dinocysts of oceanic aspect.
The Eemian stratotype is defined in The Netherlands in the Eem Valley in the city of Amersfoort. Obviously, the Eemian in Amersfoort is not complete and therefore it was neccessary to designate one or more parastratotypes. The infill of the Amsterdam glacial basin was selected as one of these. After a preinvestigation the locality was selected for a cored research borehole which was succesfully drilled in 1997. This borehole, named 'Amsterdam-Terminal-25E913' has been subjected to a multidisciplinary investigation. Between 28.00 and 62.00 meters below surface 43 samples were analysed for their molluscan content.
The molluscan analysis of these samples provided the following results: All samples show assemblages from different marine environments, only in the upper three meters additionally several non-marine species occur in low numbers.The sequence may be subdivided into three main zones, each of which can be subdivided into smaller units. The main zones are characterised by molluscan assemblages pointing to:- well vegetated, shallow and lower saline conditions in the lower zone,- less to non-vegetated, deep and euhaline conditions in the middle zone,- well vegetated, shallow and lower saline conditions in the upper zone. In the molluscan assemblages cold indicators are virtually missing, warm species on the contrary are present from bottom to top. Species considered to be characteristic of an Eemian age in the North Sea Basin are e.g. Acanthocardia paucicostata, Modiolus adriaticus, Mytilaster lineatus, Venerupis aurea senescens, Gastrana fragilis, Parvicardium exiguum, Bittium reticulatum, Turboella radiata balkei, Hinia pygmaea, Hyala vitrea etc. The designated zones are the expression of the rise in sea level and the subsequent shallowing of the basin by sedimentation. The upper zone probably reflects also the lowering of the sealevel in the second part of the interglacial. The sequence investigated in borehole Amsterdam-Terminal is characteristic of the deeper parts of the Amsterdam glacial basin where a clayey facies predominates. Essentially the same assemblages were found in borehole Beursplein-25 G943, although there only the upper part of the same sequence is present. Near the margins of the basin the facies is more sandy and the encountered assemblages differ considerably from those described of Amsterdam-Terminal-25E913. In these places intertidal assemblages are found, even with elements living near high tide level and still higher from the supra-littoral zone. This was found in boring Zunderdorp-25E344. Characteristic supralittoral to high sublittoral species found in the deposits of this boring are: Littorina littorea, Peringia ulvae, Bittium reticulatum, Retusa obtusa, Ovatella myosotis, Acanthochitona fascicularis, Abra tenuis etc. From this latter borehole amino acid razemisation data were published (Miller & Mangerud, 1985). Additional ratios have been obtained last year from Macoma balthica. Of this species two different sets of ratios are present. Between 24.00-29.00 m a mean D/L-ratio of 0.116 was obtained, while between 35.00-29.20 m D/L-ratios vary between 0.176-0.189.
In the first half of the 19th century a temperate lusitanian diatom flora and mollusc fauna older than the Holocene one was discovered. They were found and described from corings for exploration of groundwater in Amsterdam. The same marine flora and fauna was also described in a valley of the Eem, a small river flowing through Amerfoort in the Netherlands. It became the type locality of the Eemian. As a result of the detailed geological mapping in the nineties new borings from the Amsterdam glacial basin became available. The studies carried out on these cored borings are multidisciplinary. As a result of this approach we got new information about the environmental changes during the infilling of the glacial basin of Amsterdam. It offers also a starting point for re-interpretation of data and for new investigations of the Eemian in the glacial basin of Amersfoort. The infilling of the basin of Amsterdam starts in a fresh water environment under cold conditions in a periglacial landscape without trees, when the treeline had not yet passed this location. Navicula jaernefeltii is the dominant species in the lower assemblage at the base of the infilling. At the time the treeline passes, a freshwater lake with an Aulacoseira italica flora develops. At an early moment during the Eemian interglacial (pollenzone E3) the first marine influence occurs as shown by the Fragilaria bloom. The following marine phase is characterized by three diatom floras, from bottom to top:-- a flora with Hyalodiscus scoticus indicating clear marine-brackish waters without tidal influence-- a flora characterized by allochtonous Stephanopyxis turris and Chaetoceros spores, indicative for cold oceanic waters and Cocconeis disculoides which is autochtonous and characteristic for marine brackish, warmer and southerly waters. There is still no tidal influence in the basin. -- a flora characterized by Cymatosira belgica showing marked similarity to the holocene North Sea flora. In this phase the tidal influence is clearly present. In contrast with the Amsterdam record, which shows a very detailed Late Saalien and Early Eemian registration, in the Amersfoort record the early marine registration is very condensed. While Amsterdam is deposited in a basin with a remarkable waterdepth, the Amersfoort sediments are deposited on the beach of a shallow bay. In Amsterdam the Hyalodiscus and Stephanodiscus floras are autochtonous, in Amersfoort both floras are displaced. The differences in registration and of paleo-environmental conditions during the first part of the Eemian and towards the end of the period in the glacial basins gives the oppertunity to reconstruct paleo-sealevels and to link them with North Atlantic Ocean records.
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