T. van der Hammen

Palaeoecology and stratigraphy of the lateglacial type section at Usselo (the Netherlands)

Abstract The present study is based on a detailed analysis of the Lateglacial type section from Usselo. In addition to pollen, seeds and insect assemblages many other hitherto unrecognized or incompletely known micro- and macrofossils are identified, described and figured and the palaeoecological significance is discussed. The new Usselo section spans most of the Lateglacial stratigraphical succession of the Netherlands. The base of the sampled section is dated c. 12,880 B.P. Since this sandy base already has thermophilous insects, it is evident that it begins after the initial climatic warming at the beginning of the Lateglacial. A considerable part of the Older Coversand II correlates with the relatively warm and treeless pre-Bolling s.s. period (= lower part of Bolling s.l.). During this interval macrophyte vegetation was very sparse. Coleoptera were rare and the most common beetles were species that live in damp sand where they feed on algae. Blue green algae of the Gloeotrichia -type apparently played a pioneer role thanks to their ability to fix nitrogen and thus make conditions in the shallow pool of Usselo suitable for other plants. At the start of the Bolling s.s. there was a great increase in the habitat diversity of both flora and fauna. Phytophagous Coleoptera show a much greater variety and there is evidence from the terrestrial beetles for a gradual reduction in the importance of the bare sand habitats. The more sandy layer which corresponds to the Earlier Dryas period does not show any marked change in either the local plant or insect communities. The sequence of aquatic macrophytes and helophytes equivalent to the older part of the Allerod period indicates that the pool was becoming progressively shallower and richer in nutrients. The continuous vegetational succession can be interpreted as due to the gradually improved nutritional status of the site leading to acidification towards the end of the Allerod. The succession of insect assemblages closely reflects that of the vegetation and the combined palaeobotanical and zoological data permit a very detailed reconstruction of successive local habitats. In contrast to the situation in western Britain where the insects suggest that the climate was warmer in Bolling time than in Allerod time, the insect sequence from Usselo suggests that the Allerod in the Netherlands was as warm as the Bolling (s.l.). Obligate cold climate insects were not found in the Earlier Dryas of Usselo. Evaluation of the existing 14 C dates led to the conclusion that the most probable age limits of the Earlier Dryas in the Netherlands are 12,150±100 and 11,900±50 B.P.. This makes clear the approximate time-correlation with the cooling of climate in western Britain and the known standstill of the Scandinavian ice sheet. It is concluded that the strong environmental changes of the Earlier Dryas in the Netherlands (extensive dune-fields, frost-phenomena, death of birch trees resulting in renewed almost treeless landscapes) cannot be caused solely by climatic dryness, but that some temperature decrease must also have been involved. Temperature estimations for the Lateglacial, based on insects, plants and frost-phenomena are given and an attempt to reconcile all the evidence is finally made.

Amazonia during the last glacial

Abstract New data on the vegetational history and Quaternary geology of Amazonia permit an improved reconstruction of past environments in Amazonia during the last glacial period. Although limited, data from Rondonia, Carajas and Guyana show that, in certain areas, savanna-type vegetation and savanna forest had replaced the rain forest during the late Pleniglacial (ca. 22,000–13,000 yr B.P.). The Amazonian forest may have been split up into one major west Amazonian and several other medium-size forest areas. This suggests a decline in rainfall of 500 to 1000 mm (a reduction of 25 to 40%). Temperatures may also have been 2° to 6°C (4±2°C) lower than today, possibly substancially influencing Amazonian vegetation. During the humid middle Pleniglacial (55,000–26,000 yr B.P.), rivers carried a lot of water and sediment, resulting in the deposition of lower terrace sediments with one dry interruption around 40,000 yr B.P. (Carajas and Katira). In Carajas, there is evidence of dry periods ocurring at about 40,000 yr B.P., and during the early Pleniglacial (ca. 60,000 yr B.P.). Rivers carried little water and incised into the low terrace sediments during the dry late Pleniglacial. Water levels rose during the late glacial (13,000–10,000 yr B.P.) or at the beginning of the Holocene (10,000 yr B.P.). Sedimentation in (and of) the present inundation valleys commenced after that.

Upper quaternary vegetational and climatic sequence of the fuquene area (Eastern Cordillera, Colombia)

Abstract In this paper the results of a pollen-analytical study of three bore holes of lake deposits in the Fuquene-Valle de Ubate area, Colombian Cordillera Oriental (5° N; elev. 2,580 m) are given. By means of 14 C dates and comparison with other pollen diagrams and dates from the Eastern Cordillera, it was possible to correlate the local pollen zones and chronostratigraphical units with the European chronostratigraphical sequence. The longest diagram (Fuquene II) represents some 30, 000 years, including part of the Middle Pleniglacial, the Upper Pleniglacial, the Late Glacial and the Holocene. During the Middle Pleniglacial, Polylepis wood was an important constituent of the vegetation. During climatic extremes of the Upper Pleniglacial an open paramo vegetation existed. During the main part of the Upper Pleniglacial (from ca. 21,000 to ca. 13,000 B.P.) the lake level was low and the climate dry. The lake level rose again in the beginning of the Late Glacial, and the area around the lake became forested. At the beginning of the El Abra Stadial (Late Dryas Stadial) there was a marked cooling and the lake became lower once more; this had a great impact on the vegetation which became partly open again. The beginning of the Holocene is marked by a gradual increase of forest elements, especially of Quercus (oak.) Oak forests dominated the area during the greater part of the Holocene. During the “hypithermal”, elements growing today at a lower level, such as Cecropia, Acalypha (and possibly Alchornea ), must have been growing in the area, intermixed with the oak forest. At the same time there was a considerable increase of Urticaceae in the undergrowth of these forests. The vegetation zones were probably situated several hundreds of metres higher than they are at the present time. The influence of man on vegetation had started already before the beginning of our era, but there is a sudden increase of this influence (e.g., increase of Dodonaea , decrease of Quercus and Urticaceae) somewhere around 2,000 B.P. Later on, the diagram shows the rapid ultimate decline of the forest elements and an increase of Gramineae. Today nothing is left of the original oak forests and their place is taken by arable land or by secondary plant communities with low brushwood. In the lake sediments, thin layers of volcanic ash were found dated approximately 9,000 B.P., 11,000 B.P., and 21,000 B.P., respectively; a series of three ash-layers was deposited between approximately 22,000 B.P. and 26,000 B.P. and one possibly at approximately 30,000 B.P. These layers have been found in several other sections in the Eastern Cordillera and are the basis for a tephrochronology.

Palynological record of the upheaval of the Northern Andes: A study of the pliocene and lower quaternary of the Colombian Eastern Cordillera and the early evolution of its high-Andean biota

The present paper deals with the stratigraphy and palynology of the Pliocene and Lower Quaternary formations of the central part of the Colombian Eastern Cordillera. Three Plio-Pleistocene formations are distinguished: the Tilata Formation is of Pliocene and Earliest Quaternary, the Subachoque Formation of Early Quaternary, and the Sabana Formation of Middle and Late Quaternary age. Four pollen zones have been distinguished the Tilata Formation, the first three being of Pliocene, the fourth of Earliest Pleistocene age. Zone I shows a purely tropical lowland flora and zone IV a high-mountain flora, whereas zones II and III show floras of intermediate altitude. A gradual elevation by more than 2,000 m has in this way been recorded palynologically for the area of the high plain of Bogota. Actually, the top level of the Tilata Formation lies at ca. 2,850 m alt. in the area east of Choconta. The top level of the Subachoque Formation lies at ca. 2,700 m alt. in the Subachoque and Guasca valleys. The sediments correspond with pollen zone V of Early Quaternary age, exhibiting a high-mountain flora and indicative of glacial conditions. Sediments corresponding with the same pollen zone have been found between 140 m and 195 m below the surface (2,580 m alt.) in a borehole in Bogota, showing that the wide central part of the high plain may have relatively subsided more than 200 m since the end of the Early Quaternary. The Sabana Formation, whose top has an actual maximum elevation of 2,600 m, corresponds with major pollen zones VI and VII and exhibits a sequence of glacials and interglacials belonging to the Middle and Late Quaternary. The pollen types important for time correlation between the tropical and mountain environments, belong the anemophilous taxa: pollen of Hedyosmum appears in zone II, of Myrica in zone III, of Alnus in zone V–VI and of Quercus in zone VII. During the process of upheaval tropical and subtropical species gradually disappeared from the area owing to the changing ecological conditions, to be gradually replaced by species better adapted to the new circumstances. It is striking that the oldest, cold to temperate flora (zone IV) is still poor in species. Apart from the dominant grasses, Aragoa, Hypericum, Valeriana, Plantago and Polylepis are amongst the oldest high-mountain (paramo) plants. This flora was later gradually enriched both by immigrants from the north (Holarctic) and the south (Antarctic) and, mostly, by endemic elements originated by the evolutionary adaptation of the local neotropical flora. In this way the Colombian Eastern Cordillera provides us, in its Plio-Pleistocene sediments, with a, so far unique, example of the gradual in situ evolution of tropical high-mountain biota. Because of the scarcity of species adapted to the High-Andean environment, it appears that the forest limit in the Early Quaternary was lower and the Andean forest belt narrower than is the case at present. The first glacial periods of the Early Pleistocene were already so severe, that a temporary, additional lowering of the forest limit by, probably, 500–800 m resulted. At the same time fluvio-glacial gravels were deposited in many places, and huge masses of the old tropical weathering products still covering the mountains at that time were removed by solifluction fromthe slopes downwards into the marginal areas of the basin of the high plain. During the Early Quaternary the large Pleistocene lake of the Sabana de Bogota was formed, and as the result of progressive and probably continuous subsidence, deposits of hundreds of metres of lake sediments accumulated in the basin during the Pleistocene. From the present data it may be concluded that the major ultimate upheaval of the Cordillera took place during the Middle-Late Pliocene, and that it probably ceased before the beginning of the Pleistocene. The average rate of upheaval may be estimated at between 1 and5 mm per year, but it may have been twice as much in the area of the Sierra Nevada del Cocuy.

Distribution and Ecology of Parent Taxa of Pollen Lodged Within the Latin American Pollen Database.

The cornerstone of palaeoecological research, concerned with vegetation dynamics over the recent geological past, is a good understanding of the present-day ecology and distribution of the taxa. This is particularly necessary in areas of high floral diversity such as Latin America. Vegetation reconstructions, based on numerous pollen records, now exist with respect to all major vegetation associations from Latin America. With this ever-increasing number of sedimentary records becoming available, there is a need to collate this information and to provide information concerning ecology and distribution of the taxa concerned. The existing Latin American Pollen Database (LAPD) meets the first of these needs. Information concerning the ecology and distribution of the parent taxa responsible for producing the pollen, presently lodged within the LAPD, is the focus of this paper. The ‘dictionary’ describes the ecology and distribution of the parent taxa responsible for producing pollen identified within sedimentary records. These descriptions are based on a wide range of literature and extensive discussions with members of the palaeoecological community working in different parts of Latin America investigating a range of different vegetation types.

The continental record of environmental conditions at 18,000 yr B.P.: An initial evaluation

Abstract The development of reliable paleoclimatic maps at a global scale requires data at the following three levels of analysis: (1) well-recorded observations of evenly positioned, well-dated geological evidence (Level I), (2) paleoclimatic estimates derived from this evidence by well-defined quantitative repeatable methods (Level II), and (3) maps synthesizing the estimates from several independent sources of geological evidence (Level III). Our paper describes much of the currently available paleoclimatic data from unglaciated terrestrial areas at ca. 18,000 yr B.P. and illustrates the quantity and quality of the data at both the Level I and the Level II stages of analysis. Although the scarcity of well-dated evidence for this time period precluded any major Level III syntheses of the information, comparisons were drawn where possible between the geological evidence and the climatic conditions simulated by general-circulation model experiments of Gates, 1976a , Gates, 1976b and Manabe and Hahn (1977) . Of the more than 320 sites with data from 18,000 yr B.P., only 65 are well-dated with bracketing dates within the interval of 23,000 to 13,000 yr B.P., whereas about 100 are undated or poorly dated. We concentrated our survey on palynological and paleobotanical evidence and also thoroughly reviewed the evidence for water levels in lakes at 18,000 yr B.P. In areas with few of these sources of evidence, data on former snowlines, periglacial features, and eolian deposits were included, but the survey of these data is far from complete. Maps of the assembled data reveal the consistency of the paleoclimatic estimates in “data-rich” areas and also show which areas required additional information. The maps show that conditions were colder than present at 18,000 yr B.P. for all sites with temperature estimates. Estimated temperature depressions varied from ca. 1° to 12°C or more, depending on the location of the sample, the type of geological evidence, and the method of temperature estimation. Interpreted hydrological conditions were more variable spatially than the temperature estimates. The southwestern U.S. was moister than present, whereas the southeast may have been drier. Europe and the northern Mediterranean across to Afghanistan were drier than present, but northwest Africa was wetter. Australia was mainly drier than present, but several sites there as well as in Africa show significant climatic changes between 21,000 and 16,000 yr B.P. This latter evidence suggests that considerable variability may have occurred during the several thousand-year period centered on 18,000 yr B.P. Accurate time control is therefore required for the geological data used to study the climate dynamics of 18,000 yr ago. Large portions of South America and Asia as well as significant portions of the other continents lack the data base, or at least the well-dated base, required to define the 18,000 yr B.P. climate. In the few areas where comparisons were made with the Ice Age climates simulated by general-circulation models, general agreement existed between the geological evidence and the model simulations. Many critical comparisons were thwarted, however, by the lack of model simulations for all seasons at 18,000 yr B.P. Difficulty in validating precipitation anomalies in the tropics also arose because surface-albedo values, which are a vital input to the general circulation models, are estimated from the same evidence that is used to validate the results of the models.

Zygnemataceae in quaternary Columbian sediments

Spores of Zygnemataceae are described from Lower Pleistocene, Upper Pleistocene and Holocene sediments from the Colombian Andes. About ten types could be distinguished, most of them referable to the genera Mougeotia, Spirogyra and Debarya. In addition records of Bulbochaete (Oedogoniaceae) and of Closterium (Desmidiaceae) are reported. Spores of Zygnemataceae appear to be of considerable importance for palaeoecological studies. Debarya aff. D. glyptosperma and several other types were up to now found only in deposits associated with a cold to cool high-mountain climate.

Glacial sequence and environmental history in the Sierra Nevada del cocuy (Colombia)

Abstract The glacial sequence in the area of the Sierra Nevada del Cocuy (Cordillera Oriental, Colombia) was studied in the field and by means of aerial photographs in relation to the environmental history, which was studied by means of pollen analysis and radiocarbon dating. At least five (possibly six) glacial drift bodies could be recognised, and on the basis of the (groups of) bordering moraines about six main glacial stades were defined and named. Drifts 2, 3, 4 and 5 are presumably all of Fuquenian (Last Glacial) age, whereas Drift 6 is of Holocene (Neoglacial) age. In the pollen diagrams the Saravita, Susaca and Guantiva interstadials are clearly reflected as periods of a somewhat higher forest limit, a more abundant growth of Polylepis and an interesting pioneer vegetation (often with Dodonaea). The greatest extension of the glaciers took place before ca. 25,000 B.P., probably in the period between 45,000 B.P. and 25,000 B.P. The climate was relatively wet at the time and the forest limit something like 800–1000 m below the present one, and during the early period of major extension of the ice, glaciers and forest may have locally been in contact (at elevations between 2200 and 2700 m), and the paramo belt was relatively narrow and wet, with Polylepis abundant in the lower parts. Between 21,000 and 14,000 B.P., the ice extension was much less, the forest limit lower, and the climate drier, which resulted in a relatively broad and dry paramo belt. The late Glacial had a relatively wet climate again, but the annual temperature increased; there are two clear “glacial stades”. There are very clear signs of a late medieval neoglaciation; glaciers started to retire from the outermost Neoglacial bordering moraines probably after 1850 A.D., and are still decreasing.

Absolute chronology of the Pliocene-Quaternary sediment sequence of the Bogota area, Colombia

Abstract A revised geochronological framework is presented for the sequence of unconsolidated sediments present in the Bogota area (Eastern Cordillera, Colombia). This is based on 11 fission track dates on zircons that were obtained both from exposed ash layers and from a series of ashes from the Funza II borehole, which reached 586 m below the surface of the high plain. The 3 dates obtained from the exposed ash layers provide control for the older part of the sequence (6-2.5 Ma). The 8 dates from the Funza II core give control for the younger part of the sequence (3-0 Ma). Acceptance of these new fission track dates on volcanic zircons means that many of the tephra dates obtained from the Funza I core (that included fission track dates on glass shards and KAr dates using the mineral fractions in the ashes, published in 1984), are rejected. It also confirms earlier climate-stratigraphical dating. The Neogene-Quaternary sediments of the Bogota area span a period of at least the last 6 Ma. The fluvial-lacustrine sediment record registers major tectonic uplift of the Eastern Cordillera for the period between 5 and 3 Ma, the development of the large sedimentary basin of Bogota after 3.5 Ma, with an important phase of tectonic adjustment at about 1 Ma, and a long period of strong climatic fluctuations that started shortly after 2.7 Ma.

Charcoal in soils of the Allerød-Younger Dryas transition were the result of natural fires and not necessarily the effect of an extra-terrestrial impact

During the warm Bolling-Allerod interstadial, tree species migrated from their refugia in southern Europe northwards into the area within the present temperate climatic zone. It is evident from high levels of charcoal in fossil records in this region that, especially during the later part of the Bolling-Allerod interstadial, many fires occurred. The start of the Younger Dryas was characterised by rapid and intense cooling and rising water tables, with catastrophic effects on the vegetation. Thermophilous pine trees could not survive the cold Younger Dryas climate. Dead wood provided an abundant source of fuel for intense, large-scale fires seen in many records as a concentration of charcoal particles in so-called ‘Usselosoils’ dated to ca 10,950 14C BP. A similar trend in increased charcoal indicating increased burning is seen at many sites across North America at this time and it has been suggested by Firestone et al. (2007) that this was caused by an explosion of extra-terrestrial material over northern North America, causing the Younger Dryas climate cooling and Megafaunal extinction. We argue that there is no need to invoke an extraterrestrial cause to explain the charcoal in the fossilized soils. The volume of forest trees that died as a result of the cold Younger Dryas climate would easily have supplied sufficient fuel for intense, large-scale fires and can be used to account for the concentration of charcoal particles. As soils were no longer covered by dense vegetation, much erosion occurred during the Younger Dryas and therefore, at many places, Usselo soils, rich in charcoal, were preserved under aeolian sand dunes.