Long pollen records from two sediment cores of the basin of Bogotá (Colombia) are presented, reflecting the dynamic history of environmental and vegetation changes in the tropical high Andes during ...the Quaternary. An astronomically tuned age model has been developed by using the visual correlation of the temperature-constrained record of Alnus with the benthic δ18O stacked record of ODP Site 846 and 849 for the last 1 million year (Ma). In addition, spectral analysis of the arboreal pollen percentages (AP%) prior to 1 Ma shows two consistent peaks at 7.6 and 9.5 m, which could either be interpreted as reflecting the 19 and 23-kyr components of the precession cycle or the 41-kyr cycle of obliquity. Evidence for precession forcing comes from the sum of the filtered 7.6 and 9.5 m cycles, revealing a long-term modulation that can be linked to the short-term (100-kyr) and long-term (400-kyr) eccentricity cycles. A precession-forced scenario, however, is not consistent with zircon fission-track ages and is difficult to explain in terms of climate forcing mechanisms. On the contrary, an obliquity control of the AP% record is consistent with the fission-track ages and with glacial–interglacial-bound temperature changes within the study region. Accordingly, it appears that the Funza09 record extends back to ∼2.25 Ma during which four periods with distinct different depositional environments could be identified, following a chronological poorly constrained interval of fluvial and fluvio-lacustrine sediments. From ∼2.25 to ∼1.47 Ma sediment deposition occurred in an area of wetlands dissected by fluvial channels and swamps. The strongest subsidence of the basin occurred between ∼1.47 and ∼1.23 Ma when a proper lake developed. Lacustrine sediments accumulated in water up to 50 m deep between ∼1.23 and ∼0.86 Ma. Hereafter, water depth was generally lower than 50 m, but fluctuated in conjunction with the 100-kyr dominated glacial–interglacial variability during the middle and late Pleistocene. The evolutionary changes of the páramo and montane forest biomes are described in terms of five characteristic stages. Most of the Pleistocene vegetation has no analogue to modern assemblages principally due to the late immigration events of Alnus (1.01 Ma) and Quercus (0.43 Ma) and we conclude that forest composition similar to modern was not established until the Last Interglacial. However, modern ecological constraints of suites of taxa that formed the vegetation of the pre-MIS 5 part of the record allow a reconstruction of environmental and climate change.
► Long continental pollen record spanning almost the entire Pleistocene. ► A 1641 points timeseries, based on published and unpublished pollen data. ► Land–sea correlation, astronomical tuning, and spectral analysis. ► Obliquity vs. precession driven vegetation dynamics on tropical mountains. ► Evolution of the Andean vegetation and basin development.
Marine Isotope Stage (MIS) 11 has been considered a potential analogue for the Holocene and its future evolution. However, a dichotomy has emerged over the precise chronological alignment of the two ...intervals, with one solution favouring a synchronization of the precession signal and another of the obliquity signal. The two schemes lead to different implications over the natural length of the current interglacial and the underlying causes of the evolution of greenhouse gas concentrations. Here, the close coupling observed between changes in southern European tree populations and atmospheric methane concentrations in previous interglacials is used to evaluate the natural vs. anthropogenic contribution to Holocene methane emissions and assess the two alignment schemes. Comparison of the vegetation trends in MIS 1 and MIS 11 favours a precessional alignment, which would suggest that the Holocene is nearing the end of its natural course. This, combined with the divergence between methane concentrations and temperate tree populations after 5 kyr BP, provides some support for the notion that the Holocene methane trend may be anomalous compared to previous interglacials. In contrast, comparison of MIS 1 with MIS 19, which may represent a closer astronomical analogue than MIS 11, leads to substantially different conclusions on the projected natural duration of the current interglacial and the extent of the anthropogenic contribution to the Holocene methane budget. As answers vary with the choice of analogue, resolution of these issues using past interglacials remains elusive.
Long continuous lacustrine sequences constitute on the continent a precious tool for coupling the long term continental environmental changes with deep sea and ice core records, using the ‘count from ...the top’ method. Moreover, they can contribute to long distance correlation on the continent itself and thus help to classify discontinuous sedimentary records. Palynostratigraphical correlation is proposed here between the Velay long sequence and the late Middle Pleistocene series from Central Europe with special attention to temperate episodes. The similarities between the Praclaux and the Holsteinian Interglacial suggest that they are both contemporaneous with marine isotopic stage (MIS) 11, even if absolute dates are still divergent. The evidence of two major warm periods with an interglacial type of vegetation between the Holsteinian and the Eemian is confirmed by the Velay record.
The palynological signature of interglacial deposits in the fragmentary European terrestrial record has often been used as the basis for determining their chronostratigraphical position and ...ultimately their age. This has placed emphasis on the presence/absence and abundance of certain characteristic taxa, but given the lack of continuous stratigraphies and independent chronologies, it has been difficult to assess the extent to which this strategy has produced reliable schemes. Here, an alternative approach is adopted whereby a chronological framework is developed for long and continuous pollen sequences from southern Europe. This in turn allows the emergence of a complete stratigraphical scheme of major vegetation events for the last 430 thousand years (ka) and the evaluation of the stage record of different taxa and their potential diagnostic value for biostratigraphical correlation. The comparison shows distinct similarities among some temperate stages of the terrestrial equivalent complexes of Marine Isotope Stages (MIS) 5 and 7 and also of MIS 9 and 11, but examination of combined records of taxa provides a possibility to differentiate between individual stages. A numerically-derived dichotomous key for the terrestrial stages based on the palynological records of 10 taxa is presented.
Carpinus,
Fagus,
Abies,
Pterocarya and
Buxus emerge as the best ‘indicator pollen types’ because of their variable behaviour from one stage to the next, possibly a result of their late expansion within a temperate stage or reduced genetic variability. The analysis shows that the palynological signature of a temperate deposit can constrain the range of chronostratigraphical possibilities, but vegetation and palynological variability arising from local factors could result in difficulties in making a definite assignment at individual sites.
An attempt is made to explore the relation between vegetation phases and orbital configurations by comparing long pollen sequences from the Mediterranean region and astronomical curves on the basis ...of their independent chronologies. Three recurring patterns emerge from this comparison: (i) all intervals with perihelion passage occurring during northern winter were associated with significant contractions of tree populations; (ii) all temperate-stage expansions of certain Mediterranean vegetation elements occurred when perihelion passage was in summer; and (iii) temperate-stage forests reached maximum extent at times of autumn perihelion. Of these patterns, the first corresponds to extreme dry/cold episodes culminating at times of March perihelion, and the second, to maximum summer temperature and evaporation regimes following June perihelion. The third, associated with the occurrence of September perihelion, appears to be more diffuse both in time and space, possibly reflecting local variations in hydrological regimes. The consistency of at least the first two cases provides some support for the notion that certain recurring vegetation patterns appear to be a result of climate changes linked to specific orbital signatures. This could eventually lead to the development of improved terrestrial chronologies through astronomical tuning.
PDF
