Palaeogeography, Palaeoclimatology, Palaeoecology
The last glacial cycle from the humid tropics of northeastern Australia: comparison of a terrestrial and a marine record
Introduction
The long, continuous palynological record from Lynch's Crater on the Atherton Tableland (Kershaw, 1976, Kershaw, 1986) has, for many years, provided the standard for late Quaternary environmental change in northeastern Australia. The record indicates substantial vegetation variation in response to climatic oscillations as well as sustained changes in taxon and community distributions. However, this small volcanic crater provides a limited spatial picture and one which may be unrepresentative of the broader region due to the steep altitudinal and rainfall gradients of the Tableland area. Of special interest is any regional expression of the decline of drier araucarian forest in association with increased burning around Lynch's Crater about 38,000 yrs BP which has been tentatively attributed to the activities of Aboriginal people. Furthermore, that part of the record beyond the effective limit of radiocarbon dating, about 40,000 yrs BP, has no firm age control.
The opportunity to produce a more regional and better dated record was provided by the retrieval of marine cores on the continental slope adjacent to the Atherton Tableland by the Ocean Drilling Program, Leg 133 (Davies et al., 1991). Preliminary palynological results indicated good pollen representation through much of the 1.4 Ma core sequence (Kershaw et al., 1993) with age control provided by a detailed oxygen isotope record for the last 700,000 years (Peerdeman et al., 1993), radiocarbon dates for the most recent sediments (Peerdeman and Davies, 1993), and marine microfossil stratigraphy (Peerdeman et al., 1993; Wei and Gartner, 1993). The pollen record suggests that the initiation of the decline in araucarian forest occurred some time between about 150,000 and 100,000 yrs BP, much earlier than around Lynch's Crater but the record resolution was too coarse to allow further comparisons. Subsequently, one of us (PTM) has been producing a detailed palynological record from the upper part of this marine sequence to provide a much firmer basis for comparative study. This paper provides a comparison for the period covering at least the last glacial cycle.
Section snippets
Regional setting
The location of sites in relation to major environmental features is shown in Fig. 1. Major physiographic features are aligned approximately north-south. From the coast inland there is a low coastal range, separated by a coastal plain from the granitic Eastern Highlands which rise to 1600 m and contain the highest mountains in Queensland. The Mulgrave River valley partly separates the Eastern Highlands from the basaltic Atherton Tableland, an undulating plateau ranging between 700 and 800 m
Sites
Lynch's Crater (17°22′S, 145°42′E) occurs at an altitude of 760 m within the southeastern part of the Atherton Tableland. The vegetation around the site, prior to clearance this century, was complex mesophyll vine forest or lowland tropical rainforest. The high-altitude occurrence of this rainforest type around the crater is due to the high nutrient status of basaltic soils which compensates for reduced temperatures. From BIOCLIM (Busby, 1991) estimates, the mean annual temperature is 20.4°C
Site records
The records presented (Figs. 2 and 3) most likely extend from the present to the later part of marine oxygen isotope stage 6 (Martinson et al., 1987). A preliminary chronology for the ODP record is provided by one of the two original interpretations of the isotope record undertaken on the core (Peerdeman et al., 1993). Dating of the Lynch's Crater record is based on extrapolation beyond the limit of radiocarbon dating, taking into account the moisture content of the sediments (Kershaw, 1980),
Modern vegetation-pollen-environmental relationships
To provide some basis for interpretation of the records, it is valuable to examine major present-day distributional features of the selected taxa.
From the comprehensive rainforest data set (644 sites × 1422 species) of Webb et al. (1984) and the predicted climatic ranges of species incorporated in the selected taxa (Busby, 1991), the ranges of the rainforest taxa in relation to mean annual temperature and rainfall, as well as mean annual precipitation of the driest month, which is an indication
Taxon abundance
In terms of relative abundance of the selected taxa, there are marked similarities between the Lynch's Crater (Fig. 2) and ODP (Fig. 3) pollen diagrams despite major differences in location, pollen catchment area, and most likely means of pollen transport. Pollen of Araucaria, Oka, Cunoniaceae, Elaeocarpus and Eucalyptus comp. have high values, at least during some phases, while Trema, Macaranga and Celtis have low representation. Exceptions to this pattern are the mangroves which are confined
Lynch's Crater
Although the Lynch's Crater pollen record has been interpreted previously and climate conditions have been quantified from BIOCLIM estimates of inferred dominant community types (Kershaw, 1986) and overlapping ranges of selected taxa (Kershaw and Nix, 1989), the modern data presented here allow some refinement of palaeovegetation and palaeoclimatic estimates. It is also important to provide reconstructions that can be compared with those from ODP 820.
The earliest period, phase L6, is
Establishment of a regional chronostratigraphy
Radiocarbon dates on the upper parts of both sequences (Table 1, Table 2) indicate chronologic integrity back to the effective limit of radiocarbon dating. Some parts of the Lynch's Crater sequence are very condensed and it is possible that there may have been short hiatuses in the upper part, particularly at the very end of the Pleistocene (oxygen isotope stage 2) and in the late Holocene. However, the fresh nature of swamp and lake sediments suggests continuous sedimentation beyond the dating
Decline of araucarian forest
Refinement of the ODP record has provided some unexpected results relating to the possible role of people in the decline of Araucaria forest. The original proposal of Kershaw et al. (1993) and Kershaw (1994), based on the Lynch's Crater and preliminary ODP records, was that Aboriginal people may have arrived about 140,000 yrs BP, during the height of the penultimate glacial period and, with their use of fire, caused the progressive elimination of araucarian forest and its replacement with fire
Conclusions
The analysis of the onshore and offshore pollen records, in relation to modern pollen samples and climatic ranges of extant taxa, has produced an interpretable pattern of vegetation and environmental change over the last glacial cycle. Dates available from the marine core have confirmed previous age estimates on the terrestrial Lynch's Crater record for key periods of time. Conversely, the continuous nature of the Lynch's Crater pollen record has helped resolve some dating uncertainties in the
Acknowledgements
We thank the Ocean Drilling Program Laboratory at College Station for providing core samples, Sander van der Kaars for advice on preparation of marine pollen samples, Chris Turney and particularly Bill Watts and Cathy Whitlock for their extremely helpful comments on the originally submitted manuscript, and Gary Swinton for drafting illustrations. The research was facilitated by a Monash Graduate Scholarship to Patrick Moss and an Australian Research Council grant to Peter Kershaw.
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