PALEOECOLOGY, PAST CLIMATE SYSTEMS, AND C3 C4PHOTOSYNTHESIS/

The geologic record shows unequivocally that the present world is unus ually cold; the so called 'greenhouse' condition has been normal for p lanet Earth for the past 500 million years. Continental positions, orb ital parameters, and atmospheric composition strongly influence global climate on timescales ranging from 10(8) to 10(2) years. Atmospheric CO2 is an important contributing factor in determining average global temperature, and is particularly important in influencing changes over shorter timescales (say < 10(5) years). Carbon sequestering on land h as varied substantially over the past 500 million years and may be cor related with changing climate. Most terrestrial carbon sequestering op erates on biological timescales (<10(5) years) rather than geological timescales (>10(5) years). Terrestrial carbon sequestering is strongly influenced by the biology of the organisms involved and it has been s hown that terrestrial carbon sequestering is greater in ever-wet condi tions. The distribution of the sites of greatest carbon sequestering s witches from low latitudes during icehouse times to higher latitudes, >40-degrees, during greenhouse times, except maritime sites. Evolution ary factors, e.g. competition, and climate change have led to major ec osystem restructuring during the past 500 million years. Pre-change bi odiversity is therefore critical in determining the nature and rate of restructuring particularly with respect to plants which are the only group of organisms capable of carbon sequestering. There exists a numb er of uncertainties as well as probabilities involved in estimating se questering ratios and climate changes; Estimates of past carbon seques tering are likely to be too low because dispersed fossil organic matte r is inadequately inventoried. Numerical climate model results are unr eliable unless evaluated against fossil and sediment data. Terrestrial carbon sequestering is unlikely to dominate tectonic controls but as it operates on a shorter time scale it has a strong short term effect and could well tip the climate balance in critical situations. Most ex tant land plants have a C3 photosynthetic pathway. However, under cond itions where photorespiration can reduce photosynthetic efficiency, wa rmth and high O2 concentrations, many unrelated plants have independen tly evolved C4 pathways. C4 plants have different water relations and competitive characteristics to C3 plants and clearly ecosystem structu re and carbon sequestering are likely to change with global warming. B y studying the different isotopic signature bequeathed by these system s the fossil record can provide critical data on the dynamics of plant s with these systems under changing climatic conditions: data that aga in are essential for effective ecosystem management strategies.