Palaeoecology, past climate systems, and C 3/C 4 photosynthesis

Abstract

The geologic record shows unequivocally that the present world is unusually cold; the so called ‘greenhouse’ condition has been normal for planet Earth for the past 500 million years. Continental positions, orbital parameters, and atmospheric composition strongly influence global climate on timescales ranging from 10 8 to 10 2 years. Atmospheric CO 2 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 has varied substantially over the past 500 million years and may be correlated with changing climate. Most terrestrial carbon sequestering operates 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 shown that terrestrial carbon sequestering is greater in ever-wet conditions. The distribution of the sites of greatest carbon sequestering switches from low latitudes during icehouse times to higher latitudes, >40°, during greenhouse times, except maritime sites. Evolutionary factors, e.g. competition, and climate change have led to major ecosystem restructuring during the past 500 million years. Pre-change biodiversity 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 number of uncertainties as well as probabilities involved in estimating sequestering ratios and climate changes; Estimates of past carbon sequestering are likely to be too low because dispersed fossil organic matter is inadequately inventoried. Numerical climate model results are unreliable 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 extant land plants have a C 3 photosynthetic pathway. However, under conditions where photorespiration can reduce photosynthetic efficiency, warmth and high O 2 concentrations, many unrelated plants have independently evolved C 4 pathways. C 4 plants have different water relations and competitive characteristics to C 3 plants and clearly ecosystem structure and carbon sequestering are likely to change with global warming. By studying the different isotopic signature bequeathed by these systems the fossil record can provide critical data on the dynamics of plants with these systems under changing climatic conditions: data that again are essential for effective ecosystem management strategies.