Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
193
high latitudes with expansion of redbeds (as over the Permian/Triassic boundary or in the
Late Cretaceous: IV.3.1). An intermittent low-latitude paralic coal zone intervening the
redbed realm, as in the terminal Cretaceous (IV.3.5), is evidence of a radical global
climate change.
VII.1.2. Isotope ratios
Isotopic compositions of carbon and oxygen, expressed as proportions of the heavier
species, δ 13 C, δ 18 O, are interrelated, reflecting productivity and the dead mass export
from terrestrial and marine ecosystems, the variables that fall under climatic control.
Their relation to climate is mediated by the influence of temperature and precipitation on
photosynthesis, biomass growth rates, continental runoff, organic and carbonate deposition,
calcification of rhizolites and skeletal structures, etc. Oxygen isotope ratios of carbonate
skeletons fluctuate with water temperature and salinity. The carbonate δ 18 O
tends to decrease with deglaciation and the influx of meltwater, hence a correlation with
deep water (in particular, NADW) production and the Heinrich events (e.g., in the Younger
Dryas: Smith et al., 1997).
Terrestrial biomass preferentially contributes the heavier oxygen and consumes the
lighter carbon, thereby inflicting a parallel evolution of δ 18 O and δ 13 C. The lighter carbon
is locked in the standing crop biomass, humus and the buried dead mass of organic-rich
deposits. It is returned to the atmosphere with plant/soil respiration, biomass burning, and
the outgassing of buried organic matter. The ratios of terrestrial to marine production are
reflected in δ 13 C fluctuations owing to their different 12 C consumption rates (Jasper &
Gagosian., 1989). Their relative contributions depend, among other factors, on sea-level
fluctuations that are in these way reflected in the δ 13 C records.
Anaerobic metabolism of methane-consuming Archaea is an additional sink for the
lighter isotope potentially leaving a signature in the δ 13 C records (Kuypers et al., 2001;
Orphan et al., 2001). However, a recent tendency to ascribe all isotopic anomalies to this
factor (e.g., Prokoph et al., 2001) does not seem warranted by the evidence, because a
much higher 13 C/ 12 C ratio has been found in other chemoautotrophic organisms of the
anaerobic ecosystems (Orphan et al., 2001) and the net effect might have been not so
prominent.
Since the globally recognized level about 7.3 Ma, but probably even earlier, the δ 13 C
is also affected by the changing ratio of C 3
:C 4
photosynthesis that depends on the atmospheric
CO 2
concentration and climate.
In the shallow-water carbonate precipitation systems, isotope fractionation is in equilibrium
with that of the organic carbon reservoir. A correlation with terrestrial productivity
is confirmed by δ 13 C carb
fluctuations with precession cycles (their induced precipitation
cycles) and, on a minor scale, with the monsoon index – rainforest production during
the El Niño cycles (Siegenthaler, 1990; Siegenthaler & Sermiento, 1993).