Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 46

Problems with this sort of analysis abound. First, there are all the dating problems
commonly associated with terrestrial palaeontology (which we touch on elsewhere in
this chapter). Second, the relationship with climate is less direct than other commonly
used indicators. The indicator really is atmospheric carbon dioxide concentration
and although carbon dioxide does force climate, climate change is known to occur
without a major change in carbon dioxide. An example here would be the Little Ice
Age (see Chapters 4 and 5), and, of course, vagaries of the weather (the day-to-day
manifestation of climate) are independent of changes in carbon dioxide. Third, even
the carbon dioxide/leaf stomata relationship produces only a coarse correlation with
carbon dioxide concentrations.
Nonetheless, analysis of leaf physiology has on occasion provided some remarkable
climatic insights. In 1999 Jenny McElwain and colleagues from Sheffield University
in the UK (McElwain et al., 1999) counted stomata densities in fossilised leaves from
dozens of species from around the Triassic/Jurassic extinction some 205 mya (see
section 3.3.6). They found lower stomata densities after the extinction than before,
which suggests that carbon dioxide levels were far higher. They calculated that carbon
dioxide levels rose from around 600 parts per million (ppm) before, to between 2100
and 2400 ppm after the extinction. This would probably have forced the global climate
to be some 4 ◦ C warmer. The resulting climate would have been too warm for large
leaves outside of high latitudes to photosynthesise properly and so could go a long
way towards accounting for the extinction of 95% of all plant species at that time.
The cause of this carbon dioxide pulse is thought to be volcanism during the breaking
up of Pangaea. The area of volcanism required to generate the carbon dioxide pulse,
the Sheffield team concluded, seems to fit with the likely area of the Pangaean split.
2.1.5 Pollenandsporeanalysis
Pollen and spores are critical parts of the life cycles of vascular plants. Because
different species of plants thrive in different climatic conditions, it is possible to
deduce past climates if one has evidence as to which species grew in those past times.
Pollen and spore utility as palaeoclimatic indicators arises out of their resilience and
durability with time. This is because they have very resistant cell walls, which in
turn is due to the outer portion of their cells (the exine) that is composed of a waxy
substance known as sporopollenin. Their cell-wall resistance and inert nature allows
the preservation of pollen and spores in sediments under a variety of conditions.
Pollen and spores typically are the most abundant, most easily identifiable and best
preserved plant remains in sediments such as bogs and clays, and in the form of
fossils in sedimentary rocks. Together it means that they can be used as a climatic
indicator for both recent and geological timescales.
Spores, as referred to here, include the reproductive bodies of lower vascular
plants such as club mosses, horsetails and ferns. The earliest occurrences of spores
produced by land plants in the fossil record are in Lower Silurian rocks (around
430 mya), slightly preceding the appearance of the first vascular plant fossils. Con-
versely, pollen grains are the gamete-carrying reproductive bodies of seed plants,
including gymnosperms (such as conifers and cycads) and angiosperms (the flower-
ing plants). Fossilised pollen first occurs in Upper Devonian rocks (about 370 mya),