Geoscience Reference
In-Depth Information
are not, and some can swim in shallow seasonal ponds and lay drought-resistant eggs.
It is therefore possible to use the proportions of different species of known salinity
tolerance to estimate past changes in lake water salinity (De Deckker,
1988
; Holmes,
2001
;Zhaietal.,
2011
). Likewise, some species prefer colder water and others
warmer water, so estimates may be made about past changes in water temperature.
All of this presupposes that particular species have retained the same levels of water
salinity and temperature tolerance through time, which may not always have been the
case.
An alternative approach to using present-day ecological tolerances to reconstruct
past changes is to analyse the stable isotopic composition of the calcareous shells,
notably the stable carbon and oxygen values. As with marine foraminifera, high
relative concentrations of the heavier isotope of oxygen (
18
O) denote evaporative
conditions in the lake water, while low concentrations denote fresh water and relatively
low rates of evaporative loss. Changes in the carbon isotope ratios can be used to infer
changes in biological productivity and carbon cycling within the lake (Ito,
2001
).
An additional method of analysis pioneered in the early 1980s involves measuring
changes in the trace element geochemistry of ostracod shells within a given lake
core. In particular, the Sr/Ca ratio in ostracod shells provides a measure of lake water
salinity, and the Mg/Ca ratio provides a measure of lake water temperature (Chivas
et al.,
1986a
;Chivasetal.,
1986b
;Chivasetal.,
1986c
;Zhaietal.,
2011
). However,
considerable finesse is needed when interpreting the results of such analyses, because
many other factors besides temperature and salinity are at play in determining ostracod
shell geochemistry (Ito et al.,
2003
; Ito and Forester,
2009
).
Cladocera
are another order of crustaceans, and they are better known as water
fleas. There are more than 600 species of cladocera, most of which live in fresh water,
although some live in the oceans. The basic principles involved in using cladocera
fossils preserved in lake and swamp sediments to reconstruct past changes in lake
temperature and salinity are the same as those used with marine foraminifera. Transfer
functions are established relating species assemblages to their freshwater habitats, and
these transfer functions are then applied to fossil assemblages in order to reconstruct
past changes in lake habitat (Korhola and Rautio,
2001
). As a general rule, a number
of other fossils should be used in addition to cladocera, so that a more complete
picture of past environments can be constructed.
Chironomids
are a family of non-biting midges. There are estimated to be more than
10,000 species worldwide, occupying a range of aquatic and semi-aquatic habitats
during the larval stage. The larvae are very sensitive to lake water temperature and
chemistry (Walker,
2001
). Transfer functions relatingwater temperature and chemistry
to assemblages of chironomid larvae have been successfully developed for North
America, Europe, New Zealand and Tasmania but not thus far for desert areas. Work is
now underway to use species assemblages and the stable oxygen isotope composition
of chironomid teeth to develop transfer functions for temperature and perhaps salinity
