Hipótesis especifica

A technique for direct, qualitative reconstmction of water depth based on the ratio of littoral to offshore fossil biota was alluded to in reference to freshwater lakes (Section 1.4.2). Although relationships with water depth are not necessarily simple, the approach is equally ^plicable to saline lakes and, in the case of diatoms, can provide complementary data against which to assess the results of palaeosalinity analysis.

The changing ratio of planktonic to benthic diatom taxa has been widely applied to reconstmct lake levels directly from fossil diatom assemblages in salt lake sequences from Africa (e.g. Gasse & Street, 1978, Gasse et al, 1987, Haberyan & Hecky, 1987, Gasse & Fontes, 1989, El Hamouti, 1989, Lamb et al, 1995) and the America's (e.g. Bradbury et al, 1981, Metcalfe et al, 1991, Hickman & Schweger,

1993). Metcalfe et al (1991) attempted to reconstmct water depth in more detail, by classifying taxa according to five categories: aerophilous (tolerant of exposure to air), epiphytic (on plants), periphytic (on lake-bed substrate) and unclassifiable. Where inferred water depth shows a clear inverse relationship with inferred water salinity (e.g. El Hamouti, 1989), the technique provides a means of

validating lake-level reconstruction from palaeosalinity data. In other cases (e.g. Gasse et ai, 1987) it can provide a useful indicator that the relationships are more complex.

In this study, the use of diatoms as direct indicators of water-level change is explored in addition to their use as quantitative palaeosalinity indicators. The validity of the approach is tested by analysis of the modem diatom training set, and it is applied in analysis of the Laguna de Medina sequence.

A range of other techniques used to assess and strengthen the reliability of Holocene palaeoclimate reconstruction is described below.

1,5.5 Ostracod shell chemistry as an indicator of palaeosalinity and/or palaeotemperature

1.5.5.1 The distribution function ('Kq value*) approach

Ostracods are microcrustacea with bivalved shells (’carapaces') of low-Mg calcite, and they are abundant in waters of widely differing salinity. The analysis of Mg/Ca and Sr/Ca ratios of ostracod shells has been used to produce continuous palaeosalinity and/or palaeotemperature records on the basis of a relationship between ostracod shell chemistry and that of the host waters. It therefore has the potential to provide an ideal test of the rehability of a diatom-inferred palaeosalinity record.

Ostracods moult up to nine times during their life-cycle, at approximately one-weekly intervals during growth (De Deckker et al., 1988). Each stage (an ’instar*) is formed rapidly from calcium carbonate secreted from the host waters (Turpen & Angell, 1971). Any relationship between ostracod shell chemistry and the aquatic environment therefore relates to water quality over a short period of time rather than an average of prevaihng conditions. In addition to Ca, the trace metals Mg and Sr are laid down, along with other elements such as P, Mn, Fe and K (Bodergat, 1985).

The most rigorous approach has been developed in work on Australian, chloride-dominated salt lakes (Chivas et al, 1983,1985,1986a). A significant, genus-dependent relationship has been found between the Mg/Ca content of ostracod shells and the Mg content of host waters, which relates to both salinity and temperature of the host waters (Chivas et al, 1983, 1986a), and between the Sr/Ca content of ostracod shells and the salinity of the host waters (Chivas et al, 1983, 1985). Since Mg is laid down preferentially in juvenile instars, analysis is restricted to fully-calcified adults or the largest instar.

In a similar vein to the development of diatom transfer functions, Chivas and co-workers have developed methods for quantitative reconstruction by establishing the relationships between modem ostracod shell chemistry and water quality. Genus-dependent distribution coefficients’, or ’Kg’ values

for Mg and Sr are derived as follows:-

Kd[M] = (M/Ca)CaCO,

(M/Ca)Hp

where is the distribution coefficient, and M is the trace element (Mg or Sr), expressed as a molar ratio with calcium in the ostracod valve and host water respectively.

The distribution coefficient is applied in calibration of fossil ostracod sequences, to calculate the unknown molar ratio of the host waters. Since the Mg/Ca profile reflects both palaeosalinity and/or palaeotemperature change and the Sr/Ca profile reflects palaeosalinity only, their combined analysis can be used to uncouple palaeosalinity and palaeotemperature profiles.

1.5,S.2 Factors affecting interpretation

Various factors can affect the reliability of the ostracod shell chemistry record. In particular, the results of recent laboratory experiments suggest that distribution coefficients are a complex function of a much wider range of parameters than previously thought (Palacios-Fest et al, 1994).

The most important of these is probably the influence of brine composition. Ostracod shell chemistry research has been extended in recent studies from chloride to sulphate and carbonate systems, where the relationships between shell chemistry and salinity can be more complex than in chloride systems. Sr is readily precipitated with aragonite since SrCOj and aragonite crystals are isomorphic (Chivas et al, 1986b); as a result the Sr/Ca profile can vary according to different carbonate phases and is truncated during aragonite precipitation (Anaddn & Julià, 1990, Engstrom & Nelson, 1991, Fritz et al, 1994, Chivas et al, 1993). As a result there has been a trend in recent studies towards abandoning the use of quantitative distribution coefficients (e.g. Bridgwater, 1995).

The reliability of the results can be assessed using a variety of techniques (Chivas et al, 1993). The most effective is analysis of ostracod stable isotope content, which is related to the isotopic composition of the host waters; the oxygen isotope composition of the host waters is a function of evaporation, temperature and also the origin of the water (groundwater versus surface input). It provides both an independent palaeotemperature record for uncoupling of the Mg/Ca signal (Chivas et al, 1993, Curtis & Hodell, 1993) and a means of assessing the influence of non-climatic factors in lakes of complex hydrology (Gasse et al, 1987). Different mineral phases in the sediment record can be identified using X-ray diffraction (e.g. Martinez & Plana, 1987), which provides another means of assessing where the ostracod shell-chemistry profile may be driven by changes independent of climate

(Fritz et ai, 1994).

Ostracod shell-chemistry analysis is carried out in this study to provide a second, continuous palaeosalinity record against which to test the diatom reconstruction. The assumptions on which it rests are less certain than those of diatom analysis, and the aim is to derive a relative salinity curve from fossil ostracod trace-metal ratios, rather than to quantify the relationship with the salinity of host waters. Ideally the study would be combined with oxygen isotope and X-ray diffraction analysis, but this was not feasible given the time constraints of this study.