Recorrido virtual

Once meteoric water reaches the ground it is often modified. Within lacustrine environments, the isotopic signature of inorganic materials such as carbonates and silicates is thought to be reflective of changes in both temperature and precipitation of the source area as well as the local precipitation/evaporation (P/E) regime.

For mineral precipitation in equilibrium, the isotopic composition of the substance is predictable by thermodynamics (Leng et al., 2005). In lacustrine environments, this process is controlled principally by the temperature and the isotopic composition of the lake water.

In theory, if there is no change in the lake water composition, then palaeotemperatures can be estimated using fractionation equations. However, in practise this is complicated as both temperature and lake water composition may be affected by different variables and changes in climate processes. The isotopic balance of a lake system can be expressed in the following mass-balance equation (Leng and Barker, 2006):

(Equation 5)

where P is the amount of direct precipitation at the lake surface, R represents surface inflow into the lake, Gi is the amount of groundwater inflow, Go is the groundwater outflow, E is the amount of evaporation from the lake surface and So is the degree of surface outflow from the lake. δP, δR, δGi, δE and δlakewater represent the isotopic compositions of precipitation, surface inflow, groundwater inflow, evaporated moisture and lakewater, respectively. As equation 5 shows, δP is only one component of a highly complex system, all facets of which should be considered.

Outside of the tropics, where the isotopic composition of precipitation entering lake systems (δP) is strongly influenced by the amount effect, δP covaries linearly with temperature. The relationship between δP and temperature change (dT) is defined by the Dansgaard equation (Dansgaard 1964):

Lakes can be classified into one of two types: open or closed. Hydrologically ‘open’ lakes have an active outflow. The dominant factor controlling δ¹⁸Olakewater in open lakes, subject to minimal evaporation, is variations in the isotopic composition of precipitation and groundwater inflow entering the lake (Clark and Fritz, 1997). In closed lake systems, especially in arid environments and where groundwater outflow is insignificant, the roles of δP and temperature are minimal relative to the importance of the ratio between precipitation and evaporation (P/E budget). In areas where the change in δP is potentially large, variations in δP may be more important than previously thought (e.g. Henderson et al., 2010). Closed lakes become enriched in ¹⁸O depending on variations in local evaporation rates, the magnitude of which are affected by changes in temperature, wind velocity and relative humidity. When assessing δ¹⁸O records from either open or closed lake systems, it is important to consider the lake’s hydrological setting including factors such as the water turnover time, catchment size, catchment processes and seasonal variations in δ¹⁸Olakewater.

2.4.2.1 Oxygen isotopes in lacustrine silicates

Diatoms are one of the principal sources of biogenic silica used for lake-based palaeoclimatic reconstructions. They are unicellular, photosynthetic, eukaryotic algae which generate siliceous cell walls that form distinctive rigid frustules (Round et al., 1990).

Diatom silica is precipitated in isotopic equilibrium with the surrounding ambient water and are later preserved within the sediment record (Leng and Barker, 2006). Biogenic silica is abundant in areas devoid of carbonate sedimentation, such as in soft-water lakes and the high-latitude oceans, and thus provide reliable isotope records to complement the existing palaeoclimate records derived from carbonates. Diatoms are ubiquitous in most lacustrine environments where levels of key nutrients such as silicon, nitrogen and phosphorus are sufficient to sustain diatom productivity. Diatom species differ from each other in their ability to successfully utilise varying nutrient levels and to withstand changes in different physical conditions, such as light availability and mixing within the water column. Diatom productivity within lacustrine environments is largely controlled by seasonal climate patterns which influence lake habitat conditions such as the depth of vertical mixing and nutrient availability. In large monomictic tropical lakes, increased mixing, and thus productivity, tends to occur during the dry season (Bootsma, 1993), whilst in dimictic temperate lakes, mixing occurs twice each year, during spring and autumn when similarities between the temperature and density of the hypolimnion and epilimnion create a strong mixing regime whereby diatom productivity takes place through the exploitation of

nutrient-rich waters (Leng and Barker, 2006). The isotopic signature of diatom silica (δ¹⁸Odiatom) is largely acquired during these growth periods which generally coincide with the dry season, and thus the seasonal productivity cycles and climatic conditions, specific to each individual lake, are an important consideration in the interpretation of records of δ¹⁸Odiatom.

Within lacustrine environments, δ¹⁸Odiatom varies as a function of temperature and/or the isotopic composition of ambient lake water (figure 2.2) which in turn is heavily influenced by the balance between precipitation and evaporation. In all lakes, a possible additional control is exerted by depth constraints within the water column. These constraints, such as vertical stratification, limit diatom productivity to the upper parts of the water column and can result in a δ¹⁸Odiatom value that is reflective of a localised δ¹⁸Owater signal (Raubitschek et al., 1999).

A number of calibration studies have attempted to define an empirical relationship between δ¹⁸Odiatom and temperature (Labeyrie, 1974; Juillet-Leclerc and Labeyrie, 1987; Matheney Figure 2.2 Schematic diagram illustrating controls on the oxygen isotope composition of lacustrine diatom silica. The importance of temperature varies depending on location and becomes more relevant for high-latitude and high-altitude sites. In low-latitude regions, the oxygen isotopic signal of precipitation and thus the lakewater in which the diatom silica precipitates becomes more important.

Modified from Leng and Barker (2006).

samples, the estimated average temperature dependence ranges from -0.2‰/°C to -0.5‰/°C.

More recently, diatom culturing experiments using individual marine diatom species (Brandriss et al., 1998) and the study of planktonic diatoms from sediment traps in a small maar lake (Lake Holzmaar) in Germany (Moschen et al., 2005) and in Lake Annecy in France (Crespin et al., 2010), have estimated the diatom-temperature coefficient to be approximately -0.2‰/°C. However, Schmidt et al. (1997) found no regular correlation between δ¹⁸Odiatom and temperature, which has led to the suggestion that the temperature-dependent oxygen isotope fractionation of some biogenic silica may have occurred during early diagenesis rather than during later growth stages (Leng and Barker, 2006).

In tropical regions, where there is little or no variation in seasonal temperatures, the effect of changes in δP and therefore variations in the isotopic composition of ambient lakewater (δ¹⁸Olakewater) become more important in determining values of δ¹⁸Odiatom. The phase change associated with evaporation results in depleted ¹⁶O isotopologues being preferentially evaporated leaving lakewater relatively enriched in the heavier ¹⁸O isotope. The degree to which evaporation increases δ¹⁸Olakewater is dependent on factors such as humidity or the residence time of a lake, which can vary according to changes in basin hydrology or groundwater flux. A number of studies have investigated the degree to which the influence of evaporation and variations in the amount of precipitation are important for driving changes in the δ¹⁸Odiatom record, particularly in tropical regions (e.g. Rietti-Shati et al., 1998;

Cole et al., 1999; Lamb et al., 2000, 2005; Barker et al., 2001, 2007; Shemesh et al., 2001a,b; Polissar et al., 2006). In addition, changes in δ¹⁸Olakewater may also occur through changes in, or at, the precipitation source area. For example, Rosqvist et al. (2004) showed that changes in the δ¹⁸Odiatom record from a lake in Northern Sweden over the past 5000 years reveal a lowering trend with stepwise spikes illustrating a 1000-yr cyclicity. Coupled with evidence for glacial advance they suggest that persistent changes in atmospheric circulation patterns and dominant air mass source could have caused the ensuing isotopic depletion. Changes in the predominance of cold polar air masses have also been invoked as the cause for variations in the δ¹⁸Odiatom record from Lake Chuna on the Kola Peninsula in Northwest Russia (Jones et al., 2004). Furthermore, changes in the δ¹⁸Odiatom record from Lake Baikal are thought to reflect differences in the composition of river inflow between northern and southern catchment areas where the variability of δP is determined by latitudinal differences (Mackay et al., 2011). A detailed and extensive review of the use of oxygen isotopes in lacustrine biogenic silica as a proxy for palaeoclimatic change can be found in Leng and Barker (2006).

Providing robust and thorough cleaning procedures (see section 4.3.3) are followed to ensure purity, δ¹⁸Odiatom offers an important palaeoclimatic proxy that provides additional information to complement our current understanding of past climatic change. Although a relatively new tool, the study of the oxygen isotope signature of diatoms preserved in the sediment record has been increasingly used in both marine and lacustrine environments (Swann and Leng, 2009). Recently however, the validity of δ¹⁸Odiatom records has been shown to be compromised by contamination remaining within purified samples, even after cleaning (Brewer et al., 2008). This is explored further in chapters 4 and 5.