Open University Uranium-Series Laboratory  
Earth and Environmental Sciences, The Open University, Milton Keynes, UK  

Secondary calcite  
Calcrete

The age of calcrete (or caliche), can give the minimum age of a soil horizon and quantifying the rate of secondary calcite development is important for sedimentology and geomorphology and glaciology (Wright and Tucker, 1991; Ludwig and Paces, 2002, Hughes, 2004).
Precipitation of different types of calcite in Secondary zones follows a set sequence and the morphological development of calcretes within coarse-grained deposits is well understood (Gile et al., 1965, 1966; Machette, 1985). Initial carbonate accumulation occurs on the undersides of clasts. These clast coatings grow progressively, coalesce and ultimately form a completely indurated horizon. The indurated nature of this horizon causes the ponding of downward percolating soil moisture at the hardpans upper surface. Evaporation and/or root activity in association with this ponded water results in the precipitation of a laminar crust. However, calcretes may contain no petrographic evidence for progressive carbonate growth and comprise a homogeneous micritic cement (i.e., the a-fabric of Wright, 1990).

 
In recent years, it has been shown that calcretes may be reliably dated by U-series disequlibrium techniques (Kelly et al., 2000; Rowe and Maher, 2000; Sharp et al., 2003) from bulk samples taken from single horizons. Resulting U-series ages are average ages of the carbonate cement present within these samples. To establish the time required for calcrete formation it is necessary to sample chronologically distinct carbonate cement phases (Ludwig and Paces, 2002). Candy et al. (2004) present sampling methods that can be used to quantify the time scales required for mature calcretes to form. The technique involves estimating the location of the oldest and youngest carbonate fractions and then dating these fractions using U-series isochron techniques. Candy et al. (2004) present a quantitative time scale for mature Secondary calcrete development and the technique is applicable to mature calcretes in semiarid/arid regions worldwide.
 
Glaciology
 
The timing of the glaciations can be confirmed by applying U-series dating to vadose zone calcite cements to provide minimum ages for host glacial deposits, particularly when glacial sediments are formed on limestone bedrock. In the Pindus Mountains, Greece, U-series dating was applied successfully by Hughes (2005) to provide minimum ages for cemented glacial deposits. In order to obtain representative ages for glacial stratigraphical units, multiple cements must be sampled. Although cements will always post-date the host glacial deposits, it is important to obtain the oldest cement age in a stratigraphical unit. Multiple samples from different cements often yield a range of U-series ages, reflecting multiple phases of cement formation and a good understanding of the geological and glaciological context is essential for effective data interpretation.
 
Alluvial fans
 
Climate amelioration can result in pluvial lake level lowering leading to dessication and salt pan development. In such dry lakes climate-related fan lobe construction, abandonment and dissection commonly show clear stratigraphic relationships to distal pluvial lake beach ridges. Dating of vadose calcite precipitates cementing the gravel of fan deposits can help to evaluate regional climate models, Stokes et al., in progress.
 
Earth worm granules
 

Calcite granules formed in the gut of earthworms can be considered as Secondary calcite These granules are very common in stratigraphically and archaeologically important sequences and have the potential to provide high-resolution chronologies for both terrestrial response to environmental change and human occupation (Candy et al., in progress).

 

Candy et al 2004 Sedimentary Geology 170 177–187
Candy et al 2006 in progress
Gile et al. 1965 Soil Science 99 74– 82
Gile et al. 1966 Soil Science 101 347– 360
Hughes 2004 Quaternary glaciation in the Pindus Mountains, Northwest Greece. Ph.D. thesis 1-364
Kelly et al. 2000 Quaternary Science Reviews 19 995– 1010
Ludwig and Paces 2002 Geochimica et Cosmochimica Acta 66 487–506
Machette 1985 Geological society of America Special Paper 203 1 – 21
Rowe and Maher 2000 Palaeogeography, Palaeoclimatology and Palaeoecology 157 109–125
Sharp et al 2003 Quaternary research 59 139– 150
Wright 1990 Soil micromorphology 401– 407
Wright et al. 1991 Calcretes: an introduction. Wright and Tucker eds. Blackwell 1– 21
 
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© Peter van Calsteren
Last updated: December 23, 2011 11:41