Diagenesis or dire genesis? The origin of columnar spar in tufa stromatolites of central Greece and the role of chironomid larvae

Abstract

Two Greek Pleistocene tufa stromatolites were examined petrographically and with stable isotope geochemistry to determine whether calcite spar is of primary or diagenetic origin. The younger (ca 100 ka) tufa from Zemeno primarily is micritic, with primary columnar calcite spar restricted to areas immediately above chironomid larval tubes. This relationship suggests that chironomid larval feeding behaviour is responsible for the development of Zemeno tufa columnar calcite, probably involving biological substances smeared onto the tufa surface. Most micritic crystals are not suitably oriented to allow later post-depositional growth resulting in columnar fabrics. The older (ca 1 Ma) predominantly sparry tufa from Nemea contains some chironomid tubes and organic cyanobacterial filaments preserved in crystal fans but also contains many fabrics found in primary speleothem spar. Columnar spar here is unlikely to be the result of post-depositional crystal growth. A comparison of stable isotopic trends between the two tufa deposits suggests that both contain interpretable seasonal trends and implies little or minor post-depositional alteration of either tufa. Consequently, there is no basis for the common assumption that sparry tufa fabrics must be of diagenetic origin

Geobody architecture, genesis and petrophysical characteristics of the Budakalász travertines, Buda Hills (Hungary)

“…The omnipresence of diagenetic fabrics as observed by petrographic analyses raises questions about possible resetting of the stable isotopic signatures, as discussed for other locations by Manfra et al (1974Manfra et al ( , 1976, Janssen et al (1999), Chafetz and Guidry (2003), Andrews andBrasier (2005), Andrews (2006), Rainey and Jones (2007), Brasier et al (2011), De Filippis et al (2013 and .…”

“…Kahle, 1977;Chafetz et al, 1994;Guo and Riding, 1994;Nicoll et al, 1999 andJones, 2007) or sparitisation (e.g. Love and Chafetz, 1988;Freytet and Verrecchia, 1999;Golubic et al, 2008 andBrasier et al, 2011). This results in highly complex fabrics, where it is often challenging to distinguish primary from secondary fabrics (Pentecost, 2004;Pedley, 2009;Brasier et al, 2015).…”

“…The driving process would be Ostwald ripening where, for increasing crystal size, the specific surface free energy of the crystals would reduce (Love and Chafetz, 1988). Brasier et al (2011), however, argued that in order for Ostwald ripening to occur, temperatures in the order of 500°C is necessary, and this preferably in closed system conditions. For porous travertines that were never substantially buried, reduction of crystal specific free surface energy was thus more likely caused by post-depositional crystal growth, where crystals nucleate upon precursor crystals .…”

High-resolution δ18O seasonality record in a French Eemian tufa stromatolite (Caours, Somme Basin)

“…In Microfacies I the preservation of cyanobacterial filaments argue against a diagenetic origin (Brasier et al, 2011), the presence of expanded fascicules suggesting growth conditions favourable for cyanobacterial colonies. In laminated European tufas, alternation of dense micritic laminae and more porous laminae containing erect cyanobacterial/algal filaments…”

Hydrogeological Significance of Secondary Terrestrial Carbonate Deposition in Karst Environments

“…The 28 White Peak was not subjected to glacial erosion during the most recent (Devensian, MIS 2-4) 29 glaciation, therefore there is a potential for an extensive record of Quaternary palaeoclimatic 30 conditions to be preserved in the secondary carbonate deposits. 31 32 Secondary terrestrial carbonate deposits include four major groups: (i) Speleothems that are 33 characteristically deposited in caves above the water table (vadose or unsaturated zone) 34 from saturated mineral solutions and can be seen at scales that range from millimetres to 35 tens of metres. Typically, they are formed of calcium carbonate by the precipitation of calcite 36 or aragonite from water as excess dissolved carbon dioxide is diffused into the cave 1 atmosphere.…”