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Seasonal records of climatic change in annually laminated tufas: short review and future prospects
Author(s) -
Andrews J. E.,
Brasier A. T.
Publication year - 2005
Publication title -
journal of quaternary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.142
H-Index - 94
eISSN - 1099-1417
pISSN - 0267-8179
DOI - 10.1002/jqs.942
Subject(s) - tufa , calcite , micrite , speleothem , aquifer , geology , trace element , seasonality , carbonate , diagenesis , precipitation , arid , environmental chemistry , hydrology (agriculture) , mineralogy , geochemistry , groundwater , paleontology , ecology , chemistry , cave , geotechnical engineering , physics , organic chemistry , facies , structural basin , meteorology , biology
Many Recent and fossil freshwater tufa stromatolites contain millimetre‐scale, alternating laminae of dense micrite and more porous or sparry crystalline calcites. These alternating laminae have been interpreted to represent seasonally controlled differences in the biotic activity of microbes, and/or seasonally controlled changes in the rate of calcification. Either way, couplets of these microbially mediated alternating calcified laminae are generally agreed to represent annual seasonality. Combined stable isotope (δ 18 O and δ 13 C) and trace element (Mg, Sr, Ba) geochemistry from Recent tufa stromatolites show that seasonal climatic information is available from these calcites. Variability in δ 18 O (and in one case Mg concentration) has been shown to be controlled primarily by stream temperature change, usually driven by solar insolation. In arid climates, seasonal evaporation can also cause δ 18 O enrichment by at least 1‰. Variability in δ 13 C results potentially from: (1) seasonal change in plant uptake of 12 C‐enriched CO 2 ; (2) seasonal change in degassing of 12 C‐enriched CO 2 in the aquifer system; and (3) precipitation of calcite along the aquifer or river flow path, a process that increases δ 13 C of dissolved inorganic carbon (DIC) in the remaining water. Mechanisms 2 and 3 are linked because calcite precipitates in aquifers where degassing occurs, e.g. air pockets. The latter mechanism for δ 13 C enrichment has also been shown to cause sympathetic variation between trace element/Ca ratios and δ 13 C because trace elements with partition coefficients much greater than 1 (e.g. Sr, Ba) remain preferentially in solution. Since degassing in air pockets will be enhanced during decreased recharge when water saturation of the aquifer is lowest, sympathetic variation in trace element/Ca ratios and δ 13 C is a possible index of recharge and therefore precipitation intensity. High‐resolution geochemical data from well‐dated tufa stromatolites have great potential for Quaternary palaeoclimate reconstructions, possibly allowing recovery of annual seasonal climatic information including water temperature variation and change in rainfall intensity. However, careful consideration of diagenetic effects, particularly aggrading neomorphism, needs to be the next step. Copyright © 2005 John Wiley & Sons, Ltd.