New System to Construct Speleothem Fabrics Time Series

25 Aug

New Comprehensive System to Construct Speleothem Fabrics Time Series

S. Frisia ¹ & A. Borsato

[1] School of Environmental and Life Sciences, University of Newcastle, Australia

SCOPE

The systematic documentation of calcite fabrics in stalagmites and flowstones provides robustness to palaeoclimate interpretation based on geochemical proxies, but its potential as climate proxy and as means by which to detect kinetic modifications or diagenesis can be fully exploited only if petrographic observations are transformed into time series comparable to geochemical profiles.

Table 1. FABRIC TYPES, SYMBOLS, CODES, CHARACTERISTICS AND ENVIRONMENT OF FORMATION

The table directly links fabric types (and codes) to the environment of formation and becomes the key to the construction and interpretation of fabric time series. The fabrics in fig 1 are arranged according to the ranking criteria (from 1 to 8) proposed in the summary table. The arrows synthesize the underpinning environments of formation. Figure 2 shows diagenetic fabric coded from 9 to 12.

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Figure 1. CRITERIA

Fabrics are transformed into numerical values
by using a ranking system based on models of fabric development.
The hierarchy of fabric is operated through criteria of:
a) progressive dripwater variability (”hydrological stress”);
b) progressive increase in supersaturation state (SIcc) of the dripwater;
c) progressive increase in Mg content and impurities;
d) diagenetic transformation.

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Figure 2. DIAGENETIC FABRICS

A) Mosaic fabric. General appearance of mosaic calcite (Mc) replacing aragonite (PPL). B). The same as A but seen at XPL. C) Relic aragonite clearly visible within the subeuhedral to euhedral mosaic of calcite spar. D) Micrite (M) associated with aragonite needles. Microsparite (Ms) appears lighter, has crystals larger than micrite and replaces aragonite needles. The mosaic calcite (Mc) almost completely blurs the original aragonite fabric. E) Elongated columnar calcite capped by acicular fabric radiating from a micrite layer. The needle-like crystals entirely consist of calcite. The crystals grew constrained by impurity-rich micrite (PPL).

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Figure 3. THE MICROSTRATIGRAPHIC LOG

The fabrics log is constructed by plotting ranked fabrics against the distance from top where they occur. Changes through time (vertical dimension) coincide with changes in depositional conditions and the graphic form allows direct comparison with the series of stable isotope ratio variability. It is evident from the fabric time series that occurrence of micrite and microsparite coincides with shift in d13C toward more positive values. Micrite suggests dry conditions and microsparite diagenesis (aggrading neomorphism).

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Figure 4. THE ISOFAB PLOT

In the IsoFab plot (Isotopes + Fabrics) fabrics are plotted against their stable isotope values. In this case, columnar fabrics have similar d13C and d18O values, which are more negative with respect to the d13C and d18O of 2 micrite and microsparite. Good correlation (R = 0.97) shows that there is progressive kinetic modification from columnar elongated to microsparite The plot highlights that Ce, whose content of Mg in the specimen is up to 20,000 ppm, has the most negative d13C and d18O values. Here, Mg has little kinetic effect on stable isotope fractionation. The d13C and d13O values more negative than in C and Co suggest that the drip rate was higher during the deposition of Ce, which suggests association of Mg with particulate.

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References: Frisia S. 2015. Microstratigraphic logging of calcite fabrics in speleothems as tool for palaeoclimate studies. International Journal of Speleology, 44(1): 1-16.

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