MINERALOGY, PETROLOGY AND GEOCHEMISTRY OF SUBGLACIALLY PRECIPITATED CARBONATE DEPOSITS AT ALVEAR ESTE AND MARTIAL GLACIERS, ANDES OF TIERRA DEL FUEGO, ARGENTINA

RABASSA, J.; SUBÍAS, I.; BIEL, C.; CORONATO, A.; PONCE, J. F.; ACEVEDO, R.D.

Neuquen

Congreso; XVIII CONGRESO GEOLÓGICO ARGENTINO; 2011

Asociación Geológica Argentina

The Alvear Este (54°41’S; 68°03’W; ca. 1200 m a.s.l.) and the Martial (54°47’S; 68°25’W; ca. 1300 m a.s.l.) glaciers are relatively small, fast-receding cirque glaciers of the Fuegian Andes. In both glaciers, CaCO3 sedimentary deposits have been found on bedrock near the ice front, over very recently deglaciated rock surfaces and underneath the ice (Rabassa et al., 2004). This was the first description of these sediments for South America, and perhaps of the Southern Hemisphere, until they were mentioned in Antarctica (Vogel et al., 2006). Deglaciated bedrock surfaces exhibit subglacial calcite deposits developed as patchy coatings up to a few cm thick on polished and striated pavements related to bedrock undulations. The agent responsible for the process is basal ice sliding over a small bedrock obstacle, by pressure melting on the stoss side and refreezing on the lee side. Thus, regelation of melt water is accompanied by a rejection of ions from the growing ice and a consequent ion concentration increase in the residual water until saturation is reached. The Alvear Este Glacier is located in the Sierra de Alvear, lying on the Lemaire Fm., including rhyolites and quartz porphyries, breccias, conglomerates and black slates, containing organic-rich facies with disseminated pyrite. This glacier has retreated more than 2 km from its “Little Ice Age” position. The Martial Glacier has also receded very fast and later fragmented into much smaller ice bodies. It is lying on slates and shales of the Yaghan Fm., Late Mesozoic metamorphosed marine sediments. Samples obtained were grounded to a fine powder in an agate mortar and pestle to form a homogenized sample for X-Ray Diffraction (XRD) characterization using a Philips PW 1710 diffractometer set at CuKα radiation, automatic slit and diffracted-beam graphite monochromator. Thin sections were made to study mineral composition and textures by transmitted light microscopy. Carbon-coated thin sections were characterized by SEM using back-scattered electron (BSE) imaging and energy-dispersive X-ray (EDX) analyses for the determination of composition of both carbonate matrix and intraclast. The oxygen and carbon isotopic compositions were measured on the micritic fraction, which was sampled after selecting homogeneous fine-grained parts of the sediments. The powdered samples were digested in 103% phosphoric acid at 25°C and the resulting CO2 gas was analyzed using a dual inject Sira II VG-Isotech mass spectrometer at University of Salamanca, Spain. The isotopic compositions of calcite are expressed by the conventional δ notation relative to the PDB reference; δ = [(Rs/Rr) - 1] x 1000, where R = 18O/16O or 13C/12C, respectively in the sample and in the reference. Analytical reproducibility (1σ) is 0.1‰ for δ 18O and δ 13C. The studied carbonate sedimentary deposits are yellowish-greenish to grey, scaly and thin layers attached to bedrock outcrops, mostly occupying ice-contact positions, with larger thickness up to 25 mm in protected places or rock surface irregularities. Their surfaces have been roughly striated by glacier movement after deposition. These sediments are calcite mudstones containing silt- and clay-sized sediments and occasionally larger sand-size particles (up to 2-4 mm in diameter). These chemical deposits disappear away from the present ice front, due to rapid subaerial weathering. Mineral composition displays over 90% calcite. Quartz is the most abundant detrital component with minor amounts of feldspars and chlorite. Under SEM, the rock is formed almost entirely by a very fine, micritic, calcite matrix that has been deposited following a xenotopic mosaic. Very thin calcareous (30 μ) laminae evidence a poor-developed planar structure. Detrital constituents are floating in the calcite matrix and they are arranged following a roughly textural alignment. Mostly, the intraclasts have a mean grain size somewhat smaller than 1/16 mm (silt). They are composed of quartz, feldspar, titanite, opaque minerals, and lithic fragments of local rocks. It may be classified as an intraclast-bearing micrite or a mud-supported mudstone. Occasionally, alternating laminae of calcite matrix and detrital grains can be observed. The space between grains is filled with a very fine-grained matrix giving a patchy appearance. Also, cements, chiefly of ferruginous nature, are rarely observed in the contact between the mentioned laminae. Flow textures evidenced by folded laminae were identified in the field as well. A calcite recrystallization to microsparite can be observed in fold hinges. Micrites from carbonate encrustations have ∂18O values ranging from -12.9 to -12.0 ‰ PDB, whereas ∂13C values range from -5.1 to -3.3 ‰ PDB. Assuming a ∂18O mean value of the study calcites and a water temperature of 0ºC, the oxygen isotope composition of water would be between -16.4 ‰ SMOW. This value is different from data from the Ushuaia weather station compiled by the Global Network of Isotopes in Precipitation (GNIP), which shows a mean of -11.0 ‰ SMOW between 1981 and 2002 (range from -15.5 to -7.3 ‰ SMOW). This isotopic shift toward more 18O-depleted compositions could be related to two main processes in the subglacial environment: a) changes of meteoric ∂18O over geological time, and b) basal freeze-on, a process able to produce such light source water. Regarding the first factor, there is no evidence of dramatic changes in oxygen isotopic composition through time at least during the last two decades (see GNIP data, http://isohis.iaea.org/). The freeze-on process indicates reservoir depletion of more than 95%. Such reservoir depletion is likely to concentrate solutes in the source water to over-saturation, causing carbonate precipitation. A freeze-on event associated with such large reservoir depletion may have also dewatered the subglacial environment enough to cause ice stream stoppage. This does not agree with observed field evidence. The interpretation of stable isotope data must take into account two considerations: (1) an isotope fractionation exists between ice and liquid water; this fractionation factor is 3.1 ‰ (a = 1.0031) and (2) it has been suggested that isotope fractionation took place between the liquid and the solid, when liquid water (formed by melting at the snow surface layer) flowed groundwards (Hashimoto et al., 2002). These authors found that the oxygen isotope concentration in the liquid part was lighter by about 2 than the solid part of the wet snow. Calculated 18O value of ice water is -7.9 ‰ SMOW and consequently the oxygen composition of melt water is -5.9 ‰ SMOW. Permissible ∂18O value for subglacial calcites in equilibrium with the mentioned ice is +16.8 ‰ SMOW. Consequently, the range of ∂18O values in subglacial carbonates could be produced from meteoric waters isotopically indistinguishable from present-day ones. At the temperature of 0°C, calculated mean permissible value of water in equilibrium with subglacial carbonates (-4.3 ‰ PDB) are -10 ‰ SMOW. These values are lighter than those reported from Antarctica atmospheric CO2 ranging from -8.0 to -7.4 ‰ during the period 1977-2002. Consequently the determined values do not reflect primary atmospheric CO2 values but possibly reflect an organic carbon contribution to precipitating fluids what is compatible with scavenging organic carbon from bedrock. These sedimentary deposits vanish away from present ice front, due to rapid subaerial weathering. Therefore, they may be taken as a good indicator for timing of recent deglaciation. The subglacially deposited carbonates in the studied areas are very promising materials for further investigation regarding the chemistry of subglacial waters, the activity of the water film at the glacier bed and the palaeoclimatic history since the beginning of the last glacierization in the Fuegian Andes. Further research may demonstrate that these deposits are also associated with glaciations and glacial sediments, both of Pleistocene or pre-Pleistocene age.