Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong region, South Australia

Abstract

Gravity cores of Holocene sediments from a shallow ephemeral lake in the Coorong region (Pellet Lake, southeastern coastal Australia) show a mineral assemblage and sequence particular to its hydrology. The mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core (i.e. organic-rich aragonite to magnesite + hydromagnesite + aragonite) followed by a relative decrease in salinity (i.e. magnesite + aragonite + hydrornagnesite to aragonite + hydromagnesite) in the upper portions of the core. This sequence is capped by -0.4 m of micritic dolomite and minor amounts of hydromagnesite, with the relative abundance of dolomite increasing upwards. Three stratigraphically and spatially distinct dolomite units (upper, lower and margin) are recognized using stable carbon and oxygen isotope data, unit cell calculations and MgC03 mole per cent data of the dolomite.Detailed X-ray diffraction (XRD) analyses of samples with more than 80% dolomite shows that the dolomite is ordered. Average unit cell parameters, calculated from the XRD patterns, indicate that the upper dolomite unit has crystal lattices expanded in the c, direction (c, = 16.09 A) relative to ideal dolomite (c, = 16.02 A) and contracted in the a, direction (a, = 4.796 A) relative to ideal dolomite (a, = 4.812 A).The mol fraction of MgC03 in the upper dolomite shows up to 4.0 f 2.0 mole per cent excess Mg in the dolomite crystal lattice (calculated from XRD). This unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated Mg/Ca ratios. Transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution. The modulated microstructure found in calcium-rich dolomite is completely lacking. Dolomite ordering reflections are present in electron diffraction patterns, but are weak.Stable oxygen and carbon isotope values of the upper dolomite are tightly grouped (ave. 6 l 8 0 x + 7.55%,, 6I3C = + 4.10%,), yet show three upward-lightening oxygen cycles. The oxygen cycles correlate with three upward decreases in the calculated Mg content of the dolomite zone. These cycles may indicate the increased importance of rain-water dilution of the brine at times when the water in the lake was at its shallowest levels.Analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit. The lower dolomite is close to stoichiometric, has less evaporitic stable isotope values than the upper dolomite, and has only a slightly expanded c,-axis. The margin dolomite is Ca-rich, has a more homogeneous microstructure, and has expanded a, and c, axes.The abundance of relatively soluble Mg-bearing phases, such as hydromagnesite and magnesite, may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment....

Revisiting geochemical controls on patterns of carbonate deposition through the lens of multiple pathways to mineralization

“…7A). SAED analysis shows these pseudofacets likely arise from partial ordering and co-alignment of the particles 43 ( Fig. 7A inset).…”

Empirical Calibration for Dolomite Stoichiometry Calculation: Application on Triassic Muschelkalk- Lettenkohle Carbonates (French Jura)

“…10). The deficiency of larger Ca ions in the unit cell of Lettenhkohle dolomites compared to Upper Muschelkalk dolomites is potentially a cause for the contraction in the a direction (see Rosen et al, 1988). The expansion of the c parameter has been previously related to a cation disordering (Reeder and Wenk, 1983) or other lattice defects (Miser et al, 1987).…”

“…The expansion of the c parameter has been previously related to a cation disordering (Reeder and Wenk, 1983) or other lattice defects (Miser et al, 1987). This is an explanation for the c variability of Upper Muschelkalk dolomites and could be caused by a faster crystallization (Rosen et al, 1988) compared to Lettenkohle dolomites.…”

Environmental significance of mid- to late Holocene sapropels in Old Man Lake, Coorong coastal plain, South Australia: An isotopic, biomarker and palaeoecological perspective

“…4) were processed using a procedure similar to that employed by Edwards et al (2006) and the resulting slides examined by optical microscopy to identify and quantify their ostracod populations. Ostracods were identified by reference to DeDeckker and Geddes (1980), DeDeckker (1981, 1988, Martens (1985) and Yasini and Jones (1995).…”

“…1) display large and, in part, contrasting variations that attest to the impacts of neotectonic uplift, and changing sea level and climate, on their individual physiographic settings (von der Borch and Altmann, 1979;Rosen et al, 1988;Warren, 1990Warren, , 1994Barnett, 1994;McKirdy et al, 1995McKirdy et al, , 2002McKirdy et al, , 2010aMee et al, 2004Mee et al, , 2007Edwards et al, 2006). Significant biogeochemical changes also have been documented in the Holocene estuarine sediments of the Coorong lagoon itself (e.g.…”