Hydromagnesite replacement of biomineralized aragonite in a new location of Holocene stromatolites, Lake Walyungup, Western Australia

Abstract

Holocene stromatolites are described from Lake Walyungup, a coastal hyposaline lake in south-western Australia. At summer low water, this groundwater-fed depression comprises two permanent shallow water bodies and an ephemeral southern pool, set within an areally extensive littoral zone of variably cemented carbonate crust. Up to 5 m of organic-rich carbonate mud has been deposited within each of these basins in less than 7000 years. Stromatolites rim the water bodies with individual columns up to 2 m tall. Stromatolite-capped tepee structures in subparallel alignment are widespread in the littoral crust, suggesting a linkage between stromatolite growth and zones of groundwater discharge. Lake Walyungup stromatolites, regardless of external morphology and setting, are coarsely laminated and have aragonitic mesoclot microfabrics. These microfabrics are similar to those from lithi®ed portions of active thrombolitic microbialites from nearby Lake Clifton.Hydromagnesite is a minor to subdominant phase (up to 47 wt%) of the carbonate mineral assemblage in Lake Walyungup. It occurs mainly in the littoral zone as a diagenetic replacement of precursor aragonite, particularly within the mesoclot fabric of stromatolites, but also in sediments (strandline and dune sand, crusts) derived mainly from erosion of stromatolites. In contrast with nonreplaced and impermeable inorganic aragonitic cements, stromatolite mesoclots are micropermeable. Micropermeability is inferred to facilitate hydromagnesite diagenesis. Dolomite is also present in minor amounts as a pore ®ll in stromatolites, and as a subdominant to dominant (up to 100 wt%) phase in thin, mudcracked micrite layers within the crust package. The layered dolomite may be precipitated directly from the lake water.Major element abundance of the lake water is: Na + > Mg 2+ » K + > Ca 2+ for cations, and Cl A » SO 2À 4 » HCO À 3 > CO 2À 3 for anions. Compared to other nearby coastal lakes, Lake Walyungup has a high pH (> 9á0), and an extremely high molar Mg/Ca ratio of > 90. Groundwater in the area has a Mg/Ca ratio generally less than 1. The unusual Mg/Ca ratio in Lake Walyungup is partially a result of in-lake processes with additional minor contribution of Mg 2+ sourced from basal marine sand because no Mgrich bedrock source has been found in the region.

Palaeoproterozoic magnesite-stromatolite-dolostone-'red bed' association, Russian Karelia: Palaeoenvironmental constraints on the 2.0 Ga-positive carbon isotope shift

“…-The magnesite lithofacies has been discussed in detail by Melezhik et al (submitted). They have reported that the Tulomozerskaya magnesite formed in sabkha (similar to sabkha magnesites of Abu Dhabi, Bush 1973) and playa environments (similar to the Coorong Lagoon district and Lake Walyungup coastal playa magnesites, Australia, Walter et al 1973;Schroll 1989;Coshell et al 1998).…”

Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments

“…Veizer & Hoefs, 1976; Veizer et al ., 1992a,b). In the Holocene, several episodes in which dolomite formed as a direct precipitate from lake water have been reported from Lake Walyungup, Western Australia (Coshell et al ., 1998), and the Coorong dolomite from South Australia has also been interpreted as a direct primary precipitate from lake water (Von der Borch, 1965; Rosen et al ., 1988, 1989).…”

“…Salda Gölü, south‐western Turkey (Braithwaite & Zedef, 1994, 1996). The association of stromatolites and both diagenetic and primary magnesite is very common and has been reported from many recent alkaline lakes (Walter et al ., 1973; Last & De Deckker, 1990; Renaut, 1993; Coshell et al ., 1998).…”

First lacustrine varve chronologies from Mexico: impact of droughts, ENSO and human activity since AD 1840 as recorded in maar sediments from Valle de Santiago

“…Indications for the involvement of biological processes are the mineralogy of the laminae (hydromagnesite and monohydrocalcite; Coshell et al 1998;Warthmann et al 2000) and the postive d 13 C carb (McKenzie 1985). On the other hand, grading from larger to smaller crystals and typical shapes such as dumbells, footballs or microspheres, as observed for bio-induced precipitation (Teranes et al 1999), were not found.…”