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| Titel |
Geochemical and stable isotope indicators of paleoenvironmental and climatic conditions from Cenozoic dolocretes in the Hamersley Basin of northwest Australia |
| VerfasserIn |
Caroline Mather, Grzegorz Skrzypek, Shawan Dogramaci, Pauline Grierson |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250137813
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| Publikation (Nr.) |
 EGU/EGU2017-653.pdf |
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| Zusammenfassung |
| Extensive shallow groundwater carbonate deposits composed of dolomite (dolocretes) are
ubiquitous within the semi-arid Hamersley Basin of NW Australia. However, dolomite is
relatively rare within surficial terrestrial carbonate deposits as precipitation is strongly
controlled by reaction kinetics and is inhibited at surface temperatures and pressures.
The presence of dolomite within the Hamersley Basin carbonate deposits indicates
that specific hydrochemical conditions occurred to overcome kinetic barriers to
precipitation, namely elevated Mg/Ca and high salinity and alkalinity. We investigated the
sedimentology, geochemistry and stable isotope compositions of dolocrete and
groundwater chemistry from several locations in the Hamersley Basin to better
understand dolocrete formation processes and produce a multi-proxy archive of the
paleoenvironmental conditions. Petrographic and mineralogical analysis is consistent with
channel dolocrete formation where dolomite has largely replaced host channel sediments.
Authigenic palygorskite was also present in all samples, indicative of the Mg-rich
environment. Modelling of the δ18O of paleogroundwater from oxygen isotope
compositions of carbonates revealed dolomite precipitated from groundwater with
considerably higher δ18O (median = -2.0‰) values compared to modern alluvial
groundwater (-8.02 ± 0.83‰). As groundwater δ18O values are strongly correlated
(R2=0.93) with salinity, this finding suggests that dolocrete formed from highly
saline groundwaters and in an arid climate that prevailed for sufficient time to form
dolocrete bodies of tens of metres thick. This ancient dolocrete formation can be
constrained by host sediments ages to forming since the mid-late Miocene. In contrast,
modern alluvial groundwater is relatively fresh and unlikely to precipitate dolomite.
A further indication of a more recent (likely Holocene) shift to fresher waters is
the dedolomitization and precipitation of calcite within shallow dolocrete (<5 m
b.g.l) in the vadose zone as well as modern precipitation of calcite from fresh and
brackish water within current drainage lines. These results demonstrate that stable
oxygen isotope signatures of dolocretes can be used to reconstruct paleogroundwater
salinity and by using a multi-proxy approach we can establish the groundwater
processes and hydrochemical conditions occurring at the time of precipitation. |
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