Palaeohydrogeology using geochemical, isotopic and mineralogical analyses: salinity and redox evolution in a deep groundwater system through Quaternary glacial cycles

Milodowski, Antoni E.; Bath, Adrian; Norris, Simon. 2018 Palaeohydrogeology using geochemical, isotopic and mineralogical analyses: salinity and redox evolution in a deep groundwater system through Quaternary glacial cycles. Applied Geochemistry, 97. 40-60. https://doi.org/10.1016/j.apgeochem.2018.07.008

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Official URL: http://dx.doi.org/10.1016/j.apgeochem.2018.07.008

Abstract/Summary

Mineralogical, geochemical and isotopic analyses of secondary calcites are interpreted as part of an investigation of deep groundwaters in fractured metavolcanic rock overlain by sedimentary rock. Drillcore rock samples and groundwater samples from deep boreholes (maximum depth 1950 m) were analysed. This produces information about the evolution of salinity and redox in relation to past groundwater movements including the impacts of climatic change through the Quaternary period. Salinities of present-day groundwaters vary from dilute to brine concentrations and are related to three distinct groundwater flow regimes. Crystal morphology, stable isotopic analyses and isotopic dating, cathodoluminescence and microanalyses of Fe, Mn and REEs in the latest generation of secondary calcite, plus other analyses, have provided insights into variations of salinity over time and of redox in past groundwaters. Interpretation suggests that groundwater in the depth range of the transition from dilute to brackish/saline concentrations has been gradually diluted over time by meteoric water ingress. 230Th/234U whole-crystal ages indicate that at least part of the late-stage calcite mineralisation in the present groundwater flow system is of Quaternary age, although the mineralisation may have been initiated much earlier by meteoric invasion in the Miocene, following regional uplift. The calcites exhibit a wide range in oxygen isotope composition (δ18OPDB −2 to −22‰), although no extremely light or heavy δ13C values indicative of microbial methane oxidation or deep methanogenesis were observed. The very light δ18O values suggest that glacial or other cold-climate waters flowed to more than 700 m depth in the centre of the study area and formed a greater proportion of groundwater at that depth than at present. Fe and Ce are interpreted as semi-quantitative proxies for past redox conditions over the period when secondary calcite was deposited. Variability of Fe and Mn contents of secondary calcites in deeper rock, presently containing saline groundwater, is evidence of reducing conditions being maintained in the long term, though the strength of negative redox has probably fluctuated due to other redox-active chemistry. Depth-wise changes of groundwater redox in the past are also indicated by Ce concentrations versus other REEs in secondary calcites. Shallow calcites show a negative Ce anomaly in some growth zones due to oxidation to CeIV whilst deeper calcites do not exhibit this Ce behaviour, indicating that reducing conditions prevailed. Distribution of Fe-Mn oxyhydroxides and pyrite confirm, at a broader scale over depth and time, the findings about redox variations that secondary calcites indicate. Mineralogical and geochemical studies add further information to the understanding of past geochemical conditions in deep groundwaters in this area. Interpretations provide semi-quantitative constraints on the evolution and likely variations and directions of movement of groundwater salinity and redox over the Quaternary timescale.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	https://doi.org/10.1016/j.apgeochem.2018.07.008
ISSN:	08832927
Additional Keywords:	GroundwaterBGS, Groundwater, Aquifer characterisation
Date made live:	10 Aug 2018 13:28 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/520690

Item Type:

Publication - Article

Digital Object Identifier (DOI):

https://doi.org/10.1016/j.apgeochem.2018.07.008

ISSN:

08832927

Additional Keywords:

GroundwaterBGS, Groundwater, Aquifer characterisation

Date made live:

10 Aug 2018 13:28 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/520690