SIDERITE SPHERULES AND CEMENTS AS PROXIES FOR PALEOGROUNDWATER EVOLUTION AND PALAEOCLIMATIC CHANGES: A COMPARISON OF APPLES AND ORANGES?

Siderite spherules and cements in the Late Albian Boulder Creek Formation (east central British Columbia) precipitated from groundwaters that were recharged by locally and regionally derived sources.  Basin proximity dictates that regionally derived waters were subject to alpine and orographic effects, resulting in more depleted d¹⁸O values.  Geochemical analyses of early diagenetic siderite cements have been used to determine depositional environments, particularly when sedimentology and stratigraphy are ambiguous, and are useful as proxies for paleoclimatic conditions.  Pedogenic sphaerosiderites are used as a proxy for d¹⁸O compositions of locally recharged meteoric phreatic groundwater systems.  The variability of groundwater d¹⁸O values and elemental chemistry recorded by paleosol sphaerosiderites in a succession of nonmarine coastal plain deposits serve as a proxy for relative changes in alluvial base level.  Recognition of different geographic ranges of paleoaquifer recharge areas is crucial for understanding paleoclimatic and paleoenvironmental implications of groundwater d¹⁸O values from terrestrial deposits.  Detailed evaluation of early diagenetic carbonate cements within a stratigraphic framework provides vital information on the evolution of groundwater flow for a region. 

We evaluated the stable isotopic (d¹⁸O and d¹³C) and elemental (Fe, Ca, Mg, and Mn) geochemistry of early diagenetic siderite cements within a lithofacies framework.  Substitution of Ca and Mg for Fe indicates mixing of marine with meteoric pore fluids, and proximity to the paleoshoreline.  These data serve as proxies for locally and regionally recharged paleoaquifers.  The d¹⁸O values of early diagenetic siderite cements range from –16.9 to –8.9‰ (VPDB).  d¹⁸O compositions of sphaerosiderites in interstratified paleosol horizons range between –12.4 and –9.3‰ (VPDB) and serve as a baseline for locally recharged paleoaquifers.  These d¹⁸O values indicate early meteoric groundwater compositions ranged between –21.9 and –11.8‰ (SMOW).  Analyses of multi-proxy early diagenetic carbonate cement phases within coastal deposits produces a more robust data set.  Careful attention to the diagenetic and facies associations of cement phases coupled with precise microsampling efforts ensures quality data.