UNIQUE STABLE ISOTOPIC SIGNATURES IN ROSSELIA SOCIALIS BURROW CONCRETIONS FROM THE UPPER CRETACEOUS HORSESHOE CANYON FORMATION, ALBERTA, CANADA

The physical and chemical heterogeneities of burrowed sediments are rarely considered during stable isotopic analysis of rock units. Calcite and siderite cements from Rosselia socialis burrow concretions and surrounding sediments - collected from a well defined shoreface sequence in the Upper Cretaceous Horseshoe Canyon Formation of Alberta, Canada - display anomalously high carbon isotopic values (3.0-16.1‰ PDB) that vary substantially within a single stratigraphic unit. A strong positive correlation between the carbon and oxygen isotope compositions suggests an early diagenetic formation for the carbonate cements. Transverse sampling of the burrow concretions revealed symmetrical deviation in 13-carbon isotope values for calcite, with highest values recorded at the burrow margins. The finding suggests that the calcite cement retained the original isotopic signal recorded during initial precipitation. Siderite cements are enriched in 13-carbon and 18-oxygen as compared to co-existing calcite, but show little to no deviation in isotopic values across the burrow. Thus, calcite and siderite cements do not appear to have formed in equilibrium. Formation of these 13-carbon-enriched cements likely occurred through a two-step, bacterially-mediated process of oxidation followed by anaerobic fermentation. This process of bacterially-mediated decomposition was facilitated by the presence of R. socialis burrows that concentrated organic matter and promoted colonization by microorganisms in the burrow micro-environment. Oxygen isotopic values, which range from 18.0‰ to 26.7‰ VSMOW, suggest that stable isotopic signals for the burrow micro-environments were altered further by subsurface groundwater discharge through a regional aquifer. Introduction of freshwater into the lower shoreface also accounts for the unexpected early diagenetic precipitation of siderite within a marine environment. Often isotopic signals from outside sources such as freshwater groundwater flow may blanket an entire rock unit, yet the presence of bioturbation can produce discrete micro-environments with unique stable isotopic signatures separate from those of the surrounding deposit. These burrow systems can therefore act as refuge habitats and preserve stable isotopic signatures.