STABLE ISOTOPIC ANALYSIS REVEALS EVIDENCE FOR GROUNDWATER-SEDIMENT-ANIMAL INTERACTIONS IN A MARGINAL-MARINE SETTING

摘要

We use isotopic analyses of authigenic siderite and calcite cements within Rosselia socialis burrows from shoreface deposits in the Upper Cretaceous Horseshoe Canyon Formation of Alberta, Canada, to reveal the early cementation history of the burrow and geochemical conditions of the initial sedimentary environment. Within the Horseshoe Canyon Formation, two forms of the Rosselia burrows are present: bulbous in situ burrows, and transported, spindlelike burrows, which display similar internal shaft diameters but smaller overall size compared to in situ forms. Transverse, incremental sampling of calcite and siderite cements in the Rosselia burrows reveals symmetrical isotopic deviation in 13C and 18O around the burrow core, representing accretionary records of evolving pore-water conditions. The number of isotopic deviations recorded in bulbous specimens is equal to those observed in spindle-shaped burrows, suggesting that in situ and transported burrows underwent similar periods of cementation. Cementation, however, was limited during each accretionary event in the spindle-shaped burrows, making them more susceptible to transport by storm waves because of their small size. Early cementation of Rosselia, thus, took place very close to the sediment-water interface at depths where storm waves could rework sediments (i.e., less than 1 m sediment depth). The enriched 13C values for calcite and siderite (3.06-9.45 PDB [Peedee belemnite]) suggest that cement precipitation followed bacterially mediated decomposition of the organic matter concentrated within Rosselia in the zone of methanogenesis. Oxygen isotope compositions are enriched also, ranging in siderite from 17.5 to 29.4 SMOW (standard mean ocean water) and in calcite from 16.8 to 23.0 SMOW, and are more akin to the composition of subsurface groundwater than marine waters. Freshwater discharging through shoreface sediments explains the 18O isotopic signature of calcite and accounts for the early diagenetic precipitation of siderite in shallow marine sediments. In addition, the coexistence of authigenic calcite and siderite cements was most likely controlled by variation in the mixing ratio of meteoric and marine fluids related to variable discharge rates for the freshwater aquifer.