CONTINENTAL HYDROLOGY AND CLIMATOLOGY OF THE CARBONIFEROUS JOGGINS FORMATION (LOWER CUMBERLAND GROUP) AT JOGGINS, NOVA-SCOTIA - EVIDENCE FROM THE GEOCHEMISTRY OF BIVALVES

The Pennsylvanian section at Joggins contains bivalves, possibly representing several species of Naiadites, in great abundance within certain beds of the Joggins Formation (Westphalian A). Shells are chalky-white and in most specimens the internal microstructure is preserved as nacreous tablets which are always aragonite. This site represents the oldest-known occurrence of Paleozoic non-marine biogenic aragonite. Geochemically the bivalves are distinct from coeval tropical marine, based on stable and radiogenic isotopes, and terrestrial counterparts based on their trace element contents. The Sr, Mg, Na, Mn and Fe contents are strongly habitat controlled by ambient environmental conditions and suggest a near-shore continental lacustrine-paludal environment. The high enrichment factors for Mn and Fe suggest slightly dysaerobic water conditions during shell secretion. The deltaC-13 (+ 2.11 to + 2.24%, PDB) and delta-O-18 (- 5.83 to - 5.72%, PDB) values, after mineralogy correction, for the Joggins bivalves are dissimilar to those of shallow-water, paleoequatorial brachiopods, but suggests that Naiadites also preferred warm, shallow waters. Furthermore, the postulated composition and temperature for annual precipitation of -5.5 to -2.3%, deltaO-18 (SMOW) and 18-24-degrees-C, respectively, support the equatorial position of this locality during the Pennsylvanian. Water reaching the Joggins locality was probably of a rheotrophic nature derived from rain in the Caledonia Highlands. This ''local'' origin of the meteoric water is further supported by its Sr isotopic composition. In addition, the Sr-87/Sr-86 ratios of the Joggins bivalves (>0.7093) are significantly different from those of coeval brachiopods and mollusks (<0.7085). This isotopic invariance between marine and non-marine invertebrates provides incontrovertible evidence and support for the non-marine paleoecology of Naiadites and thus the enclosing sediments of the upper Joggins Formation.