RECONSTRUCTING BASIN BOTTOM-WATER EVOLUTION FROM MICROBIAL TIME CAPSULES: RARE EARTH ELEMENT GEOCHEMISTRY OF ORGANOGENIC DOLOMITE IN THE MIOCENE MONTEREY FORMATION

Early diagenetic dolomite with unusual d¹³C (-20 to +10‰) compositions (organogenic dolomite) is well known in marine continental margin sequences rich in organic matter, notably the Miocene Monterey Formation of coastal California. Such extreme d¹³C compositions implicate the role of microbial organic matter decomposition, chiefly by sulfate reduction (light C) or methanogenesis (heavy C), in elevating pore water alkalinity toward dolomitization. Because SO₄^2- ion diffusion is an essential control governing the relative dominance of these processes, dolomitizing pore waters likely evolved from bottom water compositions near the seafloor. If so, early-formed organogenic dolomites may retain primary geochemical aspects of basin bottom waters. To evaluate this hypothesis we determined rare earth element (REE) compositions for 90 organogenic dolomite intervals from a Monterey core recovered from Santa Barbara-Ventura Basin (South Elwood Field) and compared shale-normalized REE patterns to seawater. The resulting chemostratigraphic record reflects changing pore water characteristics accompanying the Monterey cycle of basin formation and fill. All patterns resemble seawater in that they are heavy REE-enriched, show negative Ce anomalies (Ce/Ce*: 0.5-0.9), and are somewhat La-enriched. Dolomites from the main Monterey depositional interval are particularly heavy REE-enriched, have low Fe contents, and light d¹³C compositions (0 to –17‰). Pattern slopes and magnitudes of negative Ce anomalies are quite similar to those from sulfidic Black Sea waters. In contrast, dolomites from the underlying Rincon Shale and uppermost Monterey Formation have distinctly flatter REE patterns, higher Fe contents, and heavy d¹³C compositions (0 to +10‰). Positive Eu anomalies (Eu/Eu*: 1-5) throughout much of the core interval demonstrate the reducing character of dolomitizing pore waters. An interval near the basal Monterey contact characterized by extreme positive Eu anomalies (Eu/Eu*: 10-35), high Ba and Sr contents, and SREE < 2ppm may indicate proximity to reducing hydrothermal waters, which preferentially enriched ambient seawater in Eu²⁺ while scavenging other trivalent REEs. Consistent with this is that the interval coincides with initial rifting and rotation of the western Transverse Ranges block.