HR: 0800h
AN: OS21A-1202    [Abstracts]
TI: Hydrogeochemistry Of A Modern Dolomite-Forming Lagoon System, Cabo Frio-RJ, Brazil
AU: * Moreira, N F
EM: nmoreira@umich.edu
AF: Department of Geological Sciences, The University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109
AU: Walter, L M
EM: lmwalter@umich.edu
AF: Department of Geological Sciences, The University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109
AB: Two hypersaline, dolomite-forming lagoons near Cabo Frio, Brazil, and associated ground- and surface waters were sampled in a comparative sediment and fluid geochemical (solutes, stable isotopes) investigation. Although microbial mediation via sulfate reducers has been invoked to explain dolomite formation in these lagoons, we showed that dolomites are associated with sulfide oxidation (Moreira et al., 2004). Sulfide oxidation is thought to promote dolomite formation by causing undersaturation for competing carbonate phases such as Mg-calcite. Herein, we consider the larger hydrogeologic and temporal setting to further elucidate hydrogeochemical and geochemical constraints on rates and mechanisms of dolomite formation in the two lagoons. The lagoons, Brejo do Espinho (BE) and Lagoa Vermelha (LV), are shallow marginal marine systems flanked by quartz sand dunes separating them from Atlantic open seawater to the south and from Araruama lagoon, a large, hypersaline water body, to the north. In both lagoons, about 1 m of high Mg-calcite and dolomite mud has accumulated over the last 5,000 years on an underlying aquifer composed of highly permeable, quartz-rich coquinas. BE has a proximal relation to recharge from Araruama lagoon, while LV is more closely associated with meteoric recharge from lacustrine and riverine systems. BE is shallower, at 0.5 m water depth, than LV (2 m), permitting BE waters to remain oxic. Oxygen isotope values and chloride mass balances of pore waters and of fluids sampled from shallow ground water wells identify the different water and solute sources the lagoons. BE overlying brines and pore waters appear to be produced by evapoconcentration of Araruama source brines and meteoric precipitation. In contrast, LV derives from evapoconcentrated seawater mixed with regional lake and ground water sources. We envision a scenario in which dense, Mg-SO$_{4}$-rich brines from Araruama migrate along a permeable flowpath in limited contact with the atmosphere. These fluids undergo progressive sulfate reduction, then are drawn up by capillary forces to the shallow BE lagoonal system, where sulfide is oxidized. Thus, the locus of dolomite formation at BE appears to occupy the interface between discharging sulfide-rich fluids and the thin layer of oxygen-rich overlying brine. In contrast, sulfide oxidation is not an important process in the LV lagoonal-sediment system. XRD profiles and sediment chemistry indicate that BE sediments indeed contain a significantly higher percentage dolomite than those of LV. Sr systematics support this conclusion and place the lagoonal dolomites in the context of carbonates throughout the rock record. The Sr contents of BE carbonates decrease markedly in the most dolomite-rich intervals, approaching a value of 800 ppm, the high end of expected values for dolomite (Land, 1980). LV carbonates fluctuate in Sr value from 2000-3000 ppm, values more typical of high Mg-calcite. Some precipitation of dolomite has occurred in both lagoons, however, reflected by increasing pore water Sr with depth as Mg-calcite is replaced by dolomite. Recent work documenting changes in the elemental compositions of the oceans, including Ca, Mg, CO$_{3}$, and SO$_{4}$, raises the potential for significant variation in marine saturation states for carbonate minerals and in carbon-sulfur cycling through time. Our model may provide clues to the relationship between marine elemental composition and varying rates of dolomitization over the geologic record.
DE: 3675 Sedimentary petrology
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1615 Biogeochemical processes (4805)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting