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Sclerosponges have great potential as temperature (T) recorders; they precipitate carbon and oxygen isotopes in apparent equilibrium with seawater. These animals lack photosynthetic symbionts, which simplifies interpretation of their isotopic record. It also allows them to live in subphotic depths, recording T below the thermocline and as deep as the carbonate compensation depth. The coralline sponge Acanthocaetetes wellsi is widespread in caves in the western Tropical Pacific. The West Pacific warm pool accumulates here before moving eastward to the Americas during El Nino events. A. wellsi has distinct growth bands averaging ∼1 mm/yr. Skeletal ∂13C and ∂18O show millimeter-scale cyclicity, apparently due to annual T variation, and large amplitude swings that occur every four to seven cycles, likely indicating El Nino events. Because individual sponges live for several centuries, they can provide high-resolution records of pre- and post-industrial El Ninos. Additionally, the caves contain both live and dead A. wellsi. Cross-correlation of successively older specimens should yield a 1000–2000-year record. The area, depth, and T distribution of prior warm pools will be defined. The velocity of their eastward movement may be determinable from East Pacific A. wellsi and American Mytilus. From these data, estimates of heat transfer rates can be derived, allowing determination of past El Nino intensities. By correlating past intensities with known impacts, refined prediction of El Nino effects will be possible.

Petrography and cement stratigraphy of four regionally extensive cement zones in Mississippian crinoidal limestones indicate that these cements precipitated in meteoric phreatic environments. Each major zone has a distinct isotopic and trace element composition. Marine cements and lime muds in bioherms associated with the crinoidal sands have marine ^dgr13CPDB (+4.0 ppm). Phreatic cements become lighter in ^dgr13C in progressively younger zones 1, 2, and 3, representing a trend toward more contribution of organically derived carbon to precipitating waters. Since zones 1, 2, and 3 were all precipitated at shallow burial depths, their trend toward lighter ^dgr18O with decreasing age suggests increasingly light waters isotopically. At 25°C, the waters responsible for zone 1 and 3 cements are estimated as ^dgr18OSMOW= +1.5 and -0.9 ppm, respectively. The ^dgr13C of zone 5 cement is interpreted as a combination of rock-derived and organic-derived carbon, some of whic probably came from overlying Pennsylvanian strata. The distinctively light ^dgr18O of zone 5 is attributed to precipitation at somewhat elevated temperatures, averaging about 45°C, a value in agreement with estimated burial depths. Zone 1 cements appear to have formed in seaward parts of a freshwater phreatic system at shallow burial depths (relatively heavy ^dgr180). Zone 1 magnesium and carbon were derived from dissolution of skeletal high-Mg calcites (high Mg and heavy ^dgr13C). The driving force for zone 1 cementation was thus the solubility difference between high-Mg crinoidal calcite and low-Mg zone 1 calcite. Zone 2 also precipitated at shallow depths but in a more widespread groundwater system that contained some organic carbon (light ^dgr13C) and had a more landward recharge area (lighter ^dgr18O). Likewise, zone 3 is formed in a shallow-phreatic lens but with the most landward (freshest) recharge (lightest ^dgr18O). The inferred importance of organ c-derived carbon in zone 3 (light ^dgr13C) suggests degassing of CO2 as a driving force for precipitation. The light ^dgr18O of zone 5 reflects precipitation at elevated temperatures of deeper burial (750); its element composition (table) suggests a variety of intraformational and extraformational sources. Table End_of_Article - Last_Page 750

The Middle Silurian pinnacle reef system of northwest Michigan consists of a narrow, northeast-southwest-trending band of isolated reefs encased in thick Upper Silurian evaporites. All reefs display a similar pattern of diagenetic evolution: neomorphism of metastable carbonate components followed by precipitation of a closure cement; modification of porosity by a combination of cementation, solution and dolomitization; and emplacement of hydrocarbons and stylolitization. The exact sequence of diagenetic events and the resulting texture of the carbonate rock vary considerably from reef to reef, but several regional trends can be identified. There is an obvious change in reef mineralogy across the reef trend, from predominantly calcitic reefs basinward to predominantly dolomitic reefs near the shelf. There appears to be a corresponding increase in ^dgr13C and ^dgr18O isotopic ratios shelfward, both in matrix limestone, according to Sears and Lucia in 1979, and in the neomorphic replacement of former marine cements. A change in the mineralogy of closure cements is observed along the reef trend, from calcite-pyrite in the southwest to pyrite or dolomite-quartz in the northeast. Oil emplacement also varies along the reef trend. Sparry dolomite and pyrite mineralization are associated with bitumen in the southwest parts of the reef trend, whereas leaching appears to accompany or just predate oil entry in the northeast. Finally, the dominant pore-filling phases in each reef vary in an irregular fashion throughout the trend from calcite and halite in some reefs to anhydrite and laminated dolomite in others. These variations on a diagenetic theme appear to be related only to the presence or absence of diagenetic fluids in the reef's history, not to the lithology involved. Differential migration of fluids, caused by different hydrostatic heads on each fluid or migration through different pathways, would account for the diversity of diagenetic sequences in the pinnacle reef system. Careful petrographic and chemical analysis of each individual reef is needed to identify the local diagenetic history, and these local histories must be correlated in order to infer the larger picture of basin development. End_of_Article - Last_Page 438

Core samples from Middle Silurian pinnacle reefs in northern Michigan exhibit a regionally consistent assemblage of late diagenetic phases including geopetal diagenetic sediment, disseminated pyrite, pyrobitumen, rhombic dolomite cement, and equant calcite spar. High (> 80°C) fluid inclusion homogenization temperatures and inclusions of Silurian-sourced oil in these diagenetic phases indicate that they formed between the Mississippian and Jurassic, when pinnacle reefs resided in the deep subsurface. Fluid inclusion salinites and stable carbon/oxygen isotopic ratios suggest that late diagenetic carbonates did not precipitate from connate fluids but from basinal brines that were conducted to pinnacle reefs by two regional carbonate aquifers. These data confirm that ate burial diagenesis can affect carbonate rocks residing in high-salinity, low-permeability fluid environments.

Two stages of pretectonic dolomitization are recorded in the upper Madison Group of the Wyoming-Utah thrust belt. Marine to hypersaline dolomitization occurred during deposition of upper Madison carbonates and evaporites in peritidal and sabkha settings. This timing is supported by the common association of fine-grained, nonluminescent dolomite with restricted, shallow-water facies, and the relatively enriched isotopic composition of the dolomite (^dgr18O = +2.0 ^pmil; ^dgr13C = +4.0 ^pmil). A second stage of dolomitization began during exposure of the Madison shelf and continued through deposition of the overlying Amsden Formation. Regressions and transgressions during this time caused migration of marine-meteoric mixing zones across the Madison shelf and widespread dolomitization of all depositional facies within the Madison Group. The second stage of dolomitization is volumetrically dominant, has a meteoric isotopic composition (^dgr18O = -2.0 ^pmil; ^dgr13C = +2.0 to +7.0 ^pmil), and in most locations has masked the isotopic record of the first-stage, synsedimentary dolomite. In addition to replacive dolomite, dolomite and calcite cements were also formed during this diagenetic stage. These three components exhibit isotopic variations concomitant with stratigraphic position that reflect spatial gradients in ground water chemistry.