ALBERT

Description: During opening of a new ocean, magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock, creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g., during the Paleocene-Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as sediment volcanoes, and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black smoker–type chimneys rather than sediment volcanoes. We have evidence for two buried and one active hydrothermal vent systems outside the rift axis. The active vent releases fluids of several hundred degrees Celsius containing abundant methane, mid-ocean ridge basalt–type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Description: High-temperature (〉300 °C) off-axis hydrothermal systems found along the slow-spreading Mid-Atlantic Ridge are apparently consistently located at outcropping fault zones. While preferential flow of hot fluids along highly permeable, fractured rocks seems intuitive, such efficient flow inevitably leads to the entrainment of cold ambient seawater. The temperature drop this should cause is difficult to reconcile with the observed high-temperature black smoker activity and formation of associated massive sulfide ore deposits. Here we combine newly acquired seismological data from the high-temperature, off-axis Logatchev 1 hydrothermal field (LHF1) with numerical modeling of hydrothermal flow to solve this apparent contradiction. The data show intense off-axis seismicity with focal mechanisms suggesting a fault zone dipping from LHF1 toward the ridge axis. Our simulations predict high-temperature venting at LHF1 only for a limited range of fault widths and permeability contrasts, expressed as the fault’s relative transmissibility (the product of the two parameters). The relative transmissibility must be sufficient to "capture" a rising hydrothermal plume and redirect it toward LHF1 but low enough to prevent extensive mixing with ambient cold fluids. Furthermore, the temperature drop associated with any high permeability zone in heterogeneous crust may explain why a significant part of hydrothermal discharge along slow-spreading ridges occurs at low temperatures.

Description: The Western Interior Seaway (WIS) was a shallow and expansive body of water that covered the central United States during the Late Cretaceous. Attempts to reconstruct temperatures in the seaway using the oxygen isotopic composition of biogenic carbonates have suffered from uncertainty in the oxygen isotopic composition of seawater ( 18 O w ) in the semi-restricted basin. We present new reconstructed temperature and 18 O w data from marine and estuarine environments in the WIS and freshwater environments in WIS source rivers, derived from clumped isotope analyses of bivalve and gastropod shells. We find temperatures of 5–21 °C, 18 O w values below contemporaneous Gulf of Mexico marine sites, and a strong correlation between 18 O w and environmental setting. We propose that decreasing 18 O w values reflect decreasing salinity driven by an increasing contribution of continental runoff. Using a two-end-member salinity- 18 O w mixing model, we estimate salinities of 29–35 psu (practical salinity units) for the deep marine, 20–32 psu for the shallow marine, and 11–26 psu for the estuarine environments of the WIS. New climate model simulations agree with reconstructed temperatures and salinities and suggest the presence of salinity driven stratification within the seaway.

Description: Experimental studies have shown that temperature, pressure, sulfur fugacity ( f S 2 ), and oxygen fugacity ( f O 2 ) influence the Fe content of sphalerite. We present compositional in situ data on sphalerite from submarine volcanic-hosted massive sulfide (VHMS) ores of hydrothermal vents from different plate tectonic settings and with variable host-rock compositions. Sphalerite from sediment-hosted vents has systematically higher S contents and Fe/Zn ratios than those of the sediment-starved vents, reflecting an influence of f S 2 and f O 2 on Fe partitioning between fluid and sphalerite. The Fe/Zn ratios of sphalerite from sediment-starved vent systems apparently increase systematically with the fluid temperatures of the corresponding vents. We conclude that the composition of sphalerite can be used to (1) distinguish between sediment-hosted and sediment-starved hydrothermal processes, and (2) estimate minimum fluid temperatures of sphalerite precipitation from inactive sediment-starved hydrothermal vent sites and fossil VHMS deposits.