Magma mix-up

Basalts from the Snake River Plain (pictured above) in Yellowstone National Park in the northern United States look like they originate from deep in Earth’s mantle. But they have the characteristic isotopic signature of rocks from shallower reaches — from the continental lithosphere. This curiosity has been explained by Barry Hanan of San Diego State University, California, and his colleagues. Using mass spectrometry on samples of volcanic rock, the authors showed that deep-mantle magma can inherit the lithosphere’s isotopic signature when it rises and picks up contamination. The findings warn geologists about assuming too much from isotopic signatures.

Source: Nature, Vol 45618/25 December 2008

Reference: Geology 36, 51–54 (2008)

WEBINAR - Kathleen R. Johnson

Reconstructing Asian Monsoon History from Chinese Speleothems

Kathleen R. Johnson
Department of Earth System Science
University of California, Irvine



While we know that modern anthropogenic climate change is superimposed upon significant natural climate variability, the instrumental record of climate is too short to capture the full range of this variability. In order to fully understand and predict future changes, therefore, high-resolution, welldated paleoclimate records are needed to extend the record. This paleoclimate data allows us to quantify natural variability and learn how the climate system responded to past changes in boundary conditions and forcings and provides a vital test for state-of-the-art coupled climate models. Cave calcite deposits (speleothems) are widely studied paleoclimate archives that have led to significantly improved records of past climate variability over a wide range of timescales (seasonal to glacialinterglacial), most notably in low-latitude and monsoon regions. Speleothems are well-suited for terrestrial climate reconstruction because: they tend to be very pure and well-preserved; they usually contain clear visible growth banding which, like tree rings, is often annual in nature; they can be very precisely dated using uranium-series radiometric dating methods; and they contain numerous types of physical and geochemical proxy data. In this lecture, I will present an introduction to speleothem based paleoclimate proxies and describe ongoing modern calibration studies we are conducting at Heshang Cave, China to test and develop new seasonal resolution proxies of Asian monsoon rainfall. In addition, I will present multiple records of Asian monsoon rainfall obtained from stable isotope and trace element variations in Chinese speleothems and discuss the important role of the Asian monsoon in the global climate system.

WEBINAR - Jared Kluesner

Geologic and Hydrologic Role of Sill Intrusion and Delineation of the Oceanic Crustal Boundary in the Central Gulf of California

Jared Kluesner
Marine Physical Laboratory
Scripps Institution of Oceanography




Geologic and Hydrologic Role of Sill Intrusion and Delineation of the Oceanic Crustal Boundary in the Central Gulf of California High-resolution multichannel profiles recently shot in the central Gulf of California display concordant and discordant (concave-upwards) sills intruded shallowly within (I) young sediments in the axial troughs of Guaymas, Carmen and Farallon Basins, (II) off-axis in the basin floors, and (III) within the sediment cover of subsided and extended continental crust. We interpret some imaged sills as 3D saucer-shaped intrusions based on their concave-upward profiles, the overlying circular and elliptical plans of domal uplifts of the present multibeam-mapped seafloor, and their striking resemblance to field-mapped and 3-D seismically imaged saucer-like sills. Vertical zones of high-amplitude, disturbed reflectors leading up from sills are probably "blow-out pipes" acting as conduits for hydrothermal fluids and gases migrating up and away from the heated sill-sediment contact aureole, forming pockmarks on the present seafloor. Bright spots, dim spots, phase reversals, and acoustic turbidity in the sediments above sill intrusions suggest the presence of hydrocarbons and fluid flow throughout the study area. Seismic evidence of sill intrusions into the shallow crust throughout the central gulf suggests melt is being delivered not just to spreading centers, but to a much broader area of oceanic and continental crust. We have improved the delineation of the oceanic/continental crustal boundary in the central and southern gulf by sampling igneous basement (tholeiitic basalt and gabbro = oceanic; granitic = continental), by identifying the extent of magnetic stripes diagnostic of seafloor spreading, by interpreting multichannel reflection profiles, and by geomorphology. Although the "boundary" is somewhat smeared by the intrusion of shallow sills (some known to be tholeiitic, most inferred to be) into the cover of both granitic and oceanic basement, we find no evidence of "transitional zones" of hybrid crust; at those sheared and rifted margins where basement is accessible, granite commonly abuts tholeiitic flows and sills. Seafloor spreading magnetic anomalies, with low amplitudes and broad transition widths, can be read out to C2Ar in Alarcon Basin, and C2An.1 in Guaymas Basin (but only on profiles that avoid major off-axis seamounts and intrusions); in both cases they indicate significantly slower accretion during the first 1 m.y. of spreading, presumably because of concurrent continued extension of the rifted margin. Widespread sill intrusion over continental basement does hamper identifying the ocean/continental boundary on seismic reflection profiles, and because the already thin Cordilleran crust was clearly highly extended during prolonged rifting we do not think that crustal thickness is a reliable criterion for the extent of oceanic crust.