No abstract

A multichannel seismic (MCS) experiment spanning 600 km across the Alarcón Rise and its conjugate rifted margins in the southern Gulf of California (western North America) provides insight into the spatial and temporal evolution of extension between Baja California and the mainland (Mexico). Stratigraphic analysis of multiple rift basins within the Alarcón spreading corridor indicates an initial stage of oblique extension starting ca. 14-12 Ma. This initial phase of extension was characterized by the formation of several large basins in the center of the gulf and on the southeast margin with negligible synrift sedimentation. A second phase of oblique extension, likely synchronous with large-scale basin opening in the central and northern Gulf of California, began ca. 8-5 Ma and was characterized by the formation of smaller half-grabens distributed across the conjugate margins that contain both synrift and postrift deposits. A key feature imaged within the MCS data is a highly refl ective, ropey layer at the top of basement, interpreted to be either volcanic rocks from the 25-12 Ma Comondú Group, and/or early rifting volcanic rocks that are between 11 and 9 Ma, or younger. This volcanic layer is extensively faulted, suggesting that it predates the episode of early extension. Upper crustal extension appears to be equally distributed across conjugate margins, forming a symmetrical continental rift. Two styles of rifted basin are observed; older basins (estimated as 14-11 Ma using sedimentation rates) show distributed extension with extensive basement faulting. In contrast, the younger basins (likely post-6 Ma) are asymmetrical with synrift deposits thickening into the basin-bounding faults. The northeastsouthwest geomorphic expression of the Tamayo bank and trough and other features provides additional evidence that northwestsoutheast oblique extension began ca. 12 Ma. These new spatial and temporal constraints, when combined with a crustal thickness profi le obtained across the entire Alarcón corridor, suggest that signifi cant northwestsoutheast oblique extension within the Gulf of California started well before 6 Ma, in contrast to earlier models.

Phosphorus accumulated in sludge‐treated soils was chemically fractionated by a sequential extraction procedure. Samples used for the study were obtained from experimental plots of a sludge land application site, where composted sludges at rates 0, 22.5, 45, and 90 t ha−1 y−1, and liquid sludges at rates of 0, 3.75, 7.5, and 15 cm/y have been applied for 5 y. There were also experimental plots whose liquid sludge treatments were terminated after 3 y. The experimental field was cropped with winter barley (Hordeum vulgare) after the sludge applications.In soils not treated with sludges, the Ca‐bound P accounted for 50–60% of the total P. An accumulation of Al‐ and Fe‐bound P occurred in all sludge‐treated soils. In the top 15 cm of surface soil, after 5 y of composted sludge treatment at a rate of 90 t ha−1 y−1 (1980), the Al‐ and Fe‐bound P was > 60% of the total P. Concentrations of water‐soluble P in sludge‐treated soils also increased. There were indications that the organically bound P in sludge‐treated soils was the most likely form to leach. Three years after the termination of sludge applications, there was little change in the distribution pattern of sludge‐borne P in the soil profile.