Abstract

Dark colored, thinly bedded, organic-rich muds accumulating under low oxygen conditions in slope basins in the Gulf of California are modern "black shales" with features similar to black shales found in the Cretaceous and offer insights into the depositional processes recorded in these mudrocks. Analysis of sediment microfabrics based on x-radio-graphs of core sediments reveal repeated shifts from laminated to semi-laminated or bioturbated sedimentation in the past several hundred years. The most recent of these transitions occurred in the 1970s. In the center of Alfonso Basin, a shallow, mid-depth, silled-basin, a thin veneer of bioturbated sediments rests upon laminated sediments. Where the transition is least disturbed by recent biotic activity, varve counts and excess 210 Pb profiles date the shift as occurring between 24-30 years ago. A similar shift in sedimentation is observed in open slope sediments off Santa Rosalia but here the recent invasion of the oxygen minimum zone by macrobenthos has more deeply disturbed the sediments. Bioturbation began between 22 to 35 years ago and possibly earlier at the deepest sites. We believe the transition in the two locations reflect the same Gulf-wide event. Downcore sediment microfabrics vary from bioturbated to laminated, defining repetitive patterns or cycles, which can be traced between cores. Despite variations in the intensity of laminations, the same pattern of down-core change can be recognized in the two locations although with some differences. In Alfonso Basin, the typical sequence is bioturbated sediments slowly becoming more laminated, followed by a rapid shift back to bioturbation. In Santa Rosalia, the pattern is more symmetrical with bioturbated sediments becoming laminated and then returning to bioturbated. The sediment microfabrics reflect biotic disturbance in the sediments and are oxygen-related biofacies. Based on the sediment fabric model of Savrda and Bottjer (1993), we interpret the variation in the microfabrics observed in the Gulf as indicative of small-scale shifts in seafloor oxygen concentration, in which the range from well-laminated to bioturbated is probably less than 0.3 ml/l of oxygen. The repetitive down core changes in microfabrics represent ventilation cycles with durations on the order of 100-200 years. A compilation of dissolved oxygen data in the Gulf collected over the past 60 years reveals that the oxygen minimum zone (OMZ) has varied in thickness and intensity. Since the 1970s, dissolved oxygen concentrations have increased, allowing the development of a diverse macrobenthos and the shift in sedimentation. The oceanographic and sediment records are in good agreement.

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