Senior Theses 2007
Amy Westman (2007). Hypoxic cyclicity in sediments of Soledad Basin, Baja Mexico: A record of high-frequency climate fluctuations?
Faculty Advisor: Gregg Brooks
The sedimentary record in Soledad Basin, 45 km west of Baja, Mexico, shows high-frequency oscillations in hypoxia, which may be linked to fluctuations in climate. Soledad Basin, a semi-enclosed basin with a sill depth of 290m, has been shown to exhibit variable levels of hypoxia throughout the geologic past. Located at the intersection of the California Current and California Undercurrent, Soledad Basin is highly responsive to changes in current strength and upwelling, which together create fluctuations in hypoxia. During climatic cool periods, the California Current increases in upwelling and biologic productivity, along the Baja Borderland. This causes increased hypoxia in Soledad Basin. The California Undercurrent is also weakened during these cooler periods, and brings less nutrients and oxygen to the basin further increasing hypoxia. Since Soledad Basin sediments are undisturbed and have accumulated rapidly, this is a prime location to study high-frequency variations in hypoxia in the sedimentary record.
The objective of this study was to examine how and to what extent hypoxic events have been recorded in the sedimentary record of Soledad Basin, and gain insight into what controls these events. Surface sediment samples and a single 1.lm gravity core were collected aboard the S.S. V. Robert C. Seamans on a SEA Semester cruise in October 2005. The core was taken at a depth of 490 m near the deepest point of the basin. The core contained laminated sediments consisting of >95% mud. Using 210Pb analysis, a sedimentation rate of 15 cm over the past 100 years was determined, which is consistent with previous research. Trace metal analyses were performed at the cm-scale on selected intervals between 0.34-0.44m and 0.78-0.92m. These intervals correspond to dark organic-rich (>15% organic content) laminations alternating with lighter layers containing less organic material «15% organic content). All sediments are enriched in molybdenum, rhenium, vanadium, cadmium, and uranium, all of which are indicative of hypoxic conditions. The highest concentrations of these trace metals correlate with the darker, organic-rich laminations, suggesting there was less oxygen present during deposition of these sediments.
The frequency and intensity of these hypoxic events is controlled by organic content, which is a function of overlying currents driving upwelling and productivity. The overlying currents may ultimately be controlled by climatic fluctuations such as ENSO events, the Pacific Decadal Oscillation (PDO) as well as seasonal changes. Correlations may exist between the darker laminations representing higher levels of hypoxia to past climatic fluctuations. Laminations are more pronounced in these surficial layers, which could potentially indicate that climatic fluctuations are becoming more frequent and intense.