Non-annual laminations and expansion of anoxic basin-floor conditions in Santa Monica Basin, California Borderland, over the past four centuries

Abstract

Sediments from box cores in the deep portion of Santa Monica Basin have been examined by X-radiography, and the upper 10–20 cm are characterized by laminations 1–5 mm in thickness. Age dating of 9 cores by 210Pb analysis has shown that accumulation rates on the central basin floor have been remarkably constant during the past century, averaging 16.0 ± 0.4 mg/cm 2/yr. Densitometer analysis of the X-radiography films from 7 cores and spectral analysis of the resulting time series indicates that most lamination couplets have a periodicity in the 3–7 year range, a range matching that of the El Niño-Southern Oscillation periodicity. The median periodicity appears to have decreased slightly during the past 3 centuries, from 7.0 years during 200–300 yrs B.P. to 5.2 yrs during 0–100 yrs B.P. This trend could be due to a decreasing sedimentation rate, an increasing forcing frequency, or an increasingly faithful response of the Basin in more recent years. Multiple processes have contributed to creation of the laminations in Santa Monica Basin. The principal factors appear to be (1) variations in porosity due to the relative abundance of organic filaments probably formed by Beggiatoa mats, and (2) compositional variations that reflect variations in carbonate/lithogenous ratios and in grain size. Both of these are expected to respond to ENSO events. The presence of near anoxic bottom waters has prevented bioturbation, permitting laminations to be preserved. Basin bottom waters became nearly anoxic about 350 years ago, at the onset of warming as the Little Ice Age waned. Expansion of the near anoxic area during the past century may have been augmented by 20th Century anthropogenic effects. The style of these non-annual laminations in Santa Monica Basin matches that of the annual laminations in the adjacent Santa Barbara Basin, and demonstrates the need for caution in interpreting all laminations as representing annual varves.