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Laminated sediments of Santa Monica Basin, California continental borderland

Abstract Laminated sediments from Santa Monica Basin differ from other laminated sediments of the California Continental Borderland (such as varved sediments of Santa Barbara Basin) because lamina couplets represent a 3–6 year time interval and are affected by distinct local oceanographic and climatic processes. Unfortunately, mechanisms which control the temporal pattern of laminations in this basin are poorly understood. In addition, this laminated sediment record has been underutilized in paleoceanographic and paleoclimatic studies. In an attempt to identify mechanisms which control lamination formation and in order to obtain paleoclimatic and paleoceanographic records from this region, a sediment core from this basin was analysed using a suite of geochemical, sedimentary, and micropaleontologic techniques. This box-core (core DOE 26) was collected from the centre of the basin and was vertically sub-sampled at one millimetre increments, providing a temporal resolution of 1–3 years per sample. A variety of paleoceanographic and paleoclimatic records were constructed via analysis of these samples and include proxy records of: (a) regional rainfall — as expressed by gross variations in downcore density profiles; (b) relative contribution of terrestrial and marine organic carbon to the basin — as expressed by downcore variations in the isotopic composition of total organic carbon (TOC); (c) Primary productivity — as expressed in downcore variations in the carbon isotopic composition of planktonic foraminifera; (d) changes in sea surface temperatures —reflected in downcore variations in the oxygen isotopic composition of N. pachyderma (dextral); (e) large-scale changes in bottom-water oxygen conditions — as expressed by downcore changes in abundance of oxygen- sensitive benthic foraminifera. To extract additional information from these proxy records, spectral analysis was also employed in this study. Although ENSO-length signals are present in several records, decadal-length signals appear to dominate records from this basin. This dominance suggests the presence of a longer-term forcing function (other than the ENSO phenomena) which strongly influences this region. Considered together, preliminary results suggest that the flux of organic carbon is a dominant factor affecting lamination variability in Santa Monica Basin. Organic carbon fluxes in this basin are mainly affected by primary productivity, and to a lesser extent, by terrestrial input. Variations in bottom oxygen conditions may not control lamination variability; rather, long-term depression of oxygen levels appears to allow preservation of this organic matter as laminated sediments.