Geochemical and sedimentologic variations in inter-annually laminated sediments from Santa Monica Basin

Abstract

Laminated sediments were recently discovered in the center of Santa Monica Basin, California Continental Borderland and extend from the present day to at least the Little Ice Age (ca. 1600 A.D.). Unlike the annual varves observed in Santa Barbara Basin, the light-dark lamination couplets observed in 210Pb-dated cores from the center of Santa Monica Basin occur at 3 to 6 year intervals, very similar in length to the El Niño Southern Oscillation (ENSO) phenomena. The mechanisms that control the temporal patterns of laminations in this basin are unknown. Potential causal mechanisms of lamination formation in Santa Monica Basin include: (1) changes in the flux of terrigenous vs. marine organic carbon and detritus; (2) surface water processes which affect marine productivity and organic carbon flux to the sediments; and (3) deep water processes which affect oxygen renewal and preservation of organic matter within the basin. A box core from the central portion of Santa Monica Basin was analyzed using a suite of geochemical, sedimentary, and micropaleontologic techniques in order to: (a) investigate which, if any, of these mechanisms has contributed to the occurrence of laminations; and (b) reconstruct proxy records of climate change. Samples were extracted from this core at 1 mm increments and provide a temporal resolution of 1–3 years per sample, a resolution which is comparable in length to the thickness of individual laminae within the core. Geochemical and micropaleontologic analyses of core samples provide paleoclimatic and paleoceanographic records which are comparable to, and extend beyond, historical regional and global climate records. In addition to direct comparisons between these records, spectral analysis was used to investigate whether there were similar patterns of variability that would implicate dominant forcing functions in the historic and proxy records. Herein we present preliminary results of records from ca. 1860 to the present. Our results suggest that organic carbon fluxes are the dominant factors that affect lamination variability. Input of organic carbon is primarily a function of primary productivity, and, to a lesser extent, a function of percentage of terrestrial input. Thus, (1) and (2) are both mechanisms of lamination formation. Stability of (3) only allows preservation of the record of these variations. Variations in Southern California rainfall appear to be linked to decadal-scale variations in downcore density profiles, and thus cause variations in the thickness and character of the laminations in the deepest portions of this basin. Thus, filtered density records can be used to reconstruct general paleo-rainfall trends which compare well with historic rainfall records. Variations in the isotopic composition of total organic carbon (TOC) are compared to historic ENSO and climate records and primarily reflect changes in the relative contribution of marine organic carbon to the basin. Variations in the carbon isotopic composition of N. pachyderma mirror changes in the sedimentary δ 13C TOC record from Santa Monica Basin, and suggest that primary productivity is the primary control on the flux of organic carbon to the sediments and exerts a strong influence on the interannual variability in lamination composition. Variations in paleo-sea surface temperatures generated from the oxygen isotopic composition of N. pachyderma correlate well with historical SST records and suggest potential usage as a proxy for general paleoSST patterns. Oxygen-sensitive assemblages of benthic foraminifera ( Bolivina argentea and B. spissa) have been used to identify potential large-scale flushing events and construct a historical record of variations in deep water oxygen levels. Spectral analysis of regional historic records indicate dominance of short-term (ENSO-length) and long-term (decadal) patterns of climate variability. The short-term ENSO signal is not dominant in the Santa Monica proxy records. However, strong decadal-length signals are identified in all of the proxy records and suggest that the dominant forcing mechanisms in Santa Monica Basin may be decadal in length.