Multi-tracer geochemical investigation of laminated diatomaceous sediments: miocene monterey formation and holocene marine environments (Saanich Inlet and Santa Barbara Basin)
Creators: name: Johnson, Kristen M.
Publisher: name: University of British Columbia
Year: 2009

description: Organic-rich hemipelagic sediments possessing millimetre-scale laminae of alternating biosiliceous and lithogenic material from Saanich Inlet, British Columbia, the Monterey Formation, California, and Santa Barbara Basin, California were studied to interpret the paleo-environmental controls on sediment composition and fabric and to determine if the sedimentary processes recorded in the laminated sediments can be linked to distinct trends in biogeochemical cycling of opal and carbon. To accomplish this, individual laminae and groups of laminae extracted from ODP cores and outcrop samples were analyzed using a multi-tracer geochemical approach. Total organic carbon, opal, C/ N ratios, stable carbon isotopes of organic matter, Rock-Eval pyrolysis and major and minor element analyses were performed. Saanich Inlet (SI) sediments (n=82) have 52 to 84% detritus, 15 to 45% opal and one to two percent organic carbon. Stable carbon isotopes and C/ N ratios indicate a marine source of the organic matter present in the sediments. Monterey Formation (MF) samples (n=22) have six to 41% detritus, 28 to 87% opal, one to five percent organic carbon, and kerogen types, C/ N ratios, and carbon isotopes suggestive of a marine organic matter source. Santa Barbara Basin (SBB) samples (n=42) have 59 to 90% detritus, six to 34% opal, three to 22% calcite, and one to five percent organic carbon. SI and SBB have similar bulk compositions with the exception of the presence of calcite in SBB samples. MF sediments differ from those of SI and SBB primarily in the significantly lower detrital and higher opal contents, suggesting that SI and SBB are not appropriate modern analogs for the MF. Subannual/seasonal changes in the fluxes of opal and lithogenic detritus are responsible for the formation of the laminated sediment fabric; biogenic laminae form as a result of prolific diatom growth and sedimentation whereas detrital laminae form during periods of higher relative detrital flux. Seasonal variability influences the deposition of opal and detritus, producing different lamination styles which were characterized according to a new classification scheme that facilitates core description and clarifies interpretation of the various mechanisms of deposition. In the Monterey Formation, opal and TOC are negatively correlated, implying that the coupling of opal and organic matter created in the surface ocean by diatom productivity is not preserved in the sediments. The observed relationship is attributed to dilution of organic matter by large quantities of opal or the respiration and/or redistribution of organic matter during diagenesis and/or weathering. In Saanich Inlet sediments, there is a strong positive correlation between opal and organic matter. However, linking trends in opal and organic carbon export to specific laminae is problematic, and is complicated by possible sampling error. Future studies would benefit by the selection of an on-land site which is known to have experienced no weathering, complete sedimentological and paleontological characterization of a measured section prior to geochemical sampling, and the retrieval of large enough samples so that multiple analyses can be performed on individual laminae.