Opal and organic carbon in laminated diatomaceous sediments: Saanich Inlet, Santa Barbara Basin and the Miocene Monterey Formation

• Published in: Marine geology Vol. 174; no. 1-4; pp. 159 - 175
• Main Authors: Johnson, K.M, Grimm, K.A
• Format: Journal Article
• Language: English
• Published: 01.03.2001
• Subjects: Marine; INE, USA, California; INE, USA, California, Santa Barbara Basin; INE, Canada, British Columbia, Vancouver I., Saanich Inlet
• ISSN: 0025-3227
• DOI: 10.1016/S0025-3227(00)00148-1

Laminated biosiliceous sediments from Saanich Inlet, British Columbia (SI), the Monterey Formation, California (MF), and Santa Barbara Basin, California (SBB) are organic-rich and were studied to assess the suitability of SI and SBB as modern analogs of the Miocene MF and to explore the controls on distributions of opal and organic carbon. To this end, 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, delta super(13)C of organic matter (OM), kerogen type and major and minor element abundances were measured. SI sediments are 52-84% detrital, 15-45% opal and 1-2% organic carbon. delta super(13)C and C:N ratios indicate a marine source of the OM present in the sediments. Samples from the upper siliceous member of the MF are 6-41% detritus, 28-87% opal, and 1-5% organic carbon. Kerogen types, C:N ratios, and delta super(13)C data are suggest mixed but predominantly marine OM. SBB samples are 59-90% detritus, 6-34% opal, 3-22% calcite, and 1-5% percent organic carbon. SI and SBB have similar bulk compositions with the exception of the presence of calcite in SBB samples. MF sediments have significantly lower detrital and higher opal contents than SI and SBB samples. These large differences suggest that SI and SBB are not appropriate modern analogs for the MF.In SI sediments, organic carbon is concentrated in opal-rich, detritus-lean laminae and intervals. This distribution suggests that the coupling of opal and OM created in the surface ocean by diatom productivity is preserved in the sediment record. In the MF, opal and TOC are negatively correlated, implying that the coupling of opal and organic carbon did not survive the processes of settling, sedimentation, diagenesis, and /or weathering. This decoupling of organic carbon and opal in the MF is attributed to dilution of OM by high opal flux rates and/or the respiration and/or redistribution of OM during diagenesis and/or weathering.