Show simple item record Schieber, Juergen Schimmelmann, Arndt 2007-06-22T18:39:29Z 2007-06-22T18:39:29Z 2007-05
dc.description Presented at the Pacific Climate (PACLIM) 2007 Workshop, Asilomar State Beach and Conference Grounds, Pacific Grove, California, May 13-16, 2007 en
dc.description.abstract Monitoring of the concentration of dissolved elemental oxygen (i.e., ‘oxygenation’) of sub-sill waters in the Santa Barbara Basin (SBB) over the last few decades has consistently demonstrated sub-oxic bottom water conditions. However, anoxic conditions and hydrogen sulfide (H2S) are present below a few millimeters depth in the sedimentary column. The absence of euxinic conditions (i.e., anoxic-sulfidic: no free O2 but free H2S) in any part of the modern SBB water column makes this basin geochemically distinct from the Black Sea and other more oxygen-depleted basins. The varved sediment record from the central SBB has been used extensively for high-resolution paleoceanographic reconstructions. The SBB and other oxygen-depleted basins are considered modern analogs for black shales. Geochemical proxies for paleo-redox conditions in black shales need to be tested against modern, well-constrained depositional systems like the SBB. Pyrite framboids are a common component in many anoxic marine sediments and are deemed to carry paleoceanographic information. Observations from modern Black Sea sediment yielded the hypotheses that (i) abundant small framboids below 5 m diameter indicate euxinic (anoxic-sulfidic) bottom waters, and (ii) mean framboid sizes above 5m indicate suboxic or normally oxygenated bottom waters. We are using Scanning Electron Microscopy to compile a detailed inventory of framboid occurrence in AD 1983-2004 SBB varves and compare framboid size distribution against the known history of SBB water column oxygenation. Our ground-truthing effort has shown that framboids in modern SBB sediment dominantly are in the 2-4 m range and thus show closely similar size distributions to those measured in Black Sea sediments underlying an euxinic water column. Of course, the lower SBB water column is suboxic, and therefore we conclude that the framboid size distribution in modern sediment and in black shales has no diagnostic value for paleo-water column oxygenation. Our data further indicate significant variability in mean framboid size between successive varves that do not correlate with historic changes in SBB water oxygenation. We are evaluating other potential factors that could affect framboid size, for example (i) the availability of iron from clastic terrestrial input from the continent, (ii) the availability of freshly deposited organic matter nourishing the microbial community and influencing the redox profile near the sediment/water interface, and (iii) the depth of burial within the near-surface sediment that fosters the precipitation of framboids. The generally lower abundance of framboids in the youngest, uppermost varves suggests that it takes several years of mineralization before framboid formation goes to completion in a given sediment layer. After about 5 years framboid abundance appears to stabilize, providing further support to the view that framboids do not form in the water column and should not be considered recorders of water column oxygenation. en
dc.format.extent 18267377 bytes
dc.format.mimetype application/pdf
dc.language.iso en_US en
dc.rights This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License. To view a copy of this license, visit or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA. en
dc.rights.uri en
dc.subject Dissolved Oxygen en
dc.subject Hydrogen Sulfide en
dc.subject Oxygen Depleted Basins en
dc.subject Anoxic Marine Sediments en
dc.subject Framboids en
dc.subject Framboid size en
dc.subject Marine Geochemistry en
dc.subject Santa Barbara Basin en
dc.title High resolution study of pyrite framboid distribution in varved Santa Barbara Basin sediments and implications for water-column oxygenation en
dc.type Presentation en

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