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Stable-Isotope Analyses of Belemnite Rostra from the Whitby Mudstone Fm., England: Surface Water Conditions during Deposition of a Marine Black Shale

Gunnar Sælen, Peter Doyle and Michael R. Talbot
PALAIOS
Vol. 11, No. 2 (Apr., 1996), pp. 97-117
DOI: 10.2307/3515065
Stable URL: http://www.jstor.org/stable/3515065
Page Count: 21
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Stable-Isotope Analyses of Belemnite Rostra from the Whitby Mudstone Fm., England: Surface Water Conditions during Deposition of a Marine Black Shale
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Abstract

Belemnite rostra from the Grey Shale and Jet Rock Members of the Lower Toarcian Whitby Mudstone Formation (England) record fluctuations in oxygen and carbon stable-isotope compositions (δ18Oand δ13C, respectively) of the ambient waters. Salinity modelling based on the rostral calcite δ13C values from the most organic-rich intervals of the Jet Rock (Beds 34 and 35) indicates that surface waters were up to $\sim5\textperthousand$ less saline compared to contemporaneous seawater. In addition to sea-surface salinity fluctuations, δ13C of the rostra probably records (a) various rates of primary production and burial of organic carbon, as well as (b) mixing of isotopically light DIC into the lower salinity and higher temperature surface water, or (c) migration of belemnites into sub-pycnoclinal waters with isotopically light DIC. A positive correlation between modelled Δt values (i.e., the density contrast between bottom and surface waters based on the belemnite temperature and salinity proxy data) and hydrogen index values of the sedimentary organic matter, indicates that water-column stratification was a significant factor in preserving the latter. Long periods of water-column stratification probably caused stagnation and concomitant dysoxia and anoxia in the bottom waters. These conditions were favourable to the preservation of marine organic matter and, combined with relatively low clastic input, resulted in high total organic carbon contents and hydrogen index values. The primary production was probably generally low, but may intermittently have been boosted by irregular upwelling of nutrient-rich intermediate and bottom-waters.

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