Abstract

Fluctuations in the input of organic matter into a deep-sea environment can be deciphered from ichnofabrics if the continuous processes of hemipelagic sedimentation and bioturbation are episodically interrupted by turbidite deposition. Below turbidites, the bioturbated zone may be preserved almost intact; the benthic food content in such frozen tiers can be interpreted from the ethology, size, penetration depth, and density of trace fossils. Especially high benthic food content is characterized by (1) dark sediment color, (2) complete bioturbation, (3) high density of burrows produced near-surface in several levels, (4) rarity or absence of graphoglyptids, and (5) deep tiers completely bioturbated by feeding burrows having an open connection to the surface. Oxygen-deficiency can be eliminated as a reason for the black color of the sediment; the size, diversity, and penetration depth of the trace fossils are similar to that of modern and fossil counterparts formed under well-oxygenated settings that are not restricted in benthic food. An increase in sedimentation rate enhances the burial of benthic food and is indicated by the downward extension of the bioturbated zone and by increasing burrow density. Our analysis suggests that several other black shale occurrences may result from increased organic matter input rather than from water-column anoxia due to sluggish circulation. Our model is supported by similarities with other deposits that contain many frozen tiers.

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