A Combined Trace- And Body-Fossil Approach Reveals High-Resolution Record of Oxygen Fluctuations in Devonian Seas

Abstract

Black shales deposited in epeiric seas preserve dynamic, bottom-water oxygen conditions as inferred from high-resolution trace-and body-fossil data. Nearly all paleoproxies are imperfect; however, this study utilizes the strength of paleobiological proxies to capture variability in bottomwater oxygen levels through aerobic and anaerobic, as well as dysaerobic (reduced but non-zero oxygen) conditions. Trace-and body-fossil data were collected at high resolution from twelve localities through central and western New York State, United States, exposing Devonian black shales. The combination of relative amount of bioturbation, estimated as ichnofabric index (ii), maximum burrow diameter measurements, and body-fossil species diversity allows relative bottom-water oxygen levels to be interpreted on a millimeter scale. Overall, trace-fossil size and diversity through the dysaerobic zone are much reduced compared with those described from younger strata. The decreased depth of bioturbation compared with younger strata, and resulting reduced overprinting of the sedimentological record by deep burrowing infauna, provide the opportunity for records of high frequency fluctuations in relative oxygen levels to be preserved. The biological signal preserved in these units reveals that bottom-water oxygen levels fluctuated considerably within a narrow stratigraphic range (on a centimeter scale) and that the dysaerobic zone is not temporally stable.

Aligned trace fossils from the Utica Shale: implications for mode of life and feeding in the trilobiteTriarthrus beckii

“…Evidence of disruption of the primary sedimentary structures (laminations) through bioturbation is rare at these localities. This is consistent with predominantly anoxic bottom‐water conditions with short‐lived oxygenation events allowing brief episodes of infaunal activity (Boyer & Droser ). The lack of a shelly benthic fauna is strong support for some sort of oxygen stress, but overall the geochemical data indicate that these intervals were not deposited under persistently anoxic bottom‐water conditions.…”

Trace fossils from the Lower Muschelkalk of Raciborowice Górne (North Sudetic Synclinorium, SW Poland) and their palaeoenvironmental interpretation

“…It is usually found in oxygenated substrates Pruss and Bottjer 2004;Uchman 2004, 2010;Giannetti and McCann 2010). Thus, the presence of Thalassinoides is considered as a reliable indicator of well-oxygenated conditions (Savrda and Bottjer 1986;Ekdale and Mason 1988;Savrda 2007;Boyer and Droser 2011;Buatois and Mángano 2011;Hofmann et al 2011).…”

Micro-trace fossils reveal pervasive reworking of Pliocene sapropels by low-oxygen-adapted benthic meiofauna

“…For example, incompletely laminated or homogeneous intervals are commonly interbedded with laminated sediments at a cm to 10 s of cm scale 54 . They are enigmatic as they often lack obvious macrofaunal trace fossils, but since these homogeneous intervals are sometimes transitional to laminated and macroscopically bioturbated intervals, they are generally interpreted to be of macrofaunal origin, produced under bottom waters sufficiently different from the laminated intervals to permit comprehensive macrofaunal bioturbation, but without preservation of trace fossils 54 . Alternatively, they are interpreted as the products of mass flows and sediment liquefaction, or simply a lack of sediment compositional change to define laminae 55 .…”