Abstract Black shales are thought to be deposited in anoxic settings. The reason to assuming this environment is based on the high amounts of organic matter in these rocks thought to only be preserved when there is no oxygen available. However, most recent settings in which black shales form are normal shelf settings and not anoxic. This discrepancy remains puzzling and, among others, bears the question of how organic matter reaches the seafloor unharmed and subsequently gets buried.

Recent observations show that marine snow is the main vehicle by which organic matter gets transported to the seafloor, independent of whether the water it settles through is oxic or anoxic. It is estimated that only about 3-4% of the organic matter traveling through the water column reach the seafloor and are preserved in the sediment; the large majority is consumed by organisms living in the oceans. There it awaits another challenge, which is surviving the sulfate reduction zone before it can be preserved in the rock record.

Looking at ancient shales (siliciclastic mudstones), the flakes of marine snow are not evident in the rock texture but likely form part of the matrix. However, other features are present that do shed some light on the question of whether the depositional environment contained oxygen or was entirely oxygen-depleted. Both burrows as well as fecal strings are frequent in shale successions likely indicating dysoxic and not completely anoxic conditions during deposition. As in recent oceans, the water column must therefore have been probably mostly if not entirely oxic, and turned only oxygen depleted at or around the sediment-water interface. Nevertheless, the burrows and fecal strings reflect some oxygen content even in the uppermost sediment layers, at least at times, that the producing organisms needed to breathe. Therefore, burrows are such important indicators in ancient sediments for detecting a rough estimate of its former oxygen content especially when no geochemical measurement of ancient sedimentary conditions is possible anymore.

Despite the burrows, there are some "real" fossils in supposedly anoxic shales such as the upper Bakken shale member. Brachiopods are common in places in this unit, and sponge remains are rare but do occur. Nevertheless, these organism remains are limited to certain stratigraphic intervals that are interpreted to be the most proximal shale environments. In the Bakken shales, these settings are very limited in vertical extent but other units such as the Ordovician Arnestad Formation in Norway, which is entirely black in outcrop consist entirely of this environment. These entirely bioturbated upper Ordovician shales show evidence of scours produced by eroding currents, and associated shell lags. At least for these proximal shale systems an overall oxic depositional environment above the reach of storms or storm-induced currents is assumed allowing for abundant bioturbation, and even the development of probably specialized brachiopod species.

About the Speaker Sven was born in Germany, and raised in Germany, Iran, and Argentina. He studied at the Universities of Clausthal-Zellerfeld in northern Germany for his Bachelor-equivalent degree in Geology, and continued at the University of beautiful Heidelberg in central-southern Germany where he completed his Diploma (MS-equivalent degree) with a study on the internal buildup of a fossil atoll in the breathtaking Italian Dolomites. Sven received his PhD from Technische Universit├Ąt Berlin, Germany, in 2000 for a basin analytical study of the Ordovician succession in southern Bolivia. After a five-year lecturer position at Technische Universit├Ąt Bergakademie Freiberg in southeastern Germany he was appointed Assistant Professor at Colorado State University in 2006, promoted to Associate Professor in 2010, and to Full Professor in 2016. Sven's areas of expertise are understanding sedimentary processes in carbonates and shales, and using them to reconstruct fossil depositional environments. His projects apply these models to characterize oil and gas reservoirs and to recognize fossil habitats of long extinct animal groups such as graptolites.