In one of my last blogs, I addressed the answers I got from the qualified geologist who is hired by Scandivanadium, i.e. whether bottom water conditions were oxygen poor or not during the formation of the Dictyonema Shale and whether a benthic (bottom-living) bivalve fauna could exist or not.
To find out more about these issues I contacted a palaeontologist, whose research is directed at the fauna in the Alum Shale, and also a geochemist, who has extensively published about the Alum Shale. And here are their answers:
Answer 1: “The Dictyonema Shale forms the uppermost part of the Alum Shale formation. It is very similar in appearance to the underlying Alum Shale, but it has a distinctly different fauna, which is overall dominated by planktonic graptolites. There are, however, a few horizons, where a benthic fauna appears. Bivalves are not present in the Dictyonema Shale, but in some horizons brachiopods with phosphate shells can be found. It is difficult to say how oxygenated the bottom waters were and how the conditions varied in this former ocean. What we know is that bottom waters were more oxygenated during the middle Cambrian, and that conditions gradually became anoxic. When the Dictyonema Shale was deposited, bottom water conditions were very likely anoxic to dysoxic. However, there is a large variability in the depositional environment as the Alum Shale in Skåne was deposited on the outer shelf, while the middle Swedish Alum Shale was deposited on the inner shelf.” (my free translation from Swedish)
These differences in depositional environment have led to the distinct geochemical differences that are observed in the Alum Shale in Närke and Västergötland as compared to the Skåne Alum Shale. For example, the uranium content is much higher in the Närke and Västergötland shales than in those in Skåne. But having said this, I need to add that the uranium content in the Alum Shale in Skåne is still distinctly higher than in the underlying sandstone and in the overlying shale and limestone.
Answer 2: “The Dictyonema Shale is part of the Alum Shale. However, there are differences in respect to geochemistry and fauna between the Cambrian Alum Shale and the lower Ordovician Dictyonema Shale“.
“The Alum Shale represents about 30 million years of time and during these 30 million years the depositional environment has without doubt varied. At some point, bottom water conditions were euxinic, but in the middle part of the Dictyonema Shale, brachipods were living on the sea floor, suggesting higher oxygen availability. Probably oxygen levels gradually improved during Dictyonema Shale time“.
More and more, I am understanding how complicated it is to reconstruct the depositional environment and the paleogeography of these 500 million year old rocks, and the former life that existed in this Cambrian-Ordovician sea. These rocks that once had been deposited as clay and mud, gradually hardened due to overburden, were buried deep below other rocks, then lifted up and eroded and buried again and lifted up again. And some time during this long long journey, dolerite dykes intruded into the shales … but more about these another time. Being able to know so much about the former depositional environment of the Alum Shale and of the geochemistry of the former ocean is fantastic, isn’t it?
But now we know for sure that the Dictyonema Shale is an Alum Shale. Two experts have confirmed this! We also know that bivalves did not exist. Bivalves are mollusks with two symmetrical shells. The animals that did exist were brachiopods, which do not have symmetrical shells and which are not bivalves. But in any case, these animals were living on the sea floor, they were actually anchored to the sea floor. And for being able to exist there, they needed oxygen. Thus, where brachiopods are found in the Dictyonema Shale, there was oxygen in the bottom water 490 million years ago. But – brachiopods are only present in some levels, and where graptolites dominate, bottom water conditions were deprived of oxygen. Generalizing and looking at the Dictyonema Shale as being just the same thing is certainly not a good idea, because there is great spatial and temporal variability. And this variability is something even Scandivanadium needs to consider.
Barbara Wohlfarth 