June 28. Corals

Lithostrotion basaltiforme

The extinction at the end of the Devonian decimated reef-building organisms such as corals. Extinction events typically see a rebound of life following the relatively brief period of extinction, and during the Mississippian corals did begin to make a comeback. We talked about the relatively small carbonate build-ups called Waulsortian Mounds on June 11. Those piles of mud may have been stabilized by the skeletons of animals such as crinoids and corals, but they were nothing like the huge edifices constructed by animals during the
Devonian, and the even larger ones to come in the Permian and even today.  

Corals were still abundant in those warm Mississippian seas, and both colonial and solitary types were common. Tabulate corals were colonial, tall columns growing together with horizontal segments, tabulae, providing the platform on which the individual zooids lived. One species, named Lithostrotion basaltiforme, has a remarkably similar appearance to the polygonal columns of basalt that form as a result of cooling – though the corals are much smaller.

There are biologic features in the Mississippian that can truly be called reefs, although purists might call them bioherms, meaning a relatively tall structure that might be a pile of material or a construction like a true reef, or a combination of both. The words “reef” and “bioherm” are sometimes used almost interchangeably, but to me a reef means a relatively large structure that is mostly composed of the actual skeletons of the animals that lived there, rather than a pile of debris. I’m not a specialist in this area, so take that as just my opinion about the terminology.

—Richard I. Gibson

Drawing from an old text (public domain)

May 10. Corals and the days of our lives

Corals are sensitive little critters. Little, as individuals, but their colonies can become immense, hundreds of miles long and many hundreds of feet high. Like brachiopods, corals thrived in the warm tropical seas that covered much of the combined North American-European continent of Laurasia. 

Given that I grew up in Michigan, it may not surprise you to hear that one of my favorite Devonian corals is a colonial coral called Hexagonaria. The individual chambers, maybe a half-centimeter across, are typically hexagonal (or pentagonal, or heptagonal) in outline, but many hundreds or thousands grew together to make the whole colony and eventually to contribute to reefs.

Petoskey stone (Hexagonaria), photo by jtmitchcock

The Devonian rocks of Michigan crop out in the northwestern part of the Lower Peninsula, around Traverse City. Wave-rounded cobbles of hexagonaria colonies are plentiful on beaches around the town of Petoskey, where they are called Petoskey Stones. They’re often already well rounded and smooth, but collectors sometimes polish them to bring out the interlocking hexagonal patterns. The Petoskey Stone is the state fossil of Michigan, and they lived in Middle Devonian time, about 380 million years ago.

So back to the coral’s sensitivity. The coral animal, the little polyp, builds its home, whether a single cup or a colony or an entire reef, by secreting calcium carbonate, the mineral calcite, to build the structure it lives in. It only does that during the day and stops secreting at night. Why? Because most shallow-water corals live in a symbiotic relationship with photosynthetic algae. The algae provide the coral with carbon in the form of simple sugars like glucose, which the coral uses for energy and to make the calcium carbonate that builds the skeletal structure that houses the coral animal. In return, the coral passes nitrogen to the algae. The nitrogen comes from floating animals and other nutrients that the coral’s tentacles collect from the surrounding water. It’s a complex relationship that benefits both the coral and the algae, and it’s sensitive to light which is necessary for the algae to photosynthesize.

That process of secreting calcite with a daily break in secretion makes a line in the coral structure every day, like a tree’s growth rings. Seasonal variations also generate annual lines even more like annual tree rings. Knowing this, paleontologist John Wells, who worked at Cornell University back in 1963, counted the daily and annual layers in corals – and confirmed that there were more days per year during the Devonian Period than there are today. About 400 rather than 365. And that count has decreased over time. This confirmed something that was already known, that the earth’s rotation is slowing down. Similar layering in other animals, such as mollusks, indicates the same thing.

Wells’ simple study, counting growth rings in fossil corals, has repercussions far beyond the idiosyncrasies of coral life. For example, it implies that the earth had its moon during the Devonian, because calculations of the count of days based on tidal drag came up with 399 days for the Devonian year – essentially identical to the 400 Wells estimated based on coral rings. And knowing the length of the year – when you are talking about millions of years – had an impact on the way we understand cyclic sedimentation patters and even calculations of the evolution of the earth’s orbit.

All this means that Devonian days were a little over 21 hours long, rather than the 24 of today, and in the future the day will be even longer, and the year will have fewer days. But never fear, you won’t need a new calendar for a long time. It’ll be more than 17 million years before the year is down to 364 days.—

—Richard I. Gibson

Further reading
Days are getting Longer
John Wells

Photo by jtmitchcock via Wikipedia under GFDL license 

April 23. Pinnacle Reefs

Coral reefs developed around the margins of the Michigan Basin during the Silurian, but even in deeper water some corals thrived. They grew upward in cylindrical, column-like structures, keeping pace with the rate of rising sea level, like modern atolls. Some reached heights of a few hundred feet, and because of their shape they’re called pinnacle reefs. 

Pinnacle reefs are especially common in a northeast-southwest trending zone across northern Lower Michigan. A belt that’s about 150 miles long and 40 miles wide. The reefs probably grew on a linear break in the sea floor – there’s some evidence in geophysical data for such a break, which would have been the easiest place for corals to get started because it was somewhat shallower than the deepest part of the basin, to the south.

When the tall cylindrical reefs were eventually buried beneath later sediments including salt as the water evaporated, the porous limestone structures built by corals became excellent traps for oil and gas. The surrounding sediment is the seal, the stuff that keeps the oil and gas from escaping from the reservoir, which is that tall little reef. They are usually no more than a half-mile across, but there are more than 700 of them across northern Lower Michigan that have been found to contain oil and gas. A typical oil field there contains around a half-million barrels of oil, which is maybe 45 minutes worth of oil consumption in the whole United States today. To have a good oil reservoir you need good porosity – that’s the holes in the rock where the oil is found – and good permeability, which is how interconnected the pore spaces are. These little reefs have both – they average about 6% pore space, but some are as much as 37% - that’s really huge, more than a third of the rock. The permeability can be as much as 500 times that of a “good” oil reservoir.

Most of those little fields were discovered after 1970, which was the peak of oil production in the United States. In 1970, the U.S. produced around 10 million barrels of oil per day. Today, even with the dramatic additions in production from North Dakota, the U.S. only produces about 7.9 million barrels per day – but that’s significantly up from the 5 million that was usual through most of the 1990s and 2000s.

There are Silurian pinnacle reefs in Illinois, Indiana, Ohio, and Ontario as well, and some contain oil, but the reefs in Michigan are the richest.

* * *

Today’s birthday is William Dean Thornbury, born April 23, 1900, in English, Indiana. He became a professor of Geomorphology at Indiana University and wrote the textbook that was used in geomorphology classes across the United States for decades. Geomorphology is the study of landforms and the processes that create them.

—Richard I. Gibson

Links:
Pinnacle reefs 
Porosity and permeability 
Map from Michigan DEQ

April 7. Coral reefs

Corals are among the animals that got their start in the Cambrian explosion and expanded during the Ordovician diversification. During the Silurian, corals began to make reefs.

There are reefs and reefs, of course – a small hill built up by the calcareous homes of corals and other animals might only be a few meters or tens of meters high. Not the Great Barrier Reef, for sure. But by Silurian time, these constructs were becoming larger and larger.

Corals are filter feeders, and many of them are colonial, meaning that the individual polyps can’t live on their own, but survive only as part of the colony. They secreted calcium carbonate, the mineral calcite, to make little cavities in which they lived. The individuals, called corallites, might only be a fraction of a centimeter across, but many thousands of them would form a single colony. Add a few thousand colonies together, and you start to have a reef. Corals weren’t the only inhabitants, or even the only ones helping to construct the reef. Bryozoans, brachiopods, crinoids, sponges, even calcareous algae and many more critters were there, contributing to the structure. Just as today, a reef was a complex, diverse, and highly supportive environment for life. Things like trilobites were not contributing directly to the reef edifice, but they were abundant enough that when they died, their bodies certainly contributed to the mass of the reef.

Also as today, Silurian reefs were located in the tropics. Because the continents were in different positions then, that means that most of the tropical Silurian reefs are in what is now North America and Europe, as well as parts of China and elsewhere in Asia. The U.S. Midwest has some of the biggest Silurian reefs.

Rugose corals

The shallow Silurian sea that covered much of central North America wasn’t absolutely uniform or flat. You might recall back in the Ordovician we talked about the warping of the continental crust that created an uplift, the Cincinnati Arch, and some deeper basins, especially the Michigan and Illinois Basins. It was in the shallow zones along the flanks of those deeper basins that Silurian reefs developed. Some of them reached sizes of a few miles in length and maybe a hundred feet high, but together, such reef systems were probably many tens of miles long, and were effectively barrier reefs in the modern sense.


Two varieties of coral developed during the Silurian. Tabulate corals are extinct, but they were plentiful then. They are colonies of many individuals, each of which builds its own little cell upward by creating a tabula – a table – upon which the animal sits. The lower segments are abandoned as the colony grows up and up. Tabulates are the most common coral in Silurian reefs.

Another coral that flourished during the Silurian was the rugose corals. Rugose means wrinkled, a good description of the outer walls of these corals. They were mostly solitary, with individuals getting up to 10 centimeters long, and rarely up to a meter. The fossil skeletons of rugose corals typically have a tapering horn-like shape, giving the common name horn coral to this group. The pointed end was down, attached to the sea floor, and the animal’s tentacles extended out of the wider, open end. They were not colonial in the way tabulate corals were, but they often grew close together enough to contribute to reef-making. They often had vertical segmentations, called septae, that divided the internal space into four chambers. The individual animals, the corallites, were often quite a bit larger than those of the tabulate colonies. Rugose corals are also extinct.

We’ll talk about a special kind of reef – pinnacle reefs – later in the Silurian.

—Richard I. Gibson

Image of rugose corals from Kunstformen der Natur (1904; public domain)


Further reading:
Stephen Hui Geological Museum