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 

May 9. A Devonian cratering event?

In the History of the Earth book, written back in 1994, I had a page for a possible Devonian cratering event when the earth-moon system might have undergone a greater-than-usual bombardment from meteorites. In 1994, Copernicus and other young craters on the moon were thought to have formed as recently as 350 to 400 million years ago, the Devonian. Now age dates put the formation of Copernicus at more like 800 million years ago, or even as old as a billion years ago – still young, as the moon goes, but back in the Precambrian in terms of earth’s time scales. 

Copernicus (NASA photo)

So was there an impact period during the Devonian on earth? It doesn’t look like it. There are a dozen or more known large features that represent Devonian impacts. Some, such as the Siljan Ring in Sweden, are pretty big – at 52 km or 32 miles across, it’s the largest known crater in Europe. Even after much erosion, the edge of the crater is still evident in the topography – a lake fills part of the crater rim. It formed about 377 million years ago, during the Late Devonian.

Some of the Devonian craters have fairly reliable age dates, ranging from about 396 million years ago to about 360 million years ago. Nine well-dated events in 36 million years. Given the error bars in the dating, three of the impacts, including the Siljan Ring, were about 380 million years ago, plus or minus five million years or so. So I don’t think we can call this anything like a “cratering event,” and we probably really can’t say the Devonian has a greater incidence of impacts than any other time in earth history.

We talked about one likely global impact event, 480 million years ago, during the Ordovician. That one was based on multiple fossil meteorites in many locations around the world and it seems reasonable to call that a specific event. 

There’s plenty of research going on to investigate the possibility that there may be some predictable periodicity to impacts on earth – obviously we have a vested interest in knowing that, even if it is on scales of many thousands of years or more. Check out a 2005 paper on this topic. This is challenging work in part because there’s a bias toward information about younger craters, simply because younger craters and associated information are better preserved and often better exposed than the old ones.

For now, I’m going to say that there was no Devonian cratering event, nor even any noteworthy increase in impacts then. But you have to realize two things – first, I’m not an expert on this, and even though I reviewed a lot of papers before making that statement, I could have missed something significant. The other thing to realize – to always realize – is the problem of sampling – the total number of impact craters known on earth is only a couple hundred. That’s really not enough to make any confident, far-reaching conclusions about periodicity or increased occurrence. And remember that the older you get, the more likely that all the evidence of an impact might be eroded away or subducted into the earth.

—Richard I. Gibson

Image credit: NASA

May 8. The Rhynie Chert

Given enough time, almost anything can happen, and even unusual things can get preserved in the rock record. Consider how uncommon geysers are on the earth today. Most of them are in Yellowstone National Park, and most of the rest are in Iceland and the Kamchatka Peninsula, with a very few in other places. If that sparse distribution is anything like it was in the past, then we would not expect to find the kinds of rocks deposited by geysers and other hot-water sources very often, and we don’t. But we do find some.

The type of deposit created by geysers or hot springs depends mostly on what kind of rock the hot water is dissolving. At Yellowstone, the hot waters mostly dissolve either limestone or volcanic rocks that are rich in silica. When dissolved limestone precipitates out of water, you get calcite and the kinds of features you see in caves – stalagmites, stalactites, and a whole range of other deposits. The terraces at Mammoth Hot Springs in Yellowstone are redeposited calcite, deposited by the water that dissolved limestone in the subsurface.

The stuff that makes most of the mounds around the geysers at Old Faithful is silica, silicon dioxide, essentially quartz, the most common mineral in the earth’s crust – or it might be amorphous opal, the same thing but with water in its molecular structure. It’s called siliceous sinter or geyserite. Note that the online Encyclopedia Britannica says Mammoth Hot Springs is composed of geyserite – but it’s not, it’s calcite.

So you could eventually get a big enough build-up of siliceous sinter to be retained in the rock record. And indeed there is one, from about 400 to 412 million years ago, early Devonian, at the village of Rhynie, Scotland, northwest of Aberdeen. The rock is called chert, which is the common term for very fine grained, cryptocrystalline (that means hidden crystals, they are so tiny) quartz.

The Rhynie Chert is quite remarkable simply because it has been preserved – and it is so well preserved that the actual geyser vents can be seen. They are the oldest geyser vents known.

But it’s even more significant than that. The Rhynie Chert is a lagerstatten – a collection of fossils that are simply amazing in their preservation and completeness. As the very fine silica was deposited, it coated and trapped plants and animals, preserving them with microscopic detail intact, right down to the cellular level. At Yellowstone you sometimes see plant stems encased in sinter. It’s like that, but with such detail that this is one of the most important localities in the world for understanding early Devonian life on land.

The Rhynie Chert is a small part of the complex system of rocks called the Old Red Sandstone that we talked about May 3. The location was well into the Caledonian Mountain belt, and the hot waters that made the chert were probably related to the mountain building and associated faulting.

The details of plant anatomy that are preserved in the Rhynie Chert are the best of this age anywhere. The plants are pretty simple – fungi, algae, and lichen, but also including some higher vascular plants with stems that are differentiated into xylem and phloem, the key structures in modern plants that allow them to transfer nutrients and water throughout their bodies. Some of them had scales extending from the body – not true leaves, but perhaps an early expression of structures that would evolve into leaves. In reconstructions, they look a little like small cacti with fleshy scales instead of spines.

Although animals are much less common in the Rhynie Chert than plants, it’s still the most diverse collection of terrestrial animals known from the Devonian or older. All the animals are arthropods, the group that includes arachnids, insects, crustaceans, eurypterids, and trilobites. One group of arachnids in the chert look a lot like spiders, and some that are about 6 or 8 millimeters long are so well preserved that things like their book-lungs, mouth parts, and muscle tendons can be examined. That simply does not happen with any kind of frequency. There are also small harvestmen – daddy-longlegs – and small shrimp, mites, centipedes, and other animals.

There’s an outstanding website with great information and photos of the Rhynie Chert and its fossils, provided by faculty at the University of Aberdeen, Scotland, and the National Museums of Scotland.

* * *

On May 8, 1902, Mont Pelée erupted, destroying St. Pierre, Martinique, in the West Indies. All but two of the 30,000 inhabitants were killed by pyroclastic flows that came into the city within minutes of the eruption. Pyroclastic means “fire-borne broken pieces,” and a pyroclastic flow or nouée ardente in French, meaning “burning cloud,” is a mass of hot incandescent gas, ash, rock, and debris that is dense enough to hug the land surface rather than being erupted high into the atmosphere. Pyroclastic flows can reach temperatures of 1,000 °C (1,830 °F) and can flow as fast as 100 miles per hour or more. St. Pierre didn’t have a chance.

—Richard I. Gibson

Rhynie Chert website