Archive for April, 2017

Wooster’s Fossils of the Week: Sponge and clam borings that revealed an ancient climate event (Upper Pleistocene of The Bahamas)

April 28th, 2017

This week’s fossils celebrate the publication today of a paper in Nature Geoscience that has been 20 years in the making. The title is: “Sea-level oscillations during the Last Interglacial highstand recorded by Bahamas coral”, and the senior author is the geochronological wizard Bill Thompson (Woods Hole Oceanographic Institution). The junior authors are my Smith College geologist friends Al Curran and Brian White and me.

The paper’s thesis is best told with an explanation of this 2006 image:
This photograph was taken on the island of Great Inagua along the coast. The flat dark surface in the foreground is the top of a fossil coral reef (“Reef I”) formed during the Last Interglacial (LIG) about 123,000 years ago. It was eroded down to this flat surface when sea-level dropped, exposing the reef to waves and eventually terrestrial weathering. The student sitting on this surface is Emily Ann Griffin (’07), one of three I.S. students who helped with parts of this project. (The others were Allison Cornett (’00) and Ann Steward (’07).) Behind Emily Ann is a coral accumulation of a reef (“Reef II”) that grew on the eroded surface after sea-level rose again about 119,000 years ago. These two reefs show, then, that sea-level dropped for about 4000 years, eroding the first reef, and then rose again to its previous level, allowing the second reef to grow. (You can see an unlabeled version of the photograph here.) The photograph at the top of this post is a small version of the same surface.

The significance of this set of reefs is that the erosion surface separating them can be seen throughout the world as evidence of a rapid global sea-level event during the Last Interglacial. Because the LIG had warm climatic conditions similar to what we will likely experience in the near future, it is crucial to know how something as important as sea-level may respond. The only way sea-level can fluctuate like this is if glacial ice volume changes, meaning there must have been an interval of global cooling (producing greater glacial ice volume) that lowered sea-level about 123,000 years ago, and then global warming (melting the ice) that raised it again within 4000 years. As we write in the paper, “This is of great scientific and societal interest because the LIG has often been cited as an analogue for future sea-level change. Estimates of LIG sea-level change, which took place in a world warmer than that of today, are crucial for estimates of future rates of rise under IPCC warming scenarios.” With our evidence we can show a magnitude and timing of an ancient sea-level fluctuation due to climate change.

Much of the paper concerns the dating techniques and issues (which is why Bill Thompson, the essential geochronologist, is the primary author). It is the dating of the corals that makes the story globally useful and significant. Here, though, I want to tell how the surface was discovered in the first place. It is a paleontological tale.

In the summer of 1991 I worked with Al Curran and Brian White on San Salvador Island in The Bahamas. They were concentrating on watery tasks that involved scuba diving, boats and the like, while I stayed on dry land (my preferred environment by far). I explored a famous fossil coral exposure called the Cockburntown Reef (Upper Pleistocene, Eemian) that Brian and Al had carefully mapped out over the past decade. The Bahamian government had recently authorized a new harbor on that part of the coastline and a large section of the fossil reef was dynamited away. The Cockburntown Reef now had a very fresh exposure in the new excavation quite different from the blackened part of the old reef we were used to. Immediately visible was a horizontal surface running through the reef marked by large clam borings called Gastrochaenolites (see below) and small borings (Entobia) made by clionaid sponges (see the image at the top of this post).
Inside the borings were long narrow bivalve shells belonging to the species Coralliophaga coralliophaga (which means “coral eater”; see below) and remnants of an ancient terrestrial soil (a paleosol). This surface was clearly a wave-cut platform later buried under a tropical soil.


My colleagues and I could trace this surface into the old, undynamited part of the Cockburntown Reef, then to other Eemian reefs on San Salvador, and then to other Bahamian islands like Great Inagua in the far south. Eventually this proved to be a global erosion surface described or at least mentioned in many papers, but its significance as an indicator of rapid eustatic sea-level fall and rise was heretofore unrecognized. Finally getting uranium-thorium radioactive dates on the corals above and below the erosion surface placed this surface in a time framework and ultimately as part of the history of global climate change.

This project began 25 years ago with the discovery of small holes left in an eroded surface by humble sponges and clams. Another example of the practical value of paleontology.

References:

Thompson, W.G., Curran, H.A., Wilson, M.A. and White, B. 2011. Sea-level oscillations during the Last Interglacial highstand recorded by Bahamas coral. Nature Geoscience (DOI: 10.1038/NGEO1253).

White, B.H., Curran, H.A. and Wilson, M.A. 1998. Bahamian coral reefs yield evidence of a brief sea-level lowstand during the last interglacial. Carbonates and Evaporites 13: 10-22.

Wilson, M.A., Curran, H.A. and White, B. 1998. Paleontological evidence of a brief global sea-level event during the last interglacial. Lethaia 31: 241-250.

[Originally posted September 11, 2011. Some updates and editing.]

Expanding Horizons by Mapping the Seafloor

April 24th, 2017

Wooster, OH – Last weekend, The College of Wooster hosted the Expanding Your Horizons conference. About 240 fifth- and sixth-grade girls participated in hands-on science workshops on computer science, math, geology, chemistry, biology, physics, and neuroscience. This year, I went back to my roots in marine geology to run a workshop on how we see what’s on the seafloor.

Pre-workshop selfie, complete with “I love rocks” name tag and photo of the Alvin submersible to jumpstart our conversations.

I put together a version of this activity about how geologists “see” under ice, the ocean, or inside the Earth. Most of the girls guessed that we use sonar to measure the depth of the ocean floor, and this short video was helpful for understanding how sonar works. Each group of girls was given a shoebox containing a mystery letter. They used their “sonar straws” to probe the bottom of the shoebox. They plotted their measured depths on their grid and used their data to interpret the letter in the box.

Poking straws into boxes seems not-at-all scientific and maybe a little silly at first, but the girls starting making and testing hypotheses pretty quickly.

You can see the map of “hits” and “misses” as they record the results of their hypothesis testing.

We found that the easiest letters to identify were those that had right angles, like “I” and “E.” Letters with triangles (like “N”) or curves (like “S” and “C”) were harder to identify.

Along the way, we learned about reproducibility and sampling strategy. As it turns out, if your data point is wrong, or all of your data are clustered in one corner of the map, it’s hard to make an interpretation. Still, each session managed to collect enough data to interpret the word “S-C-I-E-N-C-E” when the groups brought their maps together.

We watched part of a video on women in oceanography and I told them about Deep Sea Dawn, an inspirational woman oceanographer who maps the ocean floor and builds Legos! The girls asked incredible questions about what it’s like to be out at sea and about my favorite rock (basalt, of course). Finally, we watched a video about how we shrink styrofoam cups when we conduct deep-sea research and I showed them some of the cups from my cruises.

Their enthusiasm and energy were the best reminders of why I do what I do. I’m so grateful to all of my colleagues and educators everywhere who work hard every day to inspire the next generation of young geoscientists.

Wooster Geologists participate in the historic March For Science on Earth Day, 2017

April 22nd, 2017

Wooster, Ohio — It was a chilly day downtown, but several hundred people gathered for the national March For Science. We were one of over 500 local events across the country advocating for science awareness, education and funding. Thank you very much for retired Wooster Professor of Biology Lyn Loveless for organizing such a complex meeting with speakers and break-out discussions in local businesses. It was a great success. Above are some of the signs held by children in attendance. Several Wooster Geologists were in the diverse crowd, and some participated directly.

One view of the attendees. We all see the distinctive profile of Dr. Wiles in the foreground. Kelli Baxstrom may recognize someone on the far right!

One of the speakers was ace Wooster physicist and former dean Dr. Shila Garg. Note her coat on this mid-April day.

I include this photo (taken by Wooster political scientist Matt Krain) of Dr. Wiles and me to show my Paleontological Society colleagues that I wore The Shirt, even if no one noticed under the jacket.

One of the break-out sessions was on climate change. Greg Wiles and Clara Deck (’17) did great outreach work explaining their research to the large gathering. Wooster’s paleoclimate and climate change research and education is making a difference. Visit the Tree-Ring Lab website to see more details about the operation.

It was an inspiring afternoon, especially seeing the many young scientists and scientists-to-be who participated. Of course, for someone my age it is astonishing that we have to advocate for something so self-evidently beneficial as science, but such are our times.

Wooster’s Fossil of the Week: A Biserial Graptolite (Middle Ordovician of Tennessee)

April 21st, 2017

This week’s fossils are graptolites (from the Greek for written rocks) I found many years ago in the Lebanon Limestone near the town of Caney Springs south of Nashville, Tennessee. They are of the genus Amplexograptus and probably belong to the species A. perexcavatus (Lapworth, 1876).

Graptolites were colonial organisms consisting of hundreds and sometimes thousands of tiny zooids (individuals) connected together in a flexible proteinaceous skeleton (the rhabdosome). They first appeared in the Late Cambrian (around 510 million years ago) and disappeared forever in the Early Carboniferous (around 350 million years ago). Amplexograptus colonies were probably attached to floats so they could drift through the ancient oceans filtering out organic particles; they would be officially “passively mobile planktonic suspension feeders”. They belong to the Phylum Hemichordata, although there have always been disputes about their actual evolutionary relationships. This matters because graptolites are important index fossils for sorting out the age relationships of Lower and Middle Paleozoic rocks.

Graptolites are usually preserved as thin carbonaceous films on dark shales, making them rather hard to see (as my paleontology students will readily agree). The great 18th Century naturalist Linnaeus even said that they were “pictures resembling fossils rather than true fossils”. Sometimes, though, they are found in lighter-colored rocks like limestones, as above. Goldman et al. (2002) found Amplexograptus in limestones preserved in three dimensions, possibly because the limestones were cemented early around them before they collapsed with decay. They even studied this same species from the Lebanon Limestone. The 3-D preservation allows for a much more detailed analysis of the tiny cups (thecae) which held the individual zooids. It is possible that I could dissolve the limestone shown above and retrieve some delicate three-dimensional graptolites — but I could also just as easily destroy them.

Amplexograptus perexcavatus was originally described in 1876 by the famous geologist Charles Lapworth (1842-1920), who referred it to the genus Diplograptus. Actually, he had two species in his D. perexcavatus group, so it took some taxonomic detective and legal work to fix the current naming system. Lapworth, who I’ve figured below with an inset of his not-very-helpful diagram of the original D. perexcavatus, is well known by paleontologists for his work with graptolites as index fossils. Scientists and historians of science know him as the man who invented the Ordovician Period in 1879 to solve a bitter dispute between Roderick Murchison and Adam Sedgwick who each claimed the same rock interval in Wales for the Silurian and Cambrian periods respectively. Lapworth’s primary biostratigraphic argument for the Ordovician as a separate period was the distribution of graptolites, including our friend Amplexograptus perexcavatus. (Murchison and Sedgwick were long gone by the time their dispute was settled.)

(Charles Lapworth. Image courtesy of The Lapworth Museum of Geology.)

References:

Goldman, D., Campbell, S.M. and Rahl, J.M. 2002. Three-dimensionally preserved specimens of Amplexograptus (Ordovician, Graptolithina) from the North American mid-continent: taxonomic and biostratigraphic significance. Journal of Paleontology 76: 921-927.

Lapworth, C. 1876. The Silurian System in the South of Scotland, p. 1–28. In: Armstrong, J. Young, J. and Robertson, D. (eds.), Catalogue of Western Scottish Fossils. Blackie and Son, Glasgow.

[Originally posted August 28, 2011]

Wooster’s Fossil of the Week: A Conulariid (Lower Carboniferous of Indiana)

April 14th, 2017

I have some affection for these odd fossils, the conulariids. When I was a student in the Invertebrate Paleontology course taught Dr. Richard Osgood, Jr., I did my research paper on them. I had recently found a specimen in the nearby Lodi City Park. It was so different from anything I had seen that I wanted to know much more. I championed the then controversial idea that they were extinct scyphozoans (a type of cnidarian including most of what we call today the jellyfish). That is now the most popular placement for these creatures today, although I arrived at the same place mostly by luck and naïveté. (I love the critical marks in that word! And yes, I always have to look them up.)

The specimen above is Paraconularia newberryi (Winchell) found somewhere in Indiana and added to the Wooster fossil collections before 1974. (The scale below it is in millimeters.) A close view (below) shows the characteristic ridges with a central seam on one of the sides.
Conulariids range from the Ediacaran (about 550 million years ago) to the Late Triassic (about 200 million years ago). They survived three major extinctions (end-Ordovician, Late Devonian, end-Permian), which is remarkable considering the company they kept in their shallow marine environments suffered greatly. Why they went extinct in the Triassic is a mystery.

The primary oddity about conulariids is their four-fold symmetry. They had four flat sides that came together something like an inverted and extended pyramid. The wide end was opened like an aperture, although sometimes closed by four flaps. Preservation of some soft tissues shows that tentacles extended from this opening. Their exoskeleton was made of a leathery periderm with phosphatic strengthening rods rather than the typical calcite or aragonite. (Some even preserve a kind of pearl in their interiors.) Conulariids may have spent at least part of their life cycle attached to a substrate as shown below, and maybe also later as free-swimming jellyfish-like forms.

It is the four-fold symmetry and preservation of tentacles that most paleontologists see as supporting the case for a scyphozoan placement of the conulariids. Debates continue, though, with some seeing them as belonging to a separate phylum unrelated to any cnidarians. This is what’s fun about extinct and unusual animals — so much room for speculative conversations!

[Thanks to Consuelo Sendino of The Natural History Museum (London) for correcting the age range of these fascinating organisms.]

References:

Hughes, N.C., Gunderson, G.D. and Weedon, M.J. 2000. Late Cambrian conulariids from Wisconsin and Minnesota. Journal of Paleontology 74: 828-838.

Van Iten, H. 1991. Evolutionary affinities of conulariids, p. 145-155; in Simonetta, A.M. and Conway Morris, S. (eds.). The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge University Press, Cambridge.

[Modified from an original post on July 31, 2011]

Wooster’s Fossils of the Week: Bivalve escape trace fossils (Devonian and Cretaceous)

April 7th, 2017

It is time again to dip into the wonderful world of trace fossils. These are tracks, trails, burrows and other evidence of organism behavior. The specimen above is an example. It is Lockeia James, 1879, from the Dakota Formation (Upper Cretaceous). These are traces attributed to infaunal (living within the sediment) bivalves trying to escape deeper burial by storm-deposited sediment. If you look closely, you can see thin horizontal lines made by the clams as they pushed upwards. These structures belong to a behavioral category called Fugichnia (from the Latin fug for “flee”). They are excellent evidence for … you guessed it … ancient storms.
The specimens above are also Lockeia, but from much older rocks (the Chagrin Shale, Upper Devonian of northeastern Ohio). Both slabs show the fossil traces preserved in reverse as sediment that filled the holes rather than the holes themselves. These are the bottoms of the sedimentary beds. We call this preservation, in our most excellent paleontological terminology, convex hyporelief. (Convex for sticking out; hyporelief for being on the underside of the bed.)

The traces we know as Lockeia are sometimes incorrectly referred to as Pelecypodichnus, but Lockeia has ichnotaxonomic priority (it was the earliest name). Maples and West (1989) sort that out for us.
Uriah Pierson James (1811-1889) named Lockeia. He was one of the great amateur Cincinnatian fossil collectors and chroniclers. In 1845, he guided the premier geologist of the time, Charles Lyell, through the Cincinnati hills examining the spectacular Ordovician fossils there. He was the father of Joseph Francis James (1857-1897), one of the early systematic ichnologists.

References:

James, U.P. 1879. The Paleontologist, No. 3. Privately published, Cincinnati, Ohio. p. 17-24.

Maples, C.G. and Ronald R. West, R.R. 1989. Lockeia, not Pelecypodichnus. Journal of Paleontology 63: 694-696.

Radley, J.D., Barker, M.J. and Munt, M.C. 1998. Bivalve trace fossils (Lockeia) from the Barnes High Sandstone (Wealden Group, Lower Cretaceous) of the Wessex Sub-basin, southern England. Cretaceous Research 19: 505-509.

[Originally published January 29, 2012]