I spent most of the morning at the poster Anna Trochim and I did on diatoms from Carmel Church, Westmoreland, and Richmond County (at the top is Anna with our poster). I’ll go into more detail in a future post, but basically we took diatom samples from those three sites (I have whales from all of them) to try to determine the age of the deposits, as well as to build up a diatom flora list. As a secondary goal, we wanted to see if the diatoms could give us any paleoenvironmental clues. We were successful in that we were able to pin down the Carmel Church and Richmond County deposits to Bed 15 of the Calvert Formation (slightly younger than I had previously thought). Our resolution was not as good with Westmoreland, but we still narrowed it down to Beds 12-15 (previously published data there indicates that it’s Bed 14/15). We also found that species that can tolerate brackish water dominate at all three sites.
What really surprised us was how different the three sites are from each other. Carmel Church was much more diverse that the other sites (28 species, vs. 19 at Westmoreland and 12 from Richmond County), while Westmoreland had a much higher number of individual specimens. There were also differences in which taxa were dominant. Paralia sulcata (upper row, below) and Melosira westii (lower row, below) were common at every location (left, Carmel Church, middle, Westmoreland, right, Richmond Co.):
Yet Grammatophora marina was dominant at Carmel Church and not found at the other sites, while Coscinodiscus radiatus was dominant at Westmoreland but not found at the other sites (below, G. marina on the left, C. radiatus on the right):
In another poster that caught my attention, Claire Still and Rowan Lockwood looked at the relationship between sexual dimorphism in ammonoids and how long a taxon survives. Ammonoids were shelled cephalopods (squid) that first show up (I think) in the Devonian, and reached incredible levels of diversity in the Mesozoic before finally going extinct at the end of the Cretaceous. Many ammonoids are notable for being sexually dimorphic, meaning the sexes are different sizes and/or shapes, as in Didymoceras below (specimen from the Wyoming Dinosaur Center):
Still and Lockwood found that sexually dimorphic taxa tended to survive for longer than monomorphic taxa. It’s not clear why this is the case, as modern birds seem to show the opposite pattern. Incidentally, this is at odds with what Still and Lockwood said in their abstract. I think their abstract was based on preliminary data, and after submission they got more data that changed their conclusion (I’ve had this happen on some of my abstracts).
All the talks I attended today were in Session T19, “Geologic and Paleoenvironmental History of the Chesapeake Bay” (the same session my poster was in). John Bratton, VeeAnn Cross, and David Foster showed video 3D animations of the flooding and exposure of the Chesapeake Bay that were based on compilations of huge sets of data about the bay’s age and bathymetry. Chesapeake Bay is simply the flooded Susquehanna River Valley, and they showed how over the last 200,000 years the mouth of the river was pushed further and further south by the growing Delmarva Peninsula, gradually capturing other rivers such as the Potomac, Rappahannock, York, and James (which all originally flowed into the Atlantic independently).
Siddhartha Mitra et al. gave a fascinating talk about black carbon in bay sediments. Black carbon is essentially burned material like charcoal and soot, and it has different chemical signatures depending on the source (coal, wood, etc.). They found that, prior to the mid 1800’s, the amount of black carbon in the bay closely correlates with dry periods, presumably because of an increased incidence of forest fires during droughts. During the 1800’s, the relationship between black carbon and climate broke down, with the amount of black carbon staying pretty high, but still coming from burning wood. They believe this was due to increased use of railroads and steam ships, and increased smelting, largely due to the American Civil War.
A. L. Edwards et al. discussed an interesting direct dating technique called amino acid racemization (AAR). This technique uses the fact almost all amino acids (the building blocks of protein) have two different forms, called L (for levorotatory) and D (dextrorotatory). Organisms force all their amino acids into the L form, but when an organism dies, its amino acids start to move to the D-state (racemization). This happens at a predictable rate (that is influenced by local conditions), so if you know the D/L ratio you can calculate the age of the specimen. However, since local conditions can affect the rate of transformation, you have to have an independent initial date (often C14 decay) to calibrate the D/L racemization rate.
Edwards et al. were comparing a variety of calibration techniques using the mollusk Mulinia lateralis (below):
The various techniques included using the two shells from one individual (one shell for the C14, and the other for the AAR), one shell cut in half (half for each technique), and using separate shells from the same bed for C14 and AAR. Somewhat surprisingly (at least to me), all three techniques gave results that were statistically identical to each other.
Rowan Lockwood gave a talk on an issue that is at the heart of any study of paleoecology; in looking at a fossil assemblage, how do you know that the ratios of fossils that you find are similar to the ratios of the living organisms at that time? Because some organisms are more likely to fossilize than others, any fossil deposit is potentially a biased sample.
Rowan attempted to see how severe this bias is in Chesapeake Bay sediments. The Chesapeake Bay Program has collected 20 years of census data on the organisms living in the bay each year. She compared this data to dead assemblages from the same sites, and found that the live and dead assemblages were remarkably similar; 77% of the live species were also in the dead assemblage, and the relative abundances of taxa in each assemblage was still similar. At least in this instance, there does not appear to be a strong bias in the dead assemblage.
As is usually the case with these meetings, I find myself at the end thoroughly exhausted, but with all kinds of new ideas swirling around in my head. Congratulations to GSA for putting on a fine meeting!