BRYN MAWR, PENNSYLVANIA–When we last saw Mae Kemsley (’16) and Meredith Mann (’16) in this blog, they were celebrating the end of their Senior Independent Study summer fieldwork on the coast of North Yorkshire, England. This weekend the three of us traveled to Bryn Mawr College and the geochemistry lab of Professor Pedro Marenco to start the geochemical analysis phase of our research. We learned a lot under Pedro’s kind and generous direction.
Both Mae and Meredith have belemnite fossils in their field collections. Meredith has just a few from the Passage Beds Member of the Coralline Oolite Formation (Upper Jurassic, Oxfordian); Mae has dozens from the Speeton Clay (Lower Cretaceous). A belemnite was a marine squid-like cephalopod that had a hard, bullet-shaped internal structure called a guard (shown above). These guards are made of almost pure calcite which took in trace elements from the seawater as they grew. The carbon and oxygen isotopes in their calcite crystals also reflect the isotopic composition of the seawater. These fossils are thus geochemical repositories from ancient seas. We are interested in what our belemnites tell us about the ambient chemical conditions in their environments, which in turn are proxies we can use to interpret paleotemperatures and other factors.
In our Wooster geology labs we cut small disks from a series of belemnites, then polished the surfaces and cleaned them thoroughly. We brought these prepared disks to Pedro’s lab in Bryn Mawr.
Mae is here in the Bryn Mawr petrography lab using a small drill to excavate fine calcite powder from the belemnite disks. This powder, measured in fractions of a gram, was then collected into sheets of weighing paper, folded like origami and taped to keep it in place.
Mae and Meredith are here weighing the powder samples with Pedro’s fancy balances. Each plastic sample vial had to be paced through an ion generator to reduce static charge and improve measurements to the microgram. A lot of chemwipes, weighing sheets, and gloves are used in the process to reduce contamination.
After dissolving the powder samples in acid, and then diluting the liquids in carefully-measured ways, we finally ended up with these precious tubes filled with essence de belemnite. We learned how much work goes into preparation of geochemistry samples — a lot!
The liquid samples are now ready for analysis in a device called an ICP-MS, which stands for Inductively Coupled Plasma Mass Spectrometer. This is the process and equipment Wooster geologists Mary Reinthal (’16) and Chloe Wallace (’17) described in their recent geochemistry blogpost. We’re doing the same thing: assessing the trace elements in our samples. Pedro will later run our samples through this magic machine and give us the results. We have a duplicate set of drilled belemnite powders to send to another lab for carbon and oxygen isotope analysis.
Thank you very much to our Bryn Mawr hosts Dr. Katherine Nicholson Marenco (’03) and Dr. Pedro Marenco. We are very much looking forward to our continuing collaboration. Thanks as well to Dr. Paul Taylor of the Natural History Museum in London who was our Essential Companion in the field.
I can’t wait to see the geochemical results.
Paul
Can you give us the details of what evidence we use to guarantee this is original calcium carbonate? It’s notoriously mobile because of its weak resistance to groundwater modification and recrystallization after burial. What evidence is used to suggest that there has been no modification since original seawater flavored its trace chemical and stable isotopic make-up?
Good question, Bill. First we look at thin-sections of the belemnites. We can visually detect some alteration of the calcite crystals in the centers (where there is a weakness often exploited by diagenetic fluids) but the crystals away from the center (and outer edge) are pristine. We will later confirm this with cathodoluminescence. Next we use the Mn/Sr ratio (from the ICP-MS analysis), which should be low with unaltered biogenic calcite. We also have the benefit of many studies on belemnite calcite that have established the parameters of isotopic work.
What about contamination from post-diagenetic fluid inclusions? I think this type of analysis begs for laser ablation or ion microprobe mass spectrometry to avoid some of these troublesome issues. I would think fluid inclusion pollution of the chemical signature would be a big problem for non-micron-scale analysis.
We’ll start with what we can do with the equipment available to us and following the procedures used by others in similar studies, then we’ll do an error analysis and assess the next possible steps to increase accuracy and limit contamination.
Don’t get me wrong, Mark, this is a fantastic experience for all the students and I’m not being critical, at all. I would just have them read Edwin Roedder’s pioneering USGS Prof. Paper 440-JJ, and write a few pages in their IS about the potential problems that fluid inclusions can cause, particularly in extraordinarily weak minerals like calcite that are prone to development of large numbers of tiny, healed-incipient-cleavage-plane, inclusions. It will give the students further insight into what can, and often does, go wrong in all sorts of geochemical environments/studies.
And, I fully understand the limitations of money and analysis. As I said, it’s a great experience for all the students to participate in this type of collaboration. All of you in the geo department do a tremendous job ensuring that the students get exposed to all sorts of interesting avenues of study, from the mentored field work, which is often far better than traditional field camp, to travel and exposure to all sorts of exotic equipment for analysis.