Monday, July 20, 2015

Week 4 Recap

On Monday, I finished sifting all of our dried sediment samples so that Jim Ingle can more easily identify foraminifera in our followup meeting. I also looked at the coral and shells samples under a microscope, which led me to subject them to one more cleaning, since there were still visible detrital sand and silt grains.

I devoted most of Tuesday and Wednesday to reading papers and reviewing equations. I also re-derived the equations for the Osmond 3D isochron, which helped clarify several points of confusion.

On Friday we met with Peter Blisniuk to talk about the potential application of oxygen isotopes on our carbonates. Delta-18-O in marine carbonates is dependent on water temperature, meaning it serves as a proxy for sea level in deep-water environments but is more difficult to interpret in shallow-water settings where high-frequency temperature flux is significant.

While Karrie is out of town and the U/Th prescreening lab work is on hold, I will powder and weigh out carbonate samples for oxygen and carbon isotope analysis to be performed next week.

I selected 30 different samples (the most that can be run for oxygen isotopes in one day) and photographed the individual shells and corals with the microscope-camera facility in the Payne lab.

I read and took notes on the following papers:

Muhs, D.R., Kennedy, G.L., and Rockwell, T.K., 1994, Uranium-Series Ages of Marine Terrace Corals from the Pacific Coast of North America and Implications for Last-Interglacial Sea Level History: Quaternary Research, v. 42, p. 72-87.

Kaufman, A. and Broecker, W.S., 1965, Comparison of 230-Th and 14-C Ages for Carbonate Materials from Lakes Lahontan and Bonneville: Journal of Geophysical Research, v. 70, p. 4039-4054.

Magnani, G., Bartolomei, P., Cavulli, F., Esposito, M., Marino, E.C., Neri, M., Rizzo, A., Scaruffi, S., and Tosi, M., 2007, U-series and radiocarbon dates on mollusc shells from the uppermost layer of the archaeological site of KHB-1, Ra's al Khabbah, Oman: Journal of Archaeological Science, v. 34, p. 749-755.

Szabo, B.J., 1979, 230-Th, 231-Pa, and open system dating of fossil corals and shells: Journal of Geophysical Research - Oceans, v. 84, p. 4927-4930.

Tuesday, July 14, 2015

Week 3 Recap

On Monday, Chris and I went to the USGS in Menlo Park and split all the larger cores from the June cruise. The core splitter is a relatively simple device, but it works much better than cutting the cores free-hand or using a bandsaw. It consists of a frame that holds the core in place while a vibratory cutter and a razor blade pass down the core liner. We were able to finish the job in only a few hours. Half of each core was kept as an archive and the other half was purposed for subsampling of carbonates.

I photographed each core and looked for carbonates. Except for two, each core yielded only one sample, since there were no defined carbonate horizons.

One core had two clear sediment layers (mud overlaying sand), and I was able to take a sample from each of those layers. I subsampled the longest core (51") in 8 different places at similarly spaced intervals. On this core, the upper five sites yielded large shells whereas the lower three yielded only small shell fragments. However, one of the fragments from the very bottom of the core may be large enough to date, which is promising.

I cleaned all of these new samples according to the methods explained at length in the Week 1 Recap.

I sorted through the grab samples collected during Chris and Simon's cruise last summer, as well. I took sediment samples from each core and dried them in Mitchell A55. The next step is to sift them, as I did with the other sediment samples last week. I was fortunate to find two balanophyllia corals that had been overlooked in one of the sample bags. Though one is too small to use for U/Th dating, it is a good candidate for radiocarbon dating.

I met with Karrie in order to discuss the next step with lab work. We calculated that dissolving 1 mg of carbonate from potential candidates for U/Th dating would be sufficient for each of the prescreening measurements (on the EM1 ICP-OES to measure Ca, Sr, Ba, Mg, Mn, & Al, and on the Nu Attom ICP-MS to measure U concentration). Karrie scheduled me instrument time on the EM1 ICP-OES on August 4.

On Thursday, I spent the day near Watsonville with Rosemary Knight and members of the Environmental Geophysics group observing a cone penetrometer test at a site I investigated in GP 190 (Near-surface Geophysics) during spring quarter.

Monday, July 6, 2015

Week 2 Recap

Simon, Chris, and I met with Jim Ingle to get his opinion on our samples, particularly the corals. He identified three of our corals as a shallow water variety called Balanophyllia elegans. He pointed out that two of these corals were in pristine shape, likely indicating that they are very young. He suggested that the rest of our corals were not actually corals but instead the calcareous deposits of an algal mat. While this means they are worse candidates than corals for U-series dating, it still may be possible to date these using an isochron method, as explored in the papers I read during week 1.

During week 1, I removed samples of mud and sand from every bagged sample. This week I dried these samples in Mitchell A55 and sifted them through a 150 micron sieve. We will meet with Jim Ingle again during week 3, and he will look at the forams in the samples I sifted in order to estimate water depth at the time of deposition.

I finished working through the cores that Chris obtained from USC's core repository. The majority of the cores we were looking for were missing, and most cores that we did have were poor in carbonates. However, three of these cores turned up substantial carbonate samples. Additionally, many of these cores had foram-rich sand, which we will also show to Jim Ingle next week.

I started going through the cores we collected earlier in June. I broke open the cores that displayed no strata or had very little material. These I sorted through for shells or corals. No corals were present, however I found several large shells in different cores. These cores were placed in bags and returned to cold storage in Mitchell. We may dry them in A55 and pick out shell fragments, though that may not be necessary.

On Monday, Chris and I will go to the USGS core lab in Menlo Park to split the remaining cores. After that I will characterize the material and have a better idea of what samples I can move forward with dating.

I skipped town on Friday and headed home for a long Independence Day weekend.

I read and took notes on the following papers:

Osmond, J.K., May, J.P., and Tanner, W.F., 1970, Age of the Cape Kennedy barrier-and-lagoon complex: Journal of Geophysical Research–Planets (1991–2012), v. 75, p. 469–479.

Kaufman, A., Broecker, W.S., Ku, T.L., and Thurber, D.L., 1971, The status of U-series methods of mollusk dating: Geochimica et Cosmochimica Acta, v. 35, p. 1155-1183.

Monday, June 29, 2015

Week 1 Recap

I intend to use this blog to post summaries of what I've done each week as a sort of progress report.

I selected seven corals from bagged samples in Mitchell cold storage and moved them to Green 044 for cleaning. These were rinsed in DI water, sonicated in DI water for 10 minutes, rinsed in DI water, sonicated in ethanol for 3 minutes, rinsed in DI water, and finally sonicated in DI water for 15 minutes. I photographed these corals using the reflected light microscope in 320-222. These photographs will be an important record after the corals are dissolved for analysis. During photography I noticed that corals 1419A and 1420 were in need of further cleaning.

I also removed small samples of the mud, sand, or shell hash from all bagged samples. I viewed these under the reflected light microscope but was not able to take useful photographs because I am not familiar enough with foraminifera.

Simon and I met with Dave Mucciarone and Sverre LeRoy to discuss radiocarbon dating. I went with Sverre to the USGS core lab in Menlo Park, and she showed me their core splitter.

We agreed to postpone the next step in U/Th dating (sample dissolution) until we had determined whether radiocarbon dates at LLNL could be used to determine which corals were older than the LGM. We have not yet discussed in detail which samples will be sent to LLNL, nor have we discussed the timeline for dating mollusk shells (whether this will be done after corals or concurrently).

I read and took notes on the following papers:

Bischoff, J.L., and Fitzpatrick, J.A., 1991, U-series dating of impure carbonates: An isochron technique using total-sample dissolution: Geochimica et Cosmochimica Acta, v. 55, p. 543–554.

Luo, S., and Ku, T.L., 1991, U-series isochron dating: A generalized method employing total-sample dissolution: Geochimica et Cosmochimica Acta, v. 55, p. 555–564.

Ludwig, K., and Titterington, D., 1994, Calculation of 230-Th/U isochrons, ages, and errors: Geochimica et Cosmochimica Acta, v. 58, p. 5031–5042.