Methods
Sampling and Scale
We sampled beds below, within, and above each Kellwasser horizon. The two black shale horizons were sampled most densely, at intervals of 3 - 10 cm. Outside of the Kellwasser horizons, beds were sampled at 20 – 50 cm intervals. 12 beds were sampled in total. Each sample was divided, allowing both trace metal analysis and OWM analysis from the same sample.
Trace Metal Analyses
Whole-rock trace metal analysis was completed using standard methods and analyzed in solution on a quadrupole ICP-MS (Varian 820MS, now Analytik Jena) at SUNY Oswego. Approximately 400 mg of powdered sample was ashed at 850°C for 12 hours and then dissolved in a multi-acid total digestion procedure (HNO3,
HF, HCl). Measurements
of solutions of > 1 ng/g yielded average within-run uncertainties of < 3
% (1s) and mean
duplicability of sample solution analyses (repeat measurement of the same
solution) over the full range of concentration encountered was ±12 %. Mo values were targeted for this study as Mo values are enriched under reduced bottom water oxygen depositional conditions, and further, they have been shown to reliably record anoxic and euxinic conditions in both modern and ancient settings (Morford
and Emerson, 1999; Lyons et al., 2003; Scott et al., 2008; Lyons et al., 2009; Boyer et al ., 2011)
OWM Extraction and Analysis
The samples were processed using established techniques for organic microfossil extraction (Green, 2001). To summarize, rock samples were scrubbed to remove all modern organic contamination, crushed to pea-sized gravel, and subjected to hydrochloric acid maceration and hydrofluoric acid maceration. Nitric acid and heavy liquids were not used. The resulting product is termed the “macerate” and contained both OWMs and amorphous organic material.
Quantifying Microfossil Abundance
We developed a method based on a technique used to quantify pollen abundance in sediment cores (Ogden, 1986). Most simply, a known volume of the sample is mixed with a known concentration of marker grains, and by counting the ratio of fossils to marker grains, fossil abundance relative to the sample can be calculated. However, we could not mix the solid rock sample with a solution of marker grains for obvious reasons. Instead, we dried and weighed the macerate material before re-suspending the material in a mixture with a known concentration of polyethylene microspheres (produced by Cospheric). This mixture was mounted on a slide, and using a standardized search pattern on a mechanized microscope stage, the ratio of fossils to microspheres was quantified. This enabled us to calculate the abundance of fossils relative to the macerate material.
[insert equation]Since the macerate material is not a consistent fraction of the rock, variation in fossil abundance within the macerate does not necessarily reflect variation in fossil abundance within the rock. To calculate fossil abundance relative to rock mass, we approximate the fraction of the rock represented by the macerate. The maceration procedure is designed to retain insoluble organic material, including fossils, while removing the non-organic matrix of the rock. Therefore, the total organic carbon (TOC) in the rock is preserved in the macerate. However, the macerate also includes non-organic matter, such as quartz grains. Consequently, two carbon measurements are necessary to estimate the fraction the rock represented by the macerate: first, the total carbon (percent by mass) in the macerate, and total organic carbon (percent by mass) in the rock. Both measurements were made using a Flash Elemental Analyzer (Flash EA).
First, fossil abundance is scaled by total carbon in the macerate (Equation 3-2). Total carbon in the macerate is only organic carbon, as all inorganic carbonate material is eliminated during HCl maceration. We assume that all organic carbon initially in the rock persists into the macerate. By scaling fossil abundance per mg of macerate to fossil abundance per mg of carbon in the macerate, we are effectively scaling fossil abundance to the fraction of organic carbon (TOC) in the rock. After scaling fossil abundance by total carbon in the macerate, we scale fossil abundance by total organic carbon in the rock to get the total fossils per mg of rock (Equation 3-3).
[insert equations]
Quantifying microfossils morphology variation
The first ~100 fossils found (while counting the ratio of microfossils to microspheres) were also photographed. Using ImageJ software (Abràmoff et al., 2004), we measured the diameter of each fossil. For selected beds, qualitative data for each fossil was also recorded.
Results
I think there should be a bit of sed/description of the samples here, which I can do and included some generalized trace fossil stuff. Samples ranged from
Trace Metal Data
As we are thinking about which samples to use, all the WC samples are enriched=anoxic to dysoxic, but not euxinic, while all of the BCP and CAM samples are crustal values=oxic. This is consistent with CAM and BCP being grey and burrowed.
The samples from the Walnut Creek locality were all enriched in Mo with respect to average crustal values (2-3ppm Mcmaus ref), ranging from 12.0 to 86.2 ppm with an average value of 40.79 ppm. These values are all consistent with dysoxic, anoxic or even intermittently euxinic bottom water conditions, but are unlikely to represent persistently euxinic conditions as they do not reach 100 ppm (see Scott et al., 2008).