The first chapter of my thesis has been published in Chemical Geology! You can read the full article HERE. I had the really cool opportunity to analyze some old archived samples from the Arabian Sea Oxygen Deficient Zone (ODZ). I analyze the dissolved metal concentrations in samples by preconcentrating them on an instrument called a seaFAST, so that the concentrations are within my limits of detection, and quantify on an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS). This method is really neat because it allows me to quantify MANY metals at no extra expense in cost or time. The Arabian Sea is one of 3 modern ODZs- an area in the ocean where oxygen is essentially 0. This means that there can be interesting patterns of trace metal chemicals found in these regions. For example, we often sea plumes of iron (Fe) under these low oxygen conditions, even though iron is otherwise limiting in the ocean. For my paper specifically, I focused on rare earth elements (REEs). These elements are often overlooked in typical mass-spec analyses, but they form a predictable trend in normal seawater. A normal seawater distribution of REEs showcases an enrichment in REEs as you move across the lanthanide series. A major exception of this is cerium, as cerium can form redox-dependent relationships with manganese, but other deviations from this trend can be useful in pinpointing various processes that are occurring. That being said, cerium is actually enriched at depths of low oxygen. This is because manganese oxides preferentially scavenge cerium. As these manganese oxides re-dissolve under low oxygen, they release cerium back into the water. This represents a weakening (i.e. higher value) of the cerium anomaly-- deviation from what the concentration of cerium should be as compared to its neighboring lanthanides. From the plot left, you can see that the highest concentrations of manganese (depicted by red) also coincide with the higher cerium anomaly value. However, the surface is also enriched in cerium. This represents aeolian (dust) deposition into the basin. Due to the chemical signals we see at the surface, it is likely that the dust was chemically modified desert varnish that dissolved in the surface ocean. But, the highest concentration of manganese is actually decoupled from the highest cerium anomaly value. This means it is not evidence for re-dissolution under low oxygen. Instead, there must be another source. This, and an anomaly in europium (where europium is elevated in comparison to its neighboring lanthanides), suggests hydrothermal input. There is a lot of hydrothermal activity in the area, and other common geochemical tracers point to this phenomenon as well, but it is still a little more puzzling. To sum up, my favorite figure was actually added during the editing portion. I had generated this during the ~6 months between submitting the paper and getting feedback from my anonymous reviewers. This plot shows how all the different variables are related to each other, and also leaves me with a lot of questions. If you read my last post, you know I recently returned from a cruise off the coast of Oregon (and TBW- to be written) is a second post about my SECOND cruise off the coast of Oregon. Here, there is seasonal hypoxia. This means oxygen gets low, but not quite as low as in places like the Arabian Sea. This project is mainly focused on iron....but like I mentioned earlier I can analyze many metals at once! I'm hoping to generate some more data to try to understand what is causing these different associations.
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December 2021
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