Investigating changes in single shell trace element chemistry of Neogloboquadrina dutertrei with increasing water depth in sediment traps from the Panama Basin
From Faith Schell
Hello, my name is Faith Schell, I’m a senior here at Oregon State University, and this is a snapshot of a project I’ve been working on with my advisor Dr. Jennifer Fehrenbacher titled “Investigating changes in single shell trace element chemistry of Neogloboquadrina dutertrei with increasing water depth in sediment traps from the Panama Basin”
Planktonic foraminifera are tiny marine protists that precipitate a calcite shell, and because of this, they’re great recorders of the geochemical compositions of their ambient environment- Fossil forams provide us with a better idea of the chemical profile of the water column in the paleo record VIA the geochemical composition of their shells, so understanding processes that can affect their shell geochemistry is necessary.
In this study, we analyzed shells from the species N. Dutertrei from sediment traps at three different depths in the Panama Basin. The shallow trap, placed at 890 meters, is well above the lysocline, which is the depth at which the dissolution of calcite increases dramatically and sits at about 2800m in the Panama Basin. The mid-depth trap is placed at 2590 meters, still slightly above the lysocline, and then the deepest trap is located at 3560 meters and sits below the lysocline. Each trap was equipped with six cups that rotated on a two-month basis for the year December 1979 to November 1980. There are two questions we’re attempting to address in this study: When and where does N. dutertrei form its final calcite crust, and is the trace element composition of the foraminiferal calcite altered with depth? This talk will mainly address our second question, not just for the sake of time, but because it produced our most interesting results.
To obtain our trace element ratios, we use Laser Ablation Inductively Coupled Plasma Mass Spectrometry or LA-ICP-MS for dutertrei shells for four cups at all three trap depths. We ran analyses for Magnesium, Barium, Strontium, Manganese, and many others, but the most significant results were present in Manganese, so that's what I’ll be focusing on in this talk.
The most interesting data we see in this study is our Manganese data: Concentrations increase with depth and are consistently elevated in our deepest trap. A 2020 study by Davis and Benitez-Nelson found that the Manganese from N. dutertrei shells from this sediment trap also increased with depth, confirming our results. Similarly, the Davis/Benitez-Nelson study uses LA-ICP-MS to obtain their trace element data, and here, we’re able to reproduce their results with a higher resolution instrument.
This paper suggests several reasons why we might see elevated Manganese without dissolution. Because other trace elements like Magnesium, Strontium, and Barium are all doing what we expect them to do in the depth profile, we have reason to pursue the idea that diagenetic alteration of the trace elemental composition of the foraminiferal calcite potentially occurs in the water column, not just in the sediment.
Past work in this area shows that dissolution is minimal, and if dissolution where the culprit, then we’d see that Mn would increase with time, which we don’t. Alternatively, alteration in shells may occur because of several factors: Smaller (or thinner) shells are more susceptible to their surrounding environment, meaning it’s easier for them to incorporate trace elements, so Manganese scavenging by sinking foraminifera, formation of carbonate overgrowths on the shells, and elevated manganese in the immediate micro-environment could all potentially affect how manganese is incorporated into these forams.
Long story short, our study paired with the Davis study provides an argument that diagenetic alteration of foraminiferal calcite might not be restricted to the sediment, but could instead be occurring, partially, within the water column, as a result of several potential processes with the mostly likely being changes in carbonate saturation state altering these elements prior to deposition on the seafloor.
Thanks for listening.