Degradation of Internal Organic Matter is the Main Control on Pteropod Shell Dissolution After Death

Abstract

The potential for preservation of thecosome pteropods is thought to be largely governed by the chemical stability of their delicate aragonitic shells in seawater. However, sediment trap studies have found that significant carbonate dissolution can occur above the carbonate saturation horizon. Here we present the results from experiments conducted on two cruises to the Scotia Sea to directly test whether the breakdown of the organic pteropod body influences shell dissolution. We find that on the timescales of 3 to 13 days, the oxidation of organic matter within the shells of dead pteropods is a stronger driver of shell dissolution than the aragonite saturation state of seawater. Three to four days after death, shells became milky white and nano scanning electron microscope images reveal smoothing of internal surface features and increased shell porosity, both indicative of aragonite dissolution. These findings have implications for the interpretation of the condition of pteropod shells from sediment traps and the fossil record, as well as for understanding the processes controlling particulate carbonate export from the surface ocean.

Determining how biotic and abiotic variables affect the shell condition and parameters of <i>Heliconoides inflatus</i> pteropods from a sediment trap in the Cariaco Basin

“…Organisms falling through the water column may decay en route to the sediment trap, which can cause dissolution in calcareous organisms (Lohmann, 1995;Milliman et al, 1999). In pteropod shells specifically, Oakes et al (2019a) found that the majority of the postmortem dissolution was associated with the biotic decay of organic material on the inside of the shell, and therefore specimens from sediment traps do not perfectly capture in-life shell conditions. A further complication of sediment trap data is that interpretation can be skewed by the presence of "swimmers", i.e., specimens that were alive when they entered the trap (Harbison and Gilmer, 1986).…”

“…Shell thickness has been used as a metric for calcification in previous studies, initially calculated from point measurements on the shell aperture from scanning electron microscope images (Bednaršek et al, 2014b;Roger et al, 2012), and later measured across entire shells from CT re-constructions (Howes et al, 2017;Oakes et al, 2019a;Peck et al, 2018). Here we use modal shell thickness to compare calcification among samples following the methods of Oakes et al (2019a). Although this method analyzes shell thickness across the entire shell, the final, or body, whorl, composed of the most recently calcified material, is the largest portion of the shell in Heliconoides inflatus pteropods ( Fig.…”

“…3). Shell thickness was measured using the BoneJ plugin (Doube et al, 2010;Hildebrand and Rüegsegger, 1997) in ImageJ (Schneider et al, 2012) following the methods of Oakes et al (2019a).…”

Testing the impact of two key scan parameters on the quality and repeatability of measurements from CT scan data

“…There is, therefore, an interest in monitoring pteropods over the coming decades (Bednaršek et al, 2014(Bednaršek et al, , 2017. One aspect of this monitoring could involve using CT scanning to measure the shell thickness of pteropods and assessing whether their ability to build and maintain their shells is affected by changes in ocean chemistry (e.g., Howes et al, 2017;Peck et al, 2018;Oakes et al, 2019a). Aside from their scientific interest, pteropod shells are ideal specimens to use for a CT methodology study because their overall size (< 1 mm) and the thickness of their shells (~10 µm) pushes the limits of most modern micro-CT scanners.…”

“…Shell volume was chosen as a comparative tool because it is sensitive to changes in scan quality, i.e., it is likely to be affected by both noise and low contrast between the different materials (shell, body, and air). Measuring pteropod shell volume in combination with shell size has practical applications as it could be used as a metric for monitoring the response of pteropods to ocean acidification in the future (e.g., Howes et al, 2017;Oakes et al, 2019a). Quality assessment method 1-SNR and CNR.…”