Nitrogen, oxygen, and carbon isotopes in the shell of the Antarctic scallop Adamussium colbecki as a proxy for sea ice cover in Antarctica and striae measurements to create a growth model
EMMA PUHALSKI, STEPHEN CAMARRA, DAVID GILLIKIN
Adamussium colbecki is a large thin-shelled Antarctic sea scallop that is well-represented in the fossil record. Shell oxygen (δ18Os), carbon (δ13Cs), and nitrogen isotopes in carbonate bound organic matter (δ15NCBOM) have the potential to record sea ice cover over time. When paired with interstrial growth trends, shell isotope data may provide information on growth-rate response to environmental changes such as sea ice state. To test this hypothesis, we analyzed A. colbecki shells from two bays with different sea ice cover in Western McMurdo Sound, Antarctica: Explorers Cove (EC) has persistent (multiannual) sea ice and Bay of Sails (BOS) has annual sea ice (melts out seasonally). Three adult shells were collected from each site in 2008 and two juveniles were later collected from Explorers Cove in 2016. Shells were serially sampled for O and C isotopes from the margin to the umbo following assumed growth lines to ensure the data represents specific time intervals of shell growth as accurately as possible. Similar δ18Os values for all shells indicated that meteoric glacial inputs do not play a significant role at these sites. A consistent decrease in δ13Cs values for all shells indicates metabolic carbon may be integrated through shell growth and does not provide accurate sea ice proxy data. Carbon isotopes may yet provide information about changes in A. colbecki growth rate. Interstrial growth trends in adults show little periodicity through ontogeny, suggesting high metabolic devotion to reproductive growth and secondary influences from environmental factors. Periodicity in young growth may be strongly environmentally controlled. It has been shown that δ15NCBOM values provide a similar proxy as soft tissue δ15N values, which record phytoplankton δ15N values. We hypothesize the significantly higher δ15NCBOM values in EC shells reflect nitrate draw-down under persistent ice cover. Nitrogen isotopes in fossil A. colbecki have a high potential to record sea ice cover.
This work is co-authored by Kelly Cronin and Sally Walker from the University of Georgia.