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(2020) Oceanographic and Biogeochemical Changes along the Labrador Shelf: Evidence from Nitrogen Isotopes in a Six-Hundred-Year-Old Coralline Alga

Doherty J, Williams B, Kline E, Adey W & Thibodeau B


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14d: Plenary Hall, View in program

Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.

Submitted by Mark Stevenson on
Thank you for your excellent presentation. I was wondering if you could use the organic geochemical method IP25 to generate an independent sea-ice proxy for comparison? What are the limitations of the current sea ice proxy you are using? I think the highly branched isopenoid - IP25 would be suitable at the temporal resolution you are focused on if it is present in your deposits. Also I was wondering if your shift around ~1800 AD is coherent with changes in any other local/regional records. In other locations of the Arctic (e.g. Greenland) we get climate changes a little later at the end of the LIA but this is a complex process and is spatially heterogeneous. What do you think the major control is on your clear change?
Thank you for the questions. It is my understanding that the IP25 method could be applied to generate another independent sea-ice proxy in this region, but I have to admit I am less familiar with this technique. The sea-ice index presented here is derived from growth extension rates and Mg/Ca analysis in coralline algae, which has been demonstrated to faithfully track sea-ice extent along both the Labrador Shelf and at higher latitudes (see Halfar et al, 2013, PNAS for example). Because the record we compare with is from the identical region as our d15N record, I feel this is the most appropriate comparison to make. One limitation is that this record is also influenced by temperature rather than purely sea-ice extent, but then again temperature and sea-ice extent are fundamentally connected to one another (i.e., see Moore et al., Sci Rep, 2017). As for the shift in 1800, I didn't have time to go into this during the presentation, but we see a statistically significant change in the relationship between the AMO and North Atlantic Oscillation (NAO) during this time period, potentially indicating a reorganization of ocean-atmosphere dynamics, which coincides with the initial smaller decrease in Labrador Sea convection around 1800. The NAO is positive here, and the trends in Labrador Sea convection seem to follow the behavior of the NAO as expected (with NAO+ corresponding to weakened convection and NAO- corresponding to stronger convection). Our d15N record is also significantly correlated with the NAO during this time period, and so we argue that the atmospherically-induced changes in convection could have also produced changes in the Labrador Current's strength and nutrient input at the shelf. This is the case until the mid-1900s, during which there has been a well-documented anthropogenic weakening of the circulation (i.e., Rahmstorf, 2015, Nature; Caesar, 2018, Nature; Thornalley, 2018, Nature; Thibodeau, 2019, GRL). Our paper is currently in review, but we made a pre-print available on ESSOAr if you are interested: https://www.essoar.org/doi/abs/10.1002/essoar.10501506.2

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