Establishing high-resolution baseline records of productivity in the Barents Sea using paleoclimate records from crustose coralline algae.

Over the past few decades, warming air and ocean temperatures has lead to declining Arctic sea-ice thickness and concentrations. Concurrent with this sea-ice decline is a marked 30% increase in satellite-based estimates of marine primary productivity driven by shoaling of the mixed layer and enhanced transmittance of solar radiation into the surface ocean. Enhanced productivity is predicted to not only impact fishery yields, but also marine ecosystem dynamics, resulting in wholesale shifts in local species abundance and diversity. However, difficulties of navigating in remote ice-laden waters and harsh polar climates have often resulted in short and incomplete records of in-situ plankton abundance.

Alternatively, information about past marine productivity may be gained through the study of trace nutrient and isotopic signals preserved in the skeleton of long-lived marine organisms. Crustose coralline algae Clathromorphum compactum are calcareous marine algae that form hard rock-like encrustations on the shallow rocky seafloor (Fig. 1). They offer an unprecedented opportunity to study past changes in high-latitude marine environments because they are: 1) widely distributed in mid-to-high latitude oceans where other paleoclimate archives are sparse; 2) long-lived (up to multiple centuries); and 3) produce clear and even annual growth increments, enabling the precise calendar dating and geochemical sampling of hard tissue (Fig. 2). In fact, barium-to-calcium trace element ratios (Ba/Ca) have recently been used to reconstruct a 365-year record of productivity in the Labrador Sea (Chan et al., 2017). In addition, isotopes of nitrogen (δ¹⁵N) can be used as an indicator for productivity, such that the enhanced uptake of NO₃^-(i.e. nitrate assimilation) is associated with higher levels of productivity and vice versa.

 



Figure 1: Coralline Algae Clathromorphum compactum. a) Underwater image of live crustose coralline algae. b) Cross section of coralline algae with the youngest (most recent) growth layers on top, and the oldest layers at the bottom of the specimen.

Figure 2: Cross section of crustose coralline algae. Red lines indicate growth layer boundaries. Black holes represent conceptacles (reproductive cavities within the algal skeleton). Calendar years are assigned using layer counting methods starting from year of collection (ex. 2010), and going back in time. Dates are also cross-checked using radiocarbon dating.

In this project, we will examine geochemical changes within long-lived annually-banded crustose coralline algae in order to establish highly-resolved natural baseline records of marine productivity associated with climatic changes off the Svalbard archipelago and in the Barents Sea. The results generated from this study will lead to a better understanding of long-term natural baseline trends in productivity prior to the onset of anthropogenic warming, which will improve our ability to inform policy makers regarding petroleum resource management and exploration in these regions.

Publications:
Chan, P., Halfar, J., Adey, W., Hetzinger, S., Zack, T., Moore, G.W.K., Wortmann, U.G., Williams, B., and Hou, A. (2017). Multicentennial Record of Labrador Sea Primary Productivity and Sea-Ice Variability Archived in Coralline Algal Barium. Nature Communications. 8, 15543. https://www.nature.com/articles/ncomms15543