Applying dendrochronology visual crossdating techniques to the marine bivalve Arctica islandica and assessing the utility of master growth chronologies as proxies for temperature and secondary productivity in the Gulf of Maine

The work that follows is aimed at providing a more comprehensive understanding of relationships between growth variability within and among populations of A. islandica in the Gulf of Maine. An essential goal of this work is to establish the level of coherence of A. islandica growth (that is the common growth signal) within the Gulf of Maine. Further, the relationships between variable growth rates and environmental conditions will be investigated. This research presents preliminary findings in the context of a larger project, with a goal to establish a master shell chronology and to reconstruct hydrographic conditions, including seawater temperatures, for the last 1000 years in the Gulf of Maine.\nIn order to determine the relationship between shell growth and potential environmental forcings, site-specific calibrations between growth and environmental conditions must be developed. First, the strength of a common growth signal (how synchronous growth is at the population level) must be determined at each site. Then, ecologically relevant comparisons with environmental can be investigated. However, prior to any proxy-based climate or environmental reconstruction, a calibration between the proxy archive and an instrumental series is required. Dendrochronology techniques were applied to the marine bivalve Arctica islandica to demonstrate the benefits of visual crossdating and replication of growth series (growth within one shell and between multiple shells in the same population). Prior to measuring the thickness of annual increments, individual shell increments were visually inspected and temporally aligned using several visual crossdating techniques (marker years, modified list method, and skeleton plots). Applying these techniques of crossdating sclerochronological archives resulted in precisely dated and a highly replicated master shell chronology (average expressed population signal = 0.94; series intercorrelation =0.76) from a site within the central Gulf of Maine (northwestern Atlantic Ocean). Such chronologies can then be more confidently compared to environmental and climate indices. For example, the master shell chronology developed from this population shows a strong relationship with spring (MAM) local bottom water temperatures (r = -0.81, p < 0.0001). This strong proxy/instrumental relationship rapidly diminishes with the introduction of chronology errors (i.e., errors that occurred prior to crossdating). Two types of errors, typically encountered when measuring increment widths in bivalves, were introduced to the absolutely dated age model. We then illustrate how these introduced errors result in decreased agreement in the proxy/instrumental series along with a corresponding decrease in statistical of the relationship. This finding illustrates that poorly developed age models (even with 1-3 errors over 60 years) will decrease the skill of the proxy archive to accurately reflect environmental conditions. Visual crossdating and replication of sclerochronological growth series is an essential step in the development of accurate master chronologies. This procedure is especially important during the process of calibrating a proxy archive with an environmental data series.\nTo investigate ocean and ecosystem variability within the Gulf of Maine two master shell chronologies were constructed from annual growth increments of the marine bivalve Arctica islandica from two sites (site 1: 44° 26’ 9.829” N, 67° 26’ 18.045” W; site 2: 43° 42’ 54” N, 69° 44’ 52” W). Both chronologies are statistically robust (site 1- series intercorrelation = 0.70, EPS =0.93, from 1954-2008; site 2 - series intercorrelation = 0.76, EPS = 0.94 from 1783-2009) showing strong synchronous growth within each site. The master shell chronologies are statistically comparable to A. islandica, tree-ring, and otolith master chronologies from which environmental reconstructions have been made. Both master shell chronologies were compared with nearby instrumental temperature records from Boothbay Harbor, Maine and Canadian Prince 5 station. A statistically strong (r = -0.81) and significant (p < 0.0001) relationship with site 2 master shell growth and Boothbay Harbor spring (MAM) bottom water temperatures was found. Using the full length of the site 2 master shell chronology, Boothbay Harbor MAM bottom water temperatures were reconstructed (1783 to 2009), yielding high year-to-year, decadal, and multi-decadal variability. A third chronology was constructed by averaging master shell chronologies from site 1 and site 2. The combined chronology was compared with continuous plankton recorder (CPR) time-series from the Gulf of Maine. A significant correlation (r = 0.49; p < 0.0002) exists between the combined chronology and the annual abundance of Calanus finmarchicus, which is the dominant zooplankton species for this region. Although each site master shell chronology individually yielded a significant relationship with the C. finmarchicus abundance, the combined shell chronology outperformed each site-specific shell chronology. This result illustrates the potential advantage of combining master shell chronologies, which in this study minimized local noise and enhanced the regional-scale productivity signal. Based on the robust and statistically significant relationships between master shell chronologies with temperature and secondary productivity indicators, these master shell chronologies developed in the Gulf of Maine have the potential to hindcast past ocean conditions beyond the instrumental record.

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