Reconstructing and understanding the spatiotemporal patterns of late Holocene climate variability remains a fundamental challenge in paleoclimatology, particularly with respect to coupled systems such as the North Atlantic Oscillation (NAO) and the Atlantic meridional overturning circulation (AMOC). Records with absolute chronologies on annual or better timescales have offered the most robust data for quantitative climate reconstructions, however such records are generally limited to terrestrial regions or the tropical oceans. This collaborative research between scientists from Iowa State University and the University of Maine uses sclerochronological analysis of the growth increments of the long-lived bivalve Arctica islandica to generate annually resolved records of temperature in the Gulf of Maine covering the last 1000 years. The PIs explore the specific relationship among the NAO mode, NAO extremes, seawater temperature, and shell growth rate and chemistry, as well as the response of the ocean-climate system in the Gulf of Maine to rapid climate changes during the last millennium. The results from this research shed light on climate change in a region that will affect large populations in the 21st century. Graduate and undergraduate students are participating in the research, and the PIs are working with a variety of regional research centers to interactive displays highlighting the research.
Intellectual Merit: Key Outcomes: The Gulf of Maine's oceanography reflects broad-scale patterns within the North Atlantic system. We have quantified the marine radiocarbon reservoir effect in the Gulf of Maine prior to the bomb-pulse of the 1950s. These data indicate a highly variable marine radiocarbon reservoir effect. The rapid rate of change in the marine radiocarbon reservoir effect further supports that the Gulf of Maine is an extremely dynamic region with highly variable source waters inputs. Specific Project Outcomes: Two master shell growth chronologies (MSC) have been developed from the Gulf of Maine (Sequin Island and Jonesport regions). Shell growth at each site is highly synchronous indicating that environmental variability is driving shell growth at the population level. At the Sequin Island site, which is the main focus of this work, we have constructed a shell chronology with a total of 24 shells that begins in AD 1683 and spans to AD 2009. A comparison with the MSC with monthly bottom water temperatures at Boothbay Harbor, Maine (located 13 km from Sequin Island) yielded a strong inverse relationship with springtime (MAM) temperatures. Spring seawater temperatures explain approximately 65% of the shell growth. Based on this relationship, a transfer function was developed and used to hindcast spring seawater temperatures from the Gulf of Maine. Several important features are noted in this temperature reconstruction including prominent variability at the multi-decadal, decadal, and interannual timescales (see Figure 1). Since AD 1800 a persistent 30-year periodicity in shell growth is evident. Comparison with large-scale climate forcings, including the Atlantic meridional overturning circulation (AMOC) index and the Florida Current, indicate that the MSC springtime seawater temperature record is sensitive to broad-scale ocean circulation variability within the North Atlantic region. For example, 15% of reconstruction (from 1961 to 2006) at Sequin Island in the Gulf of Maine can be explained by the spring AMOC index from the previous year. The AMOC index values during March of the previous year yields the strongest relationship with the MSC derived reconstruction (r = - 0.52; p = 0.0003; N= 45). Comparatively, ~ 29% of annual surface seawater temperatures at Boothbay Harbor (from 1961 to 2006) can be explained by the spring AMOC index from the previous year. The relationships between the winter North Atlantic Oscillation (NAO) and the Boothbay Harbor surface seawater temperature record and the MAM seawater temperature reconstruction are relatively weak. The best relationship among these parameters occurs when the NAO series leads the Gulf of Maine temperature records by two years. Since AD 1824, only 4% of the MAM seawater temperature reconstruction can be explained by the winter NAO. This relationship improves slightly from AD 1950 to present (r = 0.27; p = 0.04; N= 57). The relationship between the winter NAO mode and the annual Boothbay Harbor surface seawater temperature record since AD 1906 is not statistically significant (r = 0.07; p = 0.68; N= 102). It appears that the winter NAO does not seem to be a major driver of shell growth or seawater water temperatures in the central Gulf of Maine during this time interval. A timeseries of marine radiocarbon reservoir effect was devloped (Figure 2). Significant radiocarbon variability exists on decadal to centennial scales, and pre-bomb values from this study site in the interior Gulf of Maine are similar to those published on Georges Bank and in the Northeast Channel, indicating that hydrography in the Gulf of Maine is likely dominated by a deepwater source. This finding is important because it illustrates that decadal-scale hydrographic variability in the Gulf of Maine is related to general conditions in the northern North Atlantic. Broader Impacts: The research formed the basis for two Master’s thesis projects for two female graduate students at Iowa State University. These students were trained in mass spectrometry techniques, chronology construction, image analysis, data analysis/data assurance, oceanography, climatology, and paleoclimatology. These students participated in three sampling and fieldwork events in the Gulf of Maine. At least five undergraduate researchers were involved in many aspects of the research. Supervised by PI Wanamaker, graduate students directed and guided undergraduates in shell preparation, image analysis, and mass spectrometry analyses. PI Wanamaker and graduate students participated in several K-12 lectures and outreach events, including Edwards Elementary Science Night. PI Wanamaker has been involved during the last four years and has co-directed the event for the last two years. Each year, graduate students from Wanamaker’s research group have had interactive exhibits on various aspects of climate change, proxy records, and paleoclimatology. In the last four years, at least 1,000 students and community members were impacted by these outreach events. PI Wanamaker and project graduate students presented a total of seven conference papers. Additionally, Wanamaker gave eight public lectures that highlighted this work. One manuscript has been published and three manuscripts are under preparation.
