Arctica islandica shell growth and geochemical records from northern Norway as North Atlantic marine climate proxies for the last millennium

Paleoclimate proxy records are needed to better understand the behavior of various components of Earth’s complex climate system across major climate transitions of the past. Such records can also provide benchmarks to test climate models and interpret the evolution of climate forcing, feedbacks, and interactions in the past, present, and future. Development of high-resolution, high-latitude records of climate change such as those presented in this dissertation, provide insight to highly sensitive regions where very few lengthy instrumental records exist. This dissertation presents shell growth and geochemical records from the long-lived marine bivalve, Arctica islandica, from northern Norway to investigate major North Atlantic marine climate of the past millennium. A 112-year Master Shell Growth Chronology and oxygen isotope time series were tested against instrumental climate indices (including the Atlantic Multidecadal Oscillation, the North Atlantic Oscillation, and Atlantic meridional overturning circulation) and local to regional sea surface temperature records. Shell growth rate and oxygen isotopic composition were found to reflect regional sea surface temperatures across a broad swath of the North Atlantic in a pattern mimicking the path of the North Atlantic Current, suggesting a causal mechanism for coherence between marine variability in northern Norway and North Atlantic climate, namely, the influence of the North Atlantic Current in the Barents Sea. Statistically robust relationships were found between the Atlantic Multidecadal Oscillation and the shell-based MultiproxyAMO Index over the instrumental record, suggesting lengthy shell-based records from northern Norway may skillfully reconstruct AMO variability of the past. Spectral analysis of a 455-year Master Shell Growth Chronology constructed by crossdating dead-collected material with the modern chronology revealed significant periodicity in the ~60 year band, reminiscent of that of Atlantic multidecadal variability. Spectral analysis of oxygen isotope ratios from the Little Ice Age, Late Little Ice Age, and Modern periods also suggest multidecadal periodicity, suggesting that multidecadal oscillations in Atlantic sea surface temperatures have persisted for at least the past five centuries. A statistically significant decrease in δ18Oshell of 0.25‰ since the Little Ice Age suggests warming or strengthening of the North Atlantic Current into the Modern climate period. Additionally, this thesis presents an investigation of variability in replicated oxygen isotope measurements and the potential for the influence of sampling imprecision. Our findings suggest that natural proxy variability combined with analytical uncertainty, and not human-related sampling error, are the primary contributors to larger than expected variability among replicated measurements. This work has important implications for previously published isotope records from biogenic archives and provides a template for estimating isotopic variability on an individual case basis

Two centuries of southwest Iceland annually-resolved marine temperature reconstructed from Arctica islandica shells

Iceland's exposure to major ocean current pathways of the central North Atlantic makes it a useful location for developing long-term proxy records of past marine climate. Such records provide more detailed understanding of the full range of past variability which is necessary to improve predictions of future changes. We constructed a 225-year (1791–2015 CE) master shell growth chronology from 29 shells of Arctica islandica collected at 100 m water depth in southwest Iceland (Faxaflói). The growth chronology provides a robust age model for shell oxygen isotope (δ18Oshell) data produced at annual resolution for 251 years (1765–2015 CE). The temperature reconstruction derived from δ18Oshell shows coherence with May–October local surface temperature records and sea surface temperatures in the North Atlantic region, suggesting it is a useful proxy indicator of water temperature variability at 100 m depth within Faxaflói. Field correlations between the shell-based records and gridded sea surface temperature data reveal strong positive correlations between the 1-year lagged shell growth and temperatures within the subpolar gyre post-1972, suggesting a delayed influence of subpolar gyre dynamics on ecological indicators in southwest Iceland in recent decades. However, the shell growth chronology and δ18Oshell record generally show relatively weak and insignificant correlations with larger region climate indices including the Atlantic Multidecadal Variability, North Atlantic Oscillation, and East Atlantic pattern. Therefore the interannual variations in the newly produced shell-based records appear to reflect more local to regional dynamics around southwest Iceland than large-scale modes of climate variability.publishedVersio

Unexpected isotopic variability in biogenic aragonite: A user issue or proxy problem?

The present study seeks to investigate sources of isotopic variability in the commonly used paleoclimate archive, the marine bivalve Arctica islandica, with an emphasis on the potential of human-induced variability arising from sampling techniques. Stable carbon (d13Ccarbonate) and oxygen (d18Ocarbonate) isotopes were analyzed for split (intra-sample) and replicate (intra- and inter-shell) samples taken from a group of laboratory-reared individuals, a natural population from northern Norway, and a natural population from the Gulf of Maine, USA. Compared to analytical uncertainty of 0.17 ‰ and 0.30 ‰ for d13C and d18O, respectively, among the natural populations, the mean difference between shell splits and shell replicates ranged from 0.12 ‰ and 0.33 ‰ for d13C and d18O, respectively. Our data suggest that heterogeneity of the carbonate material (i.e., large range of isotopic composition within one sample due to seasonal environmental variability) may contribute to “unexpected” variability more than human-induced error from sampling imprecision when collecting whole annual increments. Furthermore, d13C from juvenile shells were highly variable (2s standard deviation = 0.65 ‰), approximately four times more variable than analytical precision. High precision among d18O measurements of the laboratory-reared shells confirm the presumption that shells reliably and consistently precipitate in isotopic equilibrium with ambient seawater. Monte Carlo simulations of measurements from this population allowed characterization of improvements in uncertainty at increasing levels of replication. Substantial reduction in uncertainty occurs when increasing from two to three shells, however replication using a total of four shells further decreased uncertainty to within the 99% confidence level. Published studies sometimes compensate for uncertainties by replicating records over multiple individuals or multiple transects within the one individual. Oftentimes, however, isotope records are constructed from single individuals or transects and therefore fail to provide thorough estimates of proxy error. Our findings suggest that replication of carbon and oxygen isotope measurements of contemporaneously produced aragonite is necessary in order to reduce proxy-derived noise. Furthermore, population-specific estimates of uncertainty related to natural variability among individuals should be investigated in order to provide more realistic representations of proxy noise when reporting isotope time series

Growth portfolios buffer climate-linked environmental change in marine systems

Large-scale, climate-induced synchrony in the productivity of fish populations is becoming more pronounced in the world's oceans. As synchrony increases, a population's “portfolio” of responses can be diminished, in turn reducing its resilience to strong perturbation. Here we argue that the costs and benefits of trait synchronization, such as the expression of growth rate, are context dependent. Contrary to prevailing views, synchrony among individuals could actually be beneficial for populations if growth synchrony increases during favorable conditions, and then declines under poor conditions when a broader portfolio of responses could be useful. Importantly, growth synchrony among individuals within populations has seldom been measured, despite well-documented evidence of synchrony across populations. Here, we used century-scale time series of annual otolith growth to test for changes in growth synchronization among individuals within multiple populations of a marine keystone species (Atlantic cod, Gadus morhua). On the basis of 74,662 annual growth increments recorded in 13,749 otoliths, we detected a rising conformity in long-term growth rates within five northeast Atlantic cod populations in response to both favorable growth conditions and a large-scale, multidecadal mode of climate variability similar to the East Atlantic Pattern. The within-population synchrony was distinct from the across-population synchrony commonly reported for large-scale environmental drivers. Climate-linked, among-individual growth synchrony was also identified in other Northeast Atlantic pelagic, deep-sea and bivalve species. We hypothesize that growth synchrony in good years and growth asynchrony in poorer years reflects adaptive trait optimization and bet hedging, respectively, that could confer an unexpected, but pervasive and stabilizing, impact on marine population productivity in response to large-scale environmental change.publishedVersio

Fundamental questions and applications of sclerochronology: Community-defined research priorities.

Horizon scanning is an increasingly common strategy to identify key research needs and frame future agendas in science. Here, we present the results of the first such exercise for the field of sclerochronology, thereby providing an overview of persistent and emergent research questions that should be addressed by future studies. Through online correspondence following the 5th International Sclerochronology Conference in 2019, participants submitted and rated questions that addressed either knowledge gaps or promising applications of sclerochronology. An initial list of 130 questions was compiled based on contributions of conference attendees and reviewed by expert panels formed during the conference. Herein, we present and discuss the 50 questions rated to be of the highest priority, determined through an online survey distributed to sclerochronology community members post the conference. The final list (1) includes important questions related to mechanisms of biological control over biomineralization, (2) highlights state of the art applications of sclerochronological methods and data for solving long-standing questions in other fields such as climate science and ecology, and (3) emphasizesthe need for common standards for data management and analysis. Although research priorities are continually reassessed, our list provides a roadmap that can be used to motivate research efforts and advance sclerochronology towardnew, and more powerful, applications.N/

Fundamental questions and applications of sclerochronology: Community-defined research priorities

Horizon scanning is an increasingly common strategy to identify key research needs and frame future agendas in science. Here, we present the results of the first such exercise for the field of sclerochronology, thereby providing an overview of persistent and emergent research questions that should be addressed by future studies. Through online correspondence following the 5th International Sclerochronology Conference in 2019, participants submitted and rated questions that addressed either knowledge gaps or promising applications of sclerochronology. An initial list of 130 questions was compiled based on contributions of conference attendees and reviewed by expert panels formed during the conference. Herein, we present and discuss the 50 questions rated to be of the highest priority, determined through an online survey distributed to sclerochronology community members post the conference. The final list: (1) includes important questions related to mechanisms of biological control over biomineralization; (2) highlights state of the art applications of sclerochronological methods and data for solving long-standing questions in other fields such as climate science and ecology: and (3) emphasizes the need for common standards for data management and analysis. Although research priorities are continually reassessed, our list provides a roadmap that can be used to motivate research efforts and advance sclerochronology toward new, and more powerful, applications

Arctica islandica shell growth and geochemical records from northern Norway as North Atlantic marine climate proxies for the last millennium

Paleoclimate proxy records are needed to better understand the behavior of various components of Earth’s complex climate system across major climate transitions of the past. Such records can also provide benchmarks to test climate models and interpret the evolution of climate forcing, feedbacks, and interactions in the past, present, and future. Development of high-resolution, high-latitude records of climate change such as those presented in this dissertation, provide insight to highly sensitive regions where very few lengthy instrumental records exist. This dissertation presents shell growth and geochemical records from the long-lived marine bivalve, Arctica islandica, from northern Norway to investigate major North Atlantic marine climate of the past millennium. A 112-year Master Shell Growth Chronology and oxygen isotope time series were tested against instrumental climate indices (including the Atlantic Multidecadal Oscillation, the North Atlantic Oscillation, and Atlantic meridional overturning circulation) and local to regional sea surface temperature records. Shell growth rate and oxygen isotopic composition were found to reflect regional sea surface temperatures across a broad swath of the North Atlantic in a pattern mimicking the path of the North Atlantic Current, suggesting a causal mechanism for coherence between marine variability in northern Norway and North Atlantic climate, namely, the influence of the North Atlantic Current in the Barents Sea. Statistically robust relationships were found between the Atlantic Multidecadal Oscillation and the shell-based MultiproxyAMO Index over the instrumental record, suggesting lengthy shell-based records from northern Norway may skillfully reconstruct AMO variability of the past. Spectral analysis of a 455-year Master Shell Growth Chronology constructed by crossdating dead-collected material with the modern chronology revealed significant periodicity in the ~60 year band, reminiscent of that of Atlantic multidecadal variability. Spectral analysis of oxygen isotope ratios from the Little Ice Age, Late Little Ice Age, and Modern periods also suggest multidecadal periodicity, suggesting that multidecadal oscillations in Atlantic sea surface temperatures have persisted for at least the past five centuries. A statistically significant decrease in δ18Oshell of 0.25‰ since the Little Ice Age suggests warming or strengthening of the North Atlantic Current into the Modern climate period. Additionally, this thesis presents an investigation of variability in replicated oxygen isotope measurements and the potential for the influence of sampling imprecision. Our findings suggest that natural proxy variability combined with analytical uncertainty, and not human-related sampling error, are the primary contributors to larger than expected variability among replicated measurements. This work has important implications for previously published isotope records from biogenic archives and provides a template for estimating isotopic variability on an individual case basis.</p

Chemostratigraphy and magnetic susceptibility of the Upper Ordovician Daravgai and Gashuunovoo formations, Gobi-Altai terrane, Shine Jinst area, southern Mongolia

Mongolia lies at the center of a very complex accretionary terrane and suffers from a lack of research due to its remoteness and limited access. Understanding the many individual terranes in the area is important in piecing together Mongolia’s geologic history. This study aims to contribute new data from a southern Mongolia Upper Ordovician sequence for the purpose of describing depositional conditions during this time period. These data can be combined with other studies to improve understanding of the tectonic processes that resulted in the amalgamation and formation of central Asia. Magnetic susceptibility and stable carbon isotopes were studied in a sequence of late Ordovician carbonate and clastic strata from the Shine Jinst Area of southern Mongolia where the Daravgai and Gashuunovoo formations are exposed. The rocks at this locality are typical of carbonate shelf deposits and primarily exhibit interbedded carbonate and shale. Six sections capped by faults were measured in this structurally complex area. Magnetic susceptibility is used as a proxy for sea level and tectonic effects that drive terrestrial weathering and detrital input to the marine system. Preliminary hyteresis measurements identify the primary iron-carrying mineral as hematite. The basal section of the Daravgai Fm. exhibits anomalously high values that average 4.01x10-8 m3/kg. The two overlying sections exhibit values averaging 1.29x10-8 m3/kg and include no major excursions. Missing section in the Daravgai Fm. make climate and sea level interpretations problematic. Meter-scale magnetic susceptibility oscillations were identified in the Gashuunovoo Formation that likely represent transgressive/regressive cycles, which are reflected lithologically as siliciclastic pulses. Significant positive excursions were identified which alternate with low value, relatively stable periods averaging 2.08x10-8 m3/kg. These data can be used in future studies for lateral correlation of other sections near Shine Jinst. Stable carbon isotope ratios are used to interpret climate trends and attempt regional and global correlation of the section. The Daravgai Fm. exhibits anomalously low carbon isotope values in the basal section averaging -2.79 ‰ followed by relatively stable values averaging 1.61 ‰ in the overlying two sections. The lower part of the Gashuunovoo Fm. exhibits an increasing trend and a major positive excursion averaging 2.32 ‰. This excursion likely corresponds to one of the five major excursions identified worldwide in the late Ordovician, and is tentatively correlated with a lower Ashgill excursion. Carbon isotopic data for the upper sections of the Gashuunovoo Fm. would enhance the dataset and strengthen the possibilities for correlation of the sequence

Unexpected isotopic variability in biogenic aragonite: A user issue or proxy problem?

The present study seeks to investigate sources of isotopic variability in the commonly used paleoclimate archive, the marine bivalve Arctica islandica, with an emphasis on the potential of human-induced variability arising from sampling techniques. Stable carbon (d13Ccarbonate) and oxygen (d18Ocarbonate) isotopes were analyzed for split (intra-sample) and replicate (intra- and inter-shell) samples taken from a group of laboratory-reared individuals, a natural population from northern Norway, and a natural population from the Gulf of Maine, USA. Compared to analytical uncertainty of 0.17 ‰ and 0.30 ‰ for d13C and d18O, respectively, among the natural populations, the mean difference between shell splits and shell replicates ranged from 0.12 ‰ and 0.33 ‰ for d13C and d18O, respectively. Our data suggest that heterogeneity of the carbonate material (i.e., large range of isotopic composition within one sample due to seasonal environmental variability) may contribute to “unexpected” variability more than human-induced error from sampling imprecision when collecting whole annual increments. Furthermore, d13C from juvenile shells were highly variable (2s standard deviation = 0.65 ‰), approximately four times more variable than analytical precision. High precision among d18O measurements of the laboratory-reared shells confirm the presumption that shells reliably and consistently precipitate in isotopic equilibrium with ambient seawater. Monte Carlo simulations of measurements from this population allowed characterization of improvements in uncertainty at increasing levels of replication. Substantial reduction in uncertainty occurs when increasing from two to three shells, however replication using a total of four shells further decreased uncertainty to within the 99% confidence level. Published studies sometimes compensate for uncertainties by replicating records over multiple individuals or multiple transects within the one individual. Oftentimes, however, isotope records are constructed from single individuals or transects and therefore fail to provide thorough estimates of proxy error. Our findings suggest that replication of carbon and oxygen isotope measurements of contemporaneously produced aragonite is necessary in order to reduce proxy-derived noise. Furthermore, population-specific estimates of uncertainty related to natural variability among individuals should be investigated in order to provide more realistic representations of proxy noise when reporting isotope time series.This is a manuscript of an article published as Mette, Madelyn J., Nina M. Whitney, Jared Ballew, and Alan D. Wanamaker Jr. "Unexpected isotopic variability in biogenic aragonite: A user issue or proxy problem?." Chemical Geology 483 (2018): 286-294. doi: 10.1016/j.chemgeo.2018.02.027. Posted with permission.</p

Research Collaboration Across Borders

As challenges in the aquatic sciences become more global in nature, international research collaborations are increasingly prevalent, with over 35% of worldwide publications produced as a result of international partnerships. Join a panel of experts that have successfully led international collaborations to discuss the unique set of challenges associated with conducting research internationally. Discussion topics will include identifying and contacting potential collaborators, strengthening ongoing collaborations, addressing language and cultural differences, and determining project goals, timelines, authorship, and logistics for fruitful collaborations