The lack of high resolution, geographically diverse proxy records from the marine realm limits our understanding of climate dynamics in the North Atlantic Ocean and Arctic during recent centuries. We investigate the impact of large-scale climate variability on the marine bivalve, Arctica islandica, (Linnaeus 1767) from northern Norway (718N). We evaluate the use of annual shell growth and geochemical records as proxies for North Atlantic and Arctic climate variability over centennial scales by developing a continuous, 113-yr master shell growth chronology and an oxygen isotope record (d 18 O) from live caught shell material. A relatively strong inverse relationship is observed between both the shell growth and isotopic proxies and large-scale North Atlantic sea surface temperatures in modern times (r 5 20.54 to 20.90; p < 0.05). This relationship is strengthened when using a combined shell growth/oxygen isotope Multiproxy Index (r 5 20.72 to 20.90; p <0.01). The regional spatial pattern of correlation resembles that of the North Atlantic Current as it bifurcates around 558N, indicating that large-scale ocean surface current dynamics play an important role in regulating local ecosystem processes and thus shell growth in northern Norway. A combined proxy index created using multiple linear regression exhibits a relatively strong and time-stable relationship with the Atlantic Multidecadal Oscillation (AMO; r 5 20.622; p < 0.001) since AD 1900. Variability in the relationship between the shell based records and the North Atlantic Oscillation coincide with variations in the AMO index, suggesting a complex relationship between atmospheric forcing on hydrographic variability and ecosystem dynamics in northern Norway.

Deep oceanic overturning circulation in the Atlantic (Atlantic Meridional Overturning Circulation, AMOC) is projected to decrease in the future in response to anthropogenic warming. Caesar et al. 1 argue that an AMOC slowdown started in the 19 th century and intensified during the mid-20th century. Although the argument and selected evidence proposed have some merits, we find that their conclusions might be different if a more complete array of data available in the North Atlantic region had been considered. We argue that the strength of AMOC over recent centuries is still poorly constrained and the expected slowdown may not have started yet.Recently, Moffa-Sanchez et al. 2 compiled a comprehensive set of paleoclimate proxy data from the North Atlantic and Arctic regions using objective criteria for identifying high-quality datasets of ocean conditions spanning the last two millennia (Figure 1). Although no direct (singular) proxy for AMOC exists, the paleoceanographic proxy data compiled by Moffa-Sanchez et al. 2 highlight the spatial and temporal complexities of ocean state in modern times and the recent past. When all the available proxy records potentially related to AMOC variability and 20th century observational datasets are considered, the time history of the AMOC system becomes less certain. In contrast, selecting only a subset of proxy records that share similar trends, as performed by Caesar et al. 1 , provides an incomplete perspective on AMOC changes through time.Increased data availability in recent decades has enabled a shift in the fields of paleoceanography and paleoclimatology toward more objective and transparent data selection in studies aimed at quantitatively reconstructing past variability. Such screening methods tend to minimize the impact of spurious or less reliable records on analyses and work to enhance the common signal in proxy records. Additionally, analyzing networks of suitable and carefully selected data enables robust uncertainty estimates on the resulting reconstructions, which is essential in providing confidence in the results and the ability to compare information across disciplines. Key to such work is identifying robust criteria and weighting schemes that objectively identify and utilize the most reliable data. Caesar et al. 1 use a variety of proxy records in their analysis, but do not identify the reasoning or criteria for selecting those records over many others that are likely related to aspects of AMOC dynamics (see the recent review 2 ).Objective and inclusive data selection standards are especially important when addressing AMOC, which is a system composed of many different components that can behave differently at different latitudes, depths, and timescales 3 and looking at any singular index of AMOC inherently oversimplifies the system. The complex signals in the available AMOC-related proxy variables over recent centuries support this notion 2 , though many of these studies were not considered by Caesar et al. 1In addition to the need for objective standards, we...

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The North Atlantic Ocean plays a key role in modulating global climate (e.g., Marshall et al., 2001). Surface currents of the North Atlantic, including the Gulf Stream and its northern extension, the North Atlantic Current, bring relatively warm, salty waters northward as the surface components of the Atlantic Meridional Overturning Circulation (AMOC). At higher latitudes, these waters exchange heat with the atmosphere and cool to form dense, sinking water masses (deep-water formation) that return southward at depth, contributing to global ocean circulation (e.g., Broecker, 1991;Lynch-Stieglitz et al., 2007). The strength and structure of these currents, the rate of deep-water formation, and ocean-atmosphere interactions across the