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This article is part of the Fall 2021 issue of the Earth Science Matters Newsletter.

The surface temperature of the North Atlantic Ocean has been observed to oscillate between relatively cool and warm periods lasting ~65–80 years over the past few centuries. This oscillation, termed Atlantic Multidecadal Variability (AMV), may impact sea level, hurricane activity, ecosystems, and weather patterns on either side of the Atlantic basin. Hypotheses about what causes AMV are currently being debated among the scientific community. Some scientists hypothesize that the AMV is related to changes in the circulation system that moves warm salty water from the tropics northward at the surface and returns cold deep water southward in the Atlantic Ocean (Atlantic Meridional Overturning Circulation; AMOC). A central question is whether the multidecadal pattern persists beyond the instrumental period of observation (before1850 CE). Developing a better understanding of the complex features of Earth’s climate system is critical for predicting and preparing for future climate change in the North Atlantic region and beyond.

Recent work sheds new light on the persistence of AMV in the southern Barents Sea, near the northernmost reach of the Gulf Stream and its extension, the North Atlantic Current. While most evidence for past AMV before the instrumental period is sourced from non-marine proxies like tree rings and ice cores, this new work uses long-lived marine bivalves (Arctica islandica) to reconstruct sea surface temperature and variability in other environmental factors that influence shell growth. Much like tree rings and corals, these common clams grow yearly bands in their shells and can live for centuries, preserving very long records of environmental conditions through physical and geochemical characteristics of the growth bands (Fig. 1). 

diagram of bivalve proxy archive
Fig. 1. Bivalve proxy archive. A) Arctica islandica shell and B) photomicrograph of a shell cross-section showing annual growth increments.

Using spectral analysis and comparison with other long-term proxy records, Mette et al. (2021) showed that shell-based data (Fig. 2) demonstrate AMV-like trends present in the southern Barents Sea for at least the past 500 years. This provides much needed marine-based evidence for AMV being a persistent and widely influential feature of the North Atlantic system. The new record is exceptional in terms of its long length, annual resolution, and high quality, providing key data to be incorporated into future large-scale reconstructions of AMV with the goal of understanding its origin and predictability.

graph of oxygen isotopes and shell growth
Fig. 2. Annual oxygen isotope (d18Oshell; blue) and growth increment width (black) data from Arctica islandica shells from the southern Barents Sea (Norway). Thin lines show annual data from individual shells, medium line shows annual average among replicates, and bold lines show a 21-year running mean of annual averages. At Ingøya, Norway, d18Oshellis primarily influenced by water temperature during shell precipitation, whereby lower values correspond to higher temperatures. Shell growth rate is related to food quality and food quantity in the local environment. Figure adapted from Mette et al. (2021).

The Gulf Stream is a surface component of AMOC and is responsible for the relatively mild climate in northwestern Europe. It originates in the Florida Keys and extends all the way to the North Atlantic Current which flows along Norway toward the Arctic (Fig. 3). The shell-based reconstruction from the Barents Sea represents a northern end member of a larger project being conducted by USGS researchers to understand changes in the Gulf Stream over the past millennium. Temperature proxy records from corals in the Florida Keys can be combined with the shell-based temperature reconstructions in the high-latitude North Atlantic to reconstruct changes at both ends of the Gulf Stream system. By continuing to develop and synthesize such annually resolved proxy records that are sensitive to North Atlantic Ocean dynamics in the Late Holocene (i.e., the past 2000 years), a detailed picture of past climate changes and their impacts will emerge. In particular, the relationship between sea surface temperature patterns (like AMV) and AMOC dynamics can be further investigated.

map of North Atlantic
Fig. 3. Map of the North Atlantic Ocean illustrating the approximate path of the Gulf Stream / North Atlantic Current system. Also labeled are study sites within the Gulf of Mexico (Dry Tortugas) and Barents Sea (Ingøya, Norway) representing southern and northern endmembers for paleoclimate research targeting reconstruction of Late Holocene surface ocean dynamics.

The paper, "Persistent multidecadal variability since the 15th century in the southern Barents Sea derived from annually resolved shell-based records" was recently published in JGR Oceans

<< Back to Fall 2021 Newsletter

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