A 250‐Year, Decadally Resolved, Radiocarbon Time History in the Gulf of Maine Reveals a Hydrographic Regime Shift at the End of the Little Ice Age

Abstract

In order to document relative changes in water mass contributions in the Gulf of Maine (GoM), we used the shell material of the long‐lived ocean quahog (Arctica islandica). A multicentury, crossdated master shell growth chronology facilitated the reconstruction of a radiocarbon Δ14C history prior to the radiocarbon bomb‐pulse of the 1950s. This reconstruction reveals a highly variable Δ14C series (mean = −56.6 ± 8.0‰ (1σ); N = 34) from CE 1685 to 1935. Δ14C values indicate a rapid shift ca. 1860 CE in source waters to the GoM. From CE 1685 to 1860, GoM waters were dominated by an admixture of Warm Slope Water primarily composed of tropical Atlantic surface waters/Gulf Stream Waters, and Scotian Shelf Water. This water regime was followed by a rapid Δ14C transition to a Labrador Slope Water endmember after CE 1860, with an apparent decrease in Scotian Shelf Water. Together, this shift is likely related to broader changes in the Arctic and the Labrador Sea, and a short‐term strengthening of the Atlantic meridional overturning circulation. Labrador Slope Water dominating GoM hydrography in the 1900s is verified by the similarities between this record and other coral‐ and shell‐derived Δ14C records influenced by waters with Labrador Sea origin. This suggests that GoM radiocarbon variability broadly reflects large‐scale ocean circulation processes in the Northwestern Atlantic. The lack of Δ14C values much below the Labrador Slope Water endmember suggests that the interior GoM gets very little to no Antarctic Intermediate Water as other studies had previously suggested.

A Multicentennial Proxy Record of Northeast Pacific Sea Surface Temperatures From the Annual Growth Increments of Panopea generosa

“…The two samples from the Modern chronology (TND031 and TND045) were used to calculate the local reservoir age correction (ΔR) following methods outlined by Lower‐Spies et al. (2020), which was applied to all floating segments to improve dating accuracy.…”

“…Subsequently, a Bayesian "wiggle matching" approach was implemented in OxCal to further refine the 14 C age estimates and uncertainties using the paired radiocarbon probability distributions and their respective known sclerochronological offsets (Figure 4 inset; de Vries, 1958;Ramsey et al, 2001). The two samples from the Modern chronology (TND031 and TND045) were used to calculate the local reservoir age correction (ΔR) following methods outlined by Lower-Spies et al (2020), which was applied to all floating segments to improve dating accuracy.…”

Exceptional 20th Century Shifts in Deep-Sea Ecosystems Are Spatially Heterogeneous and Associated With Local Surface Ocean Variability

“…Our ability to link the timing of benthic foraminifera changes to specific changes in North Atlantic ocean circulation is hindered by limitations in our knowledge of when, what and how circulation changed during the industrial era. Notwithstanding these limitations, and by greatly simplifying the evidence, existing proxy data suggest that there have been two major industrial era shifts, the first (Phase 1) occurring at ∼1850-1900, and then a series of shifts occurring through the mid-late 20th century (Phase 2) with evidence for more marked changes at ∼1970 onward in some datasets, which may be linked to increased freshwater in the subarctic Atlantic (Figure 2; Rahmstorf et al, 2015;Moore et al, 2017;Caesar et al, 2018Caesar et al, , 2021Thornalley et al, 2018;Osman et al, 2019;Lower-Spies et al, 2020;Spooner et al, 2020). There has also been large amplitude recent decadal variability through the 1990s and early 21st century (Robson et al, 2016;Holliday et al, 2020).…”

Seawater Cadmium in the Florida Straits Over the Holocene and Implications for Upper AMOC Variability

“…A decline in AMOC over the industrial era (nineteenth and twentieth centuries) has been inferred from many paleoclimate proxy records (Caesar et al., 2021), but the timing of the weakening differs among records (Caesar et al., 2018; Lower‐Spies et al., 2020; Thornalley et al., 2018), and there are even some records that do not suggest a weakening (Lund et al., 2006; Wanamaker et al., 2012). AMOC decline over the late twentieth century has been inferred from North Atlantic sea surface temperature data (Rahmstorf et al., 2015), sea‐level variations across the Florida Current (Piecuch, 2020), and from partial records of circulation and sea surface height (Mercier et al., 2015).…”