Exceptional 20th Century Ocean Circulation in the Northeast Atlantic

Abstract

The North Atlantic subpolar gyre (SPG) connects tropical and high-latitude waters, playing a leading role in deep-water formation, propagation of Atlantic water into the Arctic, and as habitat for many ecosystems. Instrumental records spanning recent decades document significant decadal variability in SPG circulation, with associated hydrographic and ecological changes. Emerging longer-term records provide circumstantial evidence that the North Atlantic also experienced centennial trends during the 20th century. Here, we use marine sediment records to show that there has been a long-term change in SPG circulation during the industrial era, largely during the 20th century. Moreover, we show that the shift and late 20th century SPG configuration were unprecedented in the last 10,000 years. Recent SPG dynamics resulted in an expansion of subtropical ecosystems into new habitats and likely also altered the transport of heat to high latitudes.Plain Language Summary The Northeast Atlantic is of crucial importance for the global climate system and marine ecosystems. We can use sediment from the bottom of the ocean to reconstruct how the Northeast Atlantic has changed over thousands of years. In this study, we present the first evidence that 20th century Northeast Atlantic surface ocean circulation was unusual compared to the last 10,000 years. This change caused a replacement of cool, subpolar waters with warmer subtropical waters near Iceland and has impacted the distribution of marine organisms. The most striking aspect of our work is the exceptional nature of the shift in the 20th century (in contrast to thousands of years of relative stability), with implications for understanding future change.

Annually resolved Atlantic sea surface temperature variability over the past 2,900 y

“…( A ) AMV based on the Ti record from SSL (AMV SSL ) compared to reconstructed AMV ( 12 ). ( B to H ) Same as A but AMV SSL compared to AMV from Mann et al ( B ) ( 13 ), ocean temperature south of Iceland from core Rapid-17–5P ( C ) ( 31 ), T. quinqueloba (light blue) from core EN539-MC14-A and T. quinqueloba (dark blue) from core MC16-A ( D ) ( 32 ), the δ 18 O from shells of the long-lived marine bivalve A. islandica ( E ) ( 33 ), the SIC off SE Greenland ( F ) ( 34 ), the BWT at Malangen Fjord ( G ) ( 35 ); and G. bulloides abundance from Cariaco Basin ( H ) ( 36 ). In D , SPG stands for subpolar gyre.…”

“…Although the marine record Rapid-17–5P has an impressive sample resolution, its latest age is ∼1793 CE. New cores from this location have been collected and now extend the chronology to the present day ( 32 ). These new data show a drastic shift from subpolar waters to warmer subtropical waters off southern Iceland during the 20th century ( 32 ).…”

“…New cores from this location have been collected and now extend the chronology to the present day ( 32 ). These new data show a drastic shift from subpolar waters to warmer subtropical waters off southern Iceland during the 20th century ( 32 ). Turborotalita quinqueloba , a foraminifer species that prefers cool and productive waters, has been declining at an accelerating pace during the past century and reached unprecedented low values in the last decade, in line with the unprecedented increase in the AMV SSL ( Fig.…”

“…4 D ). This suggests a reduced influence of the subpolar gyre in recent decades compared to the past several millennia ( 32 ). Similarly, off the northern coast of Iceland, an annual record based on δ 18 O from shells of the long-lived marine bivalve Arctic islandica is also correlated to AMV SSL ( Fig.…”

A new method for isolating and analysing coccospheres within sediment

“…The field sample consisted of marine sediment from the North Atlantic core EN539-16MC (61° 29 N, 19° 32 W, 2311 m water depth). A sample was taken at 12–12.5 cm depth (estimated age is 1939 based on combined 210 Pb, radiocarbon and water content 70 ) and wet sieved at 63 μm using deionised Milli-Q water and the fine-fraction was retained. ~ 100 mg of sediment was agitated with 15 ml deionised Milli-Q water then filtered through a 100 μm cell strainer to remove aggregates.…”