Cold-water coral records of temperature and circulation in the Northeast Atlantic intermediate waters over the past 35 thousand years

Abstract

<p>Geochemical evidence suggests that Atlantic circulation during the Last Glacial Maximum (LGM) was considerably different from modern and promoted carbon accumulation in the deep. During the last deglaciation, atmospheric CO<sub>2</sub> concentration and temperature rose significantly, while the radiocarbon (<sup>14</sup>C) content dropped. Marine records indicate that ocean circulation may have influenced these atmospheric parameters, for instance via outgassing of the carbon-rich and <sup>14</sup>C-depleted glacial oceanic reservoir. Temperature is also suggested to impact ocean circulation through the redistribution of heat, particularly during climate swings of the last deglaciation. Despite the crucial role that intermediate waters play in linking the deep sea to the atmosphere they remain understudied. Here, we use precisely dated (U/Th) cold-water corals to reconstruct the seawater radiocarbon, temperature, and barium concentration ([Ba]<sub>sw</sub>) of the intermediate depths at Tropic Seamount (tropical Northeast Atlantic). We analysed <sup>14</sup>C, Li/Mg and Ba/Ca ratios of corals distributed from 970 m to 1800 m and dating from 32.7 thousand years (ka) to 0.2 ka. Our results highlight the dynamic behaviour of the intermediate ocean and suggest climate driven variability with distinct features during Heinrich Event 3 (HS3, ~30 ka), Last Glacial Maximum (~22 to 18 ka) and the deglaciation (~18 to 11 ka). Overall, seawater radiocarbon values were higher and temperatures lower in the LGM compared to the deglaciation. We observe a rapid (~500 yr) decrease in intermediate water radiocarbon and temperature during the mid-LGM. This variation and an increase in [Ba]<sub>sw</sub> support a change on the water column configuration, and likely a shift on the boundary between intermediate and deep waters. During the deglaciation, we find larger radiocarbon and temperature changes, suggesting that a warmer and well-ventilated intermediate water was established in parallel with Southern Hemisphere warming, but prior to Northern hemisphere temperature spike at 14.5 ka. These findings reinforce the importance of the intermediate ocean as a vector of climate change and emphasize the advantage of multi-proxy approaches in cold-water corals to investigate environmental conditions.</p>