ALBERT

Description: Negative stable carbon isotopic excursions have been observed throughout most of the mid-depth (~1000–3000m) Atlantic Ocean during Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). Although there is agreement that these mid-depth excursions were in some way associated with a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), there is still no consensus on the precise mechanism(s). Here, we present benthic stable carbon and oxygen isotopic (δ 13 C and δ 18 O) records from five cores from the western equatorial Atlantic (WEA). Together with published benthic isotopic records from nearby cores, we produced a WEA depth transect (~800–2500m). We compare HS1 and YD data from this transect with data from previously published North- and South Atlantic cores and demonstrate that the largest negative δ 13 C excursions occurred in the WEA during these times. Moreover, our benthic δ 18 O records require the presence of two water masses flowing from the Southern Ocean, bisected by a Northern Component Water (NCW). Given that δ 18 O is a conservative water mass tracer, we suggest that δ 13 C was decoupled from water mass composition and does not correspond to simple alternations between northern and southern sourced waters. Instead, δ 13 C behaved non-conservatively during HS1 and the YD. Consistently with our new 231 Pa/ 230 Th record from the WEA transect, that allowed the reconstruction of AMOC strength, we hypothesize that the negative δ 13 C excursions reflect an increase in the residence time of NCW in response to a weakened AMOC, allowing for a marked accumulation of 13 C-depleted respired carbon at the mid-depth WEA.