Reconstructing the depth of the permanent thermocline through the morphology and geochemistry of the deep dwelling planktonic foraminifer Globorotalia truncatulinoides

Geochemical and morphological characteristics of Globorotalia truncatulinoides, a deep dwelling planktonic foraminifer, have been used since the mid-1950s to infer (paleo)oceanographic conditions of the upper ocean. The coiling ratio has been linked to different water masses and stable oxygen isotope signal of this species to changes in depth habitat and/or season. Here we show that the isotopic composition of single specimens covering Termination III of multiple size fractions of North Atlantic G. truncatulinoides<inf>sinistral</inf> is indicative of a deeper calcification depth in the water column compared to G. truncatulinoides<inf>dextral</inf> as previously indirectly inferred in a plankton tow study. Furthermore, the change in coiling ratio from dominantly G. truncatulinoides<inf>dextral</inf> (95%) to brief episodes of dominantly G. truncatulinoides<inf>sinistral</inf> (80%) gives a strong indication of deepening of the permanent thermocline during periods in which G. truncatulinoides<inf>sinistral</inf> was dominant. The position of the permanent thermocline during marine isotope stages 8 and 7 echoes the relative strength of the Atlantic meridional overturning circulation (AMOC), dominated by interglacial-glacial dynamics. We demonstrate that Glacial Heinrich (ice-rafted debris) events appear to proceed a permanent thermocline shoaling, whereas interglacial Heinrich events follow the shoaling of the permanent thermocline, likely a result of a weakened AMOC. Key Points Coiling ratio of G. truncatulinoides indicative of depth of the thermocline Single-specimen isotopes of G. truncatulinoides confirm inferred depth habitat