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

Abstract

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 sinistral is indicative of a deeper calcification depth in the water column compared to G. truncatulinoides dextral as previously indirectly inferred in a plankton tow study. Furthermore, the change in coiling ratio from dominantly G. truncatulinoides dextral (95%) to brief episodes of dominantly G. truncatulinoides sinistral (80%) gives a strong indication of deepening of the permanent thermocline during periods in which G. truncatulinoides sinistral 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.

Oxygen Isotope Variability of Planktonic Foraminifera Provide Clues to Past Upper Ocean Seasonal Variability

“…The raw single‐shell data (Feldmeijer et al, ; Metcalfe et al, ), including the new single‐shell measurements of N. pachyderma , are presented in Figure ; the computed mean and range in δ 18 O for the seven analyzed planktonic species and one epibenthic species are shown in Figure , for the core interval representing 200 to 260 ka. The absolute values, ignoring potential vital effects, of G. bulloides , G. inflata , and G. truncatulinoides , are similar to those of N. incompta and G. glutinata while N. pachyderma has similar values to the benthic species C. wuellerstorfi (vertical bars in Figure a).…”

“…Single‐shell δ 18 O analysis of G. bulloides , G. inflata (Metcalfe et al, ), and the coiling ratio (sinistral and dextral) of G. truncatulinoides (Feldmeijer et al, ) have been analyzed previously; here new single‐shell δ 18 O data of N. pachyderma were generated. N. pachyderma is an important indicator of “polar” conditions (Schiebel et al, ), being associated with the southward migration of the polar front during glacial conditions (Penaud et al, ; Ruddiman and McIntyre, ) and its occurrence either precedes or follows ice‐rafting during Heinrich events (Kretschmer et al, ).…”

“…Between 728‐ and 828‐cm core T90‐9p was sliced into 1‐cm samples at ~4‐cm intervals; this section of T90‐9P (Feldmeijer et al, ; Lototskaya et al, ; Lototskaya & Ganssen, ; Metcalfe et al, ; van Kreveld et al, ) covers the transition from MIS 8 and MIS 7 (Figures and ). The age depth relationship (Table ) is that of Feldmeijer et al (), with the downcore record of G. inflata tuned to the “planktonic foraminifera” depth‐derived δ 18 O stack of Huybers (Huybers, ) to both avoid circular reasoning and preclude the evaluation of orbital parameters (Huybers, ; Huybers & Wunsch, ).…”

“…Raw single‐specimen oxygen isotope values. (a) Age versus oxygen isotope (δ 18 O) values of Globorotalia inflata (Feldmeijer et al, ; Metcalfe et al, ) with the record selected as a tuning target, the planktonic stack of Huybers (), plotted alongside. (b) Marine isotope stages (MIS) of the core interval analyzed for single specimens of (c) G. bulloides (Metcalfe et al, ) and (d) N. pachyderma (this paper).…”

“…For single‐shell analysis, two surface dwelling species, G. bulloides and N. pachyderma (sinistral/left‐coiling also referred to as NPS), and two deeper dwellers, G. inflata and G. truncatulinoides , were picked out from narrow size fractions to exclude potential vertical migration during ontogeny (250–300 μm for N. pachyderma and 300–355 μm for others). Single‐specimen data of G. bulloides , G. inflata , and G. truncatulinoides (sinistral and dextral coiling direction) from the 300–355 μm have been published previously (Feldmeijer et al, ; Metcalfe et al, ). In conjunction with these previous measurements, 20 individuals of N. pachyderma from the 250–300 μm were picked from each sample, weighed in groups of 5 using a Sartorius Microbalance, measured for size using a Nikon digital research microscope, and analyzed singularly for stable isotopes.…”