Neogloboquadrinids in the mid-latitudinal, subtropical NE Atlantic during the Early-to-Middle Pleistocene: evidence for “not (sub)polar” variants and one extinction

The Gulf of Cadiz (NE Atlantic) is dominantly influenced by subtropical gyre waters that are advected by the Azores Current, as reflected by the strong contribution of subtropical species to the planktonic foraminifera fauna. Late Pleistocene sediment records from the Gulf of Cadiz also revealed incursions of subpolar waters based on the presence of Neogloboquadrina pachyderma during abrupt stadial events when the Atlantic meridional overturning circulation was reduced and the subarctic front moved into the lower mid-latitudes of the North Atlantic. To expand our regional knowledge on the interplay between climate variability and planktonic foraminifera faunas, we generated planktonic foraminifera assemblage and other paleoclimate data at millennial-scale resolution between Marine Isotope Stage (MIS) 52 (1.54 Ma) and MIS 18 (0.74 Ma) at IODP Site U1387 (36°48´N, 7°43´W) on the southern Portuguese margin. Here we focus on the evolution of the Neogloboquadrinids, namely N. atlantica dextral, N. incompta and N. pachyderma.

N. atlantica dextral was present continuously since MIS 52 until early MIS 40, when it went extinct around 1.3 Ma ago. This extinction event falls into the period when abundances of N. pachyderma were greatly reduced in the North Atlantic and the Mediterranean Sea, where this period was dated between 1.37 and 1.21 Ma (Lourens et al., 1996, doi:10.1029/96PA02691). At Site U1387, the onset of low N. pachyderma abundances coincided exactly with the Mediterranean Sea date, but abundances started to increase already around 1.22 Ma, i.e. with the first abrupt cold events during the MIS 37/MIS 36 transition.

On the southern Portuguese margin, MIS 36 was a peculiar glacial period with relative warm sea-surface temperatures throughout. We, nevertheless, observed the continuous presence of N. pachyderma and N. incompta. In late MIS 36 and extending into MIS 35, a larger sized N. incompta variant with 4 to 4.5 chambers and a thicker shell structure, more reminiscent of N. dutertrei or N. atlantica, appeared. This variant co-occurred with noticeable contributions of tropical species in the assemblages leading us to interpret it as a subtropical gyre variant. During late MIS 35, also larger sized N. pachyderma specimens started appearing in the assemblages and their contributions became prominent during MIS 28, in particular between 1.01 and 1 Ma, before slowly diminishing towards the end of the early Pleistocene. Already since MIS 52, we detected the co-occurrence of N. pachyderma with subtropical and tropical species. During some of the warmer interglacial periods like MIS 51, MIS 49, MIS 47, and MIS 31, when the subtropical fauna dominated (≥30 %), N. pachyderma specimens were present nearly continuously. Given the strong influence of subtropical waters reflected in the faunas, we believe that those N. pachyderma specimens also present a subtropical gyre variant, potentially predecessors of the modern-day genotypes found in the mid-latitudinal subtropical North Atlantic. Overall, the Site U1387 time series reveal that the early Pleistocene appears to be an important period for Neogloboquadrinid evolution and that the potential occurrence of subtropical water variants cannot be overlooked when interpreting faunal data from that period.