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Diatom and silicoflagellate abundances

The total abundance of diatoms and silicoflagellates is presented in Figure F2. Although most of the studied interval is barren of both siliceous microfossils, they co-occur with abundances that are on the same order of magnitude (106) as encountered in surface (approximately modern) sediments of the Portuguese margin (Abrantes, 1988) in two specific intervals, 249–252 and 263–265 cmcd (e.g., from the MIS 26/MIS 25 transition to the middle of the interglacial period of MIS 25 and, again during early MIS 27 (middle of MIS 27c) (Fig. F2).

High diatom abundance in the sediments is a good indicator of increased primary production in the surface waters. However, given that several environmental conditions can lead to increased production, it is important to evaluate the assemblage’s composition. Figure F3 shows the contribution of the different species/groups to the total assemblage in 6 of the 8 samples containing a sufficient number (≥95) of diatoms. Resting spores of the genus Chaetoceros dominate the assemblage (45%–65%) at all levels, although their highest abundance occurs at 252.00 cmcd, which according to the isotopic stratigraphy corresponds with MIS 26/25 transition. Other significant contributors to the assemblage are Thalassionema nitzschioides and Paralia sulcata, with T. nitzschioides being important at the interglacial MIS 25 maximum and P. sulcata at the early MIS 27 maximum. Large (>125 µm) Coscinodiscus, mainly Coscinodiscus asteromphalus (Fig. F4A) and few Coscinodiscus apiculatus and Coscinodiscus cf. gigas (Fig. F4B), were observed at two levels (249.68 and 249.91 cmcd) during the MIS 25 interglacial period and in the shipboard core catcher sample of Core 339-U1387B-24X. Although abundant at both levels, higher numbers were observed at 249.91 cmcd in Sample 339-U1387B-24X-5, 124.5–126.5 cm, in which specimens >63 µm but <125 µm were also common but were mostly pyritized.

In respect to ecologic groups, the cold-water pelagic forms never contribute >4% and the warm-water pelagic forms have higher contribution during early MIS 27 (Fig. F3A). The low abundance of benthic forms (<2%) is an important indication that the downcore diatom signal is likely to be a record of local oceanographic conditions rather than the result of downslope transport from shallower depths (<200 m) caused either by bottom currents or sedimentological processes, although the identified forms are relatively resistant to dissolution (Fig. F3B).

All three important components of the diatom assemblage (Chaetoceros resting spore, T. nitzschioides, and P. sulcata) are common in the shelf and upper slope sediments of coastal upwelling regions. On the Portuguese coast, species of the Chaetoceros genus are mostly important during the seasonal upwelling period, and the highest abundance of their resting spores in the sediments mark the inner upwelling front (Abrantes, 1988; Abrantes and Moita, 1999; Moita, 2001). T. nitzschioides is a species commonly found in and around productive coastal upwelling regions in particular off Peru (de Mendiola, 1981) and in the Atlantic and equatorial Pacific (Hasle, 1959). On the Portuguese margin, it is present in the water column throughout the year and is homogeneously distributed in the Portuguese shelf sediments. P. sulcata is rare in the water column where it appears in near-coast upwelling centers but is common in the shelf sediments.

Notable to this record is the presence of Coscinodiscus asteromphalus, a large-diameter species (Fig. F4A) associated with stratified low-nutrient water not common in the water column nor in recent or late quaternary sediments of the Portuguese and northwest African margins (Abrantes, 1991a, 1991b; Nave et al., 2003; Romero et al., 2008). However, in the Gulf of California, C. asteromphalus forms large blooms during the fall that sink when the thermocline breaks down (Round, 1968; Kemp, 1995; Kemp et al., 2006). The species has also been described in Peru sediments where Chaetoceros spores are less abundant (Schuette and Schrader, 1981; Fleury 2015).

Although the high abundances of Chaetoceros resting spores and T. nitzschioides suggest oceanographic conditions favorable to increased primary productivity from the MIS 26/25 transition to the interglacial period of MIS 25, the presence of C. asteromphalus implies incursions of open-ocean water into the southern Portuguese coast and low-wind conditions during the interglacial period of MIS 25.