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doi:10.2204/iodp.proc.342.204.2016

Results

Abundance and preservation

Group abundance, preservation, and total number of benthic foraminiferal specimens examined in each sample are shown in Figure F2. Whereas total number of specimens examined is typically ~100–150 from Samples 342-U1406A-1H-CC (6.23 m CCSF) through 26X-CC (246.36 m CCSF), the number decreases downhole. The scarcity in benthic foraminifers is also seen in the group abundance (Fig. F2), which is due to the increasing abundance of planktonic foraminifers in carbonate-rich sediments of lithostratigraphic Units III and IV (Fig. F2) (see the “Site U1406” chapter [Norris et al., 2014c]). Carbonate content significantly decreases at ~270 m CCSF, and benthic foraminiferal group abundance increases uphole from ~260 m CCSF to the seafloor. Group abundance shows the highest values between 120 and 250 m CCSF. The preservation of benthic foraminifers seems to be affected by the carbonate content as well. Whereas moderate or poor preservation is occasionally found in samples deeper than 250 m CCSF, the preservation is predominantly very good in shallower samples. Because cores shallower than 250 m CCSF contain less carbonate and more clay minerals (see the “Site U1406” chapter [Norris et al., 2014c]), the preservation of benthic foraminifers seems to be enhanced in the less permeable clay-rich sediments. On the other hand, in carbonate-rich deeper cores, precipitation of secondary calcite overgrowth has often been found.

Relative abundance of benthic morphotypes

Among the five different benthic foraminiferal morphotypes identified, trochospiral is the dominant type throughout the studied interval (Fig. F2). In contrast, the planispiral morphotype is the least abundant, with values usually <10% (average = 5.9%). Agglutinated foraminifers are also a minor constituent of the investigated faunas, showing average and maximum abundances of 7.7% and 27.3%, respectively. Compared to the agglutinated and planispiral morphotypes, elongated and tapered forms are relatively abundant, and their abundance is characterized by large variability downhole (Fig. F2). Thereby, maximum abundances are 68.8% for the elongated morphotype and 57.8% for the tapered morphotype.

Deeper than 239 m CCSF (except for samples between 267 and 272 m CCSF), the trochospiral morphotype is especially abundant (>40%), whereas tapered morphotype abundance is low. In Samples 342-U1406A-28X-4, 100–102 cm (267.30 m CCSF), 28X-6, 100–102 cm (270.30 m CCSF), and 28X-CC (271.69 m CCSF), the elongated morphotype suddenly becomes very abundant and reaches 68.8% of the studied fauna, which is the highest value throughout the studied interval. In parallel, the trochospiral and tapered morphotypes decrease to <20%. This drastic change in relative abundances can be correlated to an abrupt decrease in carbonate content (Fig. F2). Whereas carbonate content between 280 and 310 m CCSF is usually >80 wt%, it decreases rapidly between 270 and 280 m CCSF, reaching ~50 wt% at 260 m CCSF. The observed increase of the elongated morphotype occurs parallel to this drop in carbonate content. However, whereas carbonate content stays low uphole (~40–60 wt%), the relative abundances of trochospiral and tapered morphotypes rapidly increase around ~260 m CCSF, adding to ~80% of the entire fauna.

At ~220 m CCSF, which is correlated to the Eocene/Oligocene boundary, abundance of the tapered morphotype significantly increases, mainly driven by the abundant occurrence of Cassidulina spp. (see the “Site U1406” chapter [Norris et al., 2014c]). High abundances of the tapered morphotype occur uphole to ~130 m CCSF, well into the late Oligocene. Across the Eocene/Oligocene boundary, the observed increase in the tapered morphotype is accompanied by a rapid increase in carbonate content (“carbonate overshot”; Coxall et al., 2005) followed by a drop in carbonate content to ~40 wt%. Although a significant increase of the elongated morphotype is identified across the carbonate decrease event at ~270 m CCSF, no significant increase of this morphotype is found across the Eocene/Oligocene boundary at Site U1406. In contrast, Ortiz and Kaminski (2012) reported an abrupt and short-lived increase of the elongated morphotype (mostly stilostomellids) at the Eocene/Oligocene boundary at Ocean Drilling Program Site 647 in the southern Labrador Sea. This apparent discrepancy may be related to the relatively lower sample resolution of this study. Shallower than ~130 m CCSF, the trochospiral morphotype becomes the dominant morphotype uphole to ~90 m CCSF, which can be correlated to the late Oligocene. The abundance of trochospiral forms decreases at ~90 m CCSF. Interestingly, the tapered morphotype also increases at ~90 m CCSF, which is correlated to the Oligocene/Miocene boundary. Cassidulina spp., which is the most significant component of the tapered morphotype (Table T1), becomes abundant again in the lower Miocene.

Although the relative abundances of each benthic foraminiferal morphotype vary with carbonate content, abundance seems to be independent of primary production. This is shown by the lack of response in morphotype abundances between 10 and 150 m CCSF, an interval characterized by high abundances of diatoms and radiolarians, probably indicating enhanced primary production (see the “Site U1406” chapter [Norris et al., 2014c]).

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