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doi:10.2204/iodp.proc.339.202.2017

Methods and materials

Diatom and other siliceous microfossil abundance was analyzed on 98 samples of Cores 339-U1387B-24X through 26X and 339-U1387A-25X and 26X (Table T1) following the corrected composite depth of the new splice for the early to Middle Pleistocene section at Site U1387 (Voelker et al., unpubl. data).

For diatom quantification, sediments were cleaned and mounted according to the method described by Abrantes et al. (2005). Diatom counting (centric, pennate, freshwater, benthic, Chaetoceros resting spores, and fragments) was undertaken according to the unit definitions by Schrader and Gersonde (1978) at 1000× magnification using a Nikon microscope with differential interference contrast illumination.

Quantitative abundance estimation is based on the median value obtained from counts of 100 random fields of view on 3 replicate slides for each sample. With this counting method, and knowing the area of the microscope field of view (FOV), the absolute number of diatom valves per gram of sediment can be calculated as

where

• N = median number in 100 FOV and 3 replicate slides,
• S = area of the evaporation tray,
• s = area of slide covered for counting,
• V = total solution,
• v = solution volume extracted to dry, and
W = weight of the treated, dried raw sample.

Absolute diatom numbers are presented as number of valves per gram of sediment and silicoflagellates as number of specimens per gram of sediment. Analysis of replicate slides reduce the analytical error on the estimate of absolute abundances to ≤10% of measured values.

Assemblage composition was determined for 8 samples selected on the basis of total diatom abundance (Fig. F2C). Percent abundance is calculated from the identification of 300 specimens, but in cases where total diatom abundance fell below 10⁶ valves/g, the number of specimens counted was reduced to 100 (Fatela and Taborda, 2002).

Diatom identification followed the descriptions of Hustedt (1964), Hasle (1977), Carmelo (1996), and Round et al. (2007), and a list of the 27 identified taxa is presented in Table T2. The dominant taxa were considered independently as well as grouped according to their ecological preferences (e.g., benthic, freshwater, marine). Within the marine grouping we also split the pelagic forms into cold- and warm-water groups (Hustedt, 1964; Hasle, 1977; Carmelo, 1996). Groups that comprise <2% of the total diatom assemblage were discarded from further analyses.

As pointed out by Voelker et al. (2015), the shipboard splice of Site U1387 is incomplete within the middle to early Pleistocene section. The higher resolution X-ray fluorescence (XRF) and foraminiferal shell stable isotope records were therefore used to revise the splice resulting in a corrected meters composite depth (cmcd) scale. The current study encompasses samples from 244 to 274 cmcd (~236–260 mcd) and the splice corrections within this interval include

1. Appending the top of Section 339-U1387A-25X-1 to the bottom of Section 339-U1387B-24X-CC;
2. Incorporating 2.94 m into the splice for a transition from Section 339-U1387B-25X-6, 125 cm, to 339-U1387A-26X-1, 5 cm; and
3. Adding 0.96 m at the subsequent splice transition for a shift from Section 339-U1387A-26X-4, 113 cm, to 339-U1387B-26X-1, 78 cm.

Samples of Core 339-U1387B-24X themselves already inherited additional 8.53 m to their mcd values (from splice corrections above).

The stratigraphy of the studied interval is based on the δ¹⁸O record of the planktonic foraminifer Globigerina bulloides (Fig. F2A).

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