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doi:10.2204/iodp.proc.341.205.2019

Methods and materials

Calcareous nannofossils

A total of 131 samples were studied for calcareous nannofossils from the Site U1418 shipboard splice, which includes material from Holes U1418A, U1418C, U1418D, and U1418E (Jaeger et al., 2014b). Samples were prepared using a standard smear slide technique and examined using a Zeiss Axioskop microscope at 1000× magnification. Fossil counts were performed until 100 specimens were reached. Occasionally, 100 specimens could not be found in the entire slide. Typically, nannofossil assemblage counts include many more specimens to reach a robust statistical significance (300+); however, fossils were so rare in these samples that we chose a lower number of specimens that was attainable for most slides. The total number of counted nannofossils is reported in Table T1.

Total calcareous nannofossil abundance and preservation was visually assessed using the protocol outlined in Clemens et al. (2016).

Total calcareous nannofossil abundance was estimated as follows:

• D = dominant (>90% of particles)
• A = abundant (>50%–90% of particles)
• C = common (>10%–50% of particles)
• F = few (1%–10% of particles)
• R = rare (<1% of particles)
• B = barren (no specimens)

Preservation of calcareous nannofossils was qualitatively described as:

• VG = very good (no evidence of dissolution and/or recrystallization, no alteration of primary morphological characteristics, and specimens identifiable to species level).
• G = good (little evidence of dissolution and/or recrystallization, primary morphological characteristics only slightly altered, and specimens identifiable to species level).
• M = moderate (specimens exhibit some etching and/or recrystallization, primary morphological characteristics somewhat altered, and most specimens identifiable to species level).
• P = poor (specimens were severely etched or overgrown, primary morphological characteristics largely destroyed, fragmentation has occurred, and specimens often could not be identified to species and/or generic level).

Calcareous nannofossil taxonomy followed that of Bown (1998), Perch-Nielsen (1985), and Nannotax3 (Young et al., 2018); full taxonomic lists can be found in these references. Additional rare species that were observed while scanning the slides but not while performing the quantitative counts are illustrated in Plates P1 and P2.

The shipboard age model suggests the studied section spans the recent to ~200 ka (Jaeger et al., 2014b), during which time there are no nannofossil zonal markers (Gradstein et al., 2012). The datum Gephyrocapsa caribbeanica/Emiliania huxleyi crossover (0.09 Ma) was not observed because G. caribbeanica was not present at this site.

Gephyrocapsa species were divided based on size and morphology. Gephyrocapsa muellerae is the prominent Gephyrocapsa species in these sediments. Smaller forms (< 3 µm) of Gephyrocapsa with a visible bar across the central area were identified separately. These small forms were most likely G. ericsonii; however, we could not confidently identify these small forms without scanning electron microscopy (SEM). Specimens that had a Gephyrocapsa-like rim but were lacking the central area bar were grouped as either “Gephyrocapsa or Emiliania spp. < 3 µm” or “Gephyrocapsa spp. > 3 µm.” Under light microscopy it was very difficult to distinguish between the rims of these two genera. Our assumption is that these were gephyrocapsids missing the central bar due to preservation, but it cannot be ruled out that they were Emiliania huxleyi specimens without detailed SEM analysis.

All reworked specimens were included in the abundance counts and grouped together as “reworked fossils.” These specimens were not used to classify preservation.

Calcium carbonate

Carbonate weight percent (CaCO₃ wt%) was measured on 159 samples from Site U1418. Samples were first dried in a 50˚C oven for at least 24 h. They were then ground using a mortar and pestle and placed back in the oven for another 24 h.

CaCO₃ was determined using an UIC Inc. model 5011 CO₂ coulometer at the International Ocean Discovery Program at Texas A&M University. Between 10 and 18 mg of dried, powered sediment was weighed on a microbalance and reacted with 2 M HCl. The resulting CO₂ was titrated and the end-point determined by a photodetector. The liberated CO₂ was assumed to be entirely from dissolved CaCO₃. The total inorganic carbon (TIC) output was given in micrograms C, and the following equation was used to calculate CaCO₃ wt%:

A replicate was run after every 5 sample measurements, and a standard (100% CaCO₃) was run after every 10 measurements to check for instrumental drift.

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