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doi:10.2204/iodp.proc.318.202.2016

Results

Calcium carbonate content

During Expedition 318, four Cores (34X–37X; 311.60–349.99 mbsf), averaging 100% recovery, were collected from Hole U1361A. These cores contained the only significant calcium carbonate (CaCO₃) preservation within the Miocene section (Fig. F3). Carbonate preservation was typically <1% for sediments from Hole U1361A. From 11.7 to 11.0 Ma, CaCO₃ increased to a maximum of 24%, averaging 16.3% (Table T1).

Methane concentration

Methane concentration in Hole U1361A is elevated during the middle to late Miocene with an average concentration of 11,632 ppmv, which is 3.2 times greater than the whole core average (Fig. F3, Table T2). The highest methane concentrations are 14,739 ppmv in Core 35X (325.7 mbsf), 13,826 ppmv in Core 36X (335.3 mbsf), and 15,362 ppmv in Core 38X (353.5 mbsf; Core 38X is not included in this study). Concentrations reach negligible amounts by 162.7 mbsf and remain low to the core top (see the “Site U1361” chapter [Expedition 318 Scientists, 2011b]).

Hole U1361A foraminiferal data

Foraminiferal abundances range from absent to common, and 76% of the 90 samples examined contain foraminifers. Planktonic and benthic foraminifers exhibit good to excellent preservation (Fig. F4), with the majority of benthic foraminifers having a glassy appearance. Benthic foraminifers were identified down to the species level to evaluate paleoenvironmental changes in the middle to late Miocene Southern Ocean (Table T3). Dominant species include Nuttallides umbonifera, Pullenia bulloides, O. umbonatus, Melonis pompilioides, G. soldani, E. exigua, C. wuellerstorfi, and C. bradyi. Specimens from the genus Oolina sp., Fissurina sp., Lagena spp., and Sphaeroidina sp. were also observed.

E. exigua is the most dominant benthic species throughout the majority of the study interval (Table T3); it inhabits cold and well-oxygenated waters with abundance positively correlated with seasonal phytodetritus (Smart, 2008). Cibicidoides spp. mainly occurs in the upper portion of the core from 325.35 to 313.23 mbsf and prefers cold waters with low organic matter flux (Poli et al., 2012).

Stable isotopes

Planktonic (G. bulloides) and benthic (O. umbonatus, G. soldani, E. exigua, C. wuellerstorfi, and C. bradyi) foraminifers were first run at Lamont-Doherty Earth Observatory for stable isotopes. The δ¹⁸O values range from –0.3‰ to 2.4‰ for planktonic foraminifers and 1.3‰ to 3.3‰ for benthic foraminifers (Table T4; Fig. F5), and δ¹³C values range from –15.5‰ to 0.4‰ for planktonic foraminifers and –9.3 to 1.0‰ for benthic foraminifers (Table T5; Fig. F5). A second set of samples were sent to Rutgers University for analysis and yielded similar results, with δ¹⁸O values from benthic foraminifers ranging from 0.8‰ to 3.4‰ (Table T4; Fig. F5) and δ¹³C values ranging from –12.1‰ to 1.1‰ (Table T5; Fig. F5).

The extremely negative δ¹³C values suggest that foraminiferal calcite was diagenetically altered by the incorporation of ¹³C-depleted inorganic carbonate, likely caused by methane seeps at the core site (Martin et al., 2004; Torres et al., 2003; Rathburn et al., 2000). Foraminiferal δ¹³C values decrease to –16‰ at Hole U1361A, whereas Southern Ocean values typically reach 0.4‰ (Wright and Miller, 1993).

Benthic foraminiferal δ¹⁸O values vary more than 2‰, which suggests unrealistically extreme changes in ice volume and temperature over short timescales (<100 ky). The dissociation of methane hydrates has been observed to cause alterations in δ¹⁸O of pore waters (Torres et al., 2003) and likely caused the alteration of foraminiferal δ¹⁸O. Interestingly, these diagenetically altered foraminifers, most of which had a glassy texture, showed little to no indication of overgrowth during visual inspection with a binocular light microscope.

A list of sample requests is found in Table T6.

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