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doi:10.2204/iodp.proc.307.202.2009

Results

Raw data and stratigraphic profiles of carbon and oxygen isotopes are shown in Table T1 and Figure F1.

δ¹³C values range from –3.2‰ to 1.2‰, which is generally higher and more stable than the δ¹³C range of the mound section (–5.3‰ to –1.5‰) (Kano et al., 2007). The values fluctuate from –1.0‰ to 1.0‰ in Subunits 1A and 1B and record the lowest value (around –3‰) in Subunit 1C. From the lower part of Subunit 1C to Subunit 3A, δ¹³C values increase gradually.

δ¹⁸O values range from –4.6‰ to 1.9‰ (Table T1; Fig. F1). The profile shows a slight increasing trend from the top of Subunit 1A to Unit 2, with three peak horizons in Subunit 1A. At the Unit 2/​Subunit 3A boundary, δ¹⁸O values represent an abrupt increase from around –2.5‰ to +1.5‰.

Cross-plots of carbon and oxygen isotopes (Fig. F2) represent differences in the values among the stratigraphic units. The most distinct pattern is the distribution of the Subunit 3A samples, which is much higher in δ¹⁸O and slightly higher in δ¹³C than other units. Most Unit 1 samples and many Unit 2 samples were plotted in a small domain of –2.0‰ to –4.5‰ in δ¹⁸O and 0.8‰ to –1.0‰ in δ¹³C.

Figure F2 also shows ranges in δ¹⁸O and δ¹³C values of potential carbonate sources, including Pleistocene benthic foraminifers in the North Atlantic (δ¹⁸O = 2.5‰ to 5.5‰; δ¹³C = –0.5‰ to –1.5‰) (Venz et al., 1999), planktonic foraminifers from Site U1317 (δ¹⁸O = –0.8‰ to 1.4‰; δ¹³C = 0.5‰ to –1.5‰) (Sakai et al., 2009), and Cretaceous chalk in Southeast England (δ¹⁸O = –1.8‰ to approximately –3.5‰; δ¹³C = 1.3‰ to ~3.0‰) (Jenkyns et al., 1994). It also shows the results of bulk sediments from on-mound Site U1317 (Takashima et al., 2006).

Variation in δ¹⁸O and δ¹³C values in our plots can be explained by variation in relative importance of source carbonates. The Miocene (Subunit 3A) plots indicated that much of the carbonate was derived from foraminifers, whereas the values (especially δ¹⁸O) of Subunits 1 and 2 are similar to the values of Cretaceous chalk (Fig. F2). This trend is consistent with the common occurrence of reworked Cretaceous nannofossils (see the “Site U1318” chapter). The 2‰ difference in δ¹³C between the bulk Site U1318 sediments and southeast England chalk (Fig. F2) suggests that there might be substantial sources with lower δ¹³C values that cannot be specified in this study. The three δ¹⁸O peak horizons in Subunit 1A may indicate the low-δ¹⁸O fraction of an eroded chalk. Sediments in Units 1 and 2 contain an abundance of terrestrial particles that are associated with ice-rafted debris (see the “Site U1318” chapter). Therefore, the changes in oxygen and carbon isotopic values from Unit 3 to Units 1 and 2 likely reflect intensified glaciation during the late Pliocene and early Pleistocene.

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