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doi:10.2204/iodp.proc.320321.208.2013

Results

Analytical issues

Major difficulties were encountered during analysis of the suite of samples selected from Sites U1331–U1333 for stable isotopes. This is because the carbonate content varies significantly across the depth intervals of interest, from 0% to 3% at Site U1331, 0% to 60% at Site U1332, and 0% to 90% at Site U1333 (see the “Site U1331,” “Site U1332,” and “Site U1333” chapters [Expedition 320/321 Scientists, 2010b, 2010c, 2010d). We tried to circumvent this issue for samples with low carbonate content by increasing the sample size and using a different mass spectrometer configuration (see “Methods”). This appears to have been successful in some cases, but not in others. Importantly, stable isotope analyses for samples examined by both instrumental configurations give fairly similar values, especially for δ¹³C.

We present all data obtained (Table T1) but flag those samples with essentially zero carbonate. Stable isotope data for these samples probably has little paleoceanographic significance, and we do not discuss them below.

Bulk carbon isotopes

Carbon isotope compositions of six samples from Site U1331 range from 0.9‰ to 1.6‰ (Table T1). According to the “Site U1331” chapter [Expedition 320/321 Scientists, 2010b), the samples lie just below the apparent Chron C23n.2n–C23r boundary (201.5 m CCSF-A; 51.74 Ma) but span the top occurrence of the calcareous nannofossil, Tribrachiatus orthostylus (203.47 m CCSF-A; 50.7 Ma). It should be noted that these interpretations lead to problematic stratigraphy. In any case, the samples likely accumulated after the start of the Early Eocene Climatic Optimum (~52.5 Ma). The δ¹³C numbers are consistent with this view, as they are similar to bulk carbonate δ¹³C values determined at other locations during the early Eocene (e.g., Shackleton, 1986).

The δ¹³C values at Site U1332 fluctuate between 1.02‰ and 5.06‰ (Fig. F2). The lower middle Eocene section, between ~118 and 152 m CCSF-A, shows a relatively steady pattern, in which δ¹³C values average 2.1‰ ± 0.2‰ (1σ). Such values are consistent with other stable isotope records of bulk sediment deposited during this time (e.g., Shackleton, 1986). However, major fluctuations in the δ¹³C record are observed in upper middle Eocene and upper Eocene sediment from ~80 to 118 m CCSF-A. In this interval, δ¹³C values average 3.2‰ ± 0.8‰ (1σ); this includes several samples with δ¹³C between 4‰ and 5‰. The extreme variance and exceptionally ¹³C-enriched conditions arise in samples from both Hole U1332A and U1332B and from both IRMS configurations; they are not a result of analytical error. On the other hand, this part of the δ¹³C record does not conform to that generated at other locations (Shackleton, 1986; Bohaty and Zachos, 2003; Bohaty et al., 2009), including at Site U1333 (Fig. F3). Samples from the uppermost Eocene and lower Oligocene (74–80 m CCSF-A) have δ¹³C values that average 1.6‰ ± 0.2‰ (1σ) that may capture the drop and rise in δ¹³C spanning the Eocene/Oligocene boundary (e.g., Shackleton, 1986; Coxall et al., 2005).

At Site U1333, δ¹³C values vary between 1.41‰ and 3.49‰. Samples from between 175 and 204 m CCSF-A, which corresponds to the lower middle Eocene, are again characterized by relatively steady values. The δ¹³C values average 2.0‰ ± 0.1‰. Above, between 136 and 175 m CCSF-A, δ¹³C values are generally higher and more variable, averaging 2.4‰ ± 0.4‰. This is similar to observations of upper middle to upper Eocene sediment at Site U1332, except the extreme positive values are absent. The uppermost interval of the studied section, between ~131 and 136 m CCSF-A, spans the Eocene/Oligocene boundary and is characterized by a prominent 1.0‰ drop followed by a 0.6‰ rise in δ¹³C, a feature observed in other bulk sediment records (e.g., Shackleton, 1986).

Bulk oxygen isotopes

The δ¹⁸O of the samples from Site U1331 range from –0.87‰ to 0.19‰ (Table T1). These values display a similar range but are slightly more depleted in ¹⁸O than corresponding samples in other bulk sediment δ¹⁸O records (Shackleton, 1986). Recall, though, that Site U1331 was near the Equator; therefore, δ¹⁸O values of carbonate should be more depleted than those from locations at higher latitude, assuming temperature influences bulk sediment oxygen isotope compositions.

At Site U1332, δ¹⁸O fluctuates between –3.63‰ and 0.58‰ (Fig. F2). The lower middle Eocene section, between 118 and 152 m CCSF-A, has δ¹⁸O values that are generally more depleted in ¹⁸O than in the rest of the studied interval, averaging –1.1‰ ± 0.6‰. These values again exhibit a similar range but are more depleted in ¹⁸O than corresponding samples from other locations. The upper middle to upper Eocene interval is characterized by δ¹⁸O values that average –0.5‰ ± 0.6‰ and generally increase with shallower depth. The interval spanning the Eocene/Oligocene boundary has an average δ¹⁸O value of –1.0‰ ± 0.7‰, with much of the variance arising from a clear 1‰ increase into the Oligocene. The values are again much more depleted in ¹⁸O than in comparable records, although the rise across the Eocene/Oligocene boundary is of similar magnitude (Shackleton, 1986).

Oxygen isotope values vary between –1.85‰ and 0.28‰ across samples at Site U1333. In the lower middle Eocene section, δ¹⁸O averages –0.9‰ ± 0.3‰. This is somewhat similar to the comparable interval at Site U1332. As at Site U1332, overlying sediments between 136 and 175 m CCSF-A have a higher average and greater range in δ¹⁸O (–0.75‰ ± 0.4‰). Samples spanning the Eocene/Oligocene boundary have an average δ¹⁸O value of –1.1‰ ± 0.6‰. As at Site U1332 and other locations, a rise of at least 1.0 ‰ explains much of the variance.

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