RESULTS

Results from stable isotope measurements are displayed in Table T1 and shown in Figure F4. The isotopic composition of bulk sediment samples from the carbonate section (Subunits U3B and U3C) falls between –1 and +1 for ¹⁸O and between 1.5 and 3 for ¹³C. Only in the immediate vicinity of the intrusive gabbroic unit (U4) are values clearly more negative, reaching values as low as –20.5 ¹⁸O and –0.4 ¹³C. This negative shift is interpreted as the result of thermal alteration caused by gabbro emplacement and is not further considered in this study.

A comparison of isotope data from the pelagic sections at Sites 1039 and 1040 (Fig. F5) reveals that ¹³C values from both sites are virtually indistinguishable, except for the interval between 310 and 330 mbsf in the composite section where the two sites show slightly different ¹³C values outside analytical error. On the other hand, ¹⁸O values of bulk sediment samples from Site 1039 are more negative in four specific intervals (marked with yellow bars in Fig. F5) but do not show significant differences in the other part of the section. A maximum deviation of ~0.75 in ¹⁸O that clearly lies above the measurement and depth transformation error is reached in the lowermost 30 m of Subunit U3C immediately above the gabbro sill.

From available data at Site 1040, there is no evidence for higher porosity or for significant lithologic or composition differences that would indicate possible fluid flow or local diagenetic alteration in the respective intervals. However, the observed interval-specific offsets in the oxygen isotopic signal between the reference site and the underthrusted section are significant and need further study to investigate their implications for diagenetic alteration and fluid flow during early subduction.

With the assumption that general trends in bulk sediment isotopic composition at the reference Site 1039 mainly reflect trends in the primary isotopic composition of the pelagic carbonaceous components (i.e., coccolithophorids and foraminifers that are well preserved in smear slides and visually do not show diagenetic imprinting; Morris, Villinger, Klaus, et al., 2003; Kimura, Silver, Blum, et al., 1997), results from stable isotope geochemistry are used to establish an isotope stratigraphy for the studied section. Data from ¹³C analyses show dominant positive peaks of up to 1.5 (e.g., at 260 mbsf at Site 1039) that can be correlated to CM events of the Monterey excursion in the middle Miocene (Woodruff and Savin, 1991). Figure F6 shows the correlation between the maximum ¹³C peaks in Unit U3 of Site 1039 and CM 1–6 measured in a pelagic section in DSDP Site 574 from the equatorial Pacific (Woodruff and Savin, 1991) and outcrop data from a section on the Maltese Islands (John et al., 2003). Site 574 was further used to correlate minima in ¹⁸O values (¹⁸O Events A–F; Woodruff and Savin, 1991), revealing a robust age control on the lower stratigraphic section of Site 1039 (Figs. F6, F7).

The absolute chronology of the middle Miocene carbonate section recovered at Site 1039 is shown as an age vs. depth diagram in Figure F8. It is compiled from all available biostratigraphic, magnetostratigraphic, and isotopic datums (Kimura, Silver, Blum, et al., 1997; Muza, 2000) (Table T2). The stable isotope stratigraphy presented here is in agreement with the age models derived from biostratigraphy (Muza, 2000) and confirms that sedimentation rate of the middle Miocene carbonate-rich Subunits U3B and U3C is constant and on the order of 50 m/m.y. This is in contrast with the very high sedimentation rates at ~12.7 Ma and lower sedimentation rates (~18 m/m.y.) in the lower part of the section between 16 and 13 Ma, as inferred from the magnetostratigraphic datums (Kimura, Silver, Blum, et al., 1997).