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doi:10.2204/iodp.proc.303306.112.2006

Stratigraphic correlation

The four holes cored at Site U1313 provided ample sediment for constructing one complete spliced stratigraphic section and a second nearly complete section. The first spliced section was built from Holes U1313B and U1313C, and the second from Holes U1313A and U1313D. Because Hole U1313D only extends down to 159.3 mbsf, this second splice contains core recovery gaps between the cores from Hole U1313A from ~174 to 334 mcd. Correlation between holes is very poor below 250 mcd, and so the completeness of both spliced sections is uncertain in that interval. Correlation between holes was excellent in lithologic Unit I (0–168.5 mcd) because of the cyclic alternations in the lithology, which is largely controlled by variations in the amount of clay (Figs. F21, F22, F23). The alternations produce variations in nearly all physical properties measured. In particular, the lightness (L*) from color reflectance measurements mimic variations in the global benthic oxygen isotope stack of Lisiecki and Raymo (2005), indicating that the darker, more clay rich sediments are associated with glacials and the lighter, more carbonate rich sediments are associated with interglacials. The correlation is even more striking than what was observed for Site U1312.

Between-hole correlation is difficult in lithologic Unit II (168.5–334 mcd) because the sediments are fairly homogeneous calcareous nannofossil ooze. Cyclic variations are evident in some of the physical properties in this unit, but the amplitude of these variations are very small and the wavelength short (less than a few meters). For example, the cyclic alternations are associated with L* variations of >20% in Unit I, whereas the variations are only ~5% or less for most of Unit II. For Unit II, these coherent L* variations are also somewhat obscured by spikes associated with dark iron sulfide patches and streaks that are common along the split-core surface.

MSCL measurements and adjustments to coring operations

As at Site U1312, we measured the susceptibility of whole-core sections with the “Fast Track” magnetic susceptibility core logger (MSCL) as soon as possible after recovery. Below ~125 mcd, magnetic susceptibility decreased to noise level of the Bartington susceptibility meter because of the high carbonate content of the sediments. Therefore, we focused our measurements on only the uppermost 16–19 cores for each hole.

Initial correlation based on these data was used to monitor the coring breaks and ensure that they did not coincide for any two holes. During coring, Hole U1313B was only offset vertically such that each core would be ~2 m deeper than that from Hole U1313A. This offset was mostly successful at filling the coring gaps from Hole U1313A, but by a very narrow margin. Based on our correlation of the MSCL data, the intended 2 m offset was only ~1.5 m at the top of Hole U1313B and became narrower with depth. A decision was made to increase the offset after Core 306-U1313B-20H by drilling ahead 2 m. This was the only adjustment made to coring based on correlation.

Meters composite depth scale and spliced stratigraphic sections

The final mcd scale (Table T25) and two spliced stratigraphic sections (Tables T26, T27) were constructed after GRA density, natural gamma radiation (NGR), magnetic susceptibility, magnetic intensity and inclination, and color reflectance data became available. As is typical of mcd scales, the Site U1313 mcd scale is expanded by ~10%–12% relative to the mbsf scale. Prior to uploading the data into Splicer for correlation, we removed those intervals that contained coring disturbance or voids (Table T28). Each of these cleaned data sets contains distinct variations useful for correlation.

In lithologic Unit I, we relied mainly on the many distinctive L* and magnetic susceptibility variations as well as geomagnetic polarity reversals recorded in the paleomagnetic inclination (Table T24). Correlation was straightforward, the mcd scale is well resolved, and both spliced sections are complete over this entire interval (Figs. F21, F22, F23).

Because correlation was more difficult in Unit II, we used whichever data set had distinctive correlative features, as no one data set was sufficient. In this interval, the inclination data and three turbidite beds (Table T2) provided the most useful, albeit rather coarse, features for correlation. Below Chronozone 3n.4n (Thvera) at ~250 mcd, virtually no feature can be confidently correlated. Thus, below 250 mcd, the composite depth scale and the spliced sections are little more than best guesses.

Age model based on correlation of lightness to marine isotopic stages

A preliminary age model was constructed by tuning the Site U1313 L* splice to the LR04 benthic isotope stack (Lisiecki and Raymo, 2005). For the correlation (Fig. F24), sharp color changes as characterized by L* variations were tied with glacial and interglacial terminations (benthic isotope maxima). These tie points are given in Table T29. Back to 3.4 Ma every isotope stage listed by Lisiecki and Raymo (2005) could be identified in the color record and tuning was straightforward. The resulting timescale (Fig. F24C, F24E) is in excellent agreement with stage boundaries and events found in magnetostratigraphy (Fig. F24B). Based on the age model and mcd depths, the sedimentation rate at Site U1313 for the last 3.4 m.y. averages 4.8 cm/k.y. and varies between 3 and 7.5 cm/k.y. (Fig. F24D, F24F). Higher sedimentation rates were found for glacial intervals.

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