CAROLINA SLOPE HOLE 1055B

Site 1055 and KNR140 GGC-56 are at exactly the same location, at ~1800 m water depth on the Carolina Slope. The gravity core has been the subject of extensive accelerator ¹⁴C dating (L.D. Keigwin, unpubl. data), from which it is clear that the Holocene rate of sedimentation is ~30 cm/k.y., the deglaciation is half that, and the LGM (at ~400 cm) is a condensed interval with a rate of ~4 cm/k.y. (Fig. F3). The LGM is pale red in other GGCs from about this depth on the Carolina Slope, as it is at ODP Site 1055. According to the shipboard visual core descriptions, pale red intervals are found at ~4.2, 6.3, 4.0-4.7, and 4.2-4.5 m at Holes 1055B, 1055A, 1055D and 1055C, respectively. At all but Hole 1055A, these red intervals are almost certainly the LGM level. At Hole 1055A, however, the red interval is >1.5 m deeper than at the other holes, and a substantial (1.35 m) correction to meters composite depth (mcd) makes the mismatch even worse.

Hole 1055B was chosen for detailed study because in addition to the LGM, sediment was pale red between 5.67 and 6.40 meters below seafloor (mbsf). I estimated this deeper interval was part of isotope Stage 3, a conjecture that is supported by the ¹⁸O on G. ruber (Fig. F3). Despite the high accumulation rates in the Holocene, the overall late Pleistocene rate of sedimentation at Site 1055 is only ~4 cm/k.y., so it is unlikely that the red interval at ~600 centimeters composite depth (cmcd) is a single Stage 3 stadial event. As expected, the Stage 3 red event (as visually identified) is marked by increased ¹⁸O, suggesting the color was associated with lower SST, some combination of oxidizing conditions on the seafloor, and (probably) southerly advection of iron minerals. However, at 500 and 600 cmcd, two intervals of maximum redness were measured by color reflectance aboard ship (Fig. F4). Unfortunately, older events may have lost their color from reduction diagenesis because shipboard results showed that the only red color at this site is in the upper 10 m of the hole. When the red:yellow balance (chromaticity a*) and lightness are compared to the ¹⁸O of G. ruber, it is evident that there must be some loss of resolution in the shipboard color reflectance data because of data smoothing. Nevertheless, the two red peaks in Stage 3 are each associated with maximum ¹⁸O. Whereas it is difficult to imagine that the distinctive but short LGM red level in these cores was not measured for reflectance, it is indeed clear that there is no red peak at ~400 cmcd (Fig. F4).