The Holocene section at Site 1098 shows clear meter-scale variation between darker and lighter sediments (Figs. F2, F3). The pattern of variation in color is continuous laterally in the distinctly laminated sediments between 0 and 25 mcd and between 42.5 and 46 mcd. Color variation between 28 and 42 mcd cannot be correlated between holes (Fig. F2). Lateral changes in sediment color in this interval may be the result of variable bioturbation with the presence of discrete burrows. However, decimeter- to meter-scale fluctuations in sediment color throughout the record match with variation in bulk density (Fig. F3). Leventer et al. (1996) showed for a piston core from the same locality as Site 1098 that, in the upper 10 meters below seafloor, laminated intervals have lower bulk density and higher biogenic silica content than bioturbated levels. The average darker colors in intervals with lower bulk density (high biogenic silica) are mainly the result of the presence of bundles of darker laminae (Fig. F3). Color values of the light laminae are similar to those in the bioturbated homogeneous intervals, representing background sediments that show relatively little variation in lightness (Fig. F4). The dark laminae have variable lightness, ranging from dark to a value that is only slightly darker than the background sediment, and have variable color values, ranging between gray, olive, and orange-brown (Barker, Camerlenghi, Acton, et al., 1999). Yet, dark laminae are commonly rich in diatoms, which causes a decrease in bulk sediment density. As a result, the linear correlation between GRA bulk density and L* (r = 0.46) is significant but not very high. Although variation in a* is low amplitude, a* is highly negatively correlated with lightness (r = -0.84), that is, darker sediments are more red. A lower correlation between L* and b* (r = -0.42) indicates that dark sediments have a more variable yellow component.
The sequence at Site 1098 contains two intervals that both have a high proportion of laminated sediments but differ in sedimentological characteristics. The interval from the top of the sequence to 25 mcd shows cyclic alternation between laminated and bioturbated sediments. The transition between dark and light laminae is gradual. Laminae occur in bundles that can be traced across all three holes (Fig. F4). Within these bundles, most individual laminae are also continuous (Fig. F5). Variation between cores appears in part a result of physical disturbance of the primary lamination, possibly caused by the coring of the very weakly consolidated sediments. The overall lateral continuity of individual laminae and the gradual transitions between dark and light are consistent with quiet deposition from pelagic rain, with dark laminae representing settling of biogenic fluxes during the growing season.
The lower interval between ~42.5 and ~46 mcd is more continuously laminated than the top 25 mcd but shows much more lateral variability (Figs. F6, F7). Laminae in Hole 1098A show high-amplitude color variation, and dark levels tend to be thick and have sharp boundaries with the surrounding lighter sediments. In Holes 1098B and 1098C, dark laminae are mostly thinner with more gradual transitions in color, but the sequence is tilted and shows signs of slumping and/or erosion in several places. Meter-scale fluctuations in average color in this part of the sequence appear to correlate between the three holes, but individual laminae cannot be traced laterally, except between Holes 1098B and 1098C between 44 and 45.6 mcd (Figs. F6, F7). Backscatter electron microscope images of impregnated sediments show indications of redeposition of diatom floras (Pike et al., Chap 18, this volume). Overall, the lack of lateral continuity indicates that the laminated interval between ~42.5 and ~46 mcd cannot be interpreted as a continuous pelagic drape, as it shows signs of disturbance, possible discontinuities, and appears influenced by current action.
The combination in the upper 25 mcd of high accumulation rates (~4 mm/yr) (Domack et al., in press) and lateral continuity of laminae with the fact that biogenic productivity occurs during a short season suggests that laminated intervals could be varved, that is, dark/light couplets could represent a single year. If that were the case, then annual-resolution age scales could be produced simply through counting varves in the best laminated intervals. To explore the amount of time represented between pairs of dark laminae, we measured the thickness of all dark laminae and all light intervals in between. For dark/light couplets, we added the thickness of each light interval to that of the overlying dark lamina, irrespective of the thickness of the light interval. We concentrate on the upper 25 mcd, which we interpret as pelagic, and for which a detailed radiocarbon age scale is available (Domack et al., in press).
The thickness of virtually all dark/light couplets throughout the upper 25 mcd exceeds the long-term average annual accumulation rates derived from radiocarbon measurements (Fig. F8). This does not necessarily mean that dark/light couplets on average represent more than a single year. Leventer et al. (1996) concluded that laminated intervals represent higher-than-average total accumulation rates, through the addition of increased diatom fluxes. In addition, the thickness of dark and biogenic-rich laminae is highly variable, ranging from 0.1 to >1.0 cm (mean = 0.34 cm, standard deviation = 0.29). Given that the composition of the biogenic-rich laminae is consistent with deposition during a single growing season, the variation in thickness indicates extreme interannual variation in biogenic fluxes. We conclude that the fact that most dark/light couplets are thicker than the expected long-term average annual flux reflects primarily a distinct increase in total biogenic sediment accumulation rates in the laminated intervals relative to the homogenous levels.
This conclusion suggests as an alternative model for sedimentation that terrigenous fluxes remained relatively constant through time, whereas biogenic fluxes were highly variable. As a first-order approximation, we equate dark laminae with biogenic event deposition and light levels with siliciclastic background sedimentation. Radiocarbon accumulation rates were therefore recalculated for the light sediments only after subtracting all dark laminae from the total sediment thickness. The result shows a long-term average annual rate of deposition of light sediment that is close to the mode in thickness of light-colored laminae (Fig. F8D). This indicates that a large number of the dark/light couplets indeed represent a single year. However, the highly skewed distribution of light laminae thickness also indicates that an equally large number of light layers in the 0.5 to 2.0 cm range are too thick to represent a single year. We assume that annual rates of accumulation have a normal distribution, with the mean given by the long-term radiocarbon-based rate and a standard deviation estimated from the left side of the histogram in Figure F8D. We can then estimate the expected number of annual layers thicker than the mean. The result indicates that out of a total of 823 light laminae in the spliced record some 460 represent deposition over two or more years. Although a rough estimate, it suggests strongly that there are many "missing years" even within the best laminated intervals, that is, years that a biogenic layer was not deposited or, if deposited, not preserved as a recognizable laminae.