18O
profile of Kennett et al. (Chap.
21, this volume) is also shown (Fig.
13D) to examine variation of the sediment composition in relation to
glacial/interglacial and stadial/interstadial changes.Temporal variations of
loadings of factors 1 through 5, which basically explains temporal variation of
sediment composition, are reconstructed based on the age model (Fig.
13; original data are listed in Appendix D).
In Figure 13, the planktonic 18O
profile of Kennett et al. (Chap.
21, this volume) is also shown (Fig.
13D) to examine variation of the sediment composition in relation to
glacial/interglacial and stadial/interstadial changes.
It is obvious from Figure
13 that loading of factor 1 is higher during the Holocene and Bølling/Allerød;
intermediate during MIS 2, 3, and 5a; and lower during the Younger Dryas,
uppermost part of MIS 3 (between 25 and 26 ka), and MIS 4 (Fig.
13A). During MIS 3, factor 1 loading is higher during interstadials and
lower during stadials. Profile of factor 2 loading is almost a mirror image of
factor 1 profile. These profiles also resemble the profile of planktonic 18O.
Because loadings of factors 1 and 2 semiquantitatively represent contribution
from finer and coarser fraction of lithogenic component, these results suggest a
strong linkage between climatic changes and variations in grain size of
hemipelagic sediments with finer grain sizes during warmer periods and coarser
sizes during cooler periods.
Observation of sedimentary structures shows that faint laminations and sharp contact surfaces occur within stadial intervals including the Younger Dryas. Parallel lamination could represent either varves formed by seasonal variations of surface productivity transmitted to the bottom sediments and preserved under anoxic bottom-water conditions or contourites formed by fluctuations of contour current velocity. Because varve-type laminations are found under anoxic conditions (e.g., Savrda et al., 1984), whereas contourites tend to be formed by highly ventilated intermediate- and deep-water flows, it is possible to distinguish the two types of lamination with evidence for anoxic conditions. We examined the relationship between DOPT and the occurrence of faint laminations and sharp contact surfaces (Fig. 12) in which positions of faint laminations and sharp contact surfaces are indicated as arrows. As is obvious from Figure 12, faint laminations and sharp contact surfaces tend to coincide with intervals of lower DOPT, suggesting more oxygenated bottom-water conditions during these intervals. This evidence argues for contourite origin of laminations and against varve origin. Lower contents of org-C and carb-C within these intervals are also against varve origin.
Contourites are characterized by well-sorted grain-size distributions with their mode in the medium- to coarse-silt size range (16-62 µm). In their study of contourites in the southern Weddell Sea, Weber et al. (1994) described laminated contourite whose principal mode is between 13 and 39 µm. On the other hand, our faintly laminated clayey silt samples are also well sorted and have the principal modal position at 13-14 µm. The agreement in the principal modal position together with the presence of faint parallel lamination suggests that our faintly laminated intervals (and also sharp contact surfaces) could be contourites. If this interpretation is correct, then compositional changes of the sediments explained by factors 1 and 2 should represent variations in grain size caused by changes in contour current intensities that were stronger during the Younger Dryas and stadials within MIS 3 and 4. Although grain-size analysis has not been conducted yet for samples from the Younger Dryas, low factor 1 loading within this interval suggests stronger contour current. Stronger contour current during the Younger Dryas is consistent with low sedimentation rate at the basal part of this interval (Fig. 2).
Kennett and Ingram (1995) and Behl and Kennett (1996) suggested the possibility of increased ventilation of NPIW during the Younger Dryas and stadials during MIS 2 and 3 based on the ventilation history of the Santa Barbara Basin. Stronger contour current intensities with higher bottom-water oxygenation levels during stadials (including the Younger Dryas) and weaker current intensities with lower bottom-water oxygenation levels during interstadials (including the Bølling/Allerød) reconstructed here are consistent with their hypothesis. Together with their results, our result strongly argues for intensification of intermediate-water circulation in the North Pacific during stadials possibly associated with Dansgaard-Oeschger cycles in the last glacial episode.
When turbidite sand samples are excluded, factor 3 in hemipelagic sediments (including contourites) basically explains excess or deficiency of biogenic carbonate that are not explained by factor 1. It can be either representing carbonate productivity or dissolution. We do not have conclusive evidence to specify either of the two possibilities. However, when comparing the vertical profile of factor 3 with that of org-C, maxima of factor 3 coincide with minima of org-C in most cases (Fig. 13B). Because factors 1 and 3 are positively correlated when turbidite sand samples are excluded, this antiphase relation is not due to grain-size effect. A more plausible explanation is the dissolution of biogenic carbonate in samples rich in labile organic matter. A prominent maximum of carb-C at 10 ka is widely recognized in the sediments all over the world including the California margin and considered as reflecting better preservation of CaCO3 (Broecker et al., 1993; Gardner et al., 1997). This interpretation is consistent with presence of prominent positive loading of factor 3 at 10 ka in the studied samples. Thus, we speculate that factor 3 may represent carbonate preservation, although we do not exclude the possibility of productivity contribution.
According to this interpretation, high carbonate preservation (or high production) occurred at 9.5, 16, 25, 46.5, and 75 ka. These high carbonate preservation (or production) events seem to coincide with those observed at Hole 1016C, ~100 km to the west of this site with water depth of 3835 m, by Yamamoto et al. (Chap. 12, this volume). However, poor age control at Site 1016 prevents us from detailed correlation. Carbonate preservation was relatively good (or production are relatively high) during the Holocene, MIS 2, and MIS 4, and low during the Bølling/Allerød, MIS 3, and MIS 5a. Higher carbonate preservation (or production) during MIS 2 is consistent with Lyle et al. (1992) who reported enhanced carbonate burial at 18 ka in a northern California margin transect.
Factor 4 is considered as representing a third detrital component that dominantly explains variations of TiO2, MnO, and P2O5 and to some extent Fe2O3, and probably represent titanomagnetite-like mineral. Significant variations in factor 4 loading are restricted to turbidite sand samples from MIS 2 that have either higher or lower loading compared to hemipelagic clayey silt and silty clay samples. We interpret this as reflecting sorting effect during transportation as described before. On the other hand, factor 4 loading of turbidite sand samples from MIS 5a does not show any deviation from adjacent hemipelagic sediments. This suggests either that turbidite has at least two different source areas or that sand composition of the source area changed with time. After excluding turbidite sand samples, factor 4 loading still shows some variations with higher loading during MIS 2 and lower loading during MIS 5a, middle of MIS 3 (approximately at 50 ka), and early part of the Holocene (Fig. 13C). Because titanomagnetite preferentially occurs in mafic and ultramafic rocks, and ultramafic rocks are mostly exposed along the coast to the north of Point Conception but not to the southeast of it, it is possible that this factor could represent a northern component of the detrital material transported along the coast. Further investigation is necessary to test this possibility.
Factor 5 explains some of the variations in Fe2O3 and K2O and is considered as representing a glauconite component. When samples from a turbidite layer at 14.7 mbsf are excluded, loading of factor 5 is not large, but its variation is still larger than the noise. Temporal variations of factor 5 loading in Figure 13C shows that it is higher during the Bølling/Allerød, and to a lesser extent the Younger Dryas and interstadials during MIS 3. Because the sediments deposited during these periods are characterized by hemipelagic clayey silt to silty clay without any interruption by turbidite, and hemipelagic samples with higher factor 5 loading do not agree with contourites, it is likely that glauconite in these sediments were formed in situ rather than reworked.
Glauconite formation is favored by a marginally oxic environment (Odin and Matter, 1981), and high concentration of glauconite grains in the surface sediments are reported from the upper edge of OMZ approximately at 500 m water depth of central California margin (Mullins et al., 1985). Mullins et al. (1985) also reported high concentration of fecal pellets at the lower edge of OMZ at ~1000 m water depth. Because fecal pellets are a good substrate for glauconitization, it is possible that fecal pellets were glauconitized to some degree at the lower edge of OMZ. If this is the case, factor 5 might reflect behavior of OMZ with higher factor 5 loading corresponding to downward shift of the lower edge of OMZ.
Org-C is frequently used as an indicator of surface productivity although limitations of its usage are frequently discussed (e.g., Lyle et al, 1992). For the hemipelagic sediments analyzed in this study, org-C variations (which is included in LOI variations) are dominantly explained by factor 1, which represents finer fraction of the sediments. Through their study of surficial sediments from Washington margin, Keil et al. (1994) showed that >90% of the total organic carbon is sorbed on the inorganic mineral surfaces, and its concentration is strongly controlled by sediment surface area. Because sediment surface area increases with decreasing grain size, negative correlation between org-C and grain size is expected. This explains the clear positive correlation between factor 1 loading and org-C as shown in Figure 14. Similar relationship between org-C and grain size is also reported from laminated contourites from the southern Weddell Sea (Weber et al., 1994). Thus, org-C in the studied sediments is considered as principally controlled by grain size of lithogenic fraction that in turn reflects intensity of contour current.
Scattering from the regression line in Figure 14 may represent additional factor(s) that control org-C. Those samples that scatter to the larger org-C side of the regression line come from six intervals. These are the late Holocene (<1.5 mbsf), later part of the Younger Dryas (2.2-2.6 mbsf), three interstadials in the early half of MIS 3 (9.2-9.4, 10.0-10.5, and 11.4-11.6 mbsf), and MIS 5a (13.2-15.1 mbsf) (Fig. 14). Org-C higher than the regression line for the Holocene samples could be explained by temporal retention of metabolizable organic matter that will be decomposed during the course of early diagenesis within the top few meters (Gardner and Dartnell, 1995). Alternatively, it could be explained by enhanced preservation of organic carbon under poorly oxygenated bottom-water conditions (e.g., DeMaison and Moore, 1980; Canfield, 1994) implied from the presence of Chondrites-like burrows within the top 1.5 m. Higher org-C trends for other five intervals do not seem to be explained by either of these two factors. First, effect of temporal retention of metabolizable organic matter is unlikely because the continuous downward decreasing trend of org-C ceased at 2.3 mbsf, below which no downward decreasing trend is observable on org-C profile. Second, DOPT values within these intervals are not necessarily high, although high DOPT values occur in a few samples. Consequently, enhanced preservation under poorly oxygenated conditions does not seem important. A more plausible explanation is increased contribution of organic matter that is not associated with fine lithogenic particles. Increases in contribution of free organic matter to the sediments could be caused either by enhanced surface productivity or by enhanced supply of terrigenous organic matter, mostly through rivers. At this stage, we do not have enough data to specify the source of organic matter within these specific intervals, but increased supply of terrigenous organic matter is a possible explanation because three of these intervals coincide with the intervals with abundant plant fragments (Fig. 2). It is interesting to note that relative enrichment of org-C is not obvious for samples from the earliest Holocene or the Bølling/Allerød where high org-C and carb-C peaks are recognized.
