The variations of TOC and CaCO3 contents and biomarker concentrations appear to respond to glacial-interglacial changes. In this study, we used C25:1 HBI alkene, alkenones, and neohop-13(18)-ene as the markers of diatoms, haptophytes, and prokaryotes, respectively, for assessing the marine ecological responses to climate changes.
The preservation degree of
biomarkers in sediments depends both on the dissolved oxygen concentration near
the water-sediment interface and sedimentation rate. A comparison of alkenone
fluxes of sediment trap samples with those of sediments indicates that the
preservation degree of alkenones is 0.25%-22% in oxic surface sediments off
Oregon (Prahl et al., 1993). The biomarkers used in the present study are
relatively stable, but even they could have suffered severe degradation in the
water and sediment columns. Burrows were observed in whole range of Core
167-1016C-1H (Fig. 2),
suggesting a constantly oxic environment during deposition. The sedimentation
rates estimated from our preliminary 
-based
age-depth model are almost constant throughout the core. The little changes of
bioturbation and constant sedimentation rate suggest that only small variations
occurred in the degree of preservation of biomarkers during the deposition of
the core. It is, therefore, possible to use the relative abundance of the
biomarkers for paleoenvironmental assessment.
A comparison of TOC and CaCO3 contents with selected biomarker concentrations is shown in Figure 12. C25:1 HBI alkene of diatom origin has the highest concentrations in warming intervals (Peaks A and H in Fig. 12). Pike et al. (unpubl. data) point out the increase of diatom productivity at deglaciation (Termination I) at Site 1019 off Oregon, based on the diatom assemblages and the mode of lamination. This is concordant with the high C25:1 HBI alkene concentrations during warming intervals at Site 1016, implying the regional increase of diatom productivity at deglaciation along the California and Oregon margins. The high diatom productivity (high concentration of C25:1 HBI alkene) was presumably related to intensified coastal upwelling and accounts for major peaks of TOC contents (Peaks A and H).
Alkenones of haptophyte origin, in contrast, have maximal concentrations in the cooling intervals of the warm period (Peaks D, F, and G in Fig. 12). The alkenone maxima correspond to the maximal peaks of TOC contents. This implies that alkenone-producing haptophyte algae, in addition to diatoms, are major sources of marine organic carbon. It is interesting to note that the concentrations of alkenones and C25:1 HBI alkene vary out of phase in a warm interval (5-8 mbsf). Because diatoms tend to dominate in a highly nutrient-fluctuating environment (Turpin and Harrison, 1979), the out-of-phase variations presumably resulted from the changes in the mode of nutrient supply to surface mixed layer, such as the changes of the intensity, seasonality, frequency, or duration of upwelling.
Neohop-13(18)-ene of prokaryote origin has maximal concentrations mostly at the maximal intervals of diatom-derived C25:1 HBI alkene and haptophyte-derived alkenone concentrations (Fig. 12). Because neohop-13(18)-ene derives from bacteria or cyanobacteria, its concentration reflects either heterotrophic eubacterial activity or productivity of cyanobacteria and/or autotrophic eubacteria. The synchronous variation of neohop-13(18)-ene with C25:1 HBI alkene and alkenones, therefore, suggests that this compound reflects heterotrophic eubacterial activity associated with the primary production by diatoms and haptophytes. The possibility of autotrophic origin, however, cannot be neglected at present, and therefore a future investigation on its carbon isotopic composition is desirable to identify its source.
Maximal CaCO3 contents (>10%) were observed in warming intervals (Peaks I and K in Fig. 12) and a cooling interval (Peak J). There is no biomarker that varies in phase with carbonate Peaks I and K in the warming intervals. On the other hand, Peak J of the cooling interval corresponds to an alkenone maximum, implying that high haptophyte productivity is related to carbonate maxima. These suggest that the processes that regulate the formation of carbonate peaks are different between warming and cooling intervals.
The background value of CaCO3 contents is nearly 0%, which suggests routine dissolution of calcium carbonate below the CCD. The changes of CCD are controlled mainly both by the changes of North Pacific deep-water chemistry and local flux of calcium carbonate. The out-of-phase responses of the Atlantic and Pacific carbonate records to climate change were attributed to a global mechanism (i.e., an increased contribution of less corrosive southern source water and decreased input of corrosive North Atlantic Deep Water during glacial periods [Berger, 1970]). Karlin et al. (1992), however, suggested that the CCD has migrated more than 1800 m between glacial and interglacial times, and they attributed this large carbonate fluctuation to some regional mechanisms, such as the glacial deep-water formation in the northern Pacific Ocean or the enhanced dissolution caused by interglacial noncarbonate productivity related to coastal upwelling. Gardner et al. (1997) found a carbonate preservation event at 10 ka in cores from the northern California margin, and they attributed it to the global mechanism, which was hypothesized by Broecker et al. (1993), that the expansion of boreal forests in northern hemisphere removed CO2 from the atmosphere and the surface water of the ocean, and in turn increased alkalinity of ocean water, which should have resulted in greater preservation of calcium carbonate.
On the other hand, the local flux of calcium carbonate can be examined based on the alkenone profile. Because alkenone-producing species compose more than 70% of total coccoliths at the adjacent Site 1017 (Tanaka and Tada, Chap. 27, this volume), it is reasonable to use alkenone concentrations to estimate the flux of calcium carbonate. Carbonate Peak J corresponds to an alkenone maximum (Fig. 12), which is concordant with the hypothesis that increased calcium carbonate production enhances its preservation. However, during the older cooling intervals (6-8 mbsf), there is no carbonate maximum that corresponds to alkenone maxima, implying that the production is not the only factor controlling CCD at this site.
According to the model of Archer (1991), the dissolution of calcium carbonate is controlled by organic carbon/carbonate carbon ratio of sinking particles, and this implies that the production of noncarbonate organisms such as diatoms tends to decrease the preservation of calcium carbonate. At Site 1016, both TOC content and C25:1 HBI alkene concentration are low at carbonate maxima (>10%), and this does not disagree with the Archer hypothesis.
Lyle et al. (Chap. 11, this volume) report the synchronous changes in calcium carbonate contents and accumulation rates in various water depths at northern sites of Leg 167 and suggest that the carbonate profiles are affected by carbonate production rather than dissolution. Site 1017, adjacent to Site 1016, shows a maximum of total coccolith abundance during early Holocene (Tanaka and Tada, Chap. 27, this volume). This might suggest that calcium carbonate production controls its preservation in the Sites 1016-1017 transect too. However, alkenones have no maximal concentration at carbonate maxima (Peaks I and K) of Site 1016 (Fig. 12): the alkenone profile of Site 1016 provides no concrete evidence that carbonate production mainly affects carbonate profile. To answer this question, a future investigation is needed about the alkenone profile at Site 1017.
We therefore still need to consider the regional factors at a northern Pacific scale (Karlin et al., 1992) or the global factors (Broecker et al., 1993). It is interesting to note that the calcium carbonate profile of a deep-sea core (4402 m deep) from the Caroline Basin, the western equatorial Pacific Ocean (Kawahata et al., 1998), somewhat resembles that of Site 1016. This suggests the importance of understanding the changes of deep-water chemistry of the northern Pacific Ocean to account for CCD changes in the California margin.
Petroleum-type compounds have been widely observed in recent sediments of the California Borderland, and they have been attributed to anthropogenic petroleum pollution as well as to input from natural sources such as submarine seep oil and weathered Monterey shales (Simoneit and Kaplan, 1980; Venkatesan et al., 1980). They were also detected in sinking particles taken by sediment traps settled in the California Borderland (Crisp et al., 1979; Venkatesan and Kaplan, 1992). A 3-cm-thick tar layer and small tar fragments were observed in Site 893 cores from the Santa Barbara Basin (Shore-based Scientific Party, 1994). Hinrichs et al. (1995) found petroleum-type compounds in late Pleistocene sediments (since 160 ka) from Site 893, and it indicated that they are solely of natural origin in older sediments.
Petroleum-type compounds were observed in Site 1016 sediments in this study, indicating that the occurrence of petroleum-type compounds of natural origin is not restricted to coastal sediments but extends to hemipelagic deep-sea sediments. The generation of petroleum in deeper parts of Site 1016 is very unlikely because of the low TOC contents (0.5%-1%) in the underlying sediments (Shipboard Scientific Party, 1997a). The presence of compounds characteristic of Monterey shales and Monterey oils indicates that their potential sources are Miocene Monterey shales exposed along the California coastal area (Pisciotto and Garrison, 1981) and/or natural seep oil in the California Borderland (Vernon and Slater, 1963).
The variation of long-chain n-alkane concentrations at Site 1016 indicates relatively low contribution of terrigenous organic matter in warming intervals (Fig. 8). On the other hand, the concentrations of petroleum-type compounds are higher in warming intervals (Fig. 11). This observation disagrees with the idea of terrestrial origin of petroleum-type compounds such as weathered Monterey shales on land. Therefore, a submarine origin is more likely.
Submarine petroleum seepages were recognized on the continental shelves of the California Borderland (e.g., Wilson et al., 1974). Natural seeps off Coal Oil Point introduce about 50 to 70 barrels of oil per day into the Santa Barbara Basin (Allen et al., 1970). There are three huge tar mounds that resulted from the accumulation of altered seep oil in the Santa Barbara Basin (Vernon and Slater, 1963). The tar mounds, built on land during the last glacial period, were washed away once by wave activities during transgression and then rebuilt in water during the Holocene (Vernon and Slater, 1963). Therefore, it is proposed that a huge amount of tar was scattered to the surrounding areas by the destruction of tar mounds during transgression. The timing of destruction of tar mounds corresponds to the periods of maximum input of petroleum-type compounds into Site 1016. The biggest tar mound is located off Point Conception (Vernon and Slater, 1963), and this area is very close to the Arguello Canyon, which sources sediments to the Arguello Fan (Chase et al., 1981). Because the lobe of the Arguello Fan extends to the south of Site 1016, a turbidity current is a possible candidate for transporting tarry materials to deep seafloor, and the deposition of the tarry turbidite sediments presumably contributed to the accumulation of petroleum-type compounds at Site 1016. Although the other transportation mechanisms such as diffusion and floating cannot be neglected, the same level of their concentrations in Site 1016 as in the Santa Barbara Basin suggests that the transportation by turbidity currents is more likely.
