RESULTS AND DISCUSSION

Table 1 and Figure 1 provide vertical profiles for total organic carbon (TOC) and C/N ratios (TOC/total nitrogen, expressed by weight), total sulfur (total S), and ¹³C of TOC in the sediment samples of Core 167-1017E-1H. TOC concentrations in Hole 1017E range from 4 to 31 mg/g (dry sediments). The C/N ratios range from 7.1 to 10.2. Significant differences in TOC concentrations, C/N values, and ¹³C of organic matter (OM) are seen between the Holocene (sections above 222.7 cm in depth; <10.1 ka) and glacial period (sections below 222.7 cm in depth), as summarized in Table 2. TOC and total N concentrations in Holocene sections are roughly 1.8-1.9 times higher than those in glacial sections. C/N values for the former (8.7) resemble those for the latter (9.2). TOCs in Holocene sections are isotopically slightly heavier than those in glacial ones.

C/N ratios for OM of marine origin generally range from 6 to 9 because of the high protein content of lower organisms such as phytoplankton and zooplankton (e.g., Müller, 1977). Higher plant-derived OM gives higher C/N values than that of marine origin because it has a high percentage of nonprotein materials (e.g., 20-546; Meyers and Ishiwatari, 1993). Thus, the present results of C/N for Hole 1017E may indicate that OM is of marine origin. ¹³C of TOC (¹³C_TOC) supports this conclusion because the ¹³C_TOC values for Core 167-1017E-1H range from -21.0 to -22.8 , which are common to the organic matter of marine origin. A similar conclusion was reached by Gardner et al. (1997), who found ¹³C_TOC in the sediment cores from a similar region (35°N, 121°W) roughly -22 over the last 60 k.y. It is noteworthy that ¹³C_TOC values in the Holocene are ~0.8 higher than those in glacial sections, and ¹³C_TOC values in glacial are relatively constant. The invariability of ¹³C_TOC values in glacial contrasts with the synchronous fluctuations in TOC, C/N, and total S. Most of the sections with low TOC concentrations, C/N, and total S concentrations in glacial are sandy (fine or coarse grained) deposits (~320, ~360, ~380, ~395, ~420, ~440, ~460, ~490, and ~507 cm in depth, respectively; Fig. 1). The stratigraphy of the sediment core is given by Tada et al. (Chap. 25, this volume). These sections are probably well oxygenated, which coincides with low total S contents. Low C/N ratios in the sandy sections may be produced as a result of contribution of inorganic ammonium sorbed by clays to total nitrogen, which often becomes important in TOC-poor sediment (Müller, 1977). A marked positive shift in ¹³C_TOC values around 10 ka can be explained as follows. Carbon isotopic composition of TOC in sediment is generally controlled by the following three factors: the nature of source material (terrestrial higher plants vs. marine phytoplankton), temperatures of the photosynthetic fixation of CO₂, and rate of photosynthesis (if we assume that TOC in sediment is derived from marine phytoplankton alone). Following Schoell et al. (1994), the relationship among the three factors can be expressed as

¹³C_i = ¹³C_DIC - _p, (1)

where ¹³C_i is the carbon isotopic composition of TOC in sediment, obtained after correction for a shift in ¹³C resulting from early diagenesis in sediments (Hayes et al., 1983); ¹³C_DIC is the isotopic composition of the dissolved CO₂; and _p is the fractionation between dissolved CO₂ and primary photosynthate, as shown above. _p is written as

, (2)

where a and b are the fractionations associated with diffusion of CO₂ into the phytoplankton (0.7) and fixation of CO₂ with the phytoplankton (27.0; Yoshioka, 1997). [CO₂]_i is intracellular CO₂ concentration, and [CO₂]_aq is extracellular concentration. The ratio [CO₂]_i /[CO₂]_aq is assumed to be photosynthetic carbon demand and should be related to the phytoplankton growth rate when transport of CO₂ into the cell is controlled by diffusion, which is common in marine systems (Shemesh et al., 1993; Laws et al., 1995). ¹³C_DIC can be estimated from ¹³C of carbonate tests of planktonic foraminifers and paleo-sea-surface temperatures (paleo-SSTs) by using the following empirical equation (Jasper et al., 1994):

¹³C_DIC = _t + 14.07 - 7050/T, (3)

where _t presents the isotopic composition of foraminifera calcite and T is paleo-SST (in Kelvin).

The difference in ¹³C for sedimentary TOC at two different times (¹³C_i) can be roughly expressed as

¹³C_i = ¹³C_DIC - _p, (4)

where ¹³C_DIC is the difference in ¹³C of dissolved CO₂ in seawater, which is estimated from the difference in ¹³C of carbonate tests of planktonic foraminifers deposited at the different times with correction of paleo-SST.

The term _p, which is related to the difference in photosynthetic carbon demand at different times, can be calculated from Equation 4. The term _p was named as a parameter related to comparative growth rate (PCGR; Ishiwatari et al., 1999). A high PCGR at a particular time indicates a low photosynthetic carbon demand (i.e., growth rate of phytoplankton would be low) relative to that at the reference time.

According to Cannariato (pers. comm., 1998), ¹³C of inorganic carbon (IC) of planktonic foraminifera (Globigerina bulloides) tests ranges roughly from -1.4 to 0.6 in 280-160 cm in depth (13-7 ka) and shows no marked trend. Sea-surface temperature (SST) estimated from alkenone analysis indicates a rapid increase from ~12°C at 230 cm in depth (10.5 ka) to ~15°C at ~210 cm in depth (9.5 ka; R. Ishiwatari et al., unpubl. data. In addition, ¹³C values of IC of planktonic foraminifera tests fluctuate around -1 and show no significant difference before and after 10 ka. Thus, ¹³C of the dissolved CO₂ is estimated to become higher by ~0.3 (¹³C_DIC = ~0.3) at the SST transition at ~10 ka. Consequently, _p equals ~-0.7. This means that the increase in growth rate of phytoplankton, which may correspond closely to planktonic primary productivity, might be responsible for the ~0.5 positive shift in ¹³C values of sedimentary TOC at ~10 ka, although a substantial range of possibilities must be considered.

Another factor that can cause a part of the positive shift in ¹³C_TOC values around 10 ka is a drop in input of terrestrial higher plant-derived OM, which is suggested by lignin analysis (R. Ishiwatari et al., unpubl. data) of the sediment core. The lignin analysis indicates that TOC-normalized concentrations of lignin phenols in Holocene sediments are one-half to one-quarter times lower than those in glacial. However, we cannot evaluate at present how the drop in input of terrestrial higher plant-derived OM contributes to the positive shift in the ¹³C_TOC value.

Sulfur concentrations are high in sections in the 306-280 cm range (25.3-13.1 ka), which matches excellently with anoxia Event 1, corresponding to the number 1 (Bølling/Allerød) of warm interstadials (15.5-12.7 ka; Behl and Kennett, 1996). Behl and Kennett (1996) evaluated the degree of oxygenation of sediments in the central Santa Barbara Basin (ODP Hole 893A: 34°17.25´N, 120°2.2´W: 576.5 m water depth), which is close to Site 1017, using the bioturbation index. An oxygenation level for the anoxia event indicates anaerobic/dysaerobic boundary conditions allowing only micro- to meiofaunal bioturbation sufficient to diffuse but not destroy primary laminations (~0.1 mL O₂/L; Behl and Kennett, 1996). The marked correlation of the high sulfur section to anoxia Event 1 suggests that the same factor (the presence of relatively old bottom waters) played a role for both events. This result seems to support the idea of Behl and Kennett (1996) on the bottom-water system in the California margin.

The section corresponding to anoxia Event 2 at 23.7-22.8 ka reported by Behl and Kennett (1996) does not show high sulfur content, probably because the oxygenation level (>0.3 mL O₂/L dysaerobic to aerobic) is not enough for producing high-sulfur sediments.

High-sulfur contents are observed for other sections (400 cm and 455-430 cm in depth). But the corresponding low benthic oxygenation was not seen in the Santa Barbara Basin (Behl and Kennett, 1996). These results suggest that short-term changes in the oxygenation level occur locally during glacial periods.