LATE QUATERNARY PALEOCEANOGRAPHIC IMPLICATIONS OF THE SITE 1014 RECORD

Major changes in the surface waters occurred in the southern California margin in response to late Quaternary climate change (Thunell and Mortyn, 1995; Kennett and Ingram, 1995; Sabin and Pisias, 1996; Prahl et al., 1995), and these changes are reflected at Site 1014 by large ¹⁸ shifts (Fig. 4, Fig. 5). Earlier work has also suggested that low SSTs off the west coast of North America during the last glacial maximum were associated with the increased influence of subarctic waters in the region, resulting in the southward displacement of currents and related atmospheric systems (Thunell and Mortyn, 1995; Sabin and Pisias, 1996; Prahl et al., 1995). Ocean-atmosphere interactions over the North Pacific Ocean determine the strength and location of currents along the California margin (Reid et al., 1958). It is reasonable to conclude that with the atmospheric reorganization accompanying the formation of large ice sheets (COHMAP members, 1988) and cooler high-latitude SSTs, the relationship would have changed between the California Current and countercurrent. Thus, large inferred temperature shifts between glacial and interglacial in surface waters in Tanner Basin probably resulted from changing dominance of a cooler, intensified California Current relative to the warm countercurrent. As well, countercurrent influence at Site 1014 would be negligible, and SSTs would be much reduced if geostrophic flow created by the surface ocean pressure difference between San Diego and Point Conception was not established during glacial times.

Site 1014 lies in the outermost basins of the California Borderland; consequently, lithogenic input is minor from the nearby continent, whereas biogenic deposition makes the largest contribution to the sediment budget of the basin (Gorsline and Teng, 1989). Because changes in biogenic sedimentation can be used to estimate changes in productivity, an almost constant sedimentation rate suggests that the deposition of biogenic material did not significantly differ between glacial and interglacial episodes. However, this simple relationship is complicated by several factors. First, the sedimentation rate reflects averaged biogenic deposition as rates are interpolated between datums, so that short-term events are not discernible. Second, carbonate dissolution reduces sedimentation rates by removing deposited biogenic sediment so that increased productivity may be masked. This particularly complicated the record during the last interglacial when deposition of organic material increased (Yamamoto et al., 1998) during marine isotope Substages 5b and 5d as did the severity of dissolution. Yet sedimentation rates remained generally constant.

The benthic ¹³C record may provide evidence of changes in surface water productivity because the interstitial waters inhabited by the foraminifers are affected by degradation of organic matter. During the cooler substages (5b and 5d) of the last interglacial, organic carbon accumulation increased when benthic ¹³C values were higher. This result is confirmed by the increased accumulation rates of alkenones (Haptophytes), dinosterol (dinoflagellates) and bishomohopanol (bacteria), and higher plant biomarkers that show similar variations to organic carbon (Yamamoto et al., 1998). The relationship is opposite to the one expected if benthic ¹³C were responding to organic material deposition. It appears that there is not a simple relationship between the production of organic material in the surface waters and benthic ¹³C. The record during marine isotope Substages 5b and 5d is further complicated by an anticorrelation between organic accumulation rate and upwelling indicators. This suggests that episodes of intensified upwelling are not accompanied by increased organic carbon deposition (Yamamoto et al., 1998).

Attempts at estimating productivity changes on the California Borderland have been published by several authors. Mortyn and Thunell (1997) suggested productivity increases during the last glacial maximum that are 1.5 to 2.0 times greater than the Holocene; they attribute this increase to intensification of the California Current and enhanced upwelling south of 40°N. On the other hand, Berger et al. (1997) observed a significant reduction in opaline deposits in the Santa Barbara Basin, which they attributed to a decrease in estuarine conditions of the North Pacific. It is possible that during glacial episodes, upwelling intensified in the California Current. However, because of the increased contribution of North Pacific sourced Intermediate Water, water being upwelled was nutrient poor, and productivity did not significantly rise. Nevertheless, during interglacials the northern component of Pacific glacial Intermediate Water diminished in relation to old, deep Pacific water. Hence, although upwelling was reduced, surface waters had a higher nutrient content (Behl and Kennett, 1996; van Geen et al., 1996). Thus, complicating factors of both the North Pacific and Tanner Basin prevent support of significant climate-induced changes in productivity during cool intervals.

The relationship between the benthic ¹³C record and ventilation of the North Pacific at Site 1014 is complicated. The record (Fig. 6) does not appear to represent changes in ventilation of the basin during the last 85 k.y. but instead follows the global ¹³C record (Shackleton and Pisias, 1985) with lower values during cool intervals. During the last interglacial (MIS 5), the response of benthic ¹³C changed, no longer conforming to the global pattern. Consequently, changes in ¹³C appear to represent a delicate balance between the dominance of the flux of organic carbon at the site, ventilation of the water mass entering the basin, and global carbon isotope production.

Benthic ¹³C might be responding to the age of the deep water mass and the resulting ventilation as mentioned above, where old, poorly ventilated water typically has lower ¹³C values than newer, well-ventilated water masses. The current paradigm involving North Pacific Intermediate Water suggests that during warmer climates, the water mass becomes increasingly poorly ventilated (Behl and Kennett, 1996; van Geen et al., 1996); thus, ¹³C should be lower. This corresponds to the ¹³C benthic record of Tanner Basin (Fig. 5), where ¹³C decreased during the warm intervals of the last interglacial (marine isotope Substages 5c and 5e). High PCO₂ content—either from old, poorly ventilated water resulting from considerable decomposition of organic material or cool water saturated in CO₂—enhanced corrosivity of a water mass. However, intervals of corrosion, which punctuate carbonate sedimentation in Tanner Basin during the cool Substages 5b and 5c, are anticorrelated with the ventilation and ¹³C relationship. Therefore, during cool intervals (marine isotope Substages 5b and 5d) of the last interglacial, PCO₂ levels of intermediate waters were not connected to the ventilation history of the water. Consequently, the Tanner Basin record agrees with others from the California margin (Kennett and Ingram, 1995; Behl and Kennett, 1996; van Geen et al., 1996), indicating that ventilation of North Pacific Intermediate Water changed in response to climate change.