LATE QUATERNARY CARBON ISOTOPE RECORD OF TANNER BASIN

A number of trends are shown in the planktonic ¹³C record at Site 1014 (Fig. 4). The most negative values (~1.2) recorded in the basin occur during the warmest intervals of the interglacials (marine isotope Substage 5e and the Holocene). During much of the last glacial, the planktonic ¹³C record ranges from ~-0.5 to ~-0.25 but was interrupted by several positive events (~0.1). A number of observations can be made about the Site 1014 G. bulloides record compared to other sites on the southern California margin. In Tanner Basin, Holocene ¹³C values (~-0.7) and those of the last glacial (~-0.5; Fig. 4) are similar to those for Santa Barbara Basin but slightly higher than values from Site 1017. A sharp ¹³C (~0.5) decrease occurred during Termination I (Fig. 4), similar to Santa Barbara Basin and Hole 1017E records over the same interval. Another negative ¹³C event lasting ~5 k.y. occurs between 30 and 25 ka and is also found at all three sites (Holes 893A and 1017E; Site 1014).

Thus, it would appear that several regional ¹³C excursions occurred on the southern California margin. Because these events are not reflected in records of N. pachyderma ¹³C from Sites 1017 and 893, G. bulloides apparently responded to a species-specific forcing, which may be related to changes in nutrient supply of surface waters or surface-water hydrology. Studies have shown that G. bulloides calcifies in disequilibria with ¹³C by -2 to -4, suggesting significant incorporation of metabolic CO₂ into the test during calcification (Sautter and Thunell, 1991). Thus, environmental information retrieved from the G. bulloides ¹³C record is limited. However, some evidence shows ¹³C becomes enriched in response to upwelling, possibly as a result of rapid growth and high metabolic activities influenced by nutrient supply, temperature, and PCO₂ (Sautter and Thunell, 1991; Berger and Vincent, 1986).

The Tanner Basin benthic ¹³C record exhibits a general increase (~-1.75 to ~-0.75) in ¹³C values during the last 200 k.y. (Fig. 6). On a smaller scale, a strong correlation between the benthic ¹³C record and global climate change is exhibited (Fig. 5) at Site 1014. During the last 85 k.y., benthic ¹³C values were higher by ~0.5 during warm intervals (MISs 1 and 3; marine isotope Substage 5a) than cool intervals (MISs 2 and 4) (Fig. 6). The opposite relationship is shown during the last interglacial (MIS 5; 130 to 85 ka). Benthic ¹³C increases to ~-1 during the cool intervals (marine isotope Substages 5b and 5d) of the interglacial and decreases to ~-1.5 during the warmest episodes (marine isotope Substages 5c and 5e) (Fig. 5). Further evidence for changes in the bottom-water chemistry of the basin during the last interglacial is provided by indications of increased corrosivity of the bottom water during the cool episodes (Substages 5b and 5d). Although Uvigerina spp. are not considered reliable foraminiferal recorders of deep-water ¹³C because of infaunal habitat, these results suggest that several processes controlled the relative concentrations of ¹²C.

Benthic ¹³C can be affected by several mechanisms. Globally, the ¹³C reservoir was affected between glacial and interglacial periods by shifts in carbon reservoirs that released more light ¹³C into the ocean, shifts in the ¹³C gradient between oceans, and the effect of lower CO₂ concentration on the carbonate ion (Shackleton and Pisias, 1985). Regionally, the ¹³C value of North Pacific Intermediate Water would have been affected by changes in ventilation, which allowed increased exchange with atmospheric CO₂ during glacial and stadial times (Behl and Kennett, 1996; van Geen et al., 1996). Increased surface ocean CO₂ exchange with the atmosphere would have the effect of enriching ¹³C through processes of both cool-water isotopic equilibrium effects and lower nutrient content as a result of the "newness" of the intermediate water. The length of time a water mass remains isolated from the atmosphere within the deep ocean is related to the quantity of organic material degradation within it, which consumes oxygen and decreases ¹³C. Finally, locally within Tanner Basin, interstitial waters were possibly low in ¹³C relative to bottom waters as a result of decay of ¹²C-enriched organic matter (Berger and Vincent, 1986). Thus, higher organic carbon flux should result in higher ¹³C between bottom waters and the sediment depth in which Uvigerina lived.

Another explanation for the unusual aspects of both the ¹⁸ and ¹³C records of Tanner Basin might be diagenesis. Diagenesis can play an important role in changing the isotopic composition of biogenic calcium carbonate after burial by encrustation of tests by secondarily precipitated calcite. This can bias isotopic paleotemperatures to cooler values (Douglas and Savin, 1978). The unusually cool last interglacial sea-surface temperatures could perhaps be explained by this process as well as the change in the relationship between benthic ¹³C and climate. However, diagenesis would affect both planktonic and benthic tests so that both records would show similar trends in isotopic behavior and would mask regional similarities between Tanner Basin and other sites along the southern California margin. This is not the case: the ¹³C record during marine isotope Substage 5e in Site 1014 is more similar to that of Holes 893A and 1017E than to the Tanner Basin benthic ¹³C record. Site 1014 benthic ¹⁸ does not show the unusual cooling suggested by the planktonic record. Finally, such diagenesis is uncommon in the shallow-buried sediments as young as those investigated in this contribution.