Climate, Ventilation and Biotic Change in the Late Quaternary: Southern California Margin

J. P. Kennett, I. Hendy, and K. Cannariato, (Department of Geological Sciences and Marine Science Institute, UC Santa Barbara; ph. 805-893- 3103), and R. J. Behl (Department of Geological Sciences, California State University, Long Beach; ph. 562-985-5850)

Records from the southern California margin show strong evidence for major instability of the marine environment and ecosystem during the latest Quaternary. We have constructed 50-70 year resolution oxygen isotopic and faunal records spanning 25 to 60 Ka from Santa Barbara Basin, ODP Hole 893A, which reflect the full sequence of D/O cycles by dramatic sea surface temperature shifts. The d18Oxygen time series of G. bulloides and N. pachyderma define an apparently complete sequence of sub-Milankovitch climate oscillations from 60 to 25 Ka which closely correlates with that of the Greenland Ice Sheet. Interstadials exhibit familiar late Quaternary sawtooth patterns due to greater magnitude of warmings than coolings. At comparable chronologic resolution to the Greenland ice core records, the two regions show remarkable similarities in speed, magnitude and character of climate changes inferring synchroneity between the north Atlantic and Pacific climatic response. Switches between stadial and interstadial episodes usually occurred within <50-70 years. These rapid global oceanic and atmospheric circulation reorganizations involved brief strong feedback mechanisms that amplified the climatic changes, resulting in inferred sea-surface temperature warmings of up to 5oC within a few decades, and brief warming overshoots of up to 3oC. The large magnitude of the inferred sea-surface temperature and oceanographic changes associated with the D/O events in Santa Barbara Basin reinforce their global significance. Furthermore, our evidence for tightly coupled atmospheric and oceanic circulation switches during the D/O events demonstrates strong interdependence between atmosphere and ocean. The rapidity of the changes suggests that the global climatic signals are transmitted via the atmosphere.

The evidence of rapid climate variability in surface waters of Santa Barbara Basin and its similarity to the Greenland Ice Sheet record provides further evidence for the global nature of the D/O events. The California margin records also suggest that the north Pacific Ocean is as sensitive to rapid climate change as the north Atlantic. The oscillatory pattern of climate change during the last ice age suggests that during much of this interval the global climate system was close to a threshold, whereby small perturbations in the climate system were amplified to produce major, rapid climate shifts. Excessive climatic amplification at the beginning of interstadials, as indicated by the remarkable overshoots in sea-surface temperature increases, can only be explained by involvement of strong greenhouse gas feedback processes of brief duration.

We have also documented a history of dramatic, rapid (average ~130 yrs) upheavals in the benthic ecosystem that occurred synchronously with the D/O cycles. These upheavals, as best recorded by benthic foraminifera, reflect the switching between two states of basinal bottom water oxygenation at the initiation and termination of interstadials during the last 60 kyrs. Benthic foraminifera oscillate between oxic (>1.5 ml L-1 O2) and dysoxic (0.1-0.3 ml L-1 O2) assemblages associated with bottom water oxygenation. Low oxygen assemblages, associated with laminated sediments mark the interstadials (warm episodes); high oxygen assemblages with nonlaminated sediments characterize stadials (cold episodes). Switching between these assemblages and inferred basinal ventilation was rapid (~40 to 400 yrs; mean ~130 yr). The D/O cycles are also marked by large (up to 5 per mil) oscillations in d13C values in benthic foraminifera.

The latest Quaternary biotic changes in the basin appear to have been widespread along the California margin in conjunction with ventilation switching. The long-term oxygenation fluctuations along the California margin appear to have been controlled by the extent and location of PIW production. When ventilation is restricted to shallow depths, as at present, older, oxygen- depleted water is upwelled along the Northeast Pacific margin. During these conditions, benthic macrofauna are excluded in Santa Barbara Basin and laminations form. When ventilation extends deeper the basin is ventilated with young, oxygenated water from the subarctic North Pacific. In this oxygenated state, bioturbating benthic macrofauna occupy the basin floor. Correlation of laminated intervals in the basin to D/O cycles suggests that fluctuations in upper PIW and the OMZ off central California are tightly linked, and sensitive to global climate change.