CLIMATIC AND ENVIRONMENTAL FLUCTUATIONS ON THE QUATERNARY AND TERTIARY CALIFORNIA MARGIN
Exceedingly detailed late Quaternary sedimentary records from Ocean Drilling Program sites in Santa Barbara Basin (ODP Site 893) and elsewhere along the California margin provide evidence for repeated, dramatic oceanographic, environmental, and ecological changes along the NE Pacific that were apparently synchronous with global climatic change over the past 60-80 kyr. At Site 893, exceedingly high sedimentation rates (>120 cm/kyr since MIS 6 and ~150 cm/kyr for the Holocene), provide the opportunity to understand climatic mechanisms and transitions at a near-decadal scale. Microfossil assemblages and d18O indicate that sea-surface temperatures repeatedly oscillated with warmings of 4-8°C in <70 years. Associated shifts in thermohaline circulation changed intermediate water character in the depositional environment. During warm intervals, dysoxic conditions in Santa Barbara Basin preserved laminations in the sediment by excluding bioturbating benthic fauna, and triggering alternate replacements of exclusive benthic ecosystems. Strong, brief, negative carbon isotope excursions (up to -6) recorded in foraminifera throughout the water column near the start of several rapid warmings relate to catastrophic release of methane from gas hydrates destabilized by warm intermediate water along the continental margin.
Similar lithologic alternations preserved in older Miocene-Pliocene sequences in California (e.g., the Monterey and Sisquoc Formations) suggest that millennial to Milankovitch band climatic cycles have long influenced the California margin. These cycles are expressed by compositional variation in silica, carbonate, and detrital components, as alternation between laminated and bioturbated sediments, and as petrophysical variation in downhole logs. These observations raise a few important questions: What oceanographic or depositional conditions are required to preserve high-resolution records? Do these requirements inherently bias the high-resolution records? Is sensitivity of the geologic record to high-frequency climatic oscillations limited to certain states of the Earth System and, consquently, to certain times in Earth's history?