The paleoclimatic record for the last 20 k.y. has been examined within a detailed radiocarbon age framework for Hole 893A, Santa Barbara Basin, California, using benthic and planktonic foraminiferal oxygen isotopes and assemblage changes in planktonic foraminifers. The last deglaciation is recorded as two steps (Terminations IA and IB) separated by a warm pause, correlative with the Bølling/Allerød Interstadial (15.2 to 13 ka), followed by Younger Dryas cooling (13 to 11.1 ka). This, in turn, was followed by the warmth of the Holocene. Changes in the planktonic foraminiferal assemblage suggest that average sea-surface temperatures in the basin increased from ~7°C during the last glacial maximum to ~15°C during the late Holocene, an estimated change larger than shown in earlier work.

Hole 893A consists of alternating laminated and non-laminated (massive) sediment intervals. Laminated intervals were deposited at times of low dissolved oxygen levels in the basin that precluded support of an active benthos. Massive intervals resulted from bioturbation associated with more oxygenated waters. The basin was relatively well ventilated during the glacial to near-glacial intervals (last glacial maximum and Younger Dryas cooling) and poorly ventilated during warm intervals (Bølling/Allerød Interstadial and Holocene during the last 11 k.y.). Thus, the sequence records oscillations between two fundamentally different states, one marked by bottom waters with low oxygen content associated with warm intervals, the other with relatively oxygenated waters associated with cold intervals.

We interpret these oscillations in sediment facies to reflect changes in the source of intermediate waters, with a greater proportion of oxygen-rich waters originating from a more proximal source during cooler intervals compared with a distal source of oxygen-poor waters during warmer intervals, including the present day. The stratigraphic records examined here strongly indicate that, as in the North Atlantic, fundamental changes occurred in North Pacific intermediate water circulation during the latest Quaternary. These were closely synchronized with global climate change and with paleoceanographic events in the North Atlantic. It is possible that changing strength of oceanic conveyor circulation, currently transporting waters from the North Atlantic to the North Pacific, may have in part caused changes in oxygen content of upper intermediate waters near the coast of North America, including the Santa Barbara Basin. Alternatively, the inter-ocean paleoceanographic changes were linked directly through global climate change transmitted through the atmosphere rather than through changes in the strength of the oceanic conveyor. In this case, severe cooling during the last glacial maximum and the Younger Dryas episode led to the production of intermediate waters at high latitudes in the Pacific Ocean that influenced Santa Barbara Basin ventilation. Of proximal origin, these were young, well oxygenated waters. A third, more likely, hypothesis is that both factors in combination played a role in influencing the ventilation history of the basin. This investigation indicates the existence of tight coupling between changes in the atmosphere-ocean-cryosphere during the latest Quaternary.

Date of initial receipt: 2 September 1994
Date of acceptance: 13 February 1995

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