Revised ～2000-year chronostratigraphy of partially varved marine sediment in Santa Barbara Basin, California

摘要

Sediment in the deep center of the Santa Barbara Basin (SBB) is almost completely laminated for the portion representing the past ~2000 years and has been utilized as an archive for high-resolution paleoceanography since the 1970s. Unequivocal proof of the presence of varves in SBB sediment throughout the 20th century has been uncritically used to assume that deeper laminations are varves as well and that they can be counted down-core to arrive at a reliable varve chronology for the past ~2000 years. The advent of radiocarbon accelerator mass-spectrometric (AMS) dating of sub-milligram-sized organic terrigenous plant fragments and charcoal enabled us to independently date SBB sediment without the underlying uncertainty of variable marine radiocarbon reservoir ages. It was determined that the traditional SBB varve-count ages remain valid from the present down to ~1700 AD, whereas not all deeper laminations represent varves. Depending on depth, the newly revised chronostratigraphy deviates from the traditional varve count by up to ~400 years. Here, we present (i) a historic overview of the SBB varve chronology, (ii) a critique of the extended, traditional "varve chronology" and (iii) the rationale behind our new chronology that overcomes the long-standing misunderstanding and bias in lamination counting that was assumed to be "varve counting" below the ~1700 AD level. Evidence from other California offshore locations indicates that the oxygenation of the deeper water column has been decreasing over the past few hundred years, and this facilitated a transition from laminated and possibly intermittently varved sediment to continuously varved sediment in the SBB.