Resolving varve and radiocarbon chronology differences during the last 2000 years in the Santa Barbara Basin sedimentary record, California

Abstract

Santa Barbara Basin (SBB) is well known for producing high quality paleoclimate reconstructions, but the validity of correlations between the basin and other regions rests upon age model accuracy. Yet, the two independent and well-established chronologies based on varve counting and radiocarbon dating do not consistently agree during the last two millennia. Here the validity of age model assumptions is tested including whether 1) the regional reservoir age of surface waters was invariably 641 ± 119 years through time, and/or 2) SBB laminae couplets are consistently annual varves and counted with sufficient precision. This high-resolution 14C study of SBB sediments compares 49 mixed planktonic foraminiferal carbonate and 20 terrestrial organic carbon 14C dates to the varve chronology, to extend the high-resolution paleoclimate chronology of the basin back ∼2000 years. Evidence indicates that regional reservoir ages do not remain constant through time with ΔR, (i.e. regional reservoir age minus variations in the global mixed-layer ocean reservoir age) fluctuating between 80 and 350 years. Second, there is a consistent (R2 = 0.96) undercounting of the varves between 150 and ∼1700 AD based on a new varying ΔR14C chronology, indicating that some laminae couplets may not be annual. Loss of varves may occur when low riverine input and infrequent winter storm activity during drought intervals fail to supply a siliciclastic layer.