Geographic coherence of millennial-scale climate cycles during the Holocene
Varve thickness measurements and variation in sediment texture have been compiled for an intermittently varved sequence of Holocene sediments at ODP Site 893 in the Santa Barbara Basin (SBB), off California. A varve-based age model is generated from the varve counts, using interpolation across homogeneous intervals. With this age model, the varve thickness and sediment texture time series are compared to published GISP2 stable oxygen isotope and aerosol data as well as to the atmospheric Δ14C record. Variation in sediment texture at Site 893 shows the presence of a ∼ 1000 year cycle, which is attributed to fluctuations in the degree of ventilation of the basin. Cross spectral analysis shows that this cycle is coherent with the GISP2 δ18O record and with atmospheric Δ14C throughout the Holocene. The correlation with the Δ14C record indicates that this ∼ 1000 year cycle is related to solar cycles, which appear to have a noticeable impact especially on circum-North Atlantic climate. The presence of the 1000 year cycle in the SBB is attributed to a tele-connection with the Northern high latitudes, most likely via intermediate water formation in the North Pacific. In contrast, the GISP2 aerosol record and variation in varve thickness at ODP Site 893 show a coherent ∼ 2750 year cycle, which does not correlate with the ∼ 1000 year cycle. Varve thickness in SBB has been shown to correlate with annual rainfall, which occurs mainly during winter when prevailing winds slacken. Both the GISP2 aerosol and SBB varve thickness records are therefore interpreted as a measure for a combination of wind stress and aridity. The correlation is negative, in that thick varves/wet climate in SBB occur during times that the aerosol load is high at GISP2 (dry). The presence of two distinct patterns of millennial scale variation points to regional trends in climate change during the Holocene. However, the fact that both patterns occur at two distant localities suggest that they represent differential behaviour of supra-regional climate systems of which the effects can be traced over large parts of the globe.