What Forced Holocene Millennial-Scale Variability? A Tale from the Western Tropical Pacific
Abstract:A number of climate archives have documented millennial-scale variability during the Holocene. It is not currently known whether these variations are forced or unforced. A solar forcing hypothesis has been put forward to explain the 1000 and 2500-year periodicities because of similarities between the spectral peaks in the proxy records and reconstructions of solar variability. Our work on a marine record of Mg/Ca-based sea surface temperatures (SSTs) within the Indo-Pacific Warm Pool (core MD98-2181) challenges the solar hypothesis because there is no coherence between the amplitude of changes in solar forcing and in the proxy SSTs. There is also a long phase delay between the peak in forcing and the maximum response.
On the other hand, there exists the possibility that the deep ocean can drive some of the Holocene variability, especially at the 1500-year timescale. This periodicity is reminiscent of the Marine Isotope Stage 3 (MIS3) cycles. To evaluate whether the Holocene and MIS3 variability does in fact arise from the same mechanism, we start with the premise that climate variability from different parts of the globe should have similar relationships in the Holocene as they did in MIS3. However, this comparison is difficult because of the poor chronologic constraints inherent to proxy records. We circumvented this problem by using the benthic foraminifera δ18O from the same MD81 samples, which provides a record of deep ocean temperature and salinities. Since these conservative properties are acquired in the Southern Ocean, the MD81 benthic δ18O is a record of high southern latitude variability. Thus, MD81 allows us to investigate the relative timing of tropical and southern ocean variability from a single location. A time series analysis of the MD81 record shows that although the 1,500-year cycle is present during both the Holocene and MIS3, the phasing of tropical Pacific and Southern Ocean variability is different. During the Holocene, tropical Pacific warming precedes Southern Ocean warming/freshening but lags during MIS3, suggesting two distinct mechanisms for the Holocene and MIS3 cycles, perhaps one originating from high latitudes and the other from low latitudes.