Vegetation Responses To Cyclical Forcing

Spectral analysis is a powerful tool for examination of cyclical variation within components of well-dated, largely marine, records and has been applied to a number of sequences within the far eastern rainforest region in order to assess responses of various proxies to potential forcing mechanisms on orbital timescales.

Analysis of ODP Site 1144 in the South China Sea was restricted to pine and herb pollen, essentially winter monsoon indicators, and demonstrated clear Milankovitch forcing with prominent 100 kyr (eccentricity), 40kyr (obliquity), and 20kyr (precession) periodicities, in phase with northern hemisphere insolation and ice volume and indicating a clear link with monsoon activity (Sun et al., 2003). It is interesting that the herbs demonstrate a closer correspondence with the ice volume signature than pine, reinforcing the suggested higher latitude source of its pollen. Pine displays a higher precessional than obliquity peak, indicating some tropical influence. It also shows a strong semi-precessional frequency that may be the result of an additional southern hemisphere tropical precessional signal resulting from changing mean position of the Intertropical Convergence Zone through time. Unfortunately, as spectral analysis was performed on the whole million-year record, there is no way of determining changes in forcing through time—that may be expected with a change in the global signal—from dominant obliquity to eccentricity forcing around the Brunhes-Matuyama boundary.

A greater range of proxies has been examined from the southern hemisphere sites of Banda Sea, Lombok Ridge, and ODP 820 (Kershaw et al., 2003) and the North Australian Basin (van der Kaars, new data). In general terms, mangroves show similar frequencies to those from associated oxygen isotope records, and indicate strong northern hemisphere forcing. With mangroves, this pattern is not surprising as they are constrained by sea level changes that relate directly to ice volume. Their closer relationship with variation in sea level rather than that of climate has been explained by Grindrod et al. (1999, 2002). It is the broad exposure of the continental shelf during marine transgressions that facilitates mangrove colonization and peak mangrove pollen representation.

The major indicators of core rainforest—pteridophytes and rainforest angio-sperms—surprisingly exhibit rather different spectral signatures. Pteridophytes exhibit strong glacial-interglacial cyclicity with a prominent obliquity signal that is not evident in the rainforest angiosperms that display a dominant precessional signal, except in the Banda Sea record. The most parsimonious explanation is that the core rainforest area is greatly influenced by the Asian monsoon and this influence extends to other marine sites in the pteridophytes due to the wide dispersal of their spores. The implication that more marginal areas are displaying a more localized tropical pre-cessional influence is well-demonstrated in the eucalypt component of the North Australian Basin, whose variation can be clearly correlated with that of southern hemisphere precession. Although the area expresses a marked monsoon climate, the source of monsoon rainfall is most probably the southern Indian Ocean that is largely divorced from the Asian system.

Rainforest angiosperms also display significant variation in the 30-kyr frequency band, and this frequency is even more clearly expressed in rainforest gymnosperms and some charcoal records. It has been proposed by Kershaw et al. (2003) that this non-Milankovitch cycle is related to ENSO, due to the correspondence of peaks in burning and associated declines in fire-sensitive araucarian forests in the ODP record with peaks in ENSO frequency derived from the modelling of Clement et al. (1999). From similar spectral signatures in the records of charcoal from cores MD97-2141 and MD97-2140 in similar West Pacific settings, Beaufort et al. (2003) and Thevenon et al. (2004) propose that the 30-kyr frequency can be attributed to the competing influences of long-term ENSO-like forcing and the glacial-interglacial cycle on the East Asian Summer Monsoon.

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