The South China Sea region

Sun et al. (2003) provide a near-continuous, high resolution (i.e., 820-year average time interval between samples) record from ODP Site 1144 (20°3"N, 117°25"E) taken from a water depth of 2,037 m equidistant from southern China, Taiwan, and the Philippines (Figures 4.1 and 4.6). The record covers marine isotope stages (MIS) 29 to 1 or the last 1.03 Myr. In terms of major ecological groups represented, there is little

South China Sea Odp

Figure 4.6. Relative abundance of major taxa and taxon groups in the pollen record from ODP Site 1144, South China Sea (after Sun et al., ^ 2003). All taxa are expressed as percentages of the dryland pollen sum, that excludes Pteridophyta and mangroves. P*

Figure 4.6. Relative abundance of major taxa and taxon groups in the pollen record from ODP Site 1144, South China Sea (after Sun et al., ^ 2003). All taxa are expressed as percentages of the dryland pollen sum, that excludes Pteridophyta and mangroves. P*

consistent change in representation and variability is generally lowest in the rainforest groups, tropical and lower montane (subtropical) taxa, montane conifers, and, to some degree, pteridophytes. There is some indication that lower altitude rainforest has generally expanded from about 600 kyr, after the Early-Middle Pleistocene transition, while there has been an overall reduction since this time in montane conifers. Throughout the record, the greatest variation is in the dominant groups: the herbs and Pinus, with the former showing highest values during the glacial periods and the latter during interglacials. It is proposed that some of this variation is a result of glacials being drier than interglacials, an interpretation supported by generally higher values of pterido-phytes during interglacials. It is suggested by Sun et al. (2003), however, that it was not a simple replacement of herb vegetation by Pinus, but that herbs were largely derived from the exposed continental shelf and Pinus from more distant mountain areas. The substantial component of Artemisia at times within the herb component might suggest also that temperatures during glacial periods were much reduced, even at sea level. However, the very high values for Artemisia steppe vegetation during the Last Glacial Maximum are interpreted not in terms of climate but considered the result of a tectonically-induced broader continental shelf relating to an uplift of the Tibetan Plateau around 150 kyr. This interpretation seems inconsistent, though, with the presence of equally high values of Artemisia through much of the period from 1,000 to 900 kyr, where no expansion of the continental shelf area is inferred.

This overview of major pollen components in the ODP Site 1144 record masks some important changes that have taken place in the representation of tropical rainforest taxa over the last million years. From a separate portrayal of relative taxon abundance in relation to a lowland and montane tropical rainforest sum, Sun et al. (2003) identify three major periods. The earliest, before 900 kyr, is characterized by relatively high values for tropical montane taxa and Altingia, suggesting cool conditions, consistent with a climatic interpretation for the high Artemisia values. The period corresponds with minimum variation in the marine isotope record. Increased temperature is inferred for the subsequent period, 900 kyr to 355 kyr, where lowland taxa including Dipterocarpaceae, Celastraceae, Macaranga/Mallotus, and Trema were more conspicuous. This interpretation appears counter to that from the accompanying isotope record where generally lower temperatures may be inferred. From about 355 kyr, the submontane taxa Quercus and Castanopsis markedly increase their dominance. The abundance of these taxa in seasonal forests in southern China suggests that the climate was more seasonal as well as cooler. The period also marks the entry into and consistent representation of Moraceae, Oleaceae, and Symplocos in the pollen record as well as marked increases in Apocynaceae, Rubiaceae, Sapindaceae, and Sapotaceae, indicating a substantial change in the composition of lower altitude rainforest. The restriction of mangrove pollen to this period may seem surprising but could relate to the achievement of higher temperatures periodically with more pronounced glacial-interglacial cyclicity or to changes in coastal configuration. However, the marine isotope record indicates that the amplitude of glacial oscillations increased about 600 kyr, much before the onset of this period.

A pollen record from the lowland Tianyang volcanic basin of the Leizhou Peninsula on the northern coast of the South China Sea and at a similar latitude to ODP Site 1144 (Zheng and Lei, 1999) provides a useful terrestrial comparison of vegetation changes for the region to the marine core for the Late Quaternary period. OSL and radiocarbon dating combined with paleomagnetic analyses have allowed tentative correlation of the record with the marine isotope record over the last 400 kyr. In contrast to the marine record, the dominant pollen types are the evergreen oaks (Quercus and Castanopsis) that derive from the mountains surrounding the site. The fact that there is only low representation of lowland rainforest taxa, despite the fact that tropical semi-evergreen rainforest surrounds the site, supports the evidence for relatively low pollen production and dispersal from this vegetation formation within modern pollen studies. However, the much lower values for Pinus and pterido-phytes—together with the fact that they tend to peak in glacial rather than interglacial periods—brings into question the regional climatic significance of these taxa in the marine record. It is inferred by Zheng and Lei (1999) that glacial periods generally remained wet, although the last glacial period was an exception, with abnormally high values for Poaceae and the only significant values for Artemisia interpreted as indicating much drier conditions than present. Temperatures are estimated, from an inferred lowering of montane forest by at least 600 m, to have been some 4°C lower than today during earlier glacial periods, and even lower during the last glacial period. However, there is little variation in lowland forest elements through the record.

4.4.2 The Coral Sea region

Marine records from the Coral Sea adjacent to the humid tropics region of northeastern Australia provide a coarse resolution coverage of much of the last 10 Myr (Kershaw et al., 1993, 2005; Martin and McMinn, 1993) (Figures 4.1 and 4.7). Throughout almost all of the period, the dry land pollen assemblages are dominated by the rainforest taxa Araucariaceae (predominantly Araucaria) and Podocarpus that, in this region, may have been abundantly represented in lowland as well as higher altitude communities, and the predominantly sclerophyll taxon Casuarinaceae. Late Miocene to Early Pliocene assemblages also contain notable percentages of other montane rainforest taxa—especially Dacrydium guillauminii type, Dacrycarpus, Phyl-locladus, and Nothofagus—while rainforest angiosperms (including lowland taxa, herbs, and mangroves) are poorly represented. The climate was wet and probably substantially cooler than today. There is a gap in the record from the Early Pliocene to the very early Quaternary, but dominance of a Late Pliocene terrestrial sequence on the Atherton Tableland by Podocarpus, Nothofagus, and Casuarinaceae, with the full complement of southern conifers, suggests a continuation of wet and cool conditions until at least close to Pleistocene times (Kershaw and Sluiter 1982). The reduced representation of Araucaria at this site can be explained by the per-humid conditions as araucarian forest is generally confined to drier rainforest margins. Conversely, Nothofagus would have thrived under the high rainfall as well as the higher altitude of the Tableland.

At the time of recommencement of pollen preservation in the marine record about 1.6 Myr, values for Casuarinaceae, Araucariaceae, and Podocarpus are maintained, but other southern conifers have much reduced percentages, with Phyllocladus having

Coral Mcminn

Figure 4.7. Representation of major and indicator taxa in pollen records from ODP Site 823 (Martin and McMinn, 1993) and ODP Site 820 (Kershaw et al., 1993) in relation to the Coral Sea marine isotope record from ODP Site 820 (Peerdeman, 1993) and inferred sea surface temperature records from ODP Sites 820 and 811, and the combined "global" isotope record of Sites 846 and 667 (Shackletoneftf/., 1995). All terrestrial pollen, excluding mangroves and pteridophyte spores, make up the pollen sum on which all percentages are based.

ODP Site 823

Figure 4.7. Representation of major and indicator taxa in pollen records from ODP Site 823 (Martin and McMinn, 1993) and ODP Site 820 (Kershaw et al., 1993) in relation to the Coral Sea marine isotope record from ODP Site 820 (Peerdeman, 1993) and inferred sea surface temperature records from ODP Sites 820 and 811, and the combined "global" isotope record of Sites 846 and 667 (Shackletoneftf/., 1995). All terrestrial pollen, excluding mangroves and pteridophyte spores, make up the pollen sum on which all percentages are based.

disappeared from the record. Nothofagus also was probably regionally extinct with occasional grains most likely derived by long-distance transport from New Guinea There are marked increases from 1.4 Myr in Poaceae, Asteraceae, Chenopodiaceae, rainforest angiosperms, and mangroves, but—as the record changes at this point from a deep-sea (ODP Site 823) to a continental slope (ODP Site 820) core—these changes may reflect differential pollen transport as much as source vegetation. However, differences between Late Tertiary and Early Pleistocene assemblages do indicate that rainfall had declined and temperatures had possibly increased. The trend in the marine isotope record from the Coral Sea towards less negative values, interpreted as an increase in sea surface temperatures, provides support for a regional temperature increase (Isern et al., 1996).

Most of the Quaternary period is characterized by very variable representation of taxa. Although sample resolution is too coarse to address the cause of this variation, a detailed record from at least 1 Myr to about 950kyr (Kershaw et al., 2005) demonstrates a relationship with oscillations in the isotope record of Peerdeman (1993) that extends back to within this phase. Rainforest conifers, Casuarinaceae, and Poaceae achieve greater relative importance than rainforest angiosperms during glacial periods, indicating they were both cooler and drier than interglacials. However, the sequence remains fairly stationary until late in the record, with the only substantial change being the loss, probably within the Mid-Pleistocene Transition, of Dacrycarpus.

The most dramatic modification of the vegetation cover of the humid tropics of Australia within the Quaternary and, in fact, during the whole of the last 10 million years, is recorded within the last 200 kyr. This modification follows a sharp decline between 350 kyr and 250 kyr in S18O values of planktonic foraminifera within ODP Site 820 (Peerdeman 1993) that correlated with a major phase of development of the present Great Barrier Reef system (Davies, 1992). It has been proposed that the iso-topic change was a result of increased sea surface temperatures within the Coral Sea (Peerdeman et al., 1993; Isern et al., 1996), but this hypothesis is not supported by alkenone paleothermometry that suggests temperatures have not varied by more than 1.5°C over the last 800,000 years and that diagenesis within foraminifera is a more likely explanation for the isotope trend. A detailed record through the last 250 kyr from ODP Site 820 (Moss, 1999; Kershaw et al., 2002) illustrates the complex nature of the vegetation changes (Figure 4.8). Dates are derived from the accompanying isotope record (Peerdeman, 1993), but—due to potential hiatuses and changes in sediment accumulation rates—are not very precise. Higher values for rainforest angiosperms in this detailed record are probably due to a reduction in sieve size during preparation, allowing the collection of small grains such as Elaeocarpus and Cunoniaceae. Glacial-interglacial cyclicity is most evident in the rainforest angios-perm and mangrove components that are highest during interglacial periods and during interglacial transgressions, respectively, but is over-ridden by stepwise changes in other major components. There is a substantial and sustained increase in Poaceae around 180 kyr with apparent compensatory decreases in pteridophyes and Arecaceae. Southern conifers decrease through the record with the last major representation of Podocarpus and Dacrydium about 190 kyr: the latter disappearing from the record around 25 kyr, and sustained decreases in Araucariaceae about 140 kyr and

South China Sea Odp

-1.0 -2S) 0 2.(1 40 Temperature (°C) (dashed line)

Figure 4.8. Selective features of the detailed Late Quaternary record from ODP Site 820 (Moss, 1999) in relation to the marine isotope record of Peerdeman et al. (1993). All taxon abundances are expressed as percentages of the dry land arboreal pollen sum (excluding aquatics, mangroves, and pteridophytes).

-1.0 -2S) 0 2.(1 40 Temperature (°C) (dashed line)

Figure 4.8. Selective features of the detailed Late Quaternary record from ODP Site 820 (Moss, 1999) in relation to the marine isotope record of Peerdeman et al. (1993). All taxon abundances are expressed as percentages of the dry land arboreal pollen sum (excluding aquatics, mangroves, and pteridophytes).

30kyr. By contrast, Eucalyptus increases from very low values around 130 kyr and increases again around 40-30 kyr, with rises corresponding to highest charcoal peaks in this record. There is little sustained change in representation of rainforest angio-sperms, although the trend towards higher values for Cunoniaceae—resulting in a greater contribution of sub-montane pollen—certainly does not support a general temperature increase. Many of these changes are identified within the later part of the record by those in the adjacent terrestrial record of Lynch's Crater: notably, the initiation of burning around the site, dated to about 45 kyr (Turney et al., 2001), with a sustained increase in Eucalyptus and decline in Araucaria a few thousand years later, and a similar age for the disappearance of Dacrydium. However, there is no evidence for sustained changes before this time, back to the initiation of the record about 220kyr (Kershaw et al., 2002).

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