Late Pliocene monsoon linkage in the tropical South China Sea

Tian, J Pak, DK Wang, PX Lea, D Cheng, XR Zhao, QH

EARTH AND PLANETARY SCIENCE LETTERS 252:1-2 72-81

The onset of Northern Hemisphere Glaciation (NHG) similar to 2.7 Ma ago coincided with prominent climate changes in the tropical regions such as the African and the Asian monsoons. However, the relationship between tropical and sub-tropical monsoonal variations and high northern latitude ice sheet expansion as well as processes such as late Pliocene tropical sea surface temperature (SST) change is not clear. Our late Pliocene (2.5-3.3 Ma) monsoon proxy records and Mg/Ca derived SST records at Ocean Drilling Program (ODP) Site 1143 from the southern South China Sea (SCS) reveal that while tropical SST shows a stepwise decrease of 2-3 degrees C during this period, the East Asian monsoon gradually strengthens in response to the onset of the NHG. At the 41-kyr and 23-kyr bands, ice volume change lags tropical SST by similar to 4 kyr, but leads the East Asian monsoon by similar to 12-17 kyr. Our finding highlights the significant role of the tropical Pacific region in driving global climate change in the late Pliocene, which has invariable leading phase relative to the ice volume change as in the late Pleistocene. However, the East Asian monsoon shows a linear response to the onset: of the NHG in the late Pliocene, with much bigger phase lagged at the 41-kyr and 23-kyr bands than in the Pleistocene, which suggests that at the obliquity and precession bands the phases of the Plio-Pleistocene East Asian monsoon variations relative to the global ice volume changes are not constant, but variable. Therefore, the East Asian monsoons are not only simply driven by northern summer insolation at the precession period but also modulated by global ice volume change in high latitudes. (c) 2006 Elsevier B.V. All rights reserved.

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The elevation history of the Tibetan Plateau and its implications for the Asian monsoon

Harris, N

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY 241:1 4-15

The determination of the evolving palaeoaltitude of the Tibetan Plateau, since the India-Eurasia collision underpins our understanding of how orography in central Asia affects the intensity of the monsoon and hence global climate change. Palacoaltitudes, however, cannot be measured directly and need to be inferred from proxy observations that are usually model-dependent. Differing tectonic models for the behaviour of the lithosphere during continental collision have contrasting implications for the elevation of the plateau. However, two techniques recently employed for determining palaeo-elevation are independent of tectonic models, the first involving the variation with altitude of oxygen isotopes in precipitation and the second involving the change of leaf morphology with moist static energy of the atmosphere. Elevation studies have focused on southern Tibet, largely due to the relative ease of access to the region. There is a remarkable unanimity amongst the diverse techniques applied that the altitude of the southern plateau has not significantly changed since at least the mid Miocene (ca. 15 Ma) arguing for an onset of the monsoon system during or before the early Miocene. A range of tectonic studies suggest that the northern and eastern parts of the plateau are younger geornorphological features, but there are few quantitative constraints of the timing of elevation from these regions of Tibet. Since both the elevation and the surface area of the plateau impact on atmospheric circulation, palacoaltitude studies need to be extended to chart the increasing areas of elevated land surface through time. (c) 2006 Elsevier B.V All rights reserved.

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Atypical delta N-15 variations at the southern boundary of the East Pacific oxygen minimum zone over the last 50 ka

Martinez, P Lamy, F Robinson, RR Pichevin, L Billy, I

QUATERNARY SCIENCE REVIEWS 25:21-22 3017-3028

We report a nitrogen isotope record (ODP Site 1233) from the southern Chile margin at 41 degrees S. The site is located slightly south of the southern boundary of the Peru-Chile upwelling system and the associated oxygen minimum zone off Peru and northern Chile. We show that our nitrogen isotope record, from the time interval 0-50 calendar kiloyears before present (ka B.P.), bears an atypical pattern both in shape and timing when compared with records obtained from either the continental margin of the eastern Pacific or the Subantarctic Zone (SAZ) of the Southern Ocean. The delta N-15 values at Site 1233 are relatively high throughout the record, varying between 9 parts per thousand. and 13 parts per thousand. The iriajor features are a pronounced delta N-15 increase at the beginning of the deglaciation, a maximum from 19 to 10 ka B.P.; thereafter a large decrease during the early Holocene, and millenial scale oscillations showing an Antarctic timing. We propose that the record results from an amalgam of low-latitude and high-latitude processes. Low-latitude processes, including a stronger advection signal of heavy nitrates from the denitrifying zones off Peru and northern Chile, would explain the timing of the deglaciation rise and the heaviest values found over this interval, excluding the Antarctic Cold Reversal period. The overall differences between site 1233 and records from Peru and northwest American margins suggest however that the origin of the delta N-15 signal off Chile is largely controlled by hydrologic and climatic changes in the Southern Ocean. We propose that the interplay between nutrient demand in the SAZ and latitudinal shifts of hydrologic fronts controlled both the concentrations and the isotopic signature of the remaining nitrate delivered to the Chile margin. Then, the glacial surface waters of the southern Chile margin were likely lower in nitrate concentration and bear a higher delta N-15 than during interglacial periods. (c) 2006 Elsevier Ltd. All rights reserved.

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