Search

Taiwan is particularly sensitive to changes in monsoonal precipitation and to typhoon-induced heavy precipitation events, however, rainfall variability in Taiwan on centennial and millennial time scales during the Holocene has not been well understood. This study describes mid-Holocene rainfall features of Taiwan based on pollen, total organic carbon (TOC), total nitrogen (TN), and C/N ratio records of core MD05-2908. The step-wise increase in sedimentation rate, fern spore percentage and concentration, TOC content, and C/N ratio suggests an increasing terrestrial material supply due to the intensified rainfall in Taiwan since 6800 cal. yr BP. This rainfall pattern shows an inverse pattern to the decreasing East Asian summer monsoon (EASM) strength represented by the multi-proxy records from North China. Variation of the East Asian summer circulation and associated moisture transport may account for the long-term rainfall changes in Taiwan. Superimposed on this trend, we interpreted three prominent rainfall changes, which focus on the periods of 6800–6600, 1090–880 and 490–190 cal. yr BP. These centennial time scale rainfall variations in our records are linked to the intensity of El-Nino Southern Oscillations.

We used multi-decadal pollen and diatom records from sediment core TFL-1 from Tsuifong Lake to reconstruct the vegetation dynamics and hydroclimate in northeastern Taiwan during the past 3500 cal BP. Coarse grained sediments in association with higher percentages of wetland pollen (Cyperaceae) and upper conifer pollen (Tsuga and Pinus) in the lower part of the core indicate low lake levels and a relatively cold/dry climate between 3500 and 2030 cal BP, reflecting a decline of the East Asian summer monsoon (EASM). Muddy sediments coupled with reduction of wetland pollen represent the rise of lake levels, implying the re-strengthening of the EASM during the past 2000 years. Paleotemperature was inferred from the variation of pollen origin from the upper and lower mountain forest, indicating the global temperature anomalies of the Medieval Warm Period (MWP) and the Little Ice Age (LIA). In comparison to the main climate forces in the North Pacific, we suggest that the long-term climatic trend in Taiwan was controlled by variations in EASM intensity, while increased precipitation over the past 2000 years may also be linked to warmer sea surface temperature (SST) of the western Pacific Warm Pool (WPWP) and increased El Nino-Southern Oscillation (ENSO) events, which increased typhoon intensity. Higher diatom-inferred pH during 2930–2030 cal BP and the LIA suggest strong hydrological disturbances, reflecting more typhoons passing over Taiwan. The frequent typhoon events could be linked by an abrupt shift of typhoon track, due to the reduction of the WPWP and expansion of the Northwestern Pacific High, which move typhoons in a more westerly direction.

article i nfo Wereportontheexistenceandnature of Holocene solar andclimaticvariationson centennialto millennial time- scales. We introduce a new solar activity proxy, based on nitrate (NO3 − ) concentration from the Talos Dome ice core, East Antarctica. We also use a new algorithm for computing multiple-cross wavelet spectra in time-fre- quency space that is generalized for multiple time series (beyond two). Our results provide a new interpretive framework for relating Holocene solaractivity variations on centennial to millennial timescales to co-varying cli- mate proxies drawn from a widespread area around the globe. Climatic proxies used represent variation in the North Atlantic Ocean, Western Pacific Warm Pool, Southern Ocean and the East Asian monsoon regions. Our wavelet analysis identifies fundamental solar modes at 2300-yr (Hallstattzeit), 1000-yr (Eddy), and 500-yr (unnamed) periodicities, leaves open the possibility that the 1500-1800-yr cycle may either be fundamental or derived, and identifies intermediary derived cycles at 700-yr and 300-yr that may mark rectified responses of the Atlantic thermohaline circulation to external solar modulation and pacing. Dating uncertainties suggest that the 1500-yr and 1800-yr cycles described in the literature may represent either the same or two separate cycles, but in either case, and irrespective too of whether it is a fundamental or derived mode in the sense of Dima and Lohmann (2009), the 1500-1800-yr periodicity is widely represented in a large number of paleoclimate proxy records. It is obviously premature to reject possible links between changing solar activity at these multiple scales and the variations that are commonly observed in paleoclimatic records.

Based on elemental geochemical data, we reconstructed the sediment provenance, surface productivity and bottom water redox conditions for the last 48 ka in the Ulleung Basin (Sea of Japan) and inferred the factors controlling them. Al2O3/TiO2 ratio and chemical index of alteration (CIA) suggest that sediment provenance changed during the glacial period (48-18 ka) compared to the deglacial (ca. 18-11 ka) and Holocene. Mass accumulation rates of total organic carbon (TOC), CaCO3, phosphorus, cadmium and excess barium reveal low paleoproductivity during low sea stand. During 18-11 ka, productivity increased due to increasing inflow of nutrient-rich water masses - the Oyashio and the East China Sea coastal water - in tandem with the rising sea level. Maximum productivity occurred during Younger Dryas and Pre-boreal periods when sea level was at �� 60 m and then gradually decreased as the Tsushima Warm Current inflow kicked off at ca. 9.3 ka, consistent with other paleoredox proxies, which reveal the presence of anoxic bottom water during ca. 12-9 ka. With the changes in paleoredox proxies and their ratios (TOC, Mo, U, Mn, C/S ratio and Uauthigenic and Mo contents), we hypothesized that the redox changes were mainly ventilation driven and were superimposed on the influence of circulation-induced productivity changes. The global climate and sea-level changes on a millennial timescale play a major role in enhancing paleoproductivity and restrict bottom water advection, subsequently driving the oxygenation of bottom water in the Ulleung Basin. Copyright # 2012 John Wiley & Sons, Ltd.

National Science Council of Taiwan [NSC 97-2629-M-001-025, NSC 99-2116-M-001-011]; National Science Foundation of China [41176059, 41121091]

Multi-proxy records in two sediment cores (R and Rd) from the subalpine Retreat Lake in NE Taiwan document 5 centennial-scale periods (∼7.3–6.9, 6.35–5.9, 5.8–5.1, 4.5–∼2.1, and 2.0–1.6 ka BP) of relatively reduced summer East Asian monsoon (EAM) precipitation that were superimposed on the long-term decreasing monsoon trend from ∼8 ka BP onwards. The first three intervals of weak summer EAM indicated by low TOC, C/N ratio and water content within the Holocene optimum (∼8.6–4.5 ka BP) correlate with solar activity, revealing that the centennial-scale summer monsoon variations in a warm climate were related to weak solar irradiance. The desiccation of the lake (4.5–∼2.1 ka BP), followed by a weak monsoon event at ∼2.0–1.6 ka BP, may coincide with decreased sea surface temperature in the tropical Pacific. High water content and low dry bulk density in core Rd at ∼7.6, 7.46, 6.8–6.54, 6.1, 5.8, 5.6, and 5.0 ka BP imply a significant increase of summer EAM during these periods, and thus suggest an unstable optimum climate in Taiwan. A possible linkage of weak summer EAM with the tropical Pacific and North Atlantic cooling during these intervals may be driven by coupled ocean–atmosphere interactions, especially reduced heat and moisture transport and enhanced El Nino-Southern Oscillation in the tropical Pacific, as well as solar activity. The centennial-scale reorganizations in the tropical Pacific climate dynamics may have played an important role in the summer EAM instability, particularly during the late Holocene.

Sedimentary total organic carbon and carbon-to-nitrogen ratio records from the subalpine Retreat Lake in NE Taiwan reveal four centennial periods (~8–8.3, 5.1–5.7, 4.5–~2.1, and 2–1.6 kyr BP) of relatively reduced summer East Asian monsoon (EAM) precipitation that were superimposed on the insolation-dependent, long-term decreasing monsoon trend during the middle and late Holocene while early Holocene monsoon strength was controlled by glacial boundary conditions. Strikingly, all weak monsoon events correlate with the timings of low sea surface temperature in the tropical Pacific, maxima of hematite stained-grains in the sediments of North Atlantic, reduced formation of North Atlantic Deep Water, and low concentrations of atmospheric methane over Greenland, suggesting a globally well-connected postglacial climate (from ca. 8.6 kyr BP onwards). Persistent linkage of weak summer EAM-tropical Pacific and North Atlantic cooling-reduced global wetland extent during these intervals is believed to be driven by coupled ocean-atmosphere interactions, especially reduced heat and moisture transport and enhanced El Niño-Southern Oscillation in the tropical Pacific, as well as solar activity. Overall similarity of summer EAM with diverse proxy records and their coincidence to abrupt changes witnessed in other paleorecords across the world imply that the centennial-scale reorganizations in the tropical Pacific climate dynamics may have been playing an important role, of course closely in phase with solar variations and North Atlantic climate, in the Holocene summer EAM and, by extension, low-latitude's monsoon instability.

[1] Sedimentary geochemical records from subalpine Retreat Lake, subtropical Taiwan, document the unstable East Asian monsoon (EAM) climate over the past ∼10,300 years, with a weak EAM between ∼10.3 and 8.6 ky B.P., EAM intensity peaks between 8.6 and 7.7 ky B.P., and then gradually decrease in response to summer insolation, heat and moisture transport. Our proxy record reveals several weak monsoon intervals that correlate to low sea surface temperatures in the western tropical Pacific and cold events in the North Atlantic, linking the tropical Pacific, North Atlantic, and Polar climates because weak EAM events at ∼8.2, 5.4 and 4.5–2.1 ky B.P. also correspond to low values of atmospheric methane and periods of reduced North Atlantic Deep Water formation. We therefore suggest that centennial to millennial scale monsoon variability during the Holocene in the northern subtropics is globally-mediated via a sun-ocean-monsoon-North Atlantic linkage.