台灣東部晚冰期紋層研究:新仙女木期年際降雨變異

Abstract

台灣東部花蓮北埔岩心深度約53至67m出現紋層沉積物，據研究推論係屬新仙女木期(Younger Dryas)期間之沉積。岩心之定年控制來自碳十四定年，深度69 m之年代為13,200 cal BP；深度40.5 m年代為10,400 cal BP，時間上涵蓋了整個新仙女木期 (12,900-11,500 BP)。為了解明暗紋層之間是否呈現季節性差異，分別分析明暗紋層沉積物之花粉種屬組成，並參考現今花蓮當地花粉曆紀錄，分析結果顯示夏秋季節開花種屬於明層中佔有較高組成比例，冬春季節開花種屬則在暗層中佔有較高比例，據此推斷紋層之明暗紋層是反映了季節性的沉積變化，一對明暗紋層即為一年間的沉積記錄。另一方面，計數岩心深度61m至63m之間的紋層層數有206層，表示此段2 m厚度沉積於206年間，估算平均沉積速率為0.97 cm/yr，此結果與碳十四定年結果推估深度40.5 m-69 m之平均沉積速率 (1.01 cm/yr)相當吻合，以上兩點皆顯示北埔紋層有相當高的可能性屬於年層。 年層紋層紀錄之每年堆積厚度反映沉積物供給之狀況，受到降雨量影響之侵蝕與搬運過程主導的沉積物供應量可做為過去該區域降雨相關之指標。現今台灣東部之降雨年際變化主要受到夏、秋季颱風降雨情形所左右，而颱風侵襲台灣之機率被認為與聖嬰南方震盪(ENSO)有關，聖嬰年颱風帶來的降雨量較少，而反聖嬰年的颱風則貢獻較高降雨量。紋泥明暗層之厚度，以代表夏秋季沉積的明層厚度較厚，層厚變動大；代表冬季沉積的暗層厚度較薄，且厚度較穩定。似與現代此區域夏秋季降雨受到颱風影響而變異性較大，冬春季降雨之變異性則較小的狀況吻合。本研究分析北埔岩心深度61至63m之連續206年明層厚度記錄，希望了解過去聖嬰南方震盪之周期性，結果顯示明層厚度變化與現代活躍的聖嬰南方震盪具有相似的周期性。 由於台灣東部年際降雨變異被認為與ENSO之間有所關連，反聖嬰狀態下(La Nina)颱風侵襲台灣機會較高，該年夏秋季帶來較多的強降雨；聖嬰狀態下(El Nino)颱風路徑遠離台灣，因颱風造成的強降雨較少。這種關係具有數十年尺度的變化，基本上與北大西洋數十年尺度震盪一起改變，當北大西洋數十年尺度震盪進入相對低溫的負相時，熱帶太平洋聖嬰-南方震盪變異性則增加，同時台灣受到聖嬰-南方震盪的影響也增加，台灣東部夏秋降雨量與聖嬰-南方震盪指數的相關性明顯地增高，北大西洋氣候震盪被認為有能力影響到熱帶太平洋的聖嬰-南方震盪系統，而台灣颱風季降雨與ENSO間相關性也顯示出隨北大西洋氣候震盪同時的變化。 本研究發現新仙女木時期台灣東部在的年解析度年層紋層沉積記錄顯示出與現代ENSO相近的年際變化周期性，這可能反映新仙女木時期大西洋經向翻轉環流(AMOC)的減弱或者北大西洋溫度降低，造成聖嬰南方震盪變異性增強。大西洋氣候變化對於熱帶太平洋聖嬰南方震盪之影響學界仍正熱列討論中，我們的記錄支持在千年尺度氣候變遷當中，聖嬰南方震盪變異性可能受到大西洋氣候變化的影響。

Constrained by radiocarbon dating, the laminated sediments from depth 53 m to 67 m in Beipu Core, Hualien, are deposits during Younger Dryas (12,900-11,500 BP). The calibrated age in depth 40.5 m is 10,400 cal BP and 69 m is 13,200 cal BP including the whole duration of Younger Dryas. The pollen analysis in dark and light layer-couplets of Beipu laminations show seasonal cycles with reference to the pollen calendar from modern Hualien. The summer- autumn blooming taxa like Alnus and Lagerstroemia are with higher percentage in light layers, and the winter-spring blooming taxa like Pinus, Abies and Tsuga are with higher percentage in dark layers, indicating the light layers were composed by sediments deposit in summer and autumn, and the dark layers were sediments deposit in winter and spring. Indirectly proved the Beipu laminations are annually layers formed under the regular seasonal variations. The sedimentation rate of 0.97 cm/yr by counting 206 annually couplets of Beipu laminations between depths 61- 63 m which is consistent with that of 1.01 cm/yr obtained from the C-14 dates. Both the sedimentation rate and pollen evidence show the laminations are seasonal deposits. Presently, Interannual rainfall variations in Eastern Taiwan are linked to El Nino Southern Oscillation (ENSO), with typhoons contribute more extreme rainfall during La Nina but less in El Nino years. When the ENSO variability in the high stand from the late 1960s to late 1990s, the correlation coefficient between typhoon season precipitation of Hualien and ENSO index is relatively highcompare with 1920 to 1960 when the ENSO variability in a low stand. This phenomenon indirectly shows the effect of the connection between typhoon season precipitation of Hualien and ENSO is link to the ENSO variability. Here we show evidence from this annually laminated sediment record from eastern Taiwan for interannual rainfall variability in the Younger Dryas, based on pollen analysis of laminated layers shows seasonal cycles in dark and light layer-couplets constrain the annual resolution, and the thickness of laminated layers as annual sediment input could be a proxy of extreme rainfall which with periodicity similar to present day active ENSO cycles. This may be attributed to the Atlantic meridional overturning circulation (AMOC) weakening or cooling in the North Atlantic Ocean during Younger Dryas enhanced the ENSO variability. These influences of Atlantic climate change on the tropical Pacific ENSO’s variability are still under intense investigation. This study may provide the record of climate variability on interannual scale which was linked to ENSO variance, and indicating El Nino Southern Oscillation was once active in Younger Dryas, a widely known cold phase in the North Atlantic Ocean, showing an analogy with the idea of temperature anomalies in Atlantic Ocean are linked to a higher ENSO variance in equatorial Pacific based on the instrumental observations for previous few decades.