加勒比海石筍稀土元素紀錄重建之過去1800年沙塵活動變化

Abstract

本研究利用位於中大西洋西部 、加勒比海前緣的 Barbados 巴貝多島Harrison’s Cave 洞穴石筍 HC-1，建立高解析度稀土元素（REE/Ca）時間序列，並結合鈾–釷定年，重建過去 1800 年跨大西洋的撒哈拉沙塵輸送事件。我亦分析了石筍 HC-1的碳、氧穩定同位素資料，對比顯示，石筍的 δ¹³C （−13.1‰ 至 −3.5‰）與 δ¹⁸O（−4.7‰ 至 −3.1‰）的變化與稀土元素總量 (ΣREE) 無顯著相關，顯示 REE/Ca 訊號主要不受在地水文或生物效應控制，而反映外來物質輸入。 石筍 REE 組成與區域潛在來源（火山灰、北美黃土）在元素比例與配分型態上明顯不同；其 Post-Archean Australian Shale (PAAS)-normalized REE pattern 呈現穩定的 LREE > MREE > HREE，與撒哈拉沙塵的典型指紋高度一致，指示 HC-1 的 REE/Ca 記錄主要反映源自北非、經長距離大氣輸送至加勒比海的沙塵通量變化。 HC-1 石筍在過去 1800 年中解析出 25次明確的撒哈拉沙塵事件峰值。其中，約 1500–1000 年前為沙塵發生最頻繁且強度最高的時段（共 10 次），此高沙塵期與南歐西班牙 Kaite Cave 石筍 Sr 同位素 (Cruz et al., 2021, Sci. Adv) 所辨識的「高撒哈拉沙塵時期」在時序上相互對應。然而，關鍵差異在於解析度與訊息量：西班牙石筍 Sr 同位素主要揭示區域尺度的沙塵「群聚期（cluster）」，而巴貝多島 HC-1 的 REE/Ca 記錄則能進一步分辨該 500 年內多次個別沙塵暴事件的發生次數、相對強度與通量起伏。 相對地，自 1000 年前至現代，HC-1 僅記錄到 15 次較弱沙塵事件，顯示此期間為低沙塵通量階段。整體而言，位於跨大西洋下風處的巴貝多島石筍 REE/Ca 記錄，不僅驗證撒哈拉沙塵的長距離輸送歷史，更提供優於歐洲端石筍 Sr 同位素的事件級（event-scale）重建能力。 本方法將可應用在不同地質時間段，例如冰期與間冰期撒哈拉沙塵發生次數與強度之比較，或是應用到其他地區，例如北非的摩洛哥和突尼斯與南歐各國。

This study used the HC-1 stalagmite collected from Harrison’s Cave on Barbados, located in the western central Atlantic at the leading edge of the Caribbean, to establish a high-resolution rare earth element (REE/Ca) time series. Combined with Uranium-thorium dating, this record is used to reconstruct trans-Atlantic Saharan dust transport events over the past ~1800 years. Stable carbon and oxygen isotope data (δ¹³C and δ¹⁸O) from the HC-1 stalagmite were also analyzed for comparison. No significant correlation is observed between δ¹³C, δ¹⁸O, and the total REE concentration (ΣREE). This indicates that the REE/Ca signal is largely independent of local hydrological or biological controls and instead reflects the input of externally derived materials. The REE composition of the stalagmite is clearly distinct from those of potential regional sources, including volcanic ash and North American loess, in both elemental ratios and fractionation patterns. The Post-Archean Australian Shale (PAAS)-normalized REE pattern of the HC-1 stalagmite exhibits a consistent LREE > MREE > HREE enrichment, which closely matches the characteristic fingerprint of Saharan dust. This correspondence indicates that the REE/Ca record of HC-1 primarily reflects variations in dust flux originating from North Africa and transported over long distances to the Caribbean via the atmosphere. The HC-1 stalagmite reveals 25 distinct Sahara dust event peaks over the past 1800 years. The period from approximately 1500 to 1000 years ago was the most frequent and intense dust event period, with 10 events. This high dust period corresponds with the "high Sahara dust period" identified in the Sr isotope record of the Kaite Cave stalagmite in southern Spain (Cruz et al., 2021, Sci. Adv), although the key difference lies in the resolution and information content: while the Sr isotopic record from Spain reveals dust "clusters" at the regional scale, the REE/Ca record from Barbados is capable of distinguishing multiple individual dust storms within a 500-year period, revealing their occurrence, relative intensity, and flux variations. In contrast, from 1000 years ago to the present, the HC-1 stalagmite records only 15 relatively weak dust events, indicating a low-dust-flux phase. Overall, the REE/Ca record from the Barbados stalagmite, located in the downwind region across the Atlantic, not only verifies the history of long-range Sahara dust transport but also provides an event-scale reconstruction ability that surpasses the Sr isotope records from European stalagmites. This method can be applied to different geological periods, such as comparing the occurrence and intensity of Sahara dust events during glacial and interglacial periods, or applied to other regions, such as Morocco and Tunisia in North Africa, and southern European countries.