Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean
Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean
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DOI：
10.1016/j.quascirev.2020.106635
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发表时间：
2020-12
影响因子：
4
通讯作者：
W. Parker;Y. Yanes;Eduardo Mesa Hernández;D. Surge
中科院分区：
文献类型：
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作者：
W. Parker;Y. Yanes;Eduardo Mesa Hernández;D. Surge
中世纪气候异常（MCA;公元900-1300年）是北方半球工业化前气候变暖的最新时期，因此对MCA信号的估计可以阐明人为气候变化的可能影响。目前高分辨率的MCA气候信号仅限于中高纬度地区，这混淆了MCA如何影响一些全球/半球气候机制（例如北大西洋涛动; NAO）的推论。为了解决这一知识差距，我们估计季节性解决海面温度（SST）的氧同位素组成（δ 18 O）的连续采样Phorcus atratusshells从考古遗址跨越MCA在加那利群岛。分析了12个考古和6个现代的P. atratusshells，并使用碳酸盐靶放射性碳测年法对考古贝壳进行了测年。SST估计使用公布的文石-水平衡分馏方程。现代贝壳类显示的平均SST为20.0 ± 1.5 °C，季节幅度为5.3 ± 0.9 °C。考古贝壳的平均SST为18.2 ± 0.7 °C，平均季节幅度为5.5 ± 1.0 °C。因此，加那利群岛横跨MCA的贝壳记录的SST比现代的要冷得多（P<0.05），这与北方半球其他地方的变暖估计和模型预测形成鲜明对比。我们建议，所观察到的冷却导致增加上升流在非洲西北部由于加强盛行西风和沿海风沿着非洲海岸线。从考古贝壳中获得的原位碳同位素数据（δ 13 C）部分支持了MCA期间上升流增强的情况，并将其与现代贝壳和环境海水中溶解无机碳的δ 13 C值进行了比较。这些结果是一致的，与其他低纬度温度/降水异常与一个积极的NAO模式，这表明过渡到一个积极的NAO指数在中期和后期MCA可能延长到公元世纪。
The Medieval Climate Anomaly (MCA; 900–1300 AD) was the most recent period of pre-industrial climatic warming in the northern hemisphere, and thus estimations of MCA signals can illuminate possible impacts of anthropogenic climate change. Current high-resolution MCA climate signals are restricted to mid- and high-latitude regions, which confounds inferences of how the MCA impacted some global/hemispheric climate mechanisms (e.g. North Atlantic Oscillation; NAO). To address this knowledge gap, we estimate seasonally-resolved sea surface temperatures (SSTs) from the oxygen isotope composition (δ18O) of serially sampledPhorcus atratusshells from archaeological sites spanning the MCA in the Canary Islands. Twelve archaeological and six modernP. atratusshells were analyzed, and archaeological shells were dated using carbonate-target radiocarbon dating. SSTs were estimated using the published aragonite-water equilibrium fractionation equation. Modern shells showed a mean SST of 20.0 ± 1.5 °C, with a seasonal amplitude of 5.3 ± 0.9 °C. Archaeological shells exhibited a mean SST of 18.2 ± 0.7 °C, with a mean seasonal amplitude of 5.5 ± 1.0 °C. Thus, shells that span the MCA in the Canary Islands recorded SSTs that were significantly cooler than the modern (P< .05), contrasting with warming estimates and model predictions elsewhere in the Northern Hemisphere. We propose that the observed cooling resulted from increased upwelling in NW Africa due to a strengthening of the prevailing westerlies and coastal winds along the African shoreline. The intensified upwelling scenario during the MCA is partially supported by in-situ carbon isotope data (δ13C) retrieved from the archaeological shells, which was compared to the δ13C values of modern shells and dissolved inorganic carbon in the ambient seawater. These results are consistent with other low-latitude temperature/precipitation anomalies associated with a positive NAO mode, suggesting a transition to a positive NAO index during the middle and late MCA that possibly extended later into the 13th century AD.