NOAA/WDS Paleoclimatology - Laguna Pumacocha, Peru, Holocene Stable Isotope Data

Oxygen isotope ratios of authigenic calcite (d18Ocal) measured at annual to decadal resolution from Laguna Pumacocha document Andean precipitation variability during the last 11,200 years. Modern limnological data show that Pumacocha d18Ocal reflects the average annual isotopic composition of the lake's surface waters (d18Olw), and that d18Olw tracks the isotopic composition of precipitation (d18Oprecip), which is largely controlled by the intensity of the South American summer monsoon (SASM). Based on these relationships we use down-core d18Ocal measurements as a proxy for d18Oprecip that varies with the intensity of SASM precipitation. Pumacocha d18Ocal increased rapidly between 11,200 and 10,300 yr B.P. from -14.5‰ to -10.5‰, reaching a maximum of -10.3‰ by 9800 yr B.P. After 9800 yr B.P., d18Ocal underwent a long-term decrease that tracked increasing Southern Hemisphere summer insolation, suggesting that enhanced SASM precipitation was linked to precessional forcing. Higher-frequency trends did not follow insolation and therefore represent other variability in the climate system. Millennial-scale trends from Pumacocha strongly resemble those from lower-resolution tropical Andean ice and lake core isotopic records, particularly the Huascaran ice core, and low elevation speleothems. These relationships suggest that tropical Andean isotopic records reflect variations in precipitation intensity related to precessional forcing rather than tropical temperatures. They also demonstrate a coherent pattern of SASM variability, although with differences between low elevation and Andean records during the late Glacial to Holocene transition and the late Holocene. Centennial and decadal SASM precipitation variability is also apparent. Reduced SASM rainfall occurred from 10,000-9200, 7000-5000, 1500-900 yr B.P. and during the last 100 years. Intensifications of the SASM occurred at 5000, 2200-1500, and 550-130 yr B.P. with the amplitude of variability increasing after 2200 yr B.P. These periods may represent SASM responses to ocean-atmosphere variability related to orbital and radiative forcing (e.g., El Niño-Southern Oscillation and the Intertropical Convergence Zone).

本数据集基于普马科查湖（Laguna Pumacocha）的沉积记录，以年-十年尺度分辨率测定了自生方解石氧同位素比值（d18Ocal），据此重建了过去11200年以来安第斯地区的降水变化历史。现代湖沼学观测数据表明，普马科查湖的d18Ocal可反映湖泊表层水的年均氧同位素组成（d18Olw），而d18Olw又与降水氧同位素组成（d18Oprecip）直接相关，后者主要受控于南美夏季风（South American Summer Monsoon, SASM）的强度。基于上述成因关联，本研究将岩芯d18Ocal作为代用指标，用以反演随SASM降水强度变化的d18Oprecip。
普马科查湖的d18Ocal在距今11200至10300年间快速上升，从-14.5‰升至-10.5‰，并在距今9800年时达到-10.3‰的峰值。距今9800年之后，d18Ocal呈现长期下降趋势，与南半球夏季日照量的增加相吻合，表明增强的SASM降水与轨道岁差强迫存在关联。较高频的同位素变化并未遵循日照量的演化规律，因此反映了气候系统的其他变率。
普马科查湖记录的千年尺度变率，与分辨率较低的热带安第斯地区冰芯、湖芯同位素记录（尤其是瓦斯卡兰冰芯（Huascaran ice core））以及低海拔洞穴碳酸盐沉积的记录高度相似。上述关联表明，热带安第斯地区的同位素记录反映的是与轨道岁差强迫相关的降水强度变化，而非热带气温变率。同时，这些记录也揭示了SASM变率的统一空间模式，但在晚冰期-全新世过渡阶段及全新世晚期，低海拔记录与安第斯山区记录之间存在一定差异。
研究还识别出百年至十年尺度的SASM降水变率：SASM降水偏弱的时段包括距今10000–9200年、7000–5000年、1500–900年以及近100年；SASM降水增强的时段则为距今5000年、2200–1500年以及550–130年，且在距今2200年之后，变率振幅有所增大。上述时段可能代表SASM对与轨道及辐射强迫相关的海-气变率的响应，例如厄尔尼诺-南方涛动（El Niño-Southern Oscillation, ENSO）与热带辐合带（Intertropical Convergence Zone, ITCZ）的活动变化。