NOAA/WDS Paleoclimatology - Tamboril Cave Geochemical Data from 10000 to 940 Cal Yr BP

Investigating controls on past variability of South American hydroclimate is critical to assessing its response to future warming scenarios. δ18O records from South America offer insight into past variability of the South American Monsoon System (SAMS). The controls, however, on precipitation δ18O values can be decoupled from precipitation amount at a given site and, thereby, limit local moisture condition reconstructions. Here we use a principal components analysis to assess the coherence of speleothem and lake core Holocene δ18O records in tropical and subtropical South America to evaluate the extent to which δ18O variability reflects changes in SAMS intensity at different sites across the region. The main mode of variability across Holocene δ18O records (PC1) closely tracks austral summertime insolation, consistent with existing work. Sites towards the periphery of the continent are heavily weighted on PC1, whereas interior sites as not. Further δ18O variability at interior sites bear little similarity to each other and implicate controls, beyond regional monsoon intensity, on these δ18O records. Further, we develop speleothem 87Sr/86Sr records spanning the Holocene from Tamboril Cave (Brazilian Highlands), Paraíso Cave (eastern Amazon Basin), Jaraguá Cave (Mato Grosso do Sul Plateau), and Botuverá Cave (Atlantic coastal plain) to investigate coupling between reconstructed monsoon variability (reflected by PC1) and local moisture conditions (interpreted from 87Sr/86Sr records). We interpret speleothem 87Sr/86Sr variability as a proxy of local moisture conditions, reflecting the degree of water-rock interaction with the cave host rock as driven by variations in water residence time. Speleothem 87Sr/86Sr records from all the sites, except Botuverá cave, do not co-vary with PC1, suggesting that local moisture conditions do not necessarily follow variations in regional monsoon intensity at these interior sites. These speleothem 87Sr/86Sr records generally suggest dry mid-Holocene conditions relative to the early- and late-Holocene, consistent with interpretations of other paleo-moisture records in the region, but timing of wet-dry transitions varies between sites. These results highlight that controls, in addition to SAMS variability, might influence δ18O variability. Further, our results suggest spatially variable local moisture conditions at interior sites that do not uniformly respond to regional monsoon intensity, and stress the need for δ18O-independent reconstructions of moisture conditions.

探究南美水文气候过去变化的控制机制，对评估其对未来增温情景的响应至关重要。来自南美洲的氧同位素δ¹⁸O记录，可为解析南美季风系统（South American Monsoon System, SAMS）的过去变化提供参考。然而，特定站点的降水δ¹⁸O值的控制因素可能与降水量脱钩，进而限制了局地湿度状况的重建工作。本研究采用主成分分析（principal components analysis, PCA），评估热带与亚热带南美洲内石笋和湖芯全新世δ¹⁸O记录的一致性，以此探究不同站点的δ¹⁸O变化在多大程度上反映了区域内SAMS强度的变化。

全新世δ¹⁸O记录的主要变化模态（主成分PC1）与南半球夏季日射量变化高度吻合，这与已有研究结果一致。该区域大陆边缘站点在PC1上的载荷值较高，而内陆站点则不然。此外，内陆站点的δ¹⁸O变化彼此间相关性极低，表明这些记录的δ¹⁸O变化除受区域季风强度影响外，还存在其他控制因素。

此外，本研究还建立了来自4个洞穴的全新世石笋⁸⁷Sr/⁸⁶Sr记录，分别位于巴西高地的坦博里尔洞（Tamboril Cave）、亚马孙盆地东部的帕拉伊索洞（Paraíso Cave）、南马托格罗索高原的雅拉瓜洞（Jaraguá Cave）以及大西洋沿岸平原的博图维拉洞（Botuverá Cave），以此探究重建的季风变化（由PC1反映）与局地湿度状况（由⁸⁷Sr/⁸⁶Sr记录解译）之间的耦合关系。我们将石笋⁸⁷Sr/⁸⁶Sr的变化视为局地湿度状况的代用指标，其反映了由水体驻留时间变化驱动的、与洞穴围岩之间的水-岩相互作用程度。

除博图维拉洞外，其余所有站点的石笋⁸⁷Sr/⁸⁶Sr记录均未与PC1同步变化，表明这些内陆站点的局地湿度状况未必会随区域季风强度的变化而变化。这些石笋⁸⁷Sr/⁸⁶Sr记录总体表明，相较于早全新世与晚全新世，中全新世整体偏干，这与该区域其他古湿度记录的解译结果一致，但不同站点的干湿转换时间存在差异。

本研究结果表明，除SAMS变化外，其他控制因素也可能影响δ¹⁸O的变化。此外，研究结果还显示，内陆站点的局地湿度状况存在空间异质性，并非统一响应区域季风强度的变化，同时也强调了开展不依赖δ¹⁸O的湿度状况重建工作的必要性。