Abstract Limited studies, particularly in the low-latitudes, exist that monitor cave ventilation, an important process dictating cave geochemical properties. To investigate cave ventilation, we present multi-year monitoring results, including observations of cave-air CO2, cave-air temperature, and cave-air relative humidity, from three Brazilian caves: Paraiso (4.07°S, 55.45°W), Tamboril (16.19°S, 46.59°W), and Jaragua (21.08°S, 56.58°W). Cave ventilation regimes on numerous temporal scales are established. Highest concentrations of cave-air CO2 are recorded during austral summers from a long-term (multi-year) monitoring effort at Jaragua Cave. Conversely, lowest cave-air CO2 concentration are recorded during austral winters. This variability in cave-air CO2 is attributed to seasonally varying temperature contrasts between surface- and cave-air, which results in buoyancy-driven cave-air CO2 exchange with surface-air CO2. Speleothem growth as calcite precipitation is expected to be biased towards the austral winter season when cave-air CO2 is lowest. This understanding of cave ventilation affects the interpretation of the geochemistry of speleothems as recorders of past climate change from caves in the low-latitudes. Next, continuous CO2 measurements near the entrance of Tamboril Cave suggest diurnal ventilation attributed to diurnal cave- and surface-air temperature differences. CO2 monitored from the deeper part of Paraiso and Jaragua Cave do not exhibit this diurnal cycle. Further, virtual temperature calculations suggest that buoyancy contrasts between cave- and surface-air drive daily cave ventilation in all three caves. The established cave ventilation regimes provide an additional framework to interpret speleothem based terrestrial environmental change spanning the low- and mid-latitudes. By investigating spatially disparate caves across Brazil, we find buoyancy driven cave ventilation to be a unifying mechanism. Our results suggest that exchange between cave- and surface-air occurs seasonally in caves across the tropics and sub-tropics. These results suggest buoyancy driven ventilation is important not only at mid-latitudes but also in the tropics where speleothems are typically suited to investigate paleomonsoon variability. Therefore, prior knowledge of cave ventilation in caves that contain speleothems suitable for paleoclimate reconstructions is critical to robustly infer subsequent geochemical trends across different timescales (seasonal- centennial).

中文翻译：

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巴西洞穴的昼夜到季节性通风

摘要 监测洞穴通风的研究非常有限，特别是在低纬度地区，这是决定洞穴地球化学特性的一个重要过程。为了研究洞穴通风，我们展示了多年监测结果，包括对三个巴西洞穴的洞穴空气二氧化碳、洞穴空气温度和洞穴空气相对湿度的观察：帕拉伊索 (4.07°S, 55.45°W)、坦博瑞尔（16.19°S，46.59°W）和 Jaragua（21.08°S，56.58°W）。建立了许多时间尺度的洞穴通风制度。在 Jaragua Cave 的长期（多年）监测工作中，在南方夏季记录了最高浓度的洞穴空气 CO2。相反，在南方冬季记录到最低的洞穴空气二氧化碳浓度。洞穴空气 CO2 的这种变化归因于地表和洞穴空气之间季节性变化的温度差异，这导致浮力驱动的洞穴空气二氧化碳与地表空气二氧化碳的交换。由于方解石降水，洞穴空气中的二氧化碳含量最低，预计洞穴生长会偏向于南方冬季。这种对洞穴通风的理解影响了对洞穴动物地球化学的解释，这些洞穴动物是低纬度洞穴过去气候变化的记录器。接下来，Tamboril Cave 入口附近的连续 CO2 测量表明昼夜通风归因于昼夜洞穴和地表空气温度差异。从帕拉伊索和 Jaragua Cave 更深处监测的 CO2 没有表现出这种昼夜循环。此外，虚拟温度计算表明，洞穴和地表空气之间的浮力对比推动了所有三个洞穴的每日洞穴通风。已建立的洞穴通风制度提供了一个额外的框架来解释基于洞穴的低纬度和中纬度陆地环境变化。通过调查巴西各地空间不同的洞穴，我们发现浮力驱动的洞穴通风是一种统一机制。我们的结果表明，洞穴和地表空气之间的交换在热带和亚热带的洞穴中季节性地发生。这些结果表明，浮力驱动的通风不仅在中纬度地区很重要，而且在热带地区也很重要，那里的洞穴动物通常适合研究古季风变率。因此，包含适合古气候重建的洞穴中洞穴通风的先验知识对于可靠地推断不同时间尺度（季节性 - 百年）的后续地球化学趋势至关重要。