Abstract

Carbonate speleothems contain measurable concentration of ²¹⁰Pb, which can be used to estimate modern growth rates up to a 150-years timescale. However, data referring to such application of this geochronological method are scarce in the Southern Hemisphere, with most studies based on temperate climate locations in the Asian continent. Such a lack of data makes it difficult to approach an investigative comparison of the main physical parameters that influence the precipitation rate of calcium carbonate in caves. In this context, the present study describes the record of longitudinal growth rates of carbonate speleothems sampled in two different Brazilian regions: Pains municipality (Midwestern Minas Gerais State) and Curitiba Metropolitan Area (CMA, eastern Paraná State), whose climates are subtropical with dry winter (Cwa) and wet temperate (Cfb), respectively. Segments of variable thickness from eight stalactites samples were subjected to HCl dissolution, ²¹⁰Po deposition onto copper discs, and ²¹⁰Po analysis by alpha spectrometry for yielding the activity concentration data. ²¹⁰Po activity concentration data provided the ²¹⁰Pb values due to radioactive equilibrium condition between these radionuclides. Although all samples exhibited a quantifiable measure of the ²¹⁰Pb activity concentration in their sectional layers, the longitudinal growth rate could not be estimated by the CF:CS method for three of the eight stalactites. However, the modified CRS method allowed determine growth rates for all speleothems. The mean ²¹⁰Pb CRS growth rate obtained at Pains and CMA sites is 1.4 ± 0.1 mm/year and 1.5 ± 0.2 mm/year, respectively, indicating coincidence at both sites and suggesting that the approach is reliable for determining modern growth rates.

中文翻译：

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采用 210Pb 年代法测定巴西碳酸盐岩洞穴的生长率

摘要

碳酸盐岩洞穴含有可测量浓度的²¹⁰ Pb，可用于估计长达 150 年时间尺度的现代增长率。然而，在南半球，涉及这种地质年代学方法应用的数据很少，大多数研究都是基于亚洲大陆的温带气候地点。由于缺乏数据，很难对影响洞穴中碳酸钙沉淀速率的主要物理参数进行调查比较。在这种背景下，本研究描述了在巴西两个不同地区采样的碳酸盐岩洞穴的纵向增长率记录：佩恩斯市（米纳斯吉拉斯州中西部）和库里蒂巴都会区（CMA，巴拉那州东部），其气候为亚热带，干燥分别为冬季（Cwa）和湿温带（Cfb）。对八个钟乳石样品的不同厚度的片段进行 HCl 溶解，将²¹⁰ Po 沉积到铜盘上，并通过 α 光谱法进行²¹⁰ Po 分析，以获得活性浓度数据。²¹⁰ Po 活度浓度数据提供了由于这些放射性核素之间的放射性平衡条件而产生的^{210 Pb 值。}尽管所有样品在其截面层中都表现出可量化的²¹⁰ Pb 活度浓度，但无法通过 CF:CS 方法估计 8 个钟乳石中的 3 个的纵向生长速率。然而，修改后的 CRS 方法可以确定所有洞穴动物的生长速率。在 Pains 和 CMA 站点获得的平均²¹⁰ Pb CRS 增长率分别为 1.4 ± 0.1 毫米/年和 1.5 ± 0.2 毫米/年，表明两个站点的一致性，并表明该方法对于确定现代增长率是可靠的。