Magnetic Mineralogy of Speleothems From Tropical-Subtropical Sites of South America
Magnetic Mineralogy of Speleothems From Tropical-Subtropical Sites of South America
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DOI：
10.3389/feart.2021.634482
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发表时间：
2021-04
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通讯作者：
Plinio Jaqueto;R. Trindade;J. Feinberg;J. Carmo;V. Novello;N. Stríkis;F. Cruz;M. Shimizu;
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作者：
Plinio Jaqueto;R. Trindade;J. Feinberg;J. Carmo;V. Novello;N. Stríkis;F. Cruz;M. Shimizu;
含铁矿物是洞穴沉积物组成的一小部分。它们起源于岩溶系统或从流域搬运到洞穴中。目前对洞穴沉积物磁性的研究主要集中在其磁性矿物学特征在特定时间段内的变化，且通常局限于单一样品。在这项研究中，我们描述了一个数据库的环境磁学参数建立在22个石笋从不同的洞穴位于巴西（南美洲）在不同的纬度，包括不同的气候和生物群落。在这些石笋中观察到的磁信号主要是由低磁化率矿物（1020 mT），其磁性类似于在成土环境中形成的磁铁矿。此外，与几个样品的土壤和洞穴的墙壁碳酸盐的比较显示出良好的协议的磁性能的洞穴积物与土壤样品，加强以前的建议，在（亚）热带制度，在洞穴积物中的主导磁相与洞穴上方的土壤。斯皮尔曼等级相关性指出磁浓度参数（质量标准化磁化率、自然回复磁化、非滞后回复磁化和等温回复磁化）之间的正强相关性。这意味着超细亚铁磁性矿物是这些（亚）热带岩溶系统中的主导相，这些岩溶系统延伸到不同的生物群落范围。虽然样品集中在萨凡纳生物群落（塞拉多）（约70%），与其他生物群落相比，大草原下的洞穴沉积物中磁性矿物的浓度较高，而潮湿阔叶林（大西洋和亚马逊生物群落）和干燥森林（卡廷加）的浓度较低。因此，降雨，生物群落和表层岩溶动力学在（亚）热带地区的洞穴沉积物中磁性矿物的浓度中发挥了重要作用，并表明它们可以成为洞穴系统古环境研究的重要目标。
Fe-bearing minerals are a tiny fraction of the composition of speleothems. They have their origin in the karst system or are transported from the drainage basin into the cave. Recent studies on the magnetism of speleothems focused on the variations of their magnetic mineralogy in specific time intervals and are usually limited to a single sample. In this study, we describe a database of environmental magnetism parameters built from 22 stalagmites from different caves located in Brazil (South America) at different latitudes, comprising different climates and biomes. The magnetic signal observed in these stalagmites is dominated by low-coercivity minerals (∼20 mT) whose magnetic properties resemble those of the magnetite formed in pedogenic environments. Also, a comparison with few samples from soils and the carbonate from cave’s walls shows a good agreement of the magnetic properties of speleothems with those of soil samples, reinforcing previous suggestions that in (sub-)tropical regimes, the dominant magnetic phase in speleothems is associated with the soil above the cave. Spearman’s rank correlation points to a positive strong correlation between magnetic concentration parameters (mass-normalized magnetic susceptibility, natural remanent magnetization, anhysteretic remanent magnetization, and isothermal remanent magnetization). This implies that ultrafine ferrimagnetic minerals are the dominant phase in these (sub-)tropical karst systems, which extend across a diverse range of biomes. Although the samples are concentrated in the savannah biome (Cerrado) (∼70%), comparison with other biomes shows a higher concentration of magnetic minerals in speleothem underlying savannahs and lower concentration in those underlying moist broadleaf forests (Atlantic and Amazon biome) and dry forests (Caatinga). Thus, rainfall, biome, and epikarst dynamics play an important role in the concentration of magnetic minerals in speleothems in (sub-)tropical sites and indicate they can be an important target for paleoenvironmental research in cave systems.