Table4_The Effect of Differential Weathering on The Magnetic Properties of Paleosols: A Case Study of Magnetic Enhancement vs. Magnetic Depletion in the Pleistocene Blackwater Draw Formation, Texas.XLSX

The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.

黑水沟组（BDF）的地层剖面主要由一系列在风成沉积物上发育的五层古土壤层以及其上的地表土壤层构成。先前的研究表明，该剖面深度方向的磁性特性（例如，χ、ARM 和 IRM）对不同古土壤表现出磁增强和磁贫化信号。为了更好地理解导致这些不同响应的磁性矿物学，对构成 BDF 地层剖面的六个土壤单元中的代表性样品进行了各种岩石磁学实验、扫描电子显微镜和莫塞巴光谱分析。我们的结果表明，亚微米级的赤铁矿（其中单畴尺寸的赤铁矿（Hc ≈ 500 mT）的贡献微乎其微）在所有土壤中以重量百分比浓度占主导地位。而低矫顽力（Hc ≈ 35 mT 或更低）的磁铁矿/磁化磁铁矿颗粒，主要处于 PSD 状态（Mr/Ms ≈ 0.14 ± 0.03588，Hcr/Hc ≈ 2.68 ± 0.298789），在磁性信号中占主导地位。磁贫化土壤显示出相对较高的针铁矿比例，而磁增强土壤则显示出 SP/SSD 磁铁矿/磁化磁铁矿相贡献的增加。通过将我们的数据集与地质化学气候函数相结合，我们将地层剖面的磁保存、磁贫化和磁增强部分与三个不同的环境阶段（I-III）相对应。第一阶段的基础沉积物显示出相对均一（既未增强也未贫化）的磁性特性，这是由于相对干旱的气候条件和来自南方的风成沙的轻微改变。相反，第二阶段至第三阶段代表了风化强度和物源的变化，导致来自南方的沙和来自北方的粉砂的混合。第二阶段经历了更高的降水水平，导致 Fe-氧化物相的溶解，从而引起磁性贫化。最上层的第三阶段经历了中等的降水强度，导致磁性增强。利用先前发表的年龄模型，我们暂时将这些变化的环境条件解释为受中期更新世过渡期（1.2-0.7 Ma）的影响，当时地球的气候周期从约 41 kyr 转变为约 100 kyr。然而，由于目前发表的年龄模型的不确定性，仍存在模糊性。由于磁性信号的复杂性，我们建议未来的研究采用整体方法，结合岩石磁学、地球化学和显微镜观察，以更准确地重建区域古环境。